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HomeMy WebLinkAboutCT 02-14; BRESSI RANCH RESIDENTIAL; SUPPLEMENTAL GEOTECHNICAL INVESTIGATION; 2001-03-14SUPPLEMENTAL GEOTECHNICAL INVESTIGATION FOR MASS GRADING, BRESSI RANCH, CARLSBAD, CALIFORNIA March 14, 2001 ProjectNo. 971009-005 Prepared For: LENNAR HOMES 5780 Fleet Street, Suite 320 Carlsbad, Califomia 92008 Leighton and Ass ocistss, Inc. A LEIGHTON GROUP COMPANY 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY March 14,2001 ProjectNo. 971009-005 To: Lennar Homes 5780 Fleet Street, Suite 320 Carlsbad, Califomia 92008 Attention: Mr. Jim Urbina Subject: Supplemental Geotechnical Investigation for Mass Grading, Bressi Ranch, Carlsbad, Califomia hi accordance with your request, we have performed a supplemental geotechnical investigation for the Bressi Ranch property located southeast of the intersection of El Camino Real and Palomar Airport Road in Carlsbad, Califomia. The purpose ofthis report was to update die results of our preliminary geotechnical report for the site (Leighton, 1997) and to evaluate the existing geologic and geotechnical aspects of the proposed mass grading ofthe site relative to the latest 200-scale tentative tract map/grading plans. This report presents the results of our supplemental subsurface investigation and geotechnical analysis, and provides a summary of our conclusions and recommendations relative to the mass grading of the site. Based on the results of our supplemental investigation and review of the previous geotechnical reports pertinent to the subject site, the proposed development of the site for commercial and residential uses is considered feasible from a geotechnical standpoint provided the recommendations summarized in this report are implemented during the site grading operations. It should be noted that additional geotechnical evaluation of the site (mcluding additional field studies, laboratory testing, and slope stability analysis) and a grading plan review will be needed after more detailed grading plans (i.e. 40-scale grading plans) for the different planning areas ofthe site are developed. If you have any questions regarding our report, please contact this office. We appreciate this opportunity to be of service. Respectfully submitted. LEIGHTON AND ASSOCIATES, INC. Tim J. Lawson, RCE 53388 Principal Consulting Engineer Randall K. WagnVr, CEG 1612 Director of Geology Distribution Addressee Lennar Homes, Attention: Mr. Dale Greenhaigh PDC, Attention: Ms. Marina Wurst 3934 Murphy Canyon Road, Suite 8205 • San Diego, CA 92123-4425 858.292.8030 • Fax 858.292.0771 • www.leightongeo.com 4971009-005 TABLE OF CONTENTS Section Page 1.0 INTRODUCTION 1 1.1 PURPOSE AND SCOPE OF SERVICES 1 1.2 SITE DESCRIPTIGN 1 1.3 PROPOSED DEVELOPMENT " " 3 1.4 SUPPLEMENTAL SURFACE INVESTIGATION AND LABORATORY TESTING 3 2.0 GEOTECHNICAL CONDITIONS 5 2.1 REGIONAL GEOLOGY 5 2.2 SlTE-SPECinC GEOLOGY 5 2.2.1 Documented Fill Soils (Map Symbol -Af) 5 2.2.2 Undocumented Fill Soils (Map Symbol -Afu) 5 2.2.3 Topsoil (Unmapped) 6 2.2.4 Alluvium/ Colluvium, Undifferentiated (Map Symbol - Qal/Qcol) 6 2.2.5 Landslide Deposits (Map Symbol - Qls) 6 2.2.6 Santiago Formation (Map Symbol -Ts) 7 2.3 GEOLOGIC STRUCTURE 8 2.4 FAULTING 8 2.5 SEISMIC CONSIDERATIONS 9 2.5.1 Liquefaction and Dynamic Settlement 9 2.5.2 Uniform Building Code Seismic Parameters p 2.6 GROUNDWATER 10 2.7 ENGINEERING CHARACTERISTICS OF ON-SITE SOILS 10 2.7.7 Expansion Potential JO 2.7.2 Soluble Sulfate Content ZllZ" /1 2.7.3 Excavation Characteristics 7 / 2.7.-^ Earthwork Shrinkage and Bulking 77 2.8 SLOPE STABILITY 12 2.9 SETTLEMENT OF DEEP FILLS 12 3.0 CONCLUSIONS 13 4.0 RECOMMENDATIONS 15 4.1 EARTHWORK 15 4.1.1 Site Preparation 75 4.1.2 Removal and Recompaction of Unsuitable Soils 7 J 4.1.3 Excavations 77 4.1.4 Cut/Fill Transition Conditions 77 4.1.5 Fill Placement and Compaction 77 4.1.6 Settlement of Deep Fills 77 4.1.7 Expansive Soils and Selective Grading 75 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 4971009-005 TABLE OF CONTENTS (Continued) Section Page 4.2 SLOPE STABILITY 18 4.2.1 Deep-Seated Stability 19 4.2.2 Sutficial Stability 27 4.3 CONTROL OF GROUND WATER AND SURFACE WATERS 21 4.3.1 Canyon Subdrains 22 4.3.2 Buttress and Stability Fill Subdrains 22 4.3.3 Cut Slope Seepage Conditions 22 4.4 SETTLEMENT MONITORINO 23 4.5 SURFACE DRAINAGE AND LOT MAINTENANCE 23 4.6 GRADED SLOPES 23 4.7 PLAN REVIEW AND CONSTRUCTION OBSERVATION 24 5.0 LIMITATIONS 25 FIGURES FIGURE 1 - SITE LOCATION MAP - PAGE 2 FIGURES 2 THROUGH 14 - GEOLOGIC CROSS-SECTIONS A-A' THROUGH M-M' - IN POCKET TABLES TABLE 1 - GEOTECHNICAL SUMMARY OF EXISTING LANDSLIDES - REAR-OF-TEXT TABLE 2 - EARTHWORK SHRINKAGE AND BULKING ESTIMATES - PAGE 11 TABLE 3 - SLOPE STABE.ITY SOIL PARAMETERS - PAGE 19 PLATES PLATE 1 - GEOTECHNICAL MAP - IN POCKET PLATE 2 - REMEDL\L GRADING MAP - IN POCKET APPENDICES APPENDDC A - REFERENCES APPENDIX B - BORING AND TRENCH LOGS APPENDIX C - LABORATORY TEST PROCEDURES AND TEST RESULTS APPENDK D - GENERAL EARTHWORK AND GRADING SPECIFICATIONS FOR ROUGH-GRADING Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 1.0 INTRODUCTION 1.1 Purpose and Scope of Services This report has been prepared in accordance with your request and presents the results of our supplemental geotechnical investigation of the Bressi Ranch for mass grading purposes. Bressi Ranch is located southeast of the intersection of El Camino Real and Palomar Airport Road in the City of Carlsbad, Califomia (Figure 1). The purpose of this report was to update the results of our preliminary geotechnical report for the site (Leighton, 1997) and to evaluate the existing geologic and geotechnical aspects of the proposed mass grading of the site relative to the latest 200-scale tentative tract map/grading plans. Our scope of services included the following: • Review of pertinent available geotechnical literature (including previous geotechnical reports of Bressi Ranch, Rancho Carrillo, and La Costa - The Greens developments), geologic maps, and aerial photographs (Appendix A). • Reconnaissance and geologic mapping of the site. • A supplemental subsurface exploration program consisting of the excavation, sampling and logging of 8 large-diameter exploratory borings and 56 exploratory trenches across the site. The large-diameter borings and trenches were excavated to evaluate the characteristics of the subsurface soils. Logs of the borings and trenches are presented in Appendix B. Included in Appendix B, are logs of previous borings and trenches excavated by Leighton and others that are pertinent to the development of Bressi Ranch. • Laboratory testing of representative samples obtained during our preliminary and supplemental subsurface exploration programs (Appendix C). • Geotechnical analysis of the data accumulated during our supplemental investigation including seismic and slope stability analysis. • Preparation of this report presenting our fmdings, conclusions and recommendations including General Earthwork and Grading Specifications for Rough Grading (Appendix D) with respect to the proposed mass grading of the site. The approximate limits of the geologic units encountered and the boring and trench locations applicable to the development of the site are presented on the Geotechnical Map (Plate 1), Remedial Grading Map (Plate 2), and Cross- Sections A-A' through M-M' (Figures 2 through 14). The 200-scale Tentative Tract Map/Grading Plan (PDC, 2001b), was utilized as base map for the Geotechnical Map (Plate 1) and the Remedial Grading Map (Plate 2). 1.2 Site Description The subject property, with a total acreage of approximately 600 acres, is located southeast of the intersection El Camino Real and Palomar Airport Road in the City of Carlsbad, Califomia (Figure 1). The site consists of an irregular-shaped piece of property bordered on the north by Palomar Airport Road, on the west by El Camino Real, on the southwest and south by the La Costa - The Greens property, and by the Rancho Carrillo development and Melrose Drive to the east. 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY NORTH BASE MAP: Thomas Bros. GeoFlnder for Windows, San Diego County, 1995, Page 1127 1000 2000 4000 1 "=2,000" Scale in Feet Lennar / Bressi Ranch Carlsbad, California SITE LOCATION MAP Project No. 971009-005 Date March 2001 I& Figure No. 1 971009-005 Topographically, the site generally consists of a an east-west trending relatively large and flat ridgeline in the northem section of the site and a north-south trending central ridge in the south central portion of the site. The north-south trending central ridge is flanked by two large north-south trending drainages and associated tributaries forming the gently sloping hillside and valley terrain in the south central portion of the property. A large east-west trending drainage is located in the far southem portion of the site. Elevations on the site range from a high of approximately 465 feet mean sea level (msl) in the north central portion of the site to a low of approximately 125 feet msl within the major east-west drainage in the southem portion of the site. Natural drainage is presently accomplished through a network of minor ravines and ultimately through the east-west trending canyon in the southem portion of the site. Vegetation on the site ranges from remnant vegetable crops, native grasses, and weeds on the flat ridge tops, wide canyon bottoms and on the hillsides; and minor to thick chaparral and trees (mainly on the steep hillsides and the narrow canyon bottoms) in the central and southwestem portions of the site. Man-made features on the site include: 1) a single-family residence in the central portion of the site; 2) a guard shack at the northem site entrance; 3) two building foundations and several relatively small detention basins (associated with prior farming activities); 4) existing SDG&E, water and sewer line easements crossing the site in westem, central and southem portions of the site; 5) several dirt roads which cross the property, and 6) fences along the perimeter of the property. 1.3 Proposed Development Detailed site grading and development plans were not available as of the date of this report. However, we understand that the proposed site development will include 13 planning areas across the site for light industrial, commercial, and residential purposes. In general, the light industrial plaiming areas are located in the northem portion of the site along Palomar Airport Road while the commercial planning areas are in the south and southeastem portion of the site. The residential planning areas are generally located in the central and southwestem portion of the site. Approximately 445 acres of the site will be graded while the remaining acreage will be left as open space. Preliminary calculations of the earthwork quantities indicate the grading will entail approximately 6 million yards of cut and fill material (PDC, 2001a). We also understand that development of the Bressi Ranch will include 1) constmction of Poinsettia Lane from its existing terminus at the southeastem portion of the site through the Bressi Ranch property and possibly to El Camino Real; 2) construction of El Fuerte Road from Palomar Airport Road to existing portion of the road south of the Bressi Ranch Property; and 3) improvements to El Camino Real along the northwestem side of the Bressi Ranch Property. In addition, the development will include relatively large open space areas, interior streets, underground utilities, and other associated improvements. 1.4 Supplemental Surface Investigation and Laboratorv Testing Our supplemental subsurface investigation consisted of the excavation, logging and sampling of 8 large-diameter borings (utilizing a bucket-auger drill rig) and the excavation of 56 exploratory trenches (in addition to the previously excavated 7 small-diameter borings, 21 large-diameter borings and 30 exploratory trenches The borings and trenches were excavated to a maximum depth of approximately 116 and 15 feet, respectively. The large-diameter borings were entered by our 4 -3- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 geologist and down-hole logged. Logs of the borings and trenches are presented in Appendix B. The approximate locations of the borings are shown on the Geoteclmical Map (Plate 1) and Cross- Sections A-A' through M-M' (Figures 2 through 14). Subsequent to the subsurface investigation, the borings and trenches were backfilled. Appropriate laboratory testing was performed on representative soil samples obtain during our preliminary and supplemental subsurface investigations. The laboratory tests included moisture/density determinations, Atterberg Limits (e.g. liquid limit and plastic limit), undisturbed and remolded direct shear, maximum dry density and expansion index tests. A discussion of the tests performed and a summary of the test results are presented in Appendix C. The density/moisture determinations of the undisturbed samples obtained from the borings are shown on the boring logs (Appendix B). 4 •4- Leighton and Associates, Inc. A LEIGHTON GROUP COIVIPANY 971009-005 2.0 GEOTECHNICAL CQNDITIONS 2.1 Regional Geology The subject site is located within the coastal subprovince of the Peninsular Ranges Geomorphic Province, near the westem edge of the southem Califomia batholith. The topography at the edge of the batholith changes from the mgged landforms developed on the batholith to the more subdued landforms that typify the softer sedimentary formations of the coastal plain. Specifically, the site is underlain by the sedimentary units of the Tertiary-aged Santiago Formation. Subsequent to the deposition ofthis unit, erosion and regional tectonic uplift created the valleys and ridges ofthe area. Recent weathering and erosional processes have produced the Quatemary surficial units including alluvium, colluvium, landslide deposits, and topsoil that mantie the site. 2.2 Site-Specific Geologv Formational materials of the Tertiary-aged Santiago Formation and surficial units consisting of landslide deposits, colluvium, alluvium, topsoil and artificial fill soils were encountered during our preliminary and supplemental investigations of the site. The areal distributions ofthe geologic units are presented on the Geotechnical Map (Plate 1). Our interpretation of the subsurface geologic conditions is also indicated on Geologic Cross Sections A-A' through M-M' (Figures 2 through 14). Each ofthe geologic units present on the site is described below (youngest to oldest). 2.2.1 Documented Fill Soils (Map Symbol - Af) As observed, the documented fill was associated with prior grading of the Poinsettia Lane, El Fuerte Road, Palomar Airport Road, and Melrose Drive roadway alignments. As-graded Geotechnical Reports documenting the fill placement and grading operations (Appendix A) indicate the fill soils were placed at 90 percent relative compactions. However, we anticipate that the upper portion of the fill may have become desiccated and minimal removals (i.e. on the order of 1 to 5 feet) may be needed prior to the placement of additional fill. In addition, we anticipate the removal of unsuitable soils along the perimeter of the fills were not made outside a 1:1 (horizontal to vertical) projection from the existing toe-of-slope down to competent formational material undemeath the fill. 2.2.2 Undocumented Fill Soils (Map Svmbol - Afli) Undocumented fill soils were observed in a number of places on the site. The undocumented fill was generally associated with the gradmg of the on-site dirt roads, retention basins, and prior farming activities. These fill soils are estimated to range from 1 to 25 or more feet in depth, and generally consist of dry to damp, loose or soft sand and clay. All existing fills located within the limits of grading should be considered potentially compressible and unsuitable in their present state for stmctural support. The undocumented fill soils should be removed to competent formational material within the limits of the proposed grading. 4 -5-Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 2.2.3 Topsoil (Unmapped) The topsoil encountered during our field investigation mantles the majority of the site. The topsoil, as observed, consisted predominantly of a light-brown to brown, damp to moist, medium dense to stiff, sandy to silty clay and some clayey to silty sands. These soils were generally massive, porous, and contained scattered roots and organics. The unsuitable topsoil is estimated to be from 1 to 4 feet in thickness; however, localized areas of thicker accumulations of topsoil may be encountered during grading. 2.2.4 Alluvium/ Colluvium. Undifferentiated (Map Svmbol - Oal/Ocol) Potentially compressible deposits of alluvium were encountered in the major and most of the tributary drainage courses on the site. In addition, our field investigation indicated that potentially coinpressible deposits of colluvium mantle the middle and lower portions of the on-site natural slopes (especially slopes comprised of the Santiago Formation claystone) and in the upper portions of the tributary drainage courses throughout the site. During our supplemental investigation, we did not differentiate the alluvial and colluvial deposits and therefore, these soils are mapped and presented as undifferentiated alluvium/colluvium on the geologic maps and cross-sections. As observed, these deposits typically consist of light orange-brown to brown sands, sandy clays and clayey sands that are porous and contain scattered organics. Both the alluvium and colluvium are considered potentially compressible in the present state. In general, the alluvium/colluvium is estimated to be 4 to 15 feet thick in the tributary canyons, however deeper accumulations may be present. The alluviimi encountered in the proposed El Fuerte Road drainage and the main east-west rending canyon in the southem portion of the site (along proposed Poinsettia Lane) have alluvial thicknesses on the order of 40 to 50 or more feet. Relatively shallow ground water (generally less than 5 to 20 feet in depth) was observed in these alluvial soils. 2.2.5 Landslide Deposits (Map Svmbol - Ols) Several landslides have been identified within the subject property. The approximate limits of these landslides are shown on the Geotechnical Map (Plate 1) and the Remedial Grading Map (Plate 2). To aid in the discussion of the landslides, each landslide and/or landslide complex has been Numbered 1 through 24 (as indicated on the Remedial Grading Map). It should be noted that several of these landslides are outside the limits of grading while others extend beyond the property boundaries. A geotechnical sunmiary including a description of each numbered landslide/landslide and the preliminary recommendations to mitigate the landslide is also presented on Table 1 (presented at the rear of the text). The landslide deposits include graben material (and associated colluvial soils), relatively undisturbed blocks of formational material and weathered formational material consisting of soils characteristic of the on-site bedrock units (i.e. silty sands and silty to sandy clays). Graben development at the head of the landslides appears to be moderate to relatively 4 •6- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 minor depending on the amount of movement of the landslide. The landslide material is generally moderately fractured and jointed at depth and highly weathered near the surface and at the toe of the landslides. The landslide basal mpture surfaces, as observed in the borings, typically consisted of a paper-thin to 1/4-inch thick remolded clay seams. In general, the landslide basal mpture surfaces appear to correspond to either an existing clay seam or a weak zone in the formational claystone. Several of the smaller landslides shown on the Geotechnical Map (Plate 1) are shallow surficial slumps/debris flow deposits (generally less than 5 to 15 feet thick). These shallow surficial slumps or debris flows appear to have developed as a result of prior landslide activation, seepage zones and/or oversteepened canyon side-walls within highly weathered portions of the formational soils. Due to potentially instability concems and compressible nature, the landslide deposits within the limits of the plaimed grading are considered unsuitable for stmctural support in their present condition and remedial measures (i.e. buttressing with fill and/or removals of the unstable and potentially compressible portions) will be required. Preliminary recommendations for the stabilization of the landslides are presented in Section 4.2, on Table 1, and indicated on the Remedial Grading Map (Plate 2). With regard to the ancient landslides, site development appears feasible from a geoteclmical standpoint. However, Landslides Numbers 5, 9, 9A, and 12, all of which, extend off-site (or outside the proposed limits of grading).may require off-site grading in order to mitigate the landslide. 2.2.6 Santiago Formation (Map Svmbol - Ts) The Tertiary-aged Santiago Formation, as encountered during our field investigation, consists primarily of massively bedded sandstones and claystones/siltstones. The siltstones and claystones generally are olive green to gray (unweathered), damp to moist, stiff to hard, moderately weathered, fractured and sheared. The sandstone generally consists of orange brown (iron oxidized staining) to light brown, damp to moist, dense to very dense, silty fine to medium grained sandstone. In previous reports this unit was classified and mapped as the Delmar Formation and Torrey Sandstone. Recent publications (including Tan and Kennedy, 1996) classify these units in north San Diego Coimty as the Santiago Formation. When reviewing the boring and trench logs from previous investigations, this recent change should be taken into account. 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 2.3 Geologic Stmcture The general stmcture of the bedrock appears to be near horizontal. Based on the subsurface data, bedding within the Santiago Formation generally exhibits variable bedding with strikes ranging from northwest to northeast and dips typically 5 to 15 degrees to the southeast and northwest. Locally, cross bedding was observed with dips steeper than 15 degrees. Clay seams and/or landslide mpture surfaces encountered in the borings generally trend parallel to the bedding. Jointing on-site is very variable, but predominantly trends subparallel to the existing slopes. Jointing dips were found to be generally moderately to steeply dipping. Jointing was mainly encountered in the upper portion of the bedrock becoming less pronounced with depth. Randomly oriented shears were encountered mainly in the Santiago Formation claystone and siltstone. Numerous wide, diffuse zones of shearing, as well as more well-defined zones, were encountered in the bedrock, and are thought to be the result of regional tectonic shearing of the relatively stiff and unyielding siltstone and claystone. 2.4 Faulting Our discussion of faults on the site is prefaced with a discussion of Califomia legislation and state policies conceming the classification and land-use criteria associated with faults. By definition of the Califomia Mining and Geology Board, an active fauh is a fault that has had surface displacement within Holocene time (about the last 11,000 years). The State Geologist has defined a potentially active fault as any fauh considered to have been active during Quatemary time (last 1,600,000 years) but that has not been proven to be active or inactive. This definition is used in delineating Fault-Rupture Hazard Zones as mandated by the Alquist-Priolo Earthquake Fault Zoning Act of 1972 and as most recently revised in 1997. The intent of this act is to assure that unwise urban development does not occur across the traces of active faults. Based on our review of the Fault-Rupture Hazard Zones, the site is not located within any Fault-Rupture Hazard Zone as created by the Alquist-Priolo Act (Hart, 1997). San Diego, like the rest of southern Califomia, is seismically active as a result of being located near the active margin between the North American and Pacific tectonic plates. The principal source of seismic activity is movement along the northwest-trending regional fault zones such as the San Andreas, San Jacinto and Elsinore Faults Zones, as well as along less active faults such as the Rose Canyon Fault Zone. Our review of geologic literature pertaining to the site and general vicinity indicates that there are no known major or active faults on or in the immediate vicinity of the site (Weber, 1982, and Jennings, 1994). Evidence for faulting was not encountered during our field investigation. The nearest known active fault is the Rose Canyon Fault Zone, which is considered a Type B Seismic Source based on the 1997 Uniform building Code (UBC), and is located approximately 7.0 miles (11.2 kilometers) west of the site. Because of the lack of known active faults on the site, the potential for surface mpture at the site is considered low. Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 2.5 Seismic Considerations The principal seismic considerations for most stmctures in southem Califomia are surface mpturing of fault traces and damage caused by ground shaking or seismically induced ground settlement. The possibility of damage due to ground rapture at the site is considered low since active faults are not known to cross the site. Hazards from seiches and tsunamis are not present as the site is located away from the immediate coastal area and there are no large standing bodies of water in or near the site. The seismic hazard most likely to impact the site is ground-shaking resultmg from an earthquake on one of the major regional faults. The effects of seismic shaking can be reduced by adhering to the most recent edition of the Uniform Building Code and design parameters of the Stractural Engineers Association of Califomia. 2.5.1 Liquefaction and Dvnamic Settlement Liquefaction and dynamic settlement of soils can be caused by strong vibratory motion due to earthquakes. Research and historical data indicate that loose granular soils underlain by a near-surface ground water table are most susceptible to liquefaction, while the stability of most silty clays and clays is not adversely affected by vibratory motion. Because of the dense nature of the underlying formational material and lack of a shallow permanent groundwater table, it is our opinion that the potential for liquefaction or seismically induced dynamic settlement across the majority of the site due to the design earthquake is low. However, relatively shallow groundwater and loose sandy soils are present in the main canyons and the potential for liquefaction of these in considered moderate unless provisions to mitigate the potential liquefaction are not performed during site grading. We anticipate that after the removal of the unsuitable material above the ground water table, placement of fill soils on the order of 15 to 20 feet above the current grades in these areas and consolidation of the saturated sandy soils occur (after the placement of the fill soils), the potential for surface manifestation of liquefaction in these areas will be low. 2.5.2 Uniform Building Code Seismic Parameters The site is located within Seismic Zone 4. The closest active fault is the Rose Canyon Fault Zone, which is considered a Type B Seismic Source (per 1997 UBC criteria) and is located approximately 7.0 miles (11.2 kilometers) west of the site. The closest Type A Seismic Source is the Julian segment of the Elsinore Fauh Zone, which is located approximately 23.5 miles (38 kilometers) east of the site. The soil profile type at the site, following the planned grading is anticipated to be Sc. Near source factors of Na = 1.0 and Nv= 1.0 are considered appropriate base on the seismic setting. Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 2.6 Ground Water Ground water was encountered in a number of the exploratory borings and trenches excavated across the site. Random seepage zones were also encountered in some of the exploratory borings and surface water was observed in the south flowing drainage on the west side of tiie site and in the large west flowing drarnage (along proposed Poinsettia Lane) in the southernmost portion of the site. The approximate depths and elevations of the encountered ground water are depicted on the boring and trench logs (Appendix B). The ground water table encountered in the main drainages is generally perched ground water within the alluvial soils. The ground water that was encountered in the mam canyons of the site at the time of our preliminary and supplemental investigations was approximately 5 to 20 feet below the existing ground surface. Ground water seepage zones in the on-site formational material was encountered at slightly elevated depths relative to the ground water depths in the adjacent drainages as the ground water table will generally follow the overlying topography, although with less relief Seasonal fluctuations of surface water and ground water should be expected. Subdrains are recommended m the canyon removal areas and the buttress and stability fills as indicated in Appendix D. The approximate location of reconimended canyon subdrains are depicted on the Remedial Grading Map (Plate 2). Specific subdrain recommendations will be made when more detailed grading plans are developed. It should be noted that ground water levels might vary at the time of constmction from those elevations encountered during our preliminary and supplemental investigations. Since the elevations at which ground water was encountered were generally below anticipated finish grade elevations, it is our opinion that ground water related problems should be minimal provided the recommendations presented in this report are incorporated into the design and constraction of the project. It is our recommendation, however, that periodic inspection be made by either our soil engineer or engineering geologist during the grading operations and/or constraction for the presence of ground water. Remedial measures, if any, can be reconimended on a case-by-case basis during the grading and constraction operations. 2.7 Engineering Characteristics of On-site Soils Based on the results of our cmrent geotechnical investigation, previous geotechnical investigations of the site (Appendix A), laboratory testmg of representative on-site soils, and our professional experience on adjacent sites with similar soils, the engineermg characteristics of the on-site soils are discussed below. 2.7.1 Expansion Potential The expansion potential of the on-site soils ranges from very low to very high. The sandstone within the Santiago Formation and sandy surficial soils are anticipated to be in the very low to moderate expansion range. The siltstone and claystone of the Santiago Formation, as well as the clayey topsoil, alluvium, and colluvium are anticipated to have a medium to very high expansion potential. Geotechnical observation and/or laboratory testing upon completion of the graded pads are recommended to determine the actual expansion potential of fmish grade soils on the graded lots. To reduce the possibility of having expansion soils at or near finish pad grades, the clayey soils should be placed in deeper fill areas or outside the limits of the building pads. In addition, building pads 10- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 consisting of highly expansive soils may be overexcavated and replaced with fill soils having a lower expansion potential. 2.7.2 Soluble Sulfate Content Based on our professional experience on adjacent sites, the on-site soils should posses a negligible to moderate soluble sulfate content. However, some of the soils may possess a high sulfate content. Laboratory testing should be performed on the soils placed at or near finish grade after completion of site grading. 2.7.3 Excavation Characteristics With the exception of localized cemented zones within the Santiago Formation, it is anticipated that the on-site sedimentary and surficial soils may be excavated with conventional heavy-duty constraction equipment. Localized cemented zones may require heavy ripping. If a significant amount of oversize material (typically over 8 inches in maximum dimension) is generated, it should be placed to prevent possible settlement of the soil around the rocks, as recommended in Section 4.1 and Appendix D. 2.7.4 Earthwork Shrinkage and Bulking The volume change of excavated on-site materials upon recompaction as fill is expected to vary with materials and location. Typically, the surficial soils and bedrock materials vary significantly in natural and compacted density, and therefore, accurate earthwork shrinkage/bulking estimate caimot be determined. However, the following factors (based on the results of our subsurface investigations, laboratory testing, geotechnical analysis and professional experience on adjacent sites) are provided on Table 2 as guideline estimates. If possible, we suggest an area where site grades can be adjusted be provided as a balance area. Table 2 Earthwork Shrmkage and Bulking Estraiates Geologic Unit Estimated Shrinkage/bulking Topsoil/Alluvium/Colluvium 5 to 15 percent shrinkage Landslide Deposits 0 to 10 percent shrinkage Santiago Formation 3 to 7 percent bulking* * The clayey and more cemented sandy soils of the Santiago Formation are anticipated to bulk more than the slightly friable sands. 11 - Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 2.8 Slope Stabilitv Our review of tiie 200-scale tentative tract map/grading plan (PDC, 2001b) indicates that the proposed cut and fill slopes on the site are proposed at a maximum inclination of 2:1 (horizontal to vertical) and up to approxmiately 50 and 70 feet in height, respectively. Our analysis indicates tiiat the proposed fill slopes will have a deep-seated factor of safety of at least 1.5. With regard to natural slopes, our geotechnical analysis indicates the natural slopes should be grossly stable provided unforeseen geologic conditions are not present. However, our analysis indicates that some ofthe proposed cut slopes (especially cut slopes composed of claystones/siltstones ofthe Santiago Formation) may be surficially unstable and remedial measures such as tiie constmction of replacement/stability fills along the slope faces will be required. Slope stability analysis was also performed in areas underlain by landslides. Our analysis indicates that buttresses will be required in order to provide a minimum factor-of-safety of 1.5 or greater for slopes or grading pads proposed m areas of landslides. The recommended preluninary butb'esses are presented on the Remedial Grading Map (Plate 2) and on tiie applicable Geologic Cross-Sections (Figures 2 through 14). It should be noted tiiat additional analysis will be required after more detailed grading plans are developed. Specific preliminary recommendations conceming the stability of the slopes are presented in Section 4.2. 2.9 Settlement of Deep Fills Settlement of deep compacted fill soils occurs in two manners. One is short-term (elastic) settlement due to the weight of the overlying fill soils compressing the soil and driving the water out ofthe soil stincture (consolidation). This typically occurs during or within a few months after the completion of grading. In general, silty to clayey soils will compress more slowly than sandy/granular soils. The second manner is long-term settlement, which typically occurs on the order of years after the fill soils are placed and is ti-iggered by tiie additional wetting ofthe soils due to irrigation/precipitation. This occurs even to properly compacted fill soils and even tiiough subdrains are installed. Silty to clayey soils typically settle as much as two times the amount tiiat sandy soils will. Preluninary recommendations are presented in Sections 4.1.6 and 4.4. 12- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 3.0 CONCLUSIONS Based on the results of our supplemental geotechnical investigation at the subject site and our review of the previous geotechnical reports applicable to the site (Appendix A), it is our professional opinion that the proposed mass grading of the Bressi Ranch property is feasible from a geotechnical standpoint, provided the following conclusions and recommendations are incorporated into the project plans, specifications, and followed during the site grading operations. The following is a summary of the geotechnical factors, which may effect development of the site. • Based on our subsurface exploration and review of pertinent geotechnical reports, the site is underlain by the Santiago Formation, landslide deposits, alluvium, colluvium, topsoil and documented and undocumented fill soils. • The undocumented fill, topsoil, colluvium, alluvium and weathered formational materials are considered unsuitable in their present state and will require removal and recompaction in areas of proposed development or future fill. • Due to potentially instability concems and compressible nature, the landslide deposits within the limits of the planned grading are considered unsuitable for stractural support in their present condition and remedial measures (i.e. buttressing with fill and/or removals of the unstable and potentially compressible portions) will be required. Preliminary recommendations for the stabilization of the landslides are presented in Section 4.2, on Table 1, and indicated on the Remedial Grading Map (Plate 2). • Siltstone and claystone soils of the Santiago Formation are highly to very highly expansive. These expansive soils should either be removed where present within 5 feet of finish pad grades and replaced with soil having a lower expansion potential or a special foundation design (i.e. post-tensioned design) should be provided. • The existing on-site soils appear to be suitable material for use as fill provided they are relatively free of rocks (larger than 8 inches in maxunum dimension), organic material and debris. • Active faults are not known to exist on or in the immediate vicmity of the site. Because of the lack of known active faults on the site, the potential for surface rapture at the site is considered low. • The main seismic hazard that may affect the site is ground shaking from one of the active regional faults. • Evidence for faulting was not encountered during our field investigation. The nearest known active fauh is the Rose Canyon Fauh Zone, which is considered a Type B seismic source based on the 1997 Uniform building Code (UBC), and is located approximately 7.0 miles (11.2 kilometers) west ofthe site. • Due to the clayey and/or relatively dense nature of the on-site soils, the potential for liquefaction and dynamic settlement ofthe site is considered unlikely, provided the recommendations for site grading (as indicated in Section 4.1 and Appendix D) are adhered to. However, relatively shallow groundwater and loose sandy soils are present in the main canyons and the potential for liquefaction of these in 13. 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 considered moderate unless provisions to mitigate the potential liquefaction are not performed during site grading. The ground water table was encountered in the on-site alluvium/colluvium at depths from approximately 5 to 20 feet below the existing ground surface of the main drainages. In general, the ground water table within the main canyon along proposed Poinsettia Lane is anticipated to be on the order of 5 to 10 feet in depth while the north-south trending canyons on the west and east sides are on the order to 10 to 20 feet in depth. Localized seeps were also observed in a number of the large-diameter borings at various elevations with the formational material. However, ground water on the site is not anticipated to be a significant factor during site grading and subsequent development. If ground water seepage conditions are encountered during site development, recommendations to mitigate the conditions can be made on a case-by-case basis at that time. The expansion potential of the on-site soils ranges from very low to very high. The sandstone within the Santiago Formation and sandy surficial soils are anticipated to be in the very low to moderate expansion range. The sihstone and claystone of the Santiago Formation, as well as the clayey topsoil, alluvium, and colluvium are anticipated to have a medium to very high expansion potential. Based on our professional experience on adjacent sites, the on-site soils should posses a negligible to moderate soluble sulfate content. However, some of the soils may possess a high sulfate content. With the exception of localized cemented zones within the Santiago Fomiation, it is anticipated that the on-site sedimentary and surficial soils may be excavated with conventional heavy-duty constraction equipment. Localized cemented zones may require heavy ripping. All oversized material should be placed in accordance with the recommendations presented in Section 4.0 and Appendix D to minimize settlement of the material around the oversized rocks. In general, when recompacted as fill soils, the surficial units (including landslide deposits, topsoil, colluvium, alluvium, etc.) are anticipated to shrink and the bedrock materials are likely to bulk. It is anticipated that any planned major cut slopes that will be comprised of siltstones and/or claystones of the Santiago Formation will require stabilization measures to mitigate potential surficial instability. A detailed discussion on slope stability is provided in Section 4.2. Potential settlement of relatively deep fills is anticipated to mamly occur during or within a few months following the completion of grading. However, areas of deep fill should be monitored to ensure that the majority of the settlement occurs prior to constraction of stractures and other settlement sensitive improvements. In addition, lots underlain by fill differential thicknesses in excess of 20 feet will require the utilization of a special foundation design (i.e. a post-tensioned slab design). The potential for long term settlement of the alluvial materials within the main canyons, if not removed during grading, is potentially significant. 4 -14- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 4.0 RECOMMENDATIONS 4.1 Earthwork We anticipate that earthwork durmg the mass grading operations at the site will consist of site preparation, removals of unsuitable soil, excavation of cut material, fill placement, and trench excavation and backflll. We recommend that earthwork on-site be performed in accordance with the following recommendations, the City of Carlsbad gradmg requirements, and the General Earthwork and Grading Specifications for Rough-Grading (GEGS) included m Appendix D. In case of conflict, the followmg recommendations shall supersede those included as part of Appendix D. 4.1.1 Site Preparation Prior to the grading of areas to receive stractural fill or engineered stractures, the areas should be cleared of surface obstractions, any existing debris, unsuitable material (such as desiccated documented fill soils, undocumented fill soils, topsoil, colluvium, alluvium, landslide deposhs, and weathered formational materials) and stripped of vegetation. Vegetation and debris should be removed and properly disposed of off-site. Holes resulting from the removal of buried obstractions that extend below finished site grades should be replaced with suitable compacted fill material. Areas to receive fill and/or other surface improvements should be scarified to a minimum depth of 6 to 12 inches, brought to an above-optimum moisture condition, and recompacted to at least 90 percent relative compaction (based on American Standard of Testing and Materials [ASTM] Test Method D1557). 4.1.2 Removal and Recompaction of Unsuitable Soils As discussed in Sections 2.2 and 3.0, portions of the site are underlain by unsuitable soils, which may settle under the surcharge of fill and/or foundation loads. These materials include desiccated documented fill soils, undocumented fill soils, topsoil, colluvium, alluvium, landslide deposits and weathered fonnational material. Compressible materials not removed by the planned grading should be excavated to competent material, moisture conditioned or dried back (as needed) to obtain an above-optimum moisture content, and then recompacted prior to additional fill placement or surface improvements. The actual depth and extent of the required removals should be determined during grading operations by the geotechnical consultant; however, estimated removal depths are summarized below. 1) Existing Documented Fill The desiccated upper portion of the existing documented fills located in the eastem and southeastem portions of the site (associated with the grading of Carrillo Ranch) should be removed to competent fill prior to placement of additional fill. These materials can be utilized as fill materials provided they are moisture conditioned and free of deleterious materials. The estimated removal depths of the desiccated documented fills are anticipated to be on the order of 1 to 5 feet. However, deeper removals may be required along the edges of the fill where left-in-place unsuitable -15- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 soils may be present below the 1:1 (horizontal to vertical) removal edge extending from the existing toe-of-slope down undemeath the fill. 2) Existing Undocumented Fill The existing undocumented fills should be completely removed prior to placement of additional fill. These materials can be utilized as fill materials provided they are moisture conditioned and free of deleterious materials. The estimated removal depths of the undocumented fill soils range from 1 to more than 25 feet in depth. All trash, constraction debris, and decomposable material should be removed and disposed of off-site. 3) Topsoil Areas to receive fill which are on slopes flatter than 5:1 (horizontal to vertical) and where normal benching would not completely removed the topsoil, should be stripped to suitable formational material prior to fill placement. Topsoil is expected to be generally 1 to 4 feet thick, although localized deeper accumulations may be encountered during grading. 4) Alluvium and Colluvium In cut and fill areas, colluvial and alluvial materials should be completely removed to competent material. Where the alluvium/colluvium is located above the water table, this will most likely entail complete removal of these materials to competent bedrock. In areas where a shallow water table prevents the removal of these materials using conventional earthmoving equipment, (i.e. scrapers), we recommend the remaining material be removed by top loading (wet removal) or be surcharged and drained with wick drains. At the present time, we recommend further investigation of the areas where shallow ground water is present within the alluvial soils in order to determine the compressibility characteristics of the alluvial soils. Based on the results of the additional evaluation, specific geotechnical reconmiendations can be provided conceming the mitigation of the deep saturated alluvium and fill soils placed in these areas. 5) Landslide Deposits The landslide deposits within the limits of the planned grading should be removed to competent material during site grading in order to remove the highly disturbed and weathered material. The actual depth of stripping or overexcavation should be determined during grading based on field observations by the geotechnical consultant. However, preliminary recommendations relative to the removals of the landslides are presented on Table 1 for each of the mapped landslides on the site. Additional and/or specific recommendations may be made during the grading plan review after the completion of more detailed grading plans are developed. -16- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 4.1.3 Excavations Excavations of the on-site sedimentary and surficial materials may generally be accomplished with conventional heavy-duty earthwork equipment. Localized cemented zones in the Santiago Formation may be encoimtered that may require heavy rippmg. All oversized rock that is encountered should be placed as fill in accordance with the recommendations presented Appendix D. 4.1.4 Cut^ill Transition Conditions In order to reduce the potential for differential settlement in areas of cut/fill ttansitions, we recommend the entire cut portion of the transition building pads be overexcavated and replaced with properly compacted fill to mitigate the transition condition beneath the proposed stracture. For transitions less steep than a 2:1 (horizontal to vertical), the overexcavation of the cut portion of the building pad should be a minimum of 4 feet below the planned fmish grade elevation of the pad. For cut/fill transitions steeper than a 2:1 (horizontal to vertical) and for transitions beneath multi-unit and large commercial stractures, site specific overexcavation recoinmendations should be made after the final grading and development plans are completed. All overexcavations should extend laterally at least 10 feet beyond the building perimeter or footprmt. 4.1.5 Fill Placement and Compaction The on-site soils are generally suitable for use as compacted fill provided they are free of organic material, trash or debris, and rock fragments larger than 8 inches in maximum dimension. All fill soils should be brought to above-optimum moisture conditions and compacted m uniform lifts to at least 90 percent relative compaction based on the laboratory maxunum dry density (ASTM Test Method D1557). The optimum lift thickness required to produce a uniformly compacted fill will depend on the type and size of compaction equipment used. In general, fill should be placed in lifts not exceeding 8 inches in compacted thickness. Placement and compaction of fill should be performed in general accordance with the current City of Carlsbad grading ordinances, sound constraction practices, and the General Earthwork and Grading Specifications of Rough-Grading presented in Appendix D. 4.1.6 Settlement of Deep Fills Subsequent to removals of compressible soils, fills of up to approximately 70 to 80 feet in thickness are planned on the site. Our experience and analysis indicates that potential settlement in these deep fill areas maybe on the order of 10 inches (depending on compactive effort) and may take as long as 6 months (or longer) to occur. Most of this settlement is anticipated to occur during gradmg. We anticipate that post constraction settlement of onsite fills to be on the order of a maximum of 3 inches. Release ofbuilding pad areas for constraction of stractures in fill areas deeper than 50 feet should be made based on the results of surface settlement monuments placed at the completion of grading. 4 17- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 Provisional information indicates the release ofbuilding pads for constmction in areas not underlain by saturated alluvium will be on the order of 3 months following grading. 4.1.7 Expansive Soils and Selective Grading The laboratory test results and our professional experience with sunilar materials on sites in the vicinity indicate that the on-site soils possess a very low to very high expansion potential. As a result, the presence of highly expansive materials within 5 vertical feet from finish grade will require special foundation and slab considerations (i.e. the use of a post- tension foundation system). In general, this condition should be limited to finish grade pads comprised of Santiago Formation claystone or sihstone (generally below an approximate elevation of 270 to 300 feet msl across the site) or where these materials are utilized as compacted fill within 5 feet of finish grade. As an altemative to the use of a post-tension foundation system on lots possessing a medium to very high expansion potential, the building pads may be overexcavated a minimum of 5 feet below finish pad grade and replace with properly compacted fill possessing very low to low expansion potential (i.e., the sandy soil of the Santiago Formation present in the higher elevations of the site. Should this altemative be chosen, the overexcavation should extend across the entire lot and be graded such that water does not accumulate beneath the stractures (by providing a minimum 2 percent fall of the overexcavation bottom towards the street or existing fill). 4.2 Slope Stabilitv Review of the 200-scale tentative fract map/grading plans (PDC, 2001b), indicates that cut and fill slopes will be constracted to maximum heights on the order of 50 and 70 feet, respectively. We understand that these finished slopes are planned at inclinations of 2:1 (horizontal to vertical) or flatter. Based on the results of our investigation and geologic interpretations, the proposed slopes were analyzed for gross stability utilizing the STABL slope stability program and surficial stability using the infinite slope equation with steady-state seepage. The parameters utilized in our slope stability analysis are based on our laboratory testing, our experience with similar soil types, and our professional judgment. The parameters utilized in the slope stability analysis are presented on Table 3. A summary of our slope stability analyses is presented below. -18-4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 Table 3 Slope Stability Soil Parameters Material Moist Unit Weight (pcf) Saturated Unit Weight (pcf) Friction Angle (degrees) Cohesion (psf) Artificial Fill Soils 125 130 25 200 Quatemary Alluvium 125 130 18 100 Landslide Material 125 130 18 100 Clay Seam/Rupture Surface 120 125 8 100 Santiago Formation (sandstone) 130 135 30 300 Santiago Formation (claystone along bedding) 130 135 13.5 100 Santiago Formation (claystone across bedding) 130 135 30 300 4.2.1 Deep-Seated Stabilitv 1) Fill Slopes The materials anticipated for use in fill slope grading will predominantly consist of sandy and silty to clayey soils of the Santiago Formation. Our analysis, assuming homogeneous slope conditions, indicates the anticipated fill slopes up to the maximum proposed heights of 70 feet will have a calculated factor of safety of 1.5 or greater with respect to potential, deep-seated failure. The proposed slopes should be constracted in accordance with the recommendations of this report, the attached General Earthwork and Grading Specifications for Rough-grading (Appendix D), and City of Carlsbad grading code requirements. 2) Cut Slopes/Landslide Stabilization Engineering analysis of the proposed 2:1 (horizontal to vertical) cut slopes up to a maximum height of approximately 50 feet indicates the deep-seated stability of the slopes, in general, are stable from a geotechnical standpoint provided adverse geologic conditions are not present. The resuks of our stability analysis indicate that proposed cut slopes have a static factor of safety in excess of 1.5 for gross stability after remedial grading is performed (i.e. the constraction of butfresses) and our geotechnical recommendations are adhered to. -19-4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 However, cut slopes consisting of Santiago Formation claystones and siltstones may be surficially unstable and may require the constraction of stability or replacement fills on the slopes. Based on our subsurface exploration (Appendix B), we anticipate that the majority of the cut slopes below an approximate elevation of 270 to 300 feet msl will consist of Santiago Formation claystones and siltstones. The stability fill keys should be constracted a minimum of 15 feet wide, at least 5 feet below the toe-of- slope grade, and have a minimum 2 percent into-the-slope inclination. The approximate locations of the stability fill keys are presented on the Remedial Grading Map (Plate 2). A typical detail for stability fill constraction is provided in the attached General Earthwork and Grading Specifications (Appendix D). In addition. Geologic Cross-Sections A-A' through M-M' (Figures 4 through 17) were prepared in areas of landslides to further evaluate slope stability and to provide butfress designs, where appropriate, to increase the overall slope static factor-of-safety to at least a 1.5. Where butfresses are recoinmended, the preliminary dimensions are presented on the appropriate cross-section. A summary of the landslides is presented on Table 1. Prior to constraction of the recoinmended butfresses presented herein, the provisional stability recommendations should be reviewed and additional butfress design analysis performed based on the actual design grading plans. We recommend the geotechnical consultant document and geologically map all excavations including cut slopes during grading. The purpose of this mappmg is to substantiate the geologic conditions assumed in our analyses. Additional investigation and stability analysis may be required if unanticipated or adverse conditions are encountered during site development. 3) Slope Face Compaction and Finishing Due to the high expansion potential of the claystones and siltstones withm the Santiago Formation, special compaction procedures will be necessary in order for the specified compaction to be achieved out to the slope face. Soils placed within 15 feet of the face of slope should consist of a mixture of clay and sand. The sole use of highly expansive clayey or clean sandy material within 15 feet of the face of slope should be avoided. Overbuildmg the slope faces a minimum of 5 feet and trimming them back or frequent back-rolling with sheepsfoot compactors (at 1-to 3-foot vertical intervals) and back-rolling the completed slope with a short-shank sheepsfoot may be utilized to achieve the specified compaction of the slope face. 4) Stabilitv for Temporary Backcut Slopes During Grading The temporary backcut slopes that will be created during removal of unsuitable materials or constraction of the butfress and/or stabilization fills should have acceptable temporary factors of safety during grading. However, since there is still a small risk of slope instability, the possibility of temporary cut slopes failures may be reduced by: 1) keeping the time between cutting and filling operations to a minimum; 2) limiting the maximum length of back cut slopes exposed at any one time; 3) making removals at the head of the landslide before performing the butfress backcut near the toe of the landslide; and 4) cutting the temporary slopes at slope inclinations no steeper than 1-1/2:1 (horizontal to vertical) in locations of adverse geologic conditions 4 •20- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 and 1:1 (horizontal to vertical) inclinations in other locations. In critical areas, we may recommend a confractor's representative observe the backcut for signs of instability during butfress constraction. Backcut safety is the responsibility of the confractor. It is of utmost importance to schedule the earthwork sequence such that the time between removal and recompaction is reduced to a minimum. Full-time geologic inspection should be performed during backcut excavation, not only to confirm the geologic conditions but also to provide early waming of incipient failure of the temporary excavations and to allow m-constraction reaction to accommodate such failures and keep their occurrence to a minimum. 4.2.2 Surficial Stabilitv Surficial stability of the 2:1 (or flatter) fill and cut slopes were evaluated during our supplemental investigation. Our calculations indicate a factor of safety in excess of 1.5 for surficial stability under 4-foot steady state seepage. However, due to the presence of potentially adverse geologic conditions, we have recommended stability fills along the slopes exposing Santiago Formation claystones or siltstones as discussed in Section 4.2.1, Part 2. 4.3 Confrol of Groimd Water and Surface Waters Based on our preliminary and supplemental geotechnical investigations, it is our opinion that a permanent shallow ground water table does not currently exist at the site. However, a perched ground water condition is present within the alluvial soils in the main drainages of the property. The confrol of ground water in a hillside development is essential in order to reduce the potential for undesirable surface flow, hydrostatic pressure and the adverse effects of ground water on slope stability. We recommend that measures be taken to properly finish grade the site such that drainage water is directed away from top-of-slopes and away from proposed stractures. No ponding of water should be permitted. Drainage design is within the purview of the design civil engineer. Even with these provisions, our experience indicates that shallow ground water/perched ground water conditions can develop in areas where no such ground water conditions existed prior to site development, especially in areas where a substantial increase in surface water infilfration results from landscape irrigation. We recommend that an engineering geologist be present during grading operations to observe and record possible fiiture seepage areas and provide field recommendations for mitigation of future potential seepage. •21 4 Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 4.3.1 Canyon Subdrains In order to help reduce the potential for ground water accumulation in the proposed fill areas, we recommend subdrains be installed in the bottoms of canyons fill areas (or on the sides of the removal if saturated alluvium is left-in-place) prior to fill placement. The canyon subdrains should consist of a 6-inch diameter PVC pipe surrounded by a minimum of 9-cubic feet (per linear foot) of 3/4-inch gravel wrapped in a filter fabric (Mirafi MON or equivalent). Where the subdrain is placed on fill in order to outlet the subdrain, the subdrain should consist of solid PVC pipe. The subdrain should have a minimum fall of at least 1 percent. Specific subdrain recoinmendations will be made upon our review of the final site grading plans. However, preliminary canyon subdrain locations are presented on the Remedial Grading Map (Plate 2). Details for subdrain constraction are provided in the attached General Earthwork and Grading Specifications (Appendix D). The actual need and/or location of canyon subdrains should be based on the evaluation of the configuration of the canyon bottoms by the geoteclmical consultant after the removal of compressible soils have been completed. A representative of the project civil engmeer should survey the installed subdrains for alignment and grade. Sufficient time should be allowed for the surveys prior to commencement of fill placement operations over the subdrain. The subdrain outlets should be installed to discharge water into positive drainage devices (e.g. storm drain boxes, natural canyon bottoms, etc.). 4.3.2 Butfress and Stabilitv Fill Subdrains Subdrains should be provided in the butfress and stability fills constracted on-site in order to minimize slope instability. The subdrains should be placed along the heel of the butfress or stability fill key (across the entire length of the key) and along the backcut at approximately 30-foot vertical intervals. The subdrains should be placed and constracted in accordance with the recommendations presented in Appendix D. 4.3.3 Cut Slope Seepage Conditions Due to the anticipated presence of relatively permeable soils (i.e. Santiago Formation sandstone) underlain by relatively impermeable formational claystone or siltstones exposed in cut slopes on the site, groundwater seepage conditions are likely at these contacts. Slopes exposing these conditions (especially when the area at the top of the slope will be irrigated and/or where stractures will be located at the toe-of-slope) should be evaluated by the geotechnical consultant to determine if some type of subdrain system should be placed to mtercept the groundwater seepage. Recommendations to mitigate the seepage conditions include installmg a toe-of-slope subdrain system, installing a subdrain system at or slightly below the contact between the permeable and impermeable materials or by replacing the slope with a stability fill (discussed in Section 4.2.1, Part 2). 4 -22- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 4.4 Settlement Monitoring We recommend that fills greater than 50 feet in depth or where alluvium is left in place beneath the fill be monitored (by the placement of settlement monuments upon completion of rough-grading and periodic surveying). The number of settlement monuments necessary for monitoring and their placement locations should be determmed following a review of the site grading plans by the project geotechnical consultant. Constraction of settlement-sensitive stractures in these deep fill areas should be postponed until anticipated settlement is within tolerable limits based on the analysis of the geotechnical consultant. 4.5 Surface Drainage and Lot Maintenance Positive drainage of surface water away from stractures is very important. No water should be allowed to pond adjacent to buildings or the top of slopes. Positive drainage may be accomplished by providing drainage away from buildings at a gradient of at least 2 percent for a distance of at least 5 feet, and further maintained by a swale of drainage path at a gradient of at least 1 percent. Where limited by 5-foot side yards, drainage should be directed away from foundations for a minimum of 3 feet and into a collective swale or pipe system. Where necessary, drarnage paths may be shortened by use of area drains and collector pipes. Eave gutters also help reduce water infilfration into the subgrade soils if the downspouts are properly connected to appropriate outlets. Property owners should be reminded of the responsibilities of hillside maintenance practices (i.e., the maintenance of proper lot drainage; the undertaking of property improvements in accordance with sound engineering practices; and the proper maintenance of vegetation, including pradent lot and slope irrigation). 4.6 Graded Slopes It is recommended that all graded slopes within the development be planted with drought-tolerant ground cover vegetation as soon as practical to protect against erosion by reducing ranoff velocity. Deep-rooted vegetation should also be established to protect against surficial slumping. Oversteepening of existing slopes should be avoided during fine grading and constraction unless supported by appropriately designed retaining stractures. We recommend terrace drains on the slopes be designed by the civil engineer and be constracted in accordance with current City of Carlsbad specifications. Design of surface drainage provisions are within the purview of the project civil engineer. 4 -23- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 4.7 Plan Review and Constraction Observation The provisional recommendations contained in this report must be confirmed once the actual grading plans are available. At that time, additional investigation and analysis maybe required. Constraction observation of all on-site excavations and field density testing of all compacted fill should be performed by a representative of this office. We recommend that a geologist map all excavations during grading for the presence of potentially adverse geologic conditions. All footing excavations should be reviewed by this office prior to placing steel or concrete. 4 •24- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-005 5.0 LIMITATIONS The conclusions and recommendations in this report are based in part upon data that were obtained from a limited number of observations, site visits, excavations, samples, and tests. Such information is by necessity incomplete. The nature of many sites is such that differing geotechnical or geological conditions can occur within small distances and under varying climatic conditions. Changes in subsurface conditions can and do occur over time. Therefore, the fmdings, conclusions, and recommendations presented in this report can be relied upon only if Leighton and Associates has the opportunity to observe the subsurface conditions during grading and constraction ofthe project, in order to confirm that our preliminary findings are representative for the site. •25- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY Table 1 971009-005 Geotechnical Summary of Existing Landslides Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 1 Eastem portion of PA-1 LandsUde is a relatively large surficial slump. The landslide appears to have moved as a relatively incoherent mass of material. Failure was probably related to saturation conditions at the base of the weathered zone in the formational material (based on evidence of ground water seepage zones observed in the area). The backscarp was identified in Exploratory Trench T-27. The thickness of landslide is unknown, but anticipated to be on the order of 10 to 15 feet. A butfress approximately 60 feet wide with a depdi of at least 10 feet below the proposed toe-of-slope is recommended to remove the landsUde and stabilize the slope. A subdrain system at the heel of the key and panel drains in areas of observed and/or potential groimd water seepage zones should also be anticipated. 2 Southeastem portion of PA-2 (Cross-Section E-E') Landslide is essentially the same as Landslide No. 1 (i.e. a relatively large surficial slump). The landslide appears to have moved as a relatively incoherent mass of material. Failure was probably related to saturation conditions of the weathered zone based on evidence of ground water seepage zones observed in the area. The basal slip surface was reportedly encountered in Exploratory Trench GT-42 at a depth of 8 feet below the ground surface. Complete removal of the relatively large surficial slump to competent fonnational material vrithin the limits of grading is recommended. The removal depth is estimated to be on the order of 10 to 15 feet. In addition, it should be anticipated that the proposed fiU slope near the bottom of the landslide will require a buttress approximately 40 feet wide with a depth of at least 15 feet below the proposed toe-of-slope. A subdrain system including possible panel drains in areas of observed and/or potential ground water seepage zones should be anticipated. 3 North central portion of OS-1 Landslide is a relatively large surficial slump. Landslide appears to have moved as a relatively incoherent mass of material. Failure was probably related to saturation conditions based on evidence of ground water seepage observed in the area. Thickness of landslide is unknown, but anticipated to be on the order of 10 to 15 feet. This landslide is outside the limits of the proposed grading; and therefore, no remedial grading is needed. However, we recommend that the fiU slope key excavation for the proposed fill slope west of this landslide be geologically mapped to identify any evidence that the landslide may encroach into the fill slope key. Table 1 Geotechnical Summary of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 4 Southeast side of OS-1 Landslide is a relatively large surficial slump. LandsUde appears to have moved as a relatively incoherent mass of material. Failure was probably related to saturation conditions based on evidence of ground water seepage observed in the area. Thickness of landslide is unknown, but anticipated to be on the order of 10 to 20 feet. Complete removal of the relatively large surficial slump to competent formational material within the limits of grading is recommended. The removal depth is estimated to be on the order of 10 to 20 feet In addition, it should be anticipated that the proposed fiU slope above the landslide on the east side wiU require a fill slope/shear key approximately 15 feet wide with a depth of at least 5 feet below the proposed toe-of-slope. 5 East side of OS-1 and West side of PA-10 Landslide is a relatively large surficial slump. Landslide appears to have moved as a relatively incoherent mass of material. Failure was probably related to saturation conditions based on evidence of ground water seepage observed in the area. Thickness of landsUde is unknown, but anticipated to be on the order of 10 to 20 feet. Complete removal of the relatively large surficial slump to competent formational material is recommended since the landslide is completely within the limits of grading. The removal depth is estimated to be on the order of 10 to 20 feet. A subdrain system including possible panel drains in areas of observed and/or potential ground water seepage zones may be needed. 6 Southwest side of PA-6 and northwest side of PA- IO (Cross-Section D-D') Landslide complex is a relatively large and deep block- sUde type landslide that may include more than one landslide mass. The landsUde complex appears to have moved as at least two semi-competent blocks along a north to northwest dipping rapture surface at an approximate elevation of 265 feet Based on Borings LB-11 and LB-12, the upper approximately 25 feet of the landslide mass was found to be potentially compressible and unsuitable for the support of fill or surface improvements. Remove the potentially compressible soil to competent landslide material in the upper portion of the landslide. The removal depth is estimated to be on the order of 20 to 25 feet. The proposed in-filling of the canyon to the northwest with compacted fiU will effectively butfress the landslide complex. Table 1 Geotechnical Summary of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 7 Northwestem portion of PA-11 (Cross-Section M-M') Landslide is assumed to be a moderately sized rotational type landslide. The landslide appears to have moved as a relatively coherent mass of material. Thickness of landslide is unknown, but anticipated to be on the order of20 to 35 feet thick. Construction of a butfress on the order of 50 feet wide with a depth of approximately 30 feet is recommended along the proposed toe of the fill slope to completely remove the landsUde. A subdrain system in fee butfress is also recommended. 8 Westem portion of PA-11 Landslide is a moderately sized surficial slump. Landslide appears to have moved as a relatively coherent mass of material. Thickness of landslide is unknown, but anticipated to be on the order of 10 to 20 feet. Complete removal of the relatively large surficial slump to competent formational material within the limits of gradmg is recommended. The removal depth is estimated to be on the order of 10 to 20 feet. The proposed fill slope on the west side of the landslide will require a fill slope/shear key approximately 15 feet wide and 5 feet deep below the toe-of-slope. 9 Southem portion of PA-11 and partially offsite to the south (Cross-Section L-L') Landslide complex is a relatively large and deep block- slide type landsUde. The landslide likely failed along a clayseam or weak claystone bed at an elevation near the bottom of the adjacent drainage to the southeast. The landslide is estimated to be up to approximately 60 feet deep. It is not known how deep or the relative extent of the graben area, which wiU be located beneath a proposed fiU slope. A second proposed fill slope is located near the bottom of the landsUde just north of the property line. Some type of butfress will be required to stabiUze the landslide and provide an adequate factor of safety for the proposed fill slopes. The estimated size of the butfress at the toe of die landslide is 100 feet wide and 15 feet deep at die toe. A stability fill/shear key is also recommended below the upper fill slope. This key should be a minimum of 15 feet wide and 10 feet deep below the proposed toe of slope. In addition, removal of the unsuitable and potentially compressible portion of the landsUde is also recommended. The estimated depth of these removals is on the order of 10 to 20 feet within the limits of the proposed grading. Additional investigation and analvsis are reconunended to better define the landsUde geometrv and the butfress design. Table 1 971009-005 Geotechnical Summarv of Existing Landslides (continued) Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 9A Southem portion of PA-11 and partially offsite to the south (Cross-Section K-K') Landslide complex is a relatively small block-slide type landsUde. Based on Geocon Boring GLB-15 located offsite, the landslide failed along a clayseam within the Santiago Formation claystone at a depth of approximately 13 feet below the ground surfece (at an approximate elevation of 237 feet). The landsUde is estimated to be up to approximately 25 to 30 feet deep. A fill slope is proposed on the upper portion of the landslide. A proposed cut slope on the ofif-site property wiU be made through the front of the landslide. Constraction of a butfress on the order of 60 feet vride with a depth of approximately 25 feet (or through the landslide rapture surface) is recommended along the proposed toe of the fill slope located north of the properfy line. In addition removal of die upper approximately 5 to 10 feet of the landslide mass to competent material is also recommended. A subdrain system in the butfress is also recommended. 10 Southeastem comer of PA-11 (Cross-Section I-I') Landslide is a relatively small block-slide type landslide. The landslide rapture surface was encountered in Boring LB-27 at an approximate depth of 16 feet below the ground surface (at an approximate elevation of 220 feet). It appears that the landslide felled along a weak clay bed within the Santiago Formation claystone. The landslide is estimated to be up to approximately 25 to 30 feet deep. A fiU slope is proposed on the lower portion of the landslide north of the property line. Constraction of a butfress on the order of 60 feet wide with a depth of approximately 10 feet (or through the landslide rapture surface) is reconunended along the proposed toe of the fill slope located north of the properfy line. In addition removal of the upper approximately 5 to 10 feet of the landsUde mass to competent material is also recommended. A subdrain system in the butfress is also recommended. 11 Southem portion of PA-8 and southeastem comer of PA-11 (Cross-Section H-H') LandsUde is similar to Landslide No. 10 (i.e. a relatively small surficial slump). The landslide rapture surfece was encountered in Boring LB-26 at a depth of approximately 25 feet below the ground surface (at an approximate elevation of 220 feet). It appears that the landsUde failed along a weak clay bed within the Santiago Formation claystone. The landslide is estimated to be up to approximately 25 to 30 feet deep. A proposed fill slope is proposed on the lower portion of the landslide north of the property line. Constraction of a butfress on the order of 60 feet wide with a depth of approximately 10 feet (or through the landsUde rapture surface) is recommended along the proposed toe of the fill slope located north of the properfy line. In addition removal of the upper approximately 5 to 10 feet of the landslide mass to competent material is also reconimended. A subdrain system in the butfress is also recommended. Table 1 Geotechnical Summarv of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 12 Southem portion of PA-8, southeastem portion of PA-11, and westem portion of PA-12 (Cross-Sections C-C and G-G') Landslide is relatively large and extends partially off- site to the south. The landslide appears to have moved as a semi-competent block on a rapture surface that is dipping ^proximately 5 degrees to the southeast. The landslide is estimated to be up to approximately 30 to 50 feet deep. The lower end of the landsUde is Ukely buried by recent alluvium/colluvium. Based on the current proposed grades of PA-12, the fiU on and in front of the lower portion of tiie landslide effectively buttresses the landslide, and therefore, no butfress key is required. However, the unsuitable and potentially compressible portion of the landslide should be removed to competent material. Removal depths of the unsuitable material are estimated to be on die order of20 to 30 feet deep. 13 Northwestem portion of PA-12 and the southem portion of OS-3 (Cross-Section F-F') LandsUde is relatively large and extends partially into an open space area (OS-3). The landslide appears to have moved as a semi-competent block on a relatively flat rapture surface. The landslide rapture surface was encountered in Boring LB-7 at a depth of approximately 32.5 feet below the ground surface (at an approximate elevation of275 feet). It appears that the landslide failed along a weak clay bed within the Santiago Formation claystone just above a sandstone unit. The landslide is estimated to be approximately 35 to 80 feet thick. Some type of butfress will be required to stabilize the landslide. However, based on the current proposed grades, unknown geometry of the upper portion of Ae landslide, and our understanding that grading cannot occur within the open space area dictates that a buttress cannot be designed within the cuirent parameters. An additional investigation and analvsis are recommended 13 Northwestem portion of PA-12 and the southem portion of OS-3 (Cross-Section F-F') LandsUde is relatively large and extends partially into an open space area (OS-3). The landslide appears to have moved as a semi-competent block on a relatively flat rapture surface. The landslide rapture surface was encountered in Boring LB-7 at a depth of approximately 32.5 feet below the ground surface (at an approximate elevation of275 feet). It appears that the landslide failed along a weak clay bed within the Santiago Formation claystone just above a sandstone unit. The landslide is estimated to be approximately 35 to 80 feet thick. to better define the landslide geometrv and the buttress 13 Northwestem portion of PA-12 and the southem portion of OS-3 (Cross-Section F-F') LandsUde is relatively large and extends partially into an open space area (OS-3). The landslide appears to have moved as a semi-competent block on a relatively flat rapture surface. The landslide rapture surface was encountered in Boring LB-7 at a depth of approximately 32.5 feet below the ground surface (at an approximate elevation of275 feet). It appears that the landslide failed along a weak clay bed within the Santiago Formation claystone just above a sandstone unit. The landslide is estimated to be approximately 35 to 80 feet thick. design and/or design grades in front of the landslide 13 Northwestem portion of PA-12 and the southem portion of OS-3 (Cross-Section F-F') LandsUde is relatively large and extends partially into an open space area (OS-3). The landslide appears to have moved as a semi-competent block on a relatively flat rapture surface. The landslide rapture surface was encountered in Boring LB-7 at a depth of approximately 32.5 feet below the ground surface (at an approximate elevation of275 feet). It appears that the landslide failed along a weak clay bed within the Santiago Formation claystone just above a sandstone unit. The landslide is estimated to be approximately 35 to 80 feet thick. need to be raised in order stabilize the landsUde. 14 Southem portion of PA-13 Landslide is a relatively smaU block-slide type landslide. The landslide rapture surfece is anticipated to be at the same approximate elevation as LandsUde 15 (i.e. approximately 190 feet) within a clayey siltstone bed of the Santiago Fonnation. The landsUde is estimated to be up to approximately 25 to 30 feet deep. The lower end of the landslide is Ukely buried by recent alluvium/colluvium. Removal the potentially compressible soil to competent landsUde material in the upper portion of the landsUde is recommended. The removal deptii is estimated to be on the order of 10 to 15 feet The proposed in-filUng of die canyon to the west will effectively butfress the landslide. In addition, it should be anticipated fliat the upper end of the landsUde will need to be completely removed to competent formational material outside the proposed grading limits m order to stabilize die natural slope. A key approximately 15 feet wide should be constracted in this area. As an altemative, a building setback from the landslide may be provided. Table 1 Geotechnical Summary of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 15 Southem portion of PA-13 (Cross-Section J-J') LandsUde is essentially the same as Landslide No. 14 (i.e. a relatively small block-slide type landslide). The landslide rapture surface was encountered in Boring LB-6 at an approximate depth of 26 feet below the ground surfece (at an approximate elevation of 190 feet). It spears that the landslide felled along clayey siltstone bed directiy below a cemented zone within the Santiago Fonnation. The landslide is estimated to be up to approximately 25 to 30 feet deep. The lower end of the landslide is likely buried by recent alluvium/coUuvium. Removal the potentially compressible soil to competent landslide material in the upper portion of the landslide is recommended. The removal depth is estimated to be on the order of 10 to 15 feet. The proposed in-filling of the canyon to the west with compacted fill will effectively buttress flie landslide. In addition, it should be anticipated that the upper end of flie landslide wiU need to be completely removed to competent formational material outside the proposed grading limits in order to stabilize flie natural slope above the proposed relatively flat biulding pad. A key approximatefy 15 feet wide should be constmcted in this area. 16 Northem portion of PA-13 Landslide is a relatively smaU block-slide type landslide. The landsUde rapture surface was encountered in Boring LB-14 at an approximate depth of 34 feet below the ground surfece (at an approximate elevation of 200 feet) near the top of a claystone bed within the Santiago Fonnation. The landslide is estimated to be up to approximately 35 to 40 feet deep. The lower end of the landslide is likely buried by recent alluvium/colluvium. Removal the potentially compressible soil to competent landsUde material in the upper portion of the landslide is recommended. The removal depth is estimated to be on flie order of 10 to 15 feet. The proposed in-filling of the canyon to the west with compacted fill will effectively butfress the landslide. 17 Northem portion of PA-13 Landslide is essentially the same as Landslide No. 16 (i.e. a relatively small block-slide type landslide). The landslide rapture surface is anticipated to be at the same approximate elevation as Landslide 16 (i.e. ^proximately 200 feet) within a clayey siltstone bed of the Santiago Formation. The landslide is estimated to be up to approximately 25 to 30 feet deep. The lower end of flie landslide is likely buried by recent alluvium/colluvium. Removal the potentially compressible soil to competent landsUde material in the upper portion of flie landslide is recommended. The removal depth is estimated to be on the order of 10 to 15 feet. The proposed in-filling of the canyon to the west with compacted fill will effectively buttress the landslide. Table 1 Geotechnical Summarv of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 18 OS-3 and OS-7? And northwest portion of PA- 13 LandsUde is a moderately sized surficial slump. Landslide appears to have moved as a relatively coherent mass of material. Thickness of landslide is unknown, but anticipated to be on the order of 10 to 20 feet. Complete removal of the relatively large surficial slump to competent formational material within flie limits of grading is recommended. A building setback may be required adjacent to the landslide (due to the unstable landsUde in the open space/natural slope above the building pad to the west). As an altemative, grading in the open space may be performed to stabilize the upper portion of the landsUde outside the Umits of grading. 19 OS-3 and OS-7? And northwest portion of PA- 13 (Cross-Section A-A') Landslide is a moderately sized block-slide type landslide. The landslide appears to have moved as a semi-competent block on a rapture surface that is dipping approximately 10 to 12 degrees to flie east. The landslide rapture surface was encountered in Boring LB-20 at an approximate depth of 18 feet below the ground surfece (at an approximate elevation of 242 feet) within a weak claystone bed in the Santiago Formation. The landslide is estimated to be up to approximately 20 to 25 feet deep. The lower end of the landslide is likely buried by recent alluvium/colluvium. The proposed in-filling of the canyon to the west with compacted fill will effectively butfress the landslide. However, the landslide should be removed to competent formational material from a 1:1 projection down and away from the limits of fill. 20a and 20b OS-4 Landslide complex appears to be a relatively large block-slide fype landslide completely located in an open space area. Since the landslide is completely within an open space area, no remedial grading is required. 2laand21b OS-4 Landslide complex appears to be a relatively large block-slide type landslide completely located in an open space area. Since the landslide is completely within an open space area, no remedial grading is required. Table 1 Geotechnical Summarv of Existing Landslides (continued) 971009-005 Landslide Reference Number Location Geologic Conditions Conclusions and Recommendations 22 OS-4 LandsUde complex appears to be a relatively large block-slide type landslide completely located ui an open space area. Since the landslide is completely within an open space area, no remedial grading is required. 23 OS-2? Landslide is a small surficial slump. The landslide appears to have moved as a relatively incoherent mass of material. The thickness of the slide is unknown but beUeved to be on the order of 5 to 15 feet thick. Complete removal of the small surficial slump to competent formational material since flie landsUde is completely within the limits of grading. The removal depth is estimated to be on the order of 5 to 15 feet 24 PA-3 LandsUde is a small surficial slump in saturated colluvium behind an earthen embankment. Completely remove flie small surficial slump along wifli the colluvium. Extend the canyon subdrain through this area.