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Home My WebLink AboutCT 03-03; Bressi Ranch Affordable Housing; Supplemental Geotechnical Investigation; 2001-03-14... -... -... ---... ------------------------• -• -• 1.1 1.2 971009-005 1.0 INTRODUCTION 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, California (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 subsurf~ce 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 ofBressi 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 findings, 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, 2001 b), was utilized as base map for the Geotechnical Map (Plate 1) and the Remedial Grading Map (Plate 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, California (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 ~ - I -Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY ---• -------------------- ------------• 971009-005 Topographically, the site generally consists of a an east-west trending relatively large and flat ridge line in the northern 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 southern 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 southern 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 southern 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 southwestern portions of the site. Man-made features on the site include: I) a single-family residence in the central portion of the site; 2) a guard shack at the northern 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 western, central and southern portions of the site; 5) several dirt roads which cross the propepty, and 6) fences along the perimeter of the property. 1.3 Proposed Development 1.4 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. fu general, the light industrial planning areas are located in the northern portion of the site along Palomar Airport Road while the commercial planning areas are in the south and southeastern portion of the site. The residential planning areas are generally located in the central and southwestern 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) construction of Poinsettia Lane from its existing terminus at the southeastern 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 northwestern side of the Bressi Ranch Property. fu addition, the development will include relatively large open space areas, interior streets, underground utilities, and other associated improvements. Supplemental Surface fuvestigation and Laboratory 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 i by our -3- 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 Symbol -Qal/Qcol) 2.2.5 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 compressible 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 alluvium encountered in the proposed El Fuerte Road drainage and the main east-west rending canyon in the southern 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. Landslide Deposits (Map Symbol-Qls) 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 summary 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 moder"ielatively -6- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY ---2.6 ----.. -.. ------------2.7 ---------.. ---• -• 971009-005 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 the site and in the large west flowing drainage (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 main 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 in the canyon removal areas and the buttress and stability fills as indicated in Appendix D. The approximate location of recommended 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 construction 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 construction 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 construction for the presence of ground water. Remedial measures, if any, can be recommended on a case-by-case basis during the grading and construction operations. Engineering Characteristics of On-site Soils Based on the results of our current geotechnical investigation, previous geotechnical investigations of the site (Appendix A), laboratory testing of representative on-site soils, and our professional experience on adjacent sites with similar soils, the engineering 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 finish 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 additioning pads -10- 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 concerns and compressible nature, the landshde deposits within the limits of the planned grading are considered unsuitable for structural 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 maximum dimension), organic material and debris. • Active faults are not known to exist on or in the immediate vicinity of the site. Because of the lack of known active faults on the site, the potential for surface rupture 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 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. • Due to the clayey and/or relatively dense nature of the on-site soils, the potential for liquefaction and dynamic settlement of the 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~ these in cr -13-Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY -----4.1 ------------- --- --------------• 971009-005 4.0 RECOMMENDATIONS Earthwork We anticipate that earthwork during 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 backfill. We recommend that earthwork on-site be performed in accordance with the following recommendations, the City of Carlsbad grading requirements, and the General Earthwork and Grading Specifications for Rough-Grading (GEGS) included in Appendix D. In case of conflict, the following recommendations shall supersede those included as part of Appendix D. 4.1.1 Site Preparation Prior to the grading of areas to receive structural fill or engineered structures, the areas should be cleared of surface obstructions, any existing debris, unsuitable material (such as desiccated documented fill soils, undocumented fill soils, topsoil, colluvium, alluvium, landslide deposits, 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 obstructions 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 Dl557). 4.1.2 Removal and Recompaction ofUnsuitable 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 formational 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. I) Existing Documented Fill The desiccated upper portion of the existing documented fills located in the eastern and southeastern 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 I to 5 feet. However, deeper removals may be required along the edges of 1he fill where left-in-pisuitable -15 -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 construction 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 constructed 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 ofthe stability fill keys are presented on the Remedial Grading Map (Plate 2). A typical detail for stability fill construction 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 buttress designs, where appropriate, to increase the overall slope static factor-of-safety to at least a 1.5. Where buttresses are recommended, the preliminary dimensions are presented on the appropriate cross-section. A summary of the landslides is presented on Table 1. Prior to construction of the recommended buttresses presented herein, the provisional stability recommendations should be reviewed and additional buttress 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 mapping 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 within 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. Overbuilding 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) Stability for Temporary Backcut Slopes During Grading The temporary backcut slopes that will be created during removal of unsuitable materials or construction of the buttress 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 buttress 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~nditions cr -20-Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY • I .-CD c.o" )> :::r r r-+- m 0 Gl :::::! :i OJ 0 :::::! za. ~)> o en c en ~ 0 0 (") ;: OJ. "'0 ...-+-~ CD -< en I I I I Landslide Reference Number 2 3 I I I I I I , ••• J Location Eastern portion ofPA-1 Southeastern portion of PA-2 (Cross-Section E-E') North central portion of OS-I I I I I I ,I . I I I I I I I I I I I I I M..-·--.· '"''""' I I I .. I I ......... , ... "~ Table 1 971009-005 Geotechnical Summary of Existing Landslides Geologic Conditions Conclusions and Recommendations Landslide is a relatively large surficial slwnp. The A buttress approximately 60 feet wide with a depth of at landslide appears to have moved as a relatively least 1 0 feet below the proposed toe-of-slope is incoherent mass of material. Failure was probably recommended to remove the landslide and stabilize the related to saturation conditions at the base of the slope. A subdrain system at the heel of the key and panel weathered zone in the formational material (based on drains in areas of observed and/or potential ground evidence of ground water seepage zones observed in the water seepage zones should also be anticipated. area). The backscarp was identified in Exploratory Trench T-27. The thickness of landslide is unknown, but anticipated to be on the order of 1 0 to 15 feet. Landslide is essentially the same as Landslide No. 1 (i.e. a relatively large surficial slwnp). 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. Landslide is a relatively large surficial slwnp. 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 I 0 to 15 feet. Complete removal of the relatively large surficial slwnp to competent formational material within the limits of grading is recommended. The removal depth is estimated to be on the order of I 0 to 15 feet. In addition, it should be anticipated that the proposed fill 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 . This landslide is outside the limits of the proposed grading; and therefore, no remedial grading is needed. However, we recommend that the fill 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. I d I I r-eo <.e" l>::J r.....,. m 0 Gl ::::l :; m 0 ::::l za.. ~)> 0 (f) c (f) ~ 0 0 (") ;;:: m· ""0 ......... ~ CD -< (f) • I •• • I Landslide Reference Number 4 5 6 •• I • I • I Location Southeast side of OS-I East side of OS-I and West side ofPA-10 Southwest side ofPA-6 and northwest side ofPA- 10 (Cross-Section D-D') I 1. I I I J I 1.-· ! I I I I .•. .......~ I I I. I I. I I I I ................. I Table 1 971009-005 Geotechnical Summary of Existing Landslides (continued) Geologic Conditions Landslide is a relatively large surficial slwnp. 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 I 0 to 20 feet. Landslide is a relatively large surficial slwnp. 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 I 0 to 20 feet. Landslide complex is a relatively large and deep block- slide type landslide that may include more than one landslide mass. The landslide complex appears to have moved as at least two semi-competent blocks along a north to northwest dipping rupture 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. Conclusions and Recommendations Complete removal of the relatively large surficial slwnp 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 fill slope above the landslide on the east side -will require a fill slope/shear key approximately 15 feet wide with a depth of at least 5 feet below the proposed toe-of-slope. Complete removal of the relatively large surficial slwnp 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. 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 fill will effectively buttress the landslide complex. I I • I I CD (.Q p ::r ........ :;; 0 Gl :=J ~ !l) 0 :=J zc... ~)> o en ; en (') 0 0 ("')_ ;;: !l) -c,...... ~ CD -< en I I I I, I I, I I, I I, I I, I I. I I. I I. I I I I ....... ' ...•... ,.,. ~ .. I I I I I I I I i.. I I ..... I I I I Table 1 971009-005 Geotechnical Summary of Existing Landslides (continued) Landslide Reference Number 9A 10 11 Location Southern portion ofPA-11 and partially offsite to the south (Cross-Section K-K') Southeastern comer of PA-ll (Cross-Section I-1') Southern portion ofPA-8 and southeastern comer of PA-ll (Cross-Section H-H') Geologic Conditions Landslide complex is a relatively small block-slide type landslide. 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 surface (at an approximate elevation of 237 feet). The landslide 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 off-site property will be made through the front of the landslide. Landslide is a relatively small block-slide type landslide. The landslide rupture 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 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 fill slope is proposed on the lower portion of the landslide north of the property line. Landslide is similar to Landslide No. 10 (i.e. a relatively small surficial slump). The landslide rupture surface 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 landslide 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. Conclusions and Recommendations Construction of a buttress on the order of 60 feet wide with a depth of approximately 25 feet (or through the landslide rupture surface) is recommended along the proposed toe of the fill slope located north of the property line. In addition removal of the upper approximately 5 to I 0 feet of the landslide mass to competent material is also recommended. A subdrain system in the buttress is also recommended. Construction of a buttress on the order of 60 feet wide with a depth of approximately 10 feet (or through the landslide rupture surface) is recommended along the proposed toe of the fill slope located north of the property line. In addition removal of the upper approximately 5 to 1 0 feet of the landslide mass to competent material is also recommended. A subdrain system in the buttress is also recommended. Construction of a buttress on the order of 60 feet wide with a depth of approximately I 0 feet (or through the landslide rupture surface) is recommended along the proposed toe of the fill slope located north of the property line. In addition removal of the upper approximately 5 to I 0 feet of the landslide mass to competent material is also recommended. A subdrain system in the buttress is also recommended. I I I CD <.C :..:::r r r-+- m 0 Gl :::::::1 =i Q) 0 :::::::1 zc... ~)> o en c en ~ 0 0 (') 3: m· "tJ r-+-~ CD -< en :::::::1 (') I I I I I I I 1. I 1. I I. I I. I J, I I, I I. I I I I I I I I I I .... ........... ..,~ ..J·-· I l I I ..• --,.,o~~ . ,.,. Table 1 971009-005 Geotechnical Summary of Existing Landslides (continued) Landslide Reference Number 12 13 14 Location Southern portion ofPA-8, southeastern portion of PA-11, and western portion ofPA-12 (Cross-Sections C-C' and G-G') Northwestern portion of PA-12 and the southern portion ofOS-3 (Cross-Section F-F') Southern portion ofPA-13 Geologic Conditions 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 rupture surface that is dipping approximately 5 degrees to the southeast. The landslide is estimated to be up to approximately 30 to 50 feet deep. The lower end of the landslide is likely buried by recent alluvium/colluvium. Landslide 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 rupture surface. The landslide rupture 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. Landslide is a relatively small block-slide type landslide. The landslide rupture surface is anticipated to be at the same approximate elevation as Landslide 15 (i.e. approximately 190 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 the landslide is likely buried by recent alluvium/colluvium. Conclusions and Recommendations Based on the current proposed grades of P A -12, the fill on and in front of the lower portion of the landslide effectively buttresses the landslide, and therefore, no buttress 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 the order of20 to 30 feet deep. Some type of buttress will be required to stabilize the landslide. However, based on the current proposed grades, unknown geometry of the upper portion of the landslide, and our understanding that grading cannot occur within the open space area dictates that a buttress cannot be designed within the current parameters. An additional investigation and analysis are recommended to better define the landslide geometiy and the buttress design and/or design grades in front of the landslide need to be raised in order stabilize the landslide. 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 I 0 to 15 feet. The proposed in-filling of the canyon to the west will effectively buttress the landslide. In addition, it should be anticipated that the upper end of the landslide will need to be completely removed to competent formational material outside the proposed grading limits in order to stabilize the natural slope. A key approximately 15 feet wide should be constructed in this area. As an alternative, a building setback from the landslide may be provided. I I •. , I I I I r-CD co· > ::::r ,.. ....... m 0 "' ~ :I: ru ... 0 ~ z Cl.. "' ::r> ;o 0 en c en "0 n 0 0 (') 3: ru· "0 > ....... z CD -< en ~ (') I 1. I I, I I I 1. I I. • I, 1. I, I I. I I, I 1. ~-c·· _, I I -1 -~ ·I I I I I I Landslide Reference Number 15 16 17 Location Southern portion ofPA-13 (Cross-Section J-J') Northern portion ofPA-13 Northern portion ofPA-13 I:IJ.··· Table 1 971009-005 Geotechnical Summary of Existing Landslides (continued) Geologic Conditions Landslide is essentially the same as Landslide No. 14 (i.e. a relatively small block-slide type landslide). The landslide rupture surface was encountered in Boring LB-6 at an approximate depth of 26 feet below the ground surface (at an approximate elevation of 190 feet). It appears that the landslide failed along clayey siltstone bed directly below a cemented zone within the Santiago Formation. 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/ colluvium. Landslide is a relatively small block-slide type landslide. The landslide rupture surface was encountered in Boring LB-14 at an approximate depth of 34 feet below the ground surface (at an approximate elevation of200 feet) near the top of a claystone bed within the Santiago Formation. 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. Landslide is essentially the same as Landslide No. 16 (i.e. a relatively small block-slide type landslide). The landslide rupture surface is anticipated to be at the same approximate elevation as Landslide 16 (i.e. approximately 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 the landslide is likely buried by recent alluvium! colluvium. Conclusions and Recommendations 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 the landslide. In addition, it should be anticipated that the upper end of the landslide will need to be completely removed to competent formational material outside the proposed grading limits in order to stabilize the natural slope above the proposed relatively flat building pad. A key approximately 15 feet wide should be constructed in this area. 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 the landslide. 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 the landslide. I I od .J I I r-CD co ,.::::::r ........ :;; 0 Gl ~ ~ Ol 0 ~ za... ~)> o en c en ~ 0 0 (") ;c w· "".-.-~ CD -< en I I I I I I Landslide Reference Number 18 19 20a and 20b 2la and 21b I I I I I I, I I .. I I I I _, I I . 1. I .. l I, I . t I 1., I I, I I , I Location OS-3 and OS-7? And northwest portion ofPA- 13 OS-3 and OS-7? And northwest portion ofPA- 13 (Cross-Section A-A') OS-4 OS-4 ., ...... ,J Table I 971009-005 Geotechnical Summary of Existing Landslides (continued) Geologic Conditions 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 1 0 to 20 feet. Landslide is a moderately sized block-slide type landslide. The landslide appears to have moved as a semi-competent block on a rupture surface that is dipping approximately 10 to 12 degrees to the east. The landslide rupture surface was encountered in Boring LB-20 at an approximate depth of 18 feet below the ground surface (at an approximate elevation of242 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. Landslide complex appears to be a relatively large block-slide type landslide completely located in an open space area. Landslide complex appears to be a relatively large block-slide type landslide completely located in an open space area. Conclusions and Recommendations Complete removal of the relatively large surficial slump to competent formational material within the limits of grading is recommended. A building setback may be required adjacent to the landslide (due to the unstable landslide in the open space/natural slope above the building pad to the west). As an alternative, grading in the open space may be performed to stabilize the upper portion ofthe landslide outside the limits of grading. The proposed in-filling of the canyon to the west with compacted fill will effectively buttress the landslide. However, the landslide should be removed to competent formational material from a 1: I projection down and away from the limits of fill. Since the landslide is completely within an open space area, no remedial grading is required. Since the landslide is completely within an open space area, no remedial grading is required. I - 1 l l i -' ..... .... ..... 971009-005 APPENDIX A References Blake, Thomas F., 1996, EQF A ULT, Version 2.2. ---, 1998,FRISKSP. Eisenberg, L.l., 1985, Pleistocene Faults and Marine Terraces, Northern San Diego County in Abbott, P.L., Editor, On the Manner of Deposition of the Eocene Strata in Northern San Diego County, San Diego Association of Geologists, Field Trip Guidebook, pp. 86-91. Eisenberg, L.I. and Abbott, P.L., 1985, Eocene Lithofacies and Geologic History, Northern San Diego County in Abbott, P.L., ed., On the Manner of Deposition of the Eocene Strata in Northern San Diego County: San Diego Association of Geologists, Field Trip Guidebook, pp. 19-35. Geocon, Inc., 1982a, Soil and Geologic Reconnaissance,Bressi Ranch, San Diego County, California, File No. D-27I4-TO I, dated March 23, I982. ----., I982b, Report of Phase I Geotechnical Study, Bressi Ranch, Carlsbad, California, File No. D- 27I4-T02, dated June 4, I982. ----, I992, Preliminary Geotechnical Investigation, Rancho Carrillo, Villages E, J, and K, Carlsbad, California, Project No. 04787-12-04, dated December 30, I992. ----, 1993, Geologic Investigation for Rancho Carrillo Project-Major Roads, Carlsbad, California, Project No. 04787-I2-0I, dated January I5, I993. ----, I996, Geologic Investigation, Rancho Carrillo -El Fuerte Detention Basin Embankment and Existing Bressi Dam, Carlsbad, California, Project No. 04787-I2-II, dated November 25, I996. 1998, Final Report of Testing and Observation Services During Site Grading, El Fuerte Detention Embankment, Rancho Carrillo, Carlsbad, California, Project No. 05845-I2-07, dated November 20, I998. ----, 2000, Supplemental Soil and Geologic Investigation, Villages of La Costa -The Greens, Carlsbad, California, Project No. 06403-I2-0 I, dated March 24, 2000. Hannan, D., I975, Faulting in the Oceanside, Carlsbad and Vista Areas, Northern San Diego County, California in Ross, A. and Dowlens, R.J., eds., Studies on the Geology of Camp Pendleton and Western San Diego County, California: San Diego Association of Geologists, pp. 56- 59. Hart, E.W., I997, Fault-Rupture Hazard Zones in California, Alquist-Priolo Special Studies Zones Act of I972 with Index to Special Studies Zones Maps: Department of Conservation, Division of Mines and Geology, Special Publication42. A-I - - .... - -. J - - i j - ... .. ... 971009-005 APPENDIX A (continued) International Conference of Building Officials (ICBO), 1997, Unifonn Building Code, Volume I - Administrative, Fire-and Life-Safety, and Field Inspection Provisions, Volume II - Structural Engineering Design Provisions, and Volume III -Material, Testing and Installation Provision, I CBO. Jennings, C.W., 1994, Fault Activity Map of California and Adjacent Areas; California Division of Mines and Geology, Geologic Data Map 6, Scale I :750,000 . Leighton and Associates, Inc., 1992, City of Carlsbad, Geotechnical Hazards Analysis and Mapping Study, 84 Sheets, dated November, 1992. ----., 1997, Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California, Project No. 4971009-002,dated July 29, 1997. ----, 1998,Recommendations for Overexcavation of Potentially Compressible Materials, for Onsite Portion of Land Outfall Sewer Relocation Project, Future Poinsettia Lane, Bressi Ranch, Carlsbad, California, Project No. 4971009-002, dated March 26, 1998. ----, 2000a, Geotechnical Review of Conceptual Grading Plans for the Wetland Restoration Area, Bressi Ranch, Carlsbad, California, Project No. 4971009-005, dated January 25, 2000. ----, 2000a, Geotechnical Review of Land-Use Plans for Bressi Ranch, Carlsbad, California, Project No. 4971009-005, dated March 2, 2000. ----, 2000c, Wetland Restoration Area Grading, Bressi Ranch, Carlsbad, California, Project No. 4971 009-005, dated April 17, 2000. ----, 2000d, Remedial Quantity Estimates, Offsite Portion of Proposed Poinsettia Lane, Bressi Ranch Development, Carlsbad, California, Project No. 971009-005, dated August 25, 2000. ----., 2001, Geotechnical Feasibility Study, Offsite Poinsettia Lane, Alicante Road and Borrow Sites Within The Greens of The Villages of La Costa, Bressi Ranch Development, Carlsbad, California, Project No. 971009-005,dated January 5, 2001. ----., Undated, Unpublished In-House Geotechnical Data. Lindvall, S.C., and Rockwell, T.K., 1995, Holocene Activity of the Rose Canyon Fault Zone in San Diego, California: Journal of Geophysical Research, V. 100, No. Bl2, p. 24, 124-24, 132. PDC, 200la, Cut/Fill Exhibit, 200 Scale, Bressi Ranch, Carlsbad, California, dated February 14,200 I . PDC, 200Ib, Tentative Tract Map/Grading Plan, 200 Scale, Bressi Ranch, Carlsbad, California, undated, received February 16,2001. A-2 "" - ... ·- - ·- - ·- ..... --, . , -. " ~ " ! n ' 971009-005 APPENDIX A (continued) Reichle, M.S., and Kahle, J.E., 1990, Planning Scenario for a Major Earthquake, San Diego-Tijuana Metropolitan Area: California Division of Mines and Geology, Special Publication 100, 180 p . Rockwell, T.K., and Lindvall, S.C., 1990, Holocene Activity of the Rose Canyon Fault in San Diego, California, Based on Trench Exposures and Tectonic Geomorphology; Geological Society of America, Abstracts with Programs. ----, 1991, Minimum Holocene Slip Rate for the Rose Canyon Fault in San Diego, California in Environmental Perils, San Diego Region: San Diego Association of Geologists, p. 37-46. Tan, S.S., and Kennedy, M.P., 1996, Geologic Maps of the Northwestern Part of San Diego County, California: California Division of Mines and Geology, DMG Open-File Report 96-02, 2 Plates. Tan, S.S., and Giffen, D.G., 1995, Landslide Hazards in the Northern Port of the San Diego Metropolitan Area, San Diego County, California, Landslide Hazard Identification Map No. 35, Division of Mines and Geology, Open-File Report No. 95-04. Treiman, J.A., 1993, The Rose Canyon Fault Zone, Southern California: California Division of Mines and Geology, Open-File Report 93-02, 45 p. Seed, H.B., and Idriss, I.M., 1982, Ground Motions and Soil Liquefaction During Earthquakes, Monogram Series, Earthquake Engineering Research Institute, Berkeley, Califomia . Seed, H.B., Idriss, I.M., and Arango, 1., 1983, Evaluation of Liquefaction Potential Using Field Performance Data, Journal of Geotechnical Engineering: ASCE, Volume I 09, March, pp. 458-482. Treiman, J .A., 1984, The Rose Canyon Fault Zone: A Review and Analysis, California Division of Mines and Geology, Funded by Federal Management Agency Cooperative Agreement EMF-83- K-0148. ----,. 1993, The Rose Canyon Fault Zone, Southern California: California Division of Mines and Geology, Open-FileReport93-2, 45p. Weber, F .H., 1982, Recent Slope Failures, Ancient Landslides and Related Geology of the Northern-Central Coastal Area, San Diego County, California: California Division of Mines and Geology, Open File Report 82-12LA, 77 p. Wilson, K.L., 1972, Eocene and Related Geology of a Portion of the San Luis Rey and Encinitas Quadrangles, San Diego County, California: Master Thesis, University of California at Riverside, 123 p. A-3