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HomeMy WebLinkAbout; 5025 Tierra Del Oro Street; Geotechnical Evaluation of Coastal Bluff Property; 1997-11-17SGC Southland Geotechnical Consultants GEOTECHNICAL EVALUATION OF COASTAL BLUFF PROPERTY PROPOSED RESIDENTIAL ADDITIONS 5025 TIERRA DEL ORO STREET CARLSBAD, CALIFORNIA Project No. 126F21 November 17, 1997 Prepared for: Adams Design Associates 839-B Second Street Encinitas, California 92024 1238 GREENFIELD DRIVE, SUITE A EL CAJON, CALIFORNIA 92O21 (619)442-8022 • FAX (619)442-7859 SGC South/and Geotechnical Consultants November 17, 1997 Project No. 126F21 To: Adams Design Associates, Inc. 829-B 2nd Street Encinitas, California 92024 Attention: Mr. Andy Wilt Subject: Geotechnical Evaluation of Coastal Bluff Property, Proposed Residential Additions, 5025 Tierra del Oro Street, Carlsbad, California Introduction In accordance with your request, Southland Geotechnical Consultants has performed a geotechnical evaluation of the subject coastal bluff property. We understand that additions to the existing single-family residence are planned at the property. This report presents a summary of our field and research studies and our conclusions and recommendations, from a geotechnical standpoint, relative to the proposed development. Purpose and Scope This report presents the results of our geotechnical evaluation of the coastal bluff property located at 5025 Tierra del Oro Street in Carlsbad. The purpose of our study was to evaluate the geotechnical conditions at the coastal bluff property and provide recommendations relative to the proposed construction. The scope of our geotechnical evaluation included the following: Review of aerial photographs, geologic/topographic maps, and geologic literature pertaining to the site and vicinity. A list of the items reviewed is presented in Appendix A. Geologic reconnaissance to observe the existing site conditions including the coastal bluff and general vicinity. Preparation of a tape and compass profile of the bluff face. Investigation of the subsurface soil conditions by manually excavating, logging and sampling three exploratory borings in the areas of the proposed additions. Geotechnical analysis of the data obtained including a computer-generated slope stability analysis of the coastal bluff. • 1238 GREENFIELD DRIVE, SUITE A EL CAJON, CALIFORNIA 92021 (619)442-8022 • FAX (619)442-7859 Project No. 126F21 Preparation of this report summarizing the results of our geotechnical evaluation of the coastal bluff property. This report includes a summary of the coastal bluff conditions and discusses the geotechnical factors affecting the proposed additions including allowable soil-bearing pressure, foundation design and other design/construction considerations. Site Description The subject coastal bluff property is known as San Diego County Assessor's Parcel Number 210-02-12. The roughly rectangular property is located at 5025 Tierra del Oro Street in the City of Carlsbad (see Figure 1). The eastern property line at the site is located along the westerly side of the Tierra del Oro Street roadway. Single-family residential developments exist on the properties to the north and south of the subject property. The relatively level bluff-top area of the property is bounded on the west by an approximately 30-foot high coastal bluff with an overall gradient of approximately 28 degrees {see Photos 1 and 2). A rock revetment exists along the base of the coastal bluff. The approximate elevation of the bluff edge is about 38 feet above sea level based on the City of Carlsbad 1991 orthophoto map (Appendix A). In general, the bluff-top area at the site slopes gently towards the east. A single- story, single-family residence occupies the majority of the bluff-top area. On November 1, 1997, a SGC representative made approximate measurements of the features located on the western portion of the bluff-top area at the site. The results, with our interpretation of the approximate location of the bluff edge, are included on Figure 2. Please note that the bluff edge on the site is obscured by vegetative growth (see Photos 3 and 4} and we emphasize that our measurements are approximate. Figure 2 is not a surveyed map. Bluff Description During our site visit on November 1, 1997, a tape and compass profile of the coastal bluff on the property was prepared. The results of our approximate measurements are presented on Figure 3 (Coastal Bluff Profile). Following is a summary of the onsite bluff conditions. The approximately 30-foot high coastal bluff slopes at an overall gradient of approximately 28 degrees (from the base of the bluff face east of the rock revetment to the upper bluff edge). There is a rock revetment ("rip- rap") at the base of the coastal bluff (see Photos 1 and 2). This rock revetment is approximately 35 feet wide and extends continuously laterally along the beach for a distance of approximately 230 feet north and for approximately 300 feet south of the subject property. A veneer of beach sand overlies the wave-cut platform to the west of the rock SGC Project No. 126F21 revetment. The wave-cut platform is developed on the Santiago Formation sandstone at the site (see Photo 2). The coastal bluff appears to be underlain by terrace deposits sands and is generally moderately to well vegetated with iceplant and other plants. A wooden stairway (with landing areas) and various slope retention structures were observed on the coastal bluff face. The largest retaining wall is an approximately 3.5-foot high block wall in the southwestern portion of the bluff. This wall extends to the north for approximately 15 feet from the existing stairway. Proposed Development Based on our review of preliminary project plans (Appendix A), it is our understanding that the existing residence will be remodelled and one- and two-story additions to the residence are proposed at the site. The additions proposed to extend beyond the perimeter of the existing residence are shown on Figure 2. We understand that the residential additions will consist of typical, relatively light, residential construction. Subsurface Exploration On November 5, 1997, an engineering geologist from our firm manually excavated, logged and sampled three exploratory borings at the approximate locations shown on Figure 2. The borings were excavated to a maximum depth of 3.7 feet. Logs of the exploratory borings are included as Figure 4. Subsequent to logging, the exploratory borings were backfilled. The soil exposed in our exploratory borings consisted of silty fine sand and is similar to soil in the general site vicinity found to have a very low expansion potential when tested in accordance with UBC Standard No. 29-2. Geologic Units Based on our review of a geologic map (Appendix A, Reference 13) and our onsite observations, the property appears to be underlain by Eocene-aged Santiago Formation sandstone at depth. The Santiago Formation is overlain by Quaternary-aged terrace deposits, which are exposed in the coastal bluff face. Surficial deposits consisting of beach deposits were observed during our site visit and fill soils may exist on the bluff face and bluff-top area. The approximate limits of these units, as observed in our onsite studies, are shown on Figure 3 and are described below: Santiago Formation - The Eocene-aged Santiago Formation is exposed in the wave-cut platform (see Photo 2) underlying the beach sand in the western portion of the site and underlies the entire site at depth (below the terrace SGC Project No. 126F21 deposits). The Santiago Formation sandstone generally consists of a light gray-- brown, well-cemented, silty, fine-grained sandstone. Terrace Deposits- Quaternary-aged terrace deposits unconformably overlie the Santiago Formation and comprise the majority of the bluff face. The terrace deposits consist of orange-brown to light brown, dense but friable, slightly silty fine- to medium-grained sand. Beach Deposits - A variable thickness of unconsolidated beach deposits occur on the beach at the base of the coastal bluff, west of the rock revetment. During our site visits, the beach deposits consisted of sand. This material is subject to addition and removal in response to storm waves and currents. Fill Soils - Relatively minor amounts of fill soils exist locally on the bluff face (primarily behind the slope retention structures). Fill soils associated with the existing site improvements on the bluff-top area also were observed. The fill soils generally consist of locally-derived terrace deposits and imported landscape soils. The limits of the relatively minor amounts of fill soils are not shown on Figure 3. Geologic Structure The Santiago Formation is exposed in the wave-cut platform in the western portion of the property {see Photo 2). The Santiago Formation at and near the subject property is nearly flat-lying. In the general site vicinity, bedding in the Quaternary terrace deposits can be observed as alternating more resistant and less resistant beds; Where observed on site and in the general site vicinity, the terrace deposits appear to be horizontally bedded with localized cross bedding. Major out-of-slope bedding components, jointing or fracturing that would be adverse to slope stability were not observed on site. Indications of deep-seated landslide features were not observed during our research studies or site visits. Review of Geotechnical Hazards/Constraint Mao Our review of Geotechnical Hazards/Constraint Map page 14 of the City of Carlsbad Geotechnical Hazards Analysis and Mapping Study (Appendix A) indicates that the site is located in Hazard Categories 42 and 53. Category 42 pertains to areas with "coastal stability" concerns and describes areas that are "generally stable: low bluff face, generally favorable structure, low to moderate erosion.1* Category 53 pertains to areas with "other terrain conditions: generally stable: relatively level mesa areas underlain by terrace deposits, sandstone or granitic/metavolcanic bedrock." Although SGC Project No. 126F21 the map is not intended for evaluation of individual sites, it can provide general indications of the presence of known geologic hazards. Faulting Our review of geologic literature {Appendix A) pertaining to the general site area indicates that there are no known "active" faults on or in the immediate vicinity of the site. An "active" fault is defined by the California Division of Mines and Geology as one which has "had surface displacement within Holocene time (about the last 11,000 years)" (Appendix A, Reference 4). Indications of active faulting were not observed in the subject coastal bluff or in nearby exposures. The nearest known active faults are the Rose Canyon fault located offshore approximately 4 miles west of the site, the Coronado Bank fault located offshore approximately 19.5 miles west, and the Elsinore fault located approximately 25 miles northeast of the site. The San Andreas fault is located approximately 62 miles northeast of the site. Tsunami and Storm Waves Tsunami are sea waves generated by submarine earthquakes, landslides or volcanic action. Submarine earthquakes are common along the edge of the Pacific Ocean and coastal areas are subject to potential inundation by tsunami. Most of the 19 tsunami recorded on the San Diego tidal gauge (between 1854 to 1872 and 1906 to 1977) have only been a few tenths of a meter in height (Appendix A, Reference 1). The largest San Diego area tidal gauge excursion (1 meter) was associated with the tsunami of May 22, 1960 and was recorded at La Jolla (Scripps Pier) (Appendix A, Reference 12). The tsunami was generated by a Richter magnitude 8.5 earthquake in Chile. For comparison, the diurnal range of tides at San Diego Bay is 1.7 meters. The possibility of a destructive tsunami along the San Diego coastline is considered low (Appendix A, Reference 5). However, tsunami or storm waves (associated with winter storms), in conjunction with high tides, may overtop the rock revetment and erode the friable terrace deposits that comprise the coastal bluff face but generally are not anticipated to have the potential for inundation of the bluff-top building site. Groundwater and Surface Water During our site visits, groundwater seepage was not observed in the coastal bluff. Based on our experience and observations, our estimation to the depth of groundwater is that it is at or near sea level and may also be perched locally on the Santiago Formation at its contact with the overlying terrace deposits. Groundwater levels can be expected to fluctuate with the tides, seasonal precipitation and irrigation. Groundwater is not expected to be a constraint to construction of the proposed SGC Project No. 126F21 residential additions. However, our experience indicates that near-surface groundwater conditions can develop in areas where no such groundwater conditions previously existed, especially in areas where a substantial increase in surface water infiltration results from landscape irrigation or unusually heavy precipitation. The bluff-top surface waters appear to primarily drain towards the east as sheet flow. The majority of the bluff-top surface waters appear to be directed into a concrete swale and area drain that apparently discharge easterly to the Tierra del Oro Street roadway. Historic Research Summary We have reviewed the literature, maps and aerial photographs of the site and general vicinity listed in Appendix A. Following is a limited outline summary of our review observations: The oldest map we found on file at the County of San Diego is an 1898 survey for the Fifth Road District (Appendix A). The bluff along the coastline is sketched on this map and the railway line is shown. The oldest photograph we reviewed was one from the 1928-29 aerial photograph set on file at the County of San Diego. The railroad and coastal highway (approximately the current Carlsbad Boulevard) are shown on this photo. A dirt road west of the coastal highway, roughly parallel to the coastline, and along the bluff top in the general site vicinity is shown. The Shore Drive roadway and some of the homes in the subdivision to the south of the subject property are shown on the 1953 aerial photographs. The subject property and Tierra del Oro subdivision appear to be in an undeveloped, mostly natural state, however, a dirt road exists along the bluff edge in the site vicinity and several trails down the bluff face were observed to the north of the site. Relatively minor erosional gullies on the bluff face in the site vicinity were observed on the photographs. The existing residence at the subject property is shown on the 1964, 1967, and 1970 photographs and no rock revetment is apparent along the base of the coastal bluff. On the 1964 photos, the bluff face appears to be unvegetated. The existing residence is shown on the 1975 photographs and a relatively leve! area is shown at the base of the coastal bluff, south of the stairway, just west of the largest retaining wall that currently exists at the site. It is unclear from the photographs if some type of structure (patio?, bathhouse?) had existed in SGC Project No. 126F21 this area. It appears that some rip-rap may have existed at the base of the coastal bluff. On the 1983 photograph, it appears that some rip-rap may have existed at the base of the coastal bluff. On the 1989 and subsequent photographs, rip-rap exists along the base of the coastal bluff and looks generally similar in configuration and size to the rip-rap {rock revetment) that was observed during our site visits. On the 1991 orthophoto mapping sheet and 1992 photographs, the bluff-face area is vegetated generally similarly to what we observed during our recent site visits. However, the area shown at the base of the coastal bluff, south of the stairway, just west of the largest retaining wall, is unvegetated. The larger retaining wall and another shorter wall are apparent in this area. Coastal Bluff Retreat The Carlsbad coastline in the vicinity of the subject property consists of a slight headland (see Figure 1). Mechanisms for coastal bluff retreat at the site may occasionally include slow abrasion and undercutting by marine erosion (wave action) of the terrace deposits. Storm surf and higher high tides contribute to the natural process of marine erosion. However, the existing rock revetment reduces the potential for erosion of the onsite coastal bluff. Other factors affecting the rate of retreat of a coastal bluff include degree of fracturing, jointing, consolidation of sediments, steepness of slope, groundwater and surface water conditions, vegetation or lack of, and intensity of pedestrian and animal traffic. Portions of coastal bluffs are also exposed to precipitation, wind, pedestrian/animal erosion (including foot traffic and burrowing rodents), variations in landscape, landscape maintenance, and other activities by humans. During our studies, we did not observe indications of deep-seated instability, such as ancient or active landslides, on the site, and the Santiago Formation sandstone that underlies the site at depth is not known to be prone to large, deep-seated failures. The terrace deposits are friable and commonly rill and ravel in oversteepened slopes, however, they are not known to be prone to large, deep-seated failures. The rate and magnitude of coastal bluff retreat at a specific site are dependent on a variety of factors, both natural and manmade. Many of these factors are ongoing processes and historic documentation can be helpful in estimating general bluff-edge retreat rates. However, there are other factors affecting coastal bluff retreat that cannot be estimated from historic documentation. Such factors include future human activities or possible extreme variations in regional weather patterns. SGC Project No. 126F21 Detrimental changes in factors affecting bluff-edge retreat, such as misdirected drainage, water line breaks, very heavy storm surf and/or precipitation, could increase the rate of future erosion. However, favorable changes in the factors affecting bluff- edge retreat could decrease the rate of future erosion. Some of these include eliminating detrimental human activities on the bluff, proper maintenance of a bluff- stabilizing vegetative cover, enhanced site drainage provisions and beach sand replenishment. Research studies along the San Diego coast and historic photograph and map review are components in providing an estimation of the rate of bluff-edge retreat. We assume that the historic retreat rate may give an indication of the future retreat rate at a particular site. However, accurate and clear photographic and map documentation for measuring retreat is not always available or is of fairly short time intervals so changes may not be noticeable. Lee and others (Appendix A, Reference 6) performed research studies of regionat historic bluff-edge retreat and estimated a maximum annual bluff-edge retreat rate of 0.22 to 0.33 feet per year. Over a 75-year period {assumed to be the economic lifetime of the new construction), this equates to a conservative estimate of bluff-edge retreat of a maximum of 16.5 to 24.8 feet. This maximum is based on research studies of regional historic bluff retreat that includes coastal bluffs with generally favorable conditions, as well as coastal bluffs that are affected by more adverse conditions (fracturing, sea caves, groundwater, human activities, etc.). The estimated values of maximum retreat are very conservative, and the actual rate of bluff retreat at the subject property is expected to be significantly less considering the site conditions, relatively shallow slope gradient, historic bluff-edge retreat, and rock revetment improvements at the site. Sea cave formation and subsequent collapse are localized factors in the bluff retreat process. Indications of sea cave development were not observed at the subject property during our site visits. Our historic photograph review (Appendix A) indicates that the coastal bluff at the subject property is generally similar in configuration in the 1929, 1953, and subsequent photos. The location of the onsite bluff edge is also generally similar on the photographs, prior to and subsequent to the placement of the rock revetment at the site. The rock revetment appears to be retarding marine erosion of the lower portion of the bluff. In addition, the coastal bluff has a relatively shallow gradient with limited areas of oversteepening. 8 SGC Project No. 126F21 Slope Stability Calculations A computer-generated slope stability analysis was performed on the coastal bluff at the site. The slope stability was analyzed using 'Janbu's Simplified Method of Slices' with the PCSTABL 5M computer program. Groundwater was included in our slope stability analyses. The slope stability calculations are included in Appendix B. The soil strength parameters used in our analysis are presented below. These values are based on laboratory test results, back-calculation, our past experience in this area, and our professional judgement. Soil Type Unit Weight Friction Angle Cohesion Terrace Deposits 120.0 pcf 35 degrees 200 psf Santiago Fm Sandstone 120.0 pcf 40 degrees 500 psf The results of the analyses (Appendix B) indicate that for the existing configuration, the calculated factor of safety against deep-seated failure is in excess of 1.5 (the generally accepted standard for the geotechnical industry). SGC Project No. 126F21 CONCLUSIONS AND RECOMMENDATIONS Based on our geotechnicat evaluation of the coastal bluff at the site, it is our opinion that the proposed residential additions {and the loading from this relatively light bluff- top construction) will not adversely impact the existing coastal bluff. In addition, it is our opinion that trie proposed construction should not be affected by anticipated coastal bluff retreat processes during its economic lifetime {assumed to be 75 years). Slope Stability and Erosion Our geotechnical evaluation of the present overall static stability on the subject property indicates that the bluff is grossly stable. In its present state, the slope has a low to moderate potential for erosion and future surficial instability. We provide the following recommendations to help reduce erosion of the bluff and to reduce potential for future instability of the bluff face. Irrigation of the landscape areas on the property should be limited to the minimum amount required to establish vegetation and maintain plant vigor. The subject coastal bluff and the bluff edge are currently well vegetated with iceplant. At this time, it is our opinion that modifications to the vegetation in these areas should not be considered. However, if landscape planting and/or plant removal on the westerly bluff-top area is performed, it should be done without significantly disturbing the bluff-top soils. The surficial stability of those portions of the bluff that are not well vegetated may be increased by planting in accordance with the recommendations of a professional landscape company experienced with coastal bluffs. Terracing or excavation of the bluff- face soils should be avoided. Based on our review of the preliminary project plans and our site observations, it appears that site development will include provisions to discharge surface waters to the east (toward the Tierra del Oro Street roadway). Drainage at the site should be directed such that surface waters discharge into non-erosive drainage provisions. Runoff at the site should not be directed over the bluff edge. Eave gutters were observed on the existing residence and should be installed on the remodelled residence and additions. Eave gutters should be properly maintained with downspouts that discharge into non-erosive drainage provisions. Pedestrian and animal traffic (and burrowing, etc.) on the bluff face and bluff edge should not be allowed. 10 SGC Project No. 126F21 Slooe/Bluff-Edae Setback We recommend a minimum horizontal setback distance from the face of the coastal bluff for all structural footings and settlement-sensitive structures. This distance is measured from the outside bottom edge of the footing horizontally to the slope face and should be a minimum of 10 feet. Due to the proximity of the existing slope, the soils within the structural setback zone possess limited lateral support characteristics, and improvements {such as patios and pools) constructed within this setback zone may be subject to lateral movement and/or differential settlement. Based on our review of the project plans, the proposed residential additions will be set back a minimum of 11.5 feet from the bluff edge. It is our opinion that the proposed setback will safeguard the proposed residential additions from slope-top lateral instability during the economic lifetime of the new construction. Seismic Considerations The principal considerations for most structures in southern California are surface rupturing of fault traces and damage caused by ground shaking or seismically-induced ground settlement or liquefaction. The possibility of damage due to ground rupture is considered minimal since no active faults are known to cross the site. It is our opinion that the potential for liquefaction or seismically-induced ground settlement at the site due to an earthquake is very low because of the dense nature of the underlying terrace deposits and Santiago Formation sandstone. The seismic hazard most likely to impact the site is ground shaking resulting from an earthquake on one of the major active regional faults. The nearest known active fault is the Rose Canyon fault located offshore approximately 4 miles west of the site. It is estimated that a maximum earthquake on this portion of the Rose Canyon fault (magnitude 6.5) could produce moderate to severe ground shaking at the site. In general, the role seismic shaking plays in bluff retreat is dependent on bluff conditions at the moment of shaking. It is possible that some of the oversteepened portions of the terrace deposits may undergo shallow failure and some ravelling of the poorly indurated bluff-face terrace deposits may also occur during ground shaking, especially on unvegetated portions of the bluff face. However, it is our opinion that the potential for deep-seated or severe, catastrophic failure of the bluff due to expected seismic ground shaking is low at the site. Site Preparation Prior to construction activities, the proposed addition areas should be cleared of vegetation, demolition debris and loose soils. Vegetation and loose debris should be 11 SGC Project No. 126F21 properly disposed of off site. Holes resulting from removal of buried obstructions (pipes, etc.) which extend below finished site grades should be filled with properly compacted fill soils. Removal/Recompaction of Compressible Soils The existing fill soils and topsoil mantling the dense terrace deposits are considered compressible and unsuitable for the support of structural loads in their present condition. We recommend that the existing fill soils and topsoil be removed in areas planned for structures, surface improvements or fill placement. As encountered in our exploratory borings, these soils apparently underlie the site to a depth of approximately 1 foot below the existing ground surface. Actual depths may vary and should be evaluated by the geotechnical consultant during removal of these unsuitable soils. These soils are considered suitable for re-use as compacted, structural fill provided they are free of organic material and deleterious debris. Structural Fill Placement Areas to receive fill and/or other surface improvements should be scarified to a minimum depth of 6 inches, brought to near-optimum moisture conditions, and recompacted to at least 90 percent relative compaction, based on laboratory standard ASTM D1557. Fill soils should be brought to near-optimum moisture conditions and compacted in uniform lifts to at least 90 percent relative compaction {ASTM D1557). The optimum lift thickness to produce a uniformly compacted fill will depend on the size and type of construction equipment used. In general, fill should be placed in loose lift thicknesses not exceeding 8 inches. Foundation and Slab Recommendations It is anticipated that the proposed one- and two-story additions will be supported by the existing foundations and/or additional foundations. The adequacy of the existing foundations to support additional loading should be evaluated by a structural engineer. New foundations and slabs should be designed in accordance with structural considerations and the following recommendations. These recommendations assume that the soils encountered during foundation excavation will consist of medium dense to dense natural terrace deposits with a very low to low expansion potential. The proposed additions may be supported on isolated or continuous footings bearing at least 6 inches into firm, natural soils at a minimum depth of 18 inches beneath the lowest adjacent grade. At this depth, footings may be designed for an allowable soil- bearing value of 1,500 pounds per square foot. This value may be increased by one- 12 SGC Project No. 126F21 third for loads of short duration, such as wind or seismic forces. Footings should have a minimum width of 12 inches, and reinforcement consisting of two No. 4 rebars {one near the top and bottom of each footing). Slabs should have a minimum thickness of 4 inches and be reinforced at midheight in the slab with No. 3 rebars at 18 inches on center each way (or No. 4 rebars at 24 inches on center each way). Slabs should be underlain by a 2-inch layer of sand which is underlain by a 10-mil moisture barrier. The potential for slab cracking may be lessened by careful control of water/cement ratios. The use of low slump concrete is recommended. Appropriate curing precautions should be taken during placement of concrete during hot weather. We recommend that a slipsheet or equivalent be used if crack-sensitive flooring is planned directly on the concrete slab. Footings and slabs founded in firm, natural soils may be designed for a passive lateral bearing pressure of 350 pounds per square foot per foot of depth. A coefficient of friction against sliding between concrete and soil of 0.4 may be assumed. These values may be increased by one-third when considering loads of short duration, such as wind or seismic forces. Lateral Resistance Lateral loads can be resisted by assuming a passive pressure of 350 psf per foot of depth and a coefficient of friction of 0.35 between concrete and soil. The lateral resistance may be taken as the sum of the passive and fractional resistance, provided the passive resistance does not exceed two-thirds of the total resistance. Other Considerations The recommendations provided in this report are based on our understanding that one- and two-story additions to an existing residence (with their relatively tight loading) are planned at the site and will be set back a minimum of 11.5 feet from the bluff edge. The site conditions and bluff edge indicated on Figures 2 and 3 have been compiled from approximate measurements made during our site visits. They should not be relied on for site development. If needed, we suggest that a licensed land surveyor be retained to prepare a site topographic plan that accurately delineates the property boundaries and bluff edge. A site drainage study may also be conducted to develop a site-specific drainage plan for the proposed development. Please note that the recommendations contained herein may be revised based on modified and/or additional information regarding the structure and improvements planned at the site. A qualified consultant should be retained to review site conditions and assess potential site impacts following significant erosion events in the future or if major changes in the bluff configuration are noticed. 13 SGC Project No. 126F21 Limitations and Uniformity of Conditions This geotechnical evaluation report addresses the coastal bluff conditions at the subject property and is based on our understanding that the proposed development consists of design and construction of additions to an existing single-family residence. The additions will be set back a minimum of 11.5 feet from the bluff edge. This report is based on our document/photograph review, surficial observations of the geologic conditions exposed at the site and vicinity. This report assumes that the geologic/soils conditions do not deviate appreciably from those observed during our site visits. The recommendations of this report pertain only to the coastal bluff property evaluated. We have hot performed an evaluation of the presence of hazardous materials/contamination at the site. The findings of this report are valid as of this date. Changes in conditions of a property can, however, occur with the passage of time, whether they be due to natural processes or the work of man on this or adjacent properties. In addition, changes in applicable or appropriate standards may occur, from legislation or the broadening of knowledge in the fields of geotechnical engineering or geology. Hence, the findings of this report may be invalidated wholly or in part by changes beyond our control. Therefore, this report should not be relied upon after a period of two years without a review by us. If there are questions regarding the information contained herein, we should be contacted. We will not be responsible for the interpretation by others of the information herein. Our services consist of professional consultation and no warranty of any kind whatsoever, express or implied, is made or intended in connection with the work performed by us. 14 SGC Project No. 126F21 If you have any questions regarding our report, please call. We appreciate this opportunity to be of service. Sincerely, SOUTHLAND GEOTECHNICAL CONSULTANTS €, Susan E. Tanges, CEG Managing Principal/E CERTIFIED ENGINEERING GEOLOGIST Steven NorrisT Project Engineer Attachments:Figure 1 - Site Location Map Figure 2 - Site Plan Figure 3 - Coastal Bluff Profile Figure 4 - Logs of Exploratory Borings Photographs 1 through 4 Appendix A - References Appendix B - Slope Stability Calculations Distribution: (3) Addressee 15 SGC N SITE LOCATION MAP Project No. 126F21 5025 Tierra del Oro Street, Carlsbad Scale (approximate): 1 inch = 100 feet Base Map: City of Carlsbad Orthophoto Topographic Map 93 dated 1991 FIGURE 1 SGC Project No. 126F21 LOGS OF EXPLORATORY BORINGS BORING NO. DEPTH DESCRIPTION Boring 1 0-0.7' Disturbed Toosoil - Brown, dry to damp, loose to medium dense, silty fine sand (SM); friable, roots, organics 0.7-1.7' Weathered Terrace Deposits - Brown, damp to moist, dense, silty fine sand (SM); friable 1.7-3.7' Terrace Deposits - Brown to red-brown, moist, silty fine sand (SM); more red-orange with depth, less weathered with depth Total depth = 3.7 feet No ground water encountered Excavated and backfilled 11-05-97 Boring 2 0-0.8' Disturbed Toosoil - Brown, dry to slightly damp, loose to medium dense, silty fine sand (SM); friable, roots 0.8-2.6' Weathered Terrace Deposits - Brown, damp, dense (increasing density with depth), silty fine sand (SM); friable 2.6-3.1' Terrace Deposits - Brown to orange-brown, moist, silty fine sand (SM); friable, more dense with increasing depth Total depth = 3.1 feet No ground water encountered Excavated and backfilled 11-05-97 Boring 3 0-1' Fill/Disturbed Topsoil - Dark brown, damp to slightly moist, loose to medium dense, silty fine sand (SM); with organics (loamy texture), roots 1 '-3' Weathered Terrace Deposits - Brown to orange-brown, damp to moist dense (increasing density with depth), silty fine sand ISM); friable, more orange with depth Total depth = 3 feet No ground water encountered Excavated and backfilled 11-05-97 FIGURE 4 SGC PHOTO 1 Coastal bluff at 5025 Tierra del Oro Street, Carlsbad PHOTO 2 Coastal bluff at 5025 Tierra del Oro Street, Carlsbad SGC PHOTO 31 Bluff edge, view northerly, 5025 Tierra del Oro Street, Carlsbad PHOTO 4 - Bluff edge, view southerly ;'&<£~i#y'r>rj 5025 Tierra del Oro Street SGC APPENDIX Project No. 126F21 APPENDIX A REFERENCES 1. Agnew, D.C., 1979, Tsunami history of San Diego, in Abbott, P.L., and Elliott, W.J., eds.. Earthquakes and Other Perils: Geological Society of America field trip guidebook. 2. California Division of Mines and Geology, 1 994, Fault activity map of California and adjacent areas: CDMG Geologic Data Map No. 6. 3. Flick, R.E., ggL., 1994, Shoreline erosion assessment and atlas of the San Diego region: California Department of Boating and Waterways and the San Diego Association of Governments publication, dated December (two volumes). 4. Hart, E.W., 1994, Fault-rupture hazard zones in California: California Division of Mines and Geology, Special Publication 42, revised. 5. Lee, L.J., 1977, Potential foundation problems associated with earthquakes in San Diego, in Abbott, P.L., and Victoria, J.K., eds.. Geologic Hazards in San Diego, Earthquakes, Landslides, and Floods: San Diego Society of Natural History John Porter Dexter Memorial Publication. 6. Lee, L., Pinckney, C., and Bemis, C., 1976, Sea bluff erosion: American Society of Civil Engineers, National Water Resources and Ocean Engineering Convention Preprint No. 2708. 7. Legg, M.R., Agnew, D.C., and Simons, R.S., 1978, Earthquake history and seismicity of coastal San Diego County, California, 1800-1976 (unpublished). 8. Southland Geotechnical Consultants, in-house geologic information. 9. Tan, S.S., and Giffen, D.G., 1995, Landslide hazards in the northern part of the San Diego metropolitan area: California Division of Mines and Geology, Open- file Report 95-04. 10. U.S. Army Corps of Engineers, 1985, Coast of California Storm and Tidal Waves Study, Shoreline Movement Data Report, Portuguese Point to Mexican Border (1852-1982) (CCSTWS 85-10), dated December. 11. U.S. Army Corps of Engineers, 1985, Coast of California Storm and Tidal Waves Study, Coastal Cliff Sediments, Sari Diego Region (CCSTWS 87-2), dated June. SGC Project No. 126F21 APPENDIX A REFERENCES (continued) 12. Van Dorn, W.G., 1979, Theoretical aspects of tsunamis along the San Diego coastline, in Abbott, P.L., and Elliott, W.J., e_ds.. Earthquakes and Other Perils: Geological Society of America field trip guidebook. 13. Weber, F.H., Jr., 1982, Recent slope failures, ancient landslides and related geology of the north-central coastal area, San Diego County, California: California Division of Mines and Geology, Open-File Report 82-12. AERIAL PHOTOGRAPHS County of San Diego, 1929, Photo 30D1 (vertical, not stereoscopic). County of San Diego, 1967, Series GS-VBTA, Flight Line 1, Photos 1-142 and 1-143, dated May 8 (vertical, stereoscopic). County of San Diego, 1970, Series SDCO, Flight Line 2, Photos 2-1 (015) and 2-2 (014), dated October 9 (vertical, stereoscopic), scale 1:24,000. County of San Diego, 1975, Flight SDPD, Flight Line 36, Photos 36-2 (129) and 36-3 (128), dated January 20 (vertical, stereoscopic), scale 1 inch = 1,000 feet. County of San Diego, 1983, Flight C11109 83059, Photos 256 (023) and 257 (024), dated November 22 (vertical, stereoscopic), scale 1 inch = 2,000 feet. County of San Diego, 1989, Series WAC-89A, Photo 3-7, dated April 7 (vertical, not stereoscopic). Geo-Tech Imagery Intl., 1992, Carlsbad Frames 3 and 4, dated November 1 (infrared, oblique, stereoscopic). U.S. Department of Agriculture, 1953, Series AXN, Flight Line 14M, Photos 17 and 18, dated May 2 (vertical, stereoscopic), scale 1:20,000. U.S. Department of Agriculture, 1964, Series AXN, Flight Line 4DD, Photo 97, dated April 9 (vertical, not stereoscopic). SGC Project No. 126F21 APPENDIX A (continued) MAPS City of Carlsbad, 1992, Geotechnical Hazards Analysis and Mapping Study, dated November. City of Carlsbad, 1991, Orthophoto Mapping, Sheet 93, scale 1" = 100' {aerial photography dated September-October 1988). County of San Diego, 1975, Orthophoto Topographic Map 350-1665, dated September 17, scale 1"=200'. County of San Diego, Assessor's Map Book, page 210-02. County of San Diego, 1898, Survey Number 148, Fifth Road District, Oceanside to Encinitas, two parts, dated October. County of San Diego, 1915, Map of Partition of a Portion of the Land Owned by the Undersigned in the Rancho Agua Hedionda, Map No. 823, dated May 1. County of San Diego, 1954, Tierra Del Oro, Map No. 3052, recordation dated February 4. SGC APPENDIX B ** PCSTABL5M ** by Purdue University —Slope Stability Analysis— Simplified Janbu, Simplified Bishop or Spencer's Method of Slices Run Date: Run By: Input Data Filename: Output Filename: Plotted Output Filename: November 17, 1997 GC CBAD3.IN CBAD3.OUT CBAD3.PLT PROBLEM DESCRIPTION: COASTAL BLUFF STABILITY ANALYSTS EATON RESIDENCE, 5025 TIERRA DEL ORO STREET, CARLSBAD, CA BOUNDARY COORDINATES 8 Top Boundaries 9 Total Boundaries Boundary NO. 1 2 3 4 5 6 7 8 9 X-Lcft (ft) 20.00 41.00 77.00 84.00 89.00 112.00 121.00 144.00 77.00 Y-Left (ft) 20.00 21.00 31.00 33.00 33.00 51.00 52.00 63.00 31.00 X-Right (ft) 41.00 77.00 84.00 89.00 112.00 121.00 144.00 200.00 200.00 Y-Right (ft) 21.00 31.00 33.00 33.00 51.00 52.00 63.00 63.00 31.00 Soil Type Below Bnd 1 1 2 2 2 2 2 2 1 ISOTROPIC SOIL PARAMETERS 2 Type(s) of Soil Soil Total Saturated Cohesion Friction Pore Pressure Piez. Type Unit Wt. Unit Wt. Intercept Angle Pressure Constant Surface No. (pcf) (pcf) {psf) (deg) Param. (psf) No. 120.0 120.0 135.0 135.0 500.0 200.0 40.0 35.0 .00 .00 .0 .0 1 PIEZOMETRIC SURFACE(S) HAVE BEEN SPECIFIED Unit Weight of Water = 62.40 Piezometric Surface No. 1 Specified by 2 Coordinate Points Point No. 1 2 X-Water (ft) 77.00 200.00 Y-Water (ft) 31.00 31.00 BOUNDARY LOAD(S) 1 Load{s) Specified Load X-Left X-Right Intensity Deflection No. {ft} (ft) (Ib/sqft) (deg) 1 169.00 170.00 1000.0 .0 NOTE - Intensity Is Specified As A Uniformly Distributed Force Acting On A Horizontally Projected Surface. A Critical Failure Surface Searching Method, Using A Random Technique For Generating Circular Surfaces, Has Been Specified, 100 Trial Surfaces Have Been Generated. 10 Surfaces Initiate From Each Of 10 Points Equally Spaced Along The Ground Surface Between X = 30.00 ft. and X = 110.00 ft. Each Surface Terminates Between X = 120.00 ft. and X = 175.00 ft. Unless Further Limitations Were Imposed, The Minimum Elevation At Which A Surface Extends Is Y = .00 ft. 5.00 ft. Line Segments Define Each Trial Failure Surface. Following Are The Five Most Critical Of The Trial Failure Surfaces Examined. They Are Ordered - Most Critical First. * * Safety Factors Are Calculated By The Modified Janbu Method * * Failure Surface Specified By 13 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 12 13 X-Surf (ft) 83 88 93 98 103 107 112 116 120 123 126 129 129 .33 .20 .18 .17 .10 .88 .42 .65 .48 .85 .71 .00 .84 Y-Surf (ft) 32.81 31.66 31.17 31.34 32.17 33.64 35.73 38.41 41.62 45.31 49.42 53.86 56.23 2.445 Individual data on the 16 slices Slice No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Width Ft(m) .7 4.2 .8 4.2 5.0 4.9 4.8 4.1 .4 4.2 3.8 .5 2.9 2.9 2.3 .8 Water Water Force Force Weight Top Bot Lbs (kg) 15.6 477.8 148.7 1836.9 4702.2 6893.4 8386.9 8227.3 876.9 8101.6 6059.7 708.9 3437.8 2576.1 1123.1 111.5 Lbs (kg) 1906. 11532. 2195. 14018. 15354. 13632. 11770. 9010. 688. 6819. 6080. 820. 4797. 4533. 3437. 1222. 3 5 5 6 0 3 9 6 5 9 5 0 1 9 1 7 Tie Force Norm Tie Force Tan Earthquake Force Surcharge Hor Ver Load Lbs (kg) Lbs (kg) Lbs (kg) Lbs (kg) Lbs (kg) Lbs (kg) 1890. 12075. 2275. 11947. 14273. 14117. 13757. 12001. 1199. 12457. 11538. 1693. 8768. 9246. 7912. 3438. 7 7 3 1 0 2 9 4 9 3 8 5 7 2 4 2 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 Failure Surface Specified By 10 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 x-surf (ft) 83.33 88.33 93.30 98.15 102.82 107.23 111.32 115.04 118.31 120.45 Y-Surf (ft) 32.81 32.81 33.42 34.63 36.42 38.77 41.64 44.99 48.76 51.94 2.552 Failure Surface Specified By 14 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X-Surf (ft) 83.33 88.13 93.07 98.07 103.04 107.90 112.57 116.95 120.99 124.61 127.75 130.36 132.39 133.03 Y-Surf (ft) 32.81 31.41 30.64 30.53 31.07 32.25 34.05 36.45 39.40 42.84 46.74 51.00 55.57 57.75 2.725 Failure Surface Specified By 11 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 X-Surf (ft) 92.22 96.99 101.96 106.94 111.77 116.26 120.26 123.61 126.21 127.95 128.37 Y-Surf (ft) 35.52 34.02 33.44 33.81 35.12 37.31 40.32 44.02 48.30 52.98 55.53 *** 2.737 *** Failure Surface Specified By 13 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 12 13 X-Surf (ft) 92. 96. 101. 106. 111. 116. 121. 125. 128. 131. 134. 135. 136. 22 81 67 65 63 47 02 18 81 82 14 68 30 Y-Surf (ft) 35. 33. 32. 31. 32. 33. 35. 38. 41. 45. 50. 55. 59. 52 54 34 97 42 70 76 54 97 96 40 15 32 2.765 Y AXIS FT .00 25.00 50.00 75.00 100.00 125.00 X . 00 + + + + + + _ * 25.00 + A 50.00 + X 75.00 * 6 6* 6* 0124 7854. I 100.00 0.312 . . - 73162. . 0781462 . . - 0.751692. .* - 075816.2.2 - 0.75831. . .* S 125.00 + 0.75.3141.. - 0.7.5.33418 0..75.5633 0.797.5.5 0..9.767. 09 * 150.00 + 90.00 9. . . 9. . 9. 91/1 F 175.00 + 200.00 + 125 188 - A X I 3 75 i 38 25 8 8 25 75 180 125 X - AXIS 158 175 288 ft X I 8 IBB • 75 58 25 . e w 8 25 58 75 IBB 125 150 175 288 X - AXIS A Critical Failure Surface Searching Method, Using A Random Technique For Generating Irregular Surfaces, Has Been Specified. Janbus Empirical Coef. is being used for the case of c & phi both > 0 100 Trial Surfaces Have Been Generated. 10 Surfaces Initiate From Each Of 10 Points Equally Spaced Along The Ground Surface Between X = 30.00 ft. and X = 110.00 ft. Each Surface Terminates Between X = 120.00 ft. and X = 175.00 ft. Unless Further Limitations Were Imposed, The Minimum Elevation At Which A Surface Extends Is Y = .00 ft. 5.00 ft. Line Segments Define Each Trial Failure Surface. Factor Of Safety Calculation Has Gone Through Ten Iterations The Trial Failure Surface In Question Is Defined By The Following 25 Coordinate Points Point x-Surf Y-Surf No. (ft) (ft) 1 65.56 27.82 2 69.87 25.29 3 73.84 22.25 4 77.49 18.83 5 81.47 15.81 6 85.49 12.83 7 89.51 9.87 8 94.19 8.11 9 98.92 9.75 10 102.00 13.69 11 102.98 18.60 12 103.11 23.59 13 103.16 28.59 14 105.08 33.21 15 105.57 38.19 16 108.27 42.40 17 111.29 46.38 18 115.96 48.18 19 120.68 49.82 20 125.00 52.34 21 129.83 53.61 22 134.38 55.68 23 139.30 56.59 24 143.66 59.04 25 146.18 63.00 Factor Of Safety For The Preceding Specified Surface = 12.878 Factor Of Safety Calculation Has Gone Through Ten Iterations The Trial Failure Surface In Question Is Defined By The Following 11 Coordinate Points Point X-Surf Y-Surf NO. (ft) (ft) 1 101.11 42.48 2 104.65 38.94 3 108.33 35.56 4 113.24 34.63 5 118.24 34.79 6 122.90 36.62 7 125.33 40.98 8 127.78 45.34 9 127.80 50.34 10 128.04 55.33 11 128.04 55.37 Factor Of Safety For The Preceding Specified Surface = 8.919 Following Are Displayed The Five Most Critical Of The Trial Failure Surfaces Examined. They Are Ordered - Most Critical First. * * Safety Factors Are Calculated By The Modified Janbu Method * * Failure Surface Specified By 12 Coordinate Points Point X-Surf Y-Surf No. (ft) (ft) 1 83.33 32.81 2 88.01 31.03 3 93.00 31.18 4 97.96 31.86 5 102.90 32.64 6 107.52 34.54 7 112.17 36.38 8 115.18 40.37 9 118.03 44.48 10 120.70 48.70 11 123.31 52.97 12 123.38 53.14 2.665 Individual data on the 15 slices Slice No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Width Ft(m) .7 4.0 1.0 4.0 5.0 4.9 4.6 4.5 .2 3.0 2.9 2.7 .3 2.3 .1 Water Water Force Force Weight Top Bot Lbs(kg) 20.0 652.6 262.1 1864.4 4382.1 6464.2 7555.1 8356.6 338.3 5198.7 3503.0 1885.2 121.4 457.6 .6 Lbs(kg) 1906. 11001. 2726. 13464. 15278. 13720. 11476. 9846. 278. 4872. 4570. 4223. 465. 3900. 115. 3 1 9 8 0 4 9 2 9 0 0 1 0 0 0 Tie Force Norm Tie Force Tan Earthquake Force Surcharge Hor Ver Load Lbs(kg) Lbs(kg) Lbs(kg) Lbs(kg) Lbs(kg) Lbs(kg) 1971 12093 2849 11470 14190 13962 13544 12495 466 12050 10787 9487 995 7166 264 .1 .7 .3 .3 .2 .4 .9 .1 .3 .8 .3 .6 .8 .4 .4 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 ' .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 Failure Surface Specified By 10 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 X-Surf (ft) 92 96 100 105 110 113 116 118 122 .22 .05 .93 .88 .13 .06 .02 .97 .37 Y-Surf (ft) 35.52 32.31 31.22 31.99 34.61 38.67 42.69 46.73 50.39 10 124.92 53.87 *** 2.812 *** Failure Surface Specified By 12 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 12 X-Surf (ft) 92 95 100 105 109 113 117 121 125 128 131 131 .22 .76 .58 .23 .32 .47 .48 .29 .19 .83 .11 .26 Y-Surf (ft) 35.52 31.99 30.64 32.48 35.35 38.15 41.13 44.36 47.50 50.93 55.38 56.91 2.861 Failure Surface Specified By 13 Coordinate Points Point No. 1 2 3 4 5 6 7 8 9 10 11 12 13 X-Surf (ft) 92.22 96.10 101.07 106.05 110.84 115.62 120.00 124.31 127.93 131.53 133.98 134.34 134.44 Y-Surf {ft} 35.52 32.36 31.88 32.36 33.78 35.25 37.67 40.19 43.64 47.11 51.47 56.46 58.43 3.023 Failure Surface Specified By 13 Coordinate Points Point No. 1 . 2 3 4 5 6 7 8 9 10 11 12 13 X-Surf (ft) 101.11 105.99 110.95 115.95 120.94 125.89 130.88 135.58 138.13 141.30 144.57 148.20 150.36 Y-Surf (ft) 42.48 41.40 40.71 40.71 40.59 41.35 41.68 43.38 47.67 51.54 55.33 58.77 63.00 3.435 .00 25.00 50.00 75.00 100.00 125.00 X . 00 + + + + + + _ * 25.00 + A 50.00 + - 7 - 7 - 7.8 708 X 75.00 78.* 78. . 08. .* 70.1* 7.8.1.2 7.8..2. . I 100.00 78.21. . .5 78..1..65 78..1 87.4156..* 0.74.1219. - 0. .745.312* S 125.00+ 7.4583.12 - 0. . .74.83.3. - 0. . .7.464.3 - 0. . .5.56.8. . 0.07..5.8.. 070.5.8* 150.00 + 7.755 0. .6 06 0 0 1/1 F 175.00 + 200.00 + 125 108 I 75 A X I S 25 i •t- 25 38 73 180 X - ftXIS 125 158 175 200 125 Iflfl -L A X I S 75 t 50 25 8 e 23 73 188 123 X - AXIS 130 173 288