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HomeMy WebLinkAboutPD 15-13; GIBRALTAR TOWNHOMES HOA; FINAL SOILS REPORT; 2016-08-05 DEBERRY ENGINEERING ASSOCIATES, INC The Clock Tower Plaza 16466 Bernardo Center Dr. Ste 136, San Diego, CA 92128 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com INVESTIGATION of SURFACE SLIDE 7495 - 7501 Gibraltar Street Carlsbad, California 92009 July 31, 2015 August 5, 2016 Updated PREPARED FOR: Gilbraltar Townhomes HOA DEBERRY ENGINEERING ASSOCIATES, INC The Clock Tower Plaza 16466 Bernardo Center Dr. Ste 136, San Diego, CA 92128 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com NO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALE N GI NEERWILLIA M J DEBE R RYCIVI LSTA TEOFCALI F O R NIANO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALENGINEERWILLIAMJDEBE R RYCIVI LS TATEOFCALI F O R NIANO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALENGINEERWILLIAMJDEBE R RYCIVI LS TATEOFCALI F O R NIANO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALENGINEERWILLIAMJDEBE R RYCIVI LS TATEOFCALI F O R NIANO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALENGINEERWILLIAMJDEBE R RYCIVI LS TATEOFCALI F O R NIANO. 34545EXPIRES 09/30/13REGISTEREDPROFESSIONALENGINEERWILLIAMJDEBE R RYCIVI LS TATEOFCALI F O R NIANO. 34545EXPIRES 09/30/17REGISTEREDPROFESSIONALE N GI NEERWILLIAM J DEB ER RYCIVI LSTATEOFCALI F O R NIAJuly 31, 2015 November 18, 2015 and April 14, 2016 Update Gibraltar Townhomes HOA c/o Mr. Mark Skoog, President SUBJECT: Investigation of Surface Slide PROJECT:7495 - 7501 Gibraltar Street Carlsbad, California 92009 Dear Mr. Skoog: At your request, a representative of DeBerry Engineering Associates, Inc. has conducted an investigation of surface slope failure of the above project on July 28, 2015. The scope of the project is outlined as follows: 1. Provide a soils report, of the slope soil, including soil profile and soil engineering parameters. To obtain this information, small diameter test borings and soil probes into the soil have been made. 2. Provide detailed engineering design for the slope repair. 3. Provide a detailed construction plan for the slope repair. Our report of the investigation is attached. The following report is understood to be an expression of professional opinion by this engineer, which is based on his knowledge, information and belief of the conditions noted at the time of the investigation. As such, it consists of neither a guarantee nor a warranty, expressed or implied as to the condition of the property or possible defects which may not have been apparent at the time of the investigation. This report does not constitute a compliance report. This report is provided solely for the exclusive use of the above noted client. If either party becomes involved in litigation arising as a result of the contents of this report, both parties agree to mediation through the Better Business Bureau. We appreciate this opportunity to be of Professional Service to you in this matter. If you have any questions regarding this report please contact us. Respectfully Submitted, DeBerry Engineering Associates William J. DeBerry, R.C.E. #34545 Attachments 7495 - 7501 Gibraltar Street pg. 3 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com 1.0 INTRODUCTION AND SCOPE OF SERVICES: The property is more particularly referred to as Lot 382 in the La Costa South No. 5 Subdivision. The parcel includes a 4 unit condominium building including APN’s 216 290 26 01, 02, 03 and 04. It was indicated that there had been a surface slope failure at the rear of the properties on December 3, 2014. The purpose of this report is to present the results of the investigation of the Geo-Technical conditions at the subject property and their influence on the surface slide of the slope at the site. The field portion of this investigation was conducted on July 28, 2015. Additional field observations were made in March 2016. This report will address the findings and provide appropriate recommendations. SITE DESCRIPTION AND OBSERVATIONS: The subject site is a developed four unit residential condominium structure. The lot grading plan for the subdivision was done in 1970. The grading plan for the LaCosta South Unit 5 Subdivision was obtained. The page 5 which includes this property was missing. Thie City of Carlsbad searched for the missing sheet, but it was not found. The finished grading plan for the lot was done in 2002. A Preliminary Soils report for the parcel was done in 1998 by Barry and Associates. The report indicated that two backhoe trenches were excavated to examine the subsurface soil. It indicated that the soil varied from moderate to low expansive potential. The slope at the rear is considered to be a filled slope. The daylight line was noted to occur near the center of the flat building pad. The front of the property is considered to face east. The parcel is an approximately rectangularly shaped lot. It is located on the west side of the cul-de sac at the north end of Gibraltar street. On the east side of the cul-de-sac there is an apartment complex with a steep downslope into this complex and continuing on to the subject property. The subject building is on a flat pad with a downslope at the rear. The existing slope has an incline of approximately 1.5:1. Current standard require filled slopes to have inclines of 2:1 or less. There are concrete patios along the rear of the building at the top of the slope down. There are downspouts from rain gutters at the rear roof eaves discharging into subterranean drains on the patios. The finished grading plan depicts a drain line extending from the north side of the rear patios, and under the rear patios of the complex to the south west corner and down the south side to exit at the curb in Gibraltar Avenue just south of the property. The President of the Home-Owners’ Association indicated that the drain line across the rear of the property had been inspected with a camera and found no issues. A surface slide was noted directly west of the patios behind units 4 and 3 and partially behind unit 2. The soil was observed to have slumped down from just below the rear patios by approximately 4 feet. The mass of soil was noted to have slid down the slope to the west. The area of the slope which was affected by the slide is approximately 55x25 feet. A structural investigation of the building had been performed by Innovative Structural Engineering in may of 2015 to evaluate if there had been any damage to the building resulting from the slide. The report indicted that there were cracks at the junction of the columns supporting the rear decks and that the base of the columns had moved toward the slope. They concluded that the existing structural columns and deck have not been structurally compromised and still have the ability ro resist the applicable loads from the light deck above. 7495 - 7501 Gibraltar Street pg. 4 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com A cursory examination of the exterior of the building revealed some separation of the decks from the building and some separation of the building from the driveway and walkways at the front. A review of the rainfall records from McClellan-Palomar Airport for November and December 2014 indicate that there was approximately 0.66” of rain fall during November and 0.49” on December 2 and 0.31” on December 3, the day the slide occurred. 2.0 DESCRIPTION OF SITE GEOLOGY AND GEOLOGIC HAZARDS: Landslide Potential and Slope Stability - A review of the State of California Department of Conservation, Division of Mines and Geology Map indicates that the property is located in an area generally susceptible to Landslides. Slopes within this area are at or near their stability limits due to a combination of weak materials and steep slopes (many slope angles exceed 15 degrees). Although most slopes do not currently contain landslide deposits, they can be expected to fail, locally, when adversely modified. There is no history of actual landslides on this site prior to the one in December of 2014. The Geologic Map of the Oceanside 30’x6’ quadrangle by Kennedy and Tan dated 2007 indicates that the property is underlain by MzU formation, which consists of Meta Sedimentary and Metavolcanic rocks, undivided. This is considered the Del Mar formation. This is described by Geologic Formations of Western San Diego County Jeffrey D. Brown, R.G., C.E.G., GEI Consultants, Inc as Lagoonal deposit; yellowish green sandy claystone interbedded with medium-gray coarse-grained sandstone; clays are expansive and landslide-prone; outcrops from Soledad Valley to Encinitas; grades vertically into Friars Formation from Los Penasquitos Canyon to La Zanja Canyon and vertically and horizontally into the Santiago Formation in Encinitas; estimated to be 200 feet thick. Liquefaction – Based on the site conditions the materials at the site are not considered to be subject to liquefaction due to such factors are soil density, grain-size distribution and groundwater conditions. 3.0 SUBSURFACE FINDINGS: A 6 inch diameter test boring was excavated near the top of the soil slippage just west of the head scarp of the slide. The soil was sampled using a Modified California sampler, ASTM D 3550. The predominant soil in the boring was found to be wet clay varying from a dark brown to a greenish blue color. The soil became progressively wetter toward the bottom of the boring, Soil Classification The predominant fill material in the borings consisted of silty clay. Groundwater Possible groundwater was encountered during the excavations Laboratory Tests The Determination of Percentage of Particles Smaller than -200 Sieve test (ASTM01140-06) aids in classification of the tested soils based on their fine materialcontent and provides qualitative information related to engineering 7495 - 7501 Gibraltar Street pg. 5 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com characteristics such as expansion potential, permeability, and shear strength. This testing indicated that the soils possess moderate expansion characteristics.Laboratory Compaction Characteristics (ASTM 01557-09) determine the laboratoryMaximum Dry Density of the soil and the Optimum Moisture content for soil compaction. The Bulk Specific Gravity using Paraffin-coated Specimens (ASTM 01188-07) test assesses the in-place density of relatively undisturbed soil samples. The results of this testing indicate that the soils encountered in the test boring in the slide material was approximately 65% of the laboratory density. The soil at the base of the slide material was approximately 85%. Direct Shear testing per ASTM D3080 was performed on a remolded sample. The cohesion was determined to be 350 psf and the friction angle was 38o. 7495 - 7501 Gibraltar Street pg. 6 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com Site Specific Seismic Parameters Seismically related design parameters obtained from the California Building Code (UBC 2013) are presented below. These design factors are based on subsurface soil and bedrock conditions and distance of the site from known active faults. Parameter Value Reference 2013 CBC Ground Motion SDS 1.2 g Fig 1613.5(1) Ground Motion SD1 0.454 g Fig 1613.5(2) Site Class D Table 1613.5.2 Site Coefficient, Fa 1.04 Table 1613.5.3(1) Site Coefficient, Fv 1.5650 Table 1613.5.3(2) Seismic Design Category D Table 1613.5.6 (1&2) Lateral Force Seismic Design Design Kh = 0.300g Kae for seismic earth pressure = 0.344 Ka for static earth pressure = 0.205 Difference: Kae - K = 0.139 Added seismic base force 617.1 # Using Mononobe-Okabe / Seed-Whitman procedure 4.0 CONCLUSIONS: Our opinions based on the result of our testing and investigation are as follows:  The likely cause of the slide was disturbance and saturation of the surface soil resulting from a combination of the rainfall on December 3 and before and possible underground water run off from the property across the street. The rainfall data indicates approximately 0.8” of rainfall on the day of and the day preceding the slide. The raingutters along the west eaves of the building, would eliminate rainfall run-off from the roof but if there was a leak in the drain line that they discharge to, this could be a contributing factor. Although the storm drain under the patio had been inspected, it is considered likely that there is some leakage or defect which was not noticeable during a camera inspection.  The following soil parameters may be used to design the retainng walls for the slope repair:  Allowable bearing for the retaining wall in competent material – 3000 psf  Active soil pressure for retaining wall design – 30 psf, for flat backfil, 46 psf for sloped.  Passive pressure for wall foundation and pier design – 600 psf, to a max of 650 psf Based on the distance of the proposed work, the impact of the grading work is considered to be negligible. The resulting re-built slope with an incline of 2:1 are considered to possess a factor of safety against sliding of 1.5. 7495 - 7501 Gibraltar Street pg. 7 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com 5.0 RECOMMENDATIONS: The washout debris should be removed and discarded. Good granular back fill soil should be used, such as decomposed granite. In the area of the failure, the slope soils should be excavated to a depth such that firm undisturbed native soils are encountered. This is anticipated to be approximately 12’ below top of soil. The depth and extent to which the repair is to be performed will be determined by this firm at the time of excavation. It is anticipated that the depth of removal will extend a minimum of one foot below the disturbed slope surface and the lateral limits of the excavation will extend at least 2 feet beyond the edge of the failure. To rebuild the slide portion of the slope, it is recommended to construct one cantilevered retaining wall directly west of the patios and a second one 15 feet down the slope to the west. The retaining wall shall be stabilized with 2 foot diameter caissons spaced at 9.5 feet along the length of the wall and approximately 12 to 18 feet into the competent soil. The wall shall include a sub-drainage system. The retaining wall backdrains shall consist of 4” diameter pvc perforated drain line with 12 of gravel backfill surrounded by filter fabric. The imported soil shall be low expansion material with and expansion index less than 50. It shall have a liquid limit less than 30 and a plasticity index less than 15. An allowable end bearing capacity of 4,000 psf may be used for piers extending into the hard damp sandy clay Del Mar Formation. This is the allowable value for sedimentary formation. The end bearing capacity has deducted the down drag force produced by the existing near surface soils. The pier weight to be considered is only one third of the actual weight of the buried pier. The actual needed pier length and embedment into the soils shall be established by the structural design based on the length needed to adequately support the total vertical and lateral loads included in the design. An increase of 1,000 psf of vertical end bearing capacity may be allowed for every additional foot of embedment into formation, up to a maximum of 6,000 psf. The area of the repair should be properly benched prior to receiving fill. The fill should be properly moistened and mechanically compacted to a minimum of 90% of the laboratory determined maximum dry density in accordance with ASTM test method D-1557. The fill shall be placed in horizontal lifts not exceeding 6 inches in thickness. Density tests should be performed by the soils engineer to ensure adequate compaction. Import soil shall be approved by the soil engineer prior to placement. Rebuilt slope areas should be planted with drought tolerant deep rooted native California vegetation. We recommend consulting with a landscape specialist to determine the most suitable species of vegetation. The concrete in the foundation and piers shall be made with sulfate resistant cement type V. Pier excavation /drilling operations shall be performed under the continued observation of a representative of our firm to confirm the penetration into proper soils. The design and construction of the piers shall be in accordance with the recommendation presented above the current CBC requirements accepted by the City of Carlsbad and also in accordance with ACI 336, 34-93, Design and construction of Drilled Piers, of the American Concrete Institute. The contractor shall follow all the safety procedures required by Cal OSHA. Pier excavations should be filled with As Built depths 26.5 - 32'. See As built Plan 7495 - 7501 Gibraltar Street pg. 8 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com NO. 34545EXPIRES 09/30/17REGISTEREDPROFESSIONAL E NGI NEERWILLIAM J DEBER RYCIVI LS TATEOFCALI F O R NIAconcrete within 2 days after the excavation to reduce the risk of soil caving or mud intrusion. Observation of work: Periodic observation by the Geotechnical Consultant, approximately one time on each day of excavation is essential during the excavation operation and during construction of the foundations, subdrains, retaining wall backfill etc. to confirm conditions anticipated by our investigation, to allow adjustments in design criteria to reflect actual field conditions exposed, and to determine that the grading proceeds in general accordance with the recommendations contained herein. It is recommended that DeBerry Engineering Associates Inc. be retained to provide continuous geotechnical engineering services during the earthwork operations. This is to observe compliance with the design concepts, specifications or recommendations and to allow design changes in the event that subsurface conditions differ from those anticipated prior to start of construction DeBerry Engineering Associates Inc., or our consultants, will not be held responsible for earthwork of any kind performed without our observation, inspection and testing. Site Drainage and Moisture Protection In order to ensure that the existing drainage system at the rear of the property is functioning properly, it is recommended that the system be checked for leakage. This should be done by blocking the south end of the drain and filling the drain line with water to a head of 5 feet. The water level in the system should be monitored for a period of 1 hour to ensure that any decrease in the level of the water is less than 6 inches in that time span. Providing and maintaining adequate site drainage and moisture protection of supporting soils is an important design consideration. Retaining wall recommendations represented herein assume proper site drainage will be established and maintained. Under no circumstances should water be allowed to pond adjacent to retaining walls. The site should be graded such that surface drainage flow is directed from structures and into swales or other controlled drainage facilities. William J. DeBerry, R.C.E. #34545 Attachments 7495 - 7501 Gibraltar Street pg. 9 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com APPENDIX This page left blank 7495 - 7501 Gibraltar Street pg. 10 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com Test No.: B1 Location: See Boring Location Diagram Boring Date: July 28, 2015 6” diameter hole. Depth In Feet Bulk Sample Apparent cohesion And Friction Angle Blow Per Foot .1.1 Description .1.2 In Place Dry Density Pcf Moisture Content, % dry wt. 0 2 4 6 8 10 12 14 16 18 20 Sample 1 Sample 2 350 psf, 38o Loose dry light brown clay CL 5’ Dark Brown wet clay CL Bluish – Green wet clay CL Fill Hard damp sandy clay SC – SM Del Mar Formation clasts End of Boring Refusal 12’ No water, no caving 126.5 7495 - 7501 Gibraltar Street pg. 11 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com P/L P/L P/L UNIT 1 UNIT 2 UNIT 3 UNIT 4 B 1 TEST BORING LOCATION 7495 - 7501 Gibraltar Street pg. 12 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com P/L P / L 60 65 70 P / L P / L FILL DELMAR FORMATION A A 55 TdAF Site Diagram D eB ER R Y E NG INE E R ING ASSO C IATE S (858) 451-0713 Diagram not to scale 16466 Bernardo Center Dr Suite 136 San Diego, California 92128-3176 N O R TH 7495 - 7501 GIBRALTAR STREETCARLSBAD, CALIFORNIA 92009 7495 - 7501 Gibraltar Street pg. 13 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com FILL Td 50 60 70 18'12'P / L 1 24 3 2 1 GRADE BEFORE WASHOUT AND PROPOSED GRADEEXISTING AND PROPOSED GRADE EXISTING PATIO GRADE AFTER WASHOUT BACKFILL BEHIND WALL TO EXISTING GRADE WALL B WALL A FILL FILL Qoa? 50 60 70 P/L 122 1 EXISTING PATIO FILL 2 TEMPORARY SHORING IF NEEDED POST AND BOARD WALL 1 CONSTRUCT 2:1 FILL SLOPE2 CONSTRUCT PVC DRAIN AT MIN 2% SLOPE PER PLAN STABILIZED WITH CAISSONS3 CONSTRUCT A MASONRY CANTELEVERED RETAINING WALL 5' MAX TALL STABILIZED WITH CAISSONS4 CONSTRUCT A MASONRY CANTELEVERED RETAINING WALL 8' MAX TALL TEMPORARY SLOPES / SHORING 1 TEMPORARY EXCAVATION REPAIR CONDITION A A TdAF ?? ? ? ? ?? ? ? ? AF Td Site Diagram D eB ER R Y E NG INE E R ING ASSO C IATE S (858) 451-0713 Diagram not to scale 16466 Bernardo Center Dr Suite 136 San Diego, California 92128-3176 7495 - 7501 GIBRALTAR STREETCARLSBAD, CALIFORNIA 92009 Qoa? 7495 - 7501 Gibraltar Street pg. 14 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com Temporary Shoring Cross section Vertical 2" X 10" pressure treated lumber 2.5" Steel Pipe Driven into soil For cuts over 5' if needed Site Diagram D eB ER R Y E NG INE E R ING ASSO C IATE S (858) 451-0713 Diagram not to scale 16466 Bernardo Center Dr Suite 136 San Diego, California 92128-3176 7495 - 7501 GIBRALTAR STREETCARLSBAD, CALIFORNIA 92009 7495 - 7501 Gibraltar Street pg. 15 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com SUBJECT SITE LOCATION OF SLIDE STEEP SLOPE AT PROPERTIES TO THE EAST GIBRALTAR STREET Google Map Aerial View of Site 7495 - 7501 Gibraltar Street pg. 16 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com California DMG L A C OS TA LEVANTE CADENCIAVENADOPIRAGUA A N ILLO FOSCA CAM SERBAL ROMERI ABOLEROAL TIS MAALTIVATORRY AVNDA VALERA NAVARRA CALLE TIMITEOGALLEONCTE DE LA VISTA LA GRAN VIA ROCIOA Z AH AR PASEO AVELLANOMADRILENAJEREZSEGOVIA HATACA CARINGA QUEBRADAPRIMAVERA ESTANCIAPASEO CANDELEROVISTA MARIANA GARBOSOLUCIACALLE MADEROMANAGUATRZA RIBERA P A S EO S AU C E D A L MORADASACADACALLE MEJORAVNDA HELECHO CALLE OLIVAGAVIOTAVIEJO CASTILLACALINACUEST A FAROLQUINTA CALLE DEL SURMURILLOAVNDA CEREZANUEVA CASTILLACALLE PINABETEPER DIZCENTELL ASI TI O AB E T OCALLE CARACASZUBARONSAVNDA DE PALAIS VIA TECASONDRACANDILPIRINEOS CTE PROMENADEVIA IRISMONTANESPRIMENTELTRZA GOYA CALLE ANDARGRADO MARBELLACTE MORERA CTE MARINCARAVALLOCARLINA FEBO CALOMA MARMOL CALLE ODESSA VIA COSCOJA VISTA HIGUERAVIVIENDOBADAJOZ SAN BRISTO VIA OPUNTIAVISTA PALMACANTERO R A N A C T E T R A D IC IO N C T E N IN A VISTA MARMATEOVISTA CANELACAM ARROYOBOTELLASI TIO GRANADOSITIO MIRTOVISTA GUYABASITIO FRESNOCAM DE AGUASDEHESAC AS TI L LA SERRANOPUEBLOGABACHODELGADOP RIV A T E DORADOALDEACORTI NAVERDE P RIVA TE DELFINAMALLORCA BRAVAGALICIA VI ST A BONI T AGIBRA L TA R AGUA DULCEVISTA GRANDESOLANODEL REY VIA DE FORTUNA DEL R IO PLAZA BONITA ALLEYLAS BRISAS # Landslide Hazard Identification Relative Landslide Susceptibility Areas email: d.deberry@worldnet.att.net DeBerry Engineering Associates, Inc. 12225 World Trade Drive, Suite H San Diego, California 92128 Phone: 858 451 0713 Fax: 858 451 2783 1 2 3-1 3-2 4-1 4-2 Least Susceptible Marginally Susceptible Generally Susceptible Most Susceptible Landslide Questionable Landslide? 7495 - 7501 Gibraltar Street pg. 17 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com REFERENCES 1. Preliminary Soils report for the parcel was done in 1998 by Barry and Associates ACI 302, “Guide for Concrete Construction. 2. California Building Code (CBC 2013). 3. Grading Plan for Gibraltar Views Condominium, Lot 382, LaCosta South Unit 5 by Masson and Associates, Inc dated 11-6-02, DWG 402 – 8A 4. ASTM Standards 5. Weather Underground Rainfall records from McClellan-Palomar Airport for November and December 2014. 6. The Geologic Map of the Oceanside 30’x6’ quadrangle by Kennedy and Tan dated 2007 7. Multi Jurisdictional Hazard Mitigation plan for San Diego County dated March 8. The State of California Division of Mines and Geology Landslide Hazard Identification Map. 9. ACI 302, “Guide for Concrete Construction. 10. Standard Specifications for Public Works Construction. 11. Geologic Formations of Western San Diego County Jeffrey D. Brown, R.G., C.E.G., GEI Consultants, Inc. 12. Google Maps 7495 - 7501 Gibraltar Street pg. 18 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com Limited Geological Investigation Proposed Caisson Slope Repair of Surface Slide 7495 - 7501 Gibraltar Street Carlsbad, California 92009 7495 - 7501 Gibraltar Street pg. 19 Office: 858.451.0713 Email: dianed3@cox.net, wjdeberry@cox.net Cell: 858.472.2783 www.deberryeng.com STEPHEN E. JACOBS, C.E.G. 1307 Engineering Geologist 2871 Sanford Lane, Carlsbad, CA 92010-6553 Phone & Fax (760) 434-8503 Cell (760) 458-5574 Email stephenejacobs3019@gmail.com August 3, 2016 Project No. 16009 Gibraltar Townhomes HOA c/o Mr. Mark Skoog, President “Skoog, Mark" <Mark.Skoog@knowles.com> Subject: Limited Geological Investigation Proposed Caisson Slope Repair of Surface Slide 7495 - 7501 Gibraltar Street Carlsbad, California 92009 Dear Mr. Skoog: In accordance with the request of Mr. William DeBerry of DeBerry Engineering Associates, Inc., I have performed a limited geological investigation of the surface slide in the subject area that is to have a caisson-retaining wall slope repair located at 7495- 7501 Gibraltar Street in the City of Carlsbad, California. The results of the limited investigation on the subject slope indicate that artificial fill, which reached an exposed depth of approximately 28 feet in an exploratory boring, revealed no detectable slip surfaces below the surface slide. On this basis it is concluded that no deeper landslides occur below the surface slide on the subject slope. The opportunity to provide consulting services to you on this project is appreciated. If you have any questions regarding the report, please contact the undersigned at your convenience. Respectfully submitted, Stephen E. Jacobs Engineering Geologist PG 3978, CEG 1307 4cc: Addressee 08/31/17 7495 - 7501 Gibraltar St., Carlsbad, CA 2 8/3/2016 STEPHEN E. JACOBS, C.E.G. William DeBerry DeBerry Engineering Associates, Inc. wjdeberry@cox.net Wayne Gifford wayne_gifford@yahoo.com City of Carlsbad Department of Building and Safety Attachments: Appendix A: References Appendix B: Text Figures LIMITED GEOLOGICAL INVESTIGATION Proposed Caisson Slope Repair of Surface Slide 7495 - 7501 Gibraltar Street Carlsbad, California 92009 INTRODUCTION This report presents the results of a limited geological investigation of a surface slide on an existing slope for a proposed caisson-retaining wall repair, located at 7495-7501 Gibraltar Street in the City of Carlsbad, San Diego County, California (Figure No. 1). The subject property lies within an area considered generally susceptible to slope failure. Accordingly, the primary purpose of this study was to examine the geologic materials and structures in the subsurface of one exploratory caisson boring in order to identify any additional slip surfaces in the materials below the slip surface of the subject surface slide. This report has been prepared for the exclusive use of Gibraltar Townhomes HOA, their representatives and direct clients, and because conditions may change over time due to earthquakes, rainstorms, construction, and other causes, this report may require an updated investigation. This report is not to be provided to any other third party without my authorization and my on-site observation. Should this report be provided to another STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 3 8/3/2016 third party without my authorization and my on-site observation, then the undersigned, will assume no liability, whatsoever. SCOPE OF WORK The scope of this investigation was limited to: surface reconnaissance, research of readily available geologic and geotechnical literature pertinent to the site, subsurface exploration, geologic analysis of the field data, and preparation of this report. More specifically, the intent of this limited investigation was to: • Identify the subsurface conditions underlying the slope at the site to the depths influenced by the proposed remedial construction. • Present my opinions in this written report, which includes in addition to my findings and recommendations, a site geologic map showing the location of my subsurface exploration, a geologic cross section, and a log of the exploratory boring. SITE DESCRIPTION The subject property is described as Lot 382 in the La Costa South No. 5 Subdivision located at the northwest corner of the cul-de-sac on 7495-7591 Gibraltar Street in the City of Carlsbad, California (Figure No. 1). The parcel contains a four-unit residential condominium structure that includes APN’s 216-290-26-01, -02, -03, and -04. The subject building is on a nearly level pad with a slope that descends westerly below the rear of the pad. The existing slope, which extends beyond the western property boundary, has a gradient ranging from approximately 1.5:1 to 2:1 (horizontal to vertical ratio) and a height of about 30 feet. The elevations on the property range from about 70 to 55 feet above mean sea level (MSL), according to the topographic base map on Figure No. 3, but they extend to an elevation of about 40 feet west of the property boundary according to the topographic map on Figure No. 1. STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 4 8/3/2016 The surface slope failure reportedly occurred at the rear of the subject properties on December 3, 2004. DeBerry Engineering and Associates (2015) performed a field investigation on the property on July 28, 2015 with additional field observations in March 2016. The surface slide was located directly west of the rear patios behind units 3 and 4 and partially behind unit 2 (see Figure No. 3). The soil was reported to have slumped down toward the west down the slope from just below the rear patios by approximately 4 feet. REVIEW OF GEOLOGIC/GEOTECHNICAL DOCUMENTS, ETC. I reviewed available published geologic literature, aerial photographs, and maps including publications by Tan and Giffen (1995), Tan and Kennedy (1996), Kennedy and Tan (2007, Figure No. 2), Jennings and Bryant (2010), and the unpublished consultant report prepared by DeBerry Engineering Associates, Inc. (2015, updated 2016). A list of references is presented in Appendix A. Text figures are presented in Appendix B, which includes a regional geologic map shown on Figure No. 2, a site geologic map shown on Figure No. 3, a geologic cross section shown on Figure No. 4, and a log of exploratory boring shown on Figure Nos. 5A and 5B. Aerial photographs from the U.S. Department of Agriculture (1953), supplemented with various dated photographs from Historic Aerials (2016), were also reviewed for this study. These photos reveal that grading of the building pad had occurred in about 2003. GENERAL GEOLOGY AND GEOLOGIC SETTING The site is within the coastal plain section of the Peninsular Ranges Geomorphic Province of California. The coastal plain generally consists of subdued landforms underlain by sedimentary bedrock. Natural alluvial and bedrock materials exposed in the vicinity of the project site are mapped as late to middle Pleistocene age old alluvial flood- plain deposits (map symbol Qoa of Kennedy and Tan, 2007, Figure No. 2) or terrace deposits (Qt2 of Tan and Kennedy, 1996), overlying the middle Eocene age Santiago Formation (map symbol Tsa of Tan and Kennedy, 1996; and Kennedy and Tan, 2007, Figure No. 2) and the Del Mar Formation (map symbol Td). Tan and Kennedy (1996) described the terrace deposits (Qt2) as “reddish brown, poorly bedded, poorly- to STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 5 8/3/2016 moderately-indurated sandstone, siltstone and conglomerate.” Subscript “2” indicates the relative terrace level with “1” the lowest elevation (youngest age) and “4” highest elevation (oldest age). Kennedy and Tan (2007, p. 6) described the old alluvial flood- plain deposits, undivided (Qoa), as “Fluvial sediments deposited on canyon floors. Consists of moderately well consolidated, poorly sorted, permeable, commonly slightly dissected gravel, sand, silt, and clay-bearing alluvium.” Tan and Kennedy (1996) described the Santiago Formation as “light-colored, poorly- bedded, poorly-indurated, fine- to medium-grained sandstone interbedded with landslide- prone siltstone and claystone. Local coarse-grained sandstone and conglomerate.” Kennedy and Tan (2007, p. 12-13) described the Santiago Formation as having “three distinctive parts. A basal member consisting of buff and brownish-gray, massive, coarse- grained, poorly sorted arkosic sandstone and conglomerate (sandstone generally predominating). In some areas the basal member is overlain by a central member that consists of gray and brownish-gray (salt and pepper) soft, medium-grained, moderately well sorted arkosic sandstone. The upper member consists of gray, coarse-grained arkosic sandstone and grit. Vertically and laterally throughout the formation there exists greenish-brown, massive claystone intervals, tongues and lenses of often fossiliferous, lagoonal claystone and siltstone. The lower part of the Santiago Formation interfingers with the Delmar Formation and Torrey Sandstone in the Encinitas quadrangle.” Bedding within the predominant sandstone of the Santiago Formation in the site vicinity was mapped with dips ranging from 10 degrees to the southwest to 20 degrees to the northeast by Tan and Kennedy (1996) and Kennedy and Tan (2007, Figure No. 2). Tan and Kennedy (1996) described the Del Mar Formation as “poorly bedded, poorly- indurated, landslide-prone sandy claystone interbedded with medium- to coarse-grained sandstone.” Kennedy and Tan (2007, p. 12) described the Del Mar Formation as “Dusky yellowish-green, sandy claystone interbedded with medium-gray, coarse-grained sandstone.” Bedding within the Del Mar Formation in the vicinity of the site was mapped with dips ranging from 5 to 17 degrees to the northwest by Tan and Kennedy (1996) and Kennedy and Tan (2007, Figure No. 2). STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 6 8/3/2016 DESCRIPTION OF SITE SUBSURFACE CONDITIONS The subject slope repair area is underlain with artificial fill as encountered in the exploratory caisson boring B1 to the explored depth of approximately 28 feet. Old alluvial flood-plain deposits are conjectured to underlie the cut portion of the building pad, and the Del Mar Formation is presumed to underlie the old alluvial flood-plain deposits and the artificial fill (see Figure Nos. 3 and 4). The old alluvial flood-plain deposits and bedrock of the Del Mar Formation were not encountered in the exploratory boring. Subsurface conditions underlying the slope on the subject property are described below. Also refer to the exploratory boring log attached as Figure Nos. 5A and 5B and the geologic cross section attached as Figure No. 4 for more detailed information on subsurface conditions. Artificial Fill (Af): Artificial fill was encountered in the exploratory caisson boring B1 to a depth of approximately 28 feet. This material consists of intercalated and mottled light gray and yellowish brown, fine- to medium-grained, clayey sand, greenish gray silty clay, and dark brown, dark reddish brown and very dark brown, sandy clay with numerous round gravels and cobbles to 8 inches and some angular metavolcanic rock fragments scattered throughout the fill. The fill was soft to firm and slightly moist to moist in the upper about 3 feet, and firm to stiff and moist to very moist below about 3 feet during my observation. SELECTED GEOLOGIC HAZARDS A seismicity evaluation for the site is beyond the scope of this report; however, a summary of relevant faults and a brief discussion of selected geologic hazards are included herein. STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 7 8/3/2016 Regional Faults: The northwesterly trending Rose Canyon fault zone is the nearest active fault system which is located about 6 miles southwest of the site. Other regional active faults, the Newport-Inglewood (offshore), Elsinore, San Jacinto and San Andreas fault zones, are about 7 miles northwest, and 23, 49, and 71 miles northeast, respectively, from the site. Landslides: As part of this investigation I reviewed the publication, “Landslide Hazards in the Northern Part of the San Diego Metropolitan Area” by Tan and Giffen (1995). This reference is a comprehensive study that classifies San Diego County into areas of relative landslide susceptibility. The subject site is located in “Area 3-1.” Land within Area 3-1 is considered to be generally susceptible to slope failures. The surface slide area on the approximately 1.5:1 westerly-facing slope that descends below the rear of the building pad on the subject property is underlain by artificial fill as encountered to the explored depth of approximately 28 feet in the caisson boring. No ancient landslides were detected on west-facing slope on the property according to the aerial photographs (USDA, 1953) reviewed for this study. No slip surfaces below the surface slide were observed within the artificial fill in the exploratory boring. Groundwater: No groundwater was encountered in the exploratory caisson boring. Localized perched groundwater can develop on the site and is usually associated with landscape irrigation or excessively heavy rainfall. CONCLUSIONS AND RECOMMENDATIONS 1. The exploratory caisson boring drilled through the existing slope on which the surface slide occurred encountered artificial fill to the explored depth of approximately 28 feet. The dark brown, dark reddish brown, and very dark brown material encountered in the boring is interpreted as reworked topsoil and subsoil. STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 8 8/3/2016 The common round gravels and cobbles are interpreted as having been derived from the old alluvial flood-plain deposits. The light gray and yellowish brown clayey sand and greenish gray silty clay with common altered gypsum veinlet pieces are interpreted as having been reworked from sandstone and claystone beds of the Del Mar Formation. 2. No slip surfaces were observed in the artificial fill below the surface slide in the exploratory boring. 3. The geotechnical engineer should provide updated foundation recommendations for the proposed caisson retaining walls on the subject slope based on the findings made from the exploratory caisson boring. LIMITATIONS This limited geological investigation report is prepared for the exclusive use of Gibraltar Townhomes HOA, and their representatives and direct clients. The opinions expressed herein are for the purpose of evaluating subsurface conditions affecting the slope on the subject parcel. This study is limited to the review of available technical published literature and observations made in the one exploratory boring. This report is intended for use only by the client named above for the purpose stated; no other use of the report is authorized, and transfer to any other person or agency without my notification or authorization is not advisable. No warranties, express or implied, are given as to the geology, soils, or foundation conditions of the subject parcel. STEPHEN E. JACOBS, C.E.G. 7495 - 7501 Gibraltar St., Carlsbad, CA 8 8/3/2016 Appendix A REFERENCES City of Carlsbad, 2015, Grading Plans for Gibraltar Townhomes HOA, 7495-7501 Gibraltar Street, Carlsbad, California, Project No. PD 15-13, dated December 28, 2015, 5 sheets. DeBerry Engineering Associates, Inc. (DBA), 2015, Investigation of Surface Slide, 7495- 7501 Gibraltar Street, Carlsbad, California 92009, dated July 31, 2015, updated November 18, 2015 and April 14, 2016. Historic Aerials by NETR Online, 2016, Aerial photographs dated 1953, 2002, 2003, and 2012, website: historicaerials.com. Jennings, C.W., and Bryant, W.A., 2010, Fault Activity Map of California: California Geological Survey, Geologic Data Map No. 6, scale 1:750,000. Kennedy, M.P., and Tan, S.S., 2007, Geologic Map of the Oceanside 30’ x 60’ Quadrangle, California: California Geological Survey, Regional Geologic Map Series, Map 2, scale 1:100,000. Tan, S.S., and Giffen, D.G., 1995, Landslide Hazards in the Northern Part of the San Diego Metropolitan Area, San Diego County, California, Landslide Hazard Identification Map No. 35: California Division of Mines and Geology Open File Report 95-04, pp. 1-6, Plate 35E-Rancho Santa Fe Quadrangle, scale 1:24,000. 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 Open File Report 96-02, Plate 2-Geologic Maps of the Encinitas and Rancho Santa Fe 7.5’ Quadrangles, scale 1:24,000. U.S. Department of Agriculture (USDA), 1953, Aerial photographs, Flight No. AXN- 8M, Frame Nos. 16, 17, and 18, vertical, black and white, stereo-pairs, scale 1:20,000. U.S. Geological Survey, 2015, Topographic Map of the Encinitas 7.5-Minute Quadrangle, California, scale 1:24,000. U.S. Geological Survey, 2015, Topographic Map of the Rancho Santa Fe 7.5-Minute Quadrangle, California, scale 1:24,000.