HomeMy WebLinkAbout2014-10-28_cityofsanrafael_b1099386f3b6af61140d63b570cf036fExhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEMHoVVESAsSOCIATES 1~lc. 1700 4th Street. 8 July 2014 Introduction This report presents the results of our geotechnical investigation of the proposed residential building site located at the above address. It conforms to the requirements of section 1803 in the 2013 California Building Code (CBC). The purpose of our investigation was to evaluate the geotechnical feasibility of the proposed development, assess the suitability of the building site, and provide detailed recommendations and conclusions as they relate to our specialty field of practice, geotechnical engineering and engineering geology. The scope of services specifically excluded any investigation needed to determine the presence or absence of issues of economic concern on the site, or of hazardous or toxic materials at the site in the soil, surface water, ground water, or air. It is understood that others have performed this work. If this report is passed onto another engineer for review it must be accompanied by the approved architectural and structural drawings so that the reviewer can evaluate the exploration and data in the context of the complete project. Ground conditions and standards of practice change; therefore, we should be contacted to update this report if construction has not been started before the next winter or one-year from the report date. For us to review the drawings for compliance with our recommendations the four following notes must be on the structural drawings: • The geotechnical engineer shall accept the footing grade prior to placing any reinforcing steel in accordance with the CRC requirements. • Drainage details may be schematic, refer to the text and drawings in the geotechnical report for actual materials and installation. • Refer to Geotechnical Report for geotechnical observation and acceptance requirements. Along with the structural drawings, to complete the review, we need the pertinent calculations from the structural engineer or the geotechnical design assumptions should be included on the drawings notes per requirements of the 2013 CBC. • It is the owner's responsibility that the contractor knows of and complies with the BMP's (Best Management Practices) of the Regional Water Quality Control Board, available at www.swrcb.ca.gov . .J water quality .J stormwater .J construction The fieldwork consisted of reconnaissance mapping of exposed geologic features on the site and in the immediate surrounding area and the drilling of three test borings. The borings were advanced using a portable hydraulic drill rig with 3-inch flight augers and sampled by Standard Penetration Tests· (see "notes to borings logs"). Fieldwork was conducted in June of 2014. During this period we reviewed select geotechnical references pertinent to the area and examined stereo-paired aerial photographs of the site, which were available from Pacific Aerial Surveys in Oakland. Discussion and Summary The site is underlain by an undetermined thickness of stiff Alluvial soil deposits; these are deposits of soil laid down by the actions of modern rivers and streams, soil, which has been transported to its present position by the action of water. The soil will provide substantial bearing for footing type foundations at a depth of 24-inches. Ground water was encountered in one boring at a depth of eight Page 2 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEMHoWESAsSOCIATES INC. 17004111 Street. 8 July 2014 foundations. Ground water was encountered in one boring at a depth of eight feet. There have been three dry years so the ground water level is bound to rise when we enter a wet period. During our investigation we did not observe any local geologic hazards that would adversely affect the site. We judge that following the recommendations in this report and standard Marin County construction practices the proposed structure can be safely constructed on this site without adversely impacting the ground stability or changing the drainage in any measurable manner. Detailed discussions and recommendations are covered in the following sections of this report. Geology and Ground Stability The area if the site has been mapped my others(1) as an undetermined thickness of Alluvium lOa] soil over Franciscan sandstone and shale bedrock. Deposits resembling those described in the literature were encountered in all of the test borings. The area is essentially flat without any stability issues. Normal precautions should be taken with any excavated slopes to prevent slope failures. Ground Water Ground water was observed in one test borings at a depth of eight feet during our investigation and there were no seeps or clumps of Pampas Grass (Cortaderia Selloana), which are indicators of high ground water. However, this is after three years of below normal rainfall and the ground water is sure to rise if we resume normal rainfall patterns. Ground water conditions vary with the seasons and annual fluctuations in weather. A general rise in ground water can be expected after one or more seasons of above average rainfall. Based on the limited time we have been able to collect ground water data on this site, it is not possible to accurately predict the range of ground water fluctuations in the future. Therefore, ground water sensitive structures such as basements, wine cellars and swimming pools should be designed to anticipate a rise in the water level that could potentially affect their function and stability. During construction it should be anticipated that ground water will be encountered below eight feet. Earthquake Hazards and Seismic Design This site is not subject to any unusual earthquake hazards, located near an active fault, within a current Alquist-Priolo Special Studies Zone or Seismic Hazards Zone as shown on the most recently published maps form the California Geologic Society. There were no geomorphic features observed in the field or on air photos, or geologic features in the literature that would suggest the presence of an active fault or splay fault traces. However, historically the entire San Francisco Bay Area has the potential for strong earthquake shaking from several fault systems, primarily the San Andreas Fault which lies approximately seven miles to the southwest and the Hayward/Rodgers Creek Faults, ten miles to the northeast. The U.S. Geologic Survey presently estimates (2) there is up to 21 percent chance of a major quake (Magnitude 8) from 2000 to 2030 on the San Francisco Bay region segment of the San Andreas Fault. The probability is lower north of San Francisco and increases to the south. However, in the same period, there is a 32 percent chance of a major event (Magnitude 7) on the Hayward fault and Rodgers Creek Faults. The total 30-year probability of one or more large earthquakes occurring in the entire San Francisco region is 70 percent (see Plate 1). Based on the bedrock and soils observed at the site, we do not anticipate those seismically induced hazards, specifically: liquefaction, settlement and differential compaction, landsliding, and flooding are present. Generally speaking structures founded on stiff soil fare far better during an earthquake than structures on fill or bay mud. Page 3 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEMHOWESAsSOCIATES INC. 1700 4"' Street. 8 July 2014 For California Building Code design purposes on this site the top 100 feet of the ground has an average Soil Profile Site of Class C per section 1613.3.2. Seismic Design Site Class and ground-motion parameters, as required by CBC and ASCE 7 may be obtained from the calculator on the USGS web site at http://earthquake.usgs.gov/research/hazmaps/design. For seismic design categories D, E or F refer to the Exception in the CBC. In California, the standard of practice requires the use of a seismic coefficient of 0.15, and minimum computed Factor of Safety of 1.5 for static and 1.1 to 1.2 for pseudo-static analysis of natural, cut and fill slopes. As a homeowner there are a number of measures one can take to limit structural damage, protect lives and valuable objects in the event of a major earthquake. To be prepared and understand the mechanics of earthquakes we strongly recommend that you purchase a very practical book entitled "Peace of Mind in Earthquake Country" by Peter Yanev. This book is written for the homeowner and, while currently out of print, used copies are available in paperback (Chronicle Books/S.F.) from Amazon.com and other locations. Foundation Conditions This is a soil site and all excavations will be in soil which can be excavated by common means. We based our recommendations on assessment of in situ testing and sampling of the soil which was performed by Standard Penetration Tests (ASTM D-1586)"'. We will continue to evaluate the ground conditions during excavation and modify our recommendation if warranted. Structures with foundations on stiff soil will not experience any measurable settlement and there are no conditions that require provisions to mitigate the effects of expansive soils, liquefaction, soil strength or adjacent loads. Except for seismic none of the requirements in CBC § 1803.5.11 and .12 apply. Design Recommendations All foundations must bear on the stiff soil by footing type foundations. Summary of Design Parameters Design parameters in this report were determined by field observations and testing and per section 1806.2 of the CBC supersede the presumptive values in the CBC table 1806.2. • Seismic Design (See Earthquake Hazards Section) Soil Profile Site Class Type C. Ground motion parameters from USGS web site at http://earthquake.usgs.gov/research/hazmaps/design with site coordinates. • Active earth pressure: (see lateral loading formula in Eq. and Seismic Design Section) In a Soil Section = 35 for level and 45 Ibs/ft3 equivalent fluid pressure for sloping backslope In a Rock Section = 35 Ibs/ft2 (pounds per square foot) • Allowable Bearing Capacity (PilllYl Use the values given in Table 1806.2 for a #3 Class of Materials and relevant notes • Foundation Drainage Include items in "Drainage Check List" Details on the application of these design values are included in the following sections of this report. Page 4 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEMHoWESAsSOCIATES INC. 17004'" Street. 8 July 20 J 4 Footings As a minimum, spread footings should conform to the requirements of Sections 1808 and1809 of the CSC. The footings should be stepped as necessary to produce level bottoms. For geotechnical considerations, since soil is a discontinuous medium, footings should be constructed in a grid like fashion by tie beams. Isolated interior and deck footings should be avoided. Sased on our site exploration footings bottomed on the stiff fill, at 24-inches depth, should conform to the requirements in table 1806.2 and of the CSC for a type 3 "Class of Materials", including the "notes" in Sections 1806 and 1809; with a lateral bearing pressure increased to 300 Ibs/ft2lft. In addition we recommend that the footings be reinforced in accordance with the minimum requirements for a member in flexure (span 10 feet and cantilever 5 feet) to mitigate the effects of ground displacement during possible long term settlement. Note: (The allowable bearing pressure and lateral bearing/sliding was based on visual soil mass classification and was calculated from SPT "N° values using 0 = 14' and V = 130 Ibs/ft) and C= 1 ksf in Figure 1 page 7.2-131 of the NAVFAC manual) Geotechnical Considerations for Slab on Grade Construction The base for slabs on grade should consist of a 4-inch capillary moisture break of clean free draining crushed rock or gravel with a gradation between 1/4 and 3/4 inch in size. The base should be compacted by a vibratory plate compactor to 90 percent maximum dry density as determined by ASTM 0-1557. A 10-mil impermeable membrane moisture vapor retarder should be placed on top of the gravel. An under-slab drain system, as shown on the attached drawing, should be installed in/under the drainrock. The gravel should be "turned down" by a vibratory roller or plate to provide a smooth surface for the membrane. Recycled material is never acceptable. Where migration of moisture vapor would be undesirable (e.g. under living spaces and areas covered by flooring) a "true" under-slab vapor barrier, such as "Stego® Wrap", should be installed. In this case one should consult an expert in waterproofing, our recommendations only apply to the geotechnical aspect of drainage and do not address the prevention of mold or flooring failures. The top of the membrane should be protected during construction from puncture .. Any punctures in the membrane will defeat its purpose. The contractor is responsible for the method of protecting the membrane and concrete placement. Drains and outlets should be provided from the slab drain rock. (See attached Drawing for Typical Under-slab Drains) Geotechnical Drainage Considerations These recommendations apply to the geotechnical aspect of the drainage as they affect the stability of the construction and land. They do not include site grading and area drainage, which is within the design responsibility of civil engineers and landscape professionals. The civil and landscape professionals should make every effort to comply with the Marin County "Stormwater Quality Manual for Development Projects In Marin County" by the Marin County Stormwater Pollution Prevention Program (MCSTOPPP www.mcstoppp.org) and Bay area Stormwater Management Agencies Association (BASMAA www.basmaa.org) when possible. The site should be graded to provide positive drainage away from the foundations at a rate of 5 percent within the first ten feet (per requirements of the CSC section1804.3). All roofs should be Page 5 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEIV1HoWESAsSOCIATES INC. 17004'" Street. 8 July 2014 equipped with gutters and downspouts that discharge into a solid drainage line. Gutters may be eliminated if roof runoff is collected by shallow surface ditches or other acceptable landscape grading. One should observe the ponding of water during winter and consult with you landscape professional for the location of surface drains and with us if subdrains are required. All cross slope foundations should have backdrainage. In compliance with section 1805.4.2 of the CSC foundation drains should be installed around the perimeter of the foundation. On sloping lots only the upslope foundation line requires a perimeter drain. Interior and downslope grade beams and foundation lines should be provided with weep holes to allow any accumulated water to pass through the foundation. The top of the drainage pipe should be a minimum of four inches below the adjacent interior grade and constructed in accordance with the attached Typical Drainage Details. All drainpipes should rest on the bottom of the trench or footing with no gravel underneath. Drain pipes with holes greater than Ys-inch should be wrapped with filter fabric, if Class 2 Permeable is used, to prevent piping of the fines into the pipe. If drain rock, other than Class 2 Permeable, is used the entire trench should be wrapped with filter fabric to prevent the large pore spaces in the drain rock from silting up. Site conditions change due to natural (e.g. rodent activity) and man related actions and during years of below average rainfall, future ground water problems may not be evident. One should expect to see changes in ground water conditions in the future that will require corrective actions. All surface and ground water collected by drains or ditches should be discharged into the 4th Street storm water collection system. All laterals carrying water to a discharge point should be SDR 35, Schedule 40 or 3000 triple wall HOPE pipe, depending on the application and should be buried. 'Flex pipe' is never acceptable. Cleanouts for stormwater drains should be installed in accordance with §11 01.12 of the CPC, without pressure testing. However, this is not a geotechnical consideration and is the responsibility of the drainage contractor. If the crawl space area is excavated below the outside site grade for joist clearance, the crawl space will act as a sump and col/ect water. If such construction is planned, the building design must provide for gravity or pumped drainage from the crawl space. If it is a concern that moisture vapor from the crawl space will affect flooring, a specialist in vapor barriers should be consulted, we only design drainage for geotechnical considerations. The owner is responsible for periodic maintenance to prevent and eliminate standing water that may lead to such problems as dry rot and mold. Construction grading will expose weak soil that will be susceptible to erosion. Erosion protection measures must be implemented during and after construction. These would include jute netting, hydromulch, silt barriers and stabilized entrances established during construction. Typically fiber rolls are installed along the contour below the work area. Refer to the current ABAG(9) manual for detailed specifications and applications. Erosion control products are available from Water Components in San Rafael. The ground should not be disturbed outside the immediate construction Page 6 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEMHoVVESAsSOCIATES INC. 1700 4th Street. 8 July 2014 area. Prevention of erosion is emphasized over containment of silt. Post construction erosion control is the responsibility of your landscape professional, It is the owner's responsibility that the contractor knows of and complies with the BMP's (Best Management Practices) of the Regional Water Quality Control Board, available at www.swrcb.ca.gov . .J water quality .J stormwater .J construction. In addition, summer construction may create considerable dust that should be controlled by the judicial application of water spray. After construction, erosion resistant vegetation must be established on all slopes to reduce sloughing and erosion this is the responsibility of a landscape professional. Periodic land maintenance should be performed to clean and maintain all drains and repair any sloughing or erosion before it becomes a major problem. Drainage Checklist Before submitting the project drawings to us for review the architect and structural engineer should be sure the following applicable drainage items are shown on the drawings: • Under-slab drains and outlets • Crawl space drainage • Cross-slope footing and grade beam weep holes • Drain pipe located at lowest part of footing • Invert of foundation drains located 4-inches below interior grade • No gravel under any drainpipe • Upslope exterior foundation drains • Drains installed in accordance with § 1101.12 of the CPC • Bentonite seals at drainpipe transition to solid pipe In lieu of the above details actually being shown on the drawings there may be a: • Note on the structural drawings: "Drainage details may be schematic and incomplete, refer to the text and drawings in the geotechnical report for actual materials and installation" Construction Inspections In order to assure that the construction work is performed in accordance with the recommendations in this report, Salem Howes Associates Inc. must perform the following applicable inspections. We will provide a full time project engineer to supervise the foundation excavation, drainage, compaction and other geotechnical concerns during construction and accept the footing grade I pier holes prior to placing any reinforcing steel in accordance with the CRC or CBC Section 1702-Definitions and Table 1704.9. Otherwise, if directed by the Owner, these inspections will be performed on an "periodic as requested basis" by the Owner or Owner's representative. We will not be responsible for construction we were not called to inspect. In this case it is the responsibility of the Owner to assure that we are notified in a timely manner to observe and accept each individual phase of the project. Key Inspection Points • Map excavations in progress to identify and record rock/soil conditions. • Accept final footing grade prior to placement of reinforcing steel. • Accept subdrainage prior to backfilling with drainage rock. • Accept drainage discharge location. Page 7 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)SALEIVI H OWEsAssoc IATES INC. 1700 41h Street 8 July 2014 Additional Engineering Services We should work closely with your project engineer and architect to interactively review the site grading plan and foundation design for conformance with the intent of these recommendations. We should provide periodic engineering inspections and testing, as outlined in this report, during the construction and upon completion to assure contractor compliance and provide a final report summarizing the work and design changes, if any. Any engineering or inspection work beyond the scope of this report would be performed at your request and at our standard fee schedule. Limitations on the Use of This Report This report is prepared for the exclusive use of Epstein Properties LLC and their design professionals for construction of the proposed new residence. This is a copyrighted document and the unauthorized copying and distribution is expressively prohibited. Our services consist of professional opinions, conclusions and recommendations developed by a Geotechnical Engineer and Engineering Geologist in accordance with generally accepted principles and practices established in this area at this time. This warranty is in lieu of all other warranties, either expressed or implied. All conclusions and recommendations in this report are contingent upon Salem Howes Associates being retained to review the geotechnical portion of the final grading and foundation plans prior to construction. The analysis and recommendations contained in this report are preliminary and based on the data obtained from the referenced subsurface explorations. The borings and exposures indicate subsurface conditions only at the specific locations and times, and only to the depths penetrated. They do not necessarily reflect strata variations that may exist between such locations. The validity of the recommendations is based on part on assumptions about the stratigraphy made by the geotechnical engineer or geologist. Such assumptions may be confirmed only during earth work and foundation construction for deep foundations. If subsurface conditions are different from those described in this report are noted during construction, recommendations in this report must be re-evaluated. It is advised that Salem Howes Associates Inc. be retained to observe and accept earthwork construction in order to help confirm that our assumptions and preliminary recommendations are valid or to modify them accordingly. SalemHowes Associates Inc. cannot assume responsibility or liability for the adequacy of recommendations if we do not observe construction. In preparation of this report it is assumed that the client will utilize the services of other licensed design professionals such as surveyors, architects and civil engineers, and will hire licensed contractors with the appropriate experience and license for the site grading and construction. We judge that construction in accordance with the recommendations in this report will be stable and that the risk of future instability is within the range generally accepted for construction on hillsides in the Marin County area. However, one must realize there is an inherent risk of instability associated with all hillside construction and, therefore, we are unable to guarantee the stability of any hillside Page 8 of 10
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)
Exhibit 5.aAugust 28, 2014 PC Meeting Exhibit 5.a (Geotechnical Inv.)