GEOTECHNICAL INVESTIGATION 8-UNIT RESIDENTIAL DEVELOPMENT 31 WHARF BOLINAS, CALIFORNIA

Prepared for Bolinas Community Land Trust 6 Wharf Road Bolinas, California 94924

March 2020 Project No. 5028-1 March 12, 2020 5028-1

Bolinas Community Land Trust RE: GEOLOGIC AND GEOTECHNICAL 6 Wharf Road INVESTIGATION Bolinas, California 94924 8-UNIT RESIDENTIAL DEVELOPMENT 31 WHARF ROAD BOLINAS, CALIFORNIA Attention: Ms. Arianne Dar

Ladies and Gentlemen:

In accordance with your request, we have provided geologic and geotechnical services for the proposed 8-unit residential development at 31 Wharf Road situated within the unincorporated community of Bolinas in Marin County, California. In particular, we have evaluated the potential for ground surface rupture by the San Andreas Fault, which is considered to be active by Marin County and the State of California. We have concluded that the potential for ground surface fault rupture is low, and thus there is no recommendation for fault setbacks. The accompanying report summarizes the results of our field exploration, geologic reconnaissance, laboratory testing, engineering analysis, and presents our geologic and geotechnical recommendations for the proposed project.

We refer you to the text of our report for specific recommendations.

Thank you for the opportunity to work with you on this project. If you have any questions or comments about our findings or recommendations, please call.

Very truly yours, ROMIG ENGINEERS, INC.

Alexander Shmurakov, G.I.T. David F. Hoexter, P.G., C.E.G.

Lucas J. Ottoboni, P.E.

Copies: Addressee (2+via email)

1390 El Camino Real, Second Floor | San Carlos, CA 94070 | (650) 591-5224 | www.romigengineers.com GEOLOGIC AND GEOTECHNICAL INVESTIGATION 8-UNIT RESIDENTIAL AND COMMERCIAL DEVELOPMENT 31 WHARF ROAD BOLINAS, CALIFORNIA 94924

PREPARED FOR: BOLINAS COMMUNITY LAND TRUST 6 WHARF ROAD BOLINAS, CALIFORNIA 94924

PREPARED BY: ROMIG ENGINEERS, INC. 1390 EL CAMINO REAL, SECOND FLOOR SAN CARLOS, CALIFORNIA 94070

FEBRUARY 2020 TABLE OF CONTENTS Page No. Letter of transmittal Cover Page TABLE OF CONTENTS INTRODUCTION ...... 1 Project Description ...... 1 Scope of Work ...... 1 Limitations ...... 2 SITE EXPLORATION AND RECONNAISSANCE ...... 3 Surface Conditions ...... 3 Subsurface Conditions ...... 3 Ground ...... 4 PREVIOUS NEARBY SITE INVESTIGATIONS ...... 4 GEOLOGIC SETTING ...... 4 Regional ...... 4 Site Geology ...... 5 Mapped Geologic Hazard Zones ...... 6 Engineering Geologic Reconnaissance ...... 7 Aerial Photographs ...... 8 Ground Surface Fault Rupture Evaluation ...... 9 Seismicity ...... 9 Table 1. Magnitudes and Historical ...... 10 Earthquake Design Parameters ...... 10 Table 2. 2019 CBC Seismic Design Criteria ...... 11 Geologic Hazards ...... 11 CONCLUSIONS...... 13 FOUNDATIONS ...... 15 Mat ...... 15 Grid Foundation System ...... 16 Basement Water Proofing ...... 17 Lateral Loads for Mat/Footings ...... 17 Settlement for Shallow Foundation System/Engineered Fill Pad ...... 17 Drilled Piers ...... 18 Lateral Loads for Drilled Piers ...... 19 Combined Foundation ...... 19 Settlement ...... 19 SLABS-ON- ...... 19 General Slab Considerations ...... 19 Exterior Flatwork ...... 20 Interior Slabs ...... 20 Subsurface Drainage ...... 21 RETAINING WALLS ...... 22 DRIVEWAY PAVEMENT ...... 23 EARTHWORK ...... 24 TABLE OF CONTENTS (Continued)

Clearing and Preparation ...... 24 Engineered Fill Pad Beneath Shallow Foundations ...... 24 Basement Excavation Support ...... 25 Material for Fill ...... 25 Compaction ...... 26 Table 3. Compaction Recommendations ...... 26 Temporary Slopes and Excavations ...... 27 Finished Slopes ...... 27 Surface Drainage ...... 27 FUTURE SERVICES ...... 28 Plan Review ...... 28 Observation and Testing ...... 29

REFERENCES FIGURE 1 - VICINITY MAP FIGURE 2 - ENGINEERING GEOLOGIC RECONNAISSANCE PLAN FIGURE 3 - VICINITY GEOLOGIC MAP FIGURE 4 - ALQUIST-PRIOLO SPECIAL STUDIES ZONE MAP FIGURE 5 - NEARBY MAP FIGURE 6 - REGIONAL FAULT AND SEISMICITY MAP FIGURE 7 - SUBSLAB DRAINAGE DETAIL

APPENDIX A - FIELD INVESTIGATION Figure A-1 - Key to Exploratory Boring Logs Figure A-2 - Key to Descriptions Exploratory Boring Logs EB-1 through EB-2

APPENDIX B - SUMMARY OF LABORATORY TESTS GEOLOGIC AND GEOTECHNICAL INVESTIGATION FOR 8-UNIT RESIDENTIAL DEVELOPMENT 31 WHARF ROAD BOLINAS, CALIFORNIA

INTRODUCTION This report presents the results of our geologic and geotechnical investigation for the proposed 8-unit development at 31 Wharf Road, situated within the unincorporated community of Bolinas in Marin County, California. The location of the property is shown on the Vicinity Map, Figure 1. The purpose of this investigation was to evaluate subsurface conditions at the site and to provide geologic and geotechnical recommendations for the project.

Project Description The project consists of constructing an 8-unit residential and commercial development at the currently vacant property. The development will be limited to the narrow, relatively flat, rectangular, lower lying portion of the parcel along Wharf Road. The development is expected to have a footprint of approximately 75 feet wide by 108 feet deep. Subterranean parking below the development will be supported by a basement along the upslope sides and will daylight at Wharf Road along the downslope side. Building “A” at the front of the property will consist of two retail spaces at the street level with two levels of residential units above. Building “B” at the rear of the proposed development will consist of two levels of residential units above a main parking level (all situated above the subterranean parking). The rear wall/excavation is expected to be up to about 15 to 20 feet high.

Scope of Work Our scope of work for this investigation was presented in detail in our agreement with you dated October 25, 2019. In order to accomplish this investigation, we performed the following work.

• Review of geologic and seismic conditions in the site vicinity. Bolinas Community Land Trust Residential Development Page 2 of 29

• Geologic reconnaissance and field mapping by our certified engineering geologist and staff.

• Review and interpretation of stereo-pair aerial photographs.

• Subsurface exploration consisting of drilling, sampling, and logging of two exploratory borings within the footprint of the proposed development.

• Laboratory testing of selected samples to aid in classification and to help evaluate the engineering properties of the soil and bedrock encountered at the site.

• Engineering analysis and evaluation of the subsurface data to develop geotechnical design criteria.

• Preparation of this report presenting our findings and geologic and geotechnical recommendations for the proposed development.

Limitations This report has been prepared for the exclusive use of the Bolinas Community Land Trust for specific application to the currently planned development located at 31 Wharf Road in an unincorporated area of Marin County near Bolinas, California. We make no warranty, expressed or implied, for the services we performed for this project. Our services were performed in accordance with geotechnical engineering principles generally accepted at this time and location. This report was prepared to provide engineering opinions and recommendations only. In the event there are any changes in the nature, design or location of the project, or if any future improvements are planned, the conclusions and recommendations contained in this report should not be considered valid unless 1) the project changes are reviewed by us, and 2) the conclusions and recommendations presented in this report are modified or verified in writing.

The analysis, conclusions, and recommendations presented in this report are based on site conditions as they existed at the time of our investigation; the currently planned improvements; review of readily available reports relevant to the site conditions; and laboratory test results. In addition, it should be recognized that certain limitations are inherent in the evaluation of subsurface conditions, and that certain conditions may not be detected during an investigation of this type. Changes in the information or data gained from any of these sources could result in changes in our conclusions or recommendations. If such changes occur, we should be advised so that we can review our report in light of those changes. Bolinas Community Land Trust Residential Development Page 3 of 29

SITE EXPLORATION AND RECONNAISSANCE Site geologic reconnaissance and subsurface exploration were performed on December 9 and December 16, 2019, respectively. Subsurface exploration was performed using a truck-mounted B-24 drill equipped with a solid flight auger. Two exploratory borings were advanced to depths of 25 and 36.5 feet. The approximate locations of the borings are shown on the Engineering Geologic Reconnaissance Plan, Figure 2. The boring logs and the results of our laboratory tests are attached in Appendices A and B, respectively.

Surface Conditions The site is located in a residential and commercial area along the northwest side of Wharf Road. The site was unoccupied with the exception of a small temporary shed. The overall property included the narrow, relatively flat, rectangular, lower elevation portion of the parcel that connects to Wharf Road at the front and a large rear area that was heavily vegetated and sloped moderately to steeply up to the northwest. There is an elevation gain of about 120 feet from the front to the rear of the property.

Based on our borings and visual observation, the surface of the relatively level front portion appeared to be comprised of fill and was approximately 7 feet higher than the adjacent Wharf Road. A short dirt driveway connected the relatively level pad down to Wharf Road. In general, the lower lying portion of the site sloped up gently towards the rear. At the intersection of the narrow, rectangular portion of the site and the rear sloping area, the site was relatively flat due to an approximately 40-foot-wide in which we observed a fenced community garden and several cuts into the native hillside. Beyond the this cut, the site sloped up to the northwest at an inclination of about 3:1 to 1.5:1 (horizontal:vertical).

We observed the crest of the hillside immediately beyond the northwestern property line, which consisted of a flat-topped knoll with no structures or topographic features. The site was generally vegetated with small to large trees and shrubs on the hillside, and generally free of vegetation on the lower lying portion of the site.

Subsurface Conditions At the location of Boring EB-1, which was advanced near the front portion of the property near Wharf Road, we encountered about 3 feet of undocumented fill comprised of firm to very stiff sandy of low plasticity. Beneath the fill we encountered about 17 feet of stiff to hard sandy silt of low plasticity, which was interpreted as slope wash/debris with residual soil (intensely weathered bedrock) at depth, underlain by very soft siltstone bedrock of the Merced Formation to sampler refusal conditions at a depth of 36.5 feet. Bolinas Community Land Trust Residential Development Page 4 of 29

At Boring EB-2, advanced further into the property, we encountered about 4 feet of fill comprised of stiff to very stiff sandy silt of low plasticity. Beneath the fill, we encountered about 12.5 feet of very stiff sandy silt of low plasticity, which was interpreted as slope wash/debris with residual soil (intensely weathered bedrock) at depth, underlain by very soft siltstone bedrock of the Merced formation to the maximum depth explored of 25 feet.

The surface and near-surface at the site were judged to have a low potential for expansion.

Ground Water Free ground water was encountered at a depth of 14 feet in Boring EB-1. We did not encounter free ground water in Boring EB-2. The borings were backfilled with grout immediately after drilling and sampling was completed, therefore a stabilized ground water level measurement was not obtained. Based on the topography in the area of the property, perched ground water conditions (similar to the conditions encountered in Boring EB-1) may develop in the soils and near the bedrock during and after significant rainfall or due to landscape watering at the property and from upslope areas. Please be cautioned that fluctuations in the level of ground water can occur due to variations in rainfall, landscaping, underground drainage patterns, and other factors.

In our opinion, ground water may seasonally rise up in the soil to approximately 10 feet below ground surface, which at the rear portion of the development will likely be above the finished floor elevation of the subterranean parking level. This should be considered in the design.

PREVIOUS NEARBY SITE INVESTIGATIONS

We were unable to locate relevant nearby site investigations.

GEOLOGIC SETTING

Regional Geology The site is located within the central region of the Coast Ranges Geomorphic Province, which extends from the Oregon border south to the Transverse Ranges. The general topography is characterized by sub-parallel, northwest trending coastal mountain ranges and intervening valleys. The region has undergone a complex geologic history of sedimentation, volcanic activity, folding, faulting, uplift and erosion. Bolinas Community Land Trust Residential Development Page 5 of 29

The relatively flat lying, alluviated San Francisco Bay Plain is situated to the southeast of the site, and uplifted Mt. Tamalpais and hills within Point Reyes National Park are located east and north of the site. The site lies west of the Bolinas Lagoon, which extends south to the Pacific Ocean.

The site is located within the San Andreas Fault Zone. The San Andreas Fault Zone a major fault system that defines the plate boundary between the North American plate and the Pacific plate. The onshore portion of the fault zone spans approximately 700 miles from the Gulf of California in the south to Mendocino County in the north. The zone is generally 1,000 feet to one mile wide. The fault zone has a slip rate greater than 5.0 millimeters per year, with a recurrence interval of approximately 200 to 400 years. The last major earthquake recorded along this portion of the fault zone occurred in 1906, where major surface deformation was observed along one of the strands of the fault adjacent to the study site.

Site Geology

The site is mapped as being underlain by early Quaternary-and late Pliocene-aged Merced Formation (QTm), as mapped by Gluskoter (1969), Blake et al (1974), Galloway (1977), Clark & Brabb (1997), and Blake, et. al (2000). The Merced Formation is characterized by soft siltstone and fine-grained silty sandstone. The siltstone contains friable, fine-grained, clayey sandstone, with some beds of shale pebble conglomerate. The siltstone observed in our borings was both blue gray (Boring EB-1) and orange- brown (Boring EB-2) and did not appear to contain shale. Merced Formation bedding plane orientations identified near the site appeared to be striking north-northwest to northwest and dipping at 7 to 30 degrees to the northeast and east. The Merced Formation in the general site vicinity is present only within the San Andreas Fault Zone. Figure 3, Vicinity Geologic Map, presents significant geologic features of the site vicinity.

The site is located at the base of the drainage and/or former inlet to the Bolinas Lagoon which separates two knolls, one northwest and one southeast of the site, each mapped as being underlain by Merced Formation bedrock. The Bolinas Lagoon is approximately 400 feet east of the site and can be characterized as a shallow wetland and estuary primarily comprised of soft and/or silty sediments. The lagoon likely was formed as a down dropped or tilted block within the San Andreas Fault Zone.

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The site is located between two strands of the San Andreas Fault, in an area where the offshore San Gregorio Fault Zone bifurcates from the San Andreas Fault Zone. The fault zone in this area is approximately 4,000 feet wide. Based on the State of California Special Studies Zones (formerly “Alquist-Priolo”) map of the Bolinas Quadrangle (Figure 4) and the previously noted published geologic maps, the western-most strand of the San Andreas Fault, located approximately 1,500 feet west of the site, has not seen indications of Holocene/recent movement but was active in the Quaternary period. The eastern strand, located approximately 1,500 feet east of the site, experienced ground surface rupture during the 1906 San Francisco Earthquake, and thus was active in Holocene (recent) period.

The State of California Special Studies Zones map also shows additional short and inferred fault segments or “splays”, generally within low-lying areas northwest of the site, including an inferred trace oriented in a northwest to southeast direction on the northeastern side of the knoll from where the site is located. This fault trace is approximately 1,400 feet long and projects away from the site. We did not observe this inferred fault trace in our field reconnaissance; however, the linearity and relative steepness of the northeast side of the knoll supports the notion that faulting is located on the east side of the knoll. Further, the orientation of this apparent fault trace would indicate that it would not produce any surface rupture within the study area.

Lawson (1908) is an extensive investigation and documentation of the effects of the 1906 San Francisco Earthquake. The current understanding of the locations of 1906 fault rupture in the Bolinas vicinity is based on the observations presented in the Lawson report. There are no indications provided by Lawson of primary impacts or damage (such as ground surface fault rupture) or secondary (such as liquefaction or seismically induced landsliding) within the immediate site vicinity.

Mapped Geologic Hazard Zones The Marin County online geographical information system (MarinMap) places the subject property near the boundary between a region designated with “few landslides” and a region designated with “mostly landslides.” This MarinMap designation appears to be based on Wentworth et. al (1977). We note that the area of the property that lies within the region designated with “mostly landslides” is the relatively flat portion of the site near Wharf Road and did not display the characteristics associated with a typical active landscape. The area of the property that lies within the “few landslides” region is moderately to steeply sloping. This apparent discrepancy relates to the construction of the map, with an over-extension of the hazard area from the southeast. Based on our site reconnaissance and air photo interpretation, there did not appear to be any evidence of previous or current landslides within the property boundaries. Bolinas Community Land Trust Residential Development Page 7 of 29

The portion of Marin County encompassing the site has not yet been evaluated by the California Geologic Survey and therefore no landslide or liquefaction hazard maps are available. According to the Liquefaction Susceptibility map by Witter et al., 2006, the site is located in a “Very Low” liquefaction susceptibility zone.

Engineering Geologic Reconnaissance The engineering geologic reconnaissance was conducted by our Senior Field Geologist, Alexander Shmurakov, and Certified Engineering Geologist, David Hoexter, on December 9, 2019. Significant features on the subject site and in the immediate vicinity are shown on Figure 2. The reconnaissance consisted of traversing the subject site and the surrounding area.

The front portion of the property within the area of the proposed development, was blanketed by 3 to 4 feet of fill material, likely placed during the original of the lot. The fill extended approximately from the front property line to the base of the rear slope.

We observed Merced Formation bedrock outcrops at two locations in the rear (higher elevations on the northwest) portion of the property. Each of the outcrops consisted of weathered siltstone bedrock. The two observed outcrops and the rock encountered in our borings appeared to be lithologically consistent. The rock encountered on site appeared to be heavily weathered but did not appear to be sheared. Based on the consistent lithology of the bedrock at the site, in our opinion, there are no indications of through- going faulting within the site.

The topographic map by Paul Krohn, P.E. (2014) identified two springs on the slope above the proposed development location. In the area of the western spring we observed a box that would likely fill up if the spring were active. The box was completely dry and vegetated, and we did not observe any evidence that this area was an active source of water. In the area where the eastern spring was mapped, we observed a large amount of overgrowth which made the exact location of the mapped spring inaccessible. There was no evidence that this spring was actively producing water. Springs often occur due to water being trapped by an impermeable barrier created by a fault. The slope direction of the hillside on which these springs are located is oriented parallel to the direction to the regional fault trend. A fault parallel to the regional trend would thus not likely induce a water barrier and thus springs. These springs are likely point sources of water rather than springs associated with faulting. Bolinas Community Land Trust Residential Development Page 8 of 29

We observed several shallow incised drainage channels in the upper hillside portions of the property. There were no indications of debris flow sources, deposits, or bowl-shaped areas which would indicate landslides, indicating that these channels were likely formed through surface runoff and erosion over time. Based on our air photo interpretation, these drainage channels are typical of the slopes within the Merced Formation in the site vicinity.

Aerial Photographs Two sets of aerial photographic stereo pairs, flown in 1972 and in 1990, were interpreted for this investigation. The image scales range from 1:12,000 to 1:24,000. The photos are listed in the References section of the report. The following discussion pertains to both sets of images, although the earlier, 1972, imagery were of greater use as there was less vegetation obscuring the ground surface.

The site appeared to be occupied by a small structure (not the current temporary shed) in the 1972 imagery. The two existing prominent trees on the lower slope were present. The slope above the site exhibited light brush growth with no trees. There were no lineations or tonal contrasts indicative of faulting on the slope or across the flat top of the knoll above the property. Shallow erosion or soil slump channels were present on the slope (see Figure 2, Engineering Geologic Reconnaissance Plan). These features headed at the top of the slope and were not present on the flat top of the knoll. Similar features, many of greater dimensions, were present on equivalent slopes in the near vicinity. We note that these erosion features may be the sources of the relatively thick sediment accumulation overlying the Merced Formation bedrock observed in our soil borings. There was a vague series of lineations trending northwest-southeast across the flat top of the knoll, suggestive of bedding (and not of faulting).

In the later 1990 imagery, the lineations were not evident, the two existing springs were not evident, and there were no indications of landslides impacting the site.

Based on our air photo interpretation, there were no indications of offsets, lineations or tonal features indicative of faulting or elevated ground water across or adjacent to the subject site. In addition, there were no indications of active landsliding within or immediately adjacent to the site. Bolinas Community Land Trust Residential Development Page 9 of 29

Ground Surface Fault Rupture Evaluation The site is shown on the State of California Special Studies Zones map of the Bolinas Quadrangle as located in between two major strands of the San Andreas Fault Zone. Our fault evaluation was intended to identify the presence (if any) of any additional strands associated with this fault zone.

Based on our review of the published geologic map (Clark and Brabb, 1997), and the California Special Studies Zone map of the Bolinas Quadrangle (1974), we determined that the site is approximately 1,500 feet away from each of the mapped strands of the San Andreas Fault.

Our subsurface investigation encountered siltstone bedrock at depth which was consistent with the surface outcrops of bedrock that we observed on the adjacent slope. We did not observe any direct indications of faulting or of secondary shears or faults which would rupture the ground surface within the footprint of the planned development or within a minimum of 50 feet of the proposed structures. Historical aerial photographs and currently available satellite imagery also do not show any evidence of previous fault rupture nor the potential for future fault rupture. In our opinion, the potential for surface rupture beneath the proposed development is low.

Seismicity The San Francisco Bay Area is located in an active seismic region. Earthquakes in the region result from strain energy constantly accumulating because of the northwestward movement of the Pacific Plate relative to the North American Plate. On average about 1.6-inches of movement occur per year. Historically, the Bay Area has experienced large, destructive earthquakes in 1838, 1868, 1906 and 1989. The faults considered most likely to produce large earthquakes in the area include the San Andreas, San Gregorio, Hayward, and Calaveras faults. The site is located within the San Andreas fault zone.

The San Gregorio Fault is located approximately 1.3 miles southwest of the site. The Rodgers Creek and West Napa faults are located approximately 22 and 30 miles to the northeast, respectively. The Hayward and Calaveras faults are located approximately 18 and 36 miles southeast of the site, respectively. These faults and significant earthquakes that have been documented in the Bay Area are listed in Table 1 below and are shown on the Regional Fault and Seismicity Map, Figure 5. Bolinas Community Land Trust Residential Development Page 10 of 29

Table 1. Earthquake Magnitudes and Historical Earthquakes Bolinas Community Land Trust Residential Development Bolinas, California

Maximum Historical Estimated Fault Magnitude Earthquakes Magnitude

San Andreas 8.3 1989 Loma Prieta 6.9 1906 San Francisco 8.3 1865 N. of 1989 Loma Prieta Earthquake 6.5 1838 San Francisco-Peninsula Segment 6.8 1836 East of Monterey 6.5

Hayward 7.3 1868 Hayward 6.8 1858 Hayward 6.8

Calaveras 7.3 1984 Morgan Hill 6.2 1911 Morgan Hill 6.2 1897 Gilroy 6.3

San Gregorio 7.3 1926 Monterey Bay 6.1 Rodgers Creek 7.3 1898 Mare Island 6.7 1969 Santa Rosa 5.6 & 5.7 West Napa 6.6 2014 Napa County 6.0

In the future, the subject property will undoubtedly experience severe ground shaking during moderate and large magnitude earthquakes produced along the San Andreas fault or other active Bay Area fault zones. Using information from recent earthquakes, improved mapping of active faults, ground motion prediction modeling, and a new model for estimating earthquake probabilities, a panel of experts convened by the U.S.G.S. have concluded there is a 72 percent chance for at least one earthquake of Magnitude 6.7 or larger in the Bay Area before 2043. The Hayward fault has the highest likelihood of an earthquake greater than or equal to magnitude 6.7 in the Bay Area, estimated at 33 percent, while the likelihood on the San Andreas and Calaveras faults is estimated at approximately 22 and 26 percent, respectively (Aagaard et al., 2016).

Earthquake Design Parameters

The State of California currently requires that buildings and structures be designed in accordance with the seismic design provisions presented in the 2019 California Building Code and in ASCE 7-16, “Minimum Design Loads for Buildings and Other Structures.” Based on site geologic conditions and on information from our subsurface exploration at the site, the site may be classified as Site Class D, stiff soil, in accordance with Chapter 20 of ASCE 7-16. Spectral Response Acceleration parameters and site coefficients may be taken directly from the U.S.G.S. website based on the longitude and latitude of the site. For site latitude (37.9101), longitude (-122.6854) and Site Class D, design parameters are presented on Table 2 on the following page.

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Table 2. 2019 CBC Seismic Design Criteria Bolinas Community Land Trust Residential Development Bolinas, California Spectral Response Acceleration Parameters Design Value Mapped Value for Short Period - SS 2.451 Mapped Value for 1-sec Period - S1 1.028 Site Coefficient - Fa 1.0 Site Coefficient - Fv 1.7* Adjusted for Site Class - SMS 1.634 Adjusted for Site Class - SM1 1.748* Value for Design Earthquake - SDS 1.634 Value for Design Earthquake - SD1 1.165*

* The values of Fv, SM1 and SD1 above are provided for calculation of Ts. A site-specific ground motion hazard analysis may be required unless the exceptions in ASCE 7-16 Section 11.4.8 apply to the project.

Geologic Hazards We briefly reviewed the potential for additional geologic hazards to impact the site, considering the geologic setting, and the soils encountered during our investigation. The results of our review are presented below:

• Fault Rupture - The site is located within a State of California Alquist-Priolo Fault Rupture Hazard Zone (1974) (formerly known as a Special Studies Zone). As discussed above, the site is situated approximately equidistant from two prominent strands of the San Andreas Fault, each located approximately 1,500 feet from the site. The eastern trace appears to coincide with ground surface rupture during the 1906 Earthquake. As discussed, based on our air photo interpretation and observed bedrock consistency across the site, there are no indications of primary or splay faulting within or projecting towards the site. In our opinion, the potential for ground surface rupture at the location of the proposed development is low.

• Ground Shaking - The site is located in an active seismic area. Moderate to large earthquakes are probable along several active faults in the greater Bay Area over a 30- to 50-year design life. Strong ground shaking should therefore be expected several times during the design life of the development, as is typical for sites throughout the Bay Area. The proposed residential development should be designed and constructed in accordance with current earthquake resistance standards. Bolinas Community Land Trust Residential Development Page 12 of 29

• Secondary Ground Deformation - As noted above, the site is located in an active seismic environment. During large earthquakes with an epicenter close to the site, a potential exists for secondary ground deformation. Secondary ground deformations include fissures, random ground cracks and ground warping not directly related to primary fault rupture. This type of deformation has been noted to occur several hundred feet or more from main fault traces. Potential damage associated with this type of deformation includes cracking in swimming pools, slabs, and even building foundations due to random ground distortions. Even by designing the development to current codes and construction standards, in our opinion, a potential for this type of damage exists at the site.

• Liquefaction - Severe ground shaking during an earthquake can cause loose to medium dense granular soils to densify. If the granular soils are below ground water, their densification can cause increases in , which can lead to soil softening, liquefaction, and ground deformation. Soils most prone to liquefaction are saturated, loose to medium dense, silty and sandy with limited drainage, and in some cases, sands and that are interbedded with or that contain seams or layers of impermeable soil. We note that based on our laboratory testing, the soils encountered in our borings below a depth of 14 feet where ground water was encountered appeared to be residual soil derived from weathered bedrock and in general appeared to have a significant amount of fines content (69 percent passing -200 sieve) or binder and were judged to have a low potential for liquefaction settlement. In addition, the site is located in a “Very Low” liquefaction susceptibility zone as mapped by the Marin County GIS liquefaction susceptibility layer and Witter et al., 2006.

• Differential Compaction - Differential compaction can occur during moderate and large earthquakes when soft or loose, natural or fill soils are densified and settle, often unevenly across a site. We note that due to the transition from stiff residual soil at the rear to firm soils and possibly fill at the front of the lower level we have recommended that an engineered fill pad (over excavation and re-compaction) be constructed at the front portion of the development along with deepening of the edges of the mat slab or footings. As an alternative, the development could be constructed on a pier supported mat foundation with piers extending into residual soils or bedrock. Provided the building is supported on an engineered fill pad or drilled piers which extend below the firm soils and/or fill across the entire footprint of the structure, in our opinion, the likelihood of significant damage to the proposed structure from differential compaction is low. Bolinas Community Land Trust Residential Development Page 13 of 29

• Slope Stability/Landsliding - Based on our site reconnaissance and interpretation of the aerial photos, there are no indications of active or older landslides within or directly adjacent to the property. According to the Reconnaissance Landslide Map (Figure 5, Wentworth and Frizzell, 1975) the site is not located within an active landslide. However, the knoll to the south, directly across Wharf Road, contains an expressed landslide. There are also multiple landslides in the general vicinity of the property, especially in the northern portions of the knoll the property is located on.

• Tsunami Hazard - We reviewed two tsunami inundation maps, prepared by the Marin County Sheriff’s Office of Emergency Services (2018) and the California Emergency Management Agency, et al (2009). Each map indicates that the site is at the lateral margin of a potential tsunami inundation area. In our opinion, the likelihood of a tsunami affecting the proposed project is low but should be further evaluated.

CONCLUSIONS From a geologic and geotechnical viewpoint, the site is suitable for the proposed development provided the recommendations presented in this report are followed during design and construction. Specific geotechnical recommendations are presented in the following sections of this report.

The primary geotechnical concerns for the proposed development are:

• The presence of firm soils and fill at and below the building pad/subgrade elevation in the front portion of the development; • The potential for seasonally shallow ground water conditions at the site; and • The potential for violent ground shaking based on the Association of Bay Area Governments at the site due to moderate to large earthquakes in the area and the proximity to the San Andreas fault.

Due to the firm soils anticipated below the front half of the development footprint, the upper 6 feet of soils below the finished floor elevation should be over-excavated and re- compacted as an engineered fill pad. Abrupt or vertical limits of the over-excavation should be avoided. Rather, the excavation should be benched to gradually transition and to allow for compaction on level benches, where possible. However, this may not be entirely feasible where the proposed structure will extend to the property lines. On-site soils, Class 2 aggregate base, or quarry fines may be used as the compacted fill material. The fill pad should be observed and tested by our staff throughout construction and compacted per the recommendations in the “Earthwork” section of this report. Bolinas Community Land Trust Residential Development Page 14 of 29

Upon completion of the engineered fill pad, the proposed development may be supported on a mat foundation (with a thickened edge along the sides and at the front) or on relatively deep continuous spread footings (constructed in a grid pattern with added reinforcing to provide a stiffer foundation) with a conventional slab-on-grade floor.

As an alternative to constructing an engineered fill pad in the front half of the footprint, the development may be supported on a pier supported mat foundation. Based on the dipping nature of the bedrock contact, the piers will likely need to be deeper at the front portion of the footprint and may be slightly shallower at the rear portion of the footprint.

Due to the potential for ground water to seasonally rise to about 10 feet from the ground surface, a full subsurface drainage system should be placed below the basement to reduce the possibility of water pressure developing below the basement floor slab and floor damp-proofing system. Due to the site gradients and likely depth/elevation of the basement floor slab, the subsurface drains should be designed to discharge/daylight by gravity at a low portion of the site. Providing adequate waterproofing of the basement mat and walls is essential for the success of the basement; however, providing water proofing recommendations is outside of our scope of services and expertise. We also note that construction of the subsurface drainage system and installation of waterproofing for a mat slab would typically be easier than a continuous grid foundation with a slab-on- grade floor or a pier supported mat.

We note that portions of the fill and upper soils encountered in our borings within the depth of the basement excavation were judged to have limited and may be prone to sloughing and/or caving if excavated near-vertical. This information should be considered by the contractor when establishing temporary shoring/cut slope criteria for the basement excavation and other temporary slopes and cuts. Protection of structures near cuts should also be the responsibility of the contractor.

Because subsurface conditions may vary from those encountered at the locations of our borings, and to observe that our recommendations are properly implemented, we recommend that we be retained to 1) review the project plans for conformance with our recommendations; and 2) observe and test during earthwork and foundation construction. Bolinas Community Land Trust Residential Development Page 15 of 29

FOUNDATIONS

Mat Foundation

In our opinion, the development may be supported on a structural mat foundation bearing on stiff native soils at the rear portion of the footprint and bearing on an engineered fill pad at the front portion of the footprint.

The mat may be designed for an average allowable bearing pressure of 2,000 pounds per square foot for combined dead plus live loads, with maximum localized bearing pressures of 3,000 pounds per square foot at column or wall loads. These pressures are net values; the weight of the mat may be neglected in design. Due to the composition of the soils at the foundation level (silts) and the potential for violent shaking due to the proximity of the San Andreas fault, we do not recommend allowing an increase for total loads including wind or seismic forces.

At the front half of the footprint, we recommend deepening the mat along the front and side perimeter edges so that it has a thickened edge that extends at least 30 inches below the bottom of the mat. In addition, we recommend that the mat be capable of spanning a distance of at least 14 feet and cantilevering a minimum distance of at least 4 feet under full dead loads.

The mat should be reinforced to provide structural continuity and to permit spanning of local irregularities. A water-proofing system designed by others should be installed below and around the edges of the mat foundation (and behind the basement walls).

A modulus of subgrade reaction (Kv1) of 45 pounds per cubic inch may be assumed for the basement subgrade. This value is based on a 1-foot square bearing area and should be scaled to account for mat foundation size effects. Alternatively, based on the anticipated building load and differential settlement, a modulus of subgrade reaction (Kv) of 15 pounds per cubic inch (pci) may be assumed for the mat subgrade.

Our representative should observe that the basement excavation extends into competent native soils and observe and test throughout construction of the engineered fill pad and to evaluate if deeper excavation is advisable. If suitable bearing material is not encountered across the basement excavation, some further excavation and supplemental recommendations likely will be required.

A sub slab drainage section should be provided below the mat as described in the section of this letter titled “Subsurface Drainage.” The sub slab drainage system could daylight by gravity drain to a low point along the north edge of the property.

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Grid Foundation System As an alternative to a mat foundation, in our opinion, the development may be supported on continuous spread footings constructed in a grid pattern bearing on stiff native soils at the rear portion of the footprint and bearing on an engineered fill pad at the front portion of the footprint.

Spread footings should have a width of at least 15 inches and should extend at least 30 inches below exterior grade, at least 24 inches below the bottom of concrete slabs-on- grade (whichever is deeper, at the front and sides of the structure) and at least 24 inches below exterior grade and at least 18 inches below concrete slabs-on-grade (whichever is deeper, in the rear half of the structure). Exterior finished grade should be considered the lowest adjacent grade within 4 feet of the downslope side of the footing excavations.

Footings with at least these minimum dimensions may be designed for an allowable bearing pressure of 2,500 pounds per square foot (psf). The weight of the footings may be neglected for design purposes. Due to the composition of the soils at the foundation level (silts) and the potential for violent shaking due to the proximity of the San Andreas fault, we do not recommend allowing and increase for total loads including wind or seismic forces. In addition, we recommend the new continuous footings be designed to be capable of spanning a distance of at least 14 feet and cantilevering a minimum distance of at least 4 feet under full dead load.

There would be a benefit to including some interior stiffening elements arranged in a grid pattern with added reinforcing to provide a stiffer foundation more capable of tolerating differential soil movement. Where applicable, we suggest that continuous footings or stiffening elements be spaced at intervals no greater than approximately 20 feet, or as determined by the structural engineer.

All footings located adjacent to utility lines should be embedded below a 1:1 plane extending up from the bottom edge of the utility . We recommend that continuous foundations be reinforced with top and bottom steel, to provide structural continuity and to permit spanning of local irregularities.

The bottom of all footing excavations should be cleaned of soft/firm or loose soil and debris. A member of our staff should observe all footing excavations prior to placement of reinforcing steel to confirm that they expose suitable material, have at least the recommended minimum dimensions, and have been properly cleaned. If old fill or soft/firm or loose soils are encountered in the foundation excavations, our field representative will require these materials to be removed and a deeper footing embedment depth before the reinforcing steel and concrete is placed. Bolinas Community Land Trust Residential Development Page 17 of 29

Basement Water Proofing We have not provided recommendations regarding the method or details for basement damp-proofing since design of damp-proofing systems is outside of our scope of services and expertise. Installing adequate damp-proofing below and behind the edges of the basement floor and behind the basement walls is essential for the success of the basement structure. Placing concrete with a low water ratio should be considered as one step of good damp-proofing as discussed in the Slab-On-Grade section below. The damp-proofing system below the basement mat may be placed directly on the sub slab drainage system rock or on a thin working slab, as determined by the water-proofing consultant.

Lateral Loads for Mat/Footings Lateral loads may be resisted by between the bottom of the mat foundation and/or footing and the supporting subgrade, and by passive soil pressure acting against the footings or mat foundation cast neat in foundation excavations or backfilled with properly compacted structural fill. The below values given for coefficient of friction and passive soil resistance are ultimate values. We recommend that a factor of safety of 1.5 be applied.

An ultimate coefficient of friction of 0.45 may be assumed for the footings bearing directly on sandy lean . An ultimate coefficient of friction of 0.55 may be assumed for the mat foundation bearing directly on a crushed rock section. However, since a water-proofing membrane is expected to be installed between the bottom of the mat and subgrade soil, the structural engineer should consult with the water-proofing consultant for the coefficient of friction between the membrane and subgrade soil.

Ultimate passive soil resistance may be simulated by an equivalent fluid pressure of 450 pounds per cubic foot beginning at the ground surface or basement subgrade, where appropriate. The ultimate passive soil resistance acting on the mat foundation should be limited to 2,500 pounds per square foot. This passive pressure assumes lateral deflection at the top of the foundation on the order of ¼- to ½-inch at the top of the basement wall.

Settlement for Shallow Foundation System/Engineered Fill Pad Thirty-year post-construction differential movement due to static loads is not expected to exceed about 1 to 1.5 inches across a horizontal distance of about 50 feet for the proposed development supported on either a mat or continuous footings bearing on stiff native soils at the rear portion of the footprint and bearing on an engineered fill pad at the front portion of the footprint. Bolinas Community Land Trust Residential Development Page 18 of 29

Drilled Piers

As an alternative to constructing an engineered fill pad in the front half of the footprint, the development may be supported on a pier supported mat foundation with piers extending into residual soils and/or bedrock.

Piers should be at least 16 inches in diameter and extend at least 10 feet below the bottom of the mat, and at least 6 feet into residual soil and/or weathered bedrock, whichever is deeper. Based on our borings, we anticipate that the piers in the front of the footprint (nearest Wharf Road) may need to extend about 20 feet below the bottom of the mat to accomplish this, the piers in the middle about 15 feet and the piers along the rear limits of the development about 10 feet below the bottom of the mat. The structural engineer may require a deeper embedment depth to resist vertical and lateral loads.

Piers may be designed for an allowable skin friction of 400 pounds per square foot in the upper 10 feet and 500 pounds per square foot below 10 feet for dead plus live loads, with a one-third increase allowed when considering additional short-term wind or seismic loading. The allowable uplift capacity of the piers may be calculated using a skin friction value of 320 pounds per square foot. Drilled piers should have a center-to-center spacing of at least three pier diameters.

In the front half of the building footprint, the vertical resistance in the upper 6 feet should be neglected in design; while the neglect of the remaining piers in the rear half of the footprint need not apply.

Pier drilling should be observed by our representative to confirm that the pier holes extend the required minimum depth, expose the anticipated competent material, and are properly cleaned of all loose or soft soil and debris and dewatered. The pier depths recommended above may require adjustment if differing conditions are encountered during drilling.

Concrete should be placed in the pier holes as soon as practical after drilling. Ground water may seep into the pier holes during pier drilling and it is possible that ground water seepage could cause some sloughing or caving of the pier holes. This can be further evaluated during drilling of the initial piers. If ground water cannot be effectively pumped from the pier holes, concrete will need to be placed in the pier holes by the method. The contractor should plan on placing concrete the same day the piers are drilled; if caving conditions occur, multiple concrete placements each day or use of drill casing may be required.

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Lateral Loads for Drilled Piers

Lateral loads may be resisted by passive earth pressure based upon an equivalent fluid pressure of 350 pounds per cubic foot, acting on 1.5 times the projected area of the pier. In the front half of the building footprint, the passive resistance in the upper 6 feet should be neglected in design; while passive resistance of the remaining piers in the rear half may be considered from the top of the piers.

Combined Foundation

In our opinion, a combined mat/spread footing and drilled pier foundation may be used for the basement retaining walls where the foundations will be bearing at least 6 feet below current site grades. An allowable bearing pressure of 2,000 pounds per square foot may be used in conjunction with the drilled pier capacity.

An ultimate coefficient of friction of 0.45 may be assumed for the footings bearing directly on sandy lean clay. An ultimate coefficient of friction of 0.55 may be assumed for the mat foundation bearing directly on a crushed rock section. However, since a water-proofing membrane is expected to be installed between the bottom of the mat and subgrade soil, the structural engineer should consult with the water-proofing consultant for the coefficient of friction between the membrane and subgrade soil.

Settlement

Thirty-year, post-construction differential settlement due to static loads is not expected to exceed ¾-inch across the proposed development supported on a combined drilled pier and mat slab foundation system, provided the foundations are designed and constructed as recommended.

SLABS-ON-GRADE

General Slab Considerations

To reduce the potential for movement of at-grade slabs, at least the upper 6-inches of the subgrade soil should be scarified and compacted at a moisture content slightly above the laboratory optimum value. The soil subgrade should be kept moist up until the time the non-expansive fill, aggregate base, and/or vapor barrier is placed. Slab subgrades and non-expansive fill should be prepared and compacted as recommended in the section of this report titled “Earthwork.”

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Overly soft, loose, overly moist and fill soils should be removed from slab-on-grade areas. Exterior flatwork and interior slabs-on-grade should be underlain by a layer of non-expansive fill as recommended below. The non-expansive fill should consist of Class 2 aggregate base or clayey soil with a Plasticity Index of 15 or less.

Considering the potential for some differential movement of the surface and near-surface soils, we expect that reinforced slabs will perform better than unreinforced slabs. Consideration should be given to using a control joint spacing on the order of 2 feet in each direction for each inch of slab thickness.

Exterior Flatwork Concrete walkways and exterior flatwork should be at least 4 inches thick and should be constructed on at least 6 inches of Class 2 aggregate base. To improve performance, exterior slabs-on-grade, such as for patios, may be constructed with a thickened edge to improve edge stiffness and to reduce the potential for water seepage under the edge of the slabs and into the underlying base and subgrade. In our opinion, the thickened edges should be at least 8 inches wide and ideally should extend at least 4 inches below the bottom of the underlying aggregate base layer.

Interior Slabs At-grade interior slab-on-grade floors (other than the slab/mat at the basement level which will be underlain as recommended in the following section titled, “Subsurface Drainage”) should be constructed on a layer of non-expansive fill at least 6 inches thick. Recycled aggregate base should not be used for non-expansive fill below interior slabs- on-grade, since adverse vapor could occur from crushed asphalt components.

In areas where dampness of concrete floor slabs would be undesirable, such as within the garage and/or building interior, concrete slabs should be underlain by at least 6 inches of free-draining , such as ½- to ¾-inch clean crushed rock with no more than 5 percent passing the ASTM No. 200 sieve. Pea gravel should not be used for this capillary break material. The crushed rock layer should be compacted and leveled with vibratory equipment. The crushed rock layer may be considered as the non-expansive fill recommended above.

Please note that the basement mat/slab floors should be underlain by a high-quality waterproofing membrane selected by your waterproofing consultant.

To reduce vapor transmission up through concrete floor slabs, the crushed rock section should be covered with a high quality vapor barrier conforming to the requirements of Bolinas Community Land Trust Residential Development Page 21 of 29

ASTM E 1745 Class A, with a water vapor transmission rate less than or equal to 0.01 perms (such as 15-mil thick “Stego Wrap Class A”) should be used. The vapor barrier should be placed directly below the . above the vapor barrier is not recommended. The vapor barrier should be installed in accordance with ASTM E 1643. All seams and penetrations of the vapor barrier should be sealed in accordance with manufacturer’s recommendations.

The permeability of concrete is affected significantly by the water cement ratio of the mix, with lower ratios producing more damp-resistant slabs (or basement retaining walls) and being stronger structurally. Where moisture protection is important and/or where the concrete will be placed directly on the vapor barrier, the water-to-cement ratio should be 0.45 or less. To increase the workability of the concrete, mid-range plasticizers can be added to the mix. Water should not be added to the mix unless the slump is less than specified and the ratio will not exceed 0.45. Other steps that may be taken to reduce moisture transmission through the slab (or mat) include moist curing for 5 to 7 days and allowing the slab to dry for a period of two months or longer prior to placing floor coverings. Also, prior to installation of the floor covering, it may be appropriate to test the slab moisture content for adherence to the manufacturer’s requirements to determine whether a longer drying time is necessary.

Subsurface Drainage

A subsurface drain system designed to discharge by gravity to a lower portion of the site should be installed below the basement mat to reduce the possibility of water pressure developing below the basement floor and floor damp-proofing system. Perforated pipes for the basement drainage system should be installed at the bottom of the basement excavation. The basement drainage system should include a minimum 4- to 8-inch-thick blanket of free-draining gravel, such as 1/2- or 3/4-inch crushed rock with no more than 5 percent passing the ASTM No. 200 sieve, below the basement mat. Prior to placing the gravel blanket, the subgrade below the gravel layer should be surface compacted and covered with a filter fabric, such as TC Mirafi 140N. The gravel drain should extend up and around the sides of the mat and basement walls.

Drain pipes around the basement walls should consist of 4-inch diameter perforated PVC pipes with perforations placed down installed at bottom of the wall excavation. The perforated pipes should preferably discharge by gravity to a low point along the north edge of the property or to a suitable sump and pump system. To minimize vapor transmission through the basement mat, a high-quality water-proof membrane should be placed over the crushed rock and around the edges of the mat foundation. A schematic sketch of the basement drainage system is presented in Figure 7.

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RETAINING WALLS

Retaining walls should be designed to support adjacent native material, fill, and backfill. Retaining walls with level backfill that are not free to deflect or rotate, such as retaining walls as part of the structures, should be designed to resist an equivalent fluid pressure of 45 pounds per cubic foot plus an additional uniform lateral pressure of 8H in pounds per square foot, where H is the height of the wall in feet. Retaining walls with level backfill that are free to rotate, such as site retaining walls, may be designed to resist an equivalent fluid pressure of 45 pounds per cubic foot.

Walls with sloping backfill should be designed for an additional equivalent fluid pressure of 1 pound per cubic foot for every 1.25 degrees of slope inclination. Where retaining walls will be subjected to surcharge loads, such as from adjacent foundations, vehicle loads, or construction, the walls should be designed for an additional uniform lateral pressure equal to one-half of the surcharge pressure.

Based on the site peak ground acceleration (PGA), on Seed and Whitman (1970); Al Atik and Sitar (2010); and Lew et al. (2010); seismic loads on walls that cannot yield, such as the basement retaining walls, may be subjected to a seismic load as high as about 21H2 (in pounds per foot, where H is the wall height in feet). This seismic surcharge line load should be assumed to act at 1/3H above the base of the wall (in addition to the active wall design pressure of 45 pounds per cubic foot for level wall backfill, with additional 1 pound per cubic foot for every 1.25 degrees of slope inclination for sloping backfill).

Where retaining walls will be subjected to surcharge loads, such as from adjacent foundations, vehicle loads, or construction, the walls should be designed for an additional uniform lateral pressure equal to one-half of the surcharge pressure.

To prevent buildup of water pressure from surface water , a subsurface drainage system should be installed behind the basement and any site retaining walls. The drainage system should consist of a 4-inch diameter perforated pipe (perforations placed down) embedded in a section of 1/2- to 3/4-inch, clean, crushed rock at least 12 inches wide. Backfill above the perforated drain line should also consist of 1/2- to 3/4- inch, clean, crushed rock to within about 1½ to 2 feet below exterior finished grade. A filter fabric should be wrapped around the crushed rock to protect it from infiltration of native soil. The upper 1½ to 2 feet of backfill should consist of compacted native clayey soil. The perforated pipe should discharge into a sump that pumps to a suitable location or preferably to daylight. Damp-proofing of the basement walls should be included in areas where wall dampness and efflorescence would be undesirable. A diagrammatic section illustrating a typical drainage system for the basement is shown on Figure 7.

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Miradrain, Enkadrain or other drainage fabrics approved by our office may be used for wall drainage as an alternative to the gravel drainage system described above. If used, the drainage fabric should extend from a depth of about 1 foot below the top of the wall backfill down to the drain pipe at the base of the wall. A minimum 12-inch wide section of ½-inch to ¾-inch clean crushed rock and filter fabric should be placed around the drainpipe, as recommended previously.

Backfill placed behind the walls should be compacted to at least 90 percent relative compaction using light compaction equipment. If is used for compaction of wall backfill, the walls should be temporarily braced.

Basement retaining walls should be supported using the foundation recommendations presented previously. If site retaining walls are planned, we should be contacted to confirm the appropriate foundation design parameters once the locations and heights of the retaining walls are determined.

DRIVEWAY PAVEMENT For light residential type traffic using asphalt concrete, we recommend the driveway pavement section consist of at least 2.5 inches of asphalt concrete on at least 8 inches of Class 2 aggregate base.

If the driveway will be constructed with concrete (PCC), we recommend the driveway pavement consist of at least 5 inches of PCC on at least 10 inches of Class 2 aggregate base. Un- for the 5-inch-thick driveway pavement should have a 28-day compressive strength of at least 3,500 psi. PCC pavements should be laterally constrained with curbs or shoulders and sufficient control joints should be incorporated in the design and construction to limit and control cracking.

The soil subgrade and aggregate base below the pavement section should be prepared and compacted as recommended below. The use of a moisture cut-off or thickened edge along the edges of the driveway would be desirable in order to reduce water seepage below the edges of the driveway and into the underlying aggregate base and subgrade, which can lead to premature pavement distress. Bolinas Community Land Trust Residential Development Page 24 of 29

EARTHWORK

Clearing and Subgrade Preparation All deleterious materials, such as existing foundations, fill soils, slabs, pavements, utilities to be abandoned, vegetation, root systems, topsoil, etc. should be cleared from areas of the site to be built on or paved. The actual stripping depth should be determined by a member of our staff at the time of construction. Excavations that extend below finish grade should be backfilled with structural fill that is water-conditioned, placed, and compacted as recommended in the section titled “Compaction.”

After the site has been properly cleared, stripped, and excavated to the required grades, exposed soil surfaces in areas to receive structural fill or slabs-on-grade should be scarified to a depth of 6 inches, moisture conditioned, and compacted as recommended for structural fill in the section titled "Compaction."

On-site soils, foundation excavations, slab subgrades, and utility should be kept in a moist condition throughout the construction period to mitigate the potential effects of the expansive on-site soils.

Large fills are generally not desirable on a hillside site like this. However, if fills are to be constructed on natural slopes (not retained by retaining walls) having an inclination steeper than 6 horizontal to 1 vertical, the fill should be benched, and a key excavated into the competent native soils, and subdrains installed if required by our field representative. If significant fills are required, we should be contacted to evaluation their feasibility and to provide benching criteria as necessary.

Engineered Fill Pad Beneath Shallow Foundations As discussed above, if a mat foundation or spread footings with a conventional slab-on- grade are selected for building support, we recommend that the upper 6 feet of soils below the finished floor elevation within the front half of the footprint be over-excavated and re-compacted as an engineered fill pad. Abrupt or vertical limits of the over- excavation should be avoided. Rather, the excavation should be benched to gradually transition and to allow for compaction on level benches, where possible. We understand this may not be entirely feasible where the proposed structure will extend to the property lines. The depth of the over excavation should be confirmed int eh filed by our field representative. Bolinas Community Land Trust Residential Development Page 25 of 29

The resulting excavation should be backfilled with -mixed and properly prepared on- site soils, imported non-expansive fill such as quarry fines or Class 2 aggregate base placed in lifts no thicker than 8-inches and compacted as recommended below. Imported backfill materials should be approved by a member of our staff prior to delivery to the site. The backfill should be moisture conditioned, and compacted as recommended in the section of this report titled "Compaction." A member of our staff should observe and test during re-working of the building pad.

Basement Excavation Support

Based on the assumed finished floor elevation of the basement, temporary excavations up to approximately 10 to 12 feet deep (depending on the finished floor elevation and foundation depth) will be required in order to construct the basement. If laying back the excavation is not possible, the walls of the basement excavation may be supported by several methods including tiebacks, soldier beams and lagging, soil nails, braced shoring or potentially other methods. The choice should be left to the contractor’s judgment since economic considerations and/or the individual contractor’s construction experience may determine which method is more economical and/or appropriate. Support of any existing structures and improvements should also be the contractor's responsibility. We recommend that the contractor forward his plan for the support system to the structural engineer and geotechnical engineer for preconstruction review. In addition, it should be the contractor’s responsibility to undertake a preconstruction survey with benchmarks and photographs of the adjacent properties.

Material for Fill

All on-site soil containing less than 3 percent organic material by weight (ASTM D2974) should be suitable for use as structural fill. Structural fill should not contain rocks or pieces larger than 6 inches in greatest dimension and no more than 15 percent larger than 2.5 inches. Imported non-expansive fill should have a Plasticity Index no greater than 15, should be predominately granular, and should have sufficient binder so as not to slough or cave into foundation excavations and utility trenches. Recycled aggregate base should not be used for non-expansive fill at building interior. A member of our staff should approve proposed import materials prior to their delivery to the site.

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Compaction

Scarified soil surfaces and all structural fill should be compacted in uniform lifts no thicker than 8-inches in pre-compacted thickness, conditioned to the appropriate moisture content, and compacted as recommended for structural fill in Table 3 below. The relative compaction and moisture content recommended in Table 3 is relative to ASTM Test D1557, latest edition.

Table 3. Compaction Recommendations Bolinas Community Land Trust Residential Development Bolinas, California

General Relative Compaction* Moisture Content*

• Scarified subgrade in areas 90 percent Above optimum to receive structural fill.

• Structural fill composed 90 percent Above optimum of native soil.

• Structural fill composed 90 percent Above optimum of non-expansive fill.

• Structural fill below a 93 percent Above optimum depth of 4 feet.

Pavement Areas • Upper 6-inches of soil 95 percent Above optimum below baserock.

• Aggregate baserock. 95 percent Near optimum

Utility Trench Backfill • On-site soil. 90 percent Above optimum

• Imported sand 95 percent Near optimum * Relative to ASTM Test D1557, latest edition.

At the start of site grading and earthwork construction, and prior to subgrade preparation and placement of non-expansive fill, representative samples of on-site soil and import material will need to be collected in order for a laboratory compaction test to be performed for use during on-site density testing. Sampling of on-site soil and proposed import material should be requested by the contractor at least 5 days prior to when our staff will be needed for density testing to allow time for soil sampling and laboratory testing to be performed prior to our on-site compaction testing.

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Temporary Slopes and Excavations

Due to the height cuts required for the crawlspace/basement excavations, temporary shoring or bracing to support these excavations will likely be necessary during construction. The contractor should be responsible for the design and construction of all temporary slopes and any required shoring. Shoring and bracing should be provided in accordance with all applicable local, state and federal safety regulations, including the current OSHA excavation and trench safety standards.

We note that portions of the fill and upper soils encountered in our borings within the depth of the basement excavation were judged to have limited cohesion and may be prone to sloughing and/or caving if excavated near-vertical. This information should be considered by the contractor when establishing temporary shoring/cut slope criteria for the basement excavation and other temporary slopes and cuts.

Because of the potential for variation of the on-site soils, field modification of temporary cut slopes may be required. Unstable materials encountered on slopes during and after excavation should be trimmed off even if this requires cutting the slopes back to a flatter inclination.

Protection of structures and slopes near cuts should also be the responsibility of the contractor. In our experience, a preconstruction survey is generally performed to document existing conditions prior to construction, with intermittent monitoring of the structures during construction. The contractor should be responsible for staging the required cuts and wall construction and the design of temporary cut slopes and/or required shoring and bracing of any adjacent structures.

Finished Slopes

Finished slopes should be cut or filled to an inclination no steeper than about 2.5:1 (horizontal:vertical). Exposed slopes may be subject to minor sloughing and erosion that may require periodic maintenance. We recommend that all slopes and soil surfaces disturbed during construction be planted with erosion-resistant vegetation.

Surface Drainage

Finished grades should be designed to prevent ponding of water and to direct surface water runoff away from foundations, and edges of slabs and pavements, and toward suitable collection and discharge facilities. Slopes of at least 2 percent are recommended for flatwork and pavement areas with 5 percent preferred in landscape areas within 8 feet

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of the structures, where possible. At a minimum, splash blocks should be provided at the discharge ends of roof downspouts to carry water away from perimeter foundations. Preferably, roof downspout water should be collected in a closed pipe system that is routed to a storm drain system or other suitable discharge outlet.

Drainage facilities should be observed to verify that they are adequate and that no adjustments need to be made, especially during first two years following construction. We recommend that an as-built plan showing the locations of surface and subsurface drain lines and clean-outs be developed. Drainage facilities should be periodically checked to verify that they are continuing to function properly. The drainage facilities will probably need to be periodically cleaned of silt/debris that may build up in the lines.

FUTURE SERVICES

Plan Review

Romig Engineers should review the completed grading and foundation plans for conformance with the recommendations contained in this report. We should be provided with these plans as soon as possible upon completion in order to limit the potential for delays in the permitting process that might otherwise be attributed to our review process. In addition, it should be noted that many of the local building and planning departments now require “clean” geotechnical plan review letters prior to acceptance of plans for their final review. Since our plan reviews typically result in recommendations for modification of the plans, our generation of a “clean” review letter often requires two iterations.

At a minimum, we recommend that the following note be added to the plans:

“Earthwork, slab and pavement subgrade and non-expansive fill preparation, foundation construction, pier drilling, basement subgrade preparation and subslab drainage, retaining wall drainage and backfilling, utility trench backfilling, and site drainage should be performed in accordance with the geotechnical report prepared by Romig Engineers, Inc., dated March 12, 2020. Romig Engineers should be notified at least 48 hours in advance of any earthwork or foundation construction and should observe and test during earthwork and foundation construction as recommended in the geotechnical report. Romig Engineers should be notified at least 5 days prior to earthwork, trench backfill and subgrade preparation work to allow time for sampling of on-site soil and laboratory compaction curve testing to be performed prior to on-site compaction density testing.”

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Construction Observation and Testing

All earthwork and foundation construction should be observed and tested by us to 1) establish that subsurface conditions are compatible with those used in the analysis and design; 2) observe compliance with the design concepts, specifications and recommendations; and 3) allow design changes in the event that subsurface conditions differ from those anticipated. The recommendations in this report are based on a limited number of borings. The nature and extent of variation across the site may not become evident until construction. If variations are exposed during construction, it will be necessary to reevaluate our recommendations.

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REFERENCES

Aerial Photographs

United States Geologic Survey Library, Menlo Park, California (USGS); WAC Corporation, Eugene, Oregon (WAC); and Pacific Aerial Surveys, Novato, California (PAS): black and white vertical stereo pairs, except as noted.

Source Imagery Date Scale

PAS AV-1013-09-16/17 1/4/72 1:24,000 PAS MRN-AV-3766-03-33/34 4/9/90 1:12,000

Unpublished Plans

Krohn, Paul, P.E, 2014, Topographic Survey May 2014, Site Improvement, Wharf Road, Bolinas, CA, APN 193-061-03, map dated May 20, 2014, Scale 1” = 20’.

Publications and Reports

Aagaard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris, R.A., Michael, A.J., Schwartz, D.P., and DiLeo, J.S., 2016, Earthquake outlook for the San Francisco Bay region 2014–2043 (ver. 1.1, August 2016): U.S. Geological Survey Fact Sheet 2016– 3020, 6 p., http://dx.doi.org/10.3133/fs20163020.

Al Atik, L., and Sitar, N., 2010, Seismic Earth Pressures on Cantilever Retaining Structures, Journal of Geotechnical and Geoenvironmental Engineering, ASCE Vol. 136, No. 10.

American Society of Civil Engineers, 2016, Minimum Design Loads for Buildings and Other Structures, ASCE Standard 7-16.

Blake, M.C. Jr, Bartow, J.A, Frizzell, V.A, Jr, Schlocker, J, Sorg, D, Wentworth, C.M, and Wright, R.H, 1974, Preliminary Geologic Map of Marin and San Francisco Counties and Parts of Alameda, Contra Costa and Sonoma Counties, California, USGS Miscellaneous Field Studies Map MF-574, HUD Series BDC 64, Scale 1:62,500.

Blake, M.C, Graymer, R.W, and Jones, D.L, 2000, Geologic Map and Map Database of Parts of Marin, San Francisco, Alameda, Contra Costa and Sonoma Counties, California, USGS Miscellaneous Field Studies Map MF-2337, Version 1.0.

California Building Standards Commission, and International Code Council, 2019 California Building Code, California Code of Regulations, Title 24, Part 2.

California Department of Conservation, Division of Mines and Geology (DMG), 1994, Fault-Rupture Hazard Zones in California, Special Publication 42.

California Division of Mines and Geology, (CDMG), 1974, State of California Special Studies Zones Maps, Bolinas Quadrangle, Official Map, Effective July 1, 1974, Scale 1:24,000.

California Emergency Management Agency, California Geological Survey, University of Southern California, 2009, Tsunami Inundation Map for Emergency Planning, State of California – County of Marin, Bolinas Quadrangle, July 1, 2009. California Geological Survey, 2002, Guidelines for Evaluating the Hazards of Surface Fault Rupture, Note 49.

Clark, Joseph C, and Brabb, Earl E, 1997, Geology of Point Reyes National Seashore and Vicinity, California: A Digital Database, USGS OFR-97-456, Scale 1:48,000.

Galloway, Alan J, 1977, Geology of the Point Reyes Peninsula, Marin County, California, CDMG Bulletin 202.

Gluskoter, Harold J, 1969, Geology of a Portion of Marin County, California, CDMG Map Sheet 11, Scale 1:48,000.

Lawson, A. C., 1908, California Earthquake of April 18, 1906, Report of The State Earthquake Investigation Commission, Carnegie Institution of Washington, Publication 87, Vol. 1 and 2.

Marin County Sheriff’s Office of Emergency Services, 2018, Tsunami Annex, Marin Operational Area Emergency Operations Plan, January 2018.

Wentworth, Carl M, and Frizzell, Virgil A, 1975, Reconnaissance Landslide Map of Parts of Marin and Sonoma Counties, California, USGS OFM 75-281, Scale 1:24,000.

Wentworth, Carl M, Graham, Scott E, Pike, Richard J, Beukelman, Gregg S, Ramsey, David W, and Barron, Andrew D, 1997, Summary Distribution of Slides and Earth Flows in Marin County, California, plot derived from USGS OFR 97-745C, Scale 1:125,000.

Witter, Robert C., Knudsen, Keith L, Sowers, Janet M, Wentworth, Carl M, Koehler, Richard D, Randolph, Carolyn E, 2006, Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California, Scale 1:200,000.

United States Geological Survey, 2018, United States Seismic Design Maps, http://earthquake.usgs.gov/designmaps/us/application.php

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SITE

Scale: 1 inch = 2000 feet Base is United States Geological Survey Bolinas 7.5 Minute Quadrangle, dated 1993. VICINITY MAP FIGURE 1 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

LEGEND

Approximate Location EB-2 of Exploratory Borings Af Artificial fill Proposed Limts of Retainign Wall Supporting Qtm Merced Formation the Subterranean Parking EB-2 Cut slope Incised drainage o X Bedrock Outcrop Qtm o Spring

X

Qtm X

o EB-2 EB-1 Af EB-1

Approximate Scale: 1 inch = 50 feet. Base is site plan prepared by Paul Krohn, P.E., dated May, 2016.

SITE PLAN FIGURE 2 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

Kfs

SITE

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LEGEND Beach sand (Holocene) Geologic Contact - dashed where approximate, dotted where inferred. Alluvium (Holocene) Fault - dashed where approximate, Marine and stream deposits (Pleistocene) dotted where inferred. Merced Formation (upper Pliocene and Pleistocene) Strike and dip of bedding Santa Cruz Mudstone (upper Miocene) Franciscan Complex Sandstone and Shale (Jurrasic and Cretaceous) Scale: 1 inch = 2000 feet Base is Geology of the Point Reyes National Seashore. (Clark and Brabb, 1997) VICINITY GEOLOGIC MAP FIGURE 3 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

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SITE

Scale: 1 inch = 2000 feet Base is State of California Special Studies Zones Bolinas Quadrangle Map, 1974. ALQUIST-PRIOLO SPECIAL STUDIES ZONE MAP FIGURE 4 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

SITE

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Landslide deposit

Block slide Scarp of uncertain origin Terrace deposit Scale: 1 inch = 500 feet Base is Reconnaissance Landslide Map of Parts of Marin and Sonoma Counties, California (Wentworth and Frizzell, 1975). NEARBY LANDSLIDES MAP FIGURE 5 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

SITE

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0 3 6 12 miles

Magnitude Year

Earthquakes with M5+ from 1900 to 1980, M2.5+ from 1980 to January 2015. Faults with activity in last 15,000 years. Based on data sources from Northern California Earthquake Data Center and USGS Quaternary Fault and Fold Database, accessed May 2015.

REGIONAL FAULT AND SEISMICITY MAP FIGURE 6 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

Basement Retaining Wall 18" - 24" Clayey Soil Cap

Water Proofing Membrane as Determined by Water Proofing Consultant

1/2" - 3/4" Clean Crushed Rock Filter Fabric

Filter Fabric Basement Slab/Mat

4" Minimum 8" Minimum 8" Minimum 4" Perforated Pipe, sloped @ 1% to Sump Pump 12" Minimum About 30 Feet Typical

SUBSLAB DRAINAGE DETAIL FIGURE 7 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

APPENDIX A

FIELD INVESTIGATION

Our representative logged the soils encountered during drilling and samples were obtained at depths appropriate to the investigation. The samples were taken to our laboratory where they were examined and classified in accordance with the Unified System. The logs of our borings, as well as a summary of the soil classification system (Figure A-1), are attached.

Several tests were performed in the field during drilling. The standard penetration test resistance was determined by dropping a 140-pound hammer through a 30-inch free fall and recording the blows required to drive the 2-inch (outside diameter) sampler 18 inches. The standard penetration test (SPT) resistance is the number of blows required to drive the sampler the last 12 inches and is recorded on the boring logs at the appropriate depths. Soil samples were also collected using a 2.5-inch and a 3.0-inch O.D. drive sampler. The blow counts shown on the logs for these samplers do not represent SPT values and have not been corrected in any way.

The locations and relative elevations of the borings were established by pacing using the topographic survey prepared by Paul Krohn, P.E., dated May 30, 2014. The locations and elevations of the borings should be considered accurate only to the degree implied by the method used.

The boring logs and related information depict our interpretation of subsurface conditions only at the specific location and time indicated. Subsurface conditions and ground water levels at other locations may differ from conditions at the location where sampling was conducted. The passage of time may also result in changes in the subsurface conditions.

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USCS SOIL CLASSIFICATION SOIL PRIMARY DIVISIONS SECONDARY DIVISIONS TYPE CLEAN GRAVEL GW Well graded gravel, gravel-sand mixtures, little or no fines. COARSE GRAVEL (< 5% Fines) GP Poorly graded gravel or gravel-sand mixtures, little or no fines. GRAINED GRAVEL with GM Silty gravels, gravel-sand-silt mixtures, non-plastic fines. SOILS FINES GC Clayey gravels, gravel-sand-clay mixtures, plastic fines. (< 50 % Fines) CLEAN SAND SW Well graded sands, gravelly sands, little or no fines. SAND (< 5% Fines) SP Poorly graded sands or gravelly sands, little or no fines. SAND SM Silty sands, sand-silt mixtures, non-plastic fines. WITH FINES SC Clayey sands, sand-clay mixtures, plastic fines. ML Inorganic silts and very fine sands, with slight plasticity. FINE SILT AND CLAY CL Inorganic clays of low to medium plasticity, lean clays. GRAINED Liquid limit < 50% OL Organic silts and organic clays of low plasticity. SOILS MH Inorganic silt, micaceous or diatomaceous fine sandy or silty soil. (> 50 % Fines) SILT AND CLAY CH Inorganic clays of high plasticity, fat clays. Liquid limit > 50% OH Organic clays of medium to high plasticity, organic silts. HIGHLY ORGANIC SOILS Pt and other highly organic soils. BEDROCK BR Weathered bedrock. RELATIVE DENSITY CONSISTENCY SAND & GRAVEL BLOWS/FOOT* SILT & CLAY STRENGTH^ BLOWS/FOOT* VERY LOOSE 0 to 4 VERY SOFT 0 to 0.25 0 to 2 LOOSE 4 to 10 SOFT 0.25 to 0.5 2 to 4 MEDIUM DENSE 10 to 30 FIRM 0.5 to 1 4 to 8 DENSE 30 to 50 STIFF 1 to 2 8 to 16 VERY DENSE OVER 50 VERY STIFF 2 to 4 16 to 32 HARD OVER 4 OVER 32

GRAIN SIZES BOULDERS COBBLES GRAVEL SAND SILT & CLAY COARSE FINE COARSE MEDIUM FINE 12 " 3" 0.75" 4 10 40 200 SIEVE OPENINGS U.S. STANDARD SERIES SIEVE

Classification is based on the Unified Soil Classification System; fines refer to soil passing a No. 200 sieve. * Standard Penetration Test (SPT) resistance, using a 140 pound hammer falling 30 inches on a 2 inch O.D. split spoon sampler; blow counts not corrected for larger diameter samplers. ^ Unconfined Compressive strength in tons/sq. ft. as estimated by SPT resistance, field and laboratory tests, and/or visual observation. KEY TO SAMPLERS z Modified California Sampler (3-inch O.D.) y Mid-size Sampler (2.5-inch O.D.) x Standard Penetration Test Sampler (2-inch O.D.)

KEY TO EXPLORATORY BORING LOGS FIGURE A-1 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

WEATHERING

Fresh Moderately Severe Rock fresh, crystals bright, few joints may show All rock except quartz discolored or stained. In granitoid rocks, slight staining. Rock rings under hammer if crystalline. all feldspars dull and discolored and majority show kaolinization. Rock shows severe loss of strength and can be excavated with geologist's pick. Rock goes "clunk" when struck. Very Slight Rock generally fresh, joints stained, some joints may Severe show thin clay coatings, crystals in broken face All rock except quartz discolored or stained. Rock "fabric" clear show bright. Rock rings under hammer if crystalline. and evident, but reduced in strength to strong soil. In granitoid rocks, all feldspars kaolinized to some extent. Some fragments of Slight strong rock usually left. Rock generally fresh, joints stained, and discoloration extends into rock up to 1 inch. Joints may contain clay. Very Severe In granitoid rocks some occasional feldspar crystals are All rock except quartz discolored and stained. Rock "fabric" dull and discolored. Crystalline rocks ring under hammer. discernible, but mass effectively reduced to "soil" with only fragments of strong rock remaining. Moderate Significant portions of rock show discoloration and Complete weathering effects. In granitoid rocks, most feldspars Rock reduced to "soil". Rock fabric not discernible or discernible are dull and discolored; some are clayey. Rock has dull only in small scattered locations. Quartz may be present as dikes sound under hammer and shows significant loss of or stringers. strength as compared with fresh rock.

HARDNESS Very hard Medium Cannot be scratched with knife or sharp pick. Hand Can be grooved or gouged 1/16 inch deep by firm pressure on knife specimens requires several hard blows of geologist's. or pick point. Can be excavated in small chips to pieces about 1 inch maximum size by hard blows of the point of a geologist's pick. Hard Can be scratched with knife or pick only with difficulty. Soft Hard blow of hammer required to detach hand Can be gouged or grooved readily with knife or pick point. Can be specimen. excavated in chips to pieces several inches in size by moderate blows of a pick point. Small thin pieces can be brocken by finger pressure. Moderately Hard Can be scratched with knife or pick. Gouges or grooves Very Soft to 1/4 inch deep can be excavated by hard blow of point Can be carved with knife. Can be excavated readily with point of of a geologist's pick. Hard specimen can be detached pick. Pieces 1 inch or more in thickness can be broken with finger by moderate blow. pressure. Can be scratched readily by fingernail. JOINT BEDDING AND FOLIATION SPACING ROCK QUALITY DESIGNATOR (RQD) Spacing Joints Bedding and Foliation RQD, as a percentage Descriptor Less than 2 in. Very Close Very Thin Exceeding 90 Excellent 2 in. to 1 ft. Close Thin 90 to 75 Good 1 ft. to 3 ft. Moderately Close Medium 75 to 50 Fair 3 ft. to 10 ft. Wide Thick 50 to 25 Poor More than 10 ft. Very Wide Very Thick Less than 25 Very Poor KEY TO BEDROCK DESCRIPTIONS FIGURE A-2 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT MARCH 2020 BOLINAS, CALIFORNIA PROJECT NO. 5028-1

DRILL TYPE: Mobile Drill B-24 with 3-1/3' Solid Flight Auger LOGGED BY: AS DEPTH TO GROUND WATER: 14 feet SURFACE ELEVATION: 15 feet DATE DRILLED: 12/16/19

(Figure A-2)

CLASSIFICATION AND DESCRIPTION Q

SOIL TYPE SOIL SYMBOL DEPTH (FEET) (%) CONTENT WATER SAMPLE INTERVAL DENSITY or ROCK UNCONFIN. COMP. (TSF)* UNCONFIN. SOIL CONSISTENCY/

(TSF)* STRENGTH SHEAR PEN. RESISTANCE (Blows/ft) RESISTANCE PEN. HARDNESS Fill: Brown to dark brown, Sandy Silt, moist to very moist, fine Firm ML 0 to coarse grained sand, fine to medium angular to rounded to gravel, low plasticity, organic debris, small rock fragments, Very z glass, shells, brick fragments. Stiff z z 24 18 3.0 Slope Wash/Debris: Dark brown, Sandy Silt, very moist, fine Stiff ML grained sand, trace sub-angular to rounded gravel, low plasticity, z some small rock fragments. z 5 z 6 22 0.5

Increase in moisture. z l 69% Passing No. 200 Sieve. l z 9 23 1.0

z z 10 z 15 23 1.3

z z z 13 23

t Ground water measured at 14 feet after drilling. t z Transitions to residual soil derived from weathered bedrock. 15 z 14 26 1.8

Note: The stratification lines represent the approximate z boundary between soil and rock types, the actual z transition may be gradual. z 14 25 2.5 *Measured using Torvane and Pocket Penetrometer devices. 20 Continued on Next Page

EXPLORATORY BORING LOG EB-1 BORING EB-1 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT PAGE 1 OF 2 BOLINAS, CALIFORNIA MARCH 2020 PROJECT NO. 5028-1

DRILL TYPE: Mobile Drill B-24 with 3-1/3' Solid Flight Auger LOGGED BY: AS DEPTH TO GROUND WATER: 14 feet SURFACE ELEVATION: 15 feet DATE DRILLED: 12/16/19

(Figure A-2)

CLASSIFICATION AND DESCRIPTION Q

SOIL TYPE SOIL SYMBOL DEPTH (FEET) WATER CONTENT (%) CONTENT WATER SAMPLE INTERVAL DENSITY or ROCK UNCONFIN. COMP. (TSF)* UNCONFIN. SOIL CONSISTENCY/

(TSF)* STRENGTH SHEAR PEN. RESISTANCE (Blows/ft) RESISTANCE PEN. HARDNESS Merced Formation: Blue-gray, Siltstone, very moist, fine Very BR 20 grained sand, very severely weathered, friable. Soft z z z 24 24 3.0

25 Some roots/organics observed. z z z 26 26 3.8

30 z z z 46 25 4.3

35 x x x 33 24 Bottom of Boring at 36.5 feet. Note: The stratification lines represent the approximate boundary between soil and rock types, the actual transition may be gradual. 40 *Measured using Torvane and Pocket Penetrometer devices.

EXPLORATORY BORING LOG EB-1 BORING EB-1 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT PAGE 2 OF 2 BOLINAS, CALIFORNIA MARCH 2020 PROJECT NO. 5028-1

DRILL TYPE: Mobile Drill B-24 with 3-1/3' Solid Flight Auger LOGGED BY: AS DEPTH TO GROUND WATER: Not Encountered SURFACE ELEVATION: 27 feet DATE DRILLED: 12/16/19

(Figure A-2)

CLASSIFICATION AND DESCRIPTION Q

SOIL TYPE SOIL SYMBOL DEPTH (FEET) WATER CONTENT (%) CONTENT WATER SAMPLE INTERVAL DENSITY or ROCK UNCONFIN. COMP. (TSF)* UNCONFIN. SOIL CONSISTENCY/

(TSF)* STRENGTH SHEAR PEN. RESISTANCE (Blows/ft) RESISTANCE PEN. HARDNESS Fill: Brown, Sandy Silt, moist, fine to coarse grained sand, low Stiff ML 0 z plasticity, brick debris. to z Very z 8 18 Stiff

z z z 19 13 Slope Wash/Debris: Brown, Sandy Silt, moist, fine grained Very ML 5 sand, low plasticity, some roots, small rock fragments, pinholes Stiff z observed. z z 19 13 >4.5

z z z 19 21 2.5 Increase in moisture. z 10 z Note: The stratification lines represent the approximate z 20 22 1.5 boundary between soil and rock types, the actual transition may be gradual.

*Measured using Torvane and Pocket Penetrometer devices.

Transitions to residual soil derived from weathered bedrock. z z 15 z 17 24 1.0

Merced Formation: Orange-brown, Siltstone, moist, very Very BR severely weathered, friable. Soft

z z 20 z 50/6" 24 Continued on Next Page

EXPLORATORY BORING LOG EB-2 BORING EB-2 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT PAGE 1 OF 2 BOLINAS, CALIFORNIA MARCH 2020 PROJECT NO. 5028-1

DRILL TYPE: Mobile Drill B-24 with 3-1/3' Solid Flight Auger LOGGED BY: AS DEPTH TO GROUND WATER: Not Encountered SURFACE ELEVATION: 27 feet DATE DRILLED: 12/16/19 (Figure A-2)

CLASSIFICATION AND DESCRIPTION Q SOIL TYPE SOIL SYMBOL DEPTH (FEET) WATER CONTENT (%) CONTENT WATER SAMPLE INTERVAL DENSITY or ROCK UNCONFIN. COMP. (TSF)* UNCONFIN. SOIL CONSISTENCY/ (TSF)* STRENGTH SHEAR PEN. RESISTANCE (Blows/ft) RESISTANCE PEN. HARDNESS Merced Formation: Orange-brown, Siltstone, moist, very severely Very BR 20 weathered, friable. Soft

x x 25 x 68 24

Bottom of Boring at 25 feet.

30

35

Note: The stratification lines represent the approximate boundary between soil and rock types, the actual transition may be gradual.

*Measured using Torvane and Pocket Penetrometer devices. 40

EXPLORATORY BORING LOG EB-2 BORING EB-2 BOLINAS LAND TRUST RESIDENTIAL DEVELOPMENT PAGE 2 OF 2 BOLINAS, CALIFORNIA MARCH 2020 PROJECT NO. 5028-1

APPENDIX B

LABORATORY TESTS

Samples from subsurface exploration were selected for tests to help evaluate the physical and engineering properties of the soils encountered at the site. The tests that were performed are briefly described below.

The natural moisture content was determined in accordance with ASTM D2216 on nearly all of the soil samples recovered from the borings. This test determines the moisture content, representative of field conditions at the time the samples were collected. The results are presented on the boring logs at the appropriate sample depths.

The amount of silt and clay-sized material present was determined on one sample of soil in accordance with ASTM D422. The result of this test is presented on the boring log at the appropriate sample depth.

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ROMIG ENGINEERS, INC. 1390 El Camino Real, 2nd Floor San Carlos, California 94070 Phone: (650) 591-5224 www.romigengineers.com