GEOTECHNICAL INVESTIGATION
FORPROPOSED NEW RESIDENCE
at
Between 131 and 151 Olympian Way
Pacifica, California
Report Prepared for:
Roundhouse Industries
Report Prepared by:
GeoForensics, Inc.
January 2021
GEOFORENSICS INC. Consulting Soil Engineering
303 Vintage Park Drive, Suite 220, Foster City, CA 94404 Phone: (650) 349-3369 Fax: (650) 571-1878 File: 220285
January 27, 2021
Roundhouse Industries
900 Rosita Road
Pacifica, CA 94044
Attention: Mike O'Connell
Subject: Olympian Way Lot
Beteen 131 and 151 Olympian Way
Pacifica, California
GEOTECHNICAL INVESTIGATION
FOR PROPOSED NEW RESIDENCE
Dear Mr. O'Connell:
In accordance with your authorization, we have performed a subsurface investigation into the geotechnical conditions present at the subject property. This report summarizes the conditions we measured and observed, and presents our opinions and recommendations for the design and construction of the proposed new residence.
Site Description
The subject site is a moderately to steeply sloping, irregularly-shaped parcel located on the east side of Olympian Way (at the approximate location shown on Figure 1). For purposes of description in this report, it is assumed that the property faces west. The property is bounded by other developed single family residential lots to the sides and rear, and Olympian Way to the west. The site is currently vacant of man-made improvements.
The ground surface in the site vicinity has an overall slope down towards the east and south (as shown on Figure 2). At the site, the ground slopes gently to moderately steeply down towards the east.
Surface gradients are approximately 3:1 (horizontal:vertical, H:V). At the property, it appears that little or no grading work was performed on the site in the past.
The grounds in the property have been covered by a variety of small to medium sized bushes and shrubs, and numerous small to large trees.
Proposed Construction
We understand that the current development for the site proposes construction of a new single-family residence, and associated improvements.
The house is to be of conventional, wood-framed construction. New foundation loads are expected to be typical for this type of structure (i.e. light).
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January 27, 2021
Excavation work at the site is expected to be limited to crawlspace and foundation excavations. No significant fill placement is anticipated as part of this work. No basement is planned for the residence.
INVESTIGATION
Scope and Purpose
The purpose of our investigation was to determine the nature of the subsurface soil conditions so that we could provide geotechnical recommendations for the construction of the proposed new residence and associated improvements. In order to achieve this purpose, we have performed the following scope of work:
1- visited the property to observe the geotechnical setting of the area to be developed; 2- reviewed relevant published geotechnical maps;
3- drilled two borings near the location of the proposed improvements; 4- assessed the collected information and prepared this report.
The findings of these work items are discussed in the following sections of this report.
Geologic Map Review
We reviewed the Geologic Map of the Montara Mountain and San Mateo County 7.5' quadrangles, san Mateo County, California: by Pampeyan, E.H., (1983) Map I-2390, and the State of California Seismic Hazards Zone Map; Montara Mountain Quadrangle (4/4/19). The relevant portion of the Pampeyan and State Seismic Hazard maps have been reproduced in Figure 3 ad 3a.
The Pampeyan map indicates that the site is underlain by Slope Wash, Revine fill, and Collovium (map symbol "Qsr"). Pampeyan describes these materials as consisting of "Unconsolidated to moderately consolidated deposits of sand, silt, clay, and rock fragments accumulated by slow downslope movement of weathered rock debris and soil. Composition dependent upon underlying rocks. Commonly unsorted and unbedded, but locally layered by downslope movements. Mapped where thickness presumed to exceed 5 ft; as thick as 20 ft on north side of San Pedro Valley." Our subsurface exploration (see below) encountered hard sandy clay and very dense clayey sand with gravel (Sandstone)which we judged to be consistent with the mapping.
The Seismic Hazards Zone Map indicates the site is outside of the areas where there have been previous occurrences of liquefaction; however, the site is mapped within an area where there has been a historic occurrence of landslide movement and where local topographic, local geological, geotechnical, and groundwater conditions would indicate a potential for permanent ground displacement such that mitigation, as defined in Public Resource Code Section 2693(c), would be required.
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The active San Andreas Fault is mapped approximately 4.1 miles (6.6 km) northeast of the site, and San Gregorio Fault is mapped approximately 2.2 miles (3.5 km) to the southwest.
Subsurface Exploration
On January 7, 2021 we drilled two borings at the site at the location shown on Figure 4. The borings were drilled using a Minute Man portable drilling rig equipped with 3.25 inch diameter helical flight augers. Logs of the soils encountered during drilling record our observations of the cuttings traveling up the augers and of relatively undisturbed samples collected from the base of the advancing hole.
The final boring logs are based upon the field log with occasional modifications made upon further laboratory examinations of the recovered samples and laboratory test results. The final logs are attached in Appendix A.
The relatively undisturbed samples were obtained by driving a 3.0 inch (outer diameter) Modified California Sampler and a Standard Penetration Sampler (as noted on logs) into the base of the advancing hole by repeated blows from a 70 pound hammer lifted 30 inches. On the logs, the number of blows required to drive the sampler the final 12 inches of the 18 inch drive, have been recorded as the Blow Counts. These blows have not been adjusted to reflect equivalent blows of any other type of sampler, or to account for the different samplers used.
Subsurface Conditions
Both borings generally encountered 3 to 8 feet of olive to yellowish brown, damp, hard sandy fat clay soils over sandstone bedrock. Boring 1 encountered refusal at 6 feet and 9 inches deep, and Boring 2 was terminated at 9 feet and 4 inches.
Please refer to Appendix A for a more detailed description of the borings.
No free groundwater was encountered during the drilling of the holes. However, during periods of heavy rain or late in the winter, groundwater seepage may exist at shallower depths through fractures within the sandstone bedrock or atop of the bedrock.
Laboratory Testing
The relatively undisturbed samples collected during the drilling process were returned to the laboratory for testing of engineering properties. In the lab, selected soil samples were tested for moisture content, density, plasticity, and strength. The results of the laboratory tests are attached to this report in Appendix B.
Plasticity Index (PI) testing performed on the site near surface clayey materials produced a PI result of 26. This testing indicated that the near surface materials have a moderate to high plasticity and are expected to be of moderate to high expansion potential. 3 File: 220285
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A staged Consolidated Undrained Triaxial Compression Test was conducted on one sample (Sample 2-1 @ 4 feet). Drawing a tangent line to the Mohr's circles showed that these materials have moderate to high strength parameters (cohesion = 100 psf, friction angle = 25.9 degrees).
Slope Stability Analysis
We have performed our preliminary analysis using the procedures identified in the "Development of a Screen Analysis Procedure for Seismic Slope Stability" presented as Appendix A in the publication Recommended Procedures for Implementation of DMG Special Publication 117A Guidelines for Analyzing and Mitigating Landslide Hazards in California by the Southern California Earthquake Center (Seismic Guidelines Report SGR-117A).
In applying the recommended procedures in SGR-117A, we have also considered design values provided in Seismic Hazard Report 128 - Seismic Hazard Zone Report for the Montara Mountain 7.5 - Minute Quadrangle, Santa Clara County California, by the Department of Conservation, California Geological Survey (Seismic Hazard Report).
The anticipated maximum peak ground acceleration (10% probability of exceedance in 50 years) for the site is 0.64g (plate 2-3 of Report 128) from a magnitude 7.9 Mw earthquake. Using USGS Unified Hazard Tool Application, the design earthquake (with a 10 percent chance of exceedance in the next 50 years) will induce an acceleration of 0.59g due to ground shaking. Our slope stability evaluation used an acceleration of 0.64g. This value should be adjusted to account for the non-linear response of the materials above the slide plane, to a value of 0.31g (SGR-117A Figure 11.1) and assuming a 15cm threshold for minimal damage.
Strength parameters provided in the Seismic Hazard Report for the Montara Mountain Quadrangle indicates a 26 degrees friction angle value should be used for the soils. We also performed one staged consolidated undrained triaxial compression test on a sample achieved at a depth of 4 feet. The laboratory test results show a friction angle of 25.9 degrees with a 100 psf cohesion value for the upper soils. We so expect the strength parameters of deeper soils increased due to over burden pressure. In our slope stability analyses we have used a 26 degrees friction angle and 100 psf cohesion value for the upper soils, while we used a conservative 26 degrees angle of friction and 500 psf cohesive value for the deeper sandstone bedrock.
No free ground water was encountered during drilling the borings. Also, Plate 1.3 of the Montara Mountain Seismic Hazard Report indicates there is no data for historically highest ground water at the site location. We have performed our analysis assuming the water table elevation is 10 feet below the ground surface.
To perform our analysis, we used the computer program GEOSTATE (version 4.30.31) to model the site conditions under seismic shaking. The geometry of the slope was obtained from Google Earth measurement tool.
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Results of Slope Stability Analyses
The results of a slope stability analyses are represented in Figures 5 and 6 as a Factor of Safety against sliding. The factor of safety is simply the sum of the factors which are resisting sliding, divided by the sum of the factors attempting to make the slope fail. If the resistance is greater than the driving forces, the factor of safety is above 1. If there is insufficient resistance available, then the factor of safety is below 1. A factor of safety of 1.0 implies that there is a balance between the driving and resisting forces, and the slope is assumed to be "on the brink" of failure.
Jurisdictions previously required that calculated factor of safety for a site to be developed with a residential structure be at least 1.5 for non-seismic conditions, and typically 1.1 for seismic conditions. At the turn of this century, the methodology for assessing the seismic stability of a slope was substantially changed as the State Division of Mines and Geology issued Special Publication 117a. Among other things, these new guidelines provided a methodology for determining whether the design earthquake is likely to cause substantial movement which may affect a residence. We have used the 15cm threshold for our analysis. Using the procedures, a resulting factor of safety of 1.0 now indicates that movements of the slide mass during the design earthquake will result in approximately 15 cm of movement.
Our calculations found that the analyzed section of the slope has adequate factors of safety for both static as well as seismic conditions. The static and seismic factors of safety are 2.44 and 1.13, respectively.
CONCLUSIONS AND RECOMMENDATIONS
General
Based upon our investigation, we believe that the proposed improvements can be safely constructed.
Geotechnical development of the site is controlled by the presence of highly expansive soils and moderately steep slopes, but aided by the shallow depth to bedrock materials.
Expansive soils derive their name from their propensity to change volume in response to changes in moisture content. When they are dry, they shrink; when they become wet, they swell. The pressures these soils can exert as they expand can be sufficiently high to move conventional residential foundations. The foundation movement induced by the soil shifting can cause wall coverings to crack, doors and windows to stick, and floors to slope. Seasonal movements of expansive soils have caused such distress to countless houses and pools in the Bay Area.
To combat seasonal expansive soil movements, it is necessary to utilize a foundation system which derives its support from the deeper, more stable soils. Typically, a drilled, cast-in-place pier foundation system is used to reach the more stable bedrock. Therefore, we have recommended that such foundation system be utilized at this site for the new residence. This type of foundation system will also eliminate any surcharge loads on the slope from the weight of the building. 5 File: 220285
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Based upon our slope stability analyses, the slope possesses adequate stability. However, as there is always a potential for shallow sloughing of topsoil materials along the downslope, we would recommend that no shallow improvements (e.g. slabs or spread footing supported elements) be located within 15 feet of the crest of any slope steeper than 5:1. Any improvement within this zone shall be supported by drilled piers designed in conformance with recommendations in this report.
The recommendations in this report should be incorporated into the design and construction of the proposed new residence and associated improvements.
Seismicity
The greater San Francisco Bay Area is recognized by Geologists and Seismologists as one of the most active seismic regions in the United States. Several major fault zones pass through the Bay Area in a northwest direction which have produced approximately 12 earthquakes per century strong enough to cause structural damage. The faults causing such earthquakes are part of the San Andreas Fault System, a major rift in the earth's crust that extends for at least 700 miles along western California.
The San Andreas Fault System includes the San Andreas, San Gregorio, Hayward, Calaveras Fault Zones, and other faults.
In 2014, seismologic and geologic experts convened by the U.S. Geological Survey, California Geological Survey, and the Southern California Earthquake Center concluded that there is a 72 percent probability for at least one "large" earthquake of magnitude 6.7 or greater to occur in the Bay Area before the year 2043. The northern portion of the San Andreas fault is estimated to have a 6 percent probability, while the Hayward and Calaveras faults are estimated to have a 14 and 7 percent probability of producing an earthquake of that magnitude or greater during that time period.
Ground Rupture - The lack of mapped active fault traces through the site, suggests that the potential for primary rupture due to fault offset on the property is low.
Ground Shaking - The subject site is likely to be subject to very strong to violent ground shaking during its life span due to a major earthquake in one of the above-listed fault zones. Current (2019) building code design may be followed by the structural engineer to minimize damages due to seismic shaking, using the following input parameters from the Structural Engineers Association of California (SEAOC) Calculator based upon ASCE 7-16 design parameters:
Site Class SDS SDI
2.097 0.799 1.677 0.745
Landsliding - The subject site and the surrounding area are gently to steeply sloping. Fortunately, the site is underlain by competent bedrock material at relatively shallow depths. The Seismic Hazards Zone Map indicates the site is mapped within an area where there is a historic occurrence of landslide movement. Our slope stability analysis showed that the hazard due to large-scale seismically-induced landsliding is, relatively low for the site. However, as with any slope, minor sloughing of the steeper 6 File: 220285
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site slopes could occur during earthquake shaking. The proposed improvements should not be affected by any such sloughing, as they will be supported by the stable bedrock materials at the site.
Liquefaction - The State of California Seismic Hazards Zones map indicates that the site is not in an area potentially subject to liquefaction. Liquefaction most commonly occurs during earthquake shaking in loose fine sands and silty sands associated with a high ground water table. These conditions were demonstrated to be absent down to the site bedrock, confirming the State's mapping.
Therefore, it is also our opinion that liquefaction is unlikely to occur on the subject property.
Ground Subsidence - Ground subsidence may occur when poorly consolidated soils densify as a result of earthquake shaking. Since the proposed building site is underlain at shallow depths by resistant materials, the hazard due to ground subsidence is, in our opinion, considered to be low.
Lateral Spreading - Lateral spreading may occur when a weak layer of material, such as a sensitive or liquefiable soil, loses its shear strength as a result of earthquake shaking. Overlying blocks of competent material may be translated laterally towards a free face. Free face conditions are not present proximate to or at the site, hence, the hazard due to lateral spreading is, in our opinion, considered to be low.
Site Preparation and Grading
Any existing underground utility lines to be abandoned should be removed from within the proposed building envelope and their ends capped outside of the building envelope. Any vegetation and organically contaminated soils should be cleared from the building area. All holes resulting from removal of tree stumps and roots, or other buried objects, should be over-excavated into firm materials and then backfilled and compacted with native materials.
The placement of fills at the site is expected to include: utility trench backfill, slab subgrade materials, finished drainage and landscaping grading. These and all other fills should be placed in conformance with the following guidelines:
Fills may use organic-free soils available at the site or import materials. Import soils should be free of construction debris or other deleterious materials and be non-expansive. A minimum of 3 days prior to the placement of any fill, our office should be supplied with a 30 pound sample (approximately a full 5 gallon bucket) of any soil or baserock to be used as fill (including native and import materials) for testing and approval.
All areas to receive fills should be stripped of organics and loose or soft near-surface soils. Fills should be placed on level benches in lifts no greater than 6 inches thick (loose) and be compacted to at least 90 percent of their Maximum Dry Density (MDD), as determined by ASTM D-1557. In pavement (concrete or asphalt) areas to receive vehicular traffic, all baserock materials should be compacted to at least 95 percent of their MDD. Also, the upper 6 inches of soil subgrade beneath any pavements should be compacted to at least 95 percent of its MDD. 7 File: 220285
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If unretained fills in excess of 3 feet thick are to be placed, our office should be contacted for further recommendations.
Temporary, dry-weather, vertical excavations should remain stable for short periods of time to heights of 3 feet. All excavations should be shored or sloped in accordance with OSHA standards.
Permanent cut and/or fill slopes should be no steeper than 2:1 (H:V). However, even at this gradient, minor sloughing of slopes may still occur in the future. Positive drainage improvements (e.g. drainage swales, catch basins, etc.) should be provided to prevent water from flowing over the tops of cut and/or fill slopes.
Foundation For The New Residence - Piers
Due to the presence of highly expansive site soils, the foundations for the new house will need to penetrate into the deeper, more stable bedrock. Therefore, we recommend a pier and grade beam foundation system be used for new residence.
Piers should penetrate a minimum of 10 feet below lowest adjacent grade, and 5 feet into the bedrock, whichever is deeper. It should be assumed that up to 3 to 8 feet of overburden will exist at the site, so nominal pier depths may range from 10 to 18 feet below lowest adjacent grade.
The piers should have a minimum diameter of 16 inches and be nominally reinforced with a minimum of four #4 bars vertically.
Actual pier depth, diameter, reinforcement, and spacing should be determined by the structural engineer based upon the following design criteria:
A friction value of 700 psf may be assumed to act on that portion of the pier in the bedrock. Lateral support may be assumed to be developed along the length of the pier in the bedrock, using a passive pressure of 450 pcf Equivalent Fluid Weight (EFW), starting at a value of 700 psf. Passive resistance may be assumed to act over 1.5 projected pier diameters. Above the bedrock, no frictional or lateral support may be assumed. These design values may be increased 1/3 for transient loads (i.e. seismic and wind).
Even though piers are designed to derive their vertical resistance through skin friction, the bases of the piers holes should be clean and firm prior to setting steel and pouring concrete. If more than 6 inches of slough exists in the base of the pier holes after drilling, then the slough should be removed.
If less than 6 inches of slough exists, the slough may be tamped to a stiff condition. Piers should not remain open for more than a few days prior to casting concrete. In the event of rain, shallow groundwater, or caving conditions it may be necessary to pour piers immediately. 8 File: 220285
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All perimeter piers, and piers under load-bearing walls, should be connected by concrete grade beams.
Perimeter grade beams should penetrate a minimum of 6 inches below grade (unless a perimeter footing drain is installed to intercept water attempting to enter around the perimeter). Interior grade beams do not need to penetrate below grade. All other isolated floor supports must also be pier supported, however, they do not need to be connected by grade beams.
In order to reduce any expansive soil uplift forces on the base of the grade beams, the beams should have either a uniform 2 inch void between their base and the soil, or should be constructed with a knife edge and triangular shaped void in a rectangular trench. The void can be created by the use of prefabricated cardboard void material (e.g. K-void, SureVoid, Carton-void), half a sonotube faced concave down, or other methods devised by the contractor and approved by our offices. The use of Styrofoam is not acceptable for creating the void.
All improvements connected directly to any pier supported structure, also need to be supported by piers. This includes, but is not limited to: porches, decks, entry stoops and columns, etc. If the designer does not wish to pier support these items, then care must be taken to structurally isolate them (with expansion joints, etc.) from the pier supported structure.
If the above recommendations are followed, total foundation settlements should be less than 1 inch, while differential settlements should be less than ½ inches.
Retaining Walls
New site retaining walls should not be structurally connected to the house. New site walls which are located on, or within 10 feet of the crest of, slopes steeper than 5:1 (H:V), should utilize a pier and grade beam foundation system.
Wall Forces - Any unrestrained retaining walls required for the proposed construction should be designed to resist an active pressure of 55 pcf Equivalent Fluid Weight (EFW) in supporting soils with retained slopes less than 4:1 (H:V). An active pressure of 75 pcf EFW should be utilized for retained slopes with an inclination of 2:1 (H:V). Where retained slopes are greater than 4:1, though less than 2:1, the designer should linearly interpolate between 55 and 75 pcf EFW.
Creep force of 35 pcf (EFW) should be applied over 3 projected pier diameters to a depth of 5 feet.
Where a retaining wall is located within a horizontal distance less than twice the height of the lower retaining wall, the lower retaining wall will need to be designed for an additional surcharge pressure from the upper wall(s). Once the geometry of such walls has been determined, please provide our office with a cross-section so that we can determine the required surcharge.
Any restrained retaining walls required should be designed for the aforementioned active pressures with an additional uniform pressure of 8H psf, where H is the height of the wall in feet. We leave it to the design professional's judgment in determining whether a wall is restrained or not. An additional 9 File: 220285
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uniform force of 10H psf may be applied to account for seismic forces on the wall, although it is our opinion that such forces need not be applied.
All retaining walls should also be designed to resist a point load applied at the midpoint of the wall, equal to ½ the maximum applied surcharge.
Drilled Piers - Any wall which is located on, or within 10 feet of the crest of, slopes steeper than 5:1 (H: V) should utilize a drilled pier foundation system. We note that pier-supported walls may not rely upon a toe footing to resist overturning forces. All vertical and lateral forces should be resisted by piers. This may require the use of a staggered, double row of piers, depending upon the wall height and any surcharges.
Drilled piers for the retaining walls should penetrate a minimum of 10 feet below lowest adjacent grade, and 5 feet into competent bedrock, whichever is deeper.
The piers should have a minimum diameter of 16 inches and be nominally reinforced with a minimum of four #4 bars vertically. Piers should be spaced no closer than 4 diameters, center to center. Actual pier depth, diameter, reinforcement, and spacing should be determined by the structural engineer.
A friction value of 700 psf may be assumed to act on that portion of the pier within bedrock. Lateral support may be assumed to be developed along the length of the pier once there is a minimum 10 feet of horizontal cover between the face of the pier and the face of slope. At that depth, a passive pressure of 400 pcf Equivalent Fluid Weight (EFW) may be used for design. Passive resistance may be assumed to act over 1.5 projected pier diameters. Above the bedrock, no frictional support may be assumed, lateral support may be assumed to start above the bedrock as discussed above. The design values may be increased 1/3 for transient loads (i.e. seismic and wind).
If drilled piers are utilized beneath a concrete or block wall, they will need to be connected by a concrete grade beam. No grade beam is required for a wood lagging wall.
Wall Drainage - The above values have been provided assuming that back-of-wall drains will be installed to prevent build-up of hydrostatic pressures behind all walls. This drainage system may This drai type system. may consist of a prefabricated drainage panel (i.e. Miradrain) or a gravel and filter fabric type system. We also recommend that any walls through which efflorescence transmission would be undesirable, should be waterproofed. The waterproofing should be specified by the designer, though we suggest the use of Bituthene, Miradri, or other similar waterproofing membrane. Surface drainage above the wall should preclude overtopping of the wall, and should also preclude ponding on the ground surface above the wall. Additionally, the ground surface above all walls should form a drainage swale to carry water to the sides of the wall and/or to area drain locations.
The back-of-wall drain systems should be installed with a minimum 3-inch diameter perforated pipe placed a minimum of 4 inches below the top of the footing (preferably at the base of the footing heel).
The pipe should not be placed on top of the heel of the wall footing unless seepage through the base of the wall is acceptable. Perforations should be placed face-down (at 5 and 7 o'clock). The 10 File: 220285
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perforated pipe should connect to a solid discharge line, which discharges away from the new structures. This solid line should not connect to surface water drain lines (i.e. downspout and area drain lines). If water transmission through the base of a wall is not a concern, then weep holes may be used in place of the pipe.
If used, the gravel system should consist of a minimum 12 inch wide column of drain rock (3% to ¾ inch clean, crushed rock) extending the full width of the wall. The rock should continue to within 12 inches of finish grade. Prior to backfilling with the drain rock, a layer of filter fabric (Mirafi 140N or approved equivalent) should be placed against all soil surfaces to separate the rock and soil. The filter fabric should wrap over the top of the gravel and then a 12 inch thick cap of native soils should be placed at the top of the drain. If concrete flatwork is to directly overlay the back-of-wall drain, or if the drain is located in a crawlspace area, then the soil cap should be eliminated.
If prefabricated drainage panels are used, a packet of filter fabric-wrapped drain rock should be placed around the perforated collector pipe at the base of the panel. The tops of the panels should be sealed and secured in accordance with the manufacturer's recommendations. The base of the drainage panels should extend down below the top of the filter fabric-wrapped drain rock.
We note that Caltrans Class II permeable rock may be utilized in lieu of clean drain rock and filter fabric. The Class Il permeable rock needs to be compacted into place, and needs to be certified by the quarry or rockery that it meets the Caltrans Class II permeable rock specifications.
Slabs-on-Grade
The new residence floors should not consist of concrete slabs due to the expansive nature of the site soils. However, driveway, and any sidewalks or patios may consist of conventional concrete slabs- on-grade, though it should be expected that some post-construction shifting of such slabs may occur.
We have provided guidelines to help reduce post-construction movements, however, it is nearly impossible to economically eliminate all shifting.
To help reduce cracking, we recommend slabs be a minimum of 5 inches thick and be nominally reinforced with #4 bars at 18 inches on center, each way. Slabs which are thinner or more lightly reinforced may experience undesirable cosmetic cracking. However, actual reinforcement and thickness should be determined by the structural engineer based upon anticipated usage and loading.
In large non-interior slabs (e.g. patios, garage, etc.), score joints should be placed at a maximum of 10 feet on center. In sidewalks, score joints should be placed at a maximum of 5 feet on center. All slabs should be separated from adjacent improvements (e.g. footings, porches, columns, etc.) with expansion joints.
It would be prudent (though not required) to underlay all slabs with at least 24 inches of non-expansive materials. This will help to reduce future expansive soil movements of the slabs. Slabs which are not underlain by this non-expansive material may undergo excessive seasonal shifting. 11 File: 220285
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The garage slab should be underlain by a minimum of 4 inches of clean ¾ inch crushed drain rock.
The drain rock should be covered by a vapor barrier which conforms to ASTM E1745-97 (e.g. Stego Wrap or an approved equivalent). The architect or structural engineer should determine if sand is required over the vapor barrier.
Slabs which will be subject to light vehicular loads and through which moisture transmission is not a concern (e.g. driveway) should be underlain by at least 6 inches of compacted baserock, in lieu of any sand and gravel. The 6 inches of granular subgrade may be included as part of the 24 inches of non- expansive materials.
Exterior landscaping flatwork (e.g. patios and sidewalks) may be placed directly on proof-rolled soil subgrade materials (e.g. no granular subgrade), however, they will be potentially subject to greater amounts of shifting and moisture transmission.
As stated previously, in pavement (concrete or asphalt) areas to receive vehicular traffic, all baserock materials should be compacted to at least 95 percent of their MDD. Also, the upper 6 inches of native soil subgrade beneath any pavements should be compacted to at least 90 percent of its MDD.
The garage slabs may be allowed to "float" independently from the perimeter grade beams if some post-construction differential movement is acceptable. If so, the slab should be separated from the grade beam with an expansion joint completely around the perimeter and at any interior isolated columns. Ideally, the grade beam at the front of the garage should continue to final floor elevation, with the slab inside the grade beam. This will help to assure that the garage doors always shut upon the grade beam, which should experience little or no movement (while the slab has the potential for greater movements).
To reduce post-construction expansive soil movements (i.e. heave) of any slabs, care should be taken to keep the subgrade moist for an extended period of time prior to pouring the slabs. Shrinkage cracks should not be allowed to develop in the soil beneath any proposed slabs.
Drainage
Surface Drainage - Adjacent to any buildings, the ground surface should slope at least 5 percent away from the foundations within 5 feet of the perimeter. Impervious surfaces should have a minimum gradient of 2 percent away from the foundation.
Surface water should be directed away from all buildings into drainage swales, or into a surface drainage system (i.e. catch basins and a solid drain line). "Trapped" planting areas should not be created next to any buildings without providing means for drainage (i.e. area drains). 12 File: 220285
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All roof eaves should be lined with gutters. The downspouts may be connected to solid drain lines, or may discharge onto paved surfaces which drain away from the structure. The downspouts may be connected to the same drain line as any catch basins, but must not connect to any perforated pipe drainage system.
Footing Drain - Due to the potential for changes to surface drainage provisions, it would be wise (though not required) to install a perimeter footing drain to intercept water attempting to enter the crawlspace. If a footing drain is not installed, some infiltration of moisture into the crawlspace may occur. Such penetration should not be detrimental to the performance of the structure, but can possibly cause humidity and mildew problems within the house. Where perimeter walls are retaining walls, the back of the walls will serve as perimeter footing drains.
The footing drain system, if installed, should consist of a 12 inch wide gravel-filled trench, dug at least 12 inches below the elevation of the adjacent crawlspace. The trench should be lined with a layer of filter fabric (Mirafi 140N or equivalent) to prevent migration of silts and clays into the gravel, but still permit the flow of water. Then 1 to 2 inches of drain rock (clean crushed rock or pea gravel) should be placed in the base of the lined trench. Next a perforated pipe (minimum 3 inch diameter) should be placed on top of the thin rock layer. The perforations in the pipe should be face down. The trench should then be backfilled with more rock to within 6 inches of finished grade. The filter fabric should be wrapped over the top of the rock. Above the filter fabric 6 inches of native soils should be used to cap the drain. If concrete slabs are to directly overlay the drain, then the gravel should continue to the base of the slab, without the 6 inch soil cap. This drain should not be connected to any surface drainage system.
Drainage Discharge - The surface drain lines should discharge at least 15 feet away from the house, preferably at the street. Care should be taken not direct concentrated flows of water towards neighboring properties. This may require the use of multiple discharge points. A percolation style dissipation system is not permitted for use at this site.
The footing drain (if installed) and any back-of-wall drain lines should discharge independently from the surface drainage system. A sump pump may be required for the footing drain discharge system.
The surface and subsurface drain systems should not be connected to one another.
Drainage Materials - Drain lines should consist of hard-walled pipes (e.g. SDR 35 or Schedule 40 PVC). In areas where vehicle loading is not a possibility, SDR 38 or HDPE pipes may be used.
Corrugated, flexible pipes may not be used in any drain system installed at the property.
Surface drain lines (e.g. downspouts, area drains, etc.) should be laid with a minimum 2 percent gradient (¼ inch of fall per foot of pipe). Any subsurface drain systems (e.g. footing drains) should be laid with a minimum 1 percent gradient (1/8 inch of fall per foot of pipe). 13 File: 220285
January 27, 2021
Utility Lines
Unless they pass through the perimeter footing drain system, all utility trenches should be backfilled with compacted native clay-rich materials or concrete plug within 5 feet of any buildings. This will help to prevent migration of surface water into trenches and then underneath the structures' perimeter.
The rest of the trenches may be compacted with other native soils or clean imported fill. Only mechanical means of compaction of trench backfill will be allowed. Jetting of sands is not acceptable.
Trench backfill should be compacted to at least 90 percent of its MDD. However, under pavements, concrete flatwork, and footings the upper 12 inches of trench backfill must be compacted to at least 95 percent of its MDD.
Pavement
The new driveway may consist of concrete, interlocking pavers, or asphaltic concrete over Caltrans Class II aggregate base (baserock). The asphalt should have a minimum thickness of 2½ inches. The baserock should have a minimum thickness of 6 inches, though 12 inches is preferable due to the weak nature of the near-surface site soils. If a flexible pavement surface is to be used (e.g. asphalt or pavers), then we would also recommend that a layer of reinforcing fabric (e.g. Mirafi 500x) be placed on the top of the compacted subgrade for best performance.
All of the upper 6 inches of soil subgrade should attain a minimum compaction of 90 percent of its Plan Review and Construction Observations
The use of the recommendations contained within this report is contingent upon our being contracted to review the plans, and to observe geotechnically relevant aspects of the construction.
We should be provided with a full set of plans to review at the same time the plans are submitted to the building/planning department for review. A minimum of one working week should be provided for review of the plans. At a minimum, our observations should include: footing excavations; pier drilling; installation of any drainage system (e.g. footing, pool, retaining wall and surface), and final grading. A minimum of 48 hours notice should be provided for all construction observations.
LIMITATIONS
This report has been prepared for the exclusive use of the addressee, and their architects and engineers for aiding in the design and construction of the proposed development. It is the addressee's responsibility to provide this report to the appropriate design professionals, building officials, and contractors to ensure correct implementation of the recommendations.
The opinions, comments and conclusions presented in this report were based upon information derived from our field investigation and laboratory testing. Conditions beyond our boring may vary 14 File: 220007
February 14, 2020
from those encountered. Such variations may result in changes to our recommendations and possibly variations in project costs. Should any additional information become available, or should there be changes in the proposed scope of work as outlined above, then we should be supplied with that information so as to make any necessary changes to our opinions and recommendations. Such changes may require additional investigation or analyses, and hence additional costs may be incurred.
Our work has been conducted in general conformance with the standard of care in the field of geotechnical engineering currently in practice in the San Francisco Bay Area for projects of this nature and magnitude. We make no other warranty either expressed or implied. By utilizing the design recommendations within this report, the addressee acknowledges and accepts the risks and limitations of development at the site, as outlined within the report.
Respectfully Submitted;
GeoForensics, Inc.
Daniel Dyckman K.Younesi
No. 002145
Daniel F. Dyckman, PE, GE * Kourosh Younesi, PE
Senior Geotechnical Engineer, GE 2145 FATE Project Engineer, PE
CALIFORKIA
CC: 1 electronic copy to client email Address
15 Pacific Brew
N
Pacifica
Pacifica State Beach
Site
Taco Bell'
Anza Dr
SHELT COVE
AN PEDRO Safeway
TERRACE
PEDRO POINT
Grand San Pedro Creek Montezuma Dr
Noriega Wast
Flores Dr
Source: Google Maps Pedro Point Headlands
GEOFORENSICS, INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure 1 - Site Location Tel: (650) 349-3369 Fax: (650) 571-1878
San Pe
Be
Site
KENT 190
PACIF
San Pedro
Point Terrace
San Pedro GRAND'AVE
Shelter Cove 300
600
GEOFORENSICS, INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure 2 - Vicinity Topography Tel: (650) 349-3369 Fax: (650) 571-1878
N
S
55-
Shelter Cov
Qb
TsI Cam
75 г
35 Osr Osr
Tsu
80 Qsr Slope Wash, Revine fill, and Collovium
Unconsolidated to moderately consolidated deposits of sand, silt, clay, and rock fragments accumulated by slow downslope movement of weathered rock debris and soil. Composition dependent upon underlying rocks.
Commonly unsorted and unbedded, but locally layered by downslope movements. Mapped where thickness presumed to exceed 5 ft; as thick as 20 ft on north side of San Pedro Valley.
Source: Pampeyan, E.H., 1994, Geologic map of the Montara Mountain and San Mateo 7.5' quadrangles, San Mateo County, California (Map I-2390)
GEOFORENSICS. INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure 3 - Geologic Map Tel: (650) 349-3369 Fax: (650) 571-1878
88. BeGg
Site
Rotni
San Arguello Blvd
CresPo
Pedro
Peralta
Linda ives!
Rosita Rd
MAP EXPLANATION
Zones of Required investigation:
Liquefaction
Areas where historic occurrence of liquefaction, or local geological geotechnical and groundwater conditions indicate a potential fo permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(c) would be required.
Earthquake-Induced Landslides
reas where previous occurrence of landslide movement, or local topographic, geological, geotechnical and subsurface water conditions indicate a potential for permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(cl would be required.
NOTE: Seismic Hazard Zones identified on this map may include developed land where delineated hazards have already been mitigated to city or county standards. Check with your local building/planning department for information regarding the location of such mitigated areas
State of California Seismic Hazard Zones; Montara Mountain Quadrangle Official Map; Released: April 4, 2019
GEOFORENSICS, INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure 3a - Seismic Hazards Map Tel: (650) 349-3369 Fax: (650) 571-1878
000
Olympian Way:
Boring 1 7
Olympian Way
• 133
155
Boring Location
GEOFORENSICS, INC.
Figure 4 - Site Photo with Approximate
303 Vintage Park Dr., Suite 220, Foster City, CA 94404
Borehole Location
Tel: (650) 349-3369 Fax: (650) 571-1878
Olympian Way Lot
Pacifica
GeoForensics Inc. 1220285 - static-2.gsd
280
No. Soil Moist Wt Sat Wt Phi Pconst Piez Surf 280
Soil
2.440 No. (pct) (pct) (pst) (deg) (ratio) (pst) No. Options
2.447 Upper Layer 120.0 132.0 100.0 26.0 0.000 0.0
2.454 Deeper Layer 120.0 132.0 500.0 26.0 0.000 0.0
2.456
2.460
260 2.475 -260
2.484
2.486
2.492
10 2.531
240- -240
B1
220- -220
B2
200-
-200
180° -180
15 35 55 75 95 115 135 155
GEOSTASE FS = 2.440
GREGORY
GEOSTASE Spencer Method GEOTECHNICAL
Slope Stability
Analvsis
GEOSTASE® by GREGORY GEOTECHNICAL S GEOFORENSICS, INC. Figure 5 - Slope Stability 561 Pilgrim Dr., Suite D, Foster City, CA 94404 Analysis Results - Static Tel: (650) 349-3369 Fax: (650) 571-1878 Condition
Olympian Way Lot
Pacifica
GeoForensics Inc. 1220285 - static -2.gsd
280 -280
No. Soil Moist Wt Sat Wt Phi Pconst Piez Surf Soil
1.128 No. (pct) (pcf) (psf) (deg) (ratio) (pst) No. Options
1.130 Upper Layer 120.0 132.0 100.0 26.0 0.000 0.0
1.134 Deeper Layer 120.0 132.0 500.0 26.0 0.000 0.0
1.138
1.138
260 1.146 ·260
1.147
1.152
1.157
1.166
240г 4240
220- 1220
200- ,200
180 :180
-5 15 35 55 75 95 115 135 155
GEOSTASE FS = 1.128
GREGORY
GEOSTASE Spencer Method GEO TECHNICAL
Slope Stability kh = 0.31000
Analvsis GEOFORENSICS, INC. Figure 6 - Slope Stability
GEOSTASE® by GREGORY GEOTECHNICAL SOF
561 Pilgrim Dr., Suite D, Foster City, CA 94404 Analysis Results - Seismic Tel: (650) 349-3369 Fax: (650) 571-1878 Condition
LOG OF BORING
(%)
DEPTH (ft) SAMPLE NUMBER SAMPLE LOC. BLOW COUNTS (12 inches) DRY DENSITY MOISTURE
MATERIAL DESCRIPTION CONTENT
Sandy fat CLAY with gravel; dark brown to
olive brown; damp, hard (CH)
1-1 66 Clayey SAND with gravel; light to olive gray; 99.3 17.4 slightly moist, very dense (SC), bedrock
1-2 = 50/3" 69
Bottom of Boring @ 6 feet and 9 inches
No Ground Water Encountered
10 15
20 25
30 Logged by: KY Minute Man Drilling Rig Mod. Cal
Job No: 220285 70 Pound Hammer Sampler
SPT"
Drilled on 1/7/21 Sampler
GEOFORENSICS, INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure A1 - Log of Boring 1 Tel: (650) 349-3369 Fax: (650) 571-1878
LOG OF BORING
DEPTH (ft) NUMBERE SAMPLE LOC. BLOW COUNTS (12 inches) DRY DENSITY CONSENT 9)
MATERIAL DESCRIPTION
Sandy CLAY with gravel; yellowish brown to
olive brown; damp, hard (CH)
2-1
Clayey SAND with gravel; light to olive gray; 10.6
slightly moist, very dense (SC), bedrock
2-2 - 50/4
10 Bottom of Boring @ 9 feet and 4 inches
No Ground Water Encountered
15 20
25 30
Logged by: KY Minute Man Drilling Rig Mod. Cal
Job No: 220285 70 Pound Hammer Sampler
SPT
Drilled on 1/7/21 Sampler
GEOFORENSICS, INC.
303 Vintage Park Dr., Suite 220, Foster City, CA 94404 Figure A2 - Log of Boring 2 Tel: (650) 349-3369 Fax: (650) 571-1878
COPER Moisture-Density-Porosity Report
Cooper Testing Labs, Inc. (ASTM D7263b)
CTL Job No: 060-2960 Project No. 220285 By: RU
Client: GeoForensics Date: 01/14/21
Project Name: Olympian Way Lot Remarks:
Boring: 1-1 1-2 2-5
Sample:
Depth, ft: 4 6.5 9
Visual Olive Olive Gray Yellowish
Description: Brown Sandy Brown
Sandy Fat CLAY w/ Sandy
CLAY Gravel CLAY
Actual
Assumed G, 2.70
Moisture, % 17.4 6.9 10.6
Wet Unit wt, pcf 116.5
Dry Unit wt, pcf 99.3
Dry Bulk Dens.pb, (g/cc) 1.59
Saturation, % 67.2
Total Porosity, % 41.2
Volumetric Water Cont,Ow,% 27.6
Volumetric Air Cont., 9a,% 13.5
Void Ratio 0.70
Series 8
Note: All reported parameters are from the as-received sample condition unless otherwise noted. If an assumed specific gravity (Gs) was used then the saturation, porosities, and void ratio should be considered approximate.
Moisture-Density
Zero Air-voids Curves, Specific Gravity
140
2.6 The Zero Air-Voids curves
2.7 . represent the dry density at •Series 1
130 100% saturation for each value
2.8 of specific gravity A Series 2
120
× Series 3
$ 110 * Series 4
Density, • Series 5
100
+ Series 6
90 - Series 7
80 - Series 8
70-
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Moisture Content, %
LIQUID AND PLASTIC LIMITS TEST REPORT (ASTM D4318)
60 Dashed line indicates the approximate
upper limit boundary for natural soils
50 CH or OH
40 30
20 CLOT
ML or OL MH or OH
10 30 50 70 90 110
LIQUID LIMIT
52.81 52.0
51.2 50.4
49.6 48.85 10 20 25 30 40
NUMBER OF BLOWS
MATERIAL DESCRIPTION LL PL PI %<#40 %<#200 USCS
• Olive Brown Sandy Fat CLAY 51 25 26
Project No. 060-2960 Client: GeoForensics Remarks:
Project: Olympian Way Lot - 220285
• Source: 1-1 Elev./Depth: 4'
LIQUID AND PLASTIC LIMITS TEST REPORT (ASTM D4318)
COOPER TESTING LABORATORY Figure
Staged Consolidated Undrained Triaxial Compression
ASTM D4767m
10 — Total Tangent
COPER
LESTONE
Shear Stress, ksf
0 5 10 15 20
Normal Stress, ksf
Stage 2 3 4
Stress-Strain Response
10000 Boring
Sample
9000
Depth 4.0
8000
Visual Dark Grayish™
7000 Brown Sand;
Description CLAY, trace Gravi
6000
5000 MC (%) 11.3
Deviator Stress,
4000 Dry Density (pch) 103.3
3000 Saturation (%) 47.0
Stage 1 Void Ratio 0.662
2000
Stage 2
Stage 3 Diameter (in) 2.41
1000 Stage 4
Height (in) 5.01
5 Final
10 15 20 25
Strain, % MC (%) 22.9 20.9 18.9
Dry Density (cr) 105.3 109.1 113.0
CTL Number: 060-2960 Saturation (%) 100.0 100.0 100.0
Client Name: GeoForensics Void Ratio 0.630 0.574 0.520
Project Name: Olympian Way Lot Diameter (in) 2.39 2.40 2.42 Project Number: 220285 Height (in) 5.01 4.80 4.55
Date: 1/21/2021 | By: MD/DC Cell Pressure (psi) 55.9 69.8 83.7 Total C 0,100 ksf Back Pressure (ps) 49.0 49.0 49.0
Total phi| 25.9 degrees Total Stresses At:
Eff. cr N/A ksf Strain (%) 5.0 5.0
Eff. Phil N/A degrees Deviator (kst) 1.721 4.368 8.842
Excess PP (Ps)
Sigma 1 (ks) 2.714 7.363 13.839
Sigma 3 (ksf) 0.994 2.995 4.997
P (ksf) 1.854 5.179 9.418
Q (kst) 0.860 2.184 4.421
Stress Ratio 2.732 2.458 2.770
Rate (in/min)| 0.0247 0.0245 0.0245