SFDB_Geotechnical_Report_Final.pdf

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Final Geotechnical Report

Sir Francis Drake Boulevard Point Reyes National Seashore

Marin County, California

CA FLAP CR109 (1)

Yeh Project No.: 214-045, 045A

September 4, 2015

Prepared for:

Jacobs Engineering Group 707 17th Street, Suite 2400 Denver, Colorado, 80202

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433

Phone: 805-481-9590 i

Final Geotechnical Report

Sir Francis Drake Boulevard Point Reyes National Seashore

Marin County, California

CA FLAP CR109 (1)

Yeh Project No.: 214-045, 045A

September 4, 2015

Prepared by:

Jonathan D. Blanchard, GE2312 Simon P. Boone Geotechnical Engineer Staff Geologist

Independent Reviewer:

Judd J. King, GE 2903 Senior Project Engineer

Geotechnical Investigation Report Project No. 214-045, 214-045A Sir Francis Drake Boulevard September 4, 2015 ii

Table of Contents

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

3. SUBSURFACE INVESTIGATION AND TESTING

3.1 HOLLOW STEM AUGER DRILLING

3.2 LABORATORY TESTING

3.3 CONE PENETRATION TESTING

4. SITE CONDITIONS

4.1 GEOLOGICAL SETTING

4.2 CLIMATE

4.3 SUBSURFACE CONDITIONS

4.3.1 STATION 101+00 TO STATION 109+00: CUT AND FILL WIDENING

4.3.2 STATION 379+00 AND 390+50: CATTLE UNDERCROSSINGS

4.3.3 STATION 491+00: SCHOONER CREEK BRIDGE

4.3.4 STATION 499+00 TO 521+00: FLOODED AREA

4.3.5 STATION 529+00: BOX CULVERT

5. GEOTECHNICAL RECOMMENDATIONS

5.1 GENERAL EARTHWORK

5.1.1 COMPACTION

5.1.2 EARTHWORK FACTORS

5.1.3 EXCAVATION AND REUSE OF ONSITE MATERIALS

5.2 SLOPE DESIGN - GENERAL

5.3 STATION 45+46 TO 70+00: FULL RECONSTRUCTION

5.4 STATION 101+00 TO 112+00: FULL RECONSTRUCTION

5.5 STATION 378+89 TO 387+75, 382+50 TO 391+85: CATTLE UNDERCROSSINGS

5.6 STATION 488+50 TO 493+75: SCHOONER CREEK ARCH CULVERT

5.6.1 GEOTECHNICAL PROPERTIES

5.6.2 SEISMIC DATA

iii

5.6.3 SCOUR CONSIDERATIONS

5.6.4 FOUNDATION DESIGN

5.6.5 FOUNDATION EXCAVATION

5.7 STATION 499+45 TO 533+00: FULL RECONSTRUCTION (FLOODED AREA)

5.8 CORROSION

5.9 SUBEXCAVATION

6. NOTES TO CONSTRUCTION

6.1 GRADING OBSERVATION

6.2 FOUNDATION EXCAVATIONS

7. LIMITATIONS

8. REFERENCES

List of Figures

FIGURE 1 - SITE LOCATION MAP

FIGURE 2: RECOMMENDED DESIGN ARS CURVE

FIGURE 3 FILL FAILURE AREA (~STA. 607+00)

List of Tables

TABLE 1: CALTRANS STANDARD PLAN CONDITIONS USED IN ANALYSIS

TABLE 2: GEOTECHNICAL PROPERTIES FOR SCHOONER CREEK ABUTMENT 1

TABLE 3: GEOTECHNICAL PROPERTIES FOR SCHOONER CREEK ABUTMENT 2

TABLE 4: RECOMMENDED PILE DATA TABLE

TABLE 5: LATERAL EARTH PRESSURES

List of Plates

No

Regional Geologic Map

Subsurface Profile at Schooner Creek Bridge file:///Z:/Projects%20-%20Grover/214-045%20045A%20ES%20Jacobs%20-%20Sir%20Francis%20Drake%20Blvd/1.5%20Final%20Geotechnical%20Report_Cal%202015.09/214-045%20SFDB%20Geotechnical%20Report%20final%20v2.docx%23_Toc436202137 file:///Z:/Projects%20-%20Grover/214-045%20045A%20ES%20Jacobs%20-%20Sir%20Francis%20Drake%20Blvd/1.5%20Final%20Geotechnical%20Report_Cal%202015.09/214-045%20SFDB%20Geotechnical%20Report%20final%20v2.docx%23_Toc436202138 file:///Z:/Projects%20-%20Grover/214-045%20045A%20ES%20Jacobs%20-%20Sir%20Francis%20Drake%20Blvd/1.5%20Final%20Geotechnical%20Report_Cal%202015.09/214-045%20SFDB%20Geotechnical%20Report%20final%20v2.docx%23_Toc436202139 iv

List of Appendices

Page

Appendix A – Boring Logs

Boring Location Plan (April 2014) ......................................................................................... A-1 to 4

Field Exploration Plan, Schooner Creek Bridge (June-July 2015) .................................................. A-5

Field Exploration Plan, CBC529 (June-July 2015) .......................................................................... A-6

Legend for Symbols used on Borehole Logs ................................................................................. A-7

Borehole Logs...................................................................................................................... A-8 to 49

Drilling Photos .................................................................................................................. A-50 to 54

Appendix B –Laboratory Test Results

Summary of Laboratory Test Results (2014) ..........................................................................B-1 to 2

Moisture and Density Measurements ..................................................................................B-3 to 4

Atterberg Limits .....................................................................................................................B-5 to 6

Proctor Compaction ...................................................................................................................... B-7

Mechanical Sieve Analysis ................................................................................................... B-8to 10

Consolidation Test ...................................................................................................................... B-11

CU Triaxial Test (Cal Poly) ................................................................................................. B-12 to 13 pH and Resistivity ........................................................................................................................ B-14

Soluble sulfates and chlorides (Cooper) ..................................................................................... B-15

Appendix C – Cone Penetration Test (CPT) Sounding Log

Log of CPT Sounding .............................................................................................................. C-1 to 6

Geotechnical Report Project No. 214-045

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates, Inc., as a sub-consultant to Jacobs

Engineering Group, Denver, Colorado was retained by the Federal Highway

Administration (FHWA), Central Federal Lands

Highway Division (CFL) to provide geotechnical and pavement design recommendations for improvements to Sir Francis

Drake Boulevard (SFDB) at the Point Reyes National

Seashore in Marin County, California. The road provides access for visitors to Point Reyes and for seven historic ranches (A Ranch through F Ranch and M Ranch), some of which are functioning dairy and poultry operations. The project location is shown on Figure 1.

The geotechnical investigation included: research of the regional and local geology, visual inspection of existing features and subsurface exploration by auger borings. Laboratory testing was performed on selected samples obtained from the borings for use in the geotechnical evaluation. Recommendations for the project were developed and are provided in this report.

Logs of the exploratory borings and results of the laboratory testing are included in the report.

This report presents geotechnical recommendations for design of the proposed widening, foundation recommendations for two proposed cattle undercrossings, grading, embankment and cut slope recommendations for approximately 1.6 miles of roadway reconstruction and realignment, a new arch culver at Schooner Creek, and a culvert replacement at East Schooner

Creek. Recommendations for pavement design are provided in a separate pavement report also being prepared by Yeh.

Figure 1 - Site Location Map

2. PROJECT UNDERSTANDING

The proposed improvements are planned for approximately 12 miles of SFDB from Milepost

30.79 (Station 644+62) to Milepost 42.90 (Station 10+00). Road improvements will generally follow the existing alignment with widening as appropriate to improve safety and minimize impacts, and realignment of the horizontal and vertical grade in selected locations as noted below. The proposed improvements include widening the road at some locations, replacing two existing cattle under crossings, raising the profile of SFDB within a flood-prone area, a new structure at Schooner Creek, a new concrete box culvert at the east fork of Schooner Creek, and rehabilitation of 12 miles of pavement. The new road will have a typical paved width of at least 24 feet with 11-foot lanes and 1-foot paved shoulders. A summary of the main improvements addressed in this report are as follows:

Station 45+00 to 70+00: Minor realignment of SFDB is proposed that will result in widening of existing cut and embankment slopes that are up to about 10 feet in height. The realignment will improve safety and drivability in a section that includes steep grades and sharp curves. Cut and fill slopes are generally designed with slopes of 1V:2H or flatter, except between 47+75 and 48+50 where the cut slope will need to be sloped to 1V:1.5H to reduce impacts to sensitive areas.

Station 101+00 to 112+00: SFDB just north of the B Ranch. The improvements will include widening an existing rock cut on the southbound (left) side, and widening and reconstruction of an embankment slope along the northbound (right) side. The existing cut slope is up to

20 feet high and is constructed at approximately 1V:1H. The cut has evidence of erosion and instability in some areas. The existing embankment slope is about 5 feet high and overlies a descending natural slope. The embankment side of the road has evidence of settlement and downslope creep that has required maintenance and patching by the County of Marin.

Station 379+00 and 390+50: Two existing cattle under crossings will be replaced at these locations. The existing structures have a timber deck with cast-in-place concrete abutments that appear to be supported on footings. Timber and concrete rubble has been placed adjacent to the abutments to help support the approaches, likely because of erosion, and are distressed and leaning in some areas. The new structures will be single span, reinforced concrete box culverts design according to Caltrans (2010) Standard Plans. The invert for the new culverts will be approximately 10 to 12 feet below the finished grade of SFDB.

Station 490+80, Schooner Creek Crossing: Two existing corrugated metal culverts at this location will be replaced with a clear-span structure. The current design that is addressed in this report is to replace the crossing with a precast concrete arch structure. A design alternative to replace the crossing with a clear-span, slab-type bridge structure is currently being evaluated; however, the scope of this report does not address the bridge alternative.

The two existing culverts are 84-inches in diameter and coated inside and out with bitumen.

The crossing is within a tidal estuary. Schooner Creek flows generally south through the existing culverts, but has tidal influences that result in stream flow in both directions. The new structure will be approximately 48 feet long (parallel to the creek), with a span of approximately 32 feet across Schooner Creek. The footings/pile cap will be founded at approximately el. -2.5 feet, about 10 feet below the surface of SFDB at the existing crossing.

Wing walls and headwalls will be provided at both the inlet and outlet of the new crossing.

Station 499+45 to 533+00: SFDB will be raised and realigned to the south within this flood-prone area. The existing alignment has evidence of subsidence and is prone to flooding, including portions of the roadway that are wet year round. The profile of the road will be raised approximately 1 to 4 feet above the existing roadway profile, and the alignment will be shifted slightly into the hillside along the eastbound (right) side of the alignment. The improvements include widening an existing rock cut near Station 506. The existing rock cut is heavily vegetated, approximately 15 feet high, and appears to have been graded to a slope inclination of about 1V:1H. There is an approximately 2-foot deep ditch along the base of the slope that provides about 4 feet of catchment beyond the existing edge of pavement.

Station 529+37: The existing culvert at Station 529+37 on East Schooner Creek will be replaced with a new reinforced concrete box culvert designed using Caltrans Standard

Plans. The existing culvert has concrete headwall, and outlets on westbound side of the road. The new culvert will be a 12-foot by 6-foot precast concrete box culvert (with the bottom buried1 foot below the streambed) with upstream and downstream headwalls. The creek flows year round with an invert about 3 to 4 feet below the existing surface of SFDB.

3. SUBSURFACE INVESTIGATION AND TESTING

The field exploration for this project consisted of two phases of work: an April 2014 program that included drilling 40 borings for both the geotechnical and pavement evaluation, and a July 2015 program for a supplemental geotechnical evaluation for the design of the Schooner Creek

Bridge and a box culvert replacement. A total of 42 borings, six cone penetration test (CPT) soundings, and supporting laboratory testing was performed for this project. The logs for 26 pavement borings (14P-01 to 26) are included in Appendix A and are also included in a

Pavement Investigation Report that is also being prepared by Yeh for this project. The approximate locations, logs of the borings, and photos of the boring locations for this geotechnical evaluation are presented in Appendix A. Laboratory test results are presented in

Appendix B. Logs of the CPT soundings are presented in Appendix C.

3.1 HOLLOW STEM AUGER DRILLING

Cascade Drilling, LP of Richmond, California was the drilling subcontractor for both phases of the field exploration program. Traffic control was provided by Road Safety, Inc. of Rocklin, California during the 2014 drilling operations. Traffic control for the 2015 field explorations was provided by Statewide Safety and Signs of Fairfield, California. The locations, logs and photos for the borings are shown on the figures in Appendix A.

The initial borings were drilled between April 14 and April 16, 2014 using a truck-mounted

CME 85 drilling rig and hollow-stem auger. Two supplemental borings were drilled on June 30, 2015. Upon completion, the borings were backfilled with auger cuttings and the pavement was patched with Portland cement concrete in compliance with the encroachment permit provided by

Marin County. The borings drilled in June 30, 2015 were tremie-grouted using a bentonite cement grout and topped with rapid set concrete in accordance with well permits issued by

Marin County Environmental Health Service.

A geologist from Yeh logged each boring, and secured samples for laboratory testing. Samples were obtained from the borings at selected intervals using a 2-inch I.D. split spoon sampler.

The sampler was driven into the subsoils with a 140-pound automatic hammer falling 30 inches.

The blow count (N-value reported on the borings logs in Appendix A) is the number of blows needed to drive the sampler 12 inches after having seated the sampler at 6 inches into the material at the bottom of the hole. The blow count can be used as a relative measure of the material stiffness or relative density. Bulk samples of auger cuttings were also obtained from the borings at selected intervals.

3.2 LABORATORY TESTING

Samples retrieved during the field exploration were returned to our laboratory for observation by the project geotechnical engineer. An applicable program of laboratory testing was developed to determine engineering properties of the subsurface materials.

Following the completion of the laboratory testing, the field descriptions were confirmed or modified as necessary and boring logs were prepared. Laboratory tests performed included the following:

Gradation (ASTM D6913, C136 and AASHTO T 27), Atterberg limits (AASHTO T 89/T 90), Moisture content (AASHTO T 265) and unit weight density (ASTM D 7263)

Corrosion tests for sulfate content (AASHTO T290), pH (ASTM D 4972/AASHTO T 289), chloride ion content (ASTM D 4327), and soil resistivity (AASHTO T 288), Consolidated undrained triaxial compressive strength (ASTM D4767), and

Consolidation (ASTM D2435).

Gradation and Atterberg limits test results were used to classify the soils in accordance with the

AASHTO classification system and the Unified Soil Classification System (USCS). Moisture content provides an estimate of the moisture conditions of the subgrade and underlying materials. Density measurements on relatively undisturbed liner samples provide an estimate of the in-situ unit weight of soil. Tests for soluble sulfate content, pH, chloride content and resistivity are used to evaluate the potential of the soil to be aggressive to concrete and to corrode buried metal. The laboratory test results are presented in Appendix B.

3.3 CONE PENETRATION TESTING

The logs for CPT soundings are presented in Appendix C. Our subcontractor for the CPT activities was Gregg Drilling & Testing (Gregg) of Martinez, California. Gregg performed six soundings for this project on July 8, 2015 that totaled about 134 feet in depth. The soundings were performed in general accordance with ASTM D-5778 using an electric piezocone penetrometer. The penetrometer was advanced into the ground using a hydraulic ram mounted inside a truck-mounted vehicle weighing approximately 20 tons. CPT soundings were advanced to depths ranging from approximately 11 to 40 feet below the ground surface. Each of the soundings was terminated once refusal was encountered as indicated when the push load became excessive or tilting was noted to avoid damaging the cone.

The piezocone penetrometer had a diameter of approximately 1.7 inches with a tip area of 15 square centimeters (cm2) and a sleeve area of 225 cm2. Cone tip resistance (qc), sleeve friction

(fs), and penetration pore water pressures measured behind the tip (in the u2 location) were recorded during penetration using an on-board computer. Data were collected from the penetrometer at approximately 5-centimeter intervals to provide a near continuous profile of the subsurface conditions encountered during penetration. The friction ratio (FR, the ratio of the sleeve friction to the tip resistance in percent) was computed for each value of qc and fs recorded. The data and soil behavior type classifications were used in subsequent geotechnical analyses and to evaluate soil types and boundaries for analyses. Upon removal of the CPT rod, the soil generally collapsed to near the groundwater level encountered. The void above that depth was filled with bentonite-cement grout and topped with cold patch.

4. SITE CONDITIONS

4.1 GEOLOGICAL SETTING

The Point Reyes Peninsula is a projection of the Marin County coast that lies west of the San

Andreas Fault. The fault zone trends southeast through Tomales Bay and along the eastern boundary of Point Reyes National Seashore. The bedrock geology consists of Tertiary marine sedimentary rocks that rest on crystalline and granitic basement rocks. Surficial deposits consist of Holocene dune sands, Pleistocene terrace deposits and older aeolian dune sands.

Plate 2 shows the surface geology in the project area as mapped by Clark and Brabb (1997).

The surface geology at the beginning of the project, near A Ranch and the Chimney Rock Road intersection, consists of late Miocene Age Santa Margarita Sandstone. North from A Ranch to near Schooner Bay, the bedrock is the Purisima Formation, consisting of Upper Miocene to

Lower Pliocene marine siltstone, sandstone and mudstone that are locally concealed by dune sand deposits or alluvium is exposed along the alignment to about Station 430+00. An outcrop of the Purisima Formation sandstone is visible in the existing cut near Station 101+00, on the left (west) side of the road. Bedrock, Santa Margarita Sandstone that is commonly concealed beneath older dune sands, is exposed from about Station 430+00 west of Schooner Bay to about Schooner Creek. In the low-lying area east of Schooner Bay, the alignment passes through recent alluvium that overlies Monterey Formation bedrock consisting of marine shales with interbeds of porcelanite and chert. The contact between the Monterey Formation and Laird

Sandstone is near Station 560+00 (near Mount Vision Road), and is exposed as isolated outcrops above the granite of the Inverness Ridge, and is the predominate bedrock exposed between Mount Vision Road and the northern project limit at Pierce Point Road. Terrain

Terrain along the SFDB alignment from A Ranch to Schooner Bay is rolling to hilly with occasional eroded drainages generally flowing south and east toward Drake's Bay. Vegetation consists of grasses and shrubs with isolated stands of cedar trees west of Schooner Creek. The area is primarily used as pasture land for dairy cattle.

At Schooner Bay, the road crosses a tidal estuary at the upper end of Schooner Bay. The terrain is marshy on both sides of the road and water ponds on the travel way at several locations east of the Schooner Creek Crossing. Sandstone outcrops are visible at several locations on slopes along the road. The slopes are heavily vegetated with oak trees, poison oak, vines and blackberry.

From about Schooner Bay to Pierce Point Road the alignment follows a narrow valley between sandstone hills as it gradually climbs eastward to the Inverness Ridge among upland and woodland areas. Vegetation consists of areas of grasses, shrubs and cedar, pine and oak trees with relatively thick willow brush along streams and drainages.

4.2 CLIMATE

Climate data is based on available records for Point Reyes Lighthouse from 1914 to 1943.

Other nearby population centers have more recent data, but because of the variation in climate between the coast and sites a few miles inland, the limited data for the lighthouse site was deemed to be more accurate for SFDB. The area has a cool climate typical of a coastal environment. The average annual precipitation for the historic period available is 17.0 inches, most of which occurred in the winter months (December through February). The average annual high temperature is 56.7o F and average annual low temperature is 48.1o F. The annual high temperatures occur in September and October and the annual low temperatures occur in

January, February and March. By contrast, the Bear Valley Visitor's Center located on the east side of the Inverness Ridge near Olema, normally receives over 60 inches of precipitation annually and has normal annual maximum temperatures about 10o F higher than at the lighthouse.

4.3 SUBSURFACE CONDITIONS

The subsurface conditions encountered in the exploratory borings drilled at the locations of the proposed cut or fill, cattle undercrossing structures, and proposed realignment areas are discussed in the sections below.

4.3.1 STATION 101+00 TO STATION 109+00: CUT AND FILL WIDENING

Borings W-1 through W-5 were drilled in the roadway north of B Ranch. Borings W-1, W-3 and

W-5 were drilled in the left (southbound) lane near the edge of the existing pavement. These borings encountered 8 ½ to 9 inches of asphalt pavement, 4 to 6 inches of aggregate base overlying 3 to 8 feet of stiff to very stiff sandy clay and clayey sand that appears to contain topsoil, weathered rock, and residual soils derived from the local bedrock exposed in the adjacent road cut. Borings W-2 and W-4 were drilled at the outside edge of the right

(northbound) lane and encountered approximately 3 to 7 feet of medium stiff to stiff sandy clay that appeared to be poorly compacted fill material. The fill was underlain by soft to moderately, claystone, mudstone and sandstone of the Purisima Formation.

Wet soil and seepage were encountered above the bedrock in two of the borings (W-3 and W-5) drilled during the April 2014 field exploration program. Laboratory test results show the sandy clay soils have high moisture content, relatively low density, very high liquid limit and medium plasticity that are characteristic of diatomaceous materials.

4.3.2 STATION 379+00 AND 390+50: CATTLE UNDERCROSSINGS

Exploratory borings were drilled at each of the cattle undercrossing (CUC) locations. The borings were drilled in the roadway, to a maximum depth of 31 feet. Two borings (B-1 on the south approach, and B-2 on the north approach) were drilled near the location of the cattle undercrossing at Station 379+00, and two borings (B-3 on the south approach and B-4 on the north approach) were drilled near the cattle crossing at Station 390+50.

CUC 379: Very loose to loose clayey and gravelly sand and medium stiff sandy clay was encountered to approximately 13 feet below the road surface in borings B-1 and B-2. The loose clayey sand transitioned to medium dense from 13 to about 18 feet in B-1. The soils appear to be a combination of poorly compacted backfill associated with the existing cattle crossings and dune sand deposits. The soils were underlain by soft sandstone and claystone of the Purisima

Formation at depths of approximately 13 to 18 feet. The formational materials consisted of moderately weathered relative soft sandstone and claystone, with field blow count values recorded during drive sampling that ranged from 33 blow per foot to more than 50 blows in 5 inches. The borings were terminated within the Purisima Formation at approximately 31 feet below the road.

Groundwater was encountered at the time of drilling in Boring B-1 at a depth of 14.5 feet and in

Boring B-2 at a depth of 16 feet. The moisture content for soil samples taken above the groundwater table ranged from 17 to 25 percent with dry unit weights that ranged between 98 and112 pounds per cubic foot (pcf). The moisture content for samples of sandstone ranged from 21 to 25 percent, with dry unit weights ranging from approximately 84 to 96 pcf. A composite soil sample from Boring B-2 at 15 to 25 feet depth had a pH of 5.6, 0.168 percent water soluble sulfates and resistivity of 401 ohm-cm.

CUC 390. Loose silty and poorly graded sand was encountered to approximately 18.5 feet below the road surface in borings B-3 and B-4. The sand appeared to be associated with existing fill and dune sand deposits. Relatively dense silty sand was encountered below 18.5 feet that appears to be older indurated dune sand deposits. The borings were terminated within the older dune deposits at depths of approximately 31 feet below the road surface.

Groundwater was encountered in each of the borings at depths of approximately 20 feet below the road surface. The moisture content for samples of dune sand ranged from approximately 7 to 8 percent, with dry unit weights that ranged from approximately 99 to 105 pcf. The moisture content for samples of the older dune sand ranged from approximately 13 to 17 percent with dry unit weights that ranged from approximately 113 to 119 pcf.

4.3.3 STATION 491+00: SCHOONER CREEK BRIDGE

Boring 15B-101 and cone penetration test soundings CPT-1 through CPT-4 were advanced at the Schooner Creek bridge location. The location of the field explorations are presented in

Appendix A. A subsurface profile summarizing the conditions encountered at Schooner Creek is presented on Plate 2. The borings was drilled to a maximum depth of 31 feet, the CPT soundings were advanced to depths of up to 40 feet.

Existing fill materials (unit Af on Plate 2) were encountered in each of the explorations. The fill was composed of the existing pavement over a mixture about 2 to 5 feet of fill material that consisted of stiff sandy clay, sand, and dense clayey gravel with varying amounts of gravel.

Relatively large rip-rap is also present along the banks of Schooner Creek at the outlet end of the existing culverts. The fill materials were underlain by estuarine deposits.

The estuarine deposits (Qe on Plate 2) were encountered beneath the fill in each of the explorations to about 10 to 12 feet below the road surface, and are exposed within the banks of

Schooner Creek and on the tidal marshlands up and downstream of the site. The estuarine deposits generally consisted of a 5- to 7-foot thickness of soft clay and elastic silt that was interbedded with layers of fibrous muck. As shown on Plate 2, the estuarine deposits were underlain by bedrock or older alluvium.

Older alluvium (Qoa) was encountered below the estuarine deposits within the west abutment area in soundings CPT-03 and 04. The older alluvium generally consisted of two predominant units (Qoa1 and Qoa2 on Plate 2). The Qoa1 unit consisted of the upper approximately 12 to

14 feet of very dense sand and gravel. The Qoa2 unit was encountered immediately below

Qoa1 and consisted of a 6 to 10 feet of stiff clay and silt. The soundings encountered refusal in apparent bedrock below the older alluvium.

Monterey Formation (Tm on Plate 2) was encountered below the estuarine deposits within the east abutment area in soundings CPT-01 and 04, and in boring 15B-101. The Monterey

Formation consisted of thinly to moderately bedded, slightly fractured, moderately to slightly weathered shale. The shale is also exposed in the streambank just upstream from the east abutment, with shale fragments common along the east bank of the creek. The regional geologic map (see Plate 1) shows an anticline through the site with its axis generally trending along Schooner Creek. As shown on Plate 2, the top of shale was encountered at about 10.5 to

14 feet below the road surface in our explorations for the east abutment. The surface of the

Monterey Formation appears to dip steeply downward to the west, and was encountered at depths of approximately 31.5 to 38.5 feet below the road in soundings CPT-02 and 03. The explorations were terminated in the shale at depths ranging from about 10.5 to 40 feet below the road surface.

Schooner Creek was flowing at the time of our April 2014 and June-July 2015 field exploration programs. The creek is influenced by the ocean and is within a tidal estuary at the bridge location. Groundwater was typically encountered at or near the high tide elevation in our explorations. Groundwater encountered at depths ranging from about 4 to 8 feet below the road surface at the locations explored (see map in Appendix A), with a groundwater elevation near el.

6 feet or lower. Soil moisture and groundwater conditions will vary seasonally and with tidal influences.

The moisture content for samples from 15B-101 were approximately 14 and 25 percent within the fill, 109 percent within the estuarine deposits, and 67 percent within the shale bedrock. The dry unit weights for samples collected from the boring were approximately 86 to 106 pcf for the fill, 38 pcf for the estuarine deposits, and 67 pcf for the shale. The low unit weights of the rock are characteristic of the organic content of the estuarine deposits and diatomaceous1 shale units typical of the Monterey Formation. A bulk sample of estuarine soil from Boring 15B-01 at 9 feet depth had pH of 7.5, and a minimum resistivity of 194 ohm-cm. Soluble chlorides were

16,118 milligrams per kilogram (mg/kg) and soluble sulfates were 1,397 mg/kg or 0.1397 percent.

4.3.4 STATION 499+00 TO 521+00: FLOODED AREA

The subsurface conditions in the flood-prone section between Station 499+00 and Station

521+00 were investigated by drilling Borings R-1 through R-5 in the travel lanes of SFDB. The borings encountered 13 to 20 feet of very loose to medium dense, silty to clayey, gravelly sand with organic soil layers. Sandstone bedrock was encountered in Boring R-2 at a depth of 18 feet, in boring R-3 at 13 feet and in Boring R-4 at 17 feet. Wet soil and groundwater were typically encountered within the upper 5 feet of the borings during drilling. Water was standing on the pavement at the location of boring R-2 (Sta. 507+50) at the time of drilling.

Results of laboratory tests on selected samples had moisture contents that ranged from 16 to

42 percent. The dry unit weight of the samples tested ranged from approximately 81 to 100 pcf.

The higher moisture contents and lower densities are likely due to organic materials in the soil samples. A bulk sample of subgrade soil from Boring R-1 at 0 to 5 feet depth had pH of 5.5,

0.027 percent water soluble sulfates, 0.007 percent water soluble chlorides and resistivity of

3,475 ohm-cm.

4.3.5 STATION 529+00: BOX CULVERT

The subsurface conditions at the proposed box culvert location on East Schooner Creek, near

Station 529+00, were evaluated by drilling Boring 15B-102 and advancing soundings CPT-05 and -06. The explorations encountered approximately 17 to 19 feet of alluvium that consisted of loose silty sand and soft clay with interbedded layers of each. The silty sand was more prevalent in 15B-102 drilled adjacent to the upstream end of the existing culvert. The soft soils were underlain by approximately 5 feet of very dense gravel at 19 feet in 15B-102. Bedrock was encountered below the alluvium at approximately 24 feet below the surface of the road.

1 “diatomite” or “diatomaceous” rock refers to sedimentary rock that is composed of the skeletal and fossilized remains of single celled aquatic algae plants called diatoms.

The bedrock consisted of soft, thinly bedded, moderately to slightly weathered shale of the

Monterey Formation. The explorations were terminated in the shale at depths of up to about

30.5 feet below the road surface.

East Schooner Creek was flowing at the time of our June-July 2015 field exploration program, with the streambed located about 4 to 5 feet from the pavement. Groundwater was typically encountered near the stream level, approximately 3 to 4 feet below the pavement on SFDB.

Results of laboratory tests on selected samples of the alluvium had moisture contents that ranged from 19 to 36 percent. The dry unit weight of a sample tested from above the water table was approximately 95 pcf. The dry unit weight of samples taken from below the water table was approximately 56 and 63 pcf. The higher moisture contents and lower densities are likely due to organic materials in the soil samples. A bulk sample of subgrade soil from Boring

15B-102 had pH of 7.0 and resistivity of 11,450 ohm-cm.

5. GEOTECHNICAL RECOMMENDATIONS

The proposed widening, structures and realignments have site-specific geologic and subsurface conditions that require geotechnical recommendations for design to be provided for each location. Recommendations for retaining structures, foundations and grading are discussed below for each of the proposed improvements.

5.1 GENERAL EARTHWORK

5.1.1 COMPACTION

Earth material encountered along the alignment generally consists of existing embankment fill, soil, and soft rock that will likely consist of soil-like material when excavated and reused as compacted fill for embankment construction. Compacted fill or embankment construction, or associated backfill, should be processed and compacted to at least 95 percent relative compaction per FP-03, Subsection 204.11.

5.1.2 EARTHWORK FACTORS

The subgrade encountered along Sir Francis Drake Boulevard generally consists of loose sandy or soft soils or cut in areas of shallow bedrock. It is recognized that disturbance to drive samples occurs due to compaction of samples during driving, loosening of soils when recovering the sampler from the drill hole and during sample trimming in the laboratory.

Differences in the soil density occur in areas that have been previously graded (the existing road subgrade), rodent borrows (particularly within the upper 2 feet of a site), and inherent differences in soil type and consistency.

A 50 percent loss in volume should be used for the upper 1 foot of ground in fallow and non-pavement areas to account for losses due to grubbing, rodent burrows, and compaction of fill beyond the limits of the existing roadway. Where on-site soil will be excavated and replaced with compacted fill, an average earthwork factor of 0.90 should be used for areas where the fill or subgrade will be placed to at least 95 percent relative compaction. Within existing cut areas, where rock is exposed along the cut, an earthwork factor of 0.97 should be used where the fill or subgrade will be compacted to at least 95 percent relative compaction. The earthwork factors are approximate, and should be checked and adjusted as-needed as the grading work progresses to properly balance earthwork operations.

5.1.3 EXCAVATION AND REUSE OF ONSITE MATERIALS

Formational materials exposed on slopes and road cuts generally consisted of soft to moderately hard sandstone, siltstone, mudstone and shale; except for slopes near the northern terminus of the project, beyond about Station 590+00, that were composed of weathered granitic rocks. Excavation of the soils and bedrock materials encountered are expected to be able to be ripped and excavated using conventional earthmoving equipment. Soil and bedrock materials derived from the cuts can be used as embankment fill. As-excavated, rock may contain large blocks and oversized material that may need to be broken down, screened or separated from the material to make it suitable for fill placement and compaction.

Soils excavated from near or below the groundwater table, such as near the estuary at

Schooner Creek and within the flooded areas (~Stations 488+50 to 533+00), may be wet and unsuitable for compaction as-excavated. If the soil is otherwise suitable for use as fill, the material can be spread and aerated to help dry the material to a moisture content suitable for compaction. However, the site is commonly subject to coastal fog, even during warm summer months, which can make drying soil slow or difficult to achieve. The use of chemical stabilizers, such as lime or cement (if permissible onsite), could be used to help accelerate drying.

The excavated soil and rock should not be considered suitable for reuse as select material such as, aggregate base course, structure backfill, or pipe bedding materials. There may be zones of sandstone or other material that may provide limited quantities of select material; however, those material should not be used unless additional testing is provided during construction to evaluate whether or not the gradation and quality of the material is suitable for the area where it would be placed.

5.2 SLOPE DESIGN - GENERAL

Cut and fill slopes should generally be graded to slopes of 1V:2H or flatter, unless otherwise recommended in the subsequent sections of this report. Graded slopes will be vulnerable to erosion. Topsoil, erosion control matting (if needed) and appropriate vegetation should be placed on graded slopes to reduce the potential for erosion. Maintenance and repairs to gullied or eroded areas should be performed promptly while vegetation is being established.

Concentrated flows of runoff should not be allowed to run over slopes. Where needed, overside drains, curbing, and other measures should be provided to control drainage and carry water to the base of slopes. Energy dissipation devices should be provided at the outlet of pipes and along ditches where concentrated flows of runoff may cause erosion.

5.3 STATION 45+46 TO 70+00: FULL RECONSTRUCTION

Grading for the proposed reconstruction of SFDB from approximately Stations 45+46 to 70+00 will include widening of the existing cuts to a maximum height of about 15 feet, raising and lowering the profile by up to 5 feet, and realignment of portions of the road. This area generally consists of a thin mantle of dune sand, residual soil or intensely weathered rock over Purisima siltstone and mudstone. The rock dips northeast to northwest between about 5 and 20 degrees as exposed at the site and mapped by Clark and Brabb (1997). Embankment slopes within this reach should be graded to 1V:2H or flatter. Cut slopes should be graded to 1V:1.5H or flatter.

5.4 STATION 101+00 TO 112+00: FULL RECONSTRUCTION

The roadway within these limits consists of westbound side cuts and eastbound side fills above a descending slope of about 20 to 25 percent grade. Widening options for this reach initially considered a left-side cut into the existing rock slope, a right-side widening of the embankment slope using a retaining wall, or a two sided widening with shallow left-side cuts and shallow right-side fills. The current design is to widen and realign the roadway by cutting into the rock slope along the left side of the alignment and subexcavation of the roadbed to a depth of 4 feet.

Cut Slope Design. The existing rock slope appears to have been cut to an inclination of about

1V:1H and to a height of up to about 20 feet. A left side ditch provides about 3 to 4 feet of catchment along the base of the slope between about Stations 101+00to 103+00. A slide appears to have occurred near Station 103+00 and there is little to no catchment beyond that point. The new rock cut should be sloped at 1V:1H or flatter with at least 3 feet of catchment beyond the edge of pavement. The new cut slope should be reviewed during construction after the vegetation has been removed. Slope rounding can be provided along top of the cut to reduce the inclination of any topsoil or weathered rock along the top of the cut.

Subexcavation. We understand from discussion with Jacobs Engineering that the outside edge of the roadway has been previously patched by the County. Borings drilled along the eastbound shoulder within this reach (W3 and W4) encountered a firm clay fill that appears to be loosely compacted. Settlement and creep of the existing embankment slope appears to have occurred within this reach as a result compression of the fill and downslope creep.

Additional distress to the edge of the roadway appears to occur as a result of off tracking vehicles entering and leaving the traveled way near Station 100+00. Subexcavation to a depth of 5 feet, or 4 feet below the roadway structural section, should be provided to improve the stability and support for the new roadway between Sta. 103+50 and 109+00. The planned realignment of the road into the cut side of the road will move the new road away from the area where the fill is impacting the existing pavement. Geogrid included in the current detail for the subexcavation with reinforced soil should conform to FP-03, Subsection 720.04 geogrid having a long term tensile strength of at least 1,000 lbs per foot.

5.5 STATION 378+89 TO 387+75, 382+50 TO 391+85: CATTLE UNDERCROSSINGS

Reinforced concrete box culverts (CBCs) will be provided as cattle undercrossings at Stations

378+89 (CBC378) and 390+42 (CBC390). The crossings will include full width reconstruction of the road from Stations 375+50 to 387+75 and 382+50 to 391+85. The culverts will be single barrel 12-foot by 8-foot standard boxes designed using Caltrans Standard Plan (SP D83). The standard plans require a minimum overfill height of at least 1 foot, and show a soil pressure of

3.7 kips per square foot including an overfill height of 10 feet and traffic live load. The new culverts will be approximately 48 feet long, extend outside the limits of the existing embankments, be embedded about 2 to 3 feet below existing grade at the ends of the culvert, within an overfill height of about 2 feet. Standard wingwall retaining walls (SP D84) will be provided at each end of the culverts. The walls will retain up to approximately 10 feet of soil at

CBC378 and 12 feet of soil at CBC390. Two (2) feet of soil fill will be placed along the invert of the box.

Site Preparation and Grading. The soils encountered near the proposed bottoms of the both culverts generally consisted of loose to medium dense silty and clayey sand. To provide uniform support for the culverts, bedding and backfill for the culverts should be provided per

Method 3 of the Caltrans Standard Plan Sheet A62G for a fill height of 2 feet or less. The bedding should be compacted to at least 95 percent relative compaction to provide a working mat for construction of the culvert.

Foundation Design. Box culverts for the undercrossings can be designed using Caltrans

(2010) Standard Plans D83 and D84 for the box culvert and wing walls. Bearing capacity and settlement were evaluated for the standard loading conditions and footing dimensions provided.

An estimated footing embedment of at least 2 feet and a resistance factor of 0.45 was used to calculate the factored bearing resistance in accordance with the 2014 AASHTO LRFD-7 Bridge

Design Specifications. The soil conditions encountered are considered suitable for the culverts and standard loading conditions. The estimated settlement for the culverts and walls is about

0.75 inches total and 0.5 inches differential across the culverts. The foundation support soil conditions are not considered vulnerable to liquefaction or seismic settlement. A summary of the standard loading conditions assumed for the crossings is presented in Table 1 below.

Table 1: Caltrans Standard Plan Conditions used in Analysis

Structure

Key Structure

Dimensions

(feet)

Soil Pressure

(ksf) Estimated Settlement

Caltrans

Standard Plan Comment

CBC378

CBC390

8’H x 12’ S Overfill <3’

3.70 ½” total

¼” differential D83A --

Wingwalls to

CBC378.

H < 10’ D = 2’ min.

3.69 ½” total

¼” differential D83B.

Designed with drained backfill conditions and 36 pcf, equivalent fluid pressure

Wingwalls to

CBC390

H < 12’ D = 2’ min.

3.89 ½” total

¼” differential D83B.

Designed with drained backfill conditions and 36 pcf, equivalent fluid pressure

H = inside of height of box culvert or retain height above footing of wall. S = inside span of culvert. D = recommended minimum footing embedment below lowest adjacent grade. Overfill = the depth of cover over the top of the culvert.

Drainage and Backfill Wingwalls. For use with Caltrans Standard Plan, Sheet D84, the wing walls should be backfilled with free-draining granular fill with positive drainage per the detail and

Standard Specifications. A vertical drain should be placed immediately behind the wall in accordance with details shown on Standard Plan Sheet D84. The structure backfill and drainage material should be terminated 2 feet below the top of the wall to allow for placement of native topsoil or embankment fill to reduce the potential for water to infiltrate directly into the backfill.

5.6 STATION 488+50 TO 493+75: SCHOONER CREEK ARCH CULVERT

A precast concrete arch will provide the new crossing at Schooner Creek near Station 490+80.

Schooner Creek flows southerly through the site toward Drake’s Bay and is surrounded by a tidal estuary. The following recommendations were prepared for use with the 2014 AASHTO

LRFD-7 Bridge Design Specifications, Caltrans amendments to those specifications, and the

Jacobs (2015) plans. A design alternative is being evaluated to replace the crossing with a clear-span slab-type bridge. The bridge alternative would have a different foundation layout and cutoff elevation. If the slab type structure is selected for design, the pile depths, recommendations and construction considerations should be updated and revised for the new design,

5.6.1 GEOTECHNICAL PROPERTIES

Plate 2 presents a summary of the subsurface profile used in our geotechnical analyses for the

Schooner Creek crossing. Geotechnical engineering properties were assigned to each unit within the profile for use in evaluating the axial and lateral load capacity of pile foundations. The following tables summarizes engineering parameters assigned to the layers shown on Plate 2.

Soil unit weights and shear strength parameters have been estimated based on measured SPT blow counts, CPT results, and laboratory testing of selected samples.

Table 2: Geotechnical Properties for Schooner Creek Abutment 1

Abutment 1 (west)

Geologic Unit

Elevations top/bottom Material Description

Unit Weight

(pcf)

Undrained Shear

Strength (psf)

Friction

Angle 50

(in/in) ks

(pci)

Af

el. 10 ft.

el. 5 ft.

Stiff Sandy CLAY and dense Clayey SAND (fill)

120 -- 37 -- 60

Qe

el. 5 ft.

el. – 2 ft.

Soft Elastic SILT (estuarine deposits) 90 500 -- 0.03 --

Qoa1

el. -2 ft.

el. -15 ft.

Very dense Sand and gravel (older alluvium)

125 -- 45 -- 125

Qoa2

el. -15 ft.

el. -28 ft.

Stiff CLAY (older alluvium)

110 1500 -- 0.007 500

Tm

el. -28 ft.

Soft Mudstone (stiff clay)1

(Monterey Formation) 70 5,000 -- 0.006 2000

1. Based on engineering properties the mudstone is modeled in ENSOFT L-Pile program as stiff clay in water

Table 3: Geotechnical Properties for Schooner Creek Abutment 2

Abutment 2 (east)

Geologic Unit

Elevations top/bottom Material Description

Unit Weight

(pcf)

Undrained Shear

Strength (psf)

Friction

Angle 50

(in/in) ks

(pci)

Af

el. 10 ft.

el. 5 ft.

Clayey SAND and GRAVEL (fill)

120 -- 37 -- 60

Qe

el. 5 ft.

el. – 1 ft.

Soft Elastic SILT

(estuarine deposits) 90 500 -- 0.03 --

Tm

el. -1 ft.

el. -5 ft.

Intensely weathered mudstone “stiff clay”

(Monterey Formation) 90 1500 -- 0.006 500

Tm

el. -5 ft.

Soft Mudstone “stiff clay”

(Monterey Formation) 70 5,000 -- 0.006 2000

5.6.2 SEISMIC DATA

A design acceleration response spectrum (ARS) curve for the site was estimated using ARS

Online. As input to ARS Online, a shear wave velocity for the site was estimated using correlations to CPT data. An average shear wave velocity of approximately 276 meters/sec was estimated for the top 100 feet of soil at the site, which corresponds to a Soil Profile Type D, stiff soil and soft rock classification. Deterministic and probabilistic spectra were estimated using

ARS Online analysis for 5 percent damping. The ARS curve for the probabilistic event was adjusted using the 2008 USGS Interactive Deaggregation calculator for a 5 percent probability of exceedance in 50 years. The design ARS curve is controlled only by the probabilistic spectrum for an approximately M7.6 earthquake, likely occurring on the San Andreas fault, having an estimated peak ground acceleration of 0.58g.

Liquefaction. Liquefaction is the loss of soil strength due to an increase in soil pore-water pressure resulting from seismic ground shaking. Liquefaction typically occurs in loose to medium dense granular soil that is below the groundwater table. The extent and severity of

Figure 2: Recommended Design ARS Curve liquefaction is dependent upon the intensity and duration of the strong ground motion.