Drakes_Beach-Schooner_Creek_Geotechnical_Report.pdf

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Geotechnical Report Sir Francis Drake Boulevard Improvements

Drake’s Beach Wetland Restoration and Parking Lot Schooner Creek Bridge

Point Reyes National Seashore Marin County, California

CA FLAP CR109 (1)

Yeh Project No.: 216-028

April 17, 2017

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

Geotechnical Report Sir Francis Drake Boulevard Improvements

Drake’s Beach Wetland Restoration and Parking Lot Schooner Creek Bridge

Point Reyes National Seashore Marin County, California

CA FLAP CR109 (1)

Yeh Project No.: 216-028

April 17, 2017

Prepared by:

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

Jamie L. Cravens Independent Reviewer:

Staff Engineer

Robert F. LaForce Judd J. King, GE 2903 Senior Materials Manager Senior Project Engineer

Geotechnical Report Project No. 216-028 Drakes Beach Wetland Restoration – Schooner Creek Bridge April 17, 2017 ii

Table of Contents

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

2.1.1 DRAKES BEACH WETLAND RESTORATION

2.1.2 RANCH D OVERFLOW PARKING LOT

2.1.3 SCHOONER CREEK BRIDGE

3. SUBSURFACE INVESTIGATION AND TESTING

3.1 DIRECT PUSH SAMPLING AND WELL INSTALLATIONS

3.2 HOLLOW STEM AUGER DRILLING

3.3 LABORATORY TESTING

4. SITE CONDITIONS

4.1 REGIONAL GEOLOGICAL SETTING

4.2 CLIMATE

4.3 SUBSURFACE CONDITIONS

4.3.1 DRAKE’S BEACH WETLAND RESTORATION AND PARKING LOT

4.3.2 RANCH D OVERFLOW PARKING LOT

4.3.3 STATION 490+80: SCHOONER CREEK BRIDGE

5. GEOTECHNICAL CONSIDERATIONS AND RECOMMENDATIONS

5.1 WETLAND RESTORATION AT DRAKES BEACH

5.2 PAVEMENT DESIGN AT DRAKES BEACH

5.2.1 SUBGRADE PREPARATION AND GRADING

5.2.2 SURFACE DRAINAGE

5.2.3 DESIGN TRAFFIC VOLUME

5.2.4 STRUCTURAL SECTION

5.3 SCHOONER CREEK BRIDGE, STATION 488+50 TO 493+75

5.3.1 GEOTECHNICAL PROPERTIES

5.3.2 SEISMIC DATA

5.3.3 SCOUR CONSIDERATIONS

iii

5.3.4 BRIDGE FOUNDATION DESIGN

5.3.5 FOUNDATION EXCAVATION

5.4 REINFORCED SOIL SLOPE - WEST APPROACH

5.4.1 GRADED SLOPES

5.4.2 SLOPE STABILITY

5.4.3 RSS DESIGN

5.5 CORROSION

5.5.1 CULVERTS

5.5.2 REINFORCED CONCRETE SUBSTRUCTURES

6. NOTES TO CONSTRUCTION

7. LIMITATIONS

8. REFERENCES

List of Figures

FIGURE 1 : SITE LOCATION MAP

FIGURE 2: SPRINGS ALONG DRAKES BEACH ACCESS (APRIL 6, 2016 PHOTO)

FIGURE 3: RECOMMENDED DESIGN ARS CURVE

FIGURE 4: AXIAL RESISTANCE OF 30-INCH DIAMETER DRILLED SHAFT

FIGURE 5: REINFORCED SOIL SLOPE

List of Tables

TABLE 1: SUMMARY OF SELECTED GEOTECHNICAL PROPERTIES – DRAKES BEACH

TABLE 2: BASELINE MONITORING WELL READINGS – DRAKES BEACH

TABLE 3: SUMMARY OF SELECTED GEOTECHNICAL PROPERTIES – OVERFLOW PARKING LOT

TABLE 4: SUMMARY OF SELECTED GEOTECHNICAL PROPERTIES – SCHOONER CREEK

TABLE 5: STRUCTURAL SECTION THICKNESS

TABLE 6: GEOTECHNICAL PROPERTIES FOR SCHOONER CREEK ABUTMENT 1

TABLE 7: GEOTECHNICAL PROPERTIES FOR SCHOONER CREEK ABUTMENT 2

TABLE 8: LATERAL EARTH PRESSURES

TABLE 9: DESIGN LIFE FOR METAL PIPES

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List of Plates

No

Site Plan

Regional Geologic Map

Field Exploration Plan – Drakes Beach Parking Lot

Field Exploration Plan – Overflow Parking Lot

Schooner Creek Bridge Plan and Profile

LPile Output – Pushover Analysis

List of Appendices

Page

Appendix A – Boring Logs

Monitoring Well Details ................................................................................................................ A-1

Boring Record Legend ........................................................................................................... A-2 to 3

Boring Logs .......................................................................................................................... A-4 to 23

Drilling Photos .................................................................................................................. A-24 to 27

Appendix B – Cone Penetration Test (CPT) Sounding Logs

Logs of CPT Soundings ...........................................................................................................B-1 to 6

Appendix C –Laboratory Test Results

Summary of Laboratory Test Results ..................................................................................... C-1 to 2

Atterberg Limits .................................................................................................................. C-3 and 4

Percent Passing No. 200 Sieve ...................................................................................................... C-5

Mechanical Sieve Analysis ..................................................................................................... C-6 to 8 pH and Resistivity .................................................................................................................C-9 to 10

Soluble sulfates and chlorides (Cooper) ......................................................................... C-11 and 12

Proctor Compaction .................................................................................................................... C-13

Unconsolidated Undrained Triaxial Test (Cal Poly) ............................................................C-14 to 19

Consolidated Undrained Triaxial Test ................................................................................C-20 to 23

Incremental Consolidation Test (Cal Poly) ...................................................................... C-24 and 25

Consolidated (CRS) Test (Cal Poly) .................................................................................. C-26 and 27

R-value Test Results (NV5) .................................................................................................C-28 to 31

Appendix D – EDR Historic Aerial Photograph Decade Package (15 pages)

Appendix E – Pavement Calculations (9 pages)

Appendix F – Slide Output (4 pages)

Appendix G – Retaining Wall Recommendations, East Approach

Recommendations ................................................................................................................ G-1 to 2

Plates:

Schooner Creek Retaining Wall – Existing Condition ...................................................... G-1

Schooner Creek Retaining Wall – Lightweight Fill .......................................................... G-2

Schooner Creek Retaining Wall – MSE Wall ................................................................... G-3

1. PURPOSE AND SCOPE OF STUDY

The project is located at the Point

Reyes National Seashore in Marin

County, California. The location of the site relative to Sir Francis

Drake Boulevard (SFDB) and local landmarks is shown on Figure 1.

Yeh and Associates, Inc., as a subconsultant to Jacobs

Engineering Group in Denver, Colorado was retained by the

Federal Highway Administration

(FHWA), Central Federal Lands

Highway Division (CFL) to provide geotechnical services for the design of improvements to restore wetlands and make parking lot improvements at Drake’s Beach, to provide an overflow parking lot for Drake’s Beach at Ranch D, and to replace the crossing of Sir Francis Drake Boulevard over Schooner Creek with a new bridge The enclosed Plate 1 shows the layout of the site and components of the project addressed in this report at the Drake’s Beach Parking Lot, Ranch D and at the crossing of Schooner Creek. A regional geologic map for the site vicinity is shown on

Plate 2. This report presents a summary of the field and laboratory testing program, a description of the subsurface conditions encountered, details of the monitoring wells that were constructed, and geotechnical recommendations for design of the proposed parking lot improvements, foundation recommendations for the new bridge and retaining wall, and for the design of new pavements at Drake’s Beach.

2. PROJECT UNDERSTANDING

Improvements addressed in this report are in addition to the design of 12-miles of roadway and drainage improvements to Sir Francis Drake Boulevard at the Point Reyes National Seashore in

Marin County, California. Yeh (2015a, 2015b) previously submitted a Geotechnical Report and a

Pavement Engineering Report for the design of those improvements. The following sections describe the improvements addressed in this report.

Figure 1 : Site Location Map

2.1.1 DRAKES BEACH WETLAND RESTORATION

Plate 3 shows the layout of the Drakes Beach Parking Lot site and layout of the improvements.

Drakes Beach is on easterly side of Point Reyes and the westerly shoreline of Drakes Bay. The

Drakes Beach Access Road is paved with asphalt and runs about 1.5 miles southeast from Sir

Francis Drake Boulevard to the beach. The access road ends at a parking lot and visitor center about 3 miles north of Point Reyes where the mouth of two converging southeast-northwest trending drainages open onto the beach (see Plates 1 and 2). The parking lot is located immediately adjacent to the beach and occupies an area of approximately 5 acres. The beach access and a smaller parking lot were originally constructed by the National Park Service around 1965. The parking lot was subsequently expanded and the wetland was filled in to construct the larger lot sometime after 1965. The sides of the drainage, beyond the limits of the parking lot and visitor center, are composed of grass covered slopes that face north and south on either side of the drainage. The slopes rise to approximately 250 feet above sea level. An unnamed lake is located west of the parking lot and at the convergence of the two arms of the drainage.

The improvements at Drakes Beach will consist of the following:

Demolition and removal of the parking lot to restore about 2 acres of wetlands on southerly half of the existing lot;

Reconstruction of the parking lot and entrance to the north, including a bus loop and turn around, a wide shoulder for a shuttle stop; and

Associated grading, paving, landscaping and surface drainage improvements.

2.1.2 RANCH D OVERFLOW PARKING LOT

Plate 4 shows the layout of the overflow parking lot improvements at Ranch D. The proposed overflow parking lot will be located on the north side of the access road to Drake’s Beach, and about 1 mile southeast of Sir Francis Drake Boulevard. The site is within a pasture on the north side of the existing ranch buildings. The ground surface within the site is at about elevation 245 feet and slopes down to the southwest at grades of approximately 3 to 4 percent. A narrow 2-to 3-foot deep drainage borders the northeast side of the parking lot (see topography on

Plate 4). The new lot will be about 120 feet wide and 250 feet long providing about 115 spaces for overflow parking. The new parking lot will have an aggregate surface and grade similar to existing site topography.

2.1.3 SCHOONER CREEK BRIDGE

Plate 5 shows an interpreted subsurface profile and layout for the new Schooner Creek Bridge on Sir Francis Drake Boulevard. The new bridge will be located on Sir Francis Drake Boulevard at station 490+80. The existing crossing consists of two 84-inch bitumen coated corrugated metal culverts that will be replaced with a new clear-span bridge. The previous design to replace the crossing with a precast concrete arch structure was addressed in the Yeh (2015a)

Geotechnical Report. The current project will consist of replacing the crossing with a new clear-span, slab-type bridge. The project also included the design of an approximately 200-foot long retaining wall along the upstream side of the east approach to the bridge (addressed in the Yeh

(2016) memorandum). Jacobs is proposing a realignment for the roadway within the vicinity of the crossing to help limit grade changes and eliminate the need for the retaining wall, and the need for potential mitigation of soft ground conditions encountered along the proposed wall alignment during this phase of work.

The crossing is within a tidal estuary. The marsh is at approximately elevation 5 feet. The approaches to the existing crossing are on embankment with a finish grade of elevation 9 feet on the approaches and about el. 11 feet at the existing crossing. Schooner Creek flows generally south through the existing culverts, but has tidal influences that result in stream flow in both directions. The invert of the creek at the existing crossing is about elevation 2 to 3 feet.

The new structure will be a clear span of approximately 53.5 feet and width of approximately 36 feet. The new bridge will have cantilever abutments seated at about el. 6 feet, about 6 feet below the surface of Sir Francis Drake Boulevard at the existing crossing. Wing walls will likely be provided to help support the end slopes at the bridge, and will likely cantilever from the abutments.

3. SUBSURFACE INVESTIGATION AND TESTING

The field exploration for the overall project has consisted of three phases of work: the initial April

2014 program included drilling 40 borings for both the geotechnical and pavement evaluation, a

June-July 2015 program for a supplemental geotechnical evaluation for the design of the

Schooner Creek crossing (an arch culvert and a box culvert replacement) that included drilling and cone penetration testing, and the most recent May 2016 program that included direct push sampling, installation of monitoring wells and pavement borings at Drakes Beach, and drilling for the slab bridge and proposed retaining walls at Schooner Creek. Memoranda describing the proposed field exploration program and summary of the field investigation were submitted by

Yeh (2016a, 2016b) prior to and following the program.

The previous logs for pavement borings (14P-01 to 26) were presented in Pavement

Investigation Report (Yeh 2015b) and Geotechnical Report (Yeh 2015a). Memoranda describing the current field exploration program was submitted by Yeh (2016). The previous data collected for the Schooner Creek bridge site is included with the new field data and laboratory test data in this report. Appendix A includes the logs of the borings and wells and photos of the boring locations for this geotechnical evaluation. Logs of the previous CPT soundings performed at Schooner Creek are presented in Appendix B. Laboratory test results for the current study are presented in Appendix C.

3.1 DIRECT PUSH SAMPLING AND WELL INSTALLATIONS

The direct push sampling and well installations at the Drakes Beach parking lot were coordinated and shared with Far Western Anthropological Research Group, Inc. of Davis, California. The wells were planned to be installed using a hollow stem auger, but were changed to direct push because Far Western needed to check the locations for cultural resources in association with the well installations. An archeologist from Far Western was onsite to log the borings for cultural resources with Yeh.

Cascade Drilling, LP of Richmond, California performed the direct push services using a

Geoprobe 6600 on a truck platform on May 3 and 4, 2016. The Geoprobe was used to take 2-inch continuous core samples using direct push and 3-foot runs at seven locations (locations

16MW-01 to 16MW-07 on Plate 3). The probes were advanced to depths ranging from 17 to 21 feet. Far Western retained the core samples for their evaluation. Yeh collected grab samples at selected intervals from the core as noted on the logs in Appendix A.

Cascade then used the same rig to install pre-packaged 2-inch (inside diameter) monitoring wells at each location. The well at 16MW-08 was installed to a depth of 5 feet by Yeh using a hand auger because of its remote location. Well construction details showing the key depths for the installations, screen lengths, seals, and riser heights are presented on Plate A-1 in Appendix

A. The well heads were secured with a traffic-rated box near the parking lot and with a metal riser or locking cap at the MW-07 and 08 locations. The approximate locations of the monitoring wells installed at the Drakes Beach parking lot are shown on Plate 3.

3.2 HOLLOW STEM AUGER DRILLING

Cascade Drilling, LP of Richmond, California was also the subcontractor for the drilling program.

Cascade remobilized to the site with a different crew equipped with a CME-85 truck-mounted drill rig equipped with 8-inch hollow stem augers to advance the pavement borings at Drakes

Beach and the borings at the Schooner Creek Bridge site on May 5 and 6, 2016. Traffic control and flagging for the borings at Schooner Creek was provided by Statewide Traffic Safety and

Signs, Inc. of Fairfield, California. Five pavement borings were drilled at Drakes Beach to depths of 6.5 to 11.5 feet. Three geotechnical borings were advanced at the Schooner Creek Bridge site to depths of 41 to 51 feet. The approximate locations of the borings at the Schooner Creek

Bridge are shown on Plate 5. The logs of the borings are presented in Appendix A. Photos of the boring locations are presented with the logs in Appendix A.

Yeh measured the thickness of the existing pavement and any base course materials, classified, labeled and secured the samples collected for subsequent laboratory testing, recorded blow counts (N-values) for drive samples, and prepared logs of subsurface conditions encountered. The borings were sampled by collecting bulk samples of subgrade materials from the auger flights, by driving 2-inch standard penetration test (SPT) and 3-inch modified

California split spoon samplers at selected intervals during drilling, and by pushing thin-walled tube samples at various intervals during drilling.

The modified California sampler contained 1-inch high brass liners. The SPT sampler was used without liners. The samplers were driven into the ground at select intervals using a 140-pound automatic trip hammer in accordance with ASTM 1586, the Standard Penetration Test.

Thin-wall samples consisted of pushing a 30-inch long steel “shelby” tube into the ground, typically about 24 inches, using the rig’s hydraulics for down pressure. The tubes had a 3-inch outside diameter, a 2-7/8-inch inside diameter, with a mandrel cutting head to reduce friction on the sample while the tube was pushed into the soil at the bottom of the hole.

The deeper borings were grouted or converted to monitoring wells in accordance with Marin

County well permit requirements. Monitoring well details are included in Appendix A. Shallower pavement borings were filled with approved native fill, and patched with rapid set concrete where drilled in pavement areas.

3.3 LABORATORY TESTING

Laboratory testing was performed on selected samples recovered from the field exploration program. Tests for moisture content, unit weight, Atterberg limits, percent passing the 200 sieve, particle size distribution by mechanical sieve analysis, pH and resistivity, compaction relationships between dry unit weight and moisture content using the modified Proctor test, consolidation, consolidated undrained triaxial compressive strength, and R-Value were performed as part of this program. Tests for classification and resistivity were performed at the

Yeh office in Grover Beach, California. Strength and consolidation tests were performed by the

GEO-E lab at the California Polytechnic State University, Civil Engineering Department in San

Luis Obispo, California. BTC Labs of Ventura, California performed R-value Testing. The results of laboratory tests are presented in Appendix C. Testing was performed accordance with applicable procedures of American Association of State Highway and Transportation

Officials (AASHTO) or ASTM. The soil descriptions were confirmed or modified as necessary on the boring logs based on laboratory test results and further review of selected samples in the laboratory. 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).

4. SITE CONDITIONS

4.1 REGIONAL 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 northwest-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 sand, and Pleistocene terrace deposits and dune sand.

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

The geology was previously described in the Yeh (2015a, 2015b) reports. The local geology in areas explored during the current study are summarized below.

The Drakes Beach Parking Lot is within a former wetland and is underlain by artificial fill (Af) materials over intermixed alluvium and estuarine deposits (Qal). The alluvium and estuarine deposits are generally sediments deposited along the drainages that pass through the site, and organic soils and sediment deposited within the previous estuary and stillwater. Beach sand

(Qs) is deposited along the shoreline of Drakes Bay. The east end of the parking lot and entrance are cut into the adjacent hillsides. The rock in this area predominantly consists of claystone and siltstone of the Purisima Formation (Tp). Thin layers of terrace deposits (Qt) and dune sand (Qd) are deposited above the Purisima Formation near the overflow parking lot. The bridge site at Schooner Creek is mapped as estuarine deposits and alluvium (Qal) overlying clay shale of the Monterey Formation (Tm)

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 inches, most of which occurred in the winter months (December through February). The average annual high temperature is 57o F and average annual low temperature is 48o 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 Drake’s Beach wetland restoration and parking lot, the Ranch D overflow parking lot, and Schooner Creek bridge sites are discussed in the sections below. Logs of the borings, CPT soundings and laboratory test results are presented in Appendices A, B and C. The approximate locations of the explorations are presented on Plates 3, 4 and 5.

4.3.1 DRAKE’S BEACH WETLAND RESTORATION AND PARKING LOT

Borings 16MW-01 through 08 were drilled at various locations within the Drake’s Beach parking lot to depths of up to 21 feet. Additionally, borings 16P01a, 16P-01b, as well as 16P-02 were drilled in existing pavement areas at the entrance and north end of the Drake’s Beach parking lot to depths up to 11.5 feet. Existing fill materials (unit Af) were encountered in each of these explorations. The existing fill is likely associated with the construction of the parking lot, access road and the filled over previous wetland. Layers of estuarine deposits (unit Qe), older alluvium

(units Qoa1), and claystone of the Purisima Formation (unit Tp) were encountered beneath the existing fill and pavements. Table 1 provides a description of the predominant geologic units encountered in the borings and a summary of selected geotechnical properties obtained from laboratory test results.

Table 1: Summary of Selected Geotechnical Properties – Drakes Beach

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

(%) R-value

% Gravel (G), sand (S) and fines)

Atterberg Limits

(LL, PI)

Af

16P-01a, 01b and 02

Existing pavement:

16P-01a: 2” asphalt over 4“ of clayey gravel base over clay subgrade

16P-01b: 2” asphalt over 4” aggregate base over clay subgrade

16P-02: 4” asphalt over 7” aggregate base over claystone subgrade

82-97 27-45

11 (CL)

15 (CL)

11 (CL)

0-7 G 23-58 S 42-69 F

43 - 58 LL

18 – 36 PI

16MW-01 to

16P-01a to 02

The existing fill consisted of existing pavement material overlying loose silty, clayey sand with gravel (SC-SM), silty sand (SM) and clayey sand (SC), and stiff clay (CL and CH) with varying amounts of sand and/or gravel. The fill thickness ranged from 2 to 14 feet in borings, with the thickness of fill in the area of the proposed wetland restoration ranging from approximately 9 to 14 feet. This unit was encountered throughout the Drake’s Beach parking lot site.

Qe

16MW-01, 02,

04, 05, 07

Estuarine deposits that consisted of an approximately 3-foot thick layer of soft peat (PT) and soft clay with interspersed fibrous organic material were encountered below the existing fill material in the parking lot.

The estuarine deposits were encountered at 9 to 11 feet below the ground surface in the parking lot, and at 5 feet below the ground surface in MW-07 advanced west of the parking lot.

60-100

Peat:

128 – 185 LL

25 - 38 PI

Clay:

49 - 56 LL

21 27 PI

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

(%) R-value

% Gravel (G), sand (S) and fines)

Atterberg Limits

(LL, PI)

Qa1 16MW-01 to

Alluvium that consisted of an approximately 5 to 8-foot thick layer of dense poorly graded sand (SP), clayey sand (SC), and/or poorly graded sand with silt (SP- SM) was encountered immediately below estuarine deposits or fill. This unit appears present at the southeast portion of the Drake’s Beach parking lot, closest to the beach.

-- 19-55 --

55-97 3-45

Tp 16P-01a 16P-02

Purisima Formation that consist of soft claystone was encountered at the subgrade elevation at the northern portion of the Drake’s Beach parking lot and its entrance. The unit underlies the upper unit of existing fill, and is exposed along the hillsides that border the entrance to the parking lot.

80-99 25-36 --

46 LL

20 PI

Groundwater levels encountered in the monitoring well borings are summarized in Table 2. Details for the wells are provided in Appendix A. Jacobs performed the monitoring for the wells after the initial installation. Wet soil conditions and groundwater was encountered in the borings at depths ranging from 3 to 15.5 feet below the ground surface. The predominant gradient is likely from the lake elevation west of the parking lot to the wetting front on the Drakes Beach. The groundwater below the existing parking lot may be confined by the clayey fill materials placed over the previous estuary. Groundwater levels recorded in the wells typically rose through moist soils above the groundwater level initially encountered in the borings.

Table 2: Baseline Monitoring Well Readings – Drakes Beach

Well No.

(MW16-#)

Date (2016)

Depth to Groundwater

(feet below ground surface)

Total Depth of Well

(feet below ground surface) Comments

May 3 15.0 -- Free water level encountered in geoprobe.

May 5 15.0 18.4

Pressure when cap removed. Left cap loose to relieve air pressure in well. Water level may continue to rise.

Traffic box. Wrench needed to open. May 6 12.5

May 3 15 -- Free water level in geoprobe.

May 5 5.7 13.5 Traffic box. Wrench needed to open.

May 3 10.5 -- Free water level in geoprobe.

May 5 8.2 10.0 Traffic box. Wrench needed to open.

May 3 14.5 -- Free water level in geoprobe.

May 5 6.0 16.5 Traffic box. Wrench needed to open.

Well No.

(MW16-#)

Date (2016)

Depth to Groundwater

(feet below ground surface)

Total Depth of Well

(feet below ground surface) Comments

May 3 10.5 -- Free water level in geoprobe.

May 5 6.0 10.0 Traffic box. Wrench needed to open.

May 3 7.0 -- Free water level in geoprobe.

May 5 6.0 13.0 Traffic box. Wrench needed to open.

May 3 2.5 -- Free water level in geoprobe.

May 5 5.0 8.0 Locking cap. Combination = 391

May 3 3.5 -- Free water level in hand auger.

May 5 3.9 5.0 Locking cap. Combination = 391

4.3.2 RANCH D OVERFLOW PARKING LOT

Borings 16P-03 and 16P-04 were drilled at the Ranch D overflow parking lot site to depths of up to 11.5 feet. The locations of the field explorations are shown on Plate 4. Topsoil and dune sand

(unit Qd) were encountered overlying soft sandstone, siltstone and claystone of the Purisima

Formation (unit Tp). Table 3 provides a description of the predominant geologic units encountered and a summary of selected geotechnical properties obtained from laboratory test results.

Table 3: Summary of Selected Geotechnical Properties – Overflow Parking Lot

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

(%) R-value

% Gravel (G), sand (S) and fines) Atterberg

Limits

(LL, PI)

Qd 16P-03 16P-04

A 1 to 3-foot layer of dune sand was encountered in the borings overlying the bedrock in the borings. The upper 12 to 18 inches of the dune consisted of a topsoil layer with roots and varying amounts of organics. The dune sand consisted of silty to poorly graded sand (SM, SP).

1 G 80 S 19 F

Tp 16P-03 16P-04

The Purisima Formation consisted of soft, slightly weathered to decomposed claystone and sandstone with an interbedded 5-foot thick layer of soft, moderately weathered siltstone encountered beneath the dune sand. The decomposed sandstone had weathered to clayey SAND (SC) as encountered in boring 16P-04.

63-103 22-61 11

43 LL

21 PI

Wet soil conditions were encountered in boring 16P-04 at a depth of 8 feet below the ground surface. Groundwater was not encountered during the exploration of boring 16P-03. Groundwater likely perches on the surface of the bedrock and within it, depending on weather conditions and runoff.

4.3.3 STATION 490+80: SCHOONER CREEK BRIDGE

Boring 15B-101 and cone penetration test soundings CPT-1 through CPT-4 were advanced at the Schooner Creek bridge location during the June-July 2015 field exploration program.

Borings 16B-02, 16W-03 and 16W-04 were drilled at the bridge site and along Sir Francis Drake

Boulevard to about 200 feet east of the Schooner Creek bridge location. The boring locations, and an interpreted subsurface profile summarizing the conditions encountered at Schooner

Creek, are presented on Plate 5. Borings 15B-101, 16B-02, 16W-03, and 16W-04 were drilled to a maximum depth of 31 feet, 41 feet, 41.5 feet, and 50.5 feet, respectively. The CPT soundings were advanced to depths of up to 40 feet. The predominant units encountered consisted of existing fill materials (Af), estuarine deposits (Qe), older alluvium (Qoa) and

Monterey Formation (Tm). Table 4 provides a description of the predominant geologic units encountered and a summary of selected geotechnical properties obtained from laboratory test results.

Table 4: Summary of Selected Geotechnical Properties – Schooner Creek

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

% Gravel (G), sand (S) and fines) Atterberg

Limits

(LL, PI)

Af

15B-101 16B-02 16W-03 16W-04

CPT-01 to 04

The existing fill consisted of existing pavement materials overlying medium dense to dense clayey gravel (GC) with varying amounts of sand and lean clay (CL). The existing fill appears to be imported material placed along the approaches in association with the construction of the existing road, pull-out parking area and culverts. The fill thicknesses encountered varied from approximately 4 to 6 feet.

57-111 14-61 --

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

% Gravel (G), sand (S) and fines) Atterberg

Limits

(LL, PI)

Qe

15B-101 16B-02 16W-03 16W-04

CPT-01 to 04

The estuarine deposits are exposed along the creek and at the surface of the Schooner Bay estuary up and downstream of the site. The units consisted of approximately 3 to 46 feet layer of very soft to soft clay (CL and CH) with interbedded layers of elastic silt (MH), peat (PT), sand, and interspersed fibrous organic material.

This unit was encountered immediately underlying the existing artificial fill and appears have been encountered, though variable in thickness, at all the subsurface exploration locations. As shown on Plate 5, the thickness of the estuarine deposits that was encountered increased from about 5 feet at the bridge to about 46 feet at the east end of the former wall.

32-89

33-141

72 - 96 LL

26 -56 PI

Qoa1

16B-01 16W-04

CPT-02 and

Older alluvium was encountered immediately below the estuarine deposits.

The upper unit of the older alluvium consisted of approximately 14 to 15 feet of medium dense to very dense poorly graded sand with clay (SP-SC) and poorly graded sand (SP). This unit appears present near the west abutment area and along the eastern portion of the proposed retaining wall location along Sir Francis Drake Boulevard. The unit was encountered above the lower unit of the older alluvium (Qoa2) or the Monterey Formation as shown on Plate 5.

103-111 17-21 0 G

90-93 S 7-10 F

Qoa2 16B-02

CPT-02 and

The lower unit of the older alluvium was encountered beneath the west approach to the proposed bridge. The unit consisted of approximately 6.5-foot thick layer of stiff clay (CL) and clayey sand (SC) encountered beneath the upper unit of the older alluvium (Qoa1).

88 32 --

Geologic Unit

Encountered in Exploration

Numbers: Description

Dry Unit Weight

(pcf)

Moisture Content

% Gravel (G), sand (S) and fines) Atterberg

Limits

(LL, PI)

Tm

15B-101 16B-02 16W-03

CPT-01 to 03

Monterey Formation was encountered beneath all other units where the explorations penetrated below the older alluvium. The formation consisted of clay shale composed of soft, slightly to moderately weathered, thinly to moderately bedded claystone and beds of hard, slightly fractured mudstone. The depth to the formation ranged from approximately 11 feet below the surface of Sir Francis Drake Boulevard to up to 39 feet at near the west abutment of the proposed crossing. The formation was not encountered to the maximum depth explored, 51.5 feet, in 16W-04 drilled about 200 feet east of the existing crossing. The bedrock unit outcrops on the east bank of Schooner Creek just upstream from the proposed east abutment. The regional geologic map (see Plate 2) shows an anticline through the site with its axis generally trending along Schooner Creek that may be associated with variable depth to bedrock that was encountered. The low unit weights of the rock are characteristic of the diatomaceous1 shale units typical of the Monterey Formation.

67-77 29-41 --

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 tides and is within a tidal estuary at the bridge location. Groundwater encountered at depths ranging from about 4 to 8 feet below the road surface at the locations explored (see map Plate 5), near the high tide elevation, with a groundwater elevation near elevation 6 feet or lower. Soil moisture and groundwater conditions will vary seasonally and with tidal influences.

5. GEOTECHNICAL CONSIDERATIONS AND 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.

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.

5.1 WETLAND RESTORATION AT DRAKES BEACH

Borings and monitoring wells were completed at the Drakes Beach parking lot. The project will involve removal of the existing parking lot and associated landfill to restore wetlands in the southerly end of the existing parking lot. The improvements should be designed to the minimum standards of FP-03.

5.2 PAVEMENT DESIGN AT DRAKES BEACH

Pavement and material recommendations for the Sir Francis Drake Boulevard project were previously submitted in the Yeh (2015) Pavement Evaluation Report. The following recommendations are limited to providing a structural section for the design of the Drakes

Beach Parking lot improvements and the overflow lot at Ranch D. The minimum design thicknesses are recommended based on pavement design standards provided in Chapter 11 of the CFLHD Project Development and Design Manual (PDDM) and the AASHTO (1993) Guide for the Design of Pavement Structures. The parking lot will be designed with new hot mix asphalt (HMA) pavement over aggregate base with Portland cement concrete pavements provided for a bus turnout by the visitor’s center. The overflow parking lot at Ranch D will have an aggregate surface. The spreadsheet calculations for our recommended traffic loading and thickness recommendations are presented in Appendix E.

5.2.1 SUBGRADE PREPARATION AND GRADING

Springs and wet subgrade conditions could be encountered near the entrance to the Drakes

Beach parking lot and bus loop. Springs daylight along the base of the west-facing hillside and edge of the existing pavement near the parking lot entrance (see Figure 2).

The subgrade conditions encountered in the proposed bus loop consisted of massive claystone. Water from the springs appears to run along the existing access road and pond on the pavement surface. Where relatively dry claystone subgrade conditions are encountered, the base course material should typically be placed on the undisturbed subgrade. If the subgrade becomes wet or yielding during compaction, the subgrade should be subexcavated to a depth of 12 inches and replaced with 12

Figure 2: Springs along Drakes Beach Access (April 6, 2016 photo) inches of crushed aggregate (per Section 703.06 of FP-03). For design, it should be assumed that 50% of the pavement area within the bus loop will require subexcavation and stabilization with the option to increase, decrease or delete quantity. The gravel should be fully encased in geotextile (per Section 714.01, Type II-B, of FP-03).

5.2.2 SURFACE DRAINAGE

Water has ponded along the main bus loop entering the Drake’s Beach parking area. The source of the water appears to be from springs located along the base of the west-facing slope that borders the southbound shoulder entering the bus loop. The springs appear to result from groundwater flowing within sandy units of the Purisima Formation that is perched above massive claystone that was encountered just below the pavement in our borings. Water from the springs flows out of the slope and onto to the existing pavement. Drainage along the southbound shoulder should drain to a drop inlet and storm drain system for the parking lot. The drop inlet was full of water and is likely plugged with sediment. The water therefore overflows and ponds on the pavement.

Surface water also appears to pond on the pavement surface in the northeast area of the Drake

Beach parking lot and along the bus loop (as evidenced by sediment stains on the pavement surface). Wet subgrade conditions below the pavement appear to have resulted in cracking and potholing of the existing pavement in these areas.

Surface drainage should be improved to help remove surface water from the pavement surface and convey water from the springs to a suitable outlet. Drainage can be improved by collecting water along the base of the slope within a lined ditch or pipe and conveying that water to suitable outlet or improved storm drain inlet. The existing drain inlets or storm drain system within the existing parking lot should either be improved to allow surface water to drain from the pavement or be replaced with a new drainage system that will allow water to drain from the pavement surface.

5.2.3 DESIGN TRAFFIC VOLUME

The design average daily traffic (ADT) into the Drake’s Beach parking lot was estimated using twice the number of spaces proposed for the parking lot improvements or approximately 800 vehicles per day. The vehicle distributions were estimated using the same percentages as were presented in the Pavement Engineering Report for Sir Francis Drake Boulevard improvements

(Yeh 2015b). The traffic mix for Sir Francis Drake Boulevard is 80.5 percent passenger cars, 16 percent pickup trucks, 0.5 percent buses and 3 percent campers, trash trucks and sweepers.

The buses and half of the larger vehicles were excluded from the mix for the parking spaces outside of the bus loop. The minimum number of 50,000 18-kip equivalent single axle loads

(ESALs) for the Drake’s Beach parking lot and 10,000 ESALs for the aggregate surfaced

Drake’s beach parking lot were not applied and only the calculated number of ESALs for those areas were applied. The ESALs for the main drive and bus pullout were estimated at 77,831 considering a design life of 20 years. More aggressive load equivalent factors were used to estimate the design ESALs for the bus pullout that resulted in 383,136 for a 30-year design life.

Calculation of ESAL loads are presented in Appendix E.

5.2.4 STRUCTURAL SECTION

The subgrade soils from the Drake’s Beach Parking Lot and overflow lot were A-7 clayey soils that are considered to provide poor subgrade support for the new pavements. R-value test results were 11, 15 and 11 for subgrade samples recovered from the borings. The recommended structural section thicknesses were estimated considering a resilient modulus

(Mr) of 3,681 psi for a clay subgrade having an R-value of 11. The Portland cement concrete section for the bus pullout is based on the minimum thickness presented in the PDDM and is likely to provide at least a 50-year design life. The recommended structural section thicknesses are presented in Table 5.

Table 5: Structural Section Thickness

Location Surface Course

(inches)

Base Course

(inches)

Required Structural Number

Design Structural Number

Drake’s Beach Parking - Bus Loop

4” HMA 8” FDR 2.66 2.72

4” HMA 7” ABC 2.66 2.74

Drake’s Beach Parking Lot

3” HMA 6” FDR 2.04 2.04

3” HMA 6” ABC 2.04 2.16

5” HMA -- 2.04 2.20

Overflow Lot 9” ABC -- -- --

Bus Pullout 8” PCC 4” ABC -- --

HMA = hot mix asphalt ABC = aggregate base course FDR = full depth reclamation/pulverized base course

5.2.4.1 FULL DEPTH RECLAMATION MATERIAL

FDR will be reclaimed for reuse within the new structural section by pulverizing the existing asphalt pavement, harvesting and stockpiling the material (if needed), and then placing the material as base course for the new structural section for the new parking lot. The thickness of the existing pavement encountered in the borings was approximately 2 inches of asphalt pavement over 4 inches of nonstandard base course in the parking lot area, and 4 inches of asphalt pavement over 7 inches of aggregate base along the bus loop entering the parking lot.

Pulverizing should extend to at least 1 inch below the existing pavement. A typical depth of 7 inches of pulverization in the parking lot and 12 inches within the bus loop can be used for design.

5.2.4.2 AGGREGATE SURFACED PARKING LOT

The aggregate thickness for the overflow parking lot is based on the FHWA (2010) guidelines for aggregate surfaced roads corresponding to low traffic flow, poor subgrade conditions and Climate

Zone I. Periodic maintenance of the aggregate surfaces should be provided to repair wash-board or eroded surfaces and maintain adequate surface drainage. The crown of the aggregate surface road should be graded to a typical cross slope of 4 percent to help minimize erosion and wash boarding and at least 2 percent in the overflow parking area. A dust palliative can be used to help reduce dust and the need for watering for dust control. The palliative should consist of lignosulfonate conforming to FP-03 Section 306.04b.

5.3 SCHOONER CREEK BRIDGE, STATION 488+50 TO 493+75

The new crossing at Schooner Creek (Station 490+80) will be a clear span slab type bridge with seated abutments. Schooner Creek flows southerly through the existing crossing 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 (2016b) plans. The Yeh (2015a) report provided recommendations for the design of an alternative reinforced concrete arch structure.

The following recommendations are for the design of the slab bridge.

5.3.1 GEOTECHNICAL PROPERTIES

Plate 5 presents a summary of the subsurface profile used in our geotechnical analyses for the

Schooner Creek crossing. Table 6 summarizes geotechnical engineering properties assigned to each unit within the profile for use in evaluating the axial and lateral load capacity of pile foundations. 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 6: 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 1,500 -- 0.007 500

Tm

el. -28 ft.

Soft Mudstone (stiff clay)1

(Monterey Formation) 70 8,000 -- 0.006 2000

1. The mudstone should modeled in L-Pile as “stiff clay in water”

Table 7: 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 1,500 -- 0.006 500

Tm

el. -5 ft.

Soft Mudstone “stiff clay”

(Monterey Formation) 70 8,000 -- 0.006 2000

5.3.2 SEISMIC DATA

The design acceleration response spectrum (ARS) curve presented in Figure 3 was estimated using Caltrans’ ARS Online website. 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 per second was estimated for the top 100 feet of 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, an approximately M7.6 earthquake likely occurring on the San

Andreas fault, and resulting in an estimated peak ground acceleration of 0.58g at the site.

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 liquefaction is dependent upon the intensity and duration of the strong ground motion. Soils encountered in the bridge foundation areas generally consisted of a shallow layer of soft estuarine deposits over dense sand and weathered rock. The soils are cohesive or dense such that they are not considered vulnerable to significant liquefaction or seismic settlement, and no special recommendations to address liquefaction or seismic settlement are recommended for design.

5.3.3 SCOUR CONSIDERATIONS

The stream hydraulics and scour depths for the project were estimated by Jacobs during the previous phase of work (Yeh 2015a). As input to their analysis, Yeh provided the following characterization of the subsurface profile.

Figure 3: Recommended Design ARS Curve

The estimated mean grain size (D50) of the active stream channel deposits within

Schooner Creek is approximately 0.1 millimeter.