Report_Geotechnical.pdf

VETERANS DRIVE IMPROVEMENTS GEOTECHNICAL REPORT

PHASE 1A - 100% PLAN SUBMITTAL

ST. THOMAS, U.S. VIRGIN ISLANDS

Submitted to:

USVI DEPARTMENT OF PUBLIC WORKS

Submitted by:

PARSONS BRINCKERHOFF, INC.

November 22, 2016

Phase 1A - 100% Plan Submittal USVI Department of Public Works Veterans Drive Improvements Geotechnical Report November 22, 2016 i

TABLE OF CONTENTS

1.0 INTRODUCTION --------------------------------------------------------------------------------------- 1

1.1 General ------------------------------------------------------------------------------------------------ 1

1.2 Project Description and Scope of Work -------------------------------------------------------- 1

2.0 SUBSURFACE INVESTIGATION PROGRAM ------------------------------------------------- 2

2.1 Field Investigation ----------------------------------------------------------------------------------- 2

2.2 Laboratory Testing ---------------------------------------------------------------------------------- 5

2.2.1 Index Property Tests ------------------------------------------------------------------------- 6

2.2.2 Consolidation Tests --------------------------------------------------------------------------- 6

2.2.3 Strength Tests --------------------------------------------------------------------------------- 6

3.0 GEOLOGY AND SUBSURFACE CONDITIONS----------------------------------------------- 7

3.1 Regional and Local Geology --------------------------------------------------------------------- 7

3.2 Subsurface Conditions ----------------------------------------------------------------------------- 7

3.2.1 Stratum 1: Fill ---------------------------------------------------------------------------------- 8

3.2.2 Stratum 2: Coastal Sediments ------------------------------------------------------------- 8

3.2.3 Stratum 3: Alluvium --------------------------------------------------------------------------- 8

3.2.4 Stratum 4: Bedrock --------------------------------------------------------------------------- 9

3.2.5 Groundwater ----------------------------------------------------------------------------------- 9

3.3 Soil and Rock Properties -------------------------------------------------------------------------- 9

3.3.1 Stratum 1: Fill -------------------------------------------------------------------------------- 10

3.3.2 Stratum 2: Coastal Sediments ----------------------------------------------------------- 10

3.3.3 Stratum 3: Alluvium ------------------------------------------------------------------------- 11

3.3.4 Stratum 4: Bedrock ------------------------------------------------------------------------- 12

4.0 DESIGN CONSIDERATIONS AND RECOMMENDATIONS ------------------------------ 12

4.1 General ---------------------------------------------------------------------------------------------- 12

4.2 Seismic Considerations -------------------------------------------------------------------------- 13

4.2.1 Design Acceleration ------------------------------------------------------------------------ 13

4.2.2 Site Effects ------------------------------------------------------------------------------------ 13

4.2.3 Seismic Performance Zone --------------------------------------------------------------- 14

4.2.4 Liquefaction ----------------------------------------------------------------------------------- 14

4.3 Retaining Walls ------------------------------------------------------------------------------------ 15

4.3.1 Overall Stability ------------------------------------------------------------------------------ 16

4.3.2 Settlement and Bearing Resistance ---------------------------------------------------- 18

4.3.3 Retaining Wall Recommendations ------------------------------------------------------ 21

4.4 Earthwork ------------------------------------------------------------------------------------------- 23

4.4.1 Pavement Subgrade Preparation-------------------------------------------------------- 24

5.0 REFERENCES --------------------------------------------------------------------------------------- 25 ii

LIST OF TABLES

Table 1 - Boring Summary ---------------------------------------------------------------------------------- 4

Table 2 - Summary of Laboratory Testing for Phase 1A ------------------------------------------ 5

Table 3 - Recommended Design Soil and Rock Parameters ------------------------------------ 9

Table 4 - Results of Slope Stability Analysis ------------------------------------------------------- 17

Table 5 - Summary of Settlement and Factored Bearing Resistance for the Wall ------ 19

Table 6 - Summary of Settlement for the Embankment ----------------------------------------- 20

Table 7 – Recommended Earth Pressure Coefficients ------------------------------------------ 22

Table 8 - Recommended Gradation Requirements for Select Granular Backfill -------- 23

Table 9 - Recommended Gradation Requirements for Gravel Backfill --------------------- 24

LIST OF APPENDICES

APPENDIX A

Figure 1 Site Location Map

Figures 2 through 5 Boring Location Plan

Figures 6 through 9 Subsurface Profiles

Figure 10 Typical Wall Section

APPENDIX B

Phase 1A Boring Logs

Soil and Rock Classification Guidelines

APPENDIX C

Laboratory Test Data

APPENDIX D

Additional Boring Logs (from areas outside of Phase 1A)

APPENDIX E

Global Stability Analysis Results

Veterans Drive Improvements Geotechnical Report November 22, 2016

1.0 INTRODUCTION

1.1 General

Parsons Brinckerhoff, Inc. has been retained by the Government of the United States Virgin Islands Department of Public Works (DPW) under the professional services contract PC087DPW11 to undertake engineering design and environmental studies for the widening of Veterans Drive along the waterfront of Charlotte Amalie on the island of St. Thomas in the US Virgin Islands (USVI). The project location is shown in Figure 1 in Appendix A. The project is divided into three segments (Phase 1A, Phase 1B and Phase 2). This geotechnical report presents the findings and includes a general discussion of the subsurface conditions and geotechnical considerations for Phase 1A only based on the project’s subsurface investigation program and laboratory testing program.

1.2 Project Description and Scope of Work

The Phase 1A portion of Veterans Drive is currently an urban, two-lane, undivided facility from about 300 feet west of Hospital Gade to approximately 400 feet west of the W.G.

Lewis/Long Bay Road intersection. Existing sidewalks, although not continuous, are located on both sides of Veterans Drive and a seawall is located on the south side of the roadway bordering St. Thomas Harbor in some sections of the project. The Phase 1A portion of the Veterans Drive Improvements Project will include reconstruction to add new lanes for a continuous four-lane roadway consisting of two lanes in each direction for a distance of 2,260.39 feet (0.37 mi). It begins at Station 236+74 and ends at Station 259+34.39.

The proposed typical section will include four 11 foot lanes with 2-to-4-foot shoulder gutters (typically on the high side of the road), where required, and an unpaved 2-foot shoulder on the low side of the road, where possible. A 10 to 15 foot wide sidewalk will be constructed between the roadway and the harbor along most of the alignment. Asphalt pavement will be completely reconstructed where the existing conditions have deteriorated. Other segments, where it is determined that the existing pavement is in good condition, will be milled, resurfaced and widened, as required. The new pavement structure will be designed for the minimum depth necessary to carry the projected loads over the design life of the pavement.

The widening into St. Thomas Harbor will require removal of existing shore protection rip-rap, installation of new retaining walls, filling behind the new retaining walls, construction of a widened roadway, and associated miscellaneous structures (signage and lighting).

No previous geotechnical investigations were performed for the project, and no existing geotechnical information was available for the project site.

The specific scope of work for Phase 1A performed by Parsons Brinckerhoff consisted of the following:

• Development of a field exploration program based on the selected roadway alignment and other proposed improvements.

Veterans Drive Improvements Geotechnical Report November 22, 2016 o The field investigation program was performed in two phases. The initial phase was performed in 2006 and included land borings and offshore borings.

A supplemental investigation phase of offshore borings was performed in 2015.

o This work included the preparation of subcontract documents for drilling and laboratory testing services, technical specifications for soil borings and sampling, rock coring and boring location plans.

• Full-time field inspection for soil borings and the necessary coordination services to ensure proper execution of the field work by the subcontractor for the initial phase of the field investigation.

• Development of a laboratory test program and coordination with the testing laboratory.

• Preparation of typed boring logs for the initial phase of the investigation. The drilling subcontractor prepared boring logs for the supplemental investigation phase.

• Preparation of this geotechnical report to describe the geotechnical conditions at the project site; provide recommended geotechnical design parameters; discuss geotechnical design methods and provide design recommendations; and discuss potential construction issues.

• Performance of geotechnical design and analysis for the roadway and other elements of the project.

Note that additional borings were performed under Parsons Brinckerhoff’s supervision outside of the Phase 1A project limits as part of the scope for other phases of this project.

Although not directly part of the scope for Phase 1A, some of these borings have been used and included for reference with regard to the broader geotechnical conditions within the project area.

2.0 SUBSURFACE INVESTIGATION PROGRAM

2.1 Field Investigation

The geotechnical field investigation for Phase 1A of the Veterans Drive Improvement project consisted of drilling 25 borings along the selected roadway alignment and other associated improvements. The land boring investigation program for Phase 1A was performed from August 10 through 17, 2006, and it consisted of 11 borings designated as PBL-5 through PBL-15. Boring depths for the land borings ranged from 3 to 75.5 feet with a total length of 309 feet drilled. The off-shore boring investigation program for Phase 1A was performed from September 21 through October 3, 2006, and it consisted of 14 borings designated as PBW-9 through PBW-22. Ten additional off-shore borings were performed from September 8 through September 13, 2015 to complete the field exploration of this phase; those borings were designated as PBW-42 through PBW-51. Boring depths for the off-shore borings ranged from 10 to 61.5 feet below the mudline with a total length of 719.8 feet drilled. A summary of the borings performed for Phase 1A of the Veterans Drive Improvement project is presented in Table 1. Detailed boring logs are presented in Appendix B. The as-drilled boring locations are shown on Figures 2 through 5. Additional borings performed within the general project area but outside of the limits of Phase 1A are included in Appendix D.

Although these borings are outside of the Phase 1A limits, the results of these borings may

Veterans Drive Improvements Geotechnical Report November 22, 2016 be relevant of the larger project area and subsurface conditions and have therefore been included for reference.

The subsurface exploration services and the barge equipment were provided by Jaca & Sierra Testing Laboratories, Inc. of San Juan, Puerto Rico, under subcontract to Parsons Brinckerhoff. The initial investigation borings (PBL 5 – 15 and PBW 9 – 22) were performed under direct, full-time field supervision of Parsons Brinckerhoff’s on-site geotechnical staff.

The land borings were advanced with a trailer-mounted Mobile B-53 drill rig utilizing hollow-stem auger techniques (2.25-inch inside diameter hollow stem auger). The off-shore borings were advanced with a barge-mounted Mobile B-53 drill rig utilizing rotary wash techniques. Where necessary, 3-inch inside diameter casing was advanced to maintain the borehole walls.

Standard Penetration Testing (SPT) was performed in general accordance with the requirements of ASTM D-1586 using a 2-inch outside diameter (OD) split spoon sampler driven by a 140-pound manual, safety-hammer falling 30 inches. The number of hammer blows necessary to produce 18 inches of sampler penetration were recorded in 6 inch increments and written on test boring logs. The number of hammer blows required to drive the sample for 12 inches after seating it six inches is defined as the SPT “N-value”. In all borings, 18-inch and 24-inch SPT split spoon samples were obtained from the ground surface or mudline at a maximum of 5-foot intervals. Typically for the land borings, continuous SPT split spoon sampling was performed to a depth of 12 feet below the ground surface or until refusal (defined as 50 blows per 6 inches or less) was encountered.

Where possible, relatively undisturbed soil samples were attempted in cohesive clay and silt deposits using 3-inch OD Shelby tubes in general accordance with ASTM D1557. The sampler was generally advanced for 24 inches; an adequate Shelby tube samples could be retrieved from borings PBW-42 and PBW-52.

Each sample was examined during drilling in the field, and the samples were visually classified according to the classification guidelines of the Burmister soil classification system (Burmister, 1951). Group symbols were determined for each split spoon sample based on the Unified Soil Classification System (USCS, ASTM Standard D2487). The definition of the terms and classification format for the Burmister soil classification system, USCS, and other information presented on the boring logs, is included in Appendix B. Representative soil specimens from each SPT sample were retained and preserved in air-tight glass jars during the investigation.

When rock was encountered, rock coring was performed using a NQ wireline core barrel system following ASTM D2113. Mud or other drilling polymers were not added to the drilling water during coring. The retrieved cores were placed into standard, wooden core boxes.

As-drilled boring locations for the initial phase of investigation were surveyed by Brian Moseley & Associates, Inc. of St. Thomas, USVI. The as-drilled land boring locations were surveyed on September 26, 2006. Off-shore borings were marked with buoys upon completion of each boring, and the buoy locations were surveyed on October 20, 2006 and September 1, 2015. In some instances, the buoys were missing at the time of locating due to either natural (wind, tide, etc.) or man-made influences. In cases where the buoys were missing, the proposed boring locations were assumed to be as-drilled locations.

Coordinates for the borings are based on the North American Datum Adjustment of 1983 (NAD83, US Survey Feet). Elevations for the land borings are referenced to the Mean

Veterans Drive Improvements Geotechnical Report November 22, 2016

Lower Low Water (MLLW) Tidal Datum. The mudline elevation for the off-shore borings are referenced to the depth below the MLLW Datum. Boring locations for the second phase of the investigation were surveyed in a similar manner.

Table 1 - Boring Summary

Boring Designation

Station (feet) Offset (feet)

Surface / Mudline

Elevation (feet)

Depth of Boring *

(feet)

Depth to Bedrock *

(feet)

PBL-5 236+68.8 12.9 RT 2.3 30.0 24.0

PBL-6 239+54.5 107.2 LT 5.0 75.5 74.0

PBL-7 239+81.4 3.1 RT 3.2 3.0 N/A

PBL-8 244+09.8 37.7 LT 4.8 20.5 N/A

PBL-9 245+74.2 53.4 LT 5.4 55.5 44.0

PBL-10 249+10.9 59.0 LT 5.5 17.5 6.0

PBL-11 252+59.0 1.3 LT 4.1 13.0 7.0

PBL-12 255+09.9 60.7 LT 7.0 6.0 1.0

PBL-13 257+32.7 44.6 LT 7.0 8.0 3.0

PBL-14 261+25.3 11.0 LT 6.7 49.5 N/A

PBL-15 261+84.6 76.3 LT 10.6 31.0 26.0

PBW-9 236+30.3 97.6 RT -5.0 40.0 29.0

PBW-10 238+34.1 77.4 RT -5.0 52.0 42.0

PBW-42 239+50.4 59.2 RT -5.0 25.0 N/A

PBW-11 240+51.7 57.0 RT -5.0 50.5 N/A

PBW-12 241+57.5 58.8 RT -4.7 50.5 N/A

PBW-43 242+53.4 55.2 RT -5.0 30.0 N/A

PBW-44 243+68.6 55.2 RT -5.0 31.0 N/A

PBW-13 244+08.1 106.9 RT -5.0 61.5 N/A

PBW-14 244+90.9 95.9 RT -6.5 60.5 N/A

PBW-45 245+99.2 99.8 RT -7.0 29.0 N/A

PBW-15 247+22.9 90.8 RT -7.0 55.3 50.0

PBW-46 247+95.3 55.2 RT -6.0 21.0 N/A

PBW-16 248+73.2 62.4 RT -7.0 31.0 22.5

PBW-17 250+03.3 41.0 RT -8.0 30.5 15.5

PBW-47 250+86.6 55.3 RT -7.0 15.0 14.0

PBW-18 251+33.3 80.0 RT -8.0 19.0 9.0

PBW-48 252+24.3 86.2 RT -10.0 10.0 4.0

Veterans Drive Improvements Geotechnical Report November 22, 2016

Boring Designation

Station (feet) Offset (feet)

Surface / Mudline

Elevation (feet)

Depth of Boring *

(feet)

Depth to Bedrock *

(feet)

PBW-49 253+28.1 86.5 RT -10.0 10.0 4.0

PBW-19 254+45.0 69.2 RT -11.0 12.5 2.5

PBW-50 255+24.8 55.3 RT -12.0 10.0 4.0

PBW-20 255+89.2 58.8 RT -11.0 11.0 1.0

PBW-51 256+74.7 55.3 RT -13.0 14.0 9.0

PBW-21 257+65.1 55.3 RT -10.0 14.0 4.0

PBW-22 259+09.4 80.4 RT -11.0 21.0 5.5

* Depth of water borings are measured from mudline.

2.2 Laboratory Testing

A series of laboratory tests, aimed at defining the geotechnical properties to be used for final design, was planned by Parsons Brinckerhoff and subsequently performed by Jaca & Sierra Testing Laboratories, Inc. The testing program on the soil samples included natural water content, Atterberg Limits, grain size analyses, organic content determinations, one-dimensional consolidation testing and unconsolidated-undrained (UU) triaxial tests.

Unconfined compression (UC) tests were performed on rock core samples. Results from the testing on soil and rock samples obtained from the land and off-shore borings are presented within this Geotechnical Report. Table 2 shows the number of laboratory tests performed and the applicable ASTM standards for the Phase 1A portion of the Veterans Drive Improvements project. Laboratory testing data is provided in Appendix C.

Due to work being performed several years ago for the entire Veterans Drive Improvements project (Phases 1A, 2 and 3), some laboratory testing results for soil samples and rock cores from borings performed outside the limits of Phase 1 are included in Appendix C along with the pertinent data.

Table 2 - Summary of Laboratory Testing for Phase 1A

Test Number of Tests Performed Applicable ASTM

Standard Land Borings Off-Shore Borings Natural Water Content 8 33 D2216

Atterberg Limits 5 19 D4318 Sieve Analyses 22 29 D422

Hydrometer Analyses 8 0 D422 Organic Content 7 0 D2974

1-D Consolidation 2 2 D2435 UU Triaxial 2 0 D2850

Unconfined Compression (rock) 18 2 D7012

A brief summary of testing procedures is provided in the following sections.

Veterans Drive Improvements Geotechnical Report November 22, 2016

2.2.1 Index Property Tests

Index property tests are generally carried out to classify the soil according to its engineering properties. Index property tests included natural moisture content, unit weight, sieve and hydrometer analyses, and Atterberg Limits.

• Natural Moisture Content: The natural moisture content is defined as the ratio of the weight of moisture present to dry weight of soil, expressed as a percentage. The natural moisture content has proved valuable in estimating engineering properties of the soil via several empirical relationships.

• Atterberg Limits: Atterberg Limits measure the Plastic Limit (PL) and the Liquid Limit (LL) of the tested soil, which are defined as the moisture content at which a cohesive soil changes from a semi-solid state to a plastic state and from a plastic state to a liquid state, respectively. The Plasticity Index (PI) is the difference between the Liquid Limit and Plastic Limit.

• Sieve and Hydrometer Analyses (grain size): The sieve analysis measures the percentage of weight of a dry soil sample passing a series of sieves, including the percent passing the No. 200 sieve, which is known as the fines (silt and clay) content.

The hydrometer test subsequently allows for the determination of grain size distribution for the portion passing the No. 200 sieve.

• Organic Content Tests: For this test, the soil sample is heated to approximately 440 degrees Centigrade (Method C), thus burning off all organic-type material, leaving only the inorganic soil minerals. The weight of the organics divided by the weight of dry soil is the percentage of organics in the sample.

2.2.2 Consolidation Tests

Consolidation tests were performed on four samples, two undisturbed samples from PBW- 42 and PBW-52 and two disturbed samples from PBL-5, to determine the compressibility properties of soils, including the coefficient of consolidation (cv), the compression index (cc), the recompression index (cr), the coefficient of secondary compression (cα), and maximum past overburden pressure. The specimens were reconstituted from samples recovered from SPT samples to the in-situ stress state prior to consolidation test. For each test, plots of axial strain versus the logarithm of the applied vertical stress were produced.

2.2.3 Strength Tests

• Unconsolidated-Undrained Triaxial Compression Tests (UU or Q tests): Single-point UU triaxial compression tests were performed to evaluate the undrained shear strength of selected disturbed soil samples under triaxial stress conditions, similar to that which would be expected under in-situ field conditions. In this test, the sample is subjected to a confining pressure approximately equal to the effective overburden pressure and then loaded axially at a constant rate of strain until failure.

• Unconfined Compression Tests: Unconfined compression tests were performed on rock core samples obtained from the test borings. In an unconfined compression test, an intact rock core specimen is subjected to an axial load with no confining pressure until failure occurs.

Veterans Drive Improvements Geotechnical Report November 22, 2016

3.0 GEOLOGY AND SUBSURFACE CONDITIONS

3.1 Regional and Local Geology

The island of St. Thomas is an exposed part of a predominantly submarine mountain range that includes the large islands of the Greater Antilles (Cuba, Hispaniola, Puerto Rico) and the southeast curving island chain of the Lesser Antilles, ending in Trinidad, off the coast of Venezuela. The Greater and Lesser Antilles island chain separates the Caribbean Sea from the Atlantic Ocean (McLaughlin, 1976). St. Thomas is a part of the Caribbean Plate and lies approximately 100 miles south of the Puerto Rico Trench, a tectonically formed subduction depression of the ocean floor that reaches depths of 30,000 feet, the deepest known part of the Atlantic Ocean. Volcanic, seismic and tsunami hazards exist throughout the region (Nealon and Dillon, 2001).

The mountainous central core of St. Thomas consists mostly of faulted and folded volcanic, volcaniclastic and sedimentary rocks of the Cretaceous and early Tertiary age. This sequence has subsequently been intruded by dioritic rocks. Sea plants and corals have formed calcareous reef deposits in the tropic zone circumventing the island. These deposits have been periodically modified by explosive volcanism (McLaughlin, 1976). The major geologic units exposed on St. Thomas are: The Louisenhoj Formation, locally known as “blue bit,” consisting mostly of a bluish toned volcanic breccia and tuff; The Tutu Formation, dominantly a tuffaceous conglomerate that contains a few limestone beds; and The Water Island Formation, consisting of extrusive volcanic rocks with minor intrusive dikes and plugs (Olcott, 1999). Hydrogeologic units on St. Thomas are of poor quality and limited to fractured volcaniclastic-, igneous-, and sedimentary-rock aquifers and minor coastal embayment aquifers formed from alluvial and beach deposits at many of the inlets along the perimeter of the island (Renken et al., 2002).

Quaternary deposits on St. Thomas consist of unconsolidated material that accumulated near the mouths of streams where they enter the ocean; beach deposits that surround ocean bays; and extensive alluvial fan, debris fan, and alluvial deposits of clay, silt, sand, and gravel. The stream deposits typically range from less than 30 to 50 feet thick and are found in valleys eroded into the volcanic-rock surface during an earlier period when sea level was lower than at present. The alluvial deposits generally are at an altitude of less than 100 feet above sea level (Olcott, 1999).

3.2 Subsurface Conditions

Generalized subsurface profiles were developed along the roadway alignment and are shown in Figures 6 through 9. Since the majority of the investigations were performed several years ago for the entire Veterans Drive Improvements project (Phases 1 and 2), information and laboratory testing results obtained from borings performed outside the limits of Phase 1A were used to complement the borings information within the Phase 1A limits.

The additional borings can be found in Appendix D. Based on the information obtained from the geotechnical investigation performed at the project site, the site is underlain by four principal soil strata, as discussed below.

Veterans Drive Improvements Geotechnical Report November 22, 2016

3.2.1 Stratum 1: Fill

Data from the geotechnical investigation revealed that existing, non-native, fill materials underlying the ground surface along several portions of the roadway alignment. The depth and consistency of the fill materials encountered across the project site varied greatly.

According to the Unified Soil Classification System USCS, these materials generally classify as SP (poorly graded SAND), SM (Silty SAND), SC (Clayey SAND), ML (SILT), and CL (lean CLAY). Concrete, brick, wood and other construction debris were present in many of the samples. The fill thickness ranged from no fill encountered to a maximum depth of 20 feet below the ground surface. Greater fill thicknesses were typically encountered behind the existing bulkhead structure where bedrock is located deep below the ground surface.

SPT N-values in the fill materials ranged from 1 to 47 blows-per-foot with an average of 13 blows-per-foot. The high blow counts encountered within the fill materials are likely indications of the presence of obstructions or gravel layers, and not necessarily areas of increased density. The recorded N-values throughout the fill are typically greater near the ground surface and above the groundwater table, indicating an increased level of compaction due to activity near the surface and/or a more effective method of placement and compaction of the fill above the groundwater table.

3.2.2 Stratum 2: Coastal Sediments

A deposit of coastal sediments consisting of soft clay and silt with seams or layers of fine sand was encountered at several locations, typically directly beneath Stratum 1. Traces of organic material including roots and peat fibers were encountered in most samples. Little to trace amounts of shells and coral fragments were also encountered within the stratum indicating its marine origin. The clay and silt is generally classified as CL (lean CLAY) according to the USCS. When the clay and silt was encountered, thicknesses of the stratum ranged from 3 to 10 feet.

SPT N-values within the stratum ranged from weight-of-hammer (WOH or zero) to 17 blows-per-foot, indicating the stratum to be very soft to very stiff.

3.2.3 Stratum 3: Alluvium

A variable deposit of alluvial sediments of clay, silt, sand, and gravel were encountered in several locations along the roadway alignment. As described in Section 3.1, alluvial deposits are typically found in valleys eroded into the volcanic-rock surface during an earlier period when sea level was lower than at present. According to the USCS, these materials generally classify as CL (lean CLAY), CH (fat CLAY), SC (clayey SAND), SC-SM (clayey, silty SAND), SM (silty SAND), and GP (poorly graded GRAVEL). Cobbles and several boulders were cored within this stratum during the field exploration program.

Where the alluvial deposits were encountered, thicknesses of the stratum ranged from 5 to over 60 feet. Greater alluvial thicknesses were typically encountered between Stations 236+00 to 247+00 where bedrock is located deep below the ground surface, as shown on Figures 2 through 5. SPT N-values within the stratum ranged from 6 blows-per-foot to refusal, with an average of 52 blow-per-foot, indicating the stratum to be very dense or hard.

Veterans Drive Improvements Geotechnical Report November 22, 2016

3.2.4 Stratum 4: Bedrock

Bedrock encountered across the site was typically classified as Breccia, a volcanic rock composed of large angular fragments encompassed by compacted volcanic ash, dust and sediments. The Breccia is part of the Louisenhoj Formation, locally known as “blue bit,” consisting mostly of a bluish toned volcanic breccia and tuff.

Typically, the upper portion of the bedrock stratum was more weathered and fractured than deeper zones. At times, decomposed bedrock or residual soil would blend with the overlying alluvium and a distinct contact surface could not be identified. Fracture spacing of the cored bedrock ranged from moderately fractured (1 to 4-inch spacing) to sound (greater than 8-inch spacing). Recovery of the rock core samples ranged between 30 and 100 percent, with an average of 82 percent. Rock Quality Designation (RQD) of the core samples ranged between 0 and 100 percent, with an average of 52 percent.

3.2.5 Groundwater

The groundwater observed during the land borings was typically encountered at an elevation of approximately 1 to 2 feet above sea level. The groundwater level throughout the site will likely vary with temperature, precipitation and tidal influence.

3.3 Soil and Rock Properties

The following is a discussion of soil properties developed from the findings of the subsurface investigation program. Table 3 presents a summary of the design parameters for each of the four strata found at the site.

Table 3 - Recommended Design Soil and Rock Parameters

Stratum 1: Fill

• Total Unit Weight, γt = 110 pounds per cubic foot (pcf)

• Buoyant Unit Weight, γb = 48 pcf

• Angle of Internal Friction, φ’ = 28° to 30°

Stratum 2: Coastal Sediments

• Total Unit Weight, γt = 105 to 115 pcf

• Buoyant Unit Weight, γb = 43 to 53 pcf

• Undrained Strength Ratio, Su/p’o = 0.20 to 0.25

• Overconsolidation Ratio, OCR ≈ 1.0

• Compression Index, Cc = 0.23 to 0.27

• Recompression Index, Cr = 0.02

• Coefficient of Vertical Consolidation, cv = 0.3 ft2/day

• Coefficient of Secondary Consolidation, cα = 0.004

Stratum 3: Alluvium

• Total Unit Weight, γt = 120 to 130 pcf

• Buoyant Unit Weight, γb = 63 to 73 pcf

• Undrained Strength, c = 2.5 to 3.5 ksf

• Drained Strength, c’ = 1.0 to 1.5 ksf, φ’ = 25° to 28°

Veterans Drive Improvements Geotechnical Report November 22, 2016

Stratum 4: Bedrock

• Total Unit Weight, γt = 150 to 160 pcf

• Buoyant Unit Weight, γb = 88 to 98 pcf

• Unconfined Compressive Strength, qu = 0.5 to 23 kips per square inch (ksi)

• Average Rock Quality Designation, RQD = 52%

3.3.1 Stratum 1: Fill

As indicated previously, Fill consists of a heterogeneous mixture of sand, silt, clay, and varying amounts of debris including concrete, brick, and wood fragments. Given the variable nature and implied density based on blow counts, Fill is interpreted to be primarily miscellaneous fill placed in a predominantly uncontrolled manner. Based on sample classifications and the results of standard penetration tests, the fill should be considered to be a cohesionless material with an angle of internal friction estimated to be approximately 28 to 30 degrees and a total unit weight of approximately 110 pcf.

3.3.2 Stratum 2: Coastal Sediments

The Coastal Sediments of Stratum 2 consists predominately of very soft organic and inorganic clay and silt to silty clay with layers or pockets of clayey, fine sand. Laboratory data indicate that its Liquid Limit (LL) varies from 32 to 42 with an average of 36, while its Plasticity Index (PI) ranges from 14 to 22 with an average of 18. The natural moisture content of this stratum ranges between 31 and 56 with an average of 38. The total unit weight is estimated to vary from about 95 pcf to 105 pcf. The unit weight will vary primarily based upon the amount of organic content and sand present. Organic contents from tests performed on selected samples from this stratum range from 3.0 percent to 5.2 percent.

Consolidation properties: Consolidation properties of the unconsolidated coastal sediments (soft silt and clays) are one of the primary factors influencing the potential settlement at the site. One-dimensional consolidation tests were performed on four samples from Stratum 2. The results of the tests show an extremely wide range of values, this is most likely due to extreme sample disturbance, the variable nature of the soft silts and clays and the amount of fine sand which could be found throughout the stratum.

The consolidation tests indicated that the coastal sediments (soft clays and silts) were slightly-to-moderately overconsolidated. However, since these sediments have likely not been subjected to higher loading in the past, and since the fill above the on-shore silt and clays were placed over 50 years ago, it should be assumed that these soils are normally-consolidated for design purposes. This is generally supported by the Atterberg limit data which indicates natural water content values within or above the range of LL values.

Therefore, any settlement of the ground due to additional loading of these soils will be in the virgin compression range and correspond to the compression index (Cc).

The compression index is a critical parameter in assessing the long term settlements resulting from the compression of the soft clays and silts. The average compression index from the consolidation tests is about 0.28. However, as discussed above, the consolidation tests were likely performed on samples with high disturbance and/or on samples with high sand contents. The compression ratio can also be estimated using an empirical correlation (Terzaghi and Peck, 1967) with the liquid limit (LL) values of the silts and clays as follows:

Cc = 0.009 (LL – 10)

Where Cc = compression index

LL = Liquid Limit

Using the LL values obtained from the laboratory testing, the empirically derived compression ratios should be on the order of about 0.23 to 0.27, which is consistent with the consolidation test results. Correlations with index test were used due to the limited laboratory test data.

The coefficient of secondary compression (cα), can be determined from the laboratory data and empirically from a correlation (Mesri, 1973) with natural water content (w) as:

wc =α

Where cα = coefficient of secondary compression over one log time cycle w = in-situ water content

Based on the equation presented above, the coefficient of secondary compression under virgin compression should be on the order of about 0.003 to 0.005. From the available laboratory data, as well as available empirical correlations, the coefficient of vertical consolidation should be on the order of about 0.3 square feet per day.

For design and analysis purposes, the following parameters are recommended for the unconsolidated coastal sediments:

• Compression Index, Cc = 0.23 to 0.27

• Recompression Index, Cr = 0.02

• Coefficient of Secondary Compression, cα = 0.004

• Coefficient of Vertical Consolidation, cv (ft2/day) = 0.3

Strength properties: Unconsolidated-undrained (UU) triaxial compression tests were performed on the coastal sediment (soft silt and clay soils) samples. The undrained shear strengths from these tests were approximately 150 and 200 pounds per square foot (psf).

The undrained shear strength (Su) corresponds to the vertical effective stress in terms of the undrained strength ratio (Su/p’o). Because of this, the strength will tend to increase with depth and with increasing degree of consolidation. For design purposes, an undrained strength ratio of approximately 0.20 to 0.25 is recommended based on the laboratory testing data and empirical correlations. For instance, if the vertical effective stress is 1,000 psf, the corresponding undrained shear strength of the soft clays and silts will be 200 to 250 psf.

3.3.3 Stratum 3: Alluvium

Stratum 3 consists predominantly of sediments of clay and silt with occasional layers of sand and gravel as well as cobbles and boulders. Based on the material classification and the standard penetration test results, the unit weight of this material will range from 115 pounds per cubic foot (pcf) to approximately 125 pcf. Samples taken in the clayey and silty alluvium

Veterans Drive Improvements Geotechnical Report November 22, 2016 show that natural water content ranges from about 16 to 29, with an average of about 22.

The LL varies from 31 to 73 with an average of 46 while the corresponding PI ranges from about 15 to 52 with an average of about 28.

Consolidation tests were not performed on the samples taken from the alluvium. However, due to the high SPT N-values encountered throughout the stratum, its relative depth compared to the amount of fill anticipated on site, and the presence of occasional dense sand and gravel layers within the stratum, consolidation settlement of the layer should be minor and need not be considered separately.

Based on sample classifications and the results of standard penetration tests, drained parameters ranging from c’=1,000 psf and φ’=25 degrees to c’=1,500 psf and φ’=28 for the alluvium are recommended. Undrained parameters ranging from c = 2,500 psf to c = 3,000 psf, and a total unit weight of approximately 115 to 125 pcf are also recommended.

3.3.4 Stratum 4: Bedrock

The unconfined compressive strength on tested intact rock samples was between 0.5 kilopounds per square inch (ksi) and 23 ksi. The amount of weathering, degree of fracturing, and the quality of the rock mass will likely dictate the engineering behavior of the rock mass.

Recovery of the rock core runs ranged between 30 and 100 percent with an average of 82 percent. Rock Quality Designation (RQD) values ranged from 0 to 100 with an average of 52.

4.0 DESIGN CONSIDERATIONS AND RECOMMENDATIONS

4.1 General

Phase 1A of the Veterans Drive Improvements Project will include reconstruction to add new lanes for a continuous four-lane roadway consisting of two lanes in each direction for a distance of 2,260.39 feet (0.37 mi.). The widening will require removal of existing shore protection rip-rap, installation of new retaining wall structures, filling behind the new retaining wall, construction of overlay, widened roadway and sidewalks, and installation of other miscellaneous structures associated with the new roadway (signage and lighting).

The site contains poor soils which will result in settlements. However, the site profile is variable, with rock and competent soils present across the entire site at depths of less than about 20 feet. In some areas rock is only a few feet deep; in other areas rock is tens of feet deep but is overlain with stiff or dense soils. To simplify the design and construction, a modular block gravity retaining wall on spread foundations is recommended. Where poor soils exist, the soils are to be over-excavated and the wall founded on the stiffer in-situ soils at the base of the excavation. The wall design is illustrated in 10. This approach will avoid the use of multiple foundation types (shallow and deep foundations) and the associated additional design and construction considerations such as pile caps, multiple foundation contractors, connections between shallow and deep foundation wall sections, construction inspection and testing of deep foundations, etc. Design and construction recommendations for the wall are presented in detail in the subsequent sections.

Veterans Drive Improvements Geotechnical Report November 22, 2016

Fill will be placed in areas of the widened roadway outside of the proposed undercuts for the retaining walls. In these areas, a construction surcharge is recommended to pre-load the compressible soils and accelerate the settlements anticipated under the final grades. The pre-load is to be performed prior to utility and roadway construction in order to mitigate the potential for damage to these final structures. The pre-load is described in detail in the subsequent sections.

4.2 Seismic Considerations

The design earthquake motions and forces specified by AASHTO LRFD 2012 are based on a low probability of their being exceeded during the normal life expectancy of the structure.

Structures designed in accordance with AASHTO LRFD 2012 guidelines may suffer damage during the design earthquake, but should have low probability of catastrophic failure.

For the purposes of this study, the Veterans Drive Improvements have been classified as “essential structures”. Essential structures are generally those that should, as a minimum, be open to emergency vehicles and for security/defense purposes immediately after the design earthquake.

4.2.1 Design Acceleration

AASHTO LRFD 2012, Section 3.10.2 provides mapped ground motion values for the peak ground acceleration (PGA) and the short- and long-period spectral acceleration coefficients (Ss and S1, respectively) to be used in the seismic design. The mapped ground motions for the project site with a seven percent exceedance in 75 years are as follows:

PGA = 0.37

Ss = 0.80 S1 = 0.24

The mapped ground motions provided by AASHTO LRFD 2012 are for ‘firm rock’ sites which do not consider the effect of soil conditions at the project location.

4.2.2 Site Effects

The subsurface conditions will affect the level of shaking the project site experiences during the design earthquake event. The soil profile of a particular project location can be classified as belonging to one of several categories of profiles as defined by AASHTO. The classification system distinguishes six site classes, A through F, based on the shear wave velocity of the soil in the uppermost 100 feet. The shear wave velocity (Vs) is the velocity that shear waves – such as those produced in an earthquake – will have as they pass through the soil or bedrock. Alternatively, AASHTO allows the use of two other methods to determine the site class. These methods are based on the SPT N-values in cohesionless soil and undrained shear strengths of cohesive soil averaged over the uppermost 100 feet.

Subsurface conditions across the project site vary. Rock is near the ground surface at some locations, but at others it is located more than 60 feet below the ground surface. Stiff

Veterans Drive Improvements Geotechnical Report November 22, 2016 cohesive soils are found to overlie the rock at some locations, but at other locations soft clays with thicknesses up to 10 feet can be found. Based on the variability of the subsurface conditions, it is recommended that a conservative approach be taken utilizing a worst-case scenario.

Site-specific measurements of shear wave velocity were not available for the project site.

However, based on the average SPT N-values taken from borings at locations where rock is relatively deep and there is a presence of soft clay, the project site can be classified as Site Class E which is distinguished by AASHTO as a soil profile having an average SPT N-value less than 15 blows-per-foot.

Site factors based on the site class were used to modify the mapped accelerations for PGA, short periods and at a 1-second period. The site factors used to modify the ‘firm rock’ motions, Fpga, Fa, and FV are a function of site class and the mapped level of shaking. The site adjusted maximum considered accelerations for a 7% probability of exceedance in 75 years used for the five-percent damped recommended design spectral acceleration are then determined to be:

As = 0.36 SDs = 0.91

SD1 = 0.73

4.2.3 Seismic Performance Zone

According to AASHTO, each structure should be assigned to one of four seismic zones which are based on the value of SD1. The seismic zones reflect the variation in seismic risk and are used to permit different requirements for method of analysis and design. The seismic zone corresponding to an SD1=0.73g is Seismic Zone 4.

4.2.4 Liquefaction

Liquefaction is a soil behavior phenomenon in which a soil located below the groundwater surface loses a substantial amount of strength due to strong earthquake ground shaking.

Some types of soil tend to compact during earthquake shaking, inducing excess pore water pressure in the saturated soil, which, in turn, causes a reduction in strength of the soil.

Recently deposited (i.e., geologically young) and relatively loose natural soils, as well as uncompacted or poorly compacted fills, are potentially susceptible to liquefaction. Loose sands are particularly susceptible. Loose silts and gravel also have potential for liquefaction.

Dense natural soils and well-compacted fills have low susceptibility to liquefaction. Clayey soils and bedrock generally are not susceptible to liquefaction.

Possible consequences of liquefaction include vertical settlement, lateral displacement, loss of bearing capacity for foundations supported by soil that liquefies, increased lateral loading on structures retaining soil that liquefies, and flotation of lightweight structures embedded in soil that liquefies.

An analysis of the potential for liquefaction at the site using the Seed-Idriss simplified procedure (Seed and Idriss (1982), Seed et. al. (1985), Youd and Idriss (1997)) was performed. This procedure utilizes SPT N-values from soil borings as measures of the relative density and susceptibility of sands to liquefaction. Seed, et al. (1985) evaluated

Veterans Drive Improvements Geotechnical Report November 22, 2016 observed occurrences or non-occurrences of liquefaction in sand deposits of various densities during numerous historical earthquakes. The study showed that sands having SPT N-values exceeding 30 blows-per-foot generally are not susceptible to liquefaction.

The study also showed that soils with higher silt and clay contents have a higher resistance to liquefaction. An empirical correlation between SPT values and cyclic stress ratio, which is computed from the peak horizontal acceleration (PGA) induced by the design earthquake, was developed to determine the “critical SPT N-value” for specified ground water levels.

The critical SPT N-value represents the value above which liquefaction would not be expected to occur, and below which liquefaction would be expected to occur. The critical SPT N-values can then be compared to observed values for the site to assess the potential for liquefaction.

Results of the analysis indicate that it is likely that Stratum 1 will exhibit liquefaction behavior during strong earthquake ground shaking. As previously discussed, Stratum 1 typically could be classified as very loose to medium dense poorly-graded sand or silty sand.

Variations throughout the stratum also included deposits of clayey sand, silt, and clay.

Concrete, brick, wood and other construction debris were present in many of the samples.

SPT N-values in the fill materials ranged from 1 to 47 blows-per-foot with an average of 13 blows-per-foot. The high blow counts encountered within the fill materials are likely indications of the presence of obstructions or gravel layers, and not necessarily areas of increased density.

Since we recommend the removal of the very loose soils in Sections 4.3.2 and 4.4, the potential liquefaction of Stratum 1 will be considerably reduced. However, consideration should be taken for very loose soils and uncompacted or poorly compacted backfills behind the wall that may be potentially susceptible to liquefaction. Recommendations to reduce the potential liquefaction are addressed in Section 4.4.

As discussed above, the design earthquake motions and forces specified by AASHTO are based on a low probability of their being exceeded during the normal life expectancy of the structure. Structures designed in accordance with AASHTO may suffer damage during the design earthquake, but should have low probability of catastrophic failure. Therefore, the proposed retaining wall should be designed such that it can withstand these deformations without a catastrophic failure. However, repair of the retaining wall may be required if an earthquake having similar ground motions to that of the design earthquake occurs.

4.3 Retaining Walls

Phase 1A of the Veteran’s Drive Improvements Project will include reconstruction to add new lanes for a continuous four-lane roadway consisting of two lanes in each direction. A typical wall section for the Phase 1A portion of the alignment is shown in Figure 10.

Construction will consist of removal of existing shore protection rip-rap, installation of new retaining wall structures, filling behind the new retaining walls, construction of a widened roadway and sidewalks, and associated miscellaneous structures (signage and lighting).

As depicted in Figure 10, the type of retaining wall structure is anticipated to be a gravity wall consisting of interlocking precast concrete blocks. These precast modular blocks are a proprietary design (similar to MSE walls), available from several manufacturers. The final design is dependent on the manufacturer chosen by the contractor. The contractor should

Veterans Drive Improvements Geotechnical Report November 22, 2016 be required to provide calculations of external and internal stability as part of the required submittals.

We recommend that the wall be founded on a bedding of #57 stone as a leveling pad. We also recommend that the toe of the wall be protected from scour with a rip-rap apron and covered with backfill soils. It is acceptable to re-use the existing rip rap for the proposed rip rap apron as long as the existing rip rap meets the specified criteria for the new rip rap.

Behind the drainage fill we recommend the use of select granular material as retained material to backfill the space between the drainage fill and the existing ground to achieve the proposed finish grades. The gradation of the select granular material backfill is recommended in Section 4.4 – Earthwork.