Amendment 0006 - P405 Geotechnical Investigation Report Rev 4.pdf

625 Holcomb Bridge Road, Norcross, GA 30071 • 770-209-0029 • unitedconsulting.com

REPORT

For Leo A. Daly

Geotechnical Exploration

P-405 Chosin Range Improvements & Modernization, Phase 3

Marine Corps Recruit Depot

(MCRD)

Parris Island, South Carolina

April 24, 2020

Revision 4 November 10, 2020

LEODL-19-SC-03665-01

April 24, 2020 Revised November 10, 2020

Mr. Ron A. Strohm, AIA, MCARB, LEED AP BD + C Associate, QAQC Manager Leo A. Daly 10 Tenth Street, NE Suite 200 Atlanta, Georgia 30309

Via Email: RAStrohm@leoadaly.com

RE: Report of Geotechnical Exploration P-405 Chosin Range Improvements & Modernization, Phase 3 Marine Corps Recruit Depot (MCRD) Parris Island, South Carolina Project No.: LEODL-19-SC-03665-01.R4

Dear Mr. Strohm:

United Consulting is pleased to submit this revised report of our Geotechnical Exploration for the above-referenced project. The work was completed in accordance with our Proposal No. P2019.3094.01 dated July 30, 2019, and Contract/Work Authorization N69450-14-D-0025 dated August 13, 2019. This revision is based on the comments from Mr. Charles Crawley dated May 29, 2020 with subsequent emails and recent telephone conversations. As requested, we have reanalyzed the subsurface data in conjunction with additional input/modification of the design of the proposed structures based on further discussion with structural engineer Mr. Michael Planer, P.E. of PES Structural Engineers, Inc. We appreciate the opportunity to assist you with this project and look forward to our continued participation. Please contact us if you have any questions or if we can be of further assistance.

Sincerely, UNITED CONSULTING

Santanu Sinharoy, P.E.

Chief Engineer

Scott D. Smelter Principal

SRT/MM/SS/SDS/nj

H:\SHAREPOINT Local Backup\2019\GEO FOLDER\LEODL-19-SC-03665-01\LEODL-19-SC-03665-01 - Geo-Rev4.docx mailto:RAStrohm@leoadaly.com

TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY

2.0 PROJECT DESCRIPTION

3.0 PURPOSE of EXPLORation

4.0 SCOPE

5.0 REGIONAL GEOLOGY

5.1 Coastal Plain Province

5.2 Historical Use Information

5.2.1 Approach:

5.2.2 Project Site:

5.2.3 Adjoining Properties:

6.0 SUBSURFACE CONDITIONS

6.1 Cone Penetration Testing

6.2 Standard Penetration Test Data

6.3 Temporary Monitoring Wells

7.0 LABORATORY TESTING PROGRAM

8.0 DISCUSSION AND RECOMMENDATIONS

8.1 Existing Fill Evaluation

8.2 Site Preparation

8.3 Excavation and Caving Considerations

8.4 Earthwork

8.5 Groundwater Considerations

8.6 Fill Induced Settlement

8.7 Seismic Consideration

8.7.1 Liquefaction Analysis

8.7.2 Liquefaction Settlement and Mitigation

8.7.3 Site Class and Spectral Accelerations

8.8 Foundation Design and Construction

8.8.1 Impact Berm Rear Retaining Wall

8.8.2 Impact Berm Front Retaining Wall

8.8.3 Target Berm Backstop Retaining Wall

8.8.4 Security Sidewall Along the Northern Boundary of the Site

8.8.4 Small Structures

8.8.5 – Pile Driving Analyzer (PDA)

8.8.6 GRLWeap Analysis

8.8.7 Lpile Analysis

8.9 Retaining Walls

8.11 Slopes

8.12 Slope Stability Analysis

8.13 Pavement Design

8.14 Utility and Pipe Construction

8.15 Fill Placement

9.0 LIMITATIONS

FIGURES

Figure 1 – Site Location Map Figure 2 – Street Map Figure 3 – USGS Topographic Map Figure 4 – Boring/Sounding Location Plan Figure 5 – Subsurface Profile A-A’ Figure 6 – Subsurface Profile B-B’ Figure 7 – Subsurface Profile C-C’ Figure 8 – Proposed PDA Location Plan

APPENDIX A

General Notes /Narrative of Drilling Operations Exploration Procedures SPT Boring Logs (46 pages) Hand Auger Boring (8 pages)

APPENDIX B – CPT Logs

Narrative of CPT Soundings (1-page) CPT Boring Logs (4-pages) CPT Field Data (15-pages)

APPENDIX C – Seismic Refraction, MASW, and ERI Survey

GEL Solutions Report dated January 24, 2020 (11 pages)

APPENDIX D – Soils Laboratory Testing

Laboratory Procedures Particle Size Distribution (43) Liquid and Plastic Test Reports (6-pages) Moisture Density Relationship (4) California Bearing Ratio Test (2 tests; 10 pages) Consolidation Test (3; 16 pages) Horticultural Test Results (5 pages)

APPENDIX E – Soil Liquefaction Analysis

Liquefaction Settlement Analysis Using Settle 3D (9-pages)

APPENDIX F – Apile Output

Driven Output for B-8 – PSC 14” Square Pile (31-pages) Driven Output for B-12 – PSC 14” Square Pile (31-pages) Driven Output for B-8 – PSC 16” Square Pile (31-pages) Driven Output for B-12 – PSC 16” Square Pile (31-pages)

Appendix G – GRLWeap Output (16-pages)

Bearing Capacity and Derivability Analysis for PSC 14” pile using B-8 (5-pages) Bearing Capacity and Derivability Analysis for PSC 14” pile using B-12 (5-pages)

Appendix H- Lpile Analysis 14” PSC Pile Fixed and Free Head Using B-8 (36-pages) 16” PSC Pile Fixed and Free Head Using B-8 (36-pages)

Appendix I – Asphalt Core Photographs (4-pages)

APPENDIX J – Historical Aerial Photograph and USGS maps.

USGS Topographic Maps (Fort Fremont, 1944; Parris Island, 1956; Fort Fremont, 1959; Parris Island, 1979; and Parris Island, 2014) Historical Aerial Photographs (1951, 1961, 1968, 1977, 1983, 1994, 2005, 2009, 2013, and 2017)

APPENDIX K - Calculations

Settlement Analysis for Impact Berm – Front and Rear Retaining Walls Using Boring B-12 (3-pages) Settlement Analysis for Impact Berm – Front and Rear Retaining Walls Using Boring B-8 (3-pages) Settlement of Typical Berm (2-page) Settlement Analysis for Target Berm Backstop Retaining Wall (2-page) Global Slope Stability Analysis for Impact Berm – Rear Retaining Wall (1-page) Global Slope Stability Analysis for Impact Berm – Front Retaining Wall (1-page) Global Slope Stability for Target Berm – Retaining Wall (1-page) Pavement Design Calculation (3-pages)

1.0 EXECUTIVE SUMMARY

United Consulting has completed a Geotechnical Exploration on the P-405 Chosin Range Improvements & Modernization, Phase 3 site at the Chosin Range located northwest of Wake Boulevard in the Marine Corps Recruits Depot at Parris Island, South Carolina. Please refer to the text of the report for a more detailed discussion of the items summarized below.

TYPICAL

DEPTHS

GENERAL SUBSURFACE CONDITIONS ENCOUNTERED

0’ to 13’

13’ to 53’

53’ to 65’+

Very loose to medium dense sand with varying amounts of silt and clay; or very soft to firm clay with varying amounts of sand and silt. (Coastal Plain or Uncontrolled Fill) (SP, CL, SM, SC, SP-SM, SC-SM).

Very loose to dense sand with varying amounts of silt and clay; or very soft to firm clay with varying amounts of sand, silt, and shell (Coastal Plain) (SM, CL, SC, CH, SP-SM, SP-SC, SP).

Loose to very dense sand with varying amounts of sand, silt, and shell. (Coastal Plain) (SM, SP, SP-SM, SP-SC, SC).

Auger Refusal CPT soundings terminated at depths of 55.6’ and 33.9’ due to very high cone tip resistance values (> 200 tsf).

Groundwater The borings encountered groundwater at depths of 2 to 6 feet below ground surface, each corresponding to approximately Elev. 1 to 6.2 feet above msl.

ANTICIPATED FUTURE CUT AND/OR FILL

Based on Review of Topographic Site Plan

Based on the provided topographic site plan, generally cuts of up to 5 feet and fills of up to 26 feet are anticipated. The pit area will be raised approximately 1-foot above existing grade to mitigate existing flood issues. The firing line will be raised approximately 5 feet, while firing line berms will be raised approximately 5 to 9 feet to be even with the target berm. While the pit area and most of the range will be below 100-year flood elevation, the firing lines and target berms will be elevated to above this elevation.

GEOTECHNICAL CONCERNS

Groundwater Groundwater is expected to be problematic over most of the site for any excavation deeper than 2 feet below existing grades. We recommend that drainage ditches be installed to temporary lower the groundwater where surface water or shallow groundwater are present. Please refer to Section 8.5 of the report for further discussion.

Lower Sand Unit

The loose and relatively clean sands are expected to liquefy during the design earthquake event. Liquefaction of soils typically cause loss of axial pile side resistance and cause down-drag forces to the pile foundation system. Soil liquefaction settlement ranging on the order of 5 to 15 inches should be expected.

TYPICAL

DEPTHS

GENERAL SUBSURFACE CONDITIONS ENCOUNTERED

FOUNDATION RECOMMENDATIONS

Shallow Foundation

United Consulting has provided shallow foundation recommendation for the small structures. All small structures shall be supported on Post-Tensioned slab or mat type foundation system. The detailed design parameters regarding these foundations are provided in the body of the report Section 8.8.3.

Deep Foundation Bearing Capacity

The Impact Berm Rear Retaining Wall will be supported on PSC pile foundations.

United Consulting has provided pile capacities and anticipated pile tip elevations for Prestressed Concrete Pile (PSC). Soil liquefaction is expected to add down-drag loads to the piles during an extreme seismic event and/or proposed fill placement. Therefore, we recommend mitigating the down-drag considerably by pretreating the piles with friction reducer such as Slickcoat, Bitumastic 300M, etc., or equivalent. More frequent expansion joints will be required to mitigate differential settlement.

Other Retaining Wall

The Impact Berm front retaining wall and Target Berm Backstop retaining wall can be supported on shallow foundations designed for up to an allowable soil bearing capacity of 1,500 pounds per square foot (psf) based on total load. More frequent expansion joints will be required to mitigate differential settlement.

SEISMIC SITE CLASS

United Consulting recommends that a seismic site classification of “Site Class D” be utilized for the site.

2.0 PROJECT DESCRIPTION

The project site is located at the Chosin small arms range which is located northwest of Wake Boulevard and approximately 3 miles due south of the main gate at the Marine Corps Recruits Depot in Parris Island, Beaufort County, South Carolina. At the time of our exploration, the site was an existing shooting range with elevated firing lines, side berms, benches, flagpoles, and the pit area. The pit area includes one male and one female head, butthouse control building, targeting hoisting equipment, a concrete retaining wall with benches and catwalk, two target storage buildings, and asphalt pavement.

The properties surrounding the project site consisted of structures, roads, and parking and drive areas associated with the Marine Corps Air Station. The site is bound to the northwest by saltwater wetlands, to the southwest by the Hue City shooting range, to the southeast by Wake Boulevard, and to the northeast by the Starlite shooting range. The general location of the project site is shown on the attached Site Location Plan (Figure 1).

Based on the 1998 USGS topographic map, the topography at the site is generally flat with elevations ranging from 2 to 20 feet above msl (Mean Sea Level).

This project will consist of the replacement of the existing range furnishings, infrastructure, and facilities;

the construction of an impact berm and concrete side safety wall along the northeastern boundary; and other exterior improvements.

The following five (5) buildings are included in the narrative:

1. Pit-Head (2)

2. Pit – Target Storage Building (2)

3. Pit – Butt Control House

In addition, the following four (4) engineered structures are included:

1. Range – Security Sidewall with no openings for passing through

2. Pit – Target Berm Retaining Wall

3. Pit – Impact Berm – Front Retaining Wall

4. Pit – Impact Berm – Rear Retaining Wall

All facilities will be concrete or reinforced concrete masonry unit structures with EIFS exterior finish and concrete slab on grade. Target area berm, retaining walls, side safety walls and 500-yard range berms will be constructed. Earth berm structures include a behind-the-target Bullet Impact Berm with side and rear retaining walls as appropriate with flat lead mining equipment maintenance surfaces. Based on the provided grading plan the fill placement will be on the order of 8 to 26 feet in the Impact Berm area, 12 feet in the Target Berm Retaining Wall area, and 10 feet in the earth berm areas.

The following conditions are considered applicable for this project.

1. The seismic design determination based on the requirements of Chapter 11.8.3 of ASCE 7-10

Based on the information provided by the structural engineer Mr. Michael Planer, P.E., of PES Structural Engineers dated June 30, 2020, the piles supporting the rear impact retaining walls are spaced at 3’-6” and 7’-0” and will be subjected to the following downward gravity loads.

Sustained Allowable Load (kips)

Sustained Ultimate Load (kips)

Ultimate Earthquake Load (kips)

140 185 130

Based on an email from Ms. Lisa J McAdam dated July 7, 2020, the two bathrooms in the pit area will each measure approximately 710 square feet; the two target storage buildings will each measure 933 square feet; and the Butts Control House will measure approximately 113 square feet. The security sidewalls will be approximately 1,500 feet in length; the target berm retaining wall will be approximately 700 feet in length; the impact berm front retaining wall will be approximately 700 feet in length; and the impact berm rear retaining wall will be approximately 740 feet in length.

Pavement include asphalt range roads, asphalt maintenance roads, Butts Control area concrete work/vehicle apron, asphalt range control tower road, and parking for approximately 64 vehicles.

The planned project is within the 100-year floodplain. We understand that a flood damage vulnerability assessment has been completed. Floodplain mitigation measures will be incorporated in this project.

Target Storage Building and head facilities are to be constructed with concrete. The pit area will be raised approximately 1-foot above existing grade to mitigate existing flood issues. The firing line will be raised approximately 5 feet, while firing line berms will be raised approximately 5 to 9 feet to be even with the target berm. The federal Emergency Management Agency (FEMA) has established a base flood elevation (BFE) of 14 feet across the site. While the pit area will be below the flood elevation, the firing lines and target berms will be elevated above this BFE.

3.0 PURPOSE OF EXPLORATION

The purpose of this geotechnical exploration was to assess potential rock, unsuitable and possible soft areas; to determine if the existing soils are suitable for reuse as engineered fill; to determine the IBC Site Class based on seismic cone penetration test soundings (SCPT) and shear wave velocity; to determine the liquefaction potential and potential settlement due to liquefaction; to provide foundation and pavement recommendations; and to discuss other geotechnical related issues deemed pertinent to this project.

4.0 SCOPE

The scope of our geotechnical exploration included the following items:

1. A visual reconnaissance of the site from a geotechnical standpoint;

2. Coordinate underground utility location with “Call Before You Dig”, site personnel, and a private utility locate sub-consultant;

3. Drilling twenty-five (25) Standard Penetration Test (SPT) borings, eight (8) hand auger borings with dynamic cone penetrometer tests, and performing two (2) Cone Penetration Test (CPT) to assess the quality and consistency of the subsurface soils;

4. Collecting five (5) surficial soil samples for horticultural soil tests;

5. Hiring a sub-consultant to complete three (3) geophysical survey cross-sections by conducting three

(3) seismic refraction lines, Multichannel Analysis of Surface Waves, and electrical resistivity imaging

(ERI);

6. Visual evaluation of the soil samples obtained during our field testing program for further identification and classification;

7. Coring the existing asphalt pavement where present at boring locations and measure and photo-document the cores;

8. Installation of three (3) temporary monitoring wells to depths of 10 feet in order to obtain 24, 48, and

72 hour groundwater readings;

9. Performing two (2) Seismic Cone Penetration Tests (SCPT);

10. Performing geotechnical laboratory testing consisting of forty three (43) soil classification tests, three

(3) consolidation tests, four (4) standard proctor tests, and two (2) soaked CBR tests;

11. Analyzing the existing soil conditions with respect to the proposed construction; and

12. Preparing this report to document the results of our field-testing program, our engineering analysis, and to provide our findings and recommendations.

5.0 REGIONAL GEOLOGY

5.1 Coastal Plain Province

The Project Site is located in the Outer Coastal Plain Physiographic Province of South Carolina within the Broad River and Beaufort River delta formation. According to the USGS geologic map, the site is underlain by peat and muck deposits ranging up to 60 feet below existing grade. The soils in this formation are generally comprised of weaker sands, silts and clays, while the older marine deposits consisting of Cretaceous Period and the Black Mingo and Santee Limestone. All of these deposits form a successively overlapping miogeosynclinal wedge, which thickens coastward from its origin along the Fall Line near Columbia, South Carolina. The Holocene deposits overlaying the basement strata consist of inter-bedded layers and mixtures of sand, silt, and clay. The unconsolidated sediments (loose sands and soft clays) have a potential for soil liquefaction during extreme seismic conditions.

Based on the Beaufort County, South Carolina Soil Survey, the site is generally underlain by Tomotley loamy fine sand (To) and Wahee fine sandy loam (Wa). The southeastern corner of the site is underlain by Seabrook fine sand (Sk). The ground surface within these soil series have about 0 to 2 percent slope.

Based on the Beaufort County soil survey, these soils have low shrink-swell potential to a depth of 80 inches. Seasonal high water table within the To series is at about 0 to 1 foot, within the Wa series is at about 0.5 to 1.5 feet, and within Sk series is 2 to 3 feet.

5.2 Historical Use Information

5.2.1 Approach:

Historical research was conducted to assess the history of the Project Site and surrounding areas. This research included reviewing available USGS topographic maps and historic aerial photographs, as readily available and outlined below.

15-Minute USGS Topographic Map: Fort Fremont, South Carolina, dated 1944

7.5-Minute USGS Topographic Map: Parris Island, South Carolina, dated 1956

15-Minute USGS Topographic Map: Fort Fremont, South Carolina, dated 1959

7.5-Minute USGS Topographic Map: Parris Island, South Carolina, dated 1979

7.5-Minute USGS Topographic Map: Parris Island, South Carolina, dated 2014

Aerial Photographs, dated 1951, 1961, 1968, 1977, 1983, 1994, 2005, 2009, 2013, 2017

No other historic maps or aerial photographs were readily available for review.

5.2.2 Project Site:

The 1951 aerial photograph appeared to show that the Project Site was used as a shooting range with primarily vacant land with a structure in the northern portion.

No changes were noted on the Project Site apart from varying grass cover in the aerial photographs from 1961 to 1977.

The 1983 aerial photograph showed development of roads in the center and along the perimeter of the Project Site, as well as addition of a few more structures in northern portion. No other changes were noted in the aerial photographs from 1994.

The 2005 aerial photograph showed development of a center road extending to the north-end of the site, as well as addition of a few more structures in northern portion. No other changes were noted in the aerial photographs from 2013 and 2017 apart from varying grass cover.

Due to the scale of the 1944 and 1959 USGS topographic maps, no structures were mapped. The 1956, 1979, and 2014 USGS topographic maps showed the Project Site was used as a shooting range.

5.2.3 Adjoining Properties:

The 1951 aerial photograph showed clear land adjoining north, structures associated with the Marine Corps Recruit Depot adjoining south, and shooting ranges adjoining northeast and southwest of the Project Site. No changes were noted in the aerial photographs from 1961 to 1994 apart from an increase in a few structures in the general surrounding area.

In the aerial photographs from 2005 through 2017, the Project Site appeared to be adjoined to the south by a few more structures as well as a reduction in wooded areas.

Due to the scale of the 1944 and 1959 USGS topographic maps, no structures were mapped. The 1956 and 1979 USGS topographic maps showed the Project Site was adjoining to the southwest and northeast by structures associated with the Marine Corps Recruit Depot shooting range. No changes were noted in the 2014 USGS topographic map.

6.0 SUBSURFACE CONDITIONS

6.1 Cone Penetration Testing

Two (2) SCPT soundings were performed to depths of 55.6 and 33.9 feet below ground surface. The CPT soundings encountered coastal plain soils.

In SCPT-1, the first layer (upper 15 feet) consisted of sand and sand mixture with silt and clay with a low to moderately high cone tip resistance (qt) ranging from 2.59 tons per square foot (tsf) to 140 tsf. Below the initial layer, the SCPT-1 sounding generally encountered sensitive fine-grained silty sand or sandy silt with clean sand from about 15 to 32 feet. This layer is characterized by having low to moderately high cone tip resistance (qt) values ranging from 4.04 tsf to 125.25 tsf. The third layer (below 32 feet) consisted of silt mixtures to silty sands to refusal depth of 55.6 feet. This layer is characterized with low to high cone tip resistance (qt) values ranging from 22.12 tsf to 294.61 tsf. SCPT sounding refusal occurred at a cone tip resistance of 294.61 tsf at a depth of 55.6 feet.

In SCPT-2, the first layer (upper 15 feet) consisted of sandy silt to silty sand with a low to moderately cone tip resistance (qt) ranging from 4.34 tsf to 68.89 tsf. Below the initial layer, the SCPT-2 sounding generally encountered sensitive fine-grained to sandy silt from about 15 to 22 feet. This layer is characterized by having low to moderate cone tip resistance (qt) values ranging from 5.45 tsf to 61.18 tsf.

The third layer (below 22 feet) consisted of stiff to dense organic material to refusal depth of 34 feet. This layer is characterized with moderately high cone tip resistance (qt) values ranging from 43.38 tsf to 396 tsf. SCPT sounding was refused at cone tip resistance of 204.47 tsf at a depth of 33.98 feet.

The SCPT soundings were terminated before the planned termination depths due to high cone tip resistance (294.61 and 204.47 tsf) at depths of 55.6 feet and 33.9 feet respectively.

For a more precise description of the conditions encountered within the SCPT soundings, we refer you to the SCPT data logs included in The Appendix.

6.2 Standard Penetration Test Data

Twenty-five (25) soil borings were drilled to depths ranging from 10 to 100 feet below ground surface.

Twelve (12) borings (P-1 through P-12) were drilled to a depth of 10 feet, five (5) borings (B-1, B-2, B-3, B-5, and B-6) were drilled to a depth of 65 feet, and eight (8) borings (B-4, B-7 through B-13) were drilled to depths ranging from 80 to 100 feet. Eight (8) hand auger borings (designated as HAP-1 through HAP-

8) were also conducted to depths ranging from 4 to 10 feet below the ground surface. Initially, the borings encountered a thin, about 2 to 4 inch layer of surficial organic soil or 0.5 to 4.5 inches of asphalt pavement.

Below the initial layer, standard penetration test (SPT) resistances (N-values) were used to determine the consistency or relative density of the soils, except for the hand auger borings. Please refer to Table 1 below for N-value definitions.

Table 1 – Soil Consistency Definition

SANDS CLAYS AND SILTS

BLOWS/FOOT DESCRIPTION BLOWS/FOOT DESCRIPTION

0-4 Very Loose 0-2 Very Soft

5-10 Loose 3-4 Soft

11-19 Firm 5-8 Firm

20-29 Medium Dense 9-15 Stiff

30-49 Dense 16-30 Very Stiff

≥ 50 Very Dense ≥ 31 Hard

Below the surficial layers, borings B-8, B-10, P-1, and P-10 encountered uncontrolled fill soils to depths of about 1 foot. The uncontrolled fill soils encountered generally consisted of loose to firm sand with varying amounts of silt and clay. The Standard Penetration Test resistances (N-values) within the uncontrolled fill ranged from 9 to 18 blows per foot (bpf).

Below the uncontrolled fill in the aforementioned borings, and the surficial layer in the remaining borings, typical coastal plain soils of the Outer Coastal Plain Province of South Carolina were encountered. The coastal soil deposits encountered generally consisted of very soft to stiff clay with varying amounts of sand and silt or very loose to very dense sand with varying amounts of silt and clay. Some layers of very soft to stiff silt with varying amounts of clay and sand was also encountered in borings B-7, B-13, and P-

7. The Standard Penetration Test resistances (N-values) within the sand deposit ranged from 2 bpf to 50 blows for 2 inch of penetration, those within the clay deposit ranged from 1 to 13 bpf, and those in the silt deposits ranged from 2 to 10 bpf. Low consistency (N-values ≤ 7) soils were generally encountered in most borings at varying depths.

A layer of very dense sand with N-values of 50 blows for 2 to 6 inches of penetration was encountered in borings B-1, B-2, B-3, and B-7 starting at depths of 53 to 55 feet, and in borings B-7 through B-13 starting at depths of at 73 to 75 feet. Auger refusal did not occur in the borings.

Shallow groundwater was encountered at the time of drilling in all the borings at depths ranging from 2 to 6 feet below existing grades (approximate elevations 1 to 6.2 feet above msl).

6.3 Temporary Monitoring Wells

Three (3) temporary monitoring wells were constructed in three of the 10 foot borings (borings P-5, P-7, and P-10). The wells were constructed with 1 inch PVC pipes with a two foot, slotted section at the base.

Properly sized, fine to medium dense sand, was placed around the slotted section followed by a bentonite seal. A bentonite seal was placed to existing grade. Stabilized groundwater level readings were taken after 24 and 48 hours of installation and the results are provided in the table below.

Table 2 – Groundwater Level Readings

Boring At Time of Drilling + 24 Hours + 48 Hours

P-5 3.9’ (elev. 5.0) 3.7’ (elev. 5.2) 3.4’ (elev. 5.5)

P-7 4’ (elev. 5.3) 4.1’ (elev. 5.2) 3.9’ (elev. 5.4)

P-10 3.3’ (elev. 4.3) 4.1’ (elev. 3.5) 3.9’ (elev. 3.7)

Note: listed depths are below existing ground surface.

7.0 LABORATORY TESTING PROGRAM

Geotechnical laboratory testing for this project included forty-three (43) soil classification tests, four (4) standard Proctor tests, and two (2) soaked CBR tests. The results of the moisture content tests are shown on the boring logs next to the respective samples tested. The results of the CBR tests are provided in the Appendix to this report. The maximum dry densities of the soil standard Proctor tests were 105.5 pcf,

110.6 pcf, 110.0 pcf, and 106.8 pcf with optimum moisture contents of 13.9%, 11.5%, 11.7%, and 12.7%, respectively. The natural moisture content for the samples within the top 10 feet tested ranged from 5.6% to 25.3% which indicated that some on-site samples have moisture contents significantly lower or higher than the optimum moisture content. A narrative description of the laboratory tests is included in The Appendix.

Additionally, three (3) consolidation tests with soil classification tests including Atterberg Limits tests were performed on three relatively undisturbed Shelby Tube samples. The results of the consolidation and soil classification testing are provided in the table below. Complete test results can be found in Appendix C.

Table 3 – Results of Consolidation Tests

Boring Dept h

NM

LL

PI

#200

Soil Type

Dry Density

(pcf)

Overbu rden (ksf)

Pc (ksf)

Cc Cr Initial Void Ratio

B-4 23’ to 25’

24.5 23 2 21.2 SM 101.2 1.68 1.4 0.08 0.01 0.665

B-8 23’ to 25’

66.5 47 30 69.7 CL 60.6 1.46 3.1 1.04 0.07 1.781

B-8 28’ to 30’

46.7 42 27 39.2 SC 73.6 1.64 3.3 0.62 0.04 1.289

LL= Liquid Limit PI = Plasticity Index

NM = Natural Moisture < #200 = Percent Passing #200 Sieve

Five surficial soil samples were also collected for horticultural soil tests. The results of these tests are also included in Appendix C

8.0 DISCUSSION AND RECOMMENDATIONS

The following recommendations are based on our understanding of the proposed construction, the data obtained in our soil test borings/sounding, a site reconnaissance, our analysis, and our experience with subsurface conditions similar to those encountered at the project site. We recommend that once the design drawings are finalized, the recommendations of this report be re-evaluated. At that time additional subsurface evaluation and analysis may be required to finalized our recommendations.

United Consulting requests the opportunity for a general review of final design documents and specifications in order to verify that earthwork and foundation recommendations have been properly interpreted and implemented in the design and specifications. We recommend that United Consulting, as the Geotechnical Engineer of Record, be consulted during construction to conduct Geotechnical Controls as the Owner's Representative.

8.1 Existing Fill Evaluation

Up to 1 foot of uncontrolled fill was encountered in localized areas of the project site, in the areas of borings B-8, B-10, P-1, and P-10. The uncontrolled fill encountered generally appeared to be free of debris and moderately compacted. The areas will generally be covered with additional fill on the order of 8 to 25 feet. The existing uncontrolled fill is largely deemed satisfactory for placement of new fill provided that it be verified to be stable during proofrolling. We recommend that the quality of the uncontrolled fill be thoroughly evaluated during construction, and the near surface uncontrolled fill be densified prior to placement of additional fill, as well as construction of the proposed building, retaining walls and pavement areas. This uncontrolled fill should be remediated as necessary as outlined in Section 8.2 Site Preparation, below.

8.2 Site Preparation

Prior to development, any existing structures, concrete slabs, concrete and asphalt pavement, foundations and other remnants of existing and prior construction should be demolished and removed from within the construction limits. Underground utility lines should be relocated or capped off outside the construction area. The excavation associated with removal of the existing building foundation and underground utilities, etc. should be backfilled with imported structural fill as outlined in Sections 8.4 Earthwork and 8.15 Fill Placement.

Also, the existing vegetation and trees including their root mat should be grubbed and removed from the area of the proposed construction. Removal of trees should include removal of their root ball, which may extend to several feet below grade. The anticipated depth of stripping and grubbing is about 8 inches below existing grade over most of the site; however, areas supporting foundation and pavement should be stripped and re-densified to 12 inches below foundation bottom or pavement subgrade elevation as applicable. Existing trash and debris or other remnants of prior construction should also be completely removed and properly disposed of.

We do not recommend that any active or non-active utility lines located below the area of the proposed structures be left in place. Existing utility lines should be re-routed outside the proposed structures. The abandoned lines should then be excavated and removed from the area of the proposed construction.

Excavations should be subsequently backfilled with properly compacted engineered fill.

After clearing the site, areas to receive fill, foundations, slabs, and pavements should be proofrolled with a heavy smooth-drum roller or a fully loaded tandem-axle dump truck. Proofrolling should be performed under the observation of the Geotechnical Engineer or his representatives so that, areas, which exhibit “pumping” (wave type displacement) during proofrolling, may be identified. These areas, if present, should be undercut to firm material and backfilled with controlled structural fill meeting classification stated in Section 8.4 below. It should be noted that proofrolling may not be feasible or practical in some areas such as small foundation excavations or excavations that extend into or near the water table.

Subgrade soils at or near the water table (i.e. excavations deeper than 2 to 4 feet BLG depending on the location) will likely be saturated and could consist of firm to stiff clay, depending on the depth, and as a result, standard compaction methods may not be possible or practical. As such, we recommend that all foundations be excavated to a depth of at least 12 to 18 inches below the design subgrade elevation and backfilled with a layer of crushed stone (tamped firmly into place with excavator bucket). This stone layer should provide a stable foundation or subgrade support.

We recommend that after stripping or exposing the subgrade, pneumatic-tired vehicles be kept off the low mangrove areas where very soft near surface soils exist. Within this area, if fill is to be placed, it should be end-dumped and spread with a tracked bulldozer. If rain is expected, the area should be rolled and sloped to promote runoff.

Due to shallow groundwater and loose near surface soil, substantial effort should be anticipated to remove organic material and soft/wet areas and provide excavation dewatering. United Consulting recommends removal of the near surface organic and soils to maximum depth of 12 inches and placement of high strength biaxial (Tensar BX 1200 or equivalent) or triaxial (Tensar TriAx 170 or equivalent) Geogrid on excavated surface, and placement of minimum 12-inch open graded aggregate to form a working surface for equipment and crane path activities. If the area is stable at the time of construction, placement of Geotextile and open graded aggregate may be eliminated. This work should be evaluated by the Geotechnical Engineer at the time of construction.

8.3 Excavation and Caving Considerations

Other than a layer of very dense sands at depths below 55 feet, no other hard excavating materials were encountered at this site. Conventional earthmoving equipment should be sufficient for any excavation at this site. Temporary slopes of 1.5(H):1(V) for shallow excavations above groundwater should be sufficient for this project. However, we recommend that the slopes be evaluated for stability by the Geotechnical Engineer during construction. We recommend that the crest of the temporary slopes be kept at least 5 feet from the edge of any existing building, foundation, pavements, or other structures.

Temporary shoring or bracing of excavation sidewalls will be required for excavations that will extend below the water table.

All excavations should be conducted in accordance with the Occupational Safety and Health Administration (OSHA) guidelines. Flattening of the excavation sidewalls and/or the use of bracing may be needed to maintain stability during construction.

8.4 Earthwork

Structural fill is defined as relatively clean sand, silty/clayey sand, or gravel (i.e. crushed stone or quarry screenings), within USCS Soil Groups “SP”, “SW”, “SP-SM”, “SP-SC”, “GW”, or “GP”, and having a maximum liquid limit (LL) of 35, maximum plasticity index (PI) of 12, and a maximum of 15 percent by weight passing the No. 200 sieve. The Structural fill to be placed under foundations and pavement subgrades when placed and compacted as outlined in the Fill Placement section 8.15 are referred to as “controlled structural fill”. Based on our visual evaluation and laboratory test data, we anticipate that about 75 percent of the near surface soil that will be excavated at the project site should meet this criterion.

Common Fill is defined as sand with silt and clay and clay mixture (SC-SM or SP-SM, SM); or clay with varying amounts of sand and silt (CL); or silt with varying amounts of sand and clay (ML). The near surface soils generally consist of sands with varying amounts of silt and clay and can be used as common fill material, if free of organic material. Common fill material should be used for construction of the impact berm and other berms to be constructed at the project site when placed with proper moisture (within 2 percent of the soils optimum moisture based on ASTM D698). However, the CL and ML soils if wet will be difficult to dry for reuse as common fill. These soils may be mixed with dry sandy soils present at the site to improve workability. Also, introduction of about 3 to 4 percent of lime into the CL and ML soils may be considered to reduce moisture and improve workability. During rainy seasons, reuse of CL and ML soils as common fill may not be practical.

Relatively Lightweight Fill consists of CL, ML, SM-CL, etc. with maximum dry density on the order of 85 to 90 pci and optimum moisture on the order of 20 to 25 percent.

Ballistic fill consists of clayey sand/gravel, or sandy/gravelly clay with USCS soil classification of SC, CL, or GC with plasticity index in the range of 7 to 20 percent, maximum particle size of ¾ inch, and fines content (% passing No. 200 sieve) of 20 to 50 percent. Soils meeting these specifications when placed and compacted in accordance with Section 8.15 are referred to as “ballistic fill”. Soils with USGC classification of poorly graded sand (SP) and high plasticity clay (CH) should not be used for construction of berms or used as ballistic fill. The fill placed in the impact berm area should be topped with a 5 feet thick layer of ballistic fill.

Positive drainage should be maintained at all times to prevent saturation of exposed soils in case of sudden rains. Rolling the surface of disturbed soils will also improve runoff and reduce the soil moisture and construction delays. The degree of soil stability problems will also be dependent upon the precautions taken by the contractor to help protect the soils from saturation during construction.

8.5 Groundwater Considerations

Groundwater was encountered in the borings at the time of drilling at depths ranging from 2 to 6 feet below the existing grade, at approximate elevation of 1 to 6 feet above msl. Groundwater is expected to be problematic for any excavations deeper than 2 feet over most of the site. Furthermore, due to the presence of a near surface clayey layer and shallow groundwater, earthwork operations could be impacted by the tendency of the exposed soil under traffic loads to result in deep ruts. We recommend that upon removal of the surficial organic soils and stripping and grubbing of the near surface soils, a drainage system consisting of ditches be installed to temporarily lower the groundwater where surface water and shallow groundwater are present. The drainage ditches should be deep enough to lower the water to at least 2 feet below the deepest cut elevation. If lowering of groundwater to the recommended depth is not feasible due to proximity of the site to open sea (Atlantic Ocean), additional and/or alternative temporary dewatering system such as sump pumps or well points may be required to lower the groundwater.

Since most the structures will be located within the 100-year floodplain, a permanent foundation drainage system consisting of perimeter strip drains discharging away from the structure or connected to storm sewer system should be constructed. This will reduce the buildup of hydrostatic pressure below the slab on grade and below grade walls and structures. Furthermore, where utility lines will be connecting to building(s) the connection points should be waterproofed.

Groundwater levels should be anticipated to fluctuate with the change of seasons, during periods of very low or high precipitation, or due to changes in the floodplain or watershed upstream from the area. We recommend that in the areas that excavations deeper than 3 feet above msl are anticipated, the contactor install a series of well points to be used to facilitate utility and foundation excavation. The lowering of the groundwater should be maintained for the duration of excavation and backfilling below the water table.

The contractor should also be prepared to control surface water, and remove groundwater or perched water as needed.

8.6 Fill Induced Settlement

Up to twenty-six (26) feet of fill (approximately 3,120 psf) is expected to be placed in the rear of the impact berm. Fill on the order of up to 10 feet (approximately 1,200 psf) will be placed in the intermediate berm and the berm traversing center of the range. These additional loads will cause consolidation (short term and long-term) of the interbedded clay soils underneath the Impact Berm Front Retaining Wall, Target Berm Retaining Wall, and intermediate berms on the order of 2 inches to about 4.5 inches. The settlement of the sand layer for these structures should occur relatively quickly, within a few weeks of the fill placement, whereas the primary consolidation of the clay could take as long as 3 to 4 years. Based on the analysis performed, we anticipate that secondary consolidation will be less than one (1) inch over a period of 50 years. These settlements will cause down-drag loads on the deep foundation system supporting the rear impact berm retaining wall, if not mitigated. Consolidation parameters used for estimating the settlement were obtained from consolidation test results and soil parameters obtained from borings B-4 and B-8 which are summarized in Table 3 and subsurface conditions encountered in the area of borings B-8 and B-12.

We understand relatively lighter density cohesive soils (ML, CL, etc.,) material may be locally available or searched for to reduce the settlement further, from the fill placed behind the walls. If an in-situ unit weight on the order of 100 to 105 pcf backfill is used, the total and differential settlement associated will reduced by about 20 to 25 percent of the calculated values provided above.

8.7 Seismic Consideration

8.7.1 Liquefaction Analysis

To help evaluate the consequence of liquefaction, we computed the Liquefactions Potential Index (LPI), which is an empirical tool used to evaluate the potential for Liquefaction to cause damage. The LPI considers the factor of safety against liquefaction, the depth to liquefiable soils, and the thickness of the liquefiable soils to compute an index that ranges from 0 to 100. An LPI of 0 means there is no risk of Liquefactions; an LPI of 100 means that the entire profile is expected to liquefy. The level of risk is generally defined as:

LPI < 5 – Surface manifestation and Liquefaction-induced damage not expected

5≤LPI≤15 – moderate liquefaction with some surface manifestation possible.

LPI>15 – severe liquefaction and foundation damage likely

The LPI for this site is about 42, indicating severe risk of liquefaction. Subsequently, United Consulting used Settle 3D and Liquefy Pro computer programs to determine the liquefiable profile and the approximate settlement due to liquefaction. The results of the evaluation are described in Section 8.7.2 below.

8.7.2 Liquefaction Settlement and Mitigation

Settle 3-D Version 4.023 copyright 2019 developed by Rocscience Inc. was used to determine the settlement due to liquefaction at borings B-8 and SCPT soundings CPT-1A and CPT-2 location. Total anticipated settlement due to liquefaction is expected to range from about 5 to 15 inches depending on CPT sounding or SPT boring data. The estimated liquefaction settlement is summarized in Table 5 for the CPT soundings and SPT borings analyzed.

Table 4: Liquefaction Settlement Estimate

Boring/CPT

ID

Approximate Liquefaction Settlement (in)

Using Settle 3D

CPT-1A 5

CPT-2 5

B-8 15

The liquefaction analysis was performed using a design earthquake magnitude 7.3 and a Geometric Peak Ground Acceleration of PGAM = 0.338g (adjusted for Site Class D) obtained from the USGS Earthquake Hazards website for the site.

The liquefaction potential for the soils based on the collected shear wave velocity data will be limited to the zone where the shear wave velocity is less than 750 ft/sec. As such, liquefaction of the soils will be limited to the top 37 feet. The results of the liquefaction evaluation and settlement calculations are displayed graphically and are included in the Appendix.

As stated above, liquefaction induced settlement on the order of 5 to 15 inches is possible at this site.

We believe that the Impact Berm Rear Retaining Wall will not be impacted by liquefaction due to being supported on deep foundations. However, the piles supporting the walls will be subjected to down-drag resulting from the reversal of skin friction in the liquefied zone. Such potential will be reduced considerably by pile treatment with friction reducer discussed later in this report. The parking and drive areas, smaller buildings and the retaining walls that are not supported on deep foundation systems will experience subsidence and distress during extreme earthquake events. Liquefaction mitigation for these structures could be accomplished through structural design techniques so that the structures can bridge over localized liquefied areas.

Furthermore, lateral spreading of the slope will occur as the angle of internal friction between the liquefied soil and the rigid non-liquefied soils layer could approach zero. Based on the results provided by the settle3D, using CPT data, lateral spreading based on Zhang et al. (2004), Yoshimine et al. (2006), and Tokimatsu & Yoshimine (1983) is on the order of 4 to 9 feet. If the stated settlement and risk of liquefaction is not acceptable, then ground improvement techniques such as installing earthquake drains or Geosynthetic Encased Columns (GECs) can be used to reduce site settlements during extreme events to 2 to 3 inches or less.

The earthquake drain ground improvement technique is a method that minimizes seismic settlements during and immediately after an earthquake event. Earthquake drains mitigate liquefaction by providing a shortened drainage path for the rapid dissipation of seismically-generated excess pore pressures before they can reach levels that potentially trigger liquefaction. Earthquake drains are prefabricated drains installed vertically through a liquefiable deposit. The drains consist of perforated high density polypropylene (HDPE) pipe wrapped in a geotextile filter fabric and typically range from 3.0 to 4.0 inches in diameter. The drains are installed on a triangular pattern with typical horizontal spacing between 4 and 6 feet. Earthquake drains generally reduce liquefaction settlements to approximately ¼ of the predicted untreated settlement. At this site, based on our conversation with an earthquake drain design/installer (Keller), settlement induced due to liquefaction could be reduced to about 2 inches. Design, installation, and spacing of the earthquake drains are performed by a specialized design/build contractor. Companies such as Menard and Hayward Baker have completed projects in the vicinity of the project site.

We recommend the client should carefully review the cost/benefit of installing Earthquake Drains or GEC upon receiving such information from the contractor. Installation of liquefaction mitigation will reduce damage to the pavement, underground utilities, and structures during extreme events. As such, careful cost benefit analysis should be performed to determine if installations of these systems are justified for this project.

8.7.3 Site Class and Spectral Accelerations

The seismic design is covered by the provisions of the 2015 International Building Code (IBC), and ASC 7, Site Class Definitions. The site categories referenced in the IBC are defined in terms of the average shear wave velocity (Vs) in the top 100 feet of the profile. Based on a multichannel analysis of surface waves (MASW) conducted at the site, the Vs estimates of the site range from 754 ft./sec to 902 ft./sec.

Based on these obtained average shear wave velocities, a seismic site classification of “Site Class D” is available. Table 5 shows the results of the MASW and the Vs across the site.

Table 5: Vs Estimates Based on MASW

Location* Vs (ft./sec) Average Vs (ft./sec)

Profile 1 / Shot Point 1 862

861 Profile 1 / Shot Point 2 848

Profile 1 / Passive MAM** 875

Profile 2 / Shot Point 1 868

880 Profile 2 / Shot Point 2 872

Profile 2 / Passive MAM 902

Profile 3 / Shot Point 1 795

776 Profile 3 / Shot Point 2 754

Profile 3 / Passive MAM 780

*Location Plan is part of the report provided in the Appendix **MAM=Micrometer Array Measurement

The report of seismic refraction, MASW, and electrical resistivity imaging (ERI) survey conducted by GEL Solutions is available in the Appendix.

Table 6 – Ground Motions Parameters

Site Class Ss’ S1 Fa Fv SDS SD1 PGA

“D” 0.481g 0.164g 1.416 2.145 0.454g 0.234g 0.338g

8.8 Foundation Design and Construction

8.8.1 Impact Berm Rear Retaining Wall

Based on the borings B-1 to B-3, B-7, B-11, and CPT-1A, presence of very dense sands were encountered at elevation (-45± feet msl) and based on borings B-8 to B-13 the elevation of dense sand was encountered at elevation of (-65± feet msl). Based on the MASW and ERI survey conducted along the profiles at the site, competent layers defined by high P-wave velocities and correlating to high N-values are present at approximately elevation (-50 msl) in most areas and range from approximately elevation (-38 to -63 feet msl) across the site. Based on the ERI data, the dense layer along the rear retaining wall (Profile 1) appears to drop approximately 15 to 20 feet in elevation at about 225 feet southwest of the northern end of the rear retaining wall. However, based on the boring data this drop occurred between borings B-7 and B-8. This ERI report is included in the Appendix. The Pile driving criteria should be confirmed with Pile Driving Analyzer (PDA) as discussed in section 8.8.5.

United Consulting used the Allowable Stress Design (ASD) method for geotechnical condition for the design of driven piles. The down-drag due to liquefaction of the soils is anticipated to occur to approximate elevation of -30 feet or so.