Base Admin Geotechnical Report (2024).pdf

ECS Southeast, LLP

Geotechnical Engineering Report

USCG Base Admin Facility

S Hobson Avenue North Charleston, South Carolina 29405

ECS Project Number 34:4619

September 20, 2023

September 18, 2023 Mr. Lee Hudson RQ Construction, LLC 1620 Faraday Avenue Carlsbad, California 92008

Reference: Report of Subsurface Exploration and Geotechnical Engineering Analysis USCG Base Admin Facility S Hobson Avenue North Charleston, South Carolina 29405

ECS Project Number 34:4619

Dear Mr. Hudson:

ECS Southeast, LLP (ECS) has completed the subsurface exploration and geotechnical engineering analyses for the above-referenced project. Our services were performed in general accordance with Engineering Agreement No. 22-1222001, dated August 8, 2023. This report presents our understanding of the geotechnical aspects of the project along with the results of the field exploration and engineering analyses conducted and our recommendations for design and construction of geotechnical related items.

It has been our pleasure to be of service to you during the design phase of this project. We would appreciate the opportunity to remain involved during the continuation of the design phase, and to provide our services during construction phase operations as well to verify the assumptions of subsurface conditions made for this report. Should you have questions concerning the information contained in this report, or if we can be of further assistance to you, please contact us.

Respectfully submitted, ECS Southeast, LLP

Winslow Goins, P.E.

Principal Engineer

Jarred Wadford, P.E.

Office Manager

USCG Base Admin Facility September 20, 2023 ECS Project Number 34:4619 Page i

TABLE OF CONTENTS

EXECUTIVE SUMMARY

1.0 INTRODUCTION

2.0 PROJECT INFORMATION

2.1 PROJECT LOCATION

2.2 CURRENT SITE CONDITIONS

2.3 PROPOSED CONSTRUCTION

3.0 FIELD EXPLORATION AND LABORATORY TESTING

3.1 FIELD EXPLORATION

3.1.1 Cone Penetration Testing (CPT) Soundings

3.2 REGIONAL/SITE GEOLOGY

3.3 SUBSURFACE CHARACTERIZATION

3.4 GROUNDWATER OBSERVATIONS

4.0 DESIGN RECOMMENDATIONS

4.1 GENERAL

4.2 SEISMIC DESIGN CONSIDERATIONS

4.2.1 Liquefaction

4.2.2 Seismic Site Classification

4.2.3 Ground Motion Parameters

4.2.4 Settlement Considerations

4.3 DRIVEN PILE RECOMMENDATIONS

4.4 FLOOR SLABS

4.5 PAVEMENT SECTIONS

4.6 SITE DRAINAGE

5.0 SITE CONSTRUCTION RECOMMENDATIONS

5.1 SUBGRADE PREPARATION

5.1.1 Stripping, Grubbing, and Demolition

5.1.2 Risk Associated with Undocumented Fill

5.1.3 Proofrolling

5.1.4 Site Temporary Dewatering

5.2 EARTHWORK OPERATIONS

5.2.1 Engineered Fill

5.3 FOUNDATION AND SLAB OBSERVATIONS

5.4 GENERAL CONSTRUCTION CONSIDERATIONS

6.0 CLOSING

ECS Project Number 34:4619 Page ii

APPENDICES

APPENDIX A – Drawings & Reports Site Location Diagram Test Location Diagram

APPENDIX B – Field Operations Reference Notes for Cone Penetration Test (CPT) Soundings Subsurface Exploration Procedure: Cone Penetration Testing (CPT) ASTM D 5778 CPT Logs (C-1 to C-6, B-3, B-4, S-1, S-2, R-1, R-2, and R-6)

APPENDIX C – Supplemental Report Documents and Calculations Allowable Axial Compressive Pile Capacities

APPENDIX D – Miscellaneous Important Information about your Geotechnical Report

ECS Project Number 34:4619 Page 1

EXECUTIVE SUMMARY

ECS has completed a subsurface exploration for the proposed facility located off S Hobson Avenue in North Charleston, South Carolina. This Executive Summary is intended as a very brief overview of the primary geotechnical conditions that are expected to affect design and construction.

Information gleaned from this executive summary should not be utilized in lieu of reading the geotechnical report. Our findings, conclusions, and recommendations are summarized below.

SUBSURFACE CONDITIONS:

Surface Materials: Approximately 5 to 16+ inches of asphalt was observed at test locations R-1, R-2, R-5, R-6, B-1, B-3, B-6.

Coastal Sedimentary Deposits: Observed below surface materials to the maximum depth explored of approximately 60 feet below the current site grades.

Groundwater: Observed during the field exploration at depths ranging from approximately 3.6 to

8.3 feet below the current ground surface.

Soundings R-1, R-2, R-5, and S-1 encountered refusal at less than 2 feet below grade.

GEOTECHNICAL CONCERNS:

Uncontrolled fill was encountered to a depth of up to approximately 10 feet below current site grades.

Very soft marine clay was encountered to a depth of approximately 40 feet below current site grades.

Due to very soft underlying clay, the placement of fill will cause excess consolidation settlement.

Due to the developed nature of the site, undocumented fill and/or buried debris may be present at unexplored areas of the site.

DESIGN & CONSTRUCTION RECOMMENDATIONS:

Seismic Design: Site Class “E” Deep Foundation System: Recommendations provided for driven pre-stressed concrete (PSC) (12- , 14-, and 16-inch square), 8 inch timber, and HP 10x42 piles.

Slabs-on-Grade: Modulus of Subgrade Reaction, k = 150 pci.

Details of our conclusions and recommendations are discussed in the report text. Should the proposed project features be changed from those described above, ECS must be consulted in order to verify that the recommendations made in this report are still valid.

ECS Project Number 34:4619 Page 2

1.0 INTRODUCTION

The purpose of this study was to provide geotechnical information for the design and construction of the proposed facility located off S Hobson Avenue in North Charleston, South Carolina. The recommendations developed for this report are based on the results of our subsurface exploration and project information supplied by Pinnacle Hospitality Partners.

Our services were provided in general accordance with Engineering Agreement No. 22-1222001, dated August 8, 2023.

This report contains the results of our subsurface exploration, site characterization, engineering analyses, and recommendations for the design and construction of the planned structures and pavements.

The report includes the following items.

Description of subsurface exploration program and test location plan.

Description of tests performed, results of tests and data collected.

Cone Penetration Test (CPT) sounding logs and soil classification in accordance with Unified Soil Classification System.

Pertinent geological data and general description of area soils.

Site class determination per 2021 International Building Code (IRC 2021), including site liquefaction analysis.

Deep foundation system.

Estimated total and differential settlement.

Recommendations on subgrade modulus for design of at-grade slabs.

Constructability recommendations including suitability of site soils for use as structural fill, compaction requirements, dewatering, and identifying undesirable subgrade material present.

ECS Project Number 34:4619 Page 3

2.0 PROJECT INFORMATION

2.1 PROJECT LOCATION

The project site is located off S Hobson Avenue in North Charleston, South Carolina, as shown below and on Figure 1 in Appendix A. The site is a portion of the larger lot which is further identified on Charleston County GIS Parcel ID No. 400-00-00-102.

2.2 CURRENT SITE CONDITIONS

At the time of our site visit, the site was developed with an asphalt parking lot and a small ancillary structure. Site specific topographic information was not available at the time of this report;

however, from observations made during our site visit the site appeared to be relatively flat.

2.3 PROPOSED CONSTRUCTION

According to conversations and correspondence with RQ Construction we understand the lot will be developed with a new three-story admin facility. Site topographic information was not available at the time of this report; however, the site appeared relatively flat.

3.0 FIELD EXPLORATION AND LABORATORY TESTING

3.1 FIELD EXPLORATION

The field exploration was planned with the objective of characterizing the project site in general geotechnical and geological terms and to evaluate subsequent field data to assist in the evaluation of recommendations for design of geotechnical related items.

ECS Project Number 34:4619 Page 4

Test locations were identified in the field by ECS personnel using hand-held GPS equipment and are shown on the Test Location Diagram in Appendix A. Prior to performing the field exploration, we contacted SC PUPS to check the test locations for potential underground utilities.

3.1.1 Cone Penetration Testing (CPT) Soundings

Fourteen (14) CPT soundings, designated R-1 to R-6, S-1 to S-2, and B-1 to B-6 were performed in the general area of the locations requested by RQ Construction. Refusal was encountered at sounding locations R-1, R-2, R-5, and S-1 at depths less than 2 feet below current site grades. The cone penetration test soundings were performed in general conformance with ASTM D5778 by our subcontractor with a track-mounted rig. The CPT sounding logs and an explanation of our exploration procedures are presented in Appendix B.

3.2 REGIONAL/SITE GEOLOGY

The natural geology of the site has been modified due to the current development of the site.

The site is located in the Chenier plain and deltas of Suwannee & Chattahoochie Rives Formation of the Coastal Plain Physiographic Province of South Carolina. The Coastal Plain is composed of seven terraces, each representing a former level of the Atlantic Ocean. Soils in this area generally consist of sedimentary materials transported from other areas by the ocean or rivers. These deposits vary in thickness from a thin veneer along the western edge of the region to more than 10,000 feet near the coast. The sedimentary deposits of the Coastal Plain rest upon consolidated rocks similar to those underlying the adjacent Piedmont Physiographic Province. In general, shallow unconfined groundwater movement within the overlying soils is largely controlled by topographic gradients. Recharge occurs primarily by infiltration along higher elevations and typically discharges into streams or other surface water bodies. The elevation of the shallow water table is transient and can vary greatly with seasonal fluctuations in precipitation.

3.3 SUBSURFACE CHARACTERIZATION

The subsurface conditions encountered were generally consistent with published geological mapping. The following sections provide generalized characterizations of the soil strata encountered during our subsurface exploration. For subsurface information at a specific location, refer to the CPT logs presented in Appendix B.

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GENERAL SUBSURFACE STRATIGRAPHY

Approximate Depth Range

(ft) Stratum Description

Estimated Ranges of CPT

N-values

0 to 1.5 N/A Asphalt depths ranging between approximately 5 to 16+ inches of asphalt was observed at a portion of our test locations. N/A

1.5 to 10 I Very loose to very dense SAND with varying amounts of very soft clay and silt (SM, SC, SP), moist to saturated

2 to 50+

10 to 40 IV Very soft CLAY (CL) with varying amounts of silt and sand, saturated 1 to 8

40 to 46 V Medium dense to very dense SAND with varying amounts of clay and silt (SM, SC, SP), saturated 8 to 50+

46 to 50 VI Loose to medium dense silty SAND/stiff to hard sandy SILT [Pre- Marl (1)], saturated 10 to 11

50 to 60 VII Loose to medium dense silty SAND/hard sandy SILT [Cooper Marl (1)], saturated 12 to 17

Notes:

(1) The Cooper Marl, locally referred to as “Marl,” is a relatively incompressible, thick ( 200 ft) stratum which underlies the area and is typically the bearing stratum for deep foundations in the greater Charleston area.

3.4 GROUNDWATER OBSERVATIONS

Groundwater depths were measured in our CPT soundings as noted on the logs in Appendix B. The groundwater depths measured at the time of our field investigation ranged from approximately 3.6 to 8.3 feet below ground surface in the CPT soundings.

Variations in the long-term water table may occur as a result of changes in precipitation, evaporation, surface water runoff, construction activities, and other factors. Due to the proximity to the Cooper River, the groundwater could be tidally influenced.

4.0 DESIGN RECOMMENDATIONS

4.1 GENERAL

The primary purpose of this geotechnical exploration was to help identify and evaluate the general subsurface conditions relative to the proposed construction. Our recommendations have been developed on the basis of the previously described project information and subsurface conditions identified during this study.

4.2 SEISMIC DESIGN CONSIDERATIONS

4.2.1 Liquefaction

When saturated soil with little to no cohesion liquefies during a major earthquake, it experiences a temporary loss of shear strength as a result of a transient rise in excess pore water pressure generated by strong ground motion. Flow failure, lateral spreading, differential settlement, loss of bearing, ground fissures, and sand boils are evidence of excess pore pressure generation and liquefaction.

ECS Project Number 34:4619 Page 6

We completed a liquefaction analysis in accordance with the 2021 International Building Code (IBC) design earthquake 1. Layers of loose to medium dense saturated sand and silty sand were encountered below the groundwater table to a depth of approximately 46 feet below the existing ground surface. ECS has compared the cyclic stress in these saturated soils to the cyclic resistance to estimate a Factor of Safety Against Liquefaction (FSAL). 2 On the basis of the results of our analyses, we conclude several of these layers have the potential to liquefy during the design seismic event.

Although the FSAL represents the liquefaction resistance of a soil stratum at a specific depth in a soil profile and are used in evaluating liquefaction-induced settlements, it does not quantify the severity of liquefaction-induced settlements or potential infrastructure damage for a site. Iwasaki et al. (1978) proposed the liquefaction potential index (LPI), which expresses liquefaction potential over an entire soil profile by integrating the product of the liquefaction potential of liquefiable soil layers and a weighting factor with respect to depth to the center of each liquefiable layer.

LPI is an empirical tool used to assess site liquefaction hazards and potential for liquefaction-related damage that ranges from 0 to 100. An LPI less than 5 indicates no anticipation of surface manifestations and low to moderate liquefaction-induced damages, LPIs ranging from 5 to 15 indicates surface manifestations and a high degree of liquefaction-induced damages are possible, and an LPI greater than 15 indicates probable surface manifestations with severe liquefaction-induced damages and that foundation damage is likely.

The LPI estimated for this site is less than 5, which indicates no risk of surface manifestations, and a low degree of liquefaction-induced damages are possible during and immediately following the design seismic event. When soils susceptible to liquefaction are located within approximately 10 feet of the surface, ground surface disruptions (i.e., sand boils) are possible. Such disruptions beneath at-grade structures would result in bearing capacity failure. Since potentially liquefiable sands are not located in the upper 10 feet at this site, there is a low risk of ground surface disruption.

Our analysis indicates that at-grade structures such as parking, slabs and shallow foundations could potentially settle on the order of up to 1 inch during and immediately following the design seismic event. Differential settlement associated with liquefaction-induced settlement is expected to be approximately 50 to 100 percent of the overall anticipated liquefaction settlement. This settlement would result from volumetric compression of the liquefiable sand layers which occurs as seismically-induced excess soil pore water pressures dissipate.

Liquefaction Mitigation: We assume the structures will likely be supported by a deep foundation system. If a deep foundation system is utilized, the effects of settlement induced by liquefaction will be likely mitigated.

1 The IBC design earthquake has a 2% probability of exceedance in 50 years. Our liquefaction analysis was based on an earthquake with a magnitude of 7.3 and ground surface acceleration of 0.972 g.

2 Analysis completed following the procedures presented in the 1996 NCEER and the 1998 NCEER/NSF workshops on the Evaluation of Liquefaction Resistance of Soils (Youd and Idriss 2001). To estimate volumetric strain and associated liquefaction-induced settlement, we used the procedures developed by Zhang et al. (2002) and a depth weighting factor proposed by Cetin (2009).

ECS Project Number 34:4619 Page 7

4.2.2 Seismic Site Classification

Section 1613 of the IBC 2021 and Section 20.3 of ASCE 7-16 classifies sites with the potential for liquefaction as Seismic Site Class F. However, Section 20.3 of ASCE 7-16 also allows the design spectral response accelerations for a site to be evaluated without regard to liquefaction provided structures have a fundamental period of less than or equal to 0.5 seconds and the risks of liquefaction are considered in design. Based on our past experience, the proposed building types should meet this criterion; however, the structural engineer must confirm this.

Based on the results of the CPT soundings and our knowledge of local geologic conditions, it is our interpretation the site may be considered a Seismic Site Classification “E,” in accordance with the

IBC 2021.

The Site Class definition should not be confused with the Seismic Design Category designation, which the structural engineer typically assesses.

4.2.3 Ground Motion Parameters

As part of the RFP for the site, the design spectral response acceleration parameters for the site were provided by a Site Specific Response Analysis performed by Terracon in the report dated September 10, 2021. The design responses for the short (0.2 second, SDS) and long period (1-second, SD1) are noted in bold at the far right end of the following table.

Site Specific Design Parameters SDS 0.956g SD1 0.968g

PGAM 0.972

4.2.4 Settlement Considerations

We understand that small equipment pads (approximately 25 square foot slabs) may not be supported on a deep foundation system. We assume minimal fill (less than 1 foot) will be required.

Conventional soil fill has a compacted in-place density typically ranging from about 115 to 120 pounds per cubic foot (psf/ft). We estimate that the use of conventional soil fill to raise grades may result in up to approximately 5 inches of post-construction settlement at a comparatively high rate in areas receiving new fill. It is important to understand that even with laboratory consolidation test data; it is difficult to accurately predict the rate and magnitude of settlement of the very soft pluff mud marine clay.

If this excess settlement is not acceptable for small equipment pads, we recommend the pads be supported on a deep foundation system.

4.3 DRIVEN PILE RECOMMENDATIONS

Due to the very soft and highly compressible underlying soils, the proposed structure will need to be supported by a deep foundation system. Therefore, to mitigate the potential for static and seismic settlement of the proposed structure, deep foundations such as driven piles are typically required. We have provided recommendations for a deep foundation system consisting of 12, 14, and 16-inch prestressed concrete (PSC), 8-inch tip timber, and HP 10x42 piles.

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Axial Pile Analysis: We performed axial compressive pile capacity calculations for 12-inch, 14-inch, 16-inch square pre-cast pre-stressed concrete (PSC), 8-inch tip timber, and HP 10x42 piles which are typically available in the area. We recommend the piles penetrate a minimum of 10 feet into the Marl. Based on the CPTs performed across the site, the Marl was encountered at a depth of approximately 50 feet below the existing site grades.

For our analysis, we have considered the depth to the top of Marl at 50 feet below the current site grades. Figure A in Appendix C provides allowable compressive pile loads for the aforementioned piles at various pile penetrations.

The table below provides the recommended allowable axial capacity for a single pile driven to a depth at least 60 feet below current site grades.

ALLOWABLE AXIAL CAPACITY (TONS)

Type Allowable Axial Capacity (tons)

12-inch PSC 56 14-inch PSC 76 16-inch PSC 102

HP 10x42 100 8-inch Tip Timber 56

As shown on Figure A in Appendix C, the allowable axial compressive capacity of the piles should increase linearly with increased penetration into the Marl.

Allowable uplift pile loads can be assumed to be approximately 70 percent of the compressive loads. Dynamic pile (PDA) testing as subsequently described must be performed to confirm the ultimate pile bearing resistance and allowable axial capacity of the driven piles.

The structural capacity and design of the piles was not considered in our analyses and should be the responsibility of the project structural engineer. Maximum allowable tensile capacities are 70% of compressive capacity. We recommend piles be spaced at least three pile diameters center-to-center, to minimize vertical capacity reductions due to pile interaction effects.

Lateral Pile Resistance: The piles should develop lateral resistance from the passive pressure acting on the upper portion of the piles and their structural rigidity. The allowable lateral capacity of the piles depends on:

1. the pile stiffness

2. the strength of the surrounding soil

3. axial load on the pile

4. the allowable deflection at the pile top and the ground surface

5. the allowable moment capacity of the pile.

The IBC 2021 states that “the allowable lateral load shall not be more than one-half of the load that produces a gross lateral movement of 1 inch at the lower of the top of foundations element and the ground surface, unless it can be shown that the predicted lateral movement shall cause neither harmful distortion of, nor instability in, the structure, nor cause any element to be loaded beyond its capacity.”

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We have assumed at this time that the maximum allowable lateral load is limited to one-half of the load that will likely produce 1 inch of deflection.

The geotechnical response of a laterally loaded 12-inch, 14-inch, 16-inch square PSC, HP 10x42, and 8-in tip timber pile was analyzed using L-Pile. The piles were modeled with both fixed-head and free-head top boundary conditions with no stick-up above the ground surface.

An axial load of 56 tons was applied to the 12-inch square PSC pile, an axial load of 76 tons was applied to the 14-inch square PSC pile, an axial load of 102 tons was applied to the 16-inch square PSC pile, an axial load of 100 tons was applied to the HP10x42 pile, and an axial load of 28 tons was applied to the 8-inch tip timber pile. The pre-stressed concrete piles were modeled with a nonlinear bending stiffness using a concrete strength of 6,000 psi and reinforcement of four (12-inch PSC pile) six (14-inch PSC pile), and eight (16-inch PSC pile) ½-inch diameter 270-k Lolax at 31,000 lbs. each.

A summary of the lateral pile analyses are presented in the following tables for the recommended piles.

SUMMARY OF LATERAL PILE ANALYSES

Pile Type Deflection

(inch)

Lateral Loads (kips)

Maximum Moment in Pile

(in-kips)

Allowable Deflection

(inch)

Allowable Lateral Load1

(kips)

Maximum Allowable

Moment in Pile

(in-kips) 12-in. PSC

Fixed-Head 1.0 15.9 901 0.17 8.0 354

12-in. PSC Free-Head 1.0 10.0 437 0.21 5.0 174

14-in. PSC Fixed-Head 1.0 19.9 1269 0.18 10.0 507

14-in PSC Free-Head 1.0 12.5 588 0.22 6.3 238

16-in PSC Free-Head 1.0 25.0 1881 0.21 12.5 720

16-in PSC Free-Head 1.0 15.1 756 0.24 7.6 311

HP 10x42 (Weak Axis) Fixed-Head 1.0 10.8 549 0.17 5.4 199

HP 10x42 (Weak Axis) Free-Head 1.0 5.6 297 0.22 2.8 101

8-in Tip Timber Fixed-Head 1.0 11.2 440 0.21 5.6 179

8-in Tip Timber Free-Head 1.0 5.9 203 0.28 3.0 80

1Defined as ½ the lateral load that produces 1-inch of top of pile displacement.

The structural capacity of the piles has not been considered, and the shear and moment distributions must be evaluated by the structural engineer. The structural engineer should also review our boundary conditions assumptions (i.e., free-head or fixed-head at the ground surface) to confirm that they are compatible with the foundations design. Once pile layouts and loading conditions are better evaluated, additional lateral pile analysis (single or group) may be performed to help optimize pile design.

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The lateral capacities are for single piles only. To account for group effects, the lateral load capacity of a single pile should be multiplied by the appropriate reduction factor. Where piles are spaced at least three pile diameters in all directions, no group reduction factors need to be applied.

Reduction for pile group spacing can be provided once the number and arrangement of piles are known.

Additional lateral load resistance can be developed by passive resistance acting against the faces of the pile caps and grade beams. To calculate the passive resistance against the vertical faces of pile caps and grade beams, we recommend an ultimate equivalent fluid weight of 240 psf/ft. This value is not factored, and the foundation designer should apply the appropriate factor of safety. The upper foot should be ignored unless it is confined by a slab.

Driven Pile Construction Considerations: Prior to the start of pile driving, a wave equation analysis should be performed to document that the proposed pile driving hammer is capable of driving the piles without damage. ECS should observe pile driving to document that the piles are driven to the minimum design depth and to note damage or other concerns during installation.

Due to the contamination of the site, we assume that pre-augering will not be an option due to the cuttings.

To confirm the required pile lengths, we recommend at least 5 test piles be driven within the proposed structure locations before production piles are ordered. The test piles can be installed in production pile locations, but the piles should extend at least 3 feet above the ground surface while still penetrating to the required tip depth. The ultimate pile bearing resistance of the test piles should be measured with a Pile Driving Analyzer (PDA) in accordance with ASTM D4945. It is our experience that piles bearing in marl will likely not develop the ultimate resistance during initial driving. PDA testing should be performed during re-driving of the test piles at least five days following initial driving.

Utilities: Utilities connected to pile supported structures should be pile supported and/or incorporate flexible connections at the buildings. In areas where future development and no fill placement is anticipated, neutral buoyancy, light weight HDPE piping can be used. We recommend the use of restrained joint connections for any utilities that use flexible connections rather than pile support.

Small Pad Shallow Foundations: Assuming that the fill heights and building loads are no greater than those assumed, liquefaction risk is accepted or mitigated, and subgrade preparation and earthwork operations are completed in strict accordance with the recommendations of this report, the proposed structures can be supported by monolithic turned down edge slabs reinforced with top and bottom steel.

The design of the foundation shall utilize the following parameters:

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Table 4-1 Shallow Foundation Design Design Parameter Column Footing Wall Footing

Net Allowable Bearing Pressure1 1,500 psf 1,500 psf Acceptable Bearing Soil Material Stratum I or Approved structural fill.

Stratum I or Approved structural fill.

Minimum Width 30 inches 18 inches

Minimum Footing Embedment Depth (below slab or exterior finished grade)

12 inches 12 inches

Estimated Total Settlement2 > 4 inches > 4 inches Estimated Differential Settlement Less than 0.5 inches between columns Less than 0.5 inches over 30 feet

1. Net allowable bearing pressure is the applied pressure in excess of the surrounding overburden soils above the base of the foundation.

2. The settlement of a structure is a function of the compressibility of the bearing materials, bearing pressure, actual structural loads, fill depths, and the bearing elevation of footings with respect to the final ground surface elevation. These settlements are in addition to the estimated liquefaction induced settlement reported in Section 4.2.1.

Estimates of settlement for foundations bearing on engineered or non-engineered fills are strongly dependent on the quality of fill placed. Factors which may affect the quality of fill include maximum loose lift thickness of the fills placed and the amount of compactive effort placed on each lift.

The final footing elevation should be evaluated by ECS personnel to document that the bearing soils are capable of supporting the recommended net allowable bearing pressure and are suitable for foundation construction. These evaluations should include visual observations, hand rod probing, and dynamic cone penetrometer (ASTM STP 399) testing, or other methods deemed appropriate by ECS at the time of construction, in each column footing excavation and at intervals not greater than 25 feet in continuous footing excavations.

If soft or unsuitable soils are observed at the footing bearing elevations, the unsuitable soils should be undercut and removed. Any undercut should be backfilled up to the original design bottom of footing elevation with one of the following:

Lean concrete (f’c 1,000 psi at 28 days).

Number 57 stone; up to 2 feet in thickness.

Compacted structural fill (with additional compaction testing and soil bearing evaluation).

4.4 FLOOR SLABS

Due to the soil conditions at the site, floor slabs are anticipated to be supported on piles to prevent excessive long-term settlement.

A vapor barrier should be installed on top of the subgrade in areas to receive moisture-sensitive floor coverings to help reduce dampness on the surface of the floor slab. A vapor barrier is generally understood to consist of a minimum 10-mil thickness, overlapping sheets of plastic without sealing the overlap between the individual sheets. If a minimum of one foot of Engineered Fill with less than 5 percent fines is placed prior to slab placement an open graded aggregate is not required under the slabs, provided that a 10 mil or thicker vapor barrier is provided.

We recommend that the perm rating of the vapor barrier be adequate to protect the rating of the floor coverings and have adequate puncture resistance according to the expected foot traffic and equipment and materials placed on the barrier. If the vapor barrier is punctured or unsealed during

ECS Project Number 34:4619 Page 12 construction, the perm rating will be greatly decreased, and vapor intrusion may occur through the slab after construction. Punctures can be caused by concrete finishing, placement of reinforcement, or by equipment and foot traffic. Openings may be caused by unsealed edges at the floor wall interface or laps.

4.5 PAVEMENT SECTIONS

We have performed analyses of the pavement sections using the SCDOT Pavement Design Guidelines (2008) and associated literature. Figure A-1 from the 2008 SCDOT Pavement Design Manual shows the relationship between the California Bearing Ratio (CBR) and the Soil Support Value (SSV). As a part of the exploration, a CBR value was estimated using the results from the CPT soundings.

Based on our experience in the area and the variation in the subgrade conditions throughout the project area, a CBR value of 7 was utilized in our analysis which corresponds to a SSV of approximately 3.0. This design CBR value assumes that topsoil and any unsuitable materials are removed during stripping and grubbing and that the pavements will be supported on either the existing fill materials or new controlled fill.

The recommended minimum pavement sections are as follows:

Table 4-2 Recommended Minimum Pavement Sections

Material Flexible Pavement Rigid Pavement

Heavy Duty Light Duty Heavy Duty Asphaltic Concrete Surface

Course (9.5 mm)* 3 inches 2 inches -

Portland Cement Concrete (f’c = 4,000 psi)

- - 6 inches

Graded Aggregate Base Course 8 inches 6 inches 4 inches

*A combination of asphaltic concrete surface course and asphaltic concrete binder course may be used.

Based on our analyses, we anticipate the pavement sections listed above can sustain design traffic loads of approximately 25,000 ESAL and 125,000 ESALs over 15 years for light duty and heavy duty flexible pavement, respectively. Light duty pavement is suitable for parking and drive areas subject only to automobile traffic. Heavy duty pavements should be used in any areas subject to heavy truck traffic. Materials and workmanship should follow the latest edition of the South Carolina Department of Transportation Standard Specifications for Highway Construction.

The heavy-duty rigid pavement sections should be a minimum of 6 inch thick concrete. Heavy duty rigid pavements are recommended for trash dumpster and other heavily trafficked areas (such as drive through lanes) where wheel loads will be concentrated. Provisions for construction traffic have not been included in our analysis.

Please note that large, front-loading trash dumpsters frequently impose concentrated front-wheel loads on pavements during loading. This type of loading typically results in rutting of bituminous pavements and ultimately pavement failures and costly repairs. Consequently, we recommend the use of a 6 inch thick, concrete slab that extends the entire length of the truck. Concrete pavements should be properly jointed and reinforced as needed to help reduce the potential for cracking and to permit proper load transfer.

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A stable subgrade is very important to pavement performance. Immediately prior to paving, the subgrade should be proofrolled and any unstable areas that are not firm and unyielding be repaired.

The base course should be compacted to at least 100% of the maximum dry density, as determined by the Modified Proctor Compaction Test (ASTM D1557). To document that the base course has been uniformly compacted, in-place field density tests should be performed by ECS and the area should be methodically proofrolled under our observation.

Unstable areas observed during proofrolling and removal of existing pavement sections or topsoil should be handled with care. Because the undocumented fill has likely been present for more than 40 years and is underlain by very soft marine clay, we recommend that excavations into the existing undocumented fill in pavement areas be limited to 12 inches, whenever practical. As needed, reinforcing fabric consisting of Mirafi HP270 or equivalent can be utilized in unstable areas below the pavement base course.

The performance of pavements will be dependent upon a number of factors, including subgrade conditions at the time of paving, rainwater runoff, and traffic. Rainwater runoff should not be allowed to seep below pavements from adjacent areas.

The above recommendations are very important for long-term performance of the pavements.

Because pavement design typically has relatively low factors of safety, it will be very important that the specifications are followed closely during pavement construction. Our analysis was based on a 15-year design life; however, some isolated areas could require repair or premature maintenance in a shorter period of time.

4.6 SITE DRAINAGE

Positive drainage should be provided around the perimeter of the structures to reduce the potential for moisture infiltration into the foundation and slab subgrade soils. We recommend that landscaped areas adjacent to the structure and pavements be sloped away from the structure. Roof drains should discharge 5 feet or more from the building perimeter or into below grade storm water piping. Paved areas should also be sloped to divert surface water away from the proposed buildings.

Site drainage is the sole responsibility of the project civil engineer.

5.0 SITE CONSTRUCTION RECOMMENDATIONS

5.1 SUBGRADE PREPARATION

We emphasize the importance of subgrade evaluations prior to engineered fill placement and/or other construction activities. These evaluations may include proofrolling the subgrade soils, performing hand auger borings, and excavation of test pits. The mentioned evaluations would help in identifying areas of soft, loose, otherwise unsuitable materials, which would require remedial activities.

5.1.1 Stripping, Grubbing, and Demolition

The subgrade preparation should consist of stripping vegetation, rootmat, topsoil, unsuitable surficial material, any remnants of previous developments, and other soft or unsuitable materials from the 10-foot expanded building pad and 5-foot expanded pavement limits and to 5 feet beyond the toe of structural fills.

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Surficial material consisting of asphalt was observed in the CPT soundings R-1, R-2, R-5, R-6, B-1, B- 3, and B-6 to approximate depths ranging between 5 and 16+ inches. Deeper topsoil, organics, surficial material, or otherwise unsuitable materials may be present at unexplored areas of the site.

ECS should observe and document that unsuitable surficial materials have been removed or are firm and unyielding with adequate bearing capacity prior to the placement of structural fill or footing construction.

5.1.2 Risk Associated with Undocumented Fill

Based on the previous development of the site, we expect there is an increased potential for portions of the site to have been modified in the past by grading activities resulting in the placement of undocumented fill materials. If existing fill soils containing under-compacted soils or pockets of organics or debris are encountered during construction and are not removed, then localized excessive differential settlements could occur in response to new structural loads and the on-going process of volume change which may still occur in the fill. If such non-uniform settlements occur, then moderate structural distress could result. As such, if existing undocumented fill soils are encountered during construction, they should be further evaluated by proofrolling and Dynamic Cone Penetrometer (DCP) testing as discussed below.

5.1.3 Proofrolling

After removing unsuitable surface materials, cutting to the proposed grade, and prior to the placement of Engineered Fill or other construction materials, the exposed subgrade should be observed by ECS. The exposed subgrade should be proofrolled with construction equipment having a minimum axle load of 10 tons (e.g., loaded tandem-axle dump truck).

The areas subject to proofrolling should be traversed by the equipment in two perpendicular (orthogonal) directions with overlapping passes of the vehicle under the observation of ECS. This procedure is intended to assist in identifying localized yielding materials. In the event that unstable or “pumping” subgrade is identified by the proofrolling, those areas should be repaired prior to the placement of Engineered Fill or other construction materials.

Loose/soft subgrade soils that cannot be improved in-place should be undercut and replaced with Engineered Fill. Methods of repair of unstable subgrade, such as stabilization with geogrid, undercutting or moisture conditioning or chemical stabilization, should be discussed with ECS to establish the appropriate procedure with regard to the existing conditions causing the instability.

A test pit(s) may be excavated to explore the shallow subsurface materials in the area of the instability to help in establishing the cause of the observed unstable materials and to assist in the evaluation of the appropriate remedial action to stabilize the subgrade.

5.1.4 Site Temporary Dewatering

The contractor shall make their own assessment of temporary dewatering needs based upon the limited subsurface groundwater information presented in this report. Soil sampling is not continuous, and thus soil and groundwater conditions may vary between sampling intervals. If the contractor believes additional subsurface information is needed to assess dewatering needs, they should obtain such information at their own expense. ECS makes no warranties or guarantees regarding the adequacy of the provided information to evaluate dewatering requirements; such recommendations are beyond our scope of services.

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Dewatering systems are a critical component of many construction projects. Dewatering systems must be selected, designed, and maintained by a qualified and experienced (specialty or other) contractor familiar with the geotechnical and other aspects of the project. The failure to properly design and maintain a dewatering system for a given project can result in delayed construction, unnecessary foundation subgrade undercuts, detrimental phenomena such as ‘running sand’ conditions, internal erosion (i.e., ‘piping’), the migration of ‘fines’ down-gradient towards the dewatering system, localized settlement of nearby infrastructure, foundations, slabs-on-grade, and pavements, etc. Water discharged from any site dewatering system shall be discharged in accordance with local, state, and federal requirements.

5.2 EARTHWORK OPERATIONS

5.2.1 Engineered Fill

Prior to placement of Engineered Fill, representative bulk samples (about 50 pounds) of on-site and off-site borrow should be submitted to ECS for laboratory testing, which may include Atterberg limits, natural moisture content, grain-size distribution, and moisture-density relationships for compaction. Import materials should be tested prior to being hauled to the site to establish if they meet project specifications.

Our estimates indicate that total settlements in excess of 8 inches may be induced considering the placement of up to 18 inches of fill. These settlements are in addition to the estimated liquefaction induced settlement previously reported. It is important to understand that even with laboratory consolidation test data; it is difficult to accurately predict the rate and magnitude of settlement of the very soft pluff mud marine clay.

Satisfactory Engineered Fill Materials: Materials satisfactory for use as Engineered Fill should consist of inorganic soils classified as SM, SC, SW, SP, GW, GP, GM, and GC, or a combination of these group symbols, per ASTM D2487. The Engineered Fill materials should be free of organic matter, debris, and should contain particle sizes less than 1 ½ inches in the largest dimension. Open graded materials, such as gravels (GP), which contain void space in their mass should not be used in Engineered Fills unless properly encapsulated with geotextile filter fabric. Engineered Fill material should consist of inorganic soils with the following engineering properties and compaction requirements.

ENGINEERED FILL INDEX PROPERTIES

Subject Property

Atterberg Limits LL < 35, PI<10

Max. Particle Size 1 ½ inches

Fines Content (% passing the #200 sieve) Max. 25 %

Max. organic content 5% by dry weight

ENGINEERED FILL COMPACTION REQUIREMENTS

Subject Requirement

Compaction Standard Modified Proctor, ASTM D1557

Required Compaction 95% of Max. Dry Density

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ENGINEERED FILL COMPACTION REQUIREMENTS

Subject Requirement

Moisture Content -3 to +3 % points of the soil’s optimum value

Loose Thickness 8 inches prior to compaction

Unsatisfactory Materials: Materials that should not be used as Engineered Fill include topsoil, organic materials (OH, OL), and low to high plasticity CLAYS and SILTS (CL, ML, CH, MH). Such materials removed during grading operations should be removed from the site and legally disposed of.

Fill Placement Considerations: Proper drainage should be maintained during the earthwork phases of construction to avoid the ponding of water which tends to degrade subgrade soils. Fill materials should not be placed on excessively wet soil. Borrow fill materials should not be excessively wet at the time of placement. Excessively wet soils or aggregates should be scarified, aerated, and moisture conditioned. At the end of each workday, fill areas should be graded to facilitate drainage of precipitation and the surface should be sealed by use of a smooth-drum roller to limit infiltration of surface water.

Alternatively, if these soils cannot be stabilized by conventional methods as previously discussed, additional modifications to the subgrade soils such as cement stabilization may be utilized to adjust the moisture content. If cement is utilized to adjust moisture contents and/or for stabilization, regular Type I/II cement can be used. The contractor should be required to minimize dusting or implement dust mitigation measures, as required. During construction, ECS can obtain additional bulk samples, perform laboratory testing and establish recommended cement contents.

We recommend that the grading contractor have equipment on site during earthwork for both drying and wetting fill soils. We do not anticipate significant problems in adjusting moisture within the fill during dry weather, but moisture adjustments may be difficult during winter months or extended periods of rain. The adjustment of moisture content of higher plasticity soils is difficult when these soils become wet. Further, such soils are easily degraded by construction traffic when the moisture content is elevated.

5.3 FOUNDATION AND SLAB OBSERVATIONS

Protection of Foundation Excavations: Exposure to the environment may weaken the soils at the footing bearing level if the foundation excavations remain open for too long a time. Therefore, foundation concrete should be placed the same day that excavations are made. If the bearing soils are softened by surface water intrusion or exposure, the softened soils must be removed from the foundation excavation bottom immediately prior to placement of concrete. If the excavation must remain open overnight, or if rainfall becomes imminent while the bearing soils are exposed, a 2 to 3-inch thick “mud mat” of “lean” concrete should be placed on the bearing soils before the placement of reinforcing steel.

Footing Subgrade Observations: It is important to have ECS observe the foundation subgrade prior to placing foundation concrete, to document that the bearing soils are what were anticipated. If loose, soft, or unsuitable soils are observed at the footing bearing elevations, these soils should be removed and replaced prior to concrete placement.

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Slab Subgrade Observation: A representative of ECS should be called on to observe slab subgrades prior to drainage layer placement to document that adequate subgrade preparation has been achieved. A proofroll using a loaded dump truck should be performed in their presence at that time.

5.4 GENERAL CONSTRUCTION CONSIDERATIONS

Construction Monitoring: ECS should be on-site full-time during earthwork and foundation construction activities to document that our recommendations are followed and to provide recommendations for remedial activities, where necessary. If we are not retained for geotechnical consulting during earthwork construction and foundation construction, ECS cannot be responsible for long-term performance of the subgrade-supported construction.

Subgrade Protection: Measures should also be taken to limit site disturbance, especially from rubber-tired heavy construction equipment, and to remove surface water from development areas, including structural and pavement areas.

Surface Drainage: Surface drainage conditions should be properly maintained. Surface water should be directed away from the construction area, and the work area should be sloped away from the construction area at a gradient of 1 percent or greater to reduce the potential of ponding water and the subsequent saturation of the surface soils. At the end of each workday, the subgrade soils should be sealed by rolling the surface with a smooth drum roller to minimize infiltration of surface water.

Erosion Control: The surface soils may be erodible. Therefore, the contractor should provide and maintain good site drainage during earthwork operations to maintain the integrity of the surface soils. Erosion and sedimentation controls should be in accordance with sound engineering practices and local requirements.

Means and Methods: Please note that the contractor is fully responsible for the means and methods employed in the construction of the project, and that the contractor shall confirm that work is conducted in accordance with OSHA standards.

Excavation Safety: Cuts or excavations may require forming or bracing, slope flattening, or other physical measures to control sloughing and/or prevent slope failures. Contractors should be familiar with applicable OSHA codes to ensure that adequate protection of the excavations and trench walls is provided.

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6.0 CLOSING

ECS has prepared this report to guide the geotechnical-related design and construction aspects of the project. We performed these services in accordance with the standard of care expected of professionals in the industry performing similar services on projects of like size and complexity at this time in the region. No other representation expressed or implied, and no warranty or guarantee is included or intended in this report.

The description of the proposed project is based on information provided to ECS by RQ Construction. If any of this information is inaccurate or changes, either because of our interpretation of the documents provided or site or design changes that may occur later, ECS should be contacted so we can review our recommendations and provide additional or alternate recommendations that reflect the proposed construction.

We recommend that ECS review the project plans and specifications so we can confirm that those plans/specifications are in accordance with the recommendations of this geotechnical report.

Field observations, and quality assurance testing during earthwork and foundation installation are an extension of, and integral to, the geotechnical design. We recommend that ECS be retained to apply our expertise throughout the geotechnical phases of construction, and to provide consultation and recommendation should issues arise.

ECS…