ARPA_Community_Center_–_Agnes_Major_Site_-_Geotechnical_Evaluation_1-26-24.pdf

The Power of Commitment i

Report of Subsurface Exploration & Geotechnical Evaluation APRA Community Center – Agnes Major Site

County Council of Beaufort County

January 26, 2024

The Power of Commitment

The Power of Commitment ii

The Power of Commitment

GHD

Our Ref: 12630716-00 | ARPA Community Center – Agnes Major Site

January 26, 2024

Mr. Eric Larson, P.E.

County Council of Beaufort County Capital Improvements Department 104 Industrial Village Drive Post Office Drawer 1228 Beaufort, South Carolina 29901

Report of Subsurface Exploration & Geotechnical Evaluation

Dear Mr. Larson:

GHD is pleased to present the results of our subsurface investigation and geotechnical evaluation for the above-referenced project. As requested, we have mobilized to the project site to perform subsurface explorations to develop an interpretation of the subsurface conditions and to provide geotechnical recommendations for the site, which include the following:

– Site preparation and shallow foundation design recommendations for the proposed community center structure

(including potential ‘static’ settlement estimates, seismic design parameters, potential liquefaction induced settlement estimates and mitigation options, if necessary); and

– Site preparation and asphalt pavement cross-section recommendations for the proposed drive and parking areas;

and

Our services were performed in general accordance with our proposal dated December 15, 2023. The results of our study are provided herein.

We appreciate the opportunity to work with you on this project. If there are any questions or comments, please do not hesitate to contact us at (843) 815-5120.

Regards

Wesley A. Marshall, GIT Robin M. Moutray, P.E.

843-815-0272 843-815-0267 wesley.marshall@ghd.com robin.moutray@ghd.com

Enclosures

57-C Sheridan Park Circle Bluffton, South Carolina 29910 United States www.ghd.com

1-26-24 http://www.ghd.com/

The Power of Commitment iv

Contents

1. Site Description / Project Understanding 1

2. Subsurface Explorations / Testing 1

2.1 Soil Test Borings 1

2.2 Hand-Auger Borings 2

2.3 Soil Sample Handling / Laboratory Analyses 2

3. Subsurface Conditions Encountered 2

3.1 Subsurface Soils 2

3.2 Encountered Groundwater Levels 3

4. Conclusions & Recommendations 3

4.1 Site Preparation 4

4.1.1 Moisture Control 4

4.1.2 Stripping and Grubbing 4

4.1.3 Building Pad Subgrade Inspection / Possible Remediation 4

4.1.4 Underground Utility / Structure Installation and Dewatering 5

4.1.5 Excavation Safety 5

4.1.6 Fill/Backfill Placement 5

4.2 Foundations Design and Construction 6

4.2.1 Shallow Foundation Sizing / Bearing Capacity 6

4.2.2 Shallow Foundation Settlement Potential 6

4.2.3 Liquefaction Potential 6

4.2.4 Seismic Design Parameters 7

4.3 Pavement Design and Construction 8

4.3.1 Suitability of Shallow Soils for Pavement 8

4.3.2 Conventional Asphalt Pavement Section Design 9

4.3.3 Subgrade and Base Preparation 9

5. Limitations 9

The Power of Commitment v

Table index

Table 3.1 Generalized Stratigraphic Profile 3

Table 3.2 Stabilized Groundwater Depths 3

Table 4.1 Fill/Backfill Placement Recommendations 5

Table 4.2 Shallow Foundation Design Parameters 6

Table 4.3 Potential Post-Construction Settlement Estimates 6

Appendices

Appendix A Figures

Appendix B Soil Test Boring Logs

Appendix C Log of Penetrometer Tests

Appendix D Seismic Design Parameters

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 1

1. Site Description / Project Understanding

GHD received project information via email correspondence with you on December 6, 2023, including a document titled "Beaufort County ARPA Community Centers – Request for Geotechnical Proposal”. The correspondence included an aerial view sketch titled, “Agnes Major Site”.

According to the information provided, the subject site is located within the northwest quadrant of an existing recreational park having a street address of 21 Agnes Major Road in Seabrook, South Carolina. The general location of the site is depicted in Figure A.1 in Appendix A. The overall existing recreation park property currently supports a basketball court, playground, a “Boys and Girls Club” recreational building, an asphalt parking lot and associated above ground and underground utilities.

The document provided indicates a new community center structure is proposed for the site with associated asphalt paved parking and driveway areas. The new asphalt paved driveway and parking area is in the area of the existing asphalt paved driveway and parking area.

We have assumed for the purpose of our analyses that the structure is preferred to be supported on a shallow foundation system having isolated and/or continuous foundation loads not exceeding 30 kips and 2.0 kips per foot, respectively. Based on our review of site topography, minimal fill (less than 18 inches) is anticipated to be required.

2. Subsurface Explorations / Testing

The scope of our geotechnical field explorations included soil test borings and hand-auger borings with associated asphalt coring and dynamic cone penetrometer testing.

The approximate locations of the explorations are shown on Figure A.2 in Appendix A. A GHD professional positioned the explorations by overlaying an aerial photograph onto the provided document to estimate global positioning system (GPS) coordinates of our explorations in Google EarthTM. The coordinates were transferred to a hand-held GPS unit and utilized the unit in the field to locate the exploration locations. Given the method of locating the borings in the field, the locations indicated on Figure A.2 should be considered approximate. Details of the explorations performed are detailed in the sections that follow.

2.1 Soil Test Borings

Two (2) soil test borings (designated as B-1 and B-2) were performed within the community center structure to depths of 25 to 50 feet below the existing ground surface on January 9 and 10, 2024. The soil test borings were advanced utilizing mud rotary drilling methods. SPT testing procedures were conducted in general accordance with ASTM

D1586. Closely spaced soil sampling was performed in the upper ten feet and at five-foot intervals thereafter. During the sampling procedure, SPT tests were performed to obtain the standard penetration value of the soil. The standard penetration value (N) is defined as the number of blows of a 140-pound hammer, falling thirty inches, required to advance the split spoon sampler one foot. The sampler is lowered to the bottom of the drill hole and the number of blows recorded for each of three successive increments of six inches penetration. The “N” value shown on the boring log is obtained by adding the second and third incremental values. The results of the SPT testing indicate the relative density and comparative consistency of the soils, and thereby provide a basis for estimating relative strength and compressibility of the soil profile components. The logs of the soil test borings are presented as Appendix B, Soil

Test Boring Logs.

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 2

2.2 Hand-Auger Borings

Three hand-auger borings (designated HA-1 through HA-3) were performed within existing / future pavement areas to depths of approximately 48 inches on January 11, 2024. The soils at each hand-auger location were examined by manually twisting a steel auger into the soil and retrieving samples of the cuttings at regular depth intervals. Our personnel visually classified the soils encountered in the field. Additionally, the soils at the hand-auger boring locations were evaluated at regular depth intervals using a Dynamic Cone Penetrometer (DCP) in general conformance with ASTM Special Technical Publication #399. In the DCP test, a 1½-inch conical point is seated 1-3/4 inches to penetrate loose cuttings, then drive two additional 1¾-inch increments with blows from a 15-pound hammer falling 20 inches. The hammer blows are recorded and provide an index to soil strength and density when properly evaluated. The logs of the hand-auger borings are presented as Appendix C, Log of Penetrometer Tests.

Prior to the hand-auger borings, our personnel extracted a core from each of the asphalt pavement surface locations

HA-1 through HA-3. The thickness of the asphalt cores were measured in the field. Below the asphalt surfacing, aggregate base course was removed and its thickness measured prior to conducting our hand-auger and DCP procedures. GHD replaced the removed pavement materials with re-heated hot mix asphalt.

2.3 Soil Sample Handling / Laboratory Analyses

The soil samples obtained from the explorations were placed in individual containers, properly sealed and marked for identification. The soils samples were then transported to our laboratory for analysis and classification by a GHD professional in general accordance with ASTM D2487 (Standard Practice for Classification of Soils for Engineering

Purposes (Unified Soil Classification System)).

Selected samples of the soils collected from the borings and hand-augers were tested in our laboratory to determine their percent fines (ASTM D1140) and natural moisture content (ASTM D2216). The laboratory data was used to aid in the classification of the soils in accordance with ASTM D2487 and to further define their engineering properties.

The laboratory test results are presented on the logs in Appendices B and C.

3. Subsurface Conditions Encountered

3.1 Subsurface Soils

A GHD professional developed the final boring log information from the field logs and visual review of the recovered soil samples in our laboratory. Similar soils were grouped into strata, with each stratum described in general accordance with the nomenclature used in ASTM D2487. Although indicated on the boring logs as distinct changes, the transition from one soil type or stratum to another may be gradual or may occur at slightly differing elevations than indicated between soil samples. Soil conditions may also vary from our findings at locations in areas of the site not explored.

For reference, a generalized stratigraphic profile developed from the explorations is provided in Table 3.1. For a more detailed description, please refer to the Appendices to this report. The logs include the SPT “N” values (where applicable), Unified Soil Classification System (USCS) symbols and groundwater levels at the time of our study.

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 3

Table 3.1 Generalized Stratigraphic Profile

Approximate Depth Below Ground Surface (feet)

Material Description Relative Density from SPT testing

0 to 2 FILL: Silty fine SAND (SM) Very Loose to Loose

2 to 8 Very clayey fine SAND (SC) Loose to Medium Dense

8 to 22 Fine SANDS with moderate to significant amounts of silt / clay (SM, SC) Very Loose to Loose

22 to 32 Variable: Fine sandy CLAY (CL) and/or Fine SAND with moderate amounts of silt / clay (SM, SC)

Medium Dense and/or Very Soft to Stiff

32 to 50 Fine SAND with moderate amounts of silt / clay (SM, SC) Medium Dense

A notable condition related to the generalized stratigraphic profile is the presence of very loose to loose sand in the upper 2 feet of the ground surface at our soil test boring locations.

3.2 Encountered Groundwater Levels

Groundwater levels encountered during our study are summarized in Table 3.2. We expect groundwater levels will fluctuate depending upon the season, recent rainfall quantities in the area, changing drainage patterns during and post-development of this and surrounding areas, as well as other factors.

Table 3.2 Stabilized Groundwater Depths

Soil Test Borings Hand-Auger Borings

2 to 2.5 feet Not Encountered

4. Conclusions & Recommendations

The following conclusions and recommendations are based on the project characteristics previously described, the data obtained in our field explorations, and our experience with similar subsurface conditions and development projects. If the final design grades are significantly different from those currently assumed, or if subsurface conditions differ from those disclosed by the borings are encountered during development, we should be notified so that we might review the following recommendations in light of such changes.

In general, it is our opinion that the subsurface conditions encountered by the explorations are suitable for support of the future development using conventional shallow foundations and conventional pavements following implementation of the site preparation and design recommendations discussed in the following sections of this report. However, the following site conditions / recommendations should be noted and which are detailed in this section of the report:

– Our analysis of the potential magnitude of settlement due to liquefaction indicates that following completion of the site preparation recommendations detailed above, settlement could be as much as 2 to 4 inches within the area of the proposed structure. Therefore, ‘improvement’ of the subsurface soils to a degree sufficient to reduce the magnitude of potential liquefaction induced settlement of the proposed structure to a lesser magnitude may be warranted. Subsurface soil improvement/ground modification options are discussed in Section 4.2.5.

– Our boring locations within the building footprint encountered very loose to loose silty fine sand (SM) in the upper

2 feet of the existing ground surface that are expected to be unstable at the time of site preparation and which were underlain by moisture sensitive very clayey fine sands (SC) that may also become unstable. Due to their moisture sensitive natures, if these near surficial soils become unstable, they will require over-excavation and replacement with suitable structural fill. Strict adherence to Section 4.1.1 will help to minimize the potential over-excavation as a result of site preparation activities.

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 4

– The soils encountered at our hand-auger boring locations contain silt/clay contents in excess of the recommended maximum of 20 percent anticipated to be encountered within the 24-inch subbase layer of the recommended pavement, such that removal and replacement should be anticipated if site elevating fill does not provide for the necessary 24-inch subbase thickness above these existing soils or 12 inches above the existing pavement surface. Due to the presence of these materials, GHD recommends the existing pavement components be removed and replaced in order to construct the 24-inch sand subbase layer where site grades cannot provide at least 12 inches above the existing asphalt pavement or aggregate base course surface.

4.1 Site Preparation

4.1.1 Moisture Control

Our explorations encountered moisture sensitive silty and clayey soils at the existing ground surface throughout the proposed development area. Strict moisture control will need to be maintained to avoid softening of the soil during site development. Failure to control moisture in silty or clayey soils may result in the need for removal and replacement of otherwise suitable soils. Moisture control methods should also be implemented even where more favorable soils are located within the upper two feet. Moisture control methods should include, but are not necessarily to be limited to:

– Staging the work to avoid excessive exposure to inclement weather;

– Maintaining positive drainage at the end of each workday or prior to inclement weather;

– Using a smooth drum roller or bulldozer to seal areas to facilitate runoff;

– And minimizing/limiting rubber-tired vehicle traffic by utilizing low contact pressure or tracked equipment whenever possible across the work area.

We highly recommend that surface water across the area be managed prior to, during and after stripping and grubbing operations to avoid excessive surface moisture which can lead to an unstable working surface and thus, undue mixing of the organic debris with the underlying soils. Therefore, it may be necessary to remove ponded surface water and to allow the moisture content of the surficial and shallow subgrade soils to dry naturally prior to initiating general site preparation procedures.

4.1.2 Stripping and Grubbing

The initial step in site preparation should be the complete removal of all surficial organic materials, major root systems

(root larger than finger size), topsoil, pavements, utilities and other deleterious materials from beneath and to three feet beyond the limits of the structure and pavement areas. Although we encountered up to approximately 2 inches of topsoil or up to 10¼ inches of pavement materials at our exploration locations (see Section 4.3.2), it should be anticipated that removal of deleterious material to greater depths may be required due to major root systems of trees, as well as due to disturbance of the surface soils by site preparation equipment during stripping, grubbing, root raking, etc.

During site clearing and while excavating for site utilities and foundations, the exposed soils should be carefully inspected for the presence of organic and/or deleterious materials that could be detrimental to foundations and slabs.

If encountered, these materials and the soils containing them should be removed and replaced with approved properly compacted structural fill.

4.1.3 Building Pad Subgrade Inspection / Possible Remediation

Once the building footprint has been cleared of vegetation and other deleterious materials, GHD should thoroughly inspect the disturbed surficial soils in the structure footprint. Our boring locations within the building footprint encountered very loose to loose silty fine sands (SM) in the upper 2 feet of the existing ground surface that are expected to be unstable at the time of site preparation and which were underlain by moisture sensitive very clayey fine sands (SC) that may also become unstable. Due to their moisture sensitive natures, if they become unstable, these

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 5 soils will require over-excavation and replacement with suitable structural fill. Strict adherence to Section 4.1.1 will help to minimize the potential over-excavation as a result of site preparation activities.

The geotechnical engineer or his or her representative should recommend the nature and extent of any necessary remedial work.

4.1.4 Underground Utility / Structure Installation and Dewatering

The soils encountered in our explorations are generally suitable for support of the anticipated underground construction, provided moisture control measures are maintained within these excavations.

We anticipate that the soils to be excavated from the excavations will be unsuitable for reuse as backfill in structural areas due to their elevated moisture and silt/clay content.

Results of our exploration indicate that dewatering will likely be required for underground construction at depths of approximately 2 to 2.5 feet below the existing ground surface elevation. This dewatering system should be installed and operated to maintain the groundwater level at a minimum of 2 feet below the bottom of the excavation.

4.1.5 Excavation Safety

All excavations should be braced in accordance with the U.S. Department of Labor, Occupational Safety and Health

Administration (OSHA) guidelines (29 CFR 1926, Subpart P, Excavations) or other applicable jurisdictional codes for permissible temporary side-slope ratios and/or shoring requirements. We recommend that the Contractor determine the actual groundwater levels at the time of the construction to determine groundwater impact, if any, on the construction procedures. Groundwater levels should be maintained at least 2 feet below any foundation or utility excavation bottom.

We recommend that the contractor exercise extreme caution in any decision to place personnel and equipment in unbraced excavations, particularly when wetted or subjected to surcharge loads or vibrational forces.

4.1.6 Fill/Backfill Placement

All fill/backfill soils in building pad and pavement areas should consist of inorganic, granular soils meeting the material and compaction recommendations as summarized in Table 4.1.

Table 4.1 Fill/Backfill Placement Recommendations

Location of Fill Composition Compaction Effort as Determined by Modified Proctor Test (ASTM D1557)

Structure <25% silt/clay 95%

≤ 24 inches below Base Course <20% silt/clay 98%

> 24 inches below Base Course <25% silt/clay 95%

The top surface of structural fill should extend a minimum of 3 feet beyond the perimeter of the structure and 2 feet beyond the limits of pavements. Fill slopes should not exceed 2 horizontal to 1 vertical to prevent possible erosion or undermining of slabs, shallow footings and/or pavements. Shallower slopes may be dictated by site grading requirements.

In-place density tests should be performed on each lift by an experienced engineering technician working under the direction of a licensed geotechnical engineer to verify that the recommended degree of compaction has been achieved.

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 6

4.2 Foundations Design and Construction

4.2.1 Shallow Foundation Sizing / Bearing Capacity

Once site preparation as detailed above is completed, our evaluation indicates that the future structure can be supported on shallow foundation systems bearing in properly compacted fill/backfill soils. Table 4.2 summarizes our recommendations for sizing and bearing capacity.

Table 4.2 Shallow Foundation Design Parameters

Foundation Type Maximum Allowable Bearing Capacity (psf)

Foundation Minimum Sizing / Depth Requirements

Continuous & Isolated 1,500 24 inches wide; 12 inches deep

All foundation elements should be excavated, formed if necessary, and have their concrete cast in the dry. It is possible that shallow groundwater and/or perched groundwater may be an issue at the time of footing excavation. Any soils at the bottom of the foundation excavations disturbed during the excavation process or soften due to shallow groundwater should be removed and immediately replaced with open graded aggregate, such as No. 57 stone, prior to placement of reinforcing steel.

Dynamic cone penetrometer (DCP) testing and inspections should be performed in the foundation excavations prior to placement of reinforcement steel to verify compliance with project specifications. The testing and inspections should be performed by an experienced engineering technician operating under the direction of a licensed professional geotechnical engineer. It should be anticipated that some selective undercutting and backfilling to the desired bottom of footing elevation with properly compacted select fill or graded aggregate may be required. If this issue is encountered during construction, GHD should be consulted to evaluate the field conditions and to determine the extent of the required undercutting and appropriate alternatives for backfill.

4.2.2 Shallow Foundation Settlement Potential

Post-construction settlement potential due to structure loading is summarized in Table 4.3.

Table 4.3 Potential Post-Construction Settlement Estimates

Foundation Type Estimated Total Potential Settlement Estimated Differential Settlement

Isolated ≤1 inch ≤1/2 inch between any 2 columns

Continuous ≤1/2 inch ≤1/2 inch in 200 feet

4.2.3 Liquefaction Potential

The subject property is located within an active seismic zone with its center in the Charleston, South Carolina area.

Although the area has not experienced significant earthquake events in the recent past, evidence of seismic event induced liquefaction has been found and geologists have mapped this area as having the potential for recurrence(s) of such an event. Considerable research is ongoing to better determine which local soils are truly liquefiable and the magnitude of settlement that might occur as a result of their liquefaction during a significant seismic event.

Based on our review of soil and groundwater conditions at the subject site, we believe there is a risk for liquefaction settlement to occur during a significant seismic event. Utilizing ‘LiquefyPro’ modelling software, we have performed a liquefaction analysis of the subject site considering the subsurface soil and groundwater conditions encountered and, in reference to the 2021 International Building Code (2021 IBC).

As previously stated, our analysis of the potential magnitude of settlement due to liquefaction indicates that following completion of the site preparation recommendations detailed above, settlement could be as much as 2 to 4 inches within the area of the proposed structure. Therefore, ‘improvement’ of the subsurface soils to a degree sufficient to

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 7 reduce the magnitude of potential liquefaction induced settlement of the proposed structure to a lesser magnitude may be warranted. Subsurface soil improvement/ground modification options are discussed in Section 4.2.5. The following additional information is offered:

– The potential liquefaction induced settlement would be due generally to the consolidation of the loose saturated sand soils below the water table variably encountered between the depths of approximately 10 to 17 feet below the existing ground surface.

– We estimate that differential liquefaction induced settlement within the building footprint would likely be on the order of 0.0015L to 0.0025L.

– Lateral spreading, which generally occurs in the presence of a sloping ground surface or permanent excavation near the structure, or permanent lateral ground displacement is not expected to occur during a significant seismic event.

4.2.4 Seismic Design Parameters

Due to the magnitude of potential liquefaction induced settlement of the soils encountered in the soil test borings at this site, it is our interpretation of the 2021 International Building Code (IBC) with reference to ASCE 7-16 that the subject site would be classified as Seismic Site Class “F”. However, based upon the soil conditions encountered, it is our understanding that the designation of the site as Seismic Site Class “D” is allowed by the 2021 IBC for structures having a design fundamental period of less than or equal to 0.5 seconds assuming the risks associated with liquefaction are considered in other design aspects of the structure. The procedure for determining the site-specific seismic design parameters follow that which is outlined in the 2021 International Building Code with reference to

ASCE 7-16. Values for Spectral Response Acceleration for short periods (0.2 seconds), SS, and for long periods (1 second), S1, were obtained from the ASCE ‘Hazards by Location’ online tool which queries the United States

Geological Survey (USGS) web servers and retrieves the seismic design variables in a report format. The values are expressed as a multiple of the acceleration of gravity. The resulting design parameters for Risk Category II is presented in Appendix D, Seismic Design Parameters.

4.2.5 ‘Subsurface Soil Improvement’ / ‘Ground Modification’

Seismic settlement potential mitigation options found to be effective in soil such as those encountered at the subject site include Rammed Aggregate Pier (RAP) systems, vibro-replacement (stone columns), vibro-compaction and earthquake drains. With the exception of earthquake drains, these procedures, or some combination of more than one of them, could potentially be utilized to both densify the deep in situ soils and to decrease the potential for liquefaction, thus significantly reducing the estimated potential ‘post-construction’ settlement of shallow spread footing type foundations due to a seismic event.

Rammed Aggregate Pier (RAP) systems and vibro-replacement (stone columns), although they utilize differing installation techniques, both reduce the settlement potential of the in situ soils by densification of the overall soil matrix between the installed elements, providing a drainage path through the installed elements to eliminate the build-up of excessive pore pressure during a seismic event, and providing a stiff, non-liquefiable load-bearing element within the soils.

Vibro-compaction uses a specially designed vibrator that is inserted (typically with the aid of jetting) into the subsurface soils and repeatedly raised and lowered in increments as it is extracted in order to densify the in situ soils.

Often the process also includes the addition of clean sand at the ground surface to compensate for the reduction in soil volume resulting from the densification process. This ground modification technique simply densifies the in situ cohesionless soils and, therefore, reduces their potential for static foundation load and/or liquefaction induced consolidation as well as increases their load bearing capacity.

Vertical composite ‘Earthquake’ drains consist of corrugated, perforated pipe wrapped in a special non-woven filter fabric. The drains are tipped with a sacrificial end plate then inserted into the ground to the desired depth within a

‘vibro probe’ or vibrating mandrel consisting of a protective thin-walled steel pipe casing. The probe casing is then vibrated back out of the ground leaving the earthquake drain in place. Installation of earthquake drains reduces the

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 8 liquefaction potential of in situ soils partially by densification of the soils during insertion of the drain casing but more significantly by providing a drainage path to eliminate the build-up of excessive pore pressure during a seismic event.

The improved soil characteristics following implementation of any of the alternative modifications techniques depends on the subsurface soils types and soil particle gradation, long term groundwater conditions, the diameter, depth and spacing of the various devices and/or vertical penetration points, intensity of vibratory action during the process, and various additional site and technique specific factors. Typically, the equipment and techniques used are proprietary to individual specialty design and installation firms. It is our recommendation that one or more such firms be consulted to determine which of the various ground modification techniques would be technically best suited and/or economically most efficient for the subject site. The goal of the ground modification program should be specified (settlement potential reduction and/or liquefaction mitigation) and the contractor(s) should be requested to submit a proposal detailing the ground modification method to be utilized and the associated costs. The proposal(s) should be evaluated by the structural design engineer and the contractor selected based upon that evaluation.

4.2.6 Floor Slabs

The floor slab subgrade soils should be compacted to meet the requirements of Section 4.1.6 and should be recompacted to meet these requirements immediately prior to placement of reinforcing steel, once utility trenches are backfilled.

Based upon the soil conditions encountered at the site, the planned fill placement, and the recommended site preparation operations presented in this report, the modulus of vertical subgrade reaction (k) for the specified fill to constitute the slab bearing soils is expected to be on the order of 125 pounds per square inch per inch of vertical deflection (pci), at the recommended structural fill compaction criteria.

We recommend the installation of a vapor retarder between the base and the concrete floor slabs. The vapor retarder should conform to the requirements of ASTM E1745 (Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs), Class A.

The structural fill soils as specified herein for use in constructing the building pad are considered to be relatively free-draining soils. It is our opinion that these soils would be classified as “drainable”. However, given the potential for groundwater to infiltrate and temporarily remain within the fill soils, we recommend a minimum of a 4-inch thick layer of open-graded aggregate, such as No. 57 stone, to act as a capillary barrier immediately below the floor slab. Note that all downspouts/roof drains should be positioned such that stormwater is directed away from the structure and that site should be constructed to meet the civil design grading requirements.

4.3 Pavement Design and Construction

The following pavement design guidelines are made without the benefit of specific traffic information and/or reference to any minimum section standards and are intended as a general guide for the design engineer’s evaluation. Site design decisions may dictate alterations to certain aspects of these guidelines. The following recommendations assume that site preparation procedures, including removal and replacement of unsuitable near surface soils/debris, have been completed where necessary. The data generated by the explorations performed for this evaluation may need to be augmented with additional data as the design process proceeds.

All pavements and base courses should be constructed in accordance with the guidelines of the latest applicable

South Carolina Department of Transportation Specifications.

4.3.1 Suitability of Shallow Soils for Pavement

The soils encountered at our hand-auger boring locations contain silt/clay contents in excess of the recommended maximum of 20 percent anticipated to be encountered within the 24-inch subbase layer of the recommended pavement, such that removal and replacement should be anticipated if site elevating fill does not provide for the necessary 24-inch subbase thickness above these existing soils or 12 inches above the existing pavement surface.

Due to the presence of these materials, GHD recommends the existing pavement components be removed and

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 9 replaced in order to construct the 24-inch sand subbase layer where site grades cannot provide at least 12 inches above the existing asphalt pavement or aggregate base course surface.

Careful consideration should be given to the potential for the moisture sensitive silty and clayey sands to become unstable due to moisture intrusion for inclement weather while exposed. Where this occurs, additional undercutting would likely be required to facilitate development. We recommend that a representative of GHD be contacted to evaluate the subbase soils during construction to verify areas requiring removal and replacement.

4.3.2 Conventional Asphalt Pavement Section Design

We anticipate an asphalt pavement section consisting of 2 inches of Type C surface course over 6 inches of graded aggregate base course. The pavement section should be underlain by a sand subbase (with a maximum of 20 percent by weight passing the No. 200 sieve) compacted to at least 98 percent of the maximum dry density as determined by the Modified Proctor test (ASTM D1557). The minimum thickness of sand subbase should be either 24 inches or 12 inches, as it pertains to site elevations as described in Section 4.3.2.

4.3.3 Subgrade and Base Preparation

Immediately prior to placing the aggregate base course, the subgrade (subbase) soils should be moisture adjusted, if necessary, and compacted to meet density requirements as outlined in Section 4.1.6. The subgrade soils should be proofrolled to check for stable conditions.

The graded aggregate base course should be compacted to at least 98 percent of the maximum dry density as determined by the ASTM D1557. The base course should be proofrolled to verify stable conditions prior to asphalt paving. We recommend the placement of a prime coat on the aggregate base course if the base will remain exposed to the elements for a period longer than 2 weeks prior to paving. A prime coat helps fill voids, stabilize the fines, protect the base course, and promotes bonding to the asphalt course.

5. Limitations

This report has been prepared by GHD for County Council of Beaufort County and may only be used and relied on by

County Council of Beaufort County for the purpose agreed between GHD and County Council of Beaufort County as set out in this report.

GHD otherwise disclaims responsibility to any person other than County Council of Beaufort County arising in connection with this report. GHD also excludes implied warranties and conditions, to the extent legally permissible.

The services undertaken by GHD in connection with preparing this report were limited to those specifically detailed in the report and are subject to the scope limitations set out in the report.

The opinions, conclusions and any recommendations in this report are based on conditions encountered and information reviewed at the date of preparation of the report. GHD has no responsibility or obligation to update this report to account for events or changes occurring subsequent to the date that the report was prepared.

The opinions, conclusions and any recommendations in this report are based on assumptions made by GHD described in this report. GHD disclaims liability arising from any of the assumptions being incorrect.

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Appendices

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Appendix A Figures

TERMINAL

DOCK SITES

Figure A.1: Site Vicinity Map January 19, 2024 GHD | Beaufort County | 12630716-00 | ARPA Community Center – Agnes A. Major Site | Subsurface Exploration and Geotechnical Evaluation A-1

Figure A.2: Exploration Location Plan – ARPA Community Center – Agnes A. Major Site January 19, 2024 GHD | Beaufort County | 12630716-00 | ARPA Community Center – Agnes A. Major Site | Subsurface Exploration and Geotechnical Evaluation A-2

NOTE: This drawing utilizes a copy of an aerial view sketch titled, “Agnes Major Site” (undated).

B-2

B-1

Designation / approximate location of

Standard Penetration Testing (SPT) Borings

B-# Designation / approximate location of Hand-Auger

Borings / Dynamic Cone Penetrometer (DCP) Testing

HA-#

HA-1

HA-2

HA-3

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 12

Appendix B Soil Test Boring Logs

GHD | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation

Key to Soil Classification

Correlation of Penetration Resistance with

Relative Density and Consistency

Sands and Gravels Silts and Clays

No. of Relative No. of Relative Blows, N Density Blows, N Density 0 – 4 Very loose 0 – 2 Very soft 5 – 10 Loose 3 – 4 Soft 11 – 30 Medium dense 5 – 8 Firm 31 – 50 Dense 9 – 15 Stiff Over 50 Very dense 16 – 30 Very stiff 31 – 50 Hard Over 50 Very hard

Particle Size Identification

(Unified Classification System)

Boulders: Diameter exceeds 8 inches Cobbles: 3 to 8 inches diameter Gravel: Coarse - 3/4 to 3 inches diameter Fine - 4.76 mm to 3/4 inch diameter Sand: Coarse - 2.0 mm to 4.76 mm diameter Medium - 0.42 mm to 2.0 mm diameter Fine - 0.074 mm to 0.42 mm diameter Silt and Clay: Less than 0.07 mm (particles cannot be seen with naked eye)

Modifiers

The modifiers provide our estimate of the amount of silt, clay or sand size particles in the soil sample.

Approximate Content

Modifiers

Field Moisture Description

5%: Trace Saturated: Usually liquid; very wet, usually from below the groundwater table

5% to 12%: Slightly silty, slightly clayey, slightly sandy

Wet: Semisolid; requires drying to attain optimum moisture

12% to 30%: Silty, clayey, sandy Moist: Solid; at or near optimum moisture

30% to 50%: Very silty, very clayey, very sandy

Dry: Requires additional water to attain optimum moisture

1.4

2.8

4.2

5.6

8.4

9.8

11.2

12.6

15.4

16.8

18.2

19.6

22.4

23.8

25.2

26.6

29.4

30.8

32.2

33.6

36.4

37.8

39.2

40.6

43.4

44.8

46.2

47.6

50.4

51.8 53.2

25.2

33.2

24.6

29.7

SM

SC

SC

SM

SC

CL

CL

SC

SM

Very loose brown and tan silty fine SAND (FILL)

Loose to medium dense tan and orange very clayey fine SAND

Loose tan and orange clayey fine SAND

Loose tan silty fine SAND

Very loose tan very clayey fine SAND

Stiff tan fine very sandy CLAY

Firm tan and gray fine very sandy CLAY

Dense brown very clayey fine SAND

Medium dense brown and gray silty fine SAND

Boring terminated at 50 feet

PROJECT: LOG OF BORING:

ARPA Community Center - Agnes A. Major B-1

DATE DRILLED:

1/9/2024

DRILLER:

A. Nelson

GROUND ELEVATION:

DRILLING METHOD: Mud Rotary BORING DEPTH: 50.00 Feet Notes:

WATER LEVEL: WATER LEVEL (24-HRS): 2.5'

M o is tu re C o n te n t

ANALYSIS

P as s in g

S ie v e

O rg a n ic C o n te n t

L iq u id

L im it

P la st ic it y

In d ex

D ep th fe et

B L

O W

C O

U N

T S

G R

O U

N D

W A

T E

R

G R

A P

H

IC

L O

G

U S

C S

GEOLOGIC DESCRIPTION

JOB NUMBER: 12630716-00 GHD SERVICES, INC. PAGE 1 OF 1

1.4

2.8

4.2

5.6

8.4

9.8

11.2

12.6

15.4

16.8

18.2

19.6

22.4

23.8

25.2

26.6

29.4

30.8

32.2

33.6

36.4

37.8

39.2

40.6

43.4

44.8

46.2

47.6

50.4

51.8 53.2

22.4

35.1

37.1

21.1

SM

SC

SC

SC

CL

SC

Loose brown silty fine SAND (FILL)

Loose to medium dense brown and orange very clayey fine SAND

Firm tan clayey fine SAND

Very loose tan clayey fine SAND

Very soft tan and brown fine sandy CLAY

Medium dense gray clayey fine SAND

Boring terminated at 30 feet

PROJECT: LOG OF BORING:

ARPA Community Center - Agnes A. Major B-2

DATE DRILLED:

1/10/2024

DRILLER:

A. Nelson

GROUND ELEVATION:

DRILLING METHOD: Mud Rotary BORING DEPTH: 30.00 Feet Notes:

WATER LEVEL: WATER LEVEL (24-HRS): 2'

M o is tu re C o n te n t

ANALYSIS

P as s in g

S ie v e

O rg a n ic C o n te n t

L iq u id

L im it

P la st ic it y

In d ex

D ep th fe et

B L

O W

C O

U N

T S

G R

O U

N D

W A

T E

R

G R

A P

H

IC

L O

G

U S

C S

GEOLOGIC DESCRIPTION

JOB NUMBER: 12630716-00 GHD SERVICES, INC. PAGE 1 OF 1

GHD | County Council of Beaufort County | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 1

Appendix C Log of Penetrometer Tests

C-1 Log of Penetrometer Tests

GHD | Client Name | Project # | Project Name | Subsurface Exploration and Geotechnical Evaluation C-1

Project: ARPA Community Center - Agnes A. Major Site Date: January 11, 2024

Personnel: W. Marshall Reference No: 12630716-00

Location: See Figure A.2

Location Depth Below

Ground Surface

Soil Description Suitability as Subbase

Depth (ft)

Blow Counts

1st 2nd 3rd Ave

HA-1

0 – 10¼” 1¼” Asphalt / 9” Granite Aggregate Base Course

0 - - - -

10¼” – 20” Dark brown silty fine SAND (SM) (FILL)

<MC=12.2; -200=26.1>

-1 16 20 22 21

20” – 30” Brown clayey fine SAND (SC) -2 10 8 5 6.5

30” – 48” Red and gray very clayey fine SAND

(SC)

-3 3 4 4 4

-4 4 5 5 5

HA-2

0 – 6¾” 1½” Asphalt / 5¼” Granite Aggregate Base Course

0 - - - -

6¾” – 12” Dark brown silty fine SAND (SM) (FILL) -1 10 18 14 16

12” – 30” Brown clayey fine SAND (SC) -2 5 5 7 6

30” – 48” Red and gray very clayey fine SAND

(SC)

<MC=23.3; -200=38.6>

-3 5 5 6 5.5

-4 6 7 7 7

HA-3

0 – 6¾” 1¼” Asphalt / 5½” Granite Aggregate Base Course

0 - - - -

6¾” – 12” Dark brown silty fine SAND (SM) (FILL) -1 14 12 12 12

12” – 24” Brown very clayey fine SAND (SC) -2 5 6 8 7

24” – 48” Red and gray very clayey fine SAND

(SC)

<MC=22.8; -200=45.2>

-3 9 9 9 9

-4 6 6 7 6.5

Comments: MC = Moisture Content; -200 = Percent Silt / Clay; Yellow = Suitable as Subbase;

Red = Not Suitable as Subbase

GHD | County Council of Beaufort County | 12630716-00 | ARPA Community Center – Agnes Major Site | Report of Subsurface Exploration & Geotechnical Evaluation 2

Appendix D Seismic Design Parameters

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The Power of Commitment iii

GHD

57-C Sheridan Park Circle

Bluffton, South Carolina 29910, United States ghd.com

Document Status

Status Code

Revision Author Reviewer Approved for issue

Name Signature Name Signature Date

S4 00 Robin M. Moutray Sean M.

McCubbins

Sean M.

McCubbins

1-26-24

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