B08_Attch_05_CAHA_158701_LHR_Trail_Geotechnical_Report.pdf

GEOTECHNICAL ENGINEERING REPORT

Lighthouse Road Trail

Buxton, North Carolina November 15, 2022

Terracon Project No. K5225040

Prepared for:

Croft and Associates Kennesaw, Georgia

Prepared by:

Terracon Consultants, Inc.

Elizabeth City, North Carolina

Terracon Consultants, Inc. 106 Capita l Trace, Unit E El izabeth City, North Carol ina 27909 P (252) 335 9765 F (252) 335 9766 terracon.com

November 15, 2022

TO: Croft and Associates 3400 Blue Springs Rd NW Ste 200 Kennesaw, GA 30144-1079

Attn: Mrs. Luisa Callura

RE: Geotechnical Engineering Report Lighthouse Road Trail Buxton, North Carolina Terracon Project No: K5225040

Dear: Mrs. Callura:

In compliance with your instructions, we have completed our Subsurface Exploration and Geotechnical Engineering Services for the above referenced project. The results of this study, together with our recommendations, are presented in this report.

Often, because of design and construction details that occur on a project, questions arise concerning subsurface conditions. Terracon would be pleased to continue its role as Geotechnical Engineer during the project implementation.

We appreciate the opportunity to work with you on this project. We trust that the information contained herein meets your immediate need, and should you have any questions or if we could be of further assistance, please do not hesitate to contact us.

Respectfully Submitted, Terracon

Gerald W. Stalls Jr., P.E.

Senior Project Engineer NC Reg. # 034336

Camille A. Kattan, P.E.

Principal Engineer NC Reg. # 014103

TABLE OF CONTENTS

EXECUTIVE SUMMARY ............................................................................................................ i

1.0 PROJECT INFORMATION

1.1 Project Authorization

1.2 Project Site Location and Description

1.3 Project Construction Description

1.4 Purpose and Scope of Services

2.0 FIELD AND LABORATORY PROCEDURES

2.1 Field Exploration

2.2 Field and Laboratory Testing

2.2.1 Soil Classification and Index Testing

3.0 SITE AND SUBSURFACE CONDITIONS

3.1 Site Geology

3.2 Recent Land Reclamation and Site Development

3.3 Subsurface Soil Conditions

3.4 Groundwater Discussion

3.5 Shrink-Swell Soils Discussion

4.0 EVALUATIONS AND RECOMMENDATIONS

4.1 Clearing and Grading – Pedestrian Trail

4.2 Subgrade Preparation – Pedestrian Trail

4.3 Structural Fill and Placement – Pedestrian Trail

4.4 Suitability of On-site Soils

4.5 Deep Foundation Design Recommendations – Wetland Overlook

4.5.1 Axial Compression Capacity Recommendations

4.5.2 Pile Group Settlement

4.5.3 Timber Piles

4.5.4 Test Pile Program

4.5.5 Establishing Pile Driving Criteria

4.5.6 Allowable Driving Stresses

4.5.7 Driven Pile Installation Monitoring

4.5.8 Production Pile/Acceptance Criteria

4.5.9 Adjacent Structures

4.6 Seismic Evaluation

4.7 Pavement Design

4.8 Review of Historical Geotechnical Report and Recommendations

5.0 CONSTRUCTION CONSIDERATIONS

5.1 Anticipated Excavation Characteristics

5.2 Excavation Stability

5.2.1 Temporary Slopes

5.2.2 Shoring

5.3 Dewatering

5.4 Site Utility Installation

6.0 REPORT LIMITATIONS

TABLE OF CONTENTS

APPENDIX A BORING LOCATION PLAN

APPENDIX B CLASSIFICATION SYSTEM FOR SOIL EXPLORATION

APPENDIX C SUMMARY OF LABORATORY CLASSIFICATION RESULTS

APPENDIX D BORING LOGS

APPENDIX E GENERALIZED SOIL PROFILE

APPENDIX F SEISMIC MAPS

APPENDIX G HISTORICAL GEOTECHNICAL REPORT

Geotechnical Engineering Report Lighthouse Road Trail ■ Buxton, NC November 15, 2022 ■ Terracon Project No: K5225040

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EXECUTIVE SUMMARY

The project site is located within the grass and/or sand covered areas throughout the previously constructed Cape Hatteras Lighthouse and Hatteras Island Visitor Center development in Buxton, North Carolina. The existing development includes the Cape Hatteras Lighthouse, the existing visitor center building, Lighthouse Road, and other various drive lanes and parking areas, as well as other associated infrastructure components.

The proposed construction consists of building new pedestrian trails and elevated wetland overlooks. Geotechnical Explorations and Engineering services were previously performed for this project (Geohydro Engineers project No.: 221305.20; report dated March 10, 2022). Following completion of these Geotechnical Engineering services, several changes were made to the planned construction. These changes included relocating a portion of the pedestrian trail to the opposite (west) side of Lighthouse Road, adding an additional wood framed wetland overlook at north end the access roadway extending beyond the north parking lot, as well as adding additional pedestrian trail alignments southeast of the Visitor Center’s parking lot. These planned changes are the subject of this report. Additionally, and at the request of the client, a cursory review of the previously completed Geotechnical Engineering Report was conducted and supplemental recommendations for subgrade preparations and structural fill placement procedures are provided in this report.

Our field exploration program included drilling seven (7) 10- to 30-foot deep Standard Penetration Test (SPT) borings within the proposed pedestrian trail and wetland overlook areas.

The initial groundwater level was measured to occur at depths of about 4 to 6 feet below the existing grades at the boring locations, corresponding to elevations generally ranging from about 0 to 2.5 feet (NAVD88) as estimated from the topographic information provided by Croft and Associates. A summary of the subsurface and groundwater conditions encountered at the boring locations is presented in Section 3 of this report.

The following evaluations and recommendations were developed based on our field exploration and laboratory-testing program:

n A field testing program during construction is recommended, which should include subgrade proofrolling, compaction testing, and pile foundation installation observations.

n The majority subsurface soils encountered at the boring locations, classified to consist of SAND (SP, SP-SM) and extending to depths of about 10 feet appear to meet the criteria recommended in this report for reuse as structural fill.

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Reliable ■ Resourceful ■ Responsive ii n This site was previously developed including the existing structures and pavement areas. As such, unsuitable existing FILL, soils containing construction debris, and/or Organic laden soils may be present at unexplored locations.

n The surficial soils encountered at the explored locations were noted to consist of approximately 2 to 3 inches of Sandy Topsoil. The existing FILL encountered within the Pedestrian Trail alignment (such as that encountered at boring B-5) is currently anticipated to be suitable to remain in place but should be further evaluated during the subgrade evaluation procedures. As such, a clearing depth of about 3 inches is anticipated to be required. Deeper cuts may be required to remove deeper organic laden soil deposits and/or otherwise unsuitable soils. The project’s budget should include an allowance for subgrade improvements (undercut and backfill with structural fill).

n The proposed wetland overlook included in our scope of services should be supported by means of a deep foundation system. More detailed information regarding the deep foundation design recommendations and the estimated settlements are provided in this report in Sections 4.5 through 4.5.2.

n Based on the information obtained at the boring locations (to a maximum depth of 30 feet) and our experience within the vicinity of the project site, it is our opinion that this site may be classified as a Site Class “D” in accordance with Table 20.3-1 of ASCE 7-16 as referenced by the 2018 International Building Code (IBC).

n Typical minimum pavement section recommendations are provided within this report based on the results of the completed site borings identified as B-1 through B-6 and our experience with similar soil conditions.

n Cursory review of the project’s original Geotechnical Engineering report (Geohydro Engineers project No.: 221305.20; report dated March 10, 2022). The results of this review along with our supplemental recommendations regarding subgrade preparation and structural fill placement procedures are provided in this report.

This summary briefly discusses some of the major topics mentioned in the attached report.

Accordingly, this report should be read in its entirety to thoroughly evaluate the contents.

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1.0 PROJECT INFORMATION

1.1 Project Authorization

Terracon has completed our subsurface exploration and geotechnical engineering services for the newly relocated and added portions of the proposed Lighthouse Road Trail project located in Buxton, North Carolina. The geotechnical engineering services were conducted in general accordance with the scope presented in Terracon Proposal No. PK522040. Authorization to proceed with our services was received from the client in the form of Task Order CAHA 2021- 418 received on the date of September 20, 2022.

1.2 Project Site Location and Description

The project site is located within the grass and/or sand covered areas throughout the existing Cape Hatteras Lighthouse and Hatteras Island Visitor Center development in Buxton, North Carolina.

The existing development includes the Cape Hatteras Lighthouse, the visitor center building, Lighthouse Road, and other various drive lanes and parking areas, as well as other associated infrastructure components. Existing site elevations within the proposed construction areas were estimated to generally range from about 4 to 12 feet based on our review of the project plans provided by Croft and Associates on the date of November 2, 2022. A site vicinity map is provided in Figure 1 with the overall project site indicated.

Figure 1: Project Site Vicinity Map

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1.3 Project Construction Description

The proposed construction at this site consists of building new pedestrian trails along with new elevated wetland overlooks. Geotechnical Explorations and Engineering services were previously performed for this project (Geohydro Engineers project No.: 221305.20; report dated March 10, 2022). Following completion of these Geotechnical Engineering services, several changes were made to the planned construction. These changes included relocating a portion of the pedestrian trail to the opposite (west) side of Lighthouse Road, adding an additional wetland overlook at north end the access roadway extending beyond the north parking lot, as well as adding additional pedestrian path alignments southeast of the Visitor Center’s parking lot. More detailed information regarding the proposed construction to occur at this site and as it pertains to the planned changes along with the contents of this report is provided in Table I below.

Table I – Project Construction Description(1)

Site Element Dimensions Structural Description Finished Grade

(ft)(2)

Existing Surface

Elevations (ft NAVD88)(3)

Maximum Pile Foundation

Loads (Tons)(2)

Pedestrian Trails

8 – 12 feet wide

Flexible asphalt and Rigid concrete pavement sections

Within 1-foot to 2 feet above existing grades 4 to 8.5 -

Wetland Overlook

200 – 400 square feet

Wood frame design supported by deep foundations.

Elevated platform about 4 to 6 Feet above existing grades

6.5 8

(1): The information provided in the table above is limited to the planned changes discussed above. Should any of this information be inaccurate, Terracon should be notified in order to determine if additional analysis is required.

(2): This information was provided by Croft and Associates.

(3): This information was estimated by Terracon based on our review of the project topographic information provided by Croft and Associates.

It is understood that the proposed trails are intended for pedestrian use only and will not be used to support vehicular traffic.

If any of the noted information is incorrect or has changed, please inform Terracon so that we may amend the recommendations presented in this report, if appropriate.

1.4 Purpose and Scope of Services

The purpose of this study was to obtain information on the general subsurface conditions at the proposed project site. The subsurface conditions encountered were then evaluated with respect to the available project characteristics. In this regard, engineering assessments for the following items were formulated:

n General assessment of the soils revealed by the borings performed at the proposed development.

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Reliable ■ Resourceful ■ Responsive n General location and description of potentially deleterious material encountered in the borings that may interfere with construction progress or structure performance, including existing fills or surficial/subsurface organics.

n Construction considerations for soil subgrade preparation (stripping, grading, and compaction) and foundation excavations. Engineering criteria for placement and compaction of approved structural fill material.

n Feasibility of utilizing a deep foundation system for support of the proposed elevated wetland overlook (boring B-7). Design parameters required for the foundation system, including foundation sizes, allowable compression capacity, embedment depths, and estimated settlements.

n Assessment of the shallow subsurface soils’ expansive properties.

n Seismic site class determination in accordance with the 2018 International Building Code.

n Typical pavement section recommendations based on the field exploration activities, laboratory testing, and our experience with similar soil conditions.

n Cursory review of the original Geotechnical Engineering report and supplemental recommendations for subgrade preparations.

The scope of services did not include an environmental assessment for determining the presence or absence of wetlands or hazardous or toxic material in the soil, bedrock, surface water, groundwater or air, on or below or around this site. Prior to development of this site, an environmental assessment is advisable.

2.0 FIELD AND LABORATORY PROCEDURES

2.1 Field Exploration

In order to explore the general subsurface soil types and to aid in developing associated design parameters and recommendations, the following exploration program was performed:

n Six (6) 10-foot deep SPT borings (designated as B-1 through B-6) were drilled within the limits of the newly proposed portions of the proposed pedestrian trail alignments.

n One (1) 30-foot deep SPT boring (designated as B-7) was drilled within the approximate limits of the newly proposed northeast elevated wetland overlook.

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Standard Penetration Tests were performed in the field in general accordance with ASTM D 1586. The tests were performed continuously from the existing ground surface to depths of 10 to 12 feet, and at 5-foot intervals thereafter, starting at a depth of 13 feet below grade. The soil samples were obtained with a standard 1.4” I.D., 2” O.D., 30” long split-spoon sampler. The sampler was driven with blows of a 140 lb. hammer falling 30 inches, using a safety hammer.

The number of blows required to drive the sampler each 6-inch increment of penetration was recorded and is shown on the boring logs. The sum of the second and third penetration increments is termed the SPT N-value (uncorrected for automatic hammer and overburden pressure). A representative portion of each disturbed split-spoon sample was collected with each SPT, placed in a sealed glass jar, and returned to our laboratory for review. Following the exploration procedures, the borings were backfilled with a neat cement grout mix in accordance with NCDENR requirements for aquifer protection.

The boring locations were established by Croft and Associates and staked in the field by a representative of Terracon by referencing the GPS coordinates noted on the project site plan (provided by Croft and Associates) and measuring from existing site features. Approximate soil boring locations are shown on the attached “Boring Location Plan” (Appendix A) which was developed by Terracon with the use of the above referenced site plan.

2.2 Field and Laboratory Testing

Soil testing provided by Terracon was performed in accordance with American Society for Testing and Materials (ASTM) standards. All soils and materials tests were performed in our AASHTO re:source (formally AMRL) certified Elizabeth City laboratory.

2.2.1 Soil Classification and Index Testing

Representative portions of all soil samples collected during drilling operations were labeled, preserved and transferred to our laboratory in accordance with ASTM D4220 for classification and analysis. Soil descriptions on the boring logs are provided using visual-manual methods in general accordance with ASTM D2488 using the Unified Soil Classification System (USCS).

Soil samples that were selected for index testing were classified in general accordance with ASTM D2487. It should be noted that some variation can be expected between samples classified using the visual-manual procedure (ASTM D2488) and the USCS (ASTM D2487). A summary of the soil classification system is provided in Appendix B.

Representative split-spoon samples were selected and subjected to natural moisture and #200 sieve wash testing in order to corroborate the visual classification. These test results are presented in Appendix C and on the soil test boring logs provided in Appendix D. Generalized subsurface soil profile is provided in Appendix E.

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3.0 SITE AND SUBSURFACE CONDITIONS

3.1 Site Geology

The project site lies within a major physiographic province called the Atlantic Coastal Plain.

Numerous transgressions and regressions of the Atlantic Ocean have deposited marine, lagoonal, and fluvial (stream lain) sediments. The regional geology is very complex, and generally consists of interbedded layers of varying mixtures of sands, silts and clays. Based on our review of existing geologic and soil boring data, the geologic stratigraphy encountered in our subsurface explorations generally consisted of marine deposited Sands as well as isolated alluvial Organic deposits.

3.2 Recent Land Reclamation and Site Development

Based on a review of historical United States Geological Survey (USGS) topographic maps of Dare County, North Carolina produced between the years of 1948 and 2019, the project site does not appear to be located within a previously reclaimed area. However, the project site was previously developed including the Cape Hatteras Lighthouse, the existing single-story visitors center structure, Lighthouse Road, and other various drive lanes and parking areas, as well as the associated infrastructure components. Additionally, subsurface deposits of existing FILL and/or Organic laden soils were encountered at the explored locations.

3.3 Subsurface Soil Conditions

The surficial soils encountered at the explored locations generally consisted of approximately 2 to 3 inches of Sandy Topsoil. As an exception, the surficial and shallow subsurface soils encountered at borings B-5 and B-7 were noted to consist of FILL and comprised of Sandy Topsoil and SAND (SP, SM) with varying amounts of Silt, Gravel, and/or Organics, which extended to depths of about 2 to 4 feet, respectively. Below the Sandy Topsoil and/or FILL, the encountered soils consisted of very loose to dense SAND (SP, SP-SM) having varying amounts of Silt and/or Marine Shell Fragments. It is noted that 1- to 2-foot thick deposits of Organic laden soils noted to consist of SAND (SP) with varying amounts of Organics and/or Organic CLAY (OL) to Clayey PEAT (PT) were encountered at borings B-1, B-2, B-5, and B-7 at depths ranging from 4 to 5 feet below existing grades and extended to depths ranging from 5.5 to 6 feet. A summary of the subsurface soil conditions encountered at the SPT boring locations is presented in Table II on the following page of this report.

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Table II – Subsurface Soil Conditions Average Depth (ft) Stratum Description Ranges of

SPT(1) N-Values to

0.2 – 0.3 Topsoil Sandy Topsoil -to

2 – 4 FILL(2) FILL: Sandy Topsoil and SAND (SP, SM) with varying amounts of Silt, Gravel -

0.2 – 4 to

10 - 30 I(3)

Granular Stratum SAND (SP, SP-SM) with varying amounts of Silt and/or Marine Shell Fragments

Organic Deposit SAND (SP) with varying amounts of Organics and/or Organic CLAY (OL) to Clayey PEAT (PT) encountered at borings B-1, B-2, B-5, and B-7 between depths generally ranging from 4 to 6 feet. Organic laden soil deposits ranged between 1 and 2 feet in thickness.

Granular Stratum:

3 – 38

Organic Deposit:

2 - 6

Note(s): (1) SPT = Standard Penetration Test, N-Values in Blows-per-foot (uncorrected)

(2) Encountered at borings B-5 and B-7 only

(3) All borings terminated in this Granular Stratum

The subsurface descriptions are of a generalized nature provided to highlight the major soil strata encountered. The records of the subsurface exploration are included in Appendix D (Boring Log sheets) and in Appendix E (Generalized Soil Profile) which should be reviewed for specific information as to the individual borings. The stratifications shown on the records of the subsurface exploration represent the conditions only at the actual boring locations. Variations may occur and should be expected between boring locations. The stratifications represent the approximate boundary between subsurface materials and the transition may be gradual. It is noted that the “Topsoil” designation references the presence of surficial organic laden soil and does not represent any particular quality specification. It is recommended that this material be tested for approval prior to use as topsoil.

3.4 Groundwater Discussion

The groundwater level was recorded at the boring locations and as observed through the relative wetness of the recovered soil samples during the drilling operations. The initial groundwater level was measured to occur at depths of about 4 to 6 feet below the existing grades at the boring locations, corresponding to elevations generally ranging from about 0 to 2.5 feet (NAVD88) as estimated from the topographic information provided by Croft and Associates.

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The soils encountered at the explored locations and at the corresponding initial groundwater levels consisted of relatively porous soils containing less than 10 percent fines (SAND: SP, SP- SM). Drilling fluids (water) are introduced into the bore holes during the drilling operations further impairing the ability to accurately determine the groundwater levels. In addition, as subsurface soils begin to dry, moisture moves upwards through the soil profile by means of capillary action. Based on the subsurface soil compositions, these initial groundwater level readings (based on the relative wetness of the soils) could be in part attributed to the capillary action of the soils. Additionally, this site is within an active Coastal Environment and the groundwater level may be affected by tidal fluctuations (including wind driven tides). As such, the reported initial groundwater levels may not be indicative of the static groundwater level. The SPT boreholes were backfilled upon completion for safety considerations as well as in accordance with NCDENR requirements for aquifer protection.

Groundwater conditions will vary with environmental variations and seasonal conditions, such as the frequency and magnitude of rainfall patterns, as well as man-made influences, such as existing swales, drainage ponds, underdrains and areas of covered soil (paved parking lots, sidewalks, etc.). In the project’s area, seasonal groundwater fluctuations of +/-2 to 3 feet or more are common; however, greater fluctuations have been documented. We recommend that the Contractor determine the actual groundwater levels at the time of the construction to determine groundwater impact on the construction procedures, if necessary.

3.5 Shrink-Swell Soils Discussion

The soils recovered during our field investigation were tested and evaluated for their potential to expand or contract with moisture changes (typically termed shrink-swell). Shallow foundations and other on-grade features constructed on expansive soils at certain depths may be subjected to detrimental uplift or horizontal forces caused by the swelling of these soils as a result of an increase in the moisture content. Conversely, as these Clays lose moisture they may shrink, adversely affecting the foundations. The depth to which soils are normally affected by moisture changes extends from the ground surface to approximately 2 to 3 feet below finished grades in this area, depending on site topography and drainage characteristics.

Based on the laboratory classification test results, the shallow (upper 4 feet) SAND (SP) soils encountered at the boring locations are Non-Liquid, Non-Plastic, and possess Sand contents of about 99%. These soils are generally estimated to be indicative of possessing no shrink-swell potential. Accordingly, subgrade improvements are not anticipated to be required as they specifically relate to shrink-swell potential soils.

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4.0 EVALUATIONS AND RECOMMENDATIONS

Our recommendations are based on the previously discussed project information, our interpretation of the soil test borings and laboratory data, and our observations during our site reconnaissance. If the proposed construction should vary from what was described, we request the opportunity to review our recommendations and make any necessary changes.

4.1 Clearing and Grading – Pedestrian Trail

Based on our site observations and the results of the completed borings, the newly proposed Pedestrian Trail construction areas (Borings B-1 through B-6) will require a clearing depth of about 2 to 3 inches to remove the surficial Sandy Topsoil. These cuts are expected to extend deeper in isolated areas to remove deeper organic laden soil deposits and/or unsuitable soils, which become evident during the clearing. It is recommended that the clearing operations extend laterally at least 2 feet from the perimeter of the proposed Pedestrian Trail alignments.

The existing granular FILL soils (SAND: SP having trace amounts of Silt and Gravel) encountered below the surficial Organic laden soils at the location of boring B-5 are currently anticipated to be suitable to remain, which should be substantiated during the subgrade evaluation procedures recommended in Section 4.2 of this report. However, this site was previously developed and may have unsuitable conditions within unexplored areas. As such, the project’s budget should include an allowance for subgrade improvements in the form of undercut and backfill with suitable structural fill to remove unsuitable soils and/or construction debris.

4.2 Subgrade Preparation – Pedestrian Trail

As indicated in this report, the relocated and/or added portions of the proposed Pedestrian Trail alignments were noted to contain existing FILL and/or subsurface Organic laden soil deposits.

The FILL encountered at the location of boring B-5 that extended to a depth of about 2 feet is currently anticipated to be suitable to remain in place. The subsurface Organic laden soils were generally encountered between depths of 4 to 6 feet at boring locations B-1, B-2, and B-5.

Although the Organic laden deposits encountered at borings B-1 and B-5 were noted to be predominantly granular in nature (SAND: SP with trace amounts of Organics), their presence indicates this site has a potential for more significant Organic deposits at other locations such as that encountered at the location of boring B-2 (Organic CLAY: OL to Clayey PEAT: PT).

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With regards to the presence of existing FILL, we have no records to indicate the degree of control, and consequently, the FILL may be unreliable for support of pavement loads. Given the primarily granular nature of the Organic laden soils encountered at borings B-1 and B-5 as well as the depth at which they occur below the proposed finished grades, settlements contributed by these soils would likely be uniform and have minor effects, whereas more visible over time within the vicinity of boring B-2 where more significant Organics were encountered. Support of pavements on or above existing fill and/or the encountered Organic laden soils is discussed in this report. However, even with the recommended construction procedures, inherent risk exists for the owner that compressible fill or unsuitable material (such as the Organic laden soils described above), will not be discovered. This risk of unforeseen conditions cannot be eliminated without completely removing the existing FILL and/or Organic laden soils but can be reduced by following the recommendations contained in this report.

If the owner elects to construct the pavements on the existing FILL and/or above the Organic laden soils to reduce initial construction costs in exchange for increased potential longer-term distress, the following protocol should be followed.

After the clearing operation, the exposed subgrade soils within all pavement areas should be densified with a large static drum roller. After the subgrade soils have been densified, they should be evaluated by Terracon for stability. Accordingly, the subgrade soils should be proofrolled to check for pockets of loose material hidden beneath a crust of better soil. Several passes should be made by a large rubber-tired roller or loaded dump truck over the construction areas. The number of passes will be determined in the field by the Geotechnical Engineer depending on the soils conditions. Any pumping or unstable areas observed during proofrolling (beyond the initial cut) should be undercut and/or stabilized at the direction of the Geotechnical Engineer.

Additionally, several 4- to 5-foot-deep test pits should be excavated within the proposed construction areas. The test pits are considered necessary to further evaluate the subgrade soils to remain as well as to determine the potential presence of near surface unsuitable soils, subsurface Organic laden soils, and/or construction debris that may go unforeseen during the clearing operations. The test pits should be performed under the observation of Terracon, who will evaluate the composition of the recovered soils and provide subgrade improvement recommendations, if necessary.

It is recommended that the subgrade soils throughout the building area be compacted to a dry density of at least 98% of the Standard Proctor maximum dry density (ASTM D 698) as tested to a depth of 12 inches as confirmed by performing compaction testing. These compaction and testing procedures should be performed following the subgrade evaluations and prior to the placement of structural fill and/or Aggregate Base Course (ABC) materials. Given the porous nature of the subgrade soils, moisture manipulation may be needed to achieve the recommended compaction. This manipulation may require the addition of water during compaction procedures.

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4.3 Structural Fill and Placement – Pedestrian Trail

Following the approval of the natural subgrade soils by the Geotechnical Engineer, the placement of the fill required to establish the design grades may begin. Any material to be used for structural fill should be evaluated and tested by Terracon prior to placement to determine if they are suitable for the intended use. Suitable structural fill material should consist of sand or gravel containing less than 20% by weight of fines (SP, SM, SW, GP, GW), having a liquid limit less than 20 and plastic limit less than 6, and should be free of rubble, organics, clay, debris and other unsuitable material.

All structural fill should be compacted to a dry density of at least 98% of the Standard Proctor maximum dry density (ASTM D 698) unless specified differently in this report. In general, the compaction should be accomplished by placing the fill in maximum 10-inch loose lifts and mechanically compacting each lift to at least the specified minimum dry density. A representative of Terracon should perform field density tests on each lift as necessary to assure that adequate compaction is achieved.

Backfill material in utility trenches within the construction areas should consist of structural fill (as described above) and should be compacted to at least 98% of ASTM D 698. This fill should be placed in 4- to 6-inch loose lifts when hand compaction equipment is used.

Care should be used when operating the compactors near existing structures to avoid transmission of the vibrations that could cause settlement damage or disturb occupants. In this regard, it is recommended that the vibratory roller remain at least 25 feet away from existing structures; these areas should be compacted with small, hand-operated compaction equipment.

4.4 Suitability of On-site Soils

Based on the laboratory testing program, the subsurface soils classified as SAND (SP, SP-SM) encountered at the boring locations and extending to depths of about 10 feet appear to meet the criteria recommended in this report for reuse as structural fill. These soils should be segregated from any Organic laden soils, unsuitable FILL, as well as Sand containing construction debris (if encountered), which are not considered to be suitable for re-use as structural fill. Finally, moisture manipulation is expected as these soils are located above and/or below the groundwater table. This manipulation may require the addition of water during placement and/or compaction. Conversely, those soils excavated below the groundwater level may require temporary stockpiling to drain excess moisture. The goal of these methods is to adjust the moisture content of the soils to within ±2 percentage points of their optimum moisture at the time of compaction.

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Further classification testing (natural moisture content, gradation analysis, and Proctor testing) should be performed in the field during construction to evaluate the suitability of excavated soils for reuse as structural fill. The project’s budget should include an allowance for imported structural fill.

4.5 Deep Foundation Design Recommendations – Wetland Overlook

The wetland overlook structure is to be supported by a deep foundation system comprised of 8-inch square timber piles. It is understood that the maximum pile foundation loads for this structure will not exceed 8 tons. If it is determined that the maximum pile foundation compression loads exceed that indicated in this report, Terracon should be consulted to determine the necessary pile foundation embedment depths.

4.5.1 Axial Compression Capacity Recommendations

We conducted pile capacity analyses using static formulas with coefficients recommended by Geoffrey Myerhoff and George Sowers. The analyses include contributions of shaft friction and end bearing to the pile capacity. The following table (Table III – Driven Pile Capacity) notes our recommendations for 8-inch square timber piles along with the necessary embedment depth. At this time, it is recommended that the design of the structure include no lateral resistance contribution from the pile foundations unless otherwise determined by a lateral capacity analysis. It is recommended that the pile foundations be analyzed by a NC registered Professional Structural Engineer for buckling and braced as needed for lateral stability.

For the reasons previously described, it may not be possible to confirm pile capacities with a simple driving criterion such as number of hammer blows per foot of advanced pile. Instead, driving criteria will likely consist of a target tip elevation and/or certain embedded length in a bearing material with specified driving resistance. The specified driving resistance should be based on a Wave Equation Analysis of the contractor’s selected hammer.

The presence of subsurface Organic soils indicates that this site has a potential to contain more significant organic conditions such as stumps and/or root mat, which often causes difficulties with pile installations. The Terracon representative should determine the necessity and/or depth of pre-augering in the field during pile installations, if difficulties arise.

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Table III – Driven Pile Capacity

Pile Type & Dimensions

Installation Method

Pile Embedment Depth (ft)(1)

Allowable Capacity in

Compression (tons) (2)

Allowable Capacity in Tension (tons) (2)

Ultimate Capacity in

Compression (tons) (2)

8-Inch Square Timber

Impact Driving 12 8 1.5 20

(1) The pile embedment depths noted above are referenced from below the existing site grade elevations noted to occur at the time of our subsurface exploration procedures.

(2) The recommendations presented above were estimated based on the completed scope of services and the soils encountered at the explored locations. Pile installation monitoring should be completed by Terracon to verify that the allowable capacities noted above are achieved.

The pile embedment depths noted in Table III are required to achieve the allowable capacities and are referenced below the existing site grade elevations. Reduction in the length of embedment will likely not be permissible.

The capacity of a group of piles spaced at least 3 pile diameters apart, center to center, can be taken as the sum of the individual capacities with no reduction factor. If closer pile spacing is anticipated, the geotechnical engineer should be contacted to evaluate the efficiency of the specific pile group. The final order lengths and tip elevations will be adjusted based on the results of the test piles and recommended load test program.

In order to confirm the required tip elevations, we recommend conducting a Test Pile Program prior to ordering production piles. The piles should be installed as described throughout this report to their minimum design tip elevation. If for some reason during construction, pile driving “refusal” is encountered before the piles reach their design tip elevations, the Geotechnical Engineer should be retained to review driving records and field reports to determine whether the pile can adequately support the design loads. If the pile driving hammer is not properly matched to the pile type, size and subsurface conditions, it may reach practical refusal before the pile reaches the design tip elevation, or the required capacity.

4.5.2 Pile Group Settlement

Based on the results of load tests performed on piles in similar soil conditions, it is anticipated that the total butt settlements (including elastic shortening) will not exceed about ½-inch, which is the settlement necessary to mobilize the soil/pile capacity in combination with the pile tip settlements due to the stress increase in the underlying soils.

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4.5.3 Timber Piles

It is recommended that the timber piles meet the requirements of ASTM D-25 for timber bearing piles. The piles should be clean peeled and pressure treated in accordance with the requirements of AWPA C3. The timber pile design stresses should be established in accordance with ASTM D-2899 and the local applicable Building Codes. Additionally, we recommend the timber piles be treated with ACA (ammoniated copper arsenate) or CCA (copper chrome arsenate) due to the location of the proposed structure in a temperature zone coastal environment. Prior to driving, it is recommended that timber piles be relatively free of defects and have a water content greater than approximately 20 percent (to minimize “breaking”) and less than about 50 percent (to minimize “brooming”).

4.5.4 Test Pile Program

We recommend that a test pile program be implemented for the purpose of assisting in the development of final tip elevations and to confirm that the contractor’s equipment and installation methods are acceptable. The test program should involve at least one (1) test pile to provide an indication of various driving and/or installation conditions. The test pile locations should be established by the Geotechnical Engineer based on the structural characteristics. It is important to note the relationship between the required testing and our design assumptions.

We chose safety factors based upon the recommended pile testing program.

The piles should be driven using the drive system submitted by the contractor and approved by the geotechnical engineer. Additional recommendations regarding driven pile installation procedures and the attempt to minimize potentially detrimental vibrations to surrounding existing structures are provided in Sections 4.5.7 and 4.5.9 of this report. Test pile lengths should be at least five (5) feet longer than anticipated production pile lengths to ensure that the required capacity is developed and to allow for refinement of estimated capacities. The indicator piles installed during the Test Pile Program, which satisfy the geotechnical engineer’s requirements for proper installation, may also be used as permanent production piles.

The contractor should include in his equipment submittal a Wave Equation Analyses (using GRLWEAP software) modeling the behavior of the test piles during driving, or what is termed by GRL as a “Drivability Study.” The primary intent of the Wave Equation Analyses is to estimate the feasibility of the contractor’s proposed pile driving system with respect to installing the piles.

Since the results of the Wave Equation Analyses are dependent on the chosen hammer, the pile type and length, and the subsurface conditions, it is likely that at least one Wave Equation Analysis per hammer will be required.

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Pile driving equipment should not be mobilized for the test piles until the Wave Equation Analyses have been submitted and approved by the geotechnical engineer. If the contractor’s proposed pile driving system is rejected, subsequent submittals of alternative drive systems should also include appropriate Wave Equation Analyses that are subject to the approval of the geotechnical engineer. The Wave Equation Analyses are also used to estimate:

· Compressive and tensile stresses experienced by the modeled pile during driving

· The total number of blows required to install the pile

· Driving resistance (in terms of blows per foot) within the various soil strata the pile is embedded in

· Driving time

4.5.5 Establishing Pile Driving Criteria

Prior to driving production piles, the geotechnical engineer should establish the criteria for pile installation. The criteria will be based on the data collected during monitoring of the test pile installation and the subsequent restriking. The intent of establishing driving criteria is to facilitate installation of the production piles without damage and to provide a means of establishing when piles have achieved the design capacities. The driving criteria may include: hammer type, hammer energy, ram weight, hammer cushion type and thickness, required tip elevations and driving resistance necessary to achieve capacities, and possibly predrilling recommendations (if the test pile results warrant the need).

4.5.6 Allowable Driving Stresses

Guidelines from the American Association of State Highway Transportation Officials (AASHTO) indicate that maximum compressive and tensile stresses, imposed on driven timber piles during installation, should be less than the following equation:

Maximum Compressive/Tensile Stress

§ Driving stress in kips per square inch (ksi) is ksi Where:

§ Resistance factor during driving for timber piles, § Reference value for compression stress parallel to grain, November 15, 2022 ■ Terracon Project No: K5225040

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4.5.7 Driven Pile Installation Monitoring

The geotechnical engineer should observe the installation of the test piles and all production piles. The purpose of the geotechnical engineer’s observations is to determine if production installations are being performed in accordance with the previously derived Pile Driving Criteria.

Continuous driving and installation records should be maintained for all piles. Production piles should be driven utilizing the approved system established as a result of the Test Program. In order to develop the allowable capacities provided in Section 4.5 of this report, it is considered necessary to install the piles by means of driving the piles to the proposed tip elevations.

The field duties of the geotechnical engineer (or a qualified engineer’s representative) should include the following:

n Being knowledgeable of the subsurface soil conditions at the site and the project-specific Pile Driving Criteria.

n Being aware of aspects of the installation including type of pile driving equipment and pile installation tolerances.

n Keeping an accurate record of pile installation and driving procedures.

n Documenting that the piles are installed to the proper depth indicative of the intended bearing stratum. Also documenting that appropriate pile splicing techniques are used, if necessary.

n Recording the number of hammer blows for each foot of driving. Generally confirming that the pile driving equipment is operating as anticipated. Record the energy rating of the hammer.

n Informing the geotechnical engineer of any unusual subsurface conditions or driving conditions.

n Notifying the contractor and structural engineer when unanticipated difficulties or conditions are encountered.

n Confirming from visual appearance that the piles are not damaged during installation and observing the piles prior to installation for defective workmanship. The geotechnical engineer should review all driving records prior to pile cap construction.

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4.5.8 Production Pile/Acceptance Criteria

The results of the WEAP analyses are highly dependent on the many input parameters related to the soil conditions, static pile capacity estimates, as well as specific characteristics associated with different makes and models of pile driving hammers.

Utilizing the driving records for the tested piles and the results of the driving conditions (WEAP), the pile capacity versus driving resistance (BPF) may be more accurately estimated; and depending on the results of these analyses a static load test or some adjustment to the design capacity, installation procedures, or depth may be necessary.

The piles should be installed to the embedment depths identified or as refined by the Geotechnical Engineer following the pile test program. These piles should be driven to the dynamic driving resistance as determined in the field by the Geotechnical Engineer, or to practical refusal, whichever occurs first.

Based on our experience with pile installation, it is essential that driving be terminated immediately if refusal is reached to prevent damage to the piles. The pile-driving hammer should be properly matched to the pile to provide sufficient energy to install the pile without damaging it.

The installation of all deep foundation systems should be in accordance with the local Building Code requirements. In addition, a qualified Geotechnical Engineer or representative should monitor the installation of all piles. General pile installation guidelines are provided below.

1. The piles should be evaluated and designed for the axial stresses by the project structural engineer. All of the piles in a foundation element should be of the same type and of equal load carrying capacity.

2. The pile-driving hammer should be capable of driving the pile to at least 3 times the design load without over-stressing the pile in tension or compression. At all times, the hammer should be operated at the chamber pressure, speed, etc. as recommended by the manufacturer.

3. Pile driving should be as continuous an operation as possible and should proceed without stopping over the last 3 feet of penetration. During pile installation, the contractor should exercise caution so as not to over stress the piles. Piles should not be driven beyond practical refusal.

4. The contractor should exercise caution in pile driving so that the effects of heave are minimized. Careful monitoring by the contractor for possible heave during driving should be exercised. All piles that heave ¼ inch or more should be re-driven unless otherwise instructed by the Geotechnical Engineer.

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5. Piles should be installed under continuous monitoring by a qualified Senior Engineering Technician or Geotechnical Engineer in order to make field judgments of pile penetration and construction and to check for satisfactory foundation support conditions. Driven piles should be monitored for penetration resistance, blow counts, and hammer action during pile driving.

4.5.9 Adjacent Structures

When considering the suitability of a driven pile foundation, consideration should be given to the integrity of nearby structures (if applicable). Due to the large amount of energy required to install driven deep foundations, vibrations of considerable magnitude are generated. These vibrations may affect nearby structures. These structures can, due to their proximity, be detrimentally affected by the construction unless proper protection measures are taken.