04_NACE_226875_Geotechnical_Eng_Report_0002.pdf

GEOTECHNICAL ENGINEERING REPORT

Oxon Cove Trail Bridge and Trail Resurfacing Oxon Hill, Maryland

Schnabel Reference 20C23028 June 9, 2021 Revised December 6, 2021 Revised February 28, 2022

Prepared for:

June 9, 2021 Revised December 6, 2021 Revised February 28, 2022

Eileen Kemp, PLA, ASLA DHM Design 900 South Broadway, Suite 300 Denver, Colorado 80209

Subject: Geotechnical Engineering Final Report, Oxon Cove Trail Bridge and Trail Resurfacing, Oxon Hill, Maryland (Schnabel Reference 20C23028)

Dear Ms. Kemp:

SCHNABEL ENGINEERING, LLC (Schnabel) is pleased to submit our geotechnical engineering final report for this project. This study was performed in accordance with our proposal dated August 31, 2020, as authorized by your office on October 14, 2020; and contract modification no. 1, as authorized on April 6, 2021. The previous revision included the results of global stability analyses for the 2H:1V slope repair options with a riprap toe key. This revision includes the revised top of bridge abutment elevations and provides the geotechnical engineering final report.

We appreciate the opportunity to be of service for this project. Please call us if you have any questions regarding this report.

Sincerely, SCHNABEL ENGINEERING, LLC

Federico Ayala, EIT Steve W. Fung, PE Senior Staff Engineer Senior Associate

Emre Biringen, PE Senior Engineer Professional Certification. I hereby certify that these documents were prepared or approved by me, and that I am a duly licensed professional engineer under the laws of the state of Maryland. License No. 38273, Expiration Date: 01/14/2024

FA:EB:SWF:jb:sam \\FFAX-FS\SCHNABEL_PROJECTS\2011-2020\2020\STERLING\PROJECTS\20C23028 - OXON COVE TRAIL\03-SE PRODUCTS\03-REPORTS\02-FINAL\20C23028 -

OXON COVE TRAIL REPORT GER.DOCX

DHM Design

February 28, 2022 Page i Schnabel Engineering, LLC Project 20C23028 ©2021, 2022 All Rights Reserved

GEOTECHNICAL ENGINEERING REPORT

OXON COVE TRAIL BRIDGE AND TRAIL RESURFACING

OXON HILL, MARYLAND

TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY

2.0 SCOPE OF SERVICES

2.1 Base Scope for Trail and Slope

2.2 Option A Scope for Bridge

3.0 DESCRIPTION OF SITE AND PROPOSED CONSTRUCTION

3.1 Site Description

3.2 Proposed Construction

4.0 SUBSURFACE EXPLORATION PROGRAM

4.1 Subsurface Exploration and Field Testing

5.0 LABORATORY TESTING

5.1 Index Testing

5.2 Compaction and CBR Testing

5.3 Chemical Testing

6.0 SITE GEOLOGY AND SUBSURFACE CONDITIONS

6.1 Site Geology

6.2 Generalized Subsurface Stratigraphy

6.3 Groundwater

6.4 Seismic Site Classification

7.0 SITE GRADING AND EARTHWORK

7.1 Compacted Fill Subgrades

7.2 Compacted Fill

8.0 PAVEMENT RECOMMENDATIONS

8.1 Pavement Observations

8.2 Pavement Recommendations for the Existing Trail

8.3 Pavement Recommendations for the Bridge

8.4 Pavement Subgrade

8.5 Pavement Drainage

9.0 FOUNDATION RECOMMENDATIONS FOR THE BRIDGE ABUTMENTS

9.1 Abutment Foundations

9.2 Lateral Earth Pressure on Abutment Walls

9.3 Fill Settlement

10.0 SLOPE STABILIZATION

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10.1 Slope Stability Analyses

10.2 Slope Drainage and Vegetation

10.3 Slope Maintenance and Inspection

11.0 CONSTRUCTION CONSIDERATIONS

11.1 Site Grading and Earthwork

11.2 Foundations

11.3 Construction Dewatering

11.4 Engineering Services During Construction

12.0 GENERAL SPECIFICATION RECOMMENDATIONS

13.0 LIMITATIONS

LIST OF FIGURES

Figure 1: Site Vicinity Map Figure 2: Boring Locations

LIST OF TABLES

Table 1: Pavement Core Summary Table 2: Summary of Water Level Readings Table 3: Seismic Design Parameters Table 4: Pavement Rehabilitation Table 5: Estimated Elevation of Suitable Subgrade Soils Table 6: Recommended Design Parameters for Abutment Walls Table 7: Soil Parameters for Backfill and Riprap

APPENDICES

Appendix A: Subsurface Exploration Data Appendix B: Soil Laboratory Test Data Appendix C: Pavement Condition Assessment Appendix D: Slope Stability Analyses

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1.0 EXECUTIVE SUMMARY

This report presents the results of our visual pavement condition assessment, subsurface exploration, laboratory testing, and geotechnical engineering analysis for the proposed Oxon Cove trail bridge and trail resurfacing project located in Oxon Cove, Maryland.

Our subsurface exploration included a total of 10 pavement cores and 12 test borings at the site to the depths of 10 ft to 40 ft below existing grade. We encountered the existing Fill soils of Stratum F in all test borings to depths of 2.5 to 11 ft. The existing Fill soils were underlain by natural soils of Alluvium, Terrace Deposits, and Potomac Group.

Our visual pavement condition assessment along the existing trail observed generally poor pavement conditions, with medium to high severity cracking, alligator cracking, and some potholes being the primary mode of pavement distress. We recommend a pavement reconstruction for the entire trail. New full depth pavement may be supported on new compacted fill, or suitable firm soils that pass a proofroll.

Pavement sections constructed as detailed herein may be designed considering a CBR of 5.0 based on our laboratory test data.

A new bridge is planned to be constructed over the small tributary of Oxon Creek. We recommend that the bridge abutments be supported on strip footings for a design soil bearing pressure of 2,500 psf, provided that the slopes adjacent to the footings are 2.5H:1V or flatter. Our analysis indicates a seismic Site Class D at the proposed bridge location.

Also, it is planned to stabilize the eroded segment of the shoreline adjacent to the existing trail and Oxon Cove. As the progression of the erosion towards the bike trail is likely to continue, we recommend repairing the existing slope by placing either riprap fill or riprap armored compacted structural fill and regulating the surface runoff water by a surface water collection system or culvert. Based on our analyses, we believe that 2H:1V or flatter slope repairs with a toe key will satisfy the minimum factors of safety of 1.3 and 1.1 under steady state and rapid drawdown conditions, respectively. The toe of the riprap will be keyed into the stream channel to a minimum depth of 2 ft and a minimum width of 2.5 ft.

It is imperative that Schnabel provide professional observation and testing of subsurface conditions revealed during construction as an extension of our engineering services. These services will also help in evaluating the Contractor's conformance with the plans and specifications in accordance with building code (special inspection) requirements.

We are providing this executive summary solely for purposes of overview. Any party that relies on this report must read the full report. This executive summary omits several details, any one of which could be very important to the proper application of the report.

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2.0 SCOPE OF SERVICES

Our proposal dated August 31, 2020 and contract modification no. 1 for environmental screening and safety measures define the following Base and Option A scope of services for this project. The Base scope of services includes recommendations for rehabilitation or replacement of the existing trail and recommendations for slope stabilization along Oxon Cove. Option A scope of services includes recommendations for support of the new pedestrian bridge.

2.1 Base Scope for Trail and Slope

• Estimated subsurface conditions, pavement thicknesses, and groundwater levels within the area explored based on data collected in the subsurface exploration.

• Recommended asphalt pavement sections or, if applicable, pavement rehabilitation options for the existing trail. These pavement sections will be designed in accordance with the National Asphalt Pavement Association design guidelines for Hot Mix Asphalt Pavements for Trails and Paths. The pavement design is based on our subsurface exploration, existing pavement observations, and laboratory California Bearing Ratio (CBR) testing.

• Evaluation of stability of the existing slope section that has shown signs of instability and recommendations for the proposed stabilization method.

• Recommended static earth pressures, subdrainage, and backfill requirements for site retaining walls.

• Earthwork recommendations for the construction of load-bearing fill including an assessment of on-site soils for use as fill, pavement subgrade preparation, and compaction criteria.

• Construction considerations related to the implementation of our recommendations.

2.2 Option A Scope for Bridge

• Estimated subsurface conditions and groundwater levels within the area explored based on data collected in the subsurface exploration.

• Foundation recommendations for spread footings for support of the proposed pedestrian bridge including a net allowable soil bearing pressure, bearing grades, estimated settlements, minimum dimensions and frost depth.

• Recommended Seismic Site Class in accordance with the latest version of IBC for use in foundation design based on an extrapolation of data collected in the subsurface exploration.

• Earthwork recommendations for the construction of load-bearing fill including an assessment of on-site soils for use as fill, pavement subgrade preparation, and compaction criteria.

• Construction considerations related to the implementation of our recommendations.

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3.0 DESCRIPTION OF SITE AND PROPOSED CONSTRUCTION

3.1 Site Description

The project site is located at the National Park Service’s (NPS’s) Oxon Cove Park in Oxon Hill, Maryland.

The park is located to the north and east of the intersection of Interstate 495 and Interstate 295, to the south and west of the Forest Heights neighborhood, and crosses Oxon Creek, which is a tributary to the Potomac River. Along the northern boundary of the park, an existing asphalt-paved bike trail is present that runs parallel to Oxon Creek (southern portion) and forks to the north and east. The northern fork of the trail crosses over Oxon Creek via a concrete bridge and continues to the boundary between the State of Maryland and the District of Columbia. The eastern fork is asphalt-surfaced and transitions to gravel/dirt and continues to the east-northeast to a shopping center in Forest Heights. Before reaching the shopping center, the trail crosses a small tributary to Oxon Creek, which we understand was previously occupied by a pedestrian bridge that was washed away after a storm in 2015. The ground surface elevation along the bike trail varies from approximately EL 5 to EL 16 on the southern portion and reaches to approximately EL 36 along the northern fork.

The existing bike trail area either borders or bisects portions of a former landfill. The construction details of the landfill constructed in the 1960s are not known. It is not clear whether a clay liner or rubber membrane (or no barrier at all) was used to enclose the landfill on all sides to contain the landfill waste.

Based on the available reports that were provided to us by the NPS and your office, the landfill was covered by a clay cap. The cap was reported to be sufficiently intact and is reported to have created a pressurized buildup of landfill gas beneath the cap. The landfill was reportedly still venting methane gas from a few penetrations located throughout the cap. Based on the historical information, surficial and subsurface soil and groundwater samples collected from the landfill area located east of Oxon Creek contained various concentrations of SVOCs, VOCs, PCBs, Pesticides and metals. Several of these contaminants exceeded their respective EPA residential risk-based screening levels (RSLs) or maximum contaminant levels (MCLs).

The existing asphalt pavement along the bike trail is in poor to fair condition with areas of pavement distress that include longitudinal and transverse cracking, alligator cracking, and potholes. Portions had wide cracks with sporadic vegetation growing through the surface of the asphalt.

Approximately 50 ft of the shoreline adjacent to the existing trail and Oxon Cove was washed away. This area is located approximately 700 ft south of the existing bridge. The shoreline slope is approximately 10 ft high. We understand that two underground 230kv electric lines in steel pipe conduits and two conduits for future communication lines owned by the Potomac electric Power Company lines cut through the trail at this location. Historical aerial images indicate that erosion of the riverbank had occurred after the removal of vegetation and trees and subsequent root system dying in order to perform excavation and burial of underground utilities.

We obtained the site information from the topographic site plan dated August 2020 and provided by DHM, and through our site visits. A Site Vicinity Map is included as Figure 1.

3.2 Proposed Construction

This project includes rehabilitation of the asphalt pavement of the Oxon Cove Trail by either resurfacing or replacement, stabilization of the unstable riverbank, and construction of a new bridge over the small

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DHM Design Oxon Cove Trail Bridge and Trail Resurfacing tributary of Oxon Creek. We understand new drainage culverts, ditches, and other associated grading are planned along the southern portion and the northern fork of the trail alignment. The scope of work does not include the eastern fork towards Forest Heights.

According to the project description in the RFP prepared by the National Park Service, the total trail length of rehabilitated area is 1.48 miles (7,814 ft). However, based on our review of the provided Oxon Trail Map, the trail segment as shown in Figure 2 is approximately 0.95 miles (5,000 ft) in length.

We understand that the bridge near the shopping center that was washed away in 2015 will be reconstructed at a higher elevation, approximately 100 ft to the east of the former bridge location. Based on the Bridge Prefab Drawing Details, dated January 22, 2021, the structure will be a vehicular bridge constructed of a pre-fabricated concrete deck and steel stringers. The bridge will have a length of 70 ft and a width of 12 ft. It is planned to support the bridge abutments on strip footings with wing walls.

Based on the structural load information provided to us, we understand that the service axial load and service moment on each bridge abutment will be 6 kips/ft and 3.8 kips-ft/ft, respectively, over the width of the bridge. The maximum lateral load on the abutments will be 1.4 kips/ft over the width of the bridge.

We understand that the existing site grades along the bridge approaches will be raised by about 1.5 ft to EL 11.5 at the western abutment and the eastern abutment.

We understand that an approximately 50 ft segment of the shoreline adjacent to the existing trail and Oxon Cove will be stabilized. The proposed stabilization measures may include placing riprap and backfilling, rock packing in overhanging portions, and long-term natural stream bank stabilization by installing live stakes in backfilled areas.

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4.0 SUBSURFACE EXPLORATION PROGRAM

We performed a subsurface exploration and field testing program to identify the subsurface stratigraphy underlying the site and to evaluate the geotechnical properties of the materials encountered. This program included test borings. Exploration methods used are discussed below. The appendices to this report contain the results of our exploration.

4.1 Subsurface Exploration and Field Testing

4.1.1 Visual Pavement Condition Assessment

Schnabel performed a visual assessment of the existing pavement conditions along the trail between April 19 and 23, 2021 to evaluate and document visible surface distresses along the bike trail. The general condition of the existing pavements at selected locations are shown in a photolog and included in Appendix C. In addition, the specific pavement conditions and distresses observed along the trail are documented in Appendix C. Schnabel provided station numbers starting from the southernmost end to the northernmost end of the trail and identified those on the boring location plan in Figure 2.

4.1.2 Test Borings and Pavement Cores

Schnabel’s subcontractor, Connelly and Associates, drilled a total of 10 pavement cores and 12 test borings at the site to the depths of 10 ft to 40 ft under our observation between April 19 and 23, 2021. The exploration along the existing asphalt-paved trail included 10 pavement cores (PC-01 through PC-10), five trail borings (SB-01 through SB-05) and three culvert borings (CB-01 through CB-03) to the depths of 10 ft. Two slope borings SP-01 and SP-02 were drilled in the vicinity of the eroded riverbank to the depths of 26 ft. Two bridge borings BR-01 and BR-02 were drilled at the proposed bridge abutment locations to the depths of 40 ft and 30 ft, respectively. The Standard Penetration Test (SPT) was performed at selected depths in the borings. Appendix A includes specific observations, remarks, logs for the borings, pavement core photologs; classification criteria; drilling methods; and sampling protocols.

Figure 2 included at the end of this report, indicates the approximate test boring locations. We will retain soil samples up to 45 days beyond the issuance of this report, unless you request other disposition.

We performed pocket penetrometer tests on collected samples of soils representing fine-grained soils of existing Fill, and Strata A and C natural soils to evaluate the shear strength of these materials. The test results are presented on boring logs in Appendix A and are summarized in Section 6.2 of this report.

The SPT samples were obtained using a hydraulically driven automatic trip hammer (ATH). Most correlations with SPT data are based on N-values collected with a safety hammer. The energy applied to the split-spoon sampler using the ATH is about 33 percent greater than that applied using the safety hammer, resulting in lower N-values. The hammer blows shown on the boring logs are uncorrected for the higher energy. However, we correct SPT N-values for the higher energy when using N-values in our analyses.

It was recommended that precautions before intrusive actions within the landfill footprint be taken due to the explosive nature of the subsurface landfill gas. Furthermore, it was recommended that landfill gas be monitored during our subsurface exploration study. During drilling of borings, after each SPT sampling, a portable combustible gas meter (a multi gas meter) was placed above the open borehole to monitor methane gas (CH4) in ambient air. The readings on the combustible gas meter represent the percentage

February 28, 2022 Page 6 Schnabel Engineering, LLC of LEL. It should be noted that LEL is stated as a percentage of concentration of a gas in the atmosphere, while the meter reading is displayed as a percentage of that concentration. A reading of 10% or more of that value is cause for concern, as stated in OSHA Standard 29 CFR 1910.146, “Permit- Required Confined Spaces,” which considers 10% or more of any LEL to be a “hazardous atmosphere.”

In borings SB-03 and SB-04, the methane gas levels higher than 10% were recorded. In borings CB-01, CB-02, CB-03, SB-02, and SB-05, pockets of methane gas with levels less than 10% were encountered.

Where methane gas was encountered, drilling stopped temporarily until the methane levels receded.

Upon completion of drilling, we immediately grouted the borings. The methane gas readings are presented on the boring logs in Appendix A.

Soil samples obtained from the borings were screened onsite with a photoionization detector (PID). The PID is an instrument used to detect volatile organic compounds (VOCs). No detectable PID readings were measured from the borings. Note that a PID reading is a measurement of the organic vapors emitted from a soil sample and is not a direct measurement of the quantity of VOCs present within the soil sample. The results of the environmental screening are presented on the boring logs in Appendix A.

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5.0 LABORATORY TESTING

Our laboratory performed tests on selected samples obtained during the subsurface exploration. The testing aided in the classification of materials encountered in the subsurface exploration and provided data for use in the development of recommendations for design of foundations, earthwork, and pavements. The results of the laboratory tests are included in Appendix B and are summarized in Section 6.2 of this report. Selected test results are also shown on the boring logs in Appendix A.

5.1 Index Testing

We performed index testing on soil samples collected as part of the exploration to provide soil classifications and to provide parameters for use with published correlations with soil properties. Index testing included performing 16 natural moisture content, 13 Atterberg Limit, and 12 gradation tests on 16 jar samples and two bulk samples of soil representing Strata F and A. The results are presented in the Summary of Laboratory Tests in Appendix B and are summarized in Section 6.2.

5.2 Compaction and CBR Testing

We performed Standard Proctor compaction tests per ASTM D698, and CBR tests per ASTM D1883 to evaluate compaction characteristics and to provide soil parameters for pavement design. The tests were performed on two bulk sample from borings SB-02 and SB-04, representing Stratum F (existing Fill soils).

The CBR specimens were compacted to 95% of maximum dry density determined from the Standard Proctor compaction. A surcharge pressure of 50 psf was applied to the specimens to simulate the pavement weight. The test results are included in Appendix B and are summarized in Section 6.2.

5.3 Chemical Testing

For soil disposal purposes, one composite soil sample was collected from the drummed investigation derived waste (IDW) and submitted to the laboratory for analysis. The composite soil sample was submitted to our subcontractor Phase Separation Science in Baltimore, Maryland for TCLP Lead analysis. The laboratory analytical report including certificates of analysis and chain of custody is provided in Appendix B.

TCLP lead was not detected above the laboratory detection limit in the composite soil sample. The drummed IDW was transported to the Covena Facility in Fairfax, VA for disposal by AEG Environmental.

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6.0 SITE GEOLOGY AND SUBSURFACE CONDITIONS

6.1 Site Geology

As part of the subsurface exploration study, we reviewed available geologic data, the test borings performed at the project site, and information in our files. Based on this review, the project site is located in the Coastal Plain physiographic province, which is generally characterized by a wedge of alternating sedimentary formations overlying residual soils and bedrock. The geologic stratigraphy in the immediate vicinity of the site generally consists of Pleistocene aged sedimentary Terrace Deposits referenced as Lowland Deposits. The Lowland Deposits formation consists of Pleistocene-aged sedimentary deposits, typically characterized as interbedded sand, gravel, and silt-clay.

During our exploration, we encountered existing Fill soils and natural sedimentary soils below the existing pavement sections and below the ground surface at the proposed bridge location. Based on our review of available information, the existing Fill soils identified as Stratum F are believed to have been placed during past earthwork operations for the trail and construction of the former landfill that once covered portions of the Oxon Cove. The natural soils underlying the site consist of Alluvium soils from Oxon Creek that overlay the Terrace Deposits and Potomac Formation. Excerpt of the geological map showing the project site is included in Plate 1 below.

Plate 1: Section of Geologic Map Showing the Project Site

Reference: Glaser, J.D. (2003) "Geologic Map of Prince George's County, Maryland"

APPROXIMATE

PROJECT SITE

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Alluvium soils identified as Stratum A typically consist of interbedded sand, silt-clay, and subordinate gravel. They are typically light- to dark-gray, tan, or brown; weathers pale-gray, yellow, or brown.

Alluvium includes very heterogeneous, commonly poorly stratified sediments, with muddy sand and silt as dominant lithology. Organic matter, including leaves, branches and logs, is a common component.

Terrace Deposits identified as Stratum B typically consist of interbedded sand, gravel, and silt-clay. They are typically tan, brown, or shades of gray; weathers to yellow, orange, or brown hues, commonly limonitic. Included under Terrace Deposits, there heterogeneous lithologies such as medium to coarse sand, pebbly sand, and subordinate silt-clay. Terrace Deposits are the product if stream erosion during the early Quaternary and are now isolated on the valley walls above the modern floodplain by renewed downcutting.

The deeper natural soils underlying Terrace Deposits consist of silt and clay facies of Potomac Group, identified as Stratum C. The Potomac Group is the oldest sedimentary deposit in the Washington, DC area. The clay soil of this deposit, in this area, tends to be over consolidated, and is known to exhibit high strength and low compressibility.

6.2 Generalized Subsurface Stratigraphy

We characterized the following generalized subsurface stratigraphy based on the exploration and laboratory test data included in the appendices. During our exploration, we encountered the following stratigraphy:

• Ground Cover

• Stratum F: Existing Fill

• Stratum A: Alluvium

• Stratum B: Terrace Deposits

• Stratum C: Potomac Group

6.2.1 Ground Cover

Approximately 2 to 3 inches of thick topsoil was encountered at the bridge, slope, and culvert borings locations beneath the grass-covered ground surface. The thicknesses may vary at other locations across the site. The topsoil depth was measured to the nearest inch and was based on our visual identification procedures.

Asphalt pavement was encountered at the ground surface in each pavement core and boring performed along the existing Oxon Cove trail. The existing pavement sections consisted of varying thickness of asphalt overlying crushed aggregate base. The table below summarizes our observations of existing asphalt concrete and crushed aggregate base thicknesses at each core/boring location.

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Table 1: Pavement Core Summary

Pavement Core ID

Trail Boring ID

Location Asphalt Concrete

Thickness (inches)

Crushed Aggregate Base

Thickness (inches) Station

PC-01 SB-01 00+25 2.0 3

PC-02 - 05+10 1.75 nm

PC-03 SB-02 10+23 2.25 5.75

PC-04 - 15+20 3.25 nm

PC-05 SB-03 20+17 2.5 5.75

PC-06 - 24+81 1.75 nm

PC-07 - 30+33 1.75 nm

PC-08 CB-03 35+55 1.75 6.25

PC-09 SB-05 40+60 2.25 5

PC-10 SB-04 45+60 2.25 5.75

Note: “nm“ indicates aggregate was observed beneath pavement core but thickness was not measured.

6.2.2 Stratum F: Existing Fill

Below the ground cover, we encountered existing fill material of Stratum F in all borings to depths of 2.5 ft to 11 ft (boring termination depth). The existing fill of Stratum F material classified as CLAYEY SAND (SC) with various amounts of gravel; LEAN CLAY (CL) with various amounts of silt, sand and gravel;

SILTY SAND (SM) with various amounts of gravel; POORLY GRADED SAND (SP) with various amounts of gravel; and SANDY SILT (ML). The SPT N-values within the existing fill soils ranged from 2 to greater than 100 blows per foot (bpf) with a median of 8 bpf. Soil samples tested with pocket penetrometer had compressive strengths ranging between 1.0 tsf and 2.5 tsf.

Nine samples were tested within this stratum in our laboratory. Index tests on the samples resulted in the following properties:

Moisture Content: 8 to 38 % % Passing No. 200 Sieve: 21 to 57 % Liquid Limit: 22 to 44 Plasticity Index: 6 to 25

CBR testing was performed in the laboratory on bulk samples collected from borings SB-02 and SB-04.

These samples were composed of existing Fill soils.

The sample obtained from SB-02 was classified as CLAYEY SAND (SC) with gravel with a Liquid Limit of 43, a Plasticity Index of 24, and percent passing the No. 200 sieve of 35.2. The standard Proctor test

February 28, 2022 Page 11 Schnabel Engineering, LLC resulted in a maximum dry density of 116.9 pcf with an optimum moisture content of 12.9 percent. The CBR value was 5.4, with a swell of 0.8 percent after submersion for four days.

The sample obtained from SB-04 was classified as CLAYEY SAND (SC) with gravel with a Liquid Limit of 39, Plasticity Index of 19, and percent passing the No. 200 sieve of 31.9. The Standard Proctor test resulted in a maximum dry density of 121.3 pcf with an optimum moisture content of 10.8 percent. The CBR value was 4.6, with a swell of 0.7 percent after submersion for four days.

6.2.3 Stratum A: Alluvium

Underlying the Fill material, we encountered the Alluvium soils of Stratum A in bridge borings BR-01 and BR-02, slope borings SP-01 and SP-02, and trail boring SB-02, to depths of 10 ft to 18.5 ft. The Alluvium soils of Stratum A material classified as SILTY SAND (SM); FAT CLAY (CH) with sand; and LEAN CLAY (CL) with various amounts of sand. The SPT N-values within the Alluvium soils varied from 2 to greater than 17 blows per foot (bpf) with a median of 5 bpf. Soil samples tested with pocket penetrometer had compressive strengths ranging between 0 tsf and 1.0 tsf.

Four samples were tested within this stratum in our laboratory. Index tests on the samples resulted in the following properties:

Moisture Content: 19 to 31 % % Passing No. 200 Sieve: 25 to 93 % Liquid Limit: NP to 55 Plasticity Index: NP to 36

6.2.4 Stratum B: Terrace Deposits

Below the Alluvium soils, we encountered the Terrace Deposits of Stratum B in bridge borings BR-01 and BR-02, and slope borings SP-01 and SP-02, to depths of 24 ft to 28.5 ft. The Terrace Deposits of Stratum B classified as SILTY GRAVEL (GM) with sand; SILTY SAND (SM) with gravel; POORLY GRADED SAND (SP) with gravel; and CLAYEY SAND (SC) with gravel. The SPT N-values within the Terrace Deposits ranged from 18 to greater than 76 blows per foot (bpf) with a median of 30 bpf.

6.2.5 Stratum C: Potomac Group

Underlying the Terrace Deposits, we encountered the Potomac Group clays of Stratum C in bridge borings of BR-01 and BR-02, and slope borings SP-01 and SP-02, to depths of 26 ft to 40 ft, which were the maximum depths explored below the ground surface. The Stratum C soils consisted of LEAN CLAY (CL) with various amounts of sand. The SPT N-values within the Potomac Group varied from 12 to 36 blows per foot (bpf) with a median of 19 bpf. Soil samples tested with pocket penetrometer had compressive strengths ranging between 2.5 tsf and 4.5 tsf.

6.3 Groundwater

The test boring logs in Appendix A include groundwater observations obtained during our subsurface exploration. These data include depths to groundwater encountered during drilling, upon drilling completion, and following completion of the boring. A summary of water level readings is included in the table below.

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Table 2: Summary of Water Level Readings

Boring

ID

Ground Surface

EL

(ft)

Encountered Completion Casing Pulled (C.P.) Caved (C.P.)

Depth (ft)

EL

(ft)

Depth (ft)

EL

(ft)

Depth (ft)

EL

(ft)

Depth (ft)

EL

(ft)

BR-01 13 12 1 11 2 10.5 2.5 - -

BR-02 6 10 -4 12 -6 6.8 -0.8 - -

CB-01 7 6 1 7.5 -0.5 Dry NA 6 1

CB-02 9 8 1 7.5 1.5 Dry NA 6.5 2.5

CB-03 9 Dry NA Dry NA Dry NA 6 3

SB-01 5 Dry NA Dry NA Dry NA 3 2

SB-02 6 9 -3 10 -4 Dry NA 6.5 -0.5

SB-03 13 Dry NA - - - - - -

SB-04 32 Dry NA - - - - - -

SB-05 20 Dry NA Dry NA Dry NA 5.5 14.5

SP-01 9 16 -7 7 2 8.2 0.8 - -

SP-02 10 12 -2 7 3 7.5 2.5 - -

Notes: “NA” Not Applicable, “-“ Not Recorded

Our drilling subcontractor installed a temporary water observation well in boring BR-01. We observed groundwater in this well at a depth of 17 ft (approximately EL -4) about two weeks after completion of the boring. We did not obtain long-term water level readings in the remaining borings because we backfilled them upon completion for safety.

The groundwater levels on the logs indicate our estimate of the hydrostatic water table at the time of our subsurface exploration. The final design should anticipate the fluctuation of the hydrostatic water table depending on variations in precipitation, surface runoff, tidal action, evaporation, stream levels, time of the year, and similar factors.

Based on our groundwater and Oxon Creek level observations, groundwater and creek water levels are expected to impact the proposed slope repair.

6.4 Seismic Site Classification

We evaluated the Seismic Site Class and Seismic Site Coefficients for this project according to ASCE 7- 16 Minimum Design Loads for Buildings and Other Structures. Our analysis indicates a Site Class D for the bridge location (BR-01 and BR-02). The Site Class was evaluated based on the Standard Penetration Tests (SPT) N-values corrected for hammer efficiency, extrapolated to a depth of 100 ft. We also assumed a Seismic Risk Category of II for the bridge, as defined by ASCE. Based on the spectral response acceleration parameters and assumed Seismic Risk Category, the site classifies as Seismic Design Category B. The seismic design parameters that are calculated based on the recommended site class and project location are summarized in the table below.

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Table 3: Seismic Design Parameters

Period Mapped Acceleration Parameters

Site Coefficient

Adjusted Maximum Spectral Response Acceleration Parameters

Design Spectral Response Acceleration Parameters

Short Ss = 0.132 g Fa =1.6 g Sms = 0.212 g Sds = 0.141 g

1 Second S1 = 0.043 g Fv = 2.4 g Sm1 = 0.104 g Sd1 = 0.069 g

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7.0 SITE GRADING AND EARTHWORK

Cuts of up to 4.5 ft are anticipated for the construction of proposed bridge abutment footings, and cuts of up to 1 ft along the trail are expected for the pavement reconstruction. Proposed slope repair will require placement of up to 8 ft of riprap and compacted structural fill. Recommendations for compacted fill subgrade preparation, fill soil requirements, and placement and compaction criteria are presented in subsequent sections.

7.1 Compacted Fill Subgrades

Subgrades to receive compacted structural fill should be stripped of vegetation, topsoil, and organic matter. Our subsurface exploration indicated topsoil to depths of 2 to 3 inches below the ground surface.

Compacted structural fill subgrades for the pavements and riverbank slopes should consist of suitable existing Fill soils of Stratum F, and natural soils of Strata A and B.

The Geotechnical Engineer should evaluate the suitability of the fill subgrades. To evaluate the subgrade suitability for support of the compacted structural fill prior to any undercutting or initiation of fill placement, the stripped subgrades should be proofrolled with a loaded dump truck or other suitable compaction equipment. Areas that exhibit excessive pumping, weaving, or rutting should be scarified, dried and recompacted, or undercut and replaced with compacted structural fill as recommended by the Geotechnical Engineer. Subgrade evaluation techniques complementary to proofrolling could include a combination of probing with a penetrometer, drilling hand augers, and/or observing test pits.

When excavation of unsuitable materials is required, it should be performed in a manner to limit disturbance of the underlying suitable material. To evaluate required excavation depths, the excavation should be performed under the observation of the Geotechnical Engineer.

Compacted structural fill subgrades should be kept free of ponded water. If springs or other flowing water is present at the compacted structural fill subgrade level, the Contractor should direct water to discharge beyond the fill limits. Recommendations for discharging springs should be provided by the Geotechnical Engineer.

Compacted structural fill subgrades should be free of snow, ice, and frozen soils. If snow, ice, or frozen soils are present at subgrade levels, these materials should be removed as recommended by the Geotechnical Engineer.

Compacted structural fill subgrades should not be steeper than about 4H:1V. If steeper slopes are present, subgrades should be benched to permit placement of horizontal lifts of fill.

7.2 Compacted Fill

Compacted structural fill and backfill behind the bridge abutments and wingwalls, at the riverbank slope, and within 2 ft of the pavement subgrade should consist of material classifying as SC, SM, SP, SW, GC, GM, GP, or GW according to ASTM D2487. Fill materials should not contain particles larger than 3 inches. In addition, fill materials should exhibit Liquid Limit and Plasticity Index values of less than 40 and 15, respectively. Most of the near-surface on-site soils are anticipated to meet the requirements for reuse as compacted fill. However, some selective excavation to separate unsuitable soils may be required.

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The remaining areas receiving fill such as the new bike trail embankment and culvert wall backfill may consist of non-organic on-site soils.

Compacted structural fill should be placed in maximum 8-inch thick horizontal, loose lifts. Fill should be compacted to at least 95 percent of the maximum dry density per ASTM D698 (Standard Proctor). Soil moisture contents at the time of compaction should be within plus or minus 3 percent of the optimum moisture content. This acceptable range of moisture contents may need to be adjusted in the field depending on results of the standard Proctor testing and the specific fill soils used.

Backfill placed in excavations, trenches, and other areas that large compaction equipment cannot access should be placed in maximum 6-inch thick lifts. Backfill should meet the material, placement, and compaction requirements outlined above.

Successful re-use of the excavated, on-site soils as compacted structural fill will depend on their natural moisture contents during excavation. Laboratory test results indicate soils encountered in proposed borrow areas are wet of the optimum moisture content. Scarifying and drying of these soils should be anticipated to achieve the recommended compaction. Drying of these soils will likely result in some delays, and may not be possible during cooler, wetter weather. We recommend that the earthwork be performed during the warmer, drier times of the year.

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8.0 PAVEMENT RECOMMENDATIONS

8.1 Pavement Observations

It is of our understanding that the National Park Service has adopted a pavement management system called the Road Inventory Program (RIP) with the objective to classify the park roads in different categories based on current conditions. As part of this program, the NPS uses the classification criteria based on Pavement Condition Rating (PCR), which is estimated using an automated pavement inspection vehicle that collects cracking, potholes, and rutting information. The conditions of existing roads are classified as Poor (PCR = 0 - 60), Fair (PCR = 61-84), Good (85 – 94), or Excellent (PCR = 95

– 100). As part of our investigation, we performed visual observation to determine the general condition of the Oxon Cove Trail and attempted to classify sections of the trail using this terminology. It is important to mention that our classification is based on visual observation only and no distress measurement using automated vehicle with Laser Crack Measurement System (LCMS) or video was performed.

Based on our visual observations and geotechnical exploration, the current condition of Oxon Cove Trail ranges from poor to fair condition, with medium to high severity transverse and longitudinal cracking, alligator cracking, and some potholes being the primary mode of pavement distress observed.

Debonding of the asphalt layers was observed in cores PC-03, PC-04, and PC-10. Debonding considerably affects the performance of the asphalt concrete layers and can decrease the structural capacity of the pavement making it susceptible to surficial alligator cracking, functional cracking, and potholes. The age of the asphalt course is also an important factor for pavement performance, as the asphalt binder is subjected to oxidation with time and high temperatures. The oxidation causes the asphalt binder to become brittle, making it more susceptible to developing cracks. Along the trail, we observed variability in the subgrade density based on the measured SPT N values ranging from 5 to 17 bpf in the top 2 ft of the subgrade. The variable in density of the subgrade layer may cause spots with very low structural capacity creating localized alligator cracking and depressions.

The southern section of the trail from Sta 0+00 to Sta 02+00 was generally observed to be in fair condition with medium to high severity transverse cracking and edge cracking. Sta 02+00 to Sta 15+00 exhibited more significant pavement distress and was categorized as in poor condition with areas of high severity alligator cracking, high severity longitudinal and transverse cracking, and potholes. A better condition pavement was observed from Sta 15+00 to Sta 20+00, classified as in fair condition the majority of the distresses observed were low to medium severity longitudinal and transverse cracks and edge cracking. The pavement condition from Sta 20+00 to 38+00 was classified as in poor condition with areas presenting high severity potholes, high severity transverse cracking, and high severity alligator cracking. Finally, the portion after the concrete bridge over Oxon Creek to the north was classified to be in fair condition with medium to high severity transverse cracking and medium severity edge cracking. A more detailed table containing our pavement assessment is attached in Appendix C.

Laboratory testing of two bulk samples, resulted in CBR values of around 5 when compacted at 95% of the maximum dry density. This CBR value is typically considered acceptable for pavement support.

However, the results of our soil test borings indicate there was some variability in density of the subgrade layer. This variability may cause spots with very low structural capacity creating localized alligator cracking and depressions. Additionally, due to the elevation of the site, closeness to the river, and topography, the trail is susceptible to flooding and surface runoff water. The exposure of the pavement layers to wet conditions will considerably affect the performance of the materials.

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8.2 Pavement Recommendations for the Existing Trail

As part of the National Park Services RIP pavement management system, a pavement treatment is recommended for each road depending on its current condition. Pavements in excellent condition require just spot repairs; pavements that are classified as in good condition require preventive maintenance;

pavement that are considered to be in fair condition require a light rehabilitation; and those roads that are classified as in poor condition require heavy rehabilitation or reconstruction.

Based on the conditions observed along the trail, we recommend a combination of two different treatments. Mill and overlay for the areas in fair condition or better, and a full-depth reconstruction for the areas in poor condition. It is important to mention that using just an asphalt overlay was not considered as a feasible treatment because the presence of wide transverse cracks throughout the whole trail.

Transverse cracks will typically reflect or propagate to the surface within two to four years if no remediation measures are taken.

The table below summarizes our recommended treatment for each portion of the trail.

Table 4: Pavement Rehabilitation Trail Area Recommended Treatment

STA 00+00 to 02+00 Mill entire asphalt concrete and resurface with 4.0 inches asphalt concrete

STA 02+00 to 15+00 Reconstruction

STA 15+00 to 20+00 Mill entire asphalt concrete and resurface with 4.0 inches asphalt concrete

STA 20+00 to 38+00 Reconstruction

STA 38+00 to 50+43 Mill entire asphalt concrete and resurface with 4.0 inches asphalt concrete

Based on the function of the trail, a pavement design analysis using conventional methodologies as AASHTO 1993 method was not possible to perform due to the extremely low loading. Therefore, we used MDSHA Standard No. 580.08 for Driveways and Bike Paths Pavement Section as the minimum thickness for the new pavement at Oxon Cove Trail.

We recommend the following pavement sections:

Full-Depth Reconstruction

Asphalt Surface Course, Superpave 9.5 mm PG64S-22, LVL 2 = 1.5 inches Asphalt Base Course, Superpave, 19.0 mm PG64S-22, LVL 2 = 2.5 inches

Graded Aggregate Base = 4.0 inches

Mill and Overlay

Mill depth (the entire asphalt layers) = 1.75 – 3.25 inches

Asphalt Base Course, Superpave, 19.0 mm PG64S-22, LVL 2 = 2.5 inches

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In areas that require full-depth reconstruction, the existing subgrade condition should be assessed by the Geotechnical Engineer during construction to determine if localized subgrade improvement is needed. In areas requiring mill and overlay, the existing aggregate base condition should be assessed by the Geotechnical Engineer during construction to determine if localized full-depth reconstruction is needed.

Following milling, a selective patching program to repair any deeper damage pavements that are still evident should again be performed.

8.3 Pavement Recommendations for the Bridge

New asphalt pavement is planned to tie the existing bike path to the proposed bridge approach. We recommend the following pavement section:

Asphalt Base Course, Superpave, 19.0 mm PG64S-22, LVL 2 = 2.5 inches

Graded Aggregate Base = 4.0 inches

8.4 Pavement Subgrade

The Contractor should prepare pavement subgrades and place compacted structural fill for pavement support as described in Section 7.0 of this report. Dense-graded aggregate placed as pavement base course should be compacted to at least 100 percent of maximum dry density according to ASTM D698, Standard Proctor. Dense-graded aggregate should be placed in maximum 4-inch-thick loose lifts.

Final pavement subgrades should be proofrolled under the observation of the Geotechnical Engineer immediately prior to placing subbase or base course aggregate to evaluate their suitability to support the pavement.

Utility excavations within pavement areas should be backfilled with compacted structural fill.

8.5 Pavement Drainage

Adequate control of surface drainage will be a very important consideration for the overall performance of this pavement design. The area surrounding pavements should be graded to direct surface water away from paved areas.

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9.0 FOUNDATION RECOMMENDATIONS FOR THE BRIDGE ABUTMENTS

We based our geotechnical engineering analysis on the information developed from our subsurface exploration and soil laboratory testing, along with the project development plans, site plans, and structural loading furnished to our office. At the time of this report, the scour depth assessment and scour protection design of the slopes and the abutments were incomplete. In an effort to develop preliminary foundation recommendations for the bridge abutments, we assumed that scour protection will be considered at the proposed bridge location.

9.1 Abutment Foundations

We understand that the existing site grade will be raised at the proposed bridge approaches by about 1.5 ft to EL 11.5 at the western abutment and the eastern abutment. The abutment wall and two wing walls on each side will retain the fill that supports the new bike trail. We also understand that the maximum axial loading on foundations will be 6 kips per linear ft over the width of the bridge (i.e., 72 kips per abutment). Based on the anticipated loads and the subsurface conditions encountered, we consider strip footings suitable for support of the proposed bridge abutments.

No documentation of the source or placement methods of the existing Fills at the site was provided to us.

There are inherent risks of unacceptable and unpredictable foundation settlements associated with founding structures directly on existing fill materials of unknown origin and placement methods.

Therefore, the existing Fill soils are not suitable for the support of footings and should be removed. We anticipate suitable natural soils will be encountered at the following elevations at the bridge abutments.

Table 5: Estimated Elevation of Suitable Subgrade Soils

Boring Number Abutment Estimated Elevation of Suitable

Subgrade Soils (ft)

BR-01 Western 4.5

BR-02 Eastern 3.5

Footings should be founded on suitable natural soils of Stratum A or Stratum B, or lean concrete. We recommend footings supported on these materials be designed for a net allowable soil bearing pressure of 2,500 psf. If embankment slopes are planned adjacent to the footings, the slopes should not be steeper than 2.5H:1V. This bearing pressure provides a factor of safety against general bearing capacity failure of at least 3.0.

The above allowable soil bearing pressure may be increased by 33 percent for wind and seismic loads when used in conjunction with load combinations defined in IBC 2021 Section 1605.2, Alternative Allowable Stress Design Load Combinations for use with allowable stress design. This increase is not applicable for other allowable stress load combinations, strength design, or load and resistance factor design.

The Geotechnical Engineer must observe the footing subgrade soils during construction to evaluate the suitability of the subgrade soils. If unsuitable soils are encountered at the design bearing grade, these soils should be removed and replaced with lean concrete.

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Settlements of shallow foundations supported on suitable natural soils and on properly placed compacted structural fill are not expected to exceed about 1 inch.