NP ROCR 503(1) 24(2) Geotechnical Engineering Report-Piney Branch Parkway _Final.pdf

GEOTECHNICAL AND PAVEMENT ENGINEERING REPORT

Piney Branch Parkway Washington, DC.

EFL Project No.: NP ROCR 503(1) 24(2)

Schnabel Reference 20C23027 July 5, 2024

Prepared for:

July 5, 2024

Ms. Martha Alunkal, PE

WSP USA

13530 Dulles Technology Drive, Suite 300 Herndon, Virginia 20171

Subject: Geotechnical and Pavement Engineering Report for Piney Branch Parkway, Rock Creek Park, Washington, D.C. NP ROCR 503(1) 24(2) (Schnabel Reference 20C23027)

Dear Ms. Alunkal:

SCHNABEL ENGINEERING, LLC (Schnabel) is pleased to submit our geotechnical and pavement engineering report for this project. This study was performed in accordance with our proposal dated May 7, 2020, as authorized by you in Task Order No. 03; and our contract modification dated March 31, 2022, as authorized by you in Amendment No. 3 to the aforementioned Task Order. This report has been revised based on September 2021 design team comments.

This report includes lateral earth pressure and foundation recommendations for the planned site retaining wall along 17th Street NW, as well as anticipated infiltration rates at the proposed stormwater management facility at the intersection of 17th Street NW and Piney Branch Parkway.

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

Lucas Hoyos, EIT Project Engineer

Christopher Clarke, PE Steve Fung, PE Senior Associate Senior Associate

FA:KD:CSC:sam:jdb:LH:jdb:ka:jdb \\EGNYTEDRIVE\EGNYTE PROJECTS\STERLING\2020\20C23027 - PINEY BRANCH PARKWAY AND WATERSIDE DRIVE\03-SE PRODUCTS\03-REPORTS\PINEY BRANCH PKWY\01-DRAFT - REV01\20C23027 - PINEY BRANCH PARKWAY GEOTECHINCAL ENGINEERING REPORT - 2024.DOCX

Project No. 20C23027 Page i Schnabel Engineering, LLC July 5, 2024 ©2024 All Rights Reserved

GEOTECHNICAL AND PAVEMENT ENGINEERING REPORT

ROCK CREEK PARK - PINEY BRANCH PARKWAY,

WASHINGTON, D.C.

TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY

2.0 SCOPE OF SERVICES

3.0 DESCRIPTION OF SITE AND PROPOSED CONSTRUCTION

3.1 Site Description

3.2 Proposed Construction

3.3 Traffic Data

3.4 Regional Geology

4.0 SITE EXPLORATION PROGRAM

4.1 Test Borings and Pavement Cores

4.2 Visual Pavement Condition Assessment (PCA)

4.3 In-situ Infiltration Testing

5.0 SOIL LABORATORY TESTING

5.1 Index Testing

5.2 Compaction and CBR Testing

5.3 Corrosivity Testing

6.0 SITE GEOLOGY, PAVEMENT, AND SUBSURFACE CONDITIONS

6.1 Site Geology

6.2 Generalized Subsurface Stratigraphy

6.3 Groundwater

6.4 In-situ Infiltration Test Results

6.5 Chemical Laboratory Test Results

7.0 SITE GRADING AND EARTHWORK

7.1 Compacted Fill Subgrades

7.2 Compacted Fill

7.3 Open Cut Excavations

8.0 RETAINING STRUCTURES RECOMMENDATIONS

8.1 Site Retaining Wall

9.0 SPREAD FOOTINGS

10.0 PAVEMENT ANALYSES AND RECOMMENDATIONS

10.1 Pavement Condition Assessment

10.2 Pavement Recommendations

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11.0 CONSTRUCTION CONSIDERATIONS

11.1 Site Grading and Earthwork

11.2 Spread Footings

11.3 General Specification Recommendations

12.0 LIMITATIONS

LIST OF FIGURES

Figure 1: Site Vicinity Map Figure 2: Lateral Earth Pressure Diagram for Site Retaining Wall Figure A1: Test Location Plan Figure A2: Pavement Condition Assessment Plan Figure 3: Geologic Map

LIST OF TABLES

Table 1: Pavement Core Summary Table 2: Summary of In-situ Infiltration Testing Results Table 3: PCI Value for Each Segment Table 4: Pavement Recommendations

APPENDICES

Appendix A: Subsurface Exploration Data Appendix B: Soil Laboratory Test Data Appendix C: PCI Survey Data Appendix D: Pavement Distress Photo Log Appendix E: Calculations

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1.0 EXECUTIVE SUMMARY

This report presents the results of our visual pavement condition assessment, subsurface explorations, laboratory testing, and geotechnical and pavement engineering analysis for the proposed pavement rehabilitation of Piney Branch Parkway and associated improvements located in the Rock Creek Park in Northwest Washington, D.C. 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.

• Our 2021 subsurface exploration and pavement cores along Piney Branch Parkway observed asphalt pavement thicknesses ranging from 5 to 18 inches with an average of 10.5 inches. Our 2023 subsurface exploration for the 17th Street improvements observed about 3 to 5 inches of topsoil/forest litter at the ground surface. We observed subsurface conditions consisting of existing fill soils (Stratum A) in 16 of the 17 test borings performed, which extended to depths ranging from 3 to 15 ft below the existing ground surface. Underlying the existing fill or the asphalt pavement, we encountered the alluvium of Stratum B in eight of the test borings to depths ranging from 6.5 to 15 ft. Below the existing fill of Stratum A we encountered Residual Soils Stratum C at a depth of 13.5 ft below the ground surface only in boring PB-08.

• Groundwater was observed during drilling in borings PB-06, PB-08, and PB-10B at depths ranging from 4 to 12 ft below the ground surface. Groundwater was not observed during drilling or after completion of the retaining wall borings (RW-01 and RW-02) or the bioretention basin borings (SWM-01 and SWM-02). However, 24-hr readings on the SWM borings observed the groundwater to be at a depth of 8.5 and 8.9 ft below the ground surface. This corresponds to approximate elevations of about EL 97.5 and EL 97.1 ft, respectively.

• The estimated in-situ infiltration rates measured during our subsurface exploration were 0.05 and

0.37 inches per hour. Typically, the minimum required infiltration rate considered suitable by the Washington, D.C. District Department of Energy and the Environment (DDOEE) is 0.52 inches/hour. Therefore, the site soils may not be suitable for infiltration.

• Based on laboratory test results, the near surface soils are considered potentially corrosive to buried metallic structures and have negligible potential for sulfate attack on concrete according to ACI standards. Thus, drainage structures to be built within these soils should be designed based on an aggressive subsurface environment for buried metals.

• We consider shallow foundations consisting of spread footings suitable for the support of the planned retaining wall or boardwalk along 17th Street NW. We recommend a design soil bearing pressure of 2,500 psf for spread footings supported by the alluvial soils of Stratum B.

• The planned site retaining wall should be designed to resist lateral earth pressures. An equivalent fluid pressure of 40H psf should be considered for design. Drainage behind the site retaining wall must be provided to reduce the possibility of hydrostatic pressures acting on the walls.

• We observed debonding of the asphalt surface layer from the asphalt base in approximately 50 percent of the extracted cores. The asphalt surface layer ranges from approximately 2 to 2.5 inches thick.

• According to the Scoping Report, the pavement was milled and resurfaced in late 2018. Specific information on milling and overlay depths or previous patching activities were not available.

Based on historic photos available online, we observed that the condition of the asphalt pavement

Project No. 20C23027 Page 2 Schnabel Engineering, LLC prior to the last rehabilitation was likely “poor” since functional cracking, fatigue cracking, and pavement patches were observed at numerous locations.

• Our recent visual pavement condition assessment observed good pavement surface conditions along the entirety of Piney Branch Parkway. Infrequent functional distresses and localized minor depressions were the only form of distress observed on the roadway surface. Furthermore, based on our recent site visits and general pavement observations, Piney Branch Parkway still appears to be in good condition as of February 2024.

• Ponded water was observed on the northbound lane from about Station 32+00 to 33+20 during the pavement survey. This drainage problem is located at the same location where we observed a pavement depression. We believe the water is penetrating into the subgrade creating loss of subgrade stability. The water source may be associated with overland drainage, and appears to originate from east of the road. This surface water should be collected before it flows onto the roadway.

• We recommend a pavement rehabilitation consisting of milling 6 inches of the existing asphalt layer and resurfacing with 6 inches asphalt overlay along the entirety of the roadway.

• Localized full depth patching will be required for the base asphalt at locations found in poor condition after milling and prior to resurfacing. We expect that full-depth patching will be required for the northbound lane from about 32+00 to 33+20 at the location where we observed water ponding on the pavement surface.

• A qualified pavement/geotechnical engineer must inspect the pavement surface after the milling operation is finished in order to identify patch locations. The patch excavations should be replaced with base asphalt to match the surrounding base asphalt depths or to the minimum of 6 inches. Based on our observations from the Google Map photos and our experience, we recommend that the project documents assume that 5 percent of the pavement surface will require full depth patches. The project documents should consider add/deduct prices for more or less patches during construction.

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2.0 SCOPE OF SERVICES

Our proposal dated May 7, 2020, our contract modification dated March 31, 2022, and the “Statement of Work” NP ROCR 503 (1) (24) 2” define the scope of services for this project. The project includes pavement and geotechnical engineering services for Piney Branch Parkway and Waterside Drive in Washington, D.C. Our scope of services for Piney Branch Parkway included our subsurface explorations, conducting a visual pavement condition assessment, associated laboratory testing and preparing this geotechnical and pavement engineering report. This report includes:

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

• The results of our in-situ infiltration testing.

• Photographs of the current pavement surface during our pavement condition survey. We have also included available historic photographs based on Google Street View images.

• Photographs of the pavement cores collected during our subsurface exploration.

• PCI results resulting from our pavement condition assessment and a map showing the plotted locations of the pavement conditions based on the evaluated segments of the roadway.

• Foundation requirements for support of the planned site retaining wall or boardwalk, including recommended allowable soil bearing pressures, bearing grades, and estimated settlements for shallow spread footings.

• Recommended lateral earth pressure diagrams for design of site retaining walls.

• Recommendations regarding handling of groundwater in design, including subdrainage design requirements.

• Earthwork recommendations for subgrade preparation and compaction criteria.

• Recommendations for pavement rehabilitation in accordance with AASHTO 1993 pavement design guidelines.

• 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 within Rock Creek Park, managed by the National Park Service (NPS), in Northwest, Washington, D.C. Piney Branch Parkway NW is a 0.84 mile, two lane, asphalt-paved roadway that connects Beach Drive NW to Arkansas Avenue NW, and is intersected by 17th Street NW. The roadway generally parallels Piney Branch Creek. Two bridge structures pass over the parkway at intersections with Park Road NW and 16th Street NW. The parkway pavements are typically 24 ft wide, with 12 ft wide merge lanes at Arkansas Ave. Other roadway features include curb and gutter pan, tight radii corners at intersections with Beach Drive, 17th Street, and Arkansas Ave., and storm drains crossing underneath the road to outfalls that drain stormwater to Piney Branch Creek. The areas surrounding Piney Branch Parkway are generally wooded with sections of gentle to steep slopes. Site grades generally slope down from the southeast towards Piney Branch Creek to the northwest. Additionally, according to the Statement of Work, surface runoff tends to pond on the roadway from about Station 32+00 to Sta. 33+20 after periods of rain causing issues to the proper flow of traffic.

Based on the Design Scoping Report (ROCR 503(1), 24(2), dated August 2019 (Scoping Report) by Eastern Federal Lands Highway Division (FHWA), we understand that Piney Branch Parkway was milled and resurfaced by DDOT in November 2018.

On the north side of Piney Branch Parkway, a gravel surface trail parallels the roadway and provides pedestrian access to Piney Branch Park. It is our understanding that this trail will be paved as part of DDOT’s Rock Creek Park Multi-Use Trail and Pedestrian Bridge Project.

Schnabel obtained the site information from the plans and reports provided by WSP, from our onsite observations during our pavement condition survey and subsurface explorations, and through our review of readily available site aerial imagery.

3.2 Proposed Construction

The project includes the rehabilitation of Piney Branch Parkway. The pavement rehabilitation will generally consist of rehabilitation treatments such as mill and resurfacing to address functional distresses generally located within the surface layer, which will also improve and maintain surface grades and improving pavement surface friction. Preservation treatments were also considered for this roadway;

however, as discussed in later sections, we believe that pavement rehabilitation is more appropriate given the pavement history.

Additionally, the project includes the construction of approximately 600 ft of sidewalk from 17th Street NW to Piney Branch Parkway including a crosswalk of Piney Branch Parkway. The planned sidewalk will be constructed parallel to the southbound lane of 17th Street NW and will be approximately 5 ft wide. To accommodate the planned sidewalk, a site retaining wall approximately 100 ft in length will be constructed. This planned site retaining wall will have a maximum retained height of 5 ft. We understand that a boardwalk is also being considered in lieu of a retaining wall. Due to the increase in impervious area, a new bioretention basin will be constructed at the southwest quadrant of the intersection of 17th Street NW and Piney Branch Parkway. This bioretention basin will be approximately 780 sq ft in plan dimension and will have an invert elevation of about EL 99.7 ft.

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From its intersection with Piney Branch Parkway, an approximately 200 ft segment of 17th street will be milled to a depth of 2.5 inches and overlain with asphalt to meet existing grade. We understand that this is a functional mill and overlay intended to resurface and connect 17th street with Piney Branch Parkway.

3.3 Traffic Data

Traffic count information was provided to us by the Eastern Federal Lands in the Scoping Report which included AADT data and 0 percent trucks and buses. The Scoping Report indicates an AADT of 11,600 in 2012 and a future ADT in 2032 of 14,600 which results in a growth rate of 1.16 percent. Based on our field exploration we did notice several small sized trucks and therefore we have assumed a single unit truck percentage of 0.5 percent for our analyses, rather than simply 0 percent trucks.

3.4 Regional Geology

We reviewed existing geologic data and the previous test borings performed within Piney Branch Parkway. Based on this information, the site is primarily underlain by Cambrian age bedrock of the Laurel Formation. The bedrock is overlain by residual soils and disintegrated rock derived from the weathering of the parent bedrock. During our subsurface exploration we also observed alluvial soils at shallow depths, likely related to the historic deposition of the adjacent Piney Branch Creek. This information was obtained from the “Geological Map of Washington West Quadrangle” by Fleming et al. (1994). An excerpt of the geologic map is included as Figure 3.

Existing fill soils were found in our test borings above the natural soils. The existing fill soils are likely associated with the construction and grading of the existing roadway and the installation of underground utilities. We expect that the roadway subgrades will mostly consist of existing fill soils.

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4.0 SITE EXPLORATION PROGRAM

We performed two subsurface explorations and field-testing programs to identify the subsurface stratigraphy underlying the project site and to evaluate the geotechnical properties of the materials encountered. The first exploration was conducted from June 21 through 24, 2021, while our recent exploration was conducted on December 20 and 21, 2023.

Our 2021 exploration program included test borings, pavement cores, and a visual Pavement Condition Assessment (PCA) of the existing pavements. Our 2023 exploration program included test borings and in-situ infiltration testing. Exploration methods used are discussed below. The appendices contain the results of our subsurface exploration (Appendix A) and PCA observations (Appendix C).

4.1 Test Borings and Pavement Cores

For the first subsurface exploration, Schnabel’s subcontractor, Connelly and Associates Drilling Services Inc., drilled 14 test borings including offset borings (PB-01 through PB-11) within the project limits under our direct observation on June 21 through 24, 2021. Three borings were offset (PB-07B, PB-09B, and PB-10B) at locations where shallow obstructions were encountered. In addition, we collected pavement cores from all borings drilled. The test boring locations were spaced along the road to obtain general subsurface information at the entire roadway. Standard Penetration Tests (SPTs) were performed at selected depths in the borings. We note that we had planned to perform a test boring at location PB-12.

During the subsurface exploration we observed a significant density of utilities at the planned test location, and it became obvious to our field engineer that some of these utilities were mismarked by the public utility companies. We elected to only perform a pavement core at this location, to avoid damaging underground utilities.

During our recent subsurface exploration, Connelly and Associates Drilling Services Inc., drilled four additional test borings under our direct observation on December 20 and 21, 2023. Two borings (RW-01 and RW-02) were drilled within the footprint of the planned site retaining wall along 17th Street NW. The other two borings (SWM-01 and SWM-02) were drilled within the planned bioretention basin on the southwest quadrant of the intersection. Standard Penetration Tests (SPTs) were also performed at selected depths in the borings. The location of these test borings are also depicted in Figure A1, Test Location Plan.

These test boring and pavement core locations are shown on the Test Location Plan included as Figure A1.

Appendix A includes specific observations, remarks, and logs for the borings; classification criteria; drilling methods; approximate test boring and pavement core locations; pavement core photographs; and sampling protocols.

4.2 Visual Pavement Condition Assessment (PCA)

Schnabel performed a visual survey of the existing pavement along Piney Branch Parkway on April 22, 2021. We utilized ASTM D6433 (Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys) to evaluate and document visible surface distresses and estimate Pavement Condition Index (PCI) values along the road. The results of our assessment of the existing pavement are shown on

Project No. 20C23027 Page 7 Schnabel Engineering, LLC the Pavement Condition Assessment Plan included as Figure A2. The specific pavement condition indices and photographs of the typical distresses observed are documented in Appendices C and D, respectively. We note that Photos 5 through 14 presented in Appendix D were obtained from Google Street and include historic photographs collected between November 2016 and September 2017, and recent photographs collected in October 2023. Some of these photos predate our 2021 and 2023 subsurface explorations. We have included these photos to present the conditions of the existing pavement prior to the milling and resurfacing performed by DDOT in November 2018 and our subsurface explorations. The photographs from October 2023 present current images of the pavement in the approximate same locations as the November 2016 and September 2017 historic photographs.

4.3 In-situ Infiltration Testing

Schnabel’s personnel conducted two in-situ infiltration tests (INF-01 and INF-02) in offset auger probes from the SWM borings performed within the planned bioretention basin during our 2023 exploration. This testing was performed using the Johnson Permeameter (constant-head testing). The Johnson Permeameter was lowered into the probe hole and water was added to the test reservoir. Water was released from the reservoir into the hole and a constant well-head height was established by continuously releasing water from the reservoir. The drop in the water level in the reservoir was measured at intervals of about two minutes to measure the rate of water infiltration. The results from the infiltration testing are provided in Appendix A and are summarized in section 6.4 of this report.

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5.0 SOIL LABORATORY TESTING

Our laboratories in Richmond, Virginia and Baltimore, Maryland, performed tests on selected soil samples collected during the subsurface explorations. This testing included index property testing, moisture density relationship (Proctor), and CBR tests on samples collected from the soil test borings. The testing aided in the classification of materials encountered during the subsurface explorations and provided data for use in the development of our recommendations for repair and replacement of the existing asphalt pavements, site retaining walls, and bioretention basin. The results of the laboratory tests are included in Appendix B and are summarized for each stratum in the Section 6.0 of this report. Selected test results are also shown on the boring logs in Appendix A.

5.1 Index Testing

We performed natural moisture content, Atterberg Limits, and gradation tests on select soil samples representative of the existing fill soils of Stratum A and the Alluvium soils of Stratum B, to provide soil classifications on soil materials anticipated at planned structure subgrades and to allow for correlations with published soil properties in general accordance with the Unified Soil Classification System Group Symbols (ASTM D2487) and AASHTO Classifications (M-145).

5.2 Compaction and CBR Testing

We performed Standard Proctor compaction (AASHTO T 99) and California Bearing Ratio (CBR) (AASHTO T193) testing on two bulk samples representing Stratum A to evaluate compaction characteristics and to provide soil parameters for pavement analysis and design.

5.3 Corrosivity Testing

We performed tests for pH, sulfides, redox potential, and resistivity testing on one bulk sample from boring SB-11 representing Stratum A. Our subcontractor, Phase Separation Science, performed water-soluble sulfate and chloride tests on the same soil sample. The test results are presented in Appendix B and are summarized in Section 6.6.

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6.0 SITE GEOLOGY, PAVEMENT, AND SUBSURFACE CONDITIONS

6.1 Site Geology

During our explorations, we encountered existing fill soils or natural alluvium directly below the existing ground cover. The existing fill soils, identified as Stratum A, are believed to have been placed during past earthwork operations for roadway construction. The natural Alluvium, identified as Stratum B, was generally encountered below the Fill of Stratum A. Also observed below the fill of Stratum A was the Disintegrated Rock of Stratum C which is derived from the in-place weathering of the underlying parent bedrock. Though not encountered during our explorations, more weathered portions of the underlying parent bedrock may be encountered at different locations onsite as Residual soils.

6.2 Generalized Subsurface Stratigraphy

We characterized the following generalized subsurface stratigraphy based on the exploration and laboratory test data included in the appendices.

6.2.1 Ground cover

Topsoil

Borings RW-01 and RW-02, and SWM-01 and SWM-02 were performed outside of the existing roadway pavements and encountered topsoil and forest litter at the ground surface to depths of about 3 to 5 inches.

Existing Asphalt Pavement

The remaining borings performed in the roadway encountered asphalt pavement at the ground surface, and pavement cores were performed. The existing pavement section generally consisted of asphalt overlying aggregate base with some exceptions where aggregate base was not observed. We observed that approximately 50 percent of the cores extracted exhibited debonding of the asphalt surface layers at various depths. During our 2021 exploration, we encountered dense aggregate base in six borings, the aggregate base thicknesses varied from 1.5 to 6 inches. The remaining asphalt pavement was supported directly on existing fill soils that were not visually identified to be dense graded aggregate base. The table below summarizes our observations of existing asphalt at each boring/core location. Photos of the extracted cores are included in Appendix A.

Table 1: Pavement Core Summary

Pavement Core ID

Location Total Asphalt

(in)

Asphalt Surface

(in)

Asphalt Base (in)

Agg.

Base

(in) Northing Easting Elev. (ft)

Approx.

Station

Lane

PB-01 462073.39 1299063.25 59

13+00 WB 10 2 8 2

PB-02 462223.65 1299397.65 62 16+67 EB 6.5 2.5 4 6

PB-03 462282.94 1299749.93 76 20+29 WB 9 2 7 3

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Pavement Core ID

Location Total Asphalt

(in)

Asphalt Surface

(in)

Asphalt Base (in)

Agg.

Base

(in) Northing Easting Elev. (ft)

Approx.

Station

Lane

PB-04 462295.07 1300122.50 83 24+03 EB 12.5 2.5 10 N/O

PB-05 462339.62 1300532.05 90 28+17 WB 10.5 2.5 8 1.5

PB-06 462496.16 1300865.42 93 31+92 EB 18 2.5 15.5 N/O

PB-07A 462641.89 1301181.24 101 35+41 WB 12 2 10 N/O

PB-07B 462647.43 1301197.20 101 35+56 WB 11.5 2.5 9 N/O

PB-08 462804.51 1301473.72 107 38+78 EB 6 3 3 6

PB-09A 462976.58 1301709.98 111 41+70 WB 12 2 10 N/O

PB-09B 462983.8 1301723.45 111 41+85 WB 13 2.5 10.5 N/O

PB-10A 463118.79 1302040.55 114 45+29 EB 12 3 9 N/O

PB-10B 463124.99 1302053.91 114 45+44 EB 13 2.5 10.5 N/O

PB-11 463355.14 1302280.99 133 48+73 WB 6 2.5 3.5 6

PB-12 463648.00 1302364.63 140 52+00 EB 5 2 3 N/M

N/O denotes not observed during subsurface exploration.

N/M denotes not measured during subsurface exploration.

6.2.2 Stratum A: Existing Fill

Below the ground cover or asphalt pavement and aggregate, we encountered existing fill material of Stratum A in 16 of the test borings. These materials extended to depths of 3 to 15 ft below the ground surface, the maximum depth explored. The existing fill material classified as LEAN CLAY (CL), CLAYEY SAND (SC), SILTY SAND (SM), WELL GRADED GRAVEL (GW), SILTY GRAVEL (GM), and POORLY GRADED SAND (SP) per the Unified Soil Classification System (USCS), and visually classified as A-6, A- 2-6, and A-2-7 per AASHTO M-145. The SPT N-values within the existing fill soils ranged from WOH (weigh of hammer - equivalent to 0 bpf) to greater than 50 blows per foot (bpf).

Laboratory tests performed on eight samples of soil collected from Stratum A recorded Liquid Limits ranging from 27 to 46 and Plasticity Indices ranging from 5 to 25. The percentage passing a No. 200 sieve was between 10.5 and 72.6 percent. The natural moisture contents of tested samples of Stratum A varied between 2.4 and 19.7 percent.

Standard Proctor compaction and CBR tests were performed on two bulk soil samples of the existing fill collected at borings PB-03 and PB-11. These samples classified as a CLAYEY SAND (SC) and CLAYEY SAND (SC) with Gravel with Liquid Limits of 32 and 43 and Plasticity Indices of 13 and 18, respectively.

These samples had percentages passing the No. 200 sieve of 39.4 and 25 percent. The aggregate in the sample collected from boring PB-11 appeared to be blast furnace slag.

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The results of Standard Proctor testing measured maximum dry densities of 122.3 and 107.6 pounds per cubic foot (pcf) at optimum moisture contents of 11.9 and 13.7 percent, respectively. The results of CBR testing measured a CBR value of 5.3 and 15.6; and a swell percentage of 0.5 and 0.2, respectively.

6.2.3 Stratum B: Alluvium Soils

Underlying the existing ground cover or the existing fill soils of Stratum A, we encountered the alluvium of Stratum B in eight test borings to depths ranging from 6.5 ft to 15 ft below the ground surface. Stratum B soils consisted of LEAN CLAY (CL), CLAYEY SAND (SC), SILTY SAND (SM), and WELL GRADED SAND (SW) per the USCS, and visually classified as A-2-4 and A-2-7 per AASHTO M-145. The alluvium of Stratum B had SPT N-values ranging from 2 to greater than 100 bpf.

6.2.4 Stratum C: Disintegrated Rock

Below the existing fill soils of Stratum A, or the alluvial soils Stratum B, we encountered disintegrated rock of Stratum C in borings PB-08, SWM-01, and SWM-02 at depths of 8.5 to 13.5 ft below the existing ground surface. This corresponds to approximate elevations of about EL 93.5 and EL 97.5, respectively.

Disintegrated rock is defined as residual material with SPT N-values between 60 blows per foot and refusal. Refusal is defined as an N-value of 50 blows for a penetration of one inch or less. The disintegrated rock of Stratum C was visually classified as SANDY SILT (ML) and SILTY SAND (SM) and contained lesser amounts of rock fragments and mica. The SPT N-values measured within this stratum were greater than 100 bpf.

Seven of the test borings had auger refusal at depths ranging from 3 to 6.5 ft below the ground surface.

We believe the augers refused on bedrock. However, it is possible the augers refused on boulders.

6.3 Groundwater

Groundwater was observed during drilling in three of the borings performed within the existing pavements of Piney Branch Parkway (PB-06, PB-08, and PB-10B) at depths ranging from 6 to 12 ft (EL 87 to EL 102) below the ground surface. Piney Branch Creek, a tributary creek of Rock Creek runs parallel, to Piney Branch Parkway and has approximate water elevations ranging from EL 101 to 108 ft. Long-term water level readings were not taken within the boring’s locations within the existing pavements of Piney Branch Parkway, as these were backfilled upon completion for safety reasons.

Groundwater was not observed during or immediately after drilling the retaining wall borings (RW-01 and RW-02) or the bioretention basin borings (SWM-01 and SWM-02). Schnabel installed 1 inch temporary PVC standpipes and measured groundwater approximately 24 hours after drilling the SWM borings.

Groundwater was measured to be at a depth of 8.5 and 8.9 ft below the ground surface 24 hours after drilling. This corresponds to approximate elevations of about EL 97.5 and EL 97.1 ft, respectively.

The groundwater levels on the logs indicate our estimate of the hydrostatic water table at the time of our subsurface exploration. Variation in groundwater levels may occur depending on variation in precipitation, surface runoff, leaking utilities, lake levels, and similar factors.

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6.4 In-situ Infiltration Test Results

Two in-situ infiltration tests (INF-01 and INF-02) were performed in offset auger probes to borings SWM- 01 and SMW-02 within the footprint of the planned bioretention basin. We utilized a Johnson Permeameter apparatus to conduct the testing. Tests were performed at depths of 6.5 ft (INF-01) and 6.8 ft (INF-02) below the existing site grades, this corresponds to approximate elevations of about EL 99.5 and EL 99.2 ft, respectively. Testing depths were selected as to maintain a distance of, at least, 2 ft above the observed groundwater.

Water level readings during the infiltration testing were collected in 2-minute intervals to measure the rate of water infiltration. The table below summarizes the results of the testing.

Table 2: Summary of In-situ Infiltration Testing Results

Test ID

Approx.

G.S.E. (1) at Test Location, (ft)

Groundwater at 24-hrs Approx. Test

Elevation (ft)

USCS Classification at Test Depth in

Adjacent Borehole

Field Saturated Infiltration Rate

(inch/hour) Depth (ft)

Approx.

EL (ft)

INF-01 EL 106 8.5 97.5 EL 99.5 CLAYEY SAND (SC) 0.05

INF-02 EL 105 8.9 97.1 EL 99.2 SILTY SAND (SM) 0.37

(1) G.S.E. = Ground Surface Elevation

The infiltration rates reported above do not include a factor of safety and the stormwater management designer should consider an appropriate factor of safety in their design. Typically, the minimum required infiltration rate considered suitable by the Washington, D.C. District Department of Energy and the Environment (DDOEE) is 0.52 inches/hour. Therefore, the site soils may not be suitable for infiltration.

It should be noted that field infiltration rates are expected to vary at different locations and depths at the site due to variable conditions of soils, compactness, gradation, soil matrix, etc. The design should account for such variations.

6.5 Chemical Laboratory Test Results

Chemical testing was performed on one bulk sample from boring PB-11 to evaluate the corrosivity of Stratum A soils. The chemical testing measured a pH value of 6.41, Redox Potential value of 2 mV, Resistivity value of 300 ohm-cm, and a negative test for sulfides. Chloride and sulfate ion tests performed on the soil sample measured 910 ppm of chlorides and 1,300 ppm of sulfates.

Based on these test results, the soils of Stratum A are considered potentially corrosive to buried metallic structures in accordance with AWWA standards and have negligible potential for sulfate attack on concrete according to ACI standards. According to these results, drainage structures to be built within the soils of Stratum A should be designed based on an aggressive subsurface environment for buried metals.

Our corrosivity calculations are included in Appendix E.

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7.0 SITE GRADING AND EARTHWORK

We expect that the majority of the pavement repairs will consist of milling and resurfacing with some full depth patching. Only small, localized sections of the roadway will likely require full-depth reconstruction in the form of patching. Other grading associated with correcting site drainage issues is expected, but we have anticipated that these will require minor cuts and fills (less than 3 ft). The subgrade may need to be removed or replaced with new compacted fill as necessary to match the proposed pavement thickness and the existing curb and gutter elevations, or to repair localized soft, loose, or wet unsuitable subgrades.

The exposed pavement subgrades must be evaluated, repaired, and approved per FP-14 Section 204.09.

Additionally, we anticipate that cuts of up to 6.5 ft will be required for the construction of the planned bioretention basin. Fills of up to 5 ft will also be required for the construction of the site retaining wall along 17th Street NW. Recommendations for compacted fill subgrade preparation, fill soil requirements, placement and compaction criteria are presented in the subsequent sections.

7.1 Compacted Fill Subgrades

Subgrades to support compacted fill or the proposed pavement sections may consist of suitable, non-deleterious existing fill soils of Stratum A, the alluvium soils of Stratum B, or the disintegrated rock of Stratum C. Subgrades that are observed to be soft, loose, or wet as identified by the Geotechnical Engineer, should be entirely removed or undercut to a depth of 2 ft, whichever is less, and replaced with new common borrow or aggregate base.

Subgrades outside of the existing pavement sections that will receive compacted structural fill should be stripped of vegetation, topsoil, and organic matter. Our 2023 subsurface exploration indicated topsoil depths of up to about 5 inches below the ground surface. However, stripping of wooded sites typically results in some disturbance and contamination of near-surface soils, particularly during periods of wet weather. Therefore, topsoil stripping depths in excess of the observed topsoil depths may be necessary.

Stumps, roots, forest litter, other perishable material, and nonperishable objects should be removed from all areas where the new fill will be placed.

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.

The Geotechnical Engineer should evaluate the suitability of the fill subgrades. To evaluate the subgrade suitability for support of the compacted fill prior to any undercutting or initiation of fill placement, the stripped subgrades should be proof rolled with a loaded dump truck or other suitable construction equipment. Areas that exhibit excessive pumping, weaving, or rutting should be scarified, dried and recompacted, or undercut and replaced with aggregate base fill as recommended by the Geotechnical Engineer. Smaller areas of exposed subgrades should be evaluated by probing with a DCP or geostick, hand excavations, and/or test pits.

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

Adequate control of surface drainage will be a very important consideration for the overall performance of the pavements. We believe the surface water from about Stations 32+00 to 33+20 is due overland flow of water to the east of the roadway, though it may also be associated with a springhead also located east of the roadway. Surface drains should collect this water before it flows onto the pavement.

The area surrounding pavements should be graded to direct surface water away from paved areas.

Compacted fill and aggregate base 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 if encountered during construction.

7.2 Compacted Fill

We recommend that compacted fill placed as backfill of undercut soils within the roadway boundary consist of compacted common borrow fill or aggregate base per FP-14 Section 703.

Compacted fill should consist of common borrow soils that are non-organic soils classifying as SC, SM, SP, SW, GM, GP, or GW according to ASTM D2487 (A-1 through A-4 according to AASHO M-145).

Compacted fill materials should exhibit Liquid Limit and Plasticity Index values of less than 45 and 20, respectively. Fill and backfill materials should not contain particles larger than 3 inches. We anticipate that the majority of the soils that will be encountered below pavement subgrades will consist of the fill soils of Stratum A. At the locations of planned retaining wall and bioretention basin, alluvial soils of Stratum B will also be encountered.

Strata A and B soils are generally expected to be suitable for reuse as compacted fill, however, portions of Stratum A soils contained deleterious materials such as wood, brick fragments, roots, and possibly blast furnace slag. Careful screening will be required if the existing fill of Stratum A will be reused as fill.

Due to the limited excavation anticipated for this project, we anticipate that most materials used as compacted fill will be imported borrow soils. Borrow soils should exhibit a minimum CBR of 5.0, this is approximately equivalent to a resilient modulus of at least 7,500 psi.

Compacted fill should be placed in maximum 8-inch-thick horizontal, compacted lifts. Fill should be compacted to at least 95 percent of the maximum dry density per AASHTO T-99. Compaction effort and soil moisture contents at the time of compaction should conform to FP-14 Section 204.11.

Backfill placed in excavations, trenches, behind site retaining walls, 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. Based on available laboratory test data, the onsite soils are expected to be wet of 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

Project No. 20C23027 Page 15 Schnabel Engineering, LLC 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.

7.3 Open Cut Excavations

Given the existing site conditions encountered during our subsurface explorations and the anticipated invert elevation for the bioretention basin, we anticipate that the excavation will be open cut.

OSHA’s Occupational Safety and Health Standards-Excavations; Final Rule (29 CFR Part 1926), applies to temporary excavations less than 20 ft deep and without groundwater seepage. The sides of the open-cut excavations should be sloped according to OSHA requirements. If the excavation will be open for an extended period of time, some sloughing may occur. Sloughing in localized areas should be repaired or the slope should be flattened as recommended by the Geotechnical Engineer. Based on the groundwater readings collected during our subsurface explorations, we expect that the side slopes discussed herein will be above the groundwater level.

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8.0 RETAINING STRUCTURES RECOMMENDATIONS

We understand that to accommodate the planned sidewalk from 17th Street NW to Piney Branch Parkway a site retaining wall approximately 100 ft in length with a maximum retained height of 5 ft will be constructed. We also understand that as an alternative, a boardwalk may be constructed to avoid adding fill to this area. The following section describes recommended earth pressures and design considerations for a potential retaining wall. The subsequent section presents spread footing recommendations for support of the retaining wall or boardwalk structure.

8.1 Site Retaining Wall

The planned site retaining wall should be designed to resist lateral earth pressures developed from the surrounding soil, backfill, and surcharge loads. Earth pressures should be calculated based on a moist unit weight of 120 pcf. Active and passive equivalent fluid pressures should be used for site retaining wall design. An equivalent fluid pressure of 40H psf should be considered. The equivalent fluid pressures are shown in Figure 2 at end of this report. The recommended equivalent fluid pressures assumes horizontal backfill. We should be contacted to provide alternative parameters if sloping ground surface conditions are anticipated.

Where applicable, the design should consider surcharge loads using a rectangular earth pressure distribution. The surcharge pressure ordinate should be obtained by multiplying the surface surcharge pressure, q, by the appropriate factor indicated on Figure 2. A friction factor of 0.3 may be utilized between the concrete footings and subgrade soils.

Backfill meeting the material requirements provided in the Site Grading and Earthwork section should be placed in the zone extending from the base of the wall heel upwards at 45 degrees. If open-grade crushed stone, such as AASHTO No. 57 stone, is used as backfill behind the wall, the Contractor may place the crushed stone backfill in maximum 12-inch-thick lifts, and each lift should be compacted using suitable vibratory equipment. Only light, hand-operated equipment should be used to compact backfill against walls. The Structural Engineer of Record should approve the size of the compaction equipment.

Drainage should be provided behind the site retaining wall, as depicted on Figure 2, to reduce the possibility of hydrostatic pressures acting on the wall. Earth pressure recommendations provided above do not include hydrostatic pressure since subdrainage will be provided behind the wall.

Subdrains for the site retaining wall should consist of 4-inch slotted, corrugated polyethylene tubing according to ASTM F405 surrounded by at least 4 inches of drainage filter material. A drainage geotextile should wrap around the drainage material. Drainage filter material should consist of AASHTO No. 57 stone. Drainage geotextile should consist of a non-woven geotextile such as Mirafi 140N or equivalent.

Subdrains should drain by gravity to an outlet, or to a sump or storm sewer. Alternatively, weepholes may be used when free-draining backfill, such as AASHTO No. 57 or 67 stone, is used. Weepholes should consist of four-inch slotted, Schedule 40 PVC pipes and be installed on 8 ft centers. A filter plug consisting of at least one cubic foot of drainage filter material wrapped in drainage geotextile should be placed at the back of each weephole as illustrated on Figure 2.

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9.0 SPREAD FOOTINGS

We consider spread footings suitable for support of the planned site retaining wall or boardwalk. Footings should be founded on suitable natural soils consisting of the alluvial soils of Stratum B or on new compacted structural fill. Based on the subsurface conditions encountered during our recent subsurface exploration, we anticipate that the alluvial soils of Stratum B will be encountered at foundation bearing grades. However, the existing fill soils of Stratum A may also be encountered at foundation bearing grades. The existing fill soils of Stratum A are undocumented and are not suitable for direct support of the site retaining wall footings. The existing fill soils of Stratum A, soft, loose, soils containing deleterious materials, and/or high plasticity soils, where encountered, should be undercut to a depth of 4 ft below spread footings, or in their entirety, whichever is less, and replaced with compacted structural fill.

Alternatively, footings may be lowered to bear on firm suitable natural soils.

We recommend footings constructed on suitable soils of Stratum B or new compacted structural fill be designed for a net allowable bearing pressure of 2,500 psf. Net bearing pressure is defined as the pressure applied to the subgrade materials due to the weight and structural loading of the structure minus the weight of the excavated overburden material. This bearing pressure provides a factor of safety against general bearing capacity failure of at least 3.0.

Settlements of shallow foundations supported on suitable natural soils and on properly placed compacted structural fill are not expected to exceed about one inch. Differential settlements between similarly loaded footings are not expected to exceed about half this value.

Footings should be at least 16 inches wide, respectively, for shear considerations. Footings should be founded at least 2.5 ft below final exterior grades for frost protection. Where bearing grades between adjacent footings vary, the slope between the bottom edges of adjacent footings should not be steeper than 30 degrees 1.75H:1V.

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10.0 PAVEMENT ANALYSES AND RECOMMENDATIONS

10.1 Pavement Condition Assessment

Schnabel performed a Pavement Condition Assessment (PCA) of the existing pavements along Piney Branch Parkway in accordance with the “Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys (ASTM D6433)”. The assessment was performed on April 23, 2021 to estimate the Pavement Condition Index (PCI) and document visible surface distresses. The Pavement Condition Assessment for Piney Branch Parkway involved:

• Piney Branch Parkway was divided into two segments within the project limits, one for the eastbound alignment and another one for the westbound alignment. The segments are shown on Figure A2 and are described in Table 3 below.

• The pavement area in each segment was subdivided into approximately 22 equal sized sections (12’ X 200’) known as sample units.

• A Schnabel engineer walked adjacent to the roadway, stopping occasionally to record his visual observations of pavement distresses (i.e. cracking area, crack width, length, surface rutting, etc.).

• At the end of the survey, the survey sheets for each sample unit were tallied for distress type and severity. We computed the Pavement Condition Index value (PCI) based on PCI deduct values.

These deduct values are derived from the deduct curves specified in ASTM D6433 for PCI calculation.

• For each segment, the average PCI value was determined based on individual sample unit PCI values.

• Table 3 below shows the overall PCI value calculated for each segment along with predominant distresses observed.

Table 3: PCI Value for Each Segment

Segment Description…