B08_3-CHOH_241449_-_Soils_Investigation_Report.pdf

Prepared for

NATIONAL PARK SERVICE

Denver Service Center

12795 W. Alameda Parkway, Lakewood CO 80225

GEOTECHNICAL REPORT FOR RESTORING THE CANAL

PRISM AND HISTORIC DRY STONE WALL NEAR MILE POST

11 TO PRESERVE INFRASTRUCTURE DESIGN SERVICES,

CHEASEPEAKE AND OHIO CANAL NHP, MARYLAND

Prepared by

412 Mt. Kemble Avenue, PO Box 1946 Morristown, New Jersey 07962-1946

December 2021

Restore Canal Prism and Historic Dry Stone Wall near Mile Post 11 to Preserve Infrastructure, MD

WSP USA December 2021

TABLE OF CONTENTS

Page

Executive Summary

1.0 Introduction

1.1 General

1.2 Project Description

1.3 Purpose and Scope of Work

2.0 Subsurface Investigation and Testing

2.1 General

2.2 Boreholes and Field Investigation Program

2.3 Borehole Inclinometer Installation

2.4 Sediment Sampling

3.0 Subsurface Conditions

3.1 General

3.2 Regional Geology of the Project Area

3.3 Site Specific Subsurface Conditions

3.4 Groundwater

3.5 Environmental Screening of Sediment

4.0 Geotechnical Evaluations and Engineering Analysis

4.1 General

4.2 Geotechnical Design Parameters

4.3 Solider Pile and Rock Anchor Design Recommendations

4.4 Stabilization by External Wall Support

4.5 Stability Analysis

5.0 Conclusions and Recommendations

6.0 Limitations

References 21

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TABLE OF CONTENTS (CONTINUED)

Figures Figure 1 Site Location Plan Figure 2 Inclinometer Boring Location Plan Figure 3 Sediment Sampling Location Plan

Tables Table 1 Boring Information Table 2 Canal Sediment Sample Summary Table 3 Groundwater Observations in Inclinometer Borings Table 4 Canal Sediment Grain Size Distribution Based on Laboratory Results Table 5 Geotechnical Design Parameters Table 6 Seismic Design Parameters Table 7 Geotechnical Design Parameters used for Stability Analysis Table 8 Global Stability Safety Factors and Maximum Deformation

Appendices Appendix A Boring Logs Appendix B Previous Boring Logs Appendix C Sediment Environmental Screening Laboratory Report.

Appendix D MDE Screening Calculator Appendix E Seismic Map Appendix F Geotechnical Analysis and Calculations Appendix G Anchor and Soldier Pile Foundation System, Design Drawings Appendix H Stability Analysis

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Executive Summary

On behalf of the National Park Service (NPS), WSP USA Inc. (WSP) conducted geotechnical design services for the final design of the restoration of the historic dry stone wall (log wall) near Mile Post 11 at Chesapeake and Ohio Canal (C&O Canal) National Historic Park, Maryland. The log wall is an approximate 1,015 foot long, on average 35 foot high dry stone wall with a tow path and historic canal located directly behind the wall. According to NPS, in December 2015 and January 2016, sinkholes developed adjacent to the towpath in the canal prism that may represent long-term stabilization issues in the log wall.. The following report includes final geotechnical design recommendations to stabilize the log wall, a summary of inclinometer installation, and environmental analytical laboratory results and recommendations for beneficial reuse of the sediment in the vicinity of the wall.

Subsurface investigation activities conducted for this program included the drilling of four (4) boreholes to depths between 30 and 47 feet below ground, installation of inclinometer casing within the boreholes, geotechnical field testing, and environmental analysis of canal sediment to assess beneficial reuse of the material. This data was used to expand on Phase 1 geotechnical activities (Louis Berger 2019) that included; six (6) boreholes to depths between 17.5 and 46.5 feet below ground, seismic refraction surveying, and a Ground Penetrating Radar (GPR) survey.

Vertical borehole inclinometers with designated name B1-I1 through B4-I4 were installed within the project area by Connelly and Associates, Inc, of Frederick, MD, between September 14 and September 24, 2021. Inclinometer boreholes were drilled from the existing grade to ten (10) feet into bedrock (a maximum depth of 47 feet below ground surface). The inclinometers consisted of four-inch diameter, Acrylonitrile Butadiene Styrene (ABS) grooved inclinometer casing, placed in each 6-inch diameter borehole, from the bottom of the borehole (socketed 10 feet into bedrock) to the existing ground surface. Grout, consisting of a mix of 9 gallon water, 95 lb Portland Cement (for the bottom 10 feet of the borehole) and 30 gallons water to one bag Portland Cement (Type II) and 25 lbs. of hydrated bentonite (left/soaked in water overnight), and was placed in the annulus between the borehole and inclinometer casing, using a tremie pipe. Subsurface lateral movement will be monitored by NPS in each inclinometer using a Digitilt AT Inclinometer System by Slope Indicator.

The geotechnical investigation program included an environmental screening of sediment within the vicinity of the wall to predetermine it’s beneficial use. This material can potentially be used as fill, to be placed at the toe of the wall, to reduce the exposed wall height and increase stability. Samples were collected via hand auguring from ground surface to the first occurrence of clay liner (material lining the canal, placed during it’s construction in 1828); encountered between 4.5 and 5.0 ft. below ground surface. Samples were analyzed by Test America Laboratories in Pittsburg, PA, for the suggested parameter analysis list in, Maryland Department of the Environment, Innovative Reuse and Beneficial Use of Dredged Material Guidance Document, August 2017. MDE Category Calculator (Appendix D) indicates analytical results for all four samples are below the Category 1 screening levels. Therefore have no environmental restriction for use within the Site.

Site-specific subsurface conditions were developed based on the data collected during the inclinometer drilling and Phase 1 activities. Based on geotechnical data collected, there are four (4) different strata at different depths/elevations below the existing ground surface. These strata include; Stratum 1- Fill (a discontinuous layer consisting of sand, silt, gravels and locally boulders to an average depth of 3 feet below the existing grade) Stratum 2- Silts, Silty Clay/Clayey Silt, Stratum 3- Gravels, Cobbles and

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Boulders, Stratum 4-Bedrock Schist-amphibolite, slightly to moderately weathered, moderately strong to very strong.

A combination of two stabilization alternatives were selected to reduce the soil pressure behind the wall. These alternative include drilled a soldier piles (micro piles) and rock anchor system and external wall support, using soil/riprap buttressing. WSP has compared the wall capacity analysis based on the recently developed wall geometry, and boreholes, and recommends soldier pile and anchor system between stations 5+20 to 7+00. The project structural engineer has selected new external wall support using soil/riprap buttressing between stations 11+20 and 13+50. The remaining wall appears to have an adequate factor of safety against a global failure.

Soldier piles should be installed by drilling (driving is not permitted), a minimum 10 feet into bedrock Schist. The micro piles should have a diameter of 12 in. and should be fully cased using 0.375 in. wall thickness 12 inch OD, steel casing pipe with one #18 bar inserted at it’s center and the annulus between the casing and the micro pile should be tremie filled with 5,000 psi (28 days) neat water-cement grout. The forces acting on the wall should be tied back to the rock using minimum 50 feet long rock anchors. The rock anchor should be installed at 40 degrees from horizontal with a minimum length of 50 feet, of which the lower 25 feet should be grouted as the load transfer zone. Each anchor bar shall consist of a 1.75 inch diameter 150 ksi all-thread bar, Williams Form Engineering Corp., or equivalent.

For visual impacts, this system should be installed starting from about 5 feet below the current (future) elevation of the tow path and once the system is fully installed the area should be backfilled and the surface be restored. For longevity and resistance to corrosion, sacrificial section was added to the wall thickness. Micro piles should be connected using a steel or concrete waling for continuity and the forces acting on the waling should be transferred using the rock anchors.

A recommended testing schedule to be implemented during construction is outlined in section 4.2.1 of this report. Each rock anchor should be tested by direct pull with a hollow ram hydraulic jack using the outlined procedure and a design strength of 5,000 psi.

Buttress support, spaced across the toe of the wall was selected by the project’s structural engineer to provide toe-support, reduce exposed wall height, and provide counter-force/reaction to control wall movement and improve stability. Buttress support would reduce the exposed wall high, provide toe support, but would require a large footprint area and large volumes of select rock. To improve the stability of the buttress foundation, benching is required and the buttress should be built bottom up using course materials at the bottom and fine riprap type materials in between coarse boulders.

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1.0 Introduction

1.1 General

On behalf of the National Park Service (NPS), WSP USA Inc. (WSP) conducted geotechnical design services for the final design of the restoration of the historic dry stone wall (log wall) near Mile Post 11 at Chesapeake and Ohio Canal (C&O Canal) National Historic Park, Maryland (Figure 1). The following report includes final geotechnical design recommendations for wall stabilization alternatives (including a solider pile with tieback anchor system and external buttress support), a summary of inclinometer installation, and environmental analytical laboratory results and recommendations for beneficial reuse of sediment within the vicinity the wall.

1.2 Project Description

The Chesapeake and Ohio (C&O) Canal National Historical Park is a 184.5-mile long corridor centered on the Potomac River and extends from Rock Creek in Georgetown, D.C. (MM 0), to the confluence of the North Fork Potomac River and Wills Creek at Cumberland, MD (MM 184.5 (Figure 1). The Chesapeake and Ohio Canal (C&O Canal) had an operational era until 1924. The Log Wall was likely constructed in the late 1820's to early 1830's based on construction reports for adjacent canal structures. If the log wall was completed sometime around 1830, then it operated with boats for approx. 94 years. The canal continued to hold water to varying degrees up until 2016 (183 years) until it was drained due to the sinkhole formation. The canal was a lifeline for communities along the Potomac River as coal, lumber, and agricultural products floated down the waterway to market. The park was established in 1961 as a National Monument to preserve the neglected remains of the Chesapeake and Ohio Canal and many of its original structures. The canal and towpath trail extends along the Potomac River from Georgetown, Washington, D.C., to Cumberland, Maryland, a distance of 184.5 miles.

The project site is located between Macarthur Boulevard and Billy Goat Trail, Northern Parking Lot in Carderock National Park, north of Vaso Island in the Potomac River. The log wall and tow path, that is the subject of this investigation, is located between river mile posts 11.0 and 11.3 on the Chesapeake & Ohio Canal National Historical Park. The log wall is located between Potomac River and the canal. The log wall, has an approximate length of 1,015 feet. It is approximately 35 feet tall in its mid-part, and tapers off towards east and west. A tow path, on average 10 feet wide, is located east and on top of the log wall. At and near the project area, the trapezoid shape canal had the maximum historic water depth of about 7 feet. Currently the depth is constrained by the Potomac Interceptor and the relic liner system, according to which the proposed grade of the canal was to be a minimum of 4' above the springline of the springline of the sewer. The width of the canal appears to be ranging from 30 feet to about 66 feet. An 84 inches diameter sanitary sewer “Potomac Interceptor” is located about 60 feet east of the edge of the canal and the tow path. NPS observations indicated that there has been movement of underlying foundation material through the wall which could pose future stability related issues and a potential for the wall and towpath to fail. According to NPS, in December 2015 and January 2016, sinkholes developed adjacent to the towpath in the canal prism which were temporarily stabilized by dewatering this reach of the canal and also using sandbag rings to prevent further water from infiltrating into the direct impact areas. Additional issues with sink holes in this area were noted in 1999 and subsequently repaired.

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1.3 Purpose and Scope of Work

The objective of this study is to provide final design recommendations for improvements/restoration, of the log wall and install vertical borehole inclinometers behind the log wall to monitor lateral subsurface movement. To achieve these objectives, the following work was performed:

1. Retained a drilling subcontractor in drill and install four (4) vertical borehole inclinometers, collection of representative soil and rock samples, groundwater observations, installation of inclinometer casing, and training for park service staff to collect measurements.

2. Collected four (4) composite sediment samples within the canal tributaries area and retain a laboratory subcontractor to analyze the samples based on Maryland beneficial reuse screening criteria, and assess the material for beneficial reuse.

3. Provided full time field inspection services by a WSP Professional for the borings, sampling and field tests.

4. Performed a geotechnical laboratory testing program on representative soil samples obtained from the borings, consisting of Atterberg limits, moisture content tests, and grain size analysis.

5. Prepared this geotechnical report that includes the following:

a. A description of the regional geologic features of the project site and subsurface investigation performed for this project;

b. Estimated depths/elevations to bedrock, which is hard and sound, and hard to remove

c. Engineering analysis and stability assessment of the log wall,

d. The results of the engineering evaluations and recommendations regarding the log wall stability, sinkhole remediation, treatment, including:

i. Generalized subsurface conditions, including depths to bedrock and groundwater;

ii. Log wall stability in different sections of the 1,015 feet long wall,

iii. Sinkhole evaluations along tow path and at the toe of the log wall,

iv. Recommended soil parameters for earthwork.

e. A discussion of construction-related issues, including:

i. Restoration of stability of the log wall and alternatives,

ii. Remedial measures for the sink holes

iii. Geotechnical design parameters for alternative remedial measures,

iv. Protection of adjacent structures and utilities; and

f. Appendices that include, test boring logs, laboratory test results and the results of the stability analysis.

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2.0 Subsurface Investigation and Testing

2.1 General

Subsurface investigation activities conducted for this program included the drilling of four (4) boreholes to depths between 30 and 47 feet below ground, installation of inclinometer casing within the boreholes, geotechnical field testing, and environmental analysis of floodplain sediment to assess beneficial reuse of the material. Data collected from this investigation was used to expand on Phase 1 geotechnical activities (Louis Berger 2019) that included six (6) boreholes to depths between 17.5 and

46.5 feet below ground, seismic refraction surveying, and a Ground Penetrating Radar (GPR) survey conducted along the face of the wall. The previous boring logs are included in Appendix B.

2.2 Boreholes and Field Investigation Program

Vertical borehole inclinometers with designated name B1-I1 through B4-I4 were installed within the project area by Connelly and Associates, Inc, of Frederick, MD, between September 14 and September 24, 2021. Inclinometer boreholes were drilled from the existing grade to ten (10) feet into bedrock (a maximum depth of 47 feet below ground surface). A WSP Professional inspected all boring operations / inclinometer casing installation, and logged the subsurface. The boring locations, coordinates, ground elevations, and total depths are tabulated in the below Table 1. Locations are displayed in Figure 2 and boring logs are included in Appendix A.

Table 1: Boring Information

Borehole # Date

Completed Northing* Easting* Elevation**

Total Drilled

Depth (ft) B1-I1 9/16/2021 477603.9 1252094.4 112.6 40.0 B2-I2 9/17/2021 477672.2 1252227.4 114.6 30.0 B3-I3 9/21/2021 477760.9 1252400.1 115.5 40.0 B4-I4 9/24/2021 477764.4 1252796.7 116.6 47.0

B1 8/27/2019 477724.0 1252333.1 116.0 32.5 B2 8/26/2019 477757.2 1252402.6 116.0 27.5 B3 8/23/2019 477779.3 1252493.5 116.0 21.5 B4 8/22/2019 477760.1 1252814.3 116.0 17.5 B5 8/21/2019 477681.7 1253018.8 116.0 46.5 B6 8/21/2019 477607.8 1253188.7 116.0 26.0

* Maryland State Plane (NAD83, Feet) ** NAVD 88 Elevation

Inclinometer boreholes were drilled using sonic drilling techniques in accordance with ASTM D6914 / D6914M-16, “Standard Practice for Sonic Drilling for Site Characterization and the Installation of Subsurface Monitoring Devices” and in general in accordance with the applicable sections of the US Department of Interior, Bureau of Reclamation (USBR) “Guidelines for Drilling and Sampling in Embankment Dams (April 2014)”. The objective of this drilling was to delineate bedrock and soil behind the dry-stone wall, and create six-inch diameter boreholes for inclinometer installation.

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Prior to any drilling activity, the drilling subcontractor, contacted Miss Utility and obtained utilities mark outs within the boring location areas. Borings were conducted using a TSi 150CC Compact Crawler TS-1237 drilling machine, using sonic drilling methods with local clean water with no additive (i.e. mud, bentonite, polymer etc.) as a circulation medium to cool the cutter core-bit, and transport cuttings to the surface.

Samples were collected during drilling, in 4.5 inch diameter core barrel at five foot intervals from ground surface to the top of bed rock. Rock coring was conducted using a HQ-sized, ten foot double-tube core barrel, in accordance with ASTM D2113 – 14. During drilling, the recovered soil samples were visually classified in accordance with Unified Soil Classification System (USCS) (ASTM D 2487) and the Burmister soil classification system (Burmister 1950). Recovered rock cores were visually classified based on rock type, degree of weathering, hardness, and degree of fracturing, and rock quality designation (RQD) and total core recovery (TCR) were calculated in accordance with ASTM D 6302.

On completion of each borehole, WSP monitored the log wall for any distress and/or drilling fluid or backfill grout stains.

2.3 Borehole Inclinometer Installation

Vertical borehole inclinometers were installed upon completion of each 6-inch diameter borehole to monitor the lateral displacement of the subsurface directly behind the face of the log wall. Inclinometer installation was conducted in accordance with manufacturer(s) requirements and ASTM D6230 - 21e1.

The installation consisted of placing four-inch diameter, Acrylonitrile Butadiene Styrene (ABS) grooved inclinometer casing in each 6-inch diameter borehole. Casing was placed from the bottom of the borehole (socketed 10 feet into bedrock) to the existing ground surface. Inclinometer casing consisted of GeoKon Model 6400, 2.75 inch outer diameter ABS pipe, with quick connect (QC) couplers, and two grooves oriented perpendicular to the wall face and extending the length of the casing. Tape was placed around each coupler to prevent material from entering during grouting and a bottom cap and top cap were placed on the top and bottom of the inclinometer casing.

Grout was placed in the annulus between the borehole and inclinometer casing, using a tremie pipe.

During grouting, an iron weight was used inside the inclinometer casing to prevent it from floating, and air/gas entrapment in grout was minimized by vibrating the casing during its removal.

Two types of grout mixtures were used to fill the annulus between the inclinometer casing and borehole:

Lowest 10 feet within bedrock: 9 gallons water to one bag Portland Cement (Type I) Above bedrock to surface: 30 gallons water to one bag Portland Cement (Type II) and 25 lbs. of hydrated bentonite (left/soaked in water overnight).

Upon completion of the installation, each inclinometer received a lockable cap, flush mount manhole cover, and was embedded within a 2 foot by 2 foot concrete pad.

Subsurface lateral movement will be monitored in each inclinometer using a Digitilt AT Inclinometer System by Slope Indicator. NPS purchased a Digitilt AT Inclinometer System and Connelly and Associates trained NPS staff on the measurement procedures. The monitoring system is comprised

WSP USA 7 December 2021 of cable, cable reel, accelerometer probe, and a digital readout unit. Inclinometer probe measures horizontal displacement in two directions with depth, A-A axis and B-B axis of inclinometer pipe were aligned perpendicular and parallel to the log wall.

2.4 Sediment Sampling

The geotechnical investigation program included an environmental screening of sediment within the project area to predetermine the beneficial use of the floodplain sediment at the base of the wall. This material can potentially be used as fill, to be placed at the toe of the wall, to reduce the exposed wall height and increase stability.

Sediment samples were collected on August 26, 2021. Composite sample were collected from four locations (locations presented in Figure 3). Samples were collected via hand auguring from ground surface to the first occurrence of clay liner (material lining the canal, placed during it’s construction in 1828); encountered between 4.5 and 5.0 inches below ground surface.

Samples were collected at 6-inch intervals, screened for volatile organic compounds (VOCs) with a photoionized detector (PID), classified based on grain size, and visually inspecting for staining, odor, or any other signs of contamination. Each 6-inch interval was then thoroughly combined, and placed in dedicated pre-cleaned laboratory containers. All samples were then labeled, logged, and placed into a cooler filled with ice, and submitted to Test America Laboratories in Pittsburg, PA for analysis.

Table 3: Sediment Sample Summary Table

Soil Sample ID Soil

Sample Depth (ft.)

S1 0.5 – 2.5 S2 0.5 - 3.5 S3 0.5 – 2.5 S4 0.5 – 2.0

The sediment was analyzed for the following analytes, in accordance with the suggested parameter analysis list in, Maryland Department of the Environment, Innovative Reuse and Beneficial Use of Dredged Material Guidance Document, August 2017

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Physical Analyses Grain Size ASTM D-422 Chemical & Nutrient Analyses Metals and Inorganic Analytes Priority Pollutant Metals SW-846 EPA Method 6020 Mercury SW-846 EPA Method 7471 Acid Volatile Sulfides/Simultaneously Extracted Metals (AVS/SEM) EPA Method 821-R-100 Total Cyanide SW-846 EPA Method 9014 Total Sulfides SW-846 EPA Method 9030 Total Phosphorus EPA Method 365.3/365.4 Ammonia-Nitrogen EPA Method 350 Total Kjeldahl Nitrogen EPA Method 351 Organic Analytes Total Organic Carbon SW-846 EPA Method 5310B modified (Lloyd Khan) Volatile Organic Compounds (VOCs) – TCL SW-846 EPA Method 8260 Semivolatile Organic Compounds (SVOCs) Hydrocarbons (PAHs) SW-846 EPA Method 8270 Pesticides SW-846 EPA Method 8081 Polychlorinated Biphenyls (PCBs) (Aroclors) SW-846 EPA Method 8082 Dioxins/Furans SW-846 EPA Methods 8280 & 8290 pH EPA 9040 Total Sulfates SW-846 EPA Method 9035

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3.0 Subsurface Conditions

3.1 General

The following subsections detail Site specific subsurface conditions based on a literature review, data collected during inclinometer drilling, sediment sampling, and Phase 1 geotechnical activities. The Phase 1 geotechnical activities include; boreholes to depths between 17.5 and 46.5 feet below ground, seismic refraction surveying, and a Ground Penetrating Radar (GPR) survey conducted along the face of the wall.

3.2 Regional Geology of the Project Area

The C&O Canal extends from Rock Creek in Georgetown, D.C. to the confluence of the North Fork Potomac River and Wills Creek at Cumberland, MD. It is located along the northern bank of the Potomac River in Maryland. The river valley transects five major physiographic provinces from which the project site is located in the Piedmont, specifically the Potomac terrane (eastern Piedmont province) (Southworth et al 2008). The Piedmont is defined by underlying Pre-Cambrian and Cambrian-age metasedimentary bedrock (Southworth et al 2008). The bedrock at or close to MM 11.0 consists of metagraywacke and schist with Phyllonite and migmatite that also occur in this interval (Southworth et al 2008). Rocks outcrops near Bear Island are relatively undeformed rocks and consist of the Sykesville Formation, which has been quarried at the Potomac Granite Mill in the early 1900’s near Cropley, From the parking area opposite Old Anglers Inn to Great Falls Tavern is the C&O Canal. Soils above bedrock are mapped as highly erodible silt loam which typically is present in a 200-foot wide strip running parallel to Potomac River.

3.3 Site Specific Subsurface Conditions

Subsurface conditions are summarized below, based on the information collected from inclinometer borings and Phase 1 activities. Inclinometer borings logs are included in Appendix A, boring locations are included in Figure 2, and logs for previously drilled borings (Louis Berger 2019) are included in Appendix B. Phase 1 borings have designated names, B1 through B6, while inclinometer borings are identified as B1-I1 through B4-I4. Based on geotechnical data collected, there are four (4) different strata with different depths/elevations below the existing ground surface. These strata are as follows:

Stratum 1- Fill, A discontinuous layer consisting of sand, silt, gravels and locally boulders to an average depth of 2 feet below the existing grade, Stratum 2- Silt, Silty Clay/Clayey Silt

Stratum 3- Gravel, Cobbles and Boulders

Stratum 4-Bedrock Schist-amphibolite, slightly to moderately weathered, moderately strong to very strong

A summary of the subsurface conditions, including a brief description of the earth materials and geologic units identified in the record of borings, is presented below.

Stratum 1 Fill (Mostly granular, Sand, Gravels and Silt): This stratum is continuous and was observed directly below the towpath to an average depth of around 3 feet below ground surface. This stratum locally extends to about 5 feet as in B-5 and B4-I4 boreholes and 6 feet as in B1-I1. Fill consist of light brown to gray to dark brown sand and gravel, lesser fractions of clay and silt, and generally some gravels and occasional cobbles, and moist. The SPT resistance “N” values (ASTM Standard D1586) in this

WSP USA 10 December 2021 stratum ranged from 21 blows/foot to 25 blows/foot indicating generally medium dense compactness.

Stratum 2 Clay (CL) and Silty Clay (CL-ML): Light brown to olive gray, Clay was observed directly below the fill material. This stratum was observed in all boreholes, except in B-3, with thickness ranging from 8 feet to 25 feet with an average 15 feet in the previously drilled boreholes and 9 feet to 13 feet in inclinometer boreholes with an average 11 ft.. This material may also be historic fill that may have been placed during construction of the canal. The SPT resistance “N” values (ASTM Standard D1586) in this stratum ranged from 2 feet to 18 blows/foot, with an average 8 blows/foot indicating soft to generally medium consistency. Laboratory tests conducted on silt and clay indicated the following index, classification limits, and consistency limits:

Liquid Limit: 24-49 Plastic Limit: 17-25 Plasticity Index 14-25 Moisture Content: 14.0-29.8%

Stratum 3 Gravels Cobbles and Boulders (GM): Underlying clay, a discontinuous layer of gravels and rock fragments were observed approximately between 5 and 30 feet below the tow path elevation. It generally consist of olive gray gravels with little to some clay, silt and little to some sand. This stratum is missing in in some boreholes, but also may have been historic fill placed as backfill during construction of the log wall and the canal. Where present its thickness range between 10 feet to 16.5 feet in the previously drilled six boreholes, while in inclinometer boreholes this stratum range between 4 feet to 20 feet with an average 11 feet.. The SPT resistance ranged from 18 blows/foot to more than 100 blows/foot with an average over 35 blows foot, indicating dense to very dense compactness.

Stratum 4 Bedrock: Schist-Amphibolite Approximately between 17.5 feet and 43 feet below the tow path elevation, schist was observed in all previously drilled boreholes.

However, in inclinometers boreholes Schist layers was observed after 30 feet (B1-I1), 20 feet (B2-I2), 30 feet (B3-I3) and 37 feet (B4-I4) below the tow path. The bedrock was described as dark gray to olive gray, fine grained, moderately weathered and moderately strong, fractured and jointed, hard and sound schist-amphibolite. Core samples collected from representative boreholes indicated the Total Core Recovery (TCR) ranged from 42 to 58 percent in previously drilled boreholes, while in the inclinometer boreholes it varies from 60 to 91.67 percent. The Rock Quality Designation (RQD) ranged from 20 to 57 percent in the previously drilled boreholes, while in inclinometer boreholes it varies from 14 to 55 percent. These values indicate relatively poor rock quality.

3.4 Groundwater

Groundwater elevations were evaluated based on the wetness of the soil samples collected and groundwater measurements in open boreholes after drilling. Groundwater was not encountered within the boring limits during this investigation. Saturated soil was not observed in any of the Phase 1 or inclinometer borings. Groundwater was observed in three of the four open inclinometers boreholes, greater than 24 hours after drilling (Table 3), however measurements were highly variable, ranging

WSP USA 11 December 2021 between 32.8 to 17.7 feet below ground surface, suggesting possible infiltrating drilling water still present greater than 24 after drilling. Therefore, groundwater is interpreted to be strongly controlled by surface water infiltration and storm events. It should be noted that the groundwater is expected to fluctuate depending on climatic factors, surface drainage conditions, as well as storm influences and other factors.

Table 3: Groundwater Observations in Inclinometer Boreholes Boreholes Water Depth from Surface (ft.) Date and Time B1-I1 32.8 September 15, 2021 at 2.30 pm B2-I2 15.0 September 17, 2021 at 8.45 am B3-I3 17.7 September 21, 2021, 8.15 am B4-I4 Dry* --

* Grout started immediately after completion of drilling

3.5 Environmental Screening of Sediment

The geotechnical investigation program included an environmental screening of sediment within the project area to predetermine the beneficial use. Sediment sampling procedures are outlined in Section

2.4 of this report. The laboratory analytical report is presented in Appendix C and the MDE Category Calculator screening criteria is preceded in Appendix D.

Analytical sample results were compared to EPA Residential and industrial soil screening criteria and placed into one of the below four categories, in accordance with the Maryland Dept of the Environment (2017) Innovative Reuse and Beneficial Use of Dredged Material Guidance document:

Category 1 – Residential Unrestricted Use Soil and Fill Material, (EPS soil RSL assuming a resident at 1E-06 risks and hazard quotient (HQ)=0.1)

Category 2 – Non-Residential Restricted Use Soil and Fill Material, EPA industrial soil RSLs assuming a worker at 1E-06 risks and HQ=0.1)

Category 3 – Restricted Use Soil and Fill Material (Cap Required), (EPA soil RSLs assuming a composite or construction worker at 1E-05 risks and HQ=0.1 (Table A3-1 in Maryland Dept of the Environment, 2017)

Category 4 – Ineligible Soil and Fill Material

MDE Category Calculator (Appendix D) indicates analytical results for all four samples are below the Category 1 screening levels. Therefore there are no environmental restrictions for use of this material within the Site. Sieve analysis of the samples indicate sediment consists of sandy clay and silt, or sandy clayey gravel. The grain size distribution of the four composite samples is displayed in Table 4, below.

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Table 4: Canal Sediment Grain Size Distribution based on Laboratory Results S1 S2 S3 S4

Gravel 65.4 6.2 2.3 6.3 Coarse Sand 9.9 1.5 1.1 6.7 Medium Sand 9.7 5.8 4.1 12.2 Fine Sand 11 19.1 41.2 26.9 Silt 2.8 40.5 40.0 32.7 Clay 1.2 26.9 11.3 15.2

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4.0 Geotechnical Evaluations and Engineering Analysis

4.1 General

During Phase 1 geotechnical evaluations, WSP evaluated the data gathered and the stability of the dry stone wall (log wall) and determined that the wall does not have an adequate factor of safety where the wall height is greater than 14 feet. Five alternatives were presented to reduce the earth pressure acting on the wall, while still maintaining the historic characteristics of log wall. These alternatives are listed below:

1. Alternative 1: Install sheeting and rock anchors below towpath to resist earth pressure,

2. Alternative 2: Install soldier piles and rock anchors below towpath to resist earth pressure,

3. Alternative 3: Injection grouting to solidify soil

4. Alternative 4: Remove upper 15 feet of clay and replace with light-weight cellular concrete.

5. Alternative 5: External wall support using soil/riprap buttressing

Of these alternatives, 2 and 5 were deemed the most cost effective, and a combination of soldier pile and rock anchor system and external wall support using soil/riprap buttressing is recommended. The project structural engineer has compared the wall capacity analysis to the existing grades and site observations, and recommends soldier pile and anchor system between stations 5+20 to 7+00 and new external wall support using soil/riprap buttressing between stations 11+20 and 13+50.

4.2 Geotechnical Design Parameters

Geotechnical designs presented in this report are based on the soil stratum outlined in section 3.3, which can be summarized as: 1) about 2 feet thick dense granular fill (Stratum 1), underlain by on average 15 feet thick medium clay (Stratum 2), over an average 10 feet thick dense gravels and locally boulders (Stratum 3) over the bedrock schist. groundwater was observed between 33 and 15 feet below ground surface in three of the four inclinometer boreholes. In design analysis, the following soil/rock parameters were used:

Table 5: Geotechnical Design Parameters

Soil/Parameters Stratum 1 Fill

Stratum 2 Clay

Stratum 3 Sand and Gravel

Stratum 4 Schist

Approximate depth below grade, ft. 0-2 2-17 17-30 >30 Angle of internal friction (0) 34 0.0 36 45 Cohesion (ksf) 0.0 0.5 0.0 5.0 Unit weight (pcf) 110 110 120 160 Submerged Unit Weight Coefficient of passive resistance Kp 3.53 1.10 3.85 5.80 Coefficient of active pressure, Ka 0.28 0.60 0.26 0.17 Coefficient of at-rest earth pressure K0

0.44 0.70 0.41 0.29

Coefficient of interface friction-ultimate 0.35 0.25 0.35 0.70

WSP USA 14 December 2021

A Seismic Site Class C with design parameters presented in Table 6, were used based on the ASCE7- 16 seismic maps (Appendix E). Site class C, Fa = 1.3, Fv = 1.5, was selected for seismic design.

Table 6 Seismic Design Parameters:

Factor/Class Value Site Class C Ss, Short Period Spectral Acceleration 0.135g S1, 1-Second Period Spectral Acceleration 0.043g Fa 1.3 Fv 1.5

4.3 Solider Pile and Rock Anchor Design Recommendations

A solider pile and rock anchor system was designed for the log wall to reduce the soil pressure behind the wall. The design consists of drilled in soldier piles (micro piles), socketed minimum 10 feet into bedrock schist to provide adequate toe support and control excessive soldier pile deflections. A steel waling (coping beam) at the top should be installed for continuity. The soldier pile spacing is to be 5.0 feet on center to allow soil arching in between the piles. Soldier piles should be installed by drilling (driving is not permitted) to minimize vibrations during installation. The micro piles should have a diameter of 12 in. and socketed a minimum 10 feet into bedrock schist. The pile should be fully cased using 0.375 in. wall thickness 12 inch OD, steel casing pipe with one #18 bar inserted at it’s center and the annulus between the casing and the micro pile should be tremie filled with 5,000 psi (at 28 days) neat water-cement grout.

The forces acting on the wall should be tied back to the rock using minimum 50 feet long rock anchors.

The rock anchor should be installed at 40 degrees from horizontal with a minimum length of 50 feet, of which the lower 25 feet should be bonded as the load transfer zone. Each anchor bar shall consist of a single 150 ksi strength hollow bar 1.75 inch diameter, Williams, or Dywidag or other approved equivalent.

For visual impacts, this system should be installed starting from about 5 feet below the current (future) elevation of the tow path and once the system is fully installed the area should be backfilled and the surface be restored. For longevity and resistance to corrosion, sacrificial section should be added to the wall thickness. Micro piles should be connecting using a steel waling for continuity and the forces acting on the waling should be transferred using the rock anchors.

The Solider Pile and Anchor System typical design drawings are presented in Appendix F, and design analysis conducted using GROUP and AMRetain software is presented in Appendix E. Design analysis (Appendix E) was conducted for the log wall with little to no pressure, with water pressure from the canal, and with the anchor system installed to withstand pressures and control wall deflections. Based on this analysis the following forces are estimated per foot of the wall for the anchor system design:

maximum moment, 186 kips.ft maximum shear force, 32 kips maximum anchor force: 40 kips maximum displacement: 0.07 inch

WSP USA 15 December 2021

4.3.1 Anchor Testing

Each anchor should be tested by direct pull with a hollow ram hydraulic jack of a model mounted to prevent bending of the anchor. Tensioning should not be allowed until the cement grout has attained the design strength of 5,000 psi. A pressure gauge shall be used to observe the pull out load and that the gauges should be calibrated and certified within one -year of the test data. All gauges should be accurate to read 100 psi changes in pressure. Dial gauges should have a precision of 0.001 inch, aligned perpendicular to the loading head, parallel to the anchor pipe, to measure elongation of the anchor pipe. The requirements for load and deflection monitoring during proof tests are the loading sequence is recommended to be as follows

AL

0.25 P

0.50 P

0.75 P

P

1.20 P

1.33 P ( = Test Load)

AL

Destress to Lock-off load LL after testing

P = Design load (as shown in Appendix F) AL = Alignment Load (0.05 P) LL = Lock-off Load (AL)

The Proof Test should be conducted by incrementally loading the anchors in accordance with the loading sequence provided above. Each load increment should be maintained for a maximum of one minute before advancing to the next load increment. Movement should be recorded at each load increment. At the test load (i.e. 1.33P), the load should be maintained constant for 10 minutes and total movement readings should be recorded at 1, 2, 3, 4, 5, 6, and 10 minutes. If the total creep movement between 1 and 10 minutes exceeds 1mm (0.040 in), the Test Load then should be maintained for an additional 50 minutes and the movement readings shall be recorded at 20, 30, 40, 50, and 60 minutes. During the load-hold periods, the hydraulic pressure should not deviate from the Test Pressure by more than 50 psi and the load should always be returned to the Test Load prior to taking the movement reading.

4.4 Stabilization by External Wall Support

The project structural engineer has selected an external wall support consisting of buttress spaced across the toe of the wall to provide toe-support, reduce exposed wall height and to provide counter-force/reaction to control wall movement and improve stability. Buttress support would reduce the exposed wall high, provide toe support, but would require a large footprint area and large volumes of select rock. To improve the stability of the buttress foundation, benching will be required and the buttress should be built bottom up using course materials at the bottom and fine riprap type materials in between coarse boulders.

WSP USA 16 December 2021

4.5 Stability Analysis

Slope stability analyses were performed using PLAXIS (2020). The following assumptions were considered for the finite element model (FEM)

The individual layer is homogeneous.

The individual layer is in the isotropic condition.

The drained condition is used for the FEMs.

Groundwater flow and solid-fluid coupled conditions are ignored.

The gravity and surcharge loads are considered in the initial state formation and deformation flow condition, respectively, during the staged construction.

The Mohr-Coulomb failure condition is used in the analysis.

The strength reduction method for the global factor of safety is used.

The finite element mesh update was followed for the deformation flow condition.

The equivalent continuum method is used for the drystone wall model.

Plane strain condition is assumed for the FEM analysis.

15-noded triangular elements were used for FEM modeling.

The precipitation rate was ignored.

In finite element modeling, the dynamic response due to earthquakes is ignored.

Six ft. equivalent water pressure is considered assuming the canal surface has an impermeable liner. Thereby, the seepage flow from the canal to the adjacent layer was ignored.

Stratum 2- Clay (CL) and Silty Clay (CL-ML) is relatively compressible compared to the other three layers. Also, the thickness and SPT values in Stratum 2 ranged from 8 feet to 25 feet and 2 feet to 18 blows/foot, respectively. Therefore, sensitivity analyses for Stratum 2 were performed to illustrate the effect in Factor of Safety and maximum deformation for 40 ft. wall height.

Model parameters for the slope stability analysis are presented in Table 7. The FEM analyses were divided under two headings: (a) without equivalent water pressure and (b) with equivalent water pressure in the canal. Details of the global factor of safety and maximum deformation for these two types of investigations are summarized in Table 8. All FEM investigation results are presented in Appendix H.

It is observed that the factor of safety decreased, and maximum deformation increased with equivalent water pressure in the canal. Also, the canal's bottom width and top widths are 39 ft. and 66 ft, respectively, while the average towpath width is 10 ft. Therefore, if the canal bottom surface is not properly impervious, the seepage pressure will reduce the global factor of safety, which also may induce maximum deformation in Stratum 2 due to the compressible nature of clay.

From the deformed mesh analyses, it is found that the top two soil layers were mainly affected the global factor of safety. With equivalent water pressure analysis, the maximum deformation was observed 2.40 in and 1.92 in. for 40 ft. wall and 30 ft. wall, respectively. Also, for 30 ft. wall, the global safety factor with and without equivalent water pressure is 1.48 and 1.71, respectively. The global factor of safety for 25 ft. wall with equivalent water pressure is 1.70. Therefore, for any height below 25 ft., remedial measure is not essential, unless there is existing sinkhole and ground deformation.

WSP USA 17 December 2021

Table 7: Geotechnical Design Parameters used for Stability Analysis Parameter Name Unit Value General Stratum

Stratum

Stratum

Stratum

Drystone Wall

Depth of Strata ft 0-2 2-17 17-30 >30 Sum of Strata

Material Model

Mohr-Coulomb

Type of material behavior

Type Drained Condition

Soil unit weight above phreatic level 𝛾𝑢𝑛𝑠𝑎𝑡 lb/cuft 110 115 120 150 140

Soil unit weight below phreatic level 𝛾𝑠𝑎𝑡 lb/cuft 112 117 125 160 145

Parameters Young’s modulus

𝐸′ lb/sqft 100000 50000 450,000 500,000 300000

Poisson’s ratio 𝜈 -- 0.27 0.3 0.25 0.2 0.3

Cohesion 𝑐𝑟𝑒𝑓′ lb/sqft 0 250/500/

0 5000 3000

Friction angle 𝜑′ Deg 34 0 36 45 40

Dilatancy angle 𝜓′ Deg 0 0 0 0 0

Table 8 Global Stability Safety Factors and Maximum Deformation Wall height Condition Cu Factor of Safety Maximum deformation

(ft) (psf) --- (in.)

40 Without equivalent water pressure 250 1.00 2.64 500 1.51 2.0 100 1.85 0.55

30 500 1.71 0.86 40 With equivalent water pressure 500 1.34 2.40

30 500 1.48 1.92 25 500 1.70 1.15

WSP USA 18 December 2021

5.0 Conclusions and Recommendations

1. On behalf of the National Park Service (NPS), WSP USA conducted a geotechnical investigation and a stability assessment as part of the design, final design (FD) for the proposed restoration of the canal prism and historic dry stone wall near Mile Post 11at Chesapeake and Ohio Canal (C&O Canal) National Historic Park, MD. The C&O Canal extends from Rock Creek in Georgetown, D.C. to the confluence of the North Fork Potomac River and Wills Creek at Cumberland, MD .

2. The Dry Stone Wall (log wall) and the tow path, subject of this investigation are located between mile posts 11.0 and 11.3 on the Chesapeake & Ohio Canal National Historical Park. log wall is located between Potomac River and the canal. log wall which is also referred to as the Log Wall was likely constructed in the late 1820's to early 1830's, and is approximately 35 feet tall in its mid-part, tapers off towards east and west and has an approximate length of 1,015 feet.

3. A tow path, on average 10 feet wide is located east and on top of the log wall, along the canal in further east. At and near the project area, the trapezoid shape canal had the historic maximum water depth of about 7 feet. The width of the canal appears to be ranging from 30 feet to about 50 feet. An 84 inches diameter sanitary sewer “Potomac Interceptor” is located about 60 feet east of the edge of the canal and the tow path.

4. This study consisted of the installation of four (4) vertical borehole inclinometers, sediment sampling to predetermine beneficial use, as well as the review of Phase 1 geotechnical activities that include: six (6) boreholes, seismic survey, Multichannel Analyses of Seismic Waves (MAWR), ground penetrating radar (GPR) survey, walkover observations, and laboratory tests. The scope also included geotechnical/stability analysis to evaluate the stability of the log wall, and to develop final design recommendations for improving/restoring stability of the log wall, mitigating measures for sinkholes at the base and along tow path, establish subsurface stratification, depth to bedrock, depth to groundwater

5. The site is located along the northern bank of the Potomac River in Maryland in the Piedmont Province. The bedrock at or close to MM 11.0 consists of metagraywacke and schist. The schist is known as the Wissahickon Schist which dates from Precambrian (600million years old) to Cambrian (500 million years old). Subsurface at the site consists of a discontinuous layer consisting of sand, silt, gravels and locally boulders to an average depth of 2 feet below the existing grade, underlain by silts, silty clay/clayey silt with an average thickness of 15 feet , over gravels and sand in thickness about 10 feet, finally underlain by the bedrock schist-amphibolite, slightly to moderately weathered, moderately strong to very strong. Based on the wetness of the samples and observations made during boring, no groundwater was observed within the boring limits.

6. WSP evaluated the data gathered and the stability of the dry stone wall (log wall) and determined that the wall does not have an adequate factor of safety where the wall height is greater than 14 feet. Stability iterations have shown that the stability is compromised because of the high earth pressure acting on the wall due to 1) about 2 feet thick dense granular fill (Stratum 1), underlain by on average 15 feet thick soft to medium clay (Stratum 2), over an average 10 feet thick dense gravels and locally boulders (Stratum 3) over the bedrock schist.

WSP USA 19 December 2021

7. Based on the surface and subsurface conditions, the wall stability was evaluated and it was determined the stability would be improved by the installation of a soldier piles and rock anchors system below towpath to resist earth pressure.

8. The drilled in soldier piles (micro piles), socketed minimum 6 inches feet into bedrock schist to provide adequate toe support and control excessive soldier pile deflections. A waling (coping beam) at the top should be installed for continuity. The soldier pile spacing is to be 5.0 feet on center to allow soil arching in between the piles. Soldier piles should be installed by drilling (driving is not permitted) to minimize vibrations during installation. The micro piles should have a diameter of 12 in. and socketed a minimum 6 inches into bedrock schist. The pile should be fully cased using 0.375 in. wall thickness 12 inch OD, steel casing pipe with one #18 bar inserted at it’s center and the annulus between the casing and the micro pile should be tremie filled with 5,000 psi (28 days) neat water-cement grout. The forces acting on the wall should be tied back to the rock using minimum 50 feet long rock anchors. The rock anchor should be installed at 40 degrees from horizontal with a minimum length of 50 feet, of which the lower 25 feet should be grouted as the load transfer zone.