B08_5 - CHOH 241450 - Rockfall Geohazard Evaluation Report.pdf

REPORT COVER PAGE Rockfall Geohazard

Evaluation Report CHOH: Improve Visitor Safety and Remediate Rockfall Hazards

Oldtown, Maryland

April 15, 2020

Terracon Project No. EY195055

Prepared for:

VHB

Williamsburg, Virginia

Prepared by:

Terracon Consultants, Inc.

Germantown, Maryland

Terracon Consultants, Inc. 20401 Seneca Meadows Parkway Germantown, MD 20876

P (301) 354 6100 F (301) 354 6101 terracon.com

Professional Certification. I hereby certify that these documents were prepared or approved by me, and that I am a duly licensed professional engineer under the laws of the State of Maryland, License No. 45404, Expiration Date: 05/19/2020

REPORT C OVER LETTER TO SIGN

April 15, 2020

VHB

351 McLaws Circle

Williamsburg, Virginia 23185

Attn: Mr. Vikrant Desai, P.E.

P: (757) 279 2832

E: vdesai@vhb.com

Re: Rockfall Geohazard Evaluation Report

CHOH: Improve Visitor Safety and Remediate Rockfall Hazards

Paw Paw Tunnel

Oldtown, Maryland

Terracon Project No. EY195055

Dear Mr. Desai, We have completed the Rockfall Geohazard Evaluation Report services at the Paw Paw Tunnel located in the Chesapeake & Ohio Canal National Historic Park (CHOH) in Oldtown, Maryland.

This study was performed in general accordance with National Park Service (NPS) IDIQ Task

Order Authorization dated September 16, 2019. This report presents the findings of our field evaluations and provides geotechnical recommendations for the remediation of rockfall issues at the project site. The geologic measurements, photogrammetry data, laboratory test information, and engineering analyses are also attached to this report in the Appendices.

We appreciate the opportunity to be of service to NPS on this project. If you have any questions concerning this report or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Tim Hastings, P.E. Nancy A. Straub, P.E., ENV SP, LEED AP

Project Engineer Senior Associate

SME Review By: Eric B. Rehwoldt, P.E., P.G., Senior Principal

Kirk Beach, Assistant National Director

TABLE OF CONTENTS

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REPORT TOPICS

SUMMARY OF KEY FINDINGS ...................................................................................... i

INTRODUCTION

BACKGROUND

SCOPE OF SERVICES

REGIONAL GEOLOGY

HISTORIC DOCUMENT REVIEW

EXPLORATION

ROCK SLOPE STABILITY ANALYSES

RECOMMENDATIONS

GENERAL COMMENTS

APPENDICES

APPENDIX A – SITE MAPS

APPENDIX B – GEOLOGIC OBSERVATIONS

APPENDIX C – HISTORIC DOCUMENTS

APPENDIX D – PHOTOGRAMMETRY & SHAPEMETRIX

APPENDIX E – GEOLOGIC MAPPING

APPENDIX F – ROCKFALL HAZARD ASSESMENT

APPENDIX G – LABORATORY TESTING

APPENDIX H – ROCK SLOPE STABILITY ANALYSES

Note: Refer to each individual Appendix for a listing of contents.

Rockfall Geohazard Evaluation Report

Oldtown, Maryland ■ April 15, 2020 ■ Terracon Project No. EY195055

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SUMMARY OF KEY FINDINGS

Terracon was authorized by the NPS to perform a geotechnical evaluation of the rockfall at the

Paw Paw Tunnel in Oldtown, Maryland. The purpose of this work is to identify and mitigate rockfall hazards along the approximately 1,000 linear feet of rockface adjacent to the C&O canal towpath and historic canal prism at mile post 155. The project also includes removal of approximately 400 cubic yards of rocks and debris that were deposited within the historic canal prism when a large rock slide occurred in April 2016.

■ VHB surveyors established survey baselines, located control targets for the photogrammetry mapping completed by Terracon, and developed a topographic map for the project site.

■ Using a combination of ground-based and Unmanned Aircraft Systems (UAS), Terracon performed a photogrammetry survey of the opposing canal prism rock slopes to obtain high resolution digital imagery of the exposed rock faces. The photogrammetry data was overlaid on the topographic data provided by VHB.

■ Terracon geologists and engineers performed geologic mapping, including obtaining field measurements using high-angle rope access techniques, confirming features identified in the aerial survey data, and making visual observations of the rockfall area and remainder of the slope.

■ Terracon geologists performed an inventory and risk assessment of rockfall hazards of the opposing rock slopes within the canal prism. All rock slopes surrounding the canal prism were inventoried and scored, using a weighted risk matrix.

■ The results of the photogrammetry survey and the geologic mapping were combined to create 2D and 3D geological engineering models for rock slope stability analysis. Software programs such as ShapeMetrix (3GSM) for characterization of rock mass, DIPS

(RocScience) for kinematic stability analysis of rock slopes, RocData (RocScience) for estimates of discontinuity interface shear strengths, SLOPE/W (Geostudio) for global stability analysis of the existing slopes, and CRSP (Colorado School of Mines and

Colorado Department of Transportation) for rockfall simulation analysis were used to analyze the slope for potential instabilities and primary modes of slope failure, and to develop potential stabilization options.

■ In our opinion, there are two primary modes of slope failure within the canal prism:

o Planar Sliding Failure – A condition of planar sliding failure within the shale and siltstone rock mass was identified within the lower portions of the east slope, controlled by bedding joints, and within the west slope, controlled by a persistent set of stress relief joints. Within both slopes, the rock mass discontinuities are unsupported, or

‘daylighted’, within the slope. Our site observations and engineering analyses indicate that tabular ‘slabs’ of rock are expected to continue to slide toward the canal and towpath over time due to natural weathering processes unless supported and/or

Responsive ■ Resourceful ■ Reliable ii contained. In our opinion, the failure mechanism for the existing landslide on the west slope of the canal prism is likely controlled by a planar sliding mechanism along stress relief joints. We expect that the larger historic landslides in the canal were controlled by planar sliding failures as well. Rock slides controlled by planar failure will continue to be a concern for safety of trail users below the east slope and are expected to be an on-going maintenance consideration for the canal prism.

o Wedge/Toppling Failure – A potential for localized toppling and wedge failure created by intersecting bedding joints and stress relief joints was identified in our site observations and engineering analyses within the upper portion of the east slope, the rock cut above the north tunnel portal, and within the west slope. Rockfall associated with local rock topple and wedge sliding failure modes are expected over time due to weathering processes. Due to the thinly bedded nature of the bedrock in the upper rock slope, most of the rockfall observed and expected from the upper east slope and above the north tunnel portal likely will be smaller in size but will still pose a significant hazard to the trail users due to the lack of catchment or control of the rockfall. Rocks falling from directly above the tunnel portal areas typically fall onto the flat catchment located above the portal face or fall on the portal steps. This reduces the height and energy that they fall but still can create a risk to trail users due to the potential for bounce over the portal structure or fall on park visitors climbing the portal stairs.

Larger-sized rockfall is expected within the west slope associated with potential toppling and wedge sliding failure modes. Our engineering analyses indicate that rockfall impacts on the towpath from the west slope are mitigated; however, as rockfall trajectories are expected to be arrested by the canal itself. Accumulation of rockfall debris over time within the canal will continue to be an on-going maintenance consideration.

■ Based on the results of our rock slope stability analysis, we evaluated five remedial actions, or Methods, to address the potential rock sliding failures and rockfall in several areas of the canal prism. Although we did consider flattening of the canal prism slopes as an option to mitigate rock sliding and rockfall control, this option does not appear to be feasible for this project due to multiple factors.

■ The recommended slope remediation Methods are presented in this report, and include:

o Method 1 (All Areas): Perform maintenance scaling of all slopes, as needed. Care must be taken by the contractor to prevent the scaling operations from destabilizing the slopes above the scaled areas. Anchored concrete shear blocks and spot rock bolting may be recommended, as needed, to support and stabilize exposed rock blocks remaining above areas to be scaled. This method is recommended to stabilize the head scarp area and remove the landslide debris within the west slope.

o Method 2 (All Areas): Following scaling, remove all fallen rock and accumulated talus from the canal area so that it can continue to function as a catchment ditch. During

Responsive ■ Resourceful ■ Reliable iii this activity, all debris from the rock slope failure near Station 37+66 should be mechanically removed and transported for disposal.

o Method 3 (Upper East Slope Areas): For the near-vertical rock cut slope within the upper portion of the east slope, we recommend a pinned mesh system that is anchored to the rock face. The pinned mesh system is recommended for this location since there is no fall out zone for the rock debris that can be maintained.

o Method 4 (Lower East Slope Area): In areas where rock bolting reinforcement has recently been completed in the lower portion of the east slope, we recommend that supplemental rock bolting be provided in a spot bolting fashion in localized areas were currently unsupported rock bedding slabs are demonstrating signs of cracking and distress. For these areas within the lower east slope, it may also be possible to extend the pinned mesh system recommended in Method 3 for the upper portion of the east slope to cover the entire east slope. We expect that we would be able to re-purpose some of the existing draped mesh system as pinned mesh within the east slope. This action may improve aesthetics along the towpath.

o Method 5 (West Slope Area): Selective rock bolting along with installation of horizontal rock drains is recommended to remediate potential planar landsliding and large block failures within higher risk rated areas for the west slope. This method may include the installation of shear blocks/braces, as appropriate. Design of bolt spacing, length, and capacity, as well as design of rock drains and shear blocks would be prescribed in the next phase of the study.

Selection of the recommended methods above, or combination of methods, in the next phase of this study will be necessarily associated with the results of the rockfall hazard ratings herein, and priorities for mitigation of rockfall hazards as set by the NPS based on the findings of this report.

Monitoring of select features, groundwater seepage, and ground movement along the canal prism slopes and towpath should be implemented to determine the rate of movement regardless of which methods are selected for possible remediation. Monitoring should be initiated immediately and continue through and after construction activities. Monitoring may consist of visual observations, mechanical monitoring systems, UAS photogrammetry, and/or use of conventional land-based survey systems, as determined to be appropriate during construction. We also recommend that annual visual monitoring of the remediated canal prism be performed to identify any changes in the condition of the slopes that may need to be revisited for maintenance scaling, and/or additional reinforcement and protection. More frequent monitoring may be recommended if slope movement/distress or immediate remedial actions warrant such action.

This summary of key findings should be used in conjunction with the entire report for design and/or construction purposes. It should be recognized that specific details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein.

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ROCKFALL GEOHAZARD EVALUATION REPORT

CHOH: IMPROVE VISITOR SAFETY AND REMEDIATE ROCKFALL HAZARDS

PAW PAW TUNNEL

OLDTOWN, MARYLAND

Terracon Project No. EY195055

April 15, 2020

INTRODUCTION

This report presents the results of a geotechnical evaluation of the rockfall at the Paw Paw Tunnel located in the Chesapeake and Ohio (C&O) Canal National Historic Park in Oldtown, Allegheny

County, Maryland. The report presents geologic mapping and rockfall hazard rating data, photogrammetry data, laboratory test information, engineering analyses, and recommendations for stabilizing the existing rock slopes and mitigating potential rockfall hazards. Recommendations include mitigation of the April 2016 landslide within the historic canal prism, that incorporates approximately 400 cubic yards of rock debris.

BACKGROUND

The Paw Paw Tunnel is a 3,118-foot long historic canal tunnel located near Oldtown, Maryland as presented on the Site

Location Map in Appendix A. The canal tunnel was constructed by the Chesapeake & Ohio Canal Company to allow boats to bypass a 6-mile-long sinuous stretch of the

Potomac River near mile post 155, known as the ‘Paw Paw

Bends’, and the adjacent steep cliffs on their way to

Cumberland. The tunnel was constructed between 1836 and

1850 and remained in operation until 1924 when it fell into disuse. The canal was purchased by the Federal Government in 1938 and by the early 1950’s, plans were being developed to repurpose the canal into a non-motorized recreational trail. In the mid-1950’s, the National Park

Service started making improvements to stabilize and repair the condition of the tunnel and the towpath, and these efforts continue today.

This section of the canal and the tunnel was designed to follow the valley of Mill Run, a former tributary of the Potomac River. Mill Run generally followed the alignment of a geologic fold axis

(i.e. anticlinal fold) in the bedrock. North of the tunnel, the canal was created by deepening and widening the valley through predominantly shale bedrock. The excavation for the canal cuts across the anticlinal axis of the fold at an oblique angle exposing potentially unstable rock slope conditions on both the east and west sides of the canal prism. Historic landslides and rockfall events have occurred within the canal prism since its construction.

Figure 1 – North portal of Paw Paw tunnel

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The National Park Service recently took measures to stabilize the east slope of the canal prism in 2013 and 2019. In 2013, in response to a landslide at the north end of the canal prism, a barrier fence was installed. In 2019, stabilization efforts including rock scaling, and installation of rock bolts, shear blocks, draped mesh, and weep drains were completed. This project builds upon that project by stabilizing the west slope of the cut and providing additional stabilization support for the east slope.

SCOPE OF SERVICES

Terracon is providing geological engineering investigation and design services to support the

CHOH project. The project is being performed in phases including Pre-Design (PD), Schematic

Design (SD), and Request for Proposal (RFP) phases to develop bidding documents for design/build construction of rock slope stabilization measures. This report presents the results of the pre-design phase of work, which included the following tasks:

■ Compile and assess existing data that pertain to site history, geologic conditions, and known rockfall hazards within the 1,000 linear-foot canal prism study area.

■ Develop a pre-field investigation work plan for safe access to the site, and to rock slopes within the canal prism study area for Terracon field staff and equipment.

■ Perform a rockfall hazard inventory that will assess and rank the existing rockfall hazards for rock slopes within the canal prism that threaten the safety of the visiting public.

■ Conduct rock mass characterization studies within the 1,000 linear-foot canal prism study area that will include multiple geologic and rock structure mapping techniques, including

UAS-supported photogrammetry, scan-line mapping of rock exposures accessible along the canal and towpath, as well as high angle rope access methods from access points along the top of the steep rock slopes. Additional geologic field measurements and observations were obtained at the south portal and north of the 1,000 linear-foot canal prism study area. These areas are beyond our scope of work and were performed for comparative purposes only.

■ Perform limited laboratory testing of selected rock samples for strength and weathering characteristics.

■ Perform engineering analysis and development of design recommendations for rockfall mitigation alternatives.

■ Prepare a summary report of findings and recommendations.

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REGIONAL GEOLOGY

Geologic Setting

The Paw Paw Tunnel is located in the Allegheny Plateau, and Valley and Ridge Physiographic province of western Maryland. Sedimentary rocks folded and faulted during the Alleghenian

Orogeny (Appalachian mountain-building period approximately 300 to 250 million years ago) created the Broadtop Synclinorium, the largest fold structure of the region. The synclinorium consists of many secondary folds.

According to the Geologic Map of the Paw Paw quadrangle, Allegany, Maryland (Glaser, J.D.

1994), bedrock at the site consists of late Devonian-age (358 to 382 million years ago) Brallier

Formation sedimentary rock. Brallier Shale is described as olive-gray to greenish-gray shale and siltstone formed from layers of mud deposited in the Kaskaskia Sea. In general, Brallier Shale is laminated and uniform, with approximately 40 to 50% of the shale formation interbedded with siltstones and fine-grained sandstones. A geologic map of the site is presented in Appendix A.

Based on a review of geologic maps available for the project site, the canal generally follows and obliquely crosses the axis of an anticlinal fold trending in a northerly direction. The axis of the anticline is overturned along the east wall of the canal prism and has been eroded or cut away leaving the east and west limbs of the anticline exposed. The alignment of the axis of the anticline appears to vary as it trends to the north, which changes how much of each limb of the anticline is exposed on the east slope face. The west slope face is incorporated in its entirety within the west limb of the anticline.

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Geologic Conditions Observed

Terracon personnel performed a site reconnaissance visit to make preliminary observations of the project site conditions. During the site visit, various conditions were observed along the canal towpath that warranted additional study during our subsequent exploration.

■ The middle to lower portion of the east slope face of the canal prism demonstrates on-going instability primarily due to planar sliding along the bedding planes in a ‘dip slope’ condition. The slope of the rock face is similar in angle and direction as the slope of the bedding planes resulting in the potential for ‘sheets’ or ‘slabs’ of rock becoming unstable and sliding into the towpath and canal.

■ Due to the alignment of the canal cutting obliquely across the overturned anticlinal fold structure in the bedrock, the bedding in the upper portion of the east slope face of the canal prism dips back into the slope face, creating a steeper, near-vertical slope. Rockfall in this portion of the rock slope is due to differential weathering of the exposed shale and siltstone along fractures and joints.

Figure 2 – Historic sketches of the general rock structure within the canal prism by William E. Davies (1999)

Oldtown, Maryland ■ April 15, 2020 ■ Terracon Project No. EY195055

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■ On the west side of the canal prism, the bedding dips into the slope. However, a persistent and closely spaced joint set is exposed within the slope and dips out of the slope unsupported, forming a condition of planar sliding along the joints. As slabs of shale and siltstone peel off the slope and into the canal, overhanging pendants (suspended rock) of more resistant siltstone formed by bedded rock dipping back into the slope, typically are exposed at the head of the slide. Further undercutting of these pendants results in rock topples into the canal. The landslide present within the west slope at the north end of the study area appears to be a result of this failure mechanism.

■ No water was observed seeping from the east slope of the canal prism. None of the existing weep drains installed in 2019 were observed producing any discharge.

■ After rain events, water was observed seeping from some joints on the west slope of the canal prism within the 100 ft of the north portal

(between Sta. 31+17 and 32+17). Between

32+17 and 41+17 along the west slope, no seepage was observed.

■ Rock debris was not noted at the base of the cut slopes above the north tunnel portal. However, differential weathering of the exposed shale and siltstones appear to pose a continued rockfall hazard above the tunnel portal.

A photographic appendix of geologic observations made at the project site is presented in

Appendix B.

HISTORIC DOCUMENT REVIEW

Terracon spent one day in the storage rooms and library of the NPS headquarters reviewing historic documents related to the construction of the Paw Paw Tunnel, as well as reviewing historic documentation of rock slides in the canal prism to the north. The documents consisted of printed photographs, photographic slides, oversize plans, and bound reports. In many cases, the photographs and slides were not labeled or dated, but site features including portal and canal geometry, towpath construction materials, and terrain allowed for identification of the subject area. A list of historic documents reviewed for the Paw Paw tunnel is presented in Appendix C.

Based on these documents, the following chronology of landslides and rockfalls has been assembled:

Figure 3 – Photo of west slope near north portal

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Year Source Closest

Portal

East/West

Slope

Estimated

Distance from

Portal (ft)

Comments

Davies

(1989)

Possibly south

“A large slide in November

1857 required almost 2 months to remove”

Undated Photograph North West 100 ft Large slide, filling entire canal section to towpath

1968 Photograph North West - Very large slide

Davies

(1989) South - -

“Another large slide almost blocked south portal of tunnel”

1974 Photograph South West 10 Small slide adjacent to portal

1975 Photograph South West 30 Small slide adjacent to portal

1975/1976 Photograph North East - “Lower Slide”

Undated Photographs North West 100 ft

A series of photographs of a large landslide near the portal. The photos likely taken in the mid-

1970’s based on clothing styles.

1977 Report North Above portal

“The north portal was generally intact but partially damaged from falling rock from the face of the cliff above the Tunnel”

1997 Newsletter - - -

“The tunnel is in good shape with some repainting needed to be done. Below the North

Portal, a few landslides need to be cleared.”

4/2/2013 NPS

news alert North East

100-ft long landslide that fell onto towpath in January 2013.

Google Earth /

Personal journal of trail user

North West

~100-ft long landslide that fell into the canal and impacted side of boardwalk

A log of historic photographs of landslides at the Paw Paw tunnel is presented in Appendix C.

Over the years, the Chesapeake & Ohio Company and later the NPS has taken a variety of activities to remediate and stabilize the slopes and the tunnel.

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Year Document Title Work Location General Description of Work Performed

Restoration of

Paw Paw Tunnel Tunnel interior Restore and repair tunnel liner and handrail

Towpath

Reconstruction &

Resurfacing

Tunnel interior and canal prism north of tunnel

■ Towpath resurfacing

■ Construct boardwalk

Stabilization: Paw

Paw Tunnel

Hollow

Canal prism north of tunnel

■ Construct drainage ditch at the north portal

■ Fill in surface depressions at shaft locations

■ Remove debris from 30-ft wide rockfall on the east slope

■ Remove debris from 100-ft long, 50-ft wide slide at North portal

■ Dismantle boardwalk

■ Construct second drainage ditch at top of north portal

■ Remove 25-ft wide talus pile from North

Portal

Paw Paw Tunnel

Portals -

Stabilization

Study

■ Add tunnel liner drains and steel drip pans below shafts in tunnel

■ Install two 36-inch diameter reinforced concrete pipes though the drilling spoil pile between shafts A and B and the north portal.

■ Rebuild portal stairs with new drainage backfill

■ Construct Lexan tunnel enclosure

■ New drainage system at portal parapets

■ New stone paving on portal parapets

■ New wood rub rails at wingwalls of North

Portal

■ Restore towpath grade and elevations

4/2/2013 NPS

news alert

Canal prism north of tunnel

■ Removing fallen rock from towpath at area of landslide

2/15/2017 NPS

news release

Canal prism north of tunnel

■ Phase 1 - Scaling slopes above towpath and boardwalk

9/27/2019 NPS

news release

Canal prism north of tunnel

■ Phase 2 - Scaling slopes above towpath and boardwalk, adding reinforcement elements

1 It is not clear what items, if any, were eventually performed.

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Based on our review of the historic plans, reports, and photographs, we make the following observations:

■ Repeated landslides have fallen from the west canal slope adjacent to the north portal. These landslides tend to be quite large but rarely land on the towpath.

Most photographs indicate that while the side of the boardwalk is impacted, the canal captures most of the rockfall debris.

■ The landslides on the west slope typically contain large slabs of rock.

■ At least two large landsides have occurred on the west side of the canal near the end of the boardwalk.

■ Photographs of landslides on the east side of the canal typically consist of large slabs of rock except for the

2013 landslide (refer to Figure 4).

■ There is one large slide that was reported at the south portal as well as a series of small, raveling landslides.

■ The debris from all landslides except the 2016 landslide have been removed (refer to Figure 5).

■ At one point, the boardwalk had been removed and subsequently replaced. Some photographs indicate that the boardwalk was replaced by a stone wall.

EXPLORATION

Photogrammetry & ShapeMetrix

Terracon performed a photogrammetry survey of the canal prism rock slopes using both terrestrial and UAS high resolution digital imagery. A certified pilot adhering to FAA regulations performed an aerial photogrammetric survey using our own UAS (DJI Phantom 4 Pro quadcopter) outfitted with a 20-megapixel camera to document the entire rock slopes including the less-accessible rock faces located high up on the slope. In addition, ground-based photogrammetry was performed to complement the UAS suite of digital photographs. Traffic control was used during UAS flights to stop visitors on the towpath for the safety of the public until the UAS could be moved to a safe location.

Following the photogrammetric survey of the canal prism rock slopes, the data was analyzed using the software program ShapeMetrix to provide an assessment of the natural rock discontinuities (i.e. bedding joints, joints, faults, etc). The natural rock discontinuities identified

Figure 4 – 2013 Landslide

Figure 5 – 2016 Landslide

Responsive ■ Resourceful ■ Reliable 9 using the ShapeMetrix software, and subsequent measurements obtained during the field geologic mapping, were used in the kinematic and limit equilibrium slope stability analyses.

Figures showing the identified natural rock discontinuities via the Shapemetrix analysis on photographs along both the east and west slopes of the canal prism is presented in Appendix D.

Geologic Measurements and Groundwater Observations

Trained and experienced Terracon geologists and engineers performed geologic mapping along the canal prism rock slopes. The purpose of the mapping was to obtain field measurements, confirm the aerial survey data, and obtain visual observations of potential rockfall hazards within the canal prism, rockfall areas, and remainder of the slope. Traffic control was used during our mapping activities to close the towpath for the safety of the public in the event that loose rocks or other debris were dislodged during roping activities.

Using Clar compasses designed to measure rock structure orientation, Terracon personnel collected data on the natural rock mass discontinuities accessible along the base of the slope (from the boardwalk and canal), and by rope access where our trained geologists and engineers used high-angle rope access techniques to rappel down the faces of the rock slopes and obtain measurements above ground level. Care was taken to avoid taking measurements with the compasses near existing rock anchors, pinned mesh, and catchment fences due to potential of magnetic interference with the compass measurements.

GPS coordinates were obtained for each mapped location using a hand-held GPS unit with an approximate accuracy of +/- 15 feet.

In addition to the rock mass discontinuity orientations and conditions, notes were made regarding the presence of groundwater along the canal prism rock slopes. After rain events, water was observed to be seeping from some bedding and/or joint discontinuities on the west slope of the canal prism within the 100 ft of the north portal

(between Sta. 31+17 and 32+17). Between Station 32+17 and Station 41+17, no seepage was observed on the west slope.

A summary of the field measurements obtained during the geologic field mapping as well as the

ShapeMetrix analysis are presented in Appendix E. Also presented in Appendix E is a plan view of the project site indicating the approximate location of where the rock structure measurements were obtained. Geologic field measurements and observations were obtained at the south portal entrance, for comparative purposes only, to determine consistency with measurements obtained at the north portal. The field measurements obtained at the south portal are also included in the summary of field measurements in Appendix E.

Figure 7 – Wood boardwalk of towpath viewed from west slope of canal prism

Figure 6 – West slope of canal prism near north portal of Paw Paw tunnel.

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Rockfall Hazard Assessment

Terracon performed a Rockfall Hazard Assessment to inventory rock slopes, identify potential hazardous rock slope conditions, and assess relative risk of those slopes to trail users.

This inventory and assessment effort included all slopes with exposed bedrock situated above the canal and towpath. The field portion of the inventory was completed by a field team consisting of a geologist and engineering geologist.

A tiered approach was employed to conduct the inventory and risk assessment work to help set priorities. A history of the development of the rockfall inventory and rating system is described in

Appendix F. All rock slopes received a preliminary rating based on basic site characteristics.

This preliminary rating permits the sites to be segregated into risk groups (low, moderate, high, very high) and determines what level of detailed data collection was required.

Methodology

Office Review

We initially reviewed existing photographs of the study area using Google Earth Pro. Both sides of the canal and the tunnel portals were assessed for the potential of rockfall. Aerial imagery along with photographs of the rock slopes taken at ground level were also reviewed. The photographs taken at the ground level included photographs taken by Terracon as well as photographs publicly available online.

Field Reconnaissance

The field team then performed a field evaluation of each rock slope within the study area and subdivided it into sections (rockfall inventory sites) for further evaluation based on field observations. For our report, a rockfall inventory site was defined as any continuous section of rock slope where the field team determines that the slope poses some level of risk to the safety of the public.

The extent of each inventory site was determined based on:

■ Limits of the exposed rock face

■ Natural breaks in the cut face or natural backslope (e.g. natural drainage features, but not man-made drainage features)

■ Limits of existing remedial actions

■ Changes in slope orientation relative to the towpath alignment

■ Changes in the slope orientation relative to discontinuities (e.g., joints)

■ Changes in the cut face angle(s)

■ Changes in the quality of the bedrock mass

The beginning point (BP) and ending point (EP) of each rockfall inventory site was marked in the field to distinguish BP and EP for inventoried sites located left/west or right/east of the towpath.

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Preliminary Ratings

Each inventory site was assigned a preliminary rating by the field team based on the following criteria: 1) the potential of a rockfall occurrence from the slope; and 2) if a rockfall was to occur, the potential of the rockfall to reach the towpath. This qualitative evaluation was based upon the best professional judgment and past experience of the field team. A rating of low, moderate, high, or very high was used for each criterion along with an associated numerical value of 1, 4, 8, or 10 respectfully. The Preliminary Rating Score is the sum of these two scores.

For example, if a site was rated as moderate for the potential of rockfall occurrence (4 points) and yields a high potential of rockfall reaching the traveling public (8 points), the preliminary rating for the site would be 12 points.

The following table presents the appropriate tiers and action levels based on the preliminary ratings.

Preliminary

Rating Score

Tier

Designation

Preliminary Assessment of Risk to Public Action Level

2 to 5 Tier 1 Low Not rated. No further evaluation needed.

8 to 11 Tier 2 Moderate Rated. Additional evaluation needed.

12 to 16 Tier 3 High Rated. Additional evaluation needed.

10 to 20 Tier 4 Very High Rated. Additional evaluation needed.

All inventoried sites required the collection of a minimum set of information and is presented in

Appendix F.

For any sites receiving a Preliminary Rating Score of 8 or greater (Tier 2 through Tier 4), additional field data was collected to supplement the minimum data listed above. This data set is also presented in Appendix F.

Calculating the Final Inventory Site Risk Score

Each parameter described above was assigned a raw value score based on the recorded field data. In some cases, equations were used to generate line item scores directly from the raw field data (RV). For each category, an exponential scoring system was used to help distinguish the highest rated sites. A weighting factor was then applied to each parameter to provide a weighted parameter score. Finally, all weighted scoring parameters values were summed to calculate the

Inventory Site Risk Score.

The paragraphs above provide a very general overview of the rating system. Full details on the rating system, terminology, formulas, and the weighting factors used in this analysis can be found in Appendix F.

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Summary of Rockfall Hazard Assessment

Thirteen inventoried sites were recorded for both sides of the canal prism, and the north portal of the Paw Paw Tunnel. Of these 13 sites, the Terracon team assigned 8 sites a preliminarily rating of moderate risk (Tier 2) and the remaining sites a rating of low risk (Tier 1). The sites rated Tier

1 did not receive additional evaluation and are not discussed further herein. The following table summarizes the moderate priority Tier 2 sites:

Site ID BP Station Position Segment

Length (ft)

Estimated

Slope

Height (ft)

Preliminary

Rating

Score

Final

Inventory

Site Risk

Score

RF001 31+16.67 Right/East 50 75 8 214

RF005 32+64.67 Right/East 176 54 8 302

RF007 34+88.67 Right/East 389 54 8 264

RF008 38+77.67 Right/East 118 48 8 237

RF010 40+45.67 Right/East 190 48 8 204

RF011 31+16.67 Left/West 530 56 9 211

RF012 36+46.67 Left/West 207 71 11 287

RF013 38+53.67 Left/West 300 66 9 156

Based on final inventory site risk scores provided in the table above, rockfall mitigation may be prioritized on the sites with higher scores. With limited funding available for remediation of geohazards, agencies utilize the risk scores to prioritize funding allocations so that the highest risk sites and/or the most cost-effective remediation projects can be undertaken while using due diligence in the selection and expenditure of funds each year.

Laboratory Testing

Terracon collected several pre-split rock samples from the talus slopes for laboratory testing. To determine physical engineering characteristics of the rock mass and the discontinuities, three unconfined Point Load Index tests (PLT) and one direct shear tests were performed in general conformance with ASTM standards. These tests are used to estimate the unconfined compressive strength of the rock and the shear strength of rock along existing fractures, respectively. Samples for laboratory testing was collected from fallen rock at the landslide location. The unconfined compressive strength of the shales ranged from 4,667 to 5,217 psi. The shear strength of the fractures, reported as a friction angle, ranged from 75 degrees for residual shear strength and 81 degrees for peak shear strength, when tested using an applied normal load/force range of 1,440 to 4,320 psf. Based on a review of the lab test results and the observations made in the field of past slope failures, we believe that these friction angles are not representative of actual shear strength values. It is believed that some asperities, or discrete bumps on the tested joint surface, may have inflated the test results. Therefore, these results were not used in our analyses. See

Appendix G for full laboratory test results.

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ROCK SLOPE STABILITY ANALYSES

Using the data collected as part of the historic data review, photogrammetry and ShapeMetrix, geologic measurements, groundwater observations, and rockfall hazard assessments, Terracon developed three representative geological engineering slope segments at the site for analysis of the rock structure kinematics, limit equilibrium slope stability, and potential rockfall trajectories and impact energies. The approximate locations of the segments are presented in Appendix H.

These segments were used to evaluate potential slope failure modes, and to help develop appropriate and effective preliminary slope stabilization/rockfall mitigation recommendations and design concepts.

Kinematic Analysis

We performed kinematic analyses using the software program DIPS by RocScience based on the geologic rock structure data collected at the site. Kinematic analysis of potential rock slope failure modes applies a graphical approach to determine potential for planar sliding, wedge sliding, flexural toppling, and direct toppling for various cut slope orientations within the rock mass relative to the dip and dip direction of rock mass discontinuities (e.g. bedding and joint planes), without reference to forces. In total, 287 individual measurements of bedding and joint planes were obtained using a combination of photogrammetry and geologic mapping by personnel at the site.

Figures showing the identified natural rock discontinuities via the photogrammetry and

Shapemetrix analysis along both the east and west slopes of the canal prism is presented in

Appendix D.

Due to the fact that the canal prism obliquely follows an overturned anticline whose axis varies in alignment, the dip and dip direction of the discontinuity (i.e. bedding joints and stress relief joints) sets vary along the length of the canal prism. The following tables indicate the mean dip and dip direction of the identified bedding joint and joint sets present within the respective segments and slopes of the canal prism. For the purposes of our analysis, bedding joints associated with the east and west limbs of the anticline have been indicated in the tables.

Segment 1 (from Station 31+16 to Station 32+50) – East Slope

Joint Set Description Mean Dip (deg) Mean Dip Direction (deg)

J1 Bedding Joint (west limb) 50 314

J2 Bedding Joint (east limb) 31 135

J5 Joint 80 246

■ Due to the orientation of the east slope face, there is a potential for planar failure of rock blocks along bedding planes and/or joint sets within the rock mass.

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■ There is a potential for wedge sliding due to intersection of J2 (the bedding joint on the east limb of the anticline) and J5 within the slope face, which can be a source of rockfall.

■ There is a potential for toppling failures.

Segment 1 (from Station 31+16 to Station 33+50) – West Slope

Joint Set Description Mean Dip (deg) Mean Dip Direction (deg)

J1 Bedding Joint (west limb) 59 313

J3 Joint 46 127

■ There is a potential for planar sliding of thin shale and siltstone beds along J3, which are unsupported in the west slope. The bedding joint, J1, is exposed in many locations along the west slope, where the planar sliding mass has separated from the rest of the rock mass along the bedding joint at the top of the slide.

■ A potential for wedge failure exists due to the intersection of the bedding joint (J1) and joint set (J3) within the slope face and can be a source of rockfall.

■ A potential for toppling failure exists due to the exposure of bedding joint set (J1) exposed within the slope as slabs of shale and siltstone slide into the canal along J3. This can be a source of rockfall.

Segment 2 (from Station 33+50 to Station 36+00) – East Slope

J1 Bedding Joint (west limb) 49 316

J5 Joint 70 243

J3 Joint 41 127

■ The bedding joint (J1) on the west limb of the anticline creates a ‘dip-slope’ condition on the lower portion of the east slope. Continued planar sliding is expected along thin slabs of shale and siltstone onto the towpath, if unsupported.

■ A potential for wedge sliding exists in the segment due to the intersection of joint sets J3 and J5 within the slope face. Rockfall from wedge sliding is possible in this area.

■ A potential for toppling failures is expected, related to differential weathering of the upper portion of the slope face, where the bedding joint (J2) structures on the east limb of the anticline are exposed and dipping back into the hill slope. Rockfall related to differential weathering and toppling failures are expected in this area.

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Segment 2 (from Station 33+50 to Station 36+00) – West Slope

Joint Set Description Mean Dip (deg) Mean Dip Direction (deg)

J1 Bedding Joint (west limb) 59 313

J3 Joint 47 128

■ There is a potential for planar sliding along joint set J3, which is exposed and unsupported in the slope.

■ There is a potential for wedge sliding to create rockfall.

■ There is a potential for toppling failures to occur. Toppling failure is associated with exposure of rock beds controlled by the J1 bedding joint in the slope, which creates a rockfall potential.

Segment 3 (from Station 36+00 to Station 43+16) – East Slope

J1 Bedding Joint (west limb) 44 315

J3 Joint 27 150

■ The bedding joint (J1) on the west limb of the anticline creates a ‘dip-slope’ condition on the lower portion of the east slope. Continued planar sliding is expected along thin slabs of shale and siltstone onto the towpath, if unsupported.

■ A potential for wedge sliding exists in the segment due to the intersection of joint sets within the slope face. Rockfall from wedge sliding is possible in this location.

■ There is a potential for toppling failures to occur related to differential weathering of the upper portion of the slope face, where the bedding joint (J2) structures on the east limb of the anticline are exposed and dipping back into the hill slope. Rockfall related to differential weathering and toppling failures are expected in this location.

Segment 3 (from Station 36+00 to Station 43+16) – West Slope

J1 Bedding Joint (west limb) 48 316

J3 Joint 44 136

J4 Joint 84 64

■ There is potential for planar sliding to along joint set J3, which is exposed and unsupported in the slope. We expect that the existing landslide within this canal prism segment on the west side of the slope likely was controlled by a planar failure mode along J3.

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■ There is potential for wedge sliding created by the intersection of joint sets within the slope face.

■ There is potential for toppling failures to occur on the West slope. Toppling failures likely would be associated with exposure of rock beds controlled by the J1 bedding joint in the slope, which creates rockfall potential.

Refer to Appendix H for full results of the kinematic analyses.

Planar Sliding Stability Analysis

For each segment, cross-sections were constructed to perform slope stability analyses for both east and west slopes of the canal prism. Planar sliding was identified as a potential failure mode for both the east and west canal slopes. Limit equilibrium stability analyses were conducted using the computer program RocPlane v3.0 developed by Rocscience. Parameters used for our planar sliding stability analyses were derived from review of previous site studies, empirical correlations, field observations and experience, and laboratory tests.

The stability of the rock slope was modeled using the

Mohr-Coulomb failure criterion based on a potential planar sliding mode, with estimates of tension crack development behind the top of the slope based on empirical assumptions and observations made in the field, interface shear strength between opposing bedding and joint planes within the rock mass, as well as assumptions of groundwater conditions. We have reviewed previous analyses presented in studies by

Schnabel Engineering1 and agree that the selected interface friction angle assumed for bedding joints and stress relief joints is reasonably conservative to represent a long-term condition for design. The unit weight used in our analyses are based on laboratory test results. The unit weight and interface shear strength parameters for the shale and siltstone rock mass used in our planar sliding stability analyses are indicated in the following table:

Parameter Value

Unit Weight (pcf) 168

Interface Cohesion (psf) 0

Interface Friction Angle (deg) 34

1 Schnabel Engineering (2019), “Design for Phase 2 – Rock Slide Remediation at the Paw Paw Tunnel”

Figure 8 – Smooth/striated bedding plane

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For our analyses, we considered both dry and wet conditions within the rock mass. The groundwater conditions were estimated assuming 30 percent of the slide plane was filled with water creating uplift, as may be expected during periods of rain. The critical tension crack location behind the top of the slope was estimated using empirical methods2.

Segment Slope

Face

Estimated

Tension Crack

Location Behind

Crest of Slope

Water

Condition

Calculated

Factor of

Safety (with water)

Water

Condition

Calculated

Factor of

Safety (dry)

West 12 ft

30% of slide plane filled with water

0.67 Dry 0.82

East Not applicable due to orientation of discontinuities not creating planar sliding potential

West 2 ft

30% of slide plane filled with water

0.00 Dry 0.79

East 6 ft

30% of slide plane filled with water

0.46 Dry 0.59

West 4 ft

30% of slide plane filled with water

0.65 Dry 0.94

East 9 ft

30% of slide plane filled with water

0.57 Dry 0.70

2 Hoek, E., Bray, J.W. (1994). Rock Slope Engineering pp. 163. Chapman & Hall.

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The calculated factor of safety results from the RocPlane analysis provided in the table above for sliding controlled by bedding joints and joint planes confirm observations that exposed joint planes near the slope face are at or near a condition of planar failure where unsupported in the slope.

This is evidenced by the historic slope failures in the canal as well as observations of the rock mass on both the east and west sides of the canal. The factor of safety is further reduced by the introduction of water pressures acting along the slide plane, which is possible due to water entering discontinuities at the top of the slope and migrating into the rock mass during the wetter periods of the year. We have analyzed the effect of the built-up water pressure on the slide plane and have provided it along with the dry condition analyses. The RocPlane output plots are included in the Analysis Results section of this report in Appendix H.

Figure 9 – Landslide along west slope of canal prism

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Global Stability

In addition to the potential kinematic failure modes identified above that are controlled by discrete rock mass discontinuities, we also considered a potential failure mode through the overall fractured rock mass using critical failure surface search routine to check the minimum Factor of

Safety (FS) against global stability failure within the slope. To model this situation, we performed our global stability analyses at cross-section locations considering potential circular and non-circular failure modes through the fractured rock mass profile. The analyses did consider tension cracks development at the top of the slope. Our global stability analysis in this case, using limit equilibrium methods, was performed using the SLOPE/W computer program developed by GeoStudio.

Global stability was checked for the drained (long-term) condition, and resulting factor of safety against failure, for static conditions, shear strength parameters assumed for the rock mass, and assumed groundwater conditions. All analyses were performed using the

Morgenstern-Price analysis method. Based on FHWA guidance3, for slopes adjacent to highways and ‘important installations’, a target factor of safety of 1.5 was used for analysis purposes.

Shear strength parameters for the soil and rock for the slope have been conservatively…