Attachment_2_-_PPNM_Stabilization_-_Appendix_A.pdf

APPENDIX A:

POMPEYS PILLAR NATIONAL MONUMENT ROCK

STABILIZATION - PHASE 1

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Pompeys Pillar National Monument

Rock Stabilization—Phase 1

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Pompeys Pillar National Monument Rock Stabilization—Phase 1

January 4, 2021 2-6202-01:20R41

Prepared For:

Bureau of Land Management Billings, MT

Prepared By:

Anya Brose, Lee Petersen & Ryan Peterson, Itasca Consulting Group Russ Sheets & Joel Swenson, Barr Engineering

Pompeys Pillar National Monument Rock Stabilization Phase 1 1/4/2021 Ref. 2-6202-01:20R41 Brose, Petersen, Peterson, Sheets & Swenson

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Executive Summary

Itasca Consulting Group (Itasca) was retained by the Bureau of Land Management (BLM) to investigate rock stability at Pompeys Pillar National Monument (PPNM). Itasca formed a team of three firms to respond to the RFP. The additional team members include Bolton and Menk, Inc.

(BMI) for scanning and photogrammetry services, and Barr Engineering (Barr) for instrumentation.

This report describes the work, findings, and recommendations resulting from the initial phase of investigation. The initial scope tasks include the following:

Obtain all the geometry data, review all the reports, assess all the prior work (GSI, Dowl, Mason, BLM areas of concern), and assess the need for additional site investigation. Itasca has partially done this work, especially the geometry data.

Conduct high-resolution scan and photogrammetry of the pillar.

Site visit and non-invasive site investigation. Install and read preliminary instrumentation.

Create a preliminary 3D geometry model of the critical areas, considering visible and hidden features. This would be a geometry model adequate for a statics-based stability assessment.

Conduct a statics-based factor of safety (FOS) for the critical blocks.

Develop a rock block monitoring program concept.

Develop site investigation plan.

Prepare a written report.

A site visit, including joint mapping, regular photography, drone photography, LIDAR scanning, rock block observations, and instrumentation installation, was conducted July 7 through 13, 2020.

Major findings from the Phase 1 work include the following:

There are four major rock layers exposed on the east, south, and west side of the Pillar. Working up from the flat at the base of the pillar, these layers are: a lower sequence of shales and siltstones with interbedded sandstone layers; the lower sandstone; an upper sequence of mixed shales and siltstones with interbedded sandstone layers; and the upper sandstone. Above the upper sandstone, there is a poorly exposed sandstone with many thin shale interbeds. The north side of the pillar does not have high vertical cliffs, but rather slopes more gradually in steps

Itasca Consulting Group, Inc. Page ii www.itascacg.com down to the base elevation. The elevation, thickness, and rock type of the major rock layers vary by location.

The drone survey and LIDAR scanning produced extensive photographs and orthoimage, videos, point clouds, and 3D meshes that were used in subsequent tasks.

Two crackmeters and two distance meters were installed in the Signature Block area. One crackmeter was installed in the Turtle Rock area. The instrument data is continuously uploaded to a cloud-based host that is accessible via a web interface.

The instrumentation data has been periodically reviewed.

Five joint sets were identified from joint mapping and from manual extraction of joint planes from the drone photogrammetry point clouds. The joint sets are steeply dipping and generally strike northeast-southwest and northwest-southeast.

Stability assessments were conducted for the areas of concern. The stability assessments for the Signature Blocks area and the Turtle Rock area were done using Itasca’s 3DEC software, considering the rock blocks as rigid and permitting sliding and separation on the joints and bedding planes.

At the Signature Blocks, the various subblocks of Block 1 were found to be the least stable. This finding matches the observed condition of Block 1. Block 1 appears to exert a stabilizing influence on Blocks 2 and 3. This conclusion was not tested during Phase 1 but is part of the recommended scope for Phase 2.

In late July 2020, Itasca recommended moving and extending the visitor barrier at the Pillar base due to the observed condition and measured movement of Block 1.

Itasca recommends that the boardwalk remain closed until some Phase 2 tasks are completed, due to the risks associated with Signature Blocks Block 2.

At Turtle Rock, Blocks 3 and 4 were found to be the least stable. The lower parts of Blocks 2c, 3, 4, and 5 are relatively fresh rock surfaces, compared to the darker, more weathered surfaces nearby. The effect of Blocks 3 and 4 instability on the other Turtle Rock blocks is uncertain, based on the Phase 1 work. Evaluating the effect on the other blocks is part of the recommended scope of Phase 2.

Three Lower Rock area blocks are the least stable, based on observations.

Other areas on the southeast and southwest faces of the Pillar are potentially unstable.

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Seven factors that must be considered in selecting the appropriate monitoring systems are discussed in Section 7.3.1. These factors must be discussed and documented before the monitoring systems may be finalized.

Monitoring options are discussed in detail. Three monitoring categories are identified: 1) essential monitoring, 2) desirable monitoring, and 3) monitoring that may be appropriate. The recommended monitoring for the site, Signature Block area, Turtle Rock area, and the Lower Rock area fit into these categories. Continued use of the cloud-based data storage system eagle.io is recommended. Development of a trigger action response plan is recommended.

The recommended next phase of site investigation includes additional joint characterization, additional sounding of the shale and siltstone layers under the upper sandstone, and some combination of core drilling and geophysics for additional characterization of the shale and siltstone.

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Table of Contents

Executive Summary ......................................................................................................................... i Table of Contents ........................................................................................................................... iv List of Figures ................................................................................................................................ vi List of Tables ................................................................................................................................ vii

1.0 Introduction and Background

2.0 Site Visit

3.0 Geologic Setting

3.1 Geology

3.2 Local Stratigraphy

4.0 LIDAR Scan and Photogrammetry

5.0 Instrumentation

5.1 Background

5.2 Instrument Hardware

5.3 Design Phase Telemetry

5.4 Data Visualization

5.5 Monitoring Data to Date

6.0 Rock Engineering Assessment

6.1 Areas of Concern

6.2 Bedding and Jointing

6.2.1 Bedding

6.2.2 Jointing

6.3 Rock Block Stability

6.3.1 Methodology

6.3.2 3DEC Software Description

6.3.3 Signature Block Area

6.3.3.1 Block Stability

6.3.3.2 Visitor Barrier Recommendations

6.3.4 Turtle Rock Area

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6.3.5 Lower Rock Area

6.4 Other Areas

7.0 Rock Block Monitoring

7.1 Geohazard Monitoring Background

7.2 Monitoring Toolbox

7.2.1 Crack Displacement Monitoring

7.2.2 Remote Displacement Monitoring

7.2.3 Weather Monitoring

7.2.4 Monitoring Hardware Specifics

7.2.4.1 Automated Techniques

7.2.4.2 Manual Techniques

7.3 Proposed Monitoring

7.3.1 Considerations in Choosing the Appropriate Monitoring

7.3.2 Site-wide Monitoring

7.3.3 Signature Blocks Area

7.3.4 Turtle Rock Area

7.3.5 Lower Rock Area

7.3.6 Monitoring Data Visualization

7.3.7 Trigger Action Response Plan

7.4 Site Investigation Plan

8.0 References

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List of Figures

Figure 1 Block names near the Clark signature Figure 2 Rock formations in the area surrounding Pompeys Pillar National Monument Figure 3 Local stratigraphy Figure 4 Elevation of the bottom of the upper sandstone Figure 5 Local stratigraphy at three locations around the Pillar Figure 6 Drone photograph of Turtle Rock Figure 7 Orthophoto of Pompeys Pillar (oriented north up) Figure 8 Instrumentation types and locations (oriented north up) Figure 9 Crack gauge locations near the signature Figure 10 Web-based data visualization plan view Figure 11 Web-based time series data visualization Figure 12 Time history for Block 1 distance meter Figure 13 Time history for Block 2 distance meter Figure 14 Time history for Block 2 east face crackmeter Figure 15 Time history for Block 2 south face crackmeter Figure 16 Time history for Turtle Rock crackmeter Figure 17 Areas of concern (oriented north up) Figure 18 South portion of the upper sandstone with full point cloud and joint planes Figure 19 South portion of the upper sandstone with segmented clouds and joint planes. 16 Figure 20 Stereonet depicting upper sandstone joints Figure 21 Stereonet depicting upper sandstone joint sets with great circles Figure 22 Joint planes and pillar shape Figure 23 3D mesh for the Signature Block area Figure 24 Signature Block joints and bedding planes (left: CADD, right: 3DEC) Figure 25 Signature Block model (left: CADD, right: 3DEC) Figure 26 Signature Block plan view illustrating model extent and shale limit Figure 27 Signature Block movements for varying amounts of shale/siltstone erosion Figure 28 History locations for the Signature Block area Figure 29 Blocks 1a, 1b, and 1c in Signature Block are the least stable Figure 30 Photographs of Block 1 and the base of Block 1a Figure 31 Block movements in the Signature Block area Figure 32 Measured displacements from Block 1 distance meter Figure 33 Revised barrier location and dimensions Figure 34 Turtle Rock area modeled in 3DEC Figure 35 Turtle Rock area block numbers Figure 36 Turtle Rock joints and bedding planes (left: CADD, right: 3DEC)

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Figure 37 Turtle Rock block model (left: CADD, right: 3DEC) Figure 38 Turtle Rock plan view illustrating model extent and shale limit Figure 39 Turtle Rock block movements for varying amounts of shale/siltstone erosion. 31 Figure 40 Turtle Rock blocks with the greatest predicted movement Figure 41 Turtle Rock area block movements Figure 42 Aerial close-up of Lower Rock area Figure 43 Shale weathering under two of the Lower Rock area blocks Figure 44 Lower area rock blocks with open back joints Figure 45 Weathered and broken areas of the Lower Rock area Figure 46 Potentially unstable blocks on the southeast face of the Pillar Figure 47 Potentially unstable blocks on the southwest face of the Pillar

List of Tables

Table 1 Summary of Drone Photos and Videos Table 2 Instrumentation Hardware Details Table 3 Joint Set Summary for the Upper Sandstone

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1.0 INTRODUCTION AND BACKGROUND

Itasca Consulting Group (Itasca) was retained by the Bureau of Land Management (BLM) to investigate rock stability at Pompeys Pillar National Monument (PPNM). Background information about the project was provided in the request for proposal (BLM, 2020):

The Department of Interior (DOI), Bureau of Land Management (BLM) manages the Pompeys Pillar National Monument (PPNM), located 30 miles east of Billings, Montana. Due to the historic significance of PPNM there is also joint interest in the site by the National Park Service (NPS) National Historic Trails Program, Lewis and Clark Trail Heritage Foundation, and Lewis and Clark Trust to name a few. This national historical site contains William Clark’s signature, scribed into the sandstone rock in the year 1806. After William Clark left his mark on the rock, various other travelers from fur trappers to homesteaders left their own inscriptions. The monument attracts 30,000 visitors each year, many of whom traverse a wooden boardwalk and stairs that ascend the high rock outcropping which contains the historic signature.

The rock stability issues were described as follows (ibid):

PPNM is a sandstone outcrop that sits on a 51-acre parcel and stands 200 feet tall adjacent to the Yellowstone river, that has been carved into by populations over hundreds or even thousands of years. The Monument is vulnerable to weather induced erosion, which has been demonstrated by the loss of prehistoric petroglyphs since the BLM took over management of the Pillar.

Itasca formed a team of three firms to respond to the RFP. The additional team members include Bolton and Menk, Inc. (BMI) for scanning and photogrammetry services, and Barr Engineering (Barr) for instrumentation.

This report describes the work, findings, and recommendations resulting from the initial phase of investigation. The initial scope tasks include the following:

Obtain all the geometry data, review all the reports, assess all the prior work (GSI, Dowl, Mason, BLM areas of concern), and assess the need for additional site investigation. Itasca has partially done this work, especially the geometry data.

Conduct high-resolution scan and photogrammetry of the pillar.

Site visit and non-invasive site investigation. Install and read preliminary instrumentation.

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Create a preliminary 3D geometry model of the critical areas, considering visible and hidden features. This would be a geometry model adequate for a statics-based stability assessment.

Conduct a statics-based factor of safety (FOS) for the critical blocks.

Develop a rock block monitoring program concept.

Develop site investigation plan.

Prepare a written report.

During the site visit, three rock blocks near Clark’s signature were named to facilitate coordination, as seen in Figure 1.

Figure 1 Block names near the Clark signature.

2.0 SITE VISIT

A site visit to Pompeys Pillar National Monument occurred from July 7 to July 10, and July 13, 2020. The purpose of the site visit was to collect geometry, observe rock characteristics and condition, collect joint data, and install monitoring equipment at several locations. Representatives from the following engineering firms were present:

Itasca Consulting Group;

Bolton and Menk; and

Barr Engineering.

Lee Petersen was the representative on site from Itasca, and lead coordination efforts among subcontractors, as well as collected manual joint data and rock observations. Bolton and Menk provided geometry capture services, including LIDAR scanning, photogrammetry, and surveying, Itasca Consulting Group, Inc. Page 3 www.itascacg.com and Barr Engineering provided the initial instrumentation. More details regarding data collected on site is provided in later sections of this report.

3.0 GEOLOGIC SETTING

3.1 Geology

Pompeys Pillar is a part of the Lance Formation, which consists of alternating beds of sandstones and shales. The Lance Formation is 700–1,500 ft thick and is underlain by the Bear Paw Shale (Hancock, 1919). Thirty percent of the Lance Formation consists of channel sandstones 20 ft or thicker and 70% is composed of thinner sandstone and finer-grained interfluvial sedimentary rocks (Connor, 1917). The geometry of the outcrop is largely formed by the meandering of the adjacent Yellowstone River. The Lance Formation is inclined downward toward the east at a rate of about 12 ft to the mile (Hancock, 1919).

Figure 2 Rock formations in the area surrounding Pompeys Pillar National Monument.

3.2 Local Stratigraphy

There are four major rock layers exposed on the east, south, and west side of the Pillar. Working up from the flat at the base of the pillar, these layers are: a lower sequence of shales and siltstones with interbedded sandstone layers; the lower sandstone; an upper sequence of mixed shales and siltstones with interbedded sandstone layers; and the upper sandstone (see Figure 3). Above the upper sandstone, there is a poorly exposed sandstone with many thin shale interbeds. The north

Itasca Consulting Group, Inc. Page 4 www.itascacg.com side of the pillar does not have high vertical cliffs, but rather slopes more gradually in steps down to the base elevation.

Figure 3 Local stratigraphy.

Figure 4 illustrates the elevation of the exposed base of the upper sandstone. The horizontal distance is the true distance around the Pillar perimeter, with the zero location near the Clark signature and progressing clockwise toward Turtle Rock at 550 ft. Excluding the Turtle Rock area, there is about a 4-ft variation in elevation. Near Turtle Rock, starting at about 500 ft, the base of the upper sandstone rises sharply from about 2941.5 ft to nearly 2948.8 ft. This abrupt rise may also be seen in the right image of Figure 5.

Figure 4 Elevation of the bottom of the upper sandstone.

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The stratigraphy varies around the Pillar, as shown in Figure 5. In the figure, the left image is on the southeast face, the middle image is on the southwest face, and the right image is on the northwest face near Turtle Rock. The images were produced from the 3D mesh generated by the photogrammetry work described in Section 4.0. The images are horizontal views, dimensionally accurate, scaled to the same scale factor, and lined up on the base of the upper sandstone.

Figure 5 Local stratigraphy at three locations around the Pillar.

The exposure at the southeast location includes the lower sandstone, the upper shale-siltstone, and the upper sandstone. The lower shale-siltstone is buried under talus. The upper shale-siltstone contains a few thin interbedded sandstone layers. The exposure at the southwest location is generally the same as at the southeast location, with the exception of the lower shale-siltstone being exposed, a thicker interbedded sandstone layer in the upper shale-siltstone, and the lower sandstone is thinner than at the southeast location. The exposure at the northwest location differs substantially from the other two. The base of the upper sandstone rises sharply. Starting between the southwest and northwest images, the interbedded sandstone layers in the upper shale-siltstone grows in thickness. By the northwest image, the upper shale-siltstone begins to pinch out, becoming much thinner.

4.0 LIDAR SCAN AND PHOTOGRAMMETRY

The three-person scanning and photogrammetry crew from BMI spent a little more than two days at the site. During this visit, they performed two tasks:

Task 1 – Drone Survey. The flights were completed by an FAA certified Remote Pilot in compliance with 14 CFR part 107. A multi-rotor drone was used to capture

Itasca Consulting Group, Inc. Page 6 www.itascacg.com high resolution photos and videos of the rock mass. A network of aerial ground control was set for geo-referencing and assessing accuracies of models generated from the collected data. The commercial structure from motion software Pix4D was used to process the photos and generate dense point clouds. Orthoimagery and textured meshes were also produced.

Task 2 – Laser Scanning and Surveying. The laser scanning services were conducted using a combination of Leica Geosystems RTC360 and C10 scan systems. The laser scans were registered to measured control points. A Trimble GNSS and a robotic total station were used in combination to provide accurate control coordinates for both drone imagery and laser scanning point clouds.

The drone work produced the photos and videos summarized in Table 1. In addition to being the basis for the photogrammetry-based point cloud and mesh, the photos and videos provided closeups of the critical Pillar areas of concern. See Figure 6 for a view of Turtle Rock from the drone.

Table 1 Summary of Drone Photos and Videos

Item Number of Files Size (GB)

Nadir and Off Nadir Photos 2404 17.1

Oblique Photos 291 2.2

Orthoimage files 3 0.1

Vertical Photos 2531 20.6

Videos 7 24.6

Figure 6 Drone photograph of Turtle Rock.

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The provided scanning and photogrammetrically derived data include:

an orthophoto (see Figure 7);

point clouds using two different filtering options, a portion of which is used in Figure 18;

a 3D mesh with image overlay (used to make the images in Figure 5); and a whole point cloud plus five segments from the laser scans.

All geometry products were provided with the following parameters:

Horizontal Projection and Datum: NAD 1983 Montana State Plane Zone 2500

Vertical Datum: NAVD 88

Unit: International Foot.

Figure 7 Orthophoto of Pompeys Pillar (oriented north up).

5.0 INSTRUMENTATION

5.1 Background

A design phase instrumentation and monitoring program was developed by Barr and Itasca. The design phase instrumentation program was intended to collect a block and joint deformation baseline. Itasca, in consultation with BLM archeology staff, identified sensor locations. Barr installed the sensors, dataloggers, and telemetry.

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5.2 Instrument Hardware

Five remote-reading rock monitoring instruments were installed on Friday, July 10, 2020. The types and locations are illustrated in Figure 8. The five instruments installed include the following:

Block 1 DM—A distance meter mounted on a rock near the rail fence and pointing at Block 1 (see Figure 1 for block names).

Block 2 DM—A distance meter mounted under Block 2.

Block 2 East Face and Block 2 South Face CM—One crackmeter each on the east and south faces of Block 2 (see Figure 9).

Turtle Rock CM—One crackmeter across the joint that separates the body of Turtle Rock from the adjacent rock mass.

Figure 8 Instrumentation types and locations (oriented north up).

Table 2 lists the details of the instrumentation hardware.

5.3 Design Phase Telemetry

The telemetry system consists of the 5 low-power, wireless dataloggers equipped with internal long-range 900MHz radios and 1 gateway with internet connection. The dataloggers are located throughout the site near the sensors collecting displacement measurements. The dataloggers record and transmit the measurements using the long-range radio to the gateway. The gateway is located in the Education Center Building located approximately 100 feet east of the pillar. The gateway has internet connectivity through a Verizon 4G cellular modem that allows the data to be transmitted to the manufacturer’s secure industrial server in tabular format. The gateway and

Itasca Consulting Group, Inc. Page 9 www.itascacg.com cellular modem have AC power through the Education Center Building, and the low power, compact wireless dataloggers are powered by C-cell lithium batteries.

5.4 Data Visualization

Data from the installed instrumentation are retrieved from the manufacturer’s industrial server and imported to the web-based data visualization platform Eagle.io (https://eagle.io/). Barr manages the telemetry, data acquisition, and web-based data presentation with direction from Itasca. Access to the monitoring website is through an individual user’s account, which can be granted access upon request.

Table 2 Instrumentation Hardware Details

Sensor Name

Block 1-DM Block 2-DM Block 2 East Face-CM

Block 2 South Face-CM

Turtle Rock-

CM

Sensor Type Laser distance meter

Laser distance meter

Crackmeter

(4420-50MM)

Crackmeter

(4420-50MM)

Crackmeter

(4420-50MM)

Sensor Manufacturer

Loadsensing Loadsensing GEOKON GEOKON GEOKON

Sensor Serial Number

32295 32227 1823702 1823703 1823724

Datalogger Type

LS-G6-LASER LS-G6-LASER LS-G6-VW-1 LS-G6-VW-1 LS-G6-VW-1

Datalogger Manufacturer

Loadsensing Loadsensing Loadsensing Loadsensing Loadsensing

Datalogger Serial Number

32295 32227 22854 22851 11259

Current Monitoring Frequency

30 minutes 30 minutes 30 minutes 30 minutes 30 minutes

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Figure 9 Crack gauge locations near the signature.

Measurements are automatically updated on the website, typically within 5 minutes of the measurement being collected, eliminating manual retrieval of data and updating of spreadsheets.

The platform allows for:

data to be viewed in a web-based, plan-view map (Figure 10);

the most current readings from the sensors;

data to be viewed in time-series plots (Figure 11); and email and text messaging notification capabilities to notify stakeholders of sensor threshold value exceedances (not currently implemented on this project).

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Figure 10 Web-based data visualization plan view.

Figure 11 Web-based time series data visualization.

5.5 Monitoring Data to Date

Figure 12 through Figure 16 show the time history of the five in-place instruments through December 21, 2020. In each figure, the temperature history is plotted in red, and the instrument data is plotted in blue.

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Figure 12 Block 1 distance meter—this instrument showed steady movement (about 60 mm) through early September then leveled off with no significant movement since then. The instrument data has dropped off from time to time since through late December 2020.

Figure 13 Block 2 distance meter—this instrument showed slight movement (about 1 mm) for the first month but has been steady through late December 2020.

Figure 14 Block 2 east face crackmeter—this instrument showed slight movement (about 0.25 mm) thru mid-August, then has shown what appears to be temperature-related movement through late December 2020.

Figure 15 Block 2 south face crackmeter—this instrument showed steady opening (about 0.2 mm) through early October, then has shown what appears to be temperature-related movement through late December 2020.

Figure 16 Turtle Rock crackmeter— this instrument showed steady opening (about 0.5 mm) through early October, then has shown what appears to be temperature-related movement through late December 2020.

Figure 12 Time history for Block 1 distance meter.

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Figure 13 Time history for Block 2 distance meter.

Figure 14 Time history for Block 2 east face crackmeter.

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Figure 15 Time history for Block 2 south face crackmeter.

Figure 16 Time history for Turtle Rock crackmeter.

6.0 ROCK ENGINEERING ASSESSMENT

6.1 Areas of Concern

The BLM has identified three areas of concern, highlighted in Figure 17:

the Signature block area, shown in Figure 1;

the Turtle Rock area, shown in Figure 6; and the Lower Rock area. The area highlighted in Figure 17 is larger than indicated in the project scope. The area extends from about 40 ft southwest of the boardwalk, Itasca Consulting Group, Inc. Page 15 www.itascacg.com to about 50 ft northeast of the boardwalk, and around the corner about 40 ft to the northwest.

Each area is addressed separately in Section 6.3.

Figure 17 Areas of concern (oriented north up).

6.2 Bedding and Jointing

6.2.1 Bedding

Bedding within the upper sandstone is location-specific and was identified during the stability analysis, see Sections 6.3.3 and 6.3.4.

6.2.2 Jointing

The point clouds, meshes, still photographs, and videos captured during the site visit, as described in Section 4.0, were used to identify, and extract the location and orientation of 72 joints. These joints ranged in size from a few feet to a few tens of feet. Once identified in the photographs and videos, Cloudcompare was used to view and segment the portions of the point clouds corresponding to each joint. A best-fit plane was determined for each point cloud segment. This procedure is best at characterizing joints that have broad exposure on the rock surface, i.e., trending in the same direction as the rock surface. Several joints trending perpendicular to the local rock surface were not captured by this means. Figure 18 and Figure 19 show the point cloud, segmented point clouds, and joint planes for a portion of the upper sandstone.

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Figure 18 South portion of the upper sandstone with full point cloud and joint planes.

Figure 19 South portion of the upper sandstone with segmented clouds and joint planes.

Figure 20 shows the orientation and clustering of the 72 joints identified on the upper sandstone on an equal-angle, lower hemisphere stereonet. On a stereonet like this, near vertical joints plot near the outer rim and near horizontal joints plot near the center. One cluster had over a 9% maximum density and others had between 8 and 9% maximum densities.

Five joint sets were identified, as shown in Figure 21. The 17 joints not included in the five sets would have been included if wider set boundaries were used.

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Table 3 Joint Set Summary for the Upper Sandstone

Set Number of Joints

Dip Dip Direction

Trend

1 14 88 317 47-227

2 15 84 211 121-301

3 11 83 247 157-337

4 11 83 163 73-253

5 4 79 112 22-202

-- 17 -- -- --

Figure 20 Stereonet depicting upper sandstone joints.

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Figure 21 Stereonet depicting upper sandstone joint sets with great circles.

Figure 22 Joint planes and pillar shape.

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6.3 Rock Block Stability

The Phase 1 scope called for statics-based factor of safety calculations to assess rock block stability. The calculations were anticipated to be spreadsheet-based. Step 1 of the anticipated approach was to use the location of the major joints to cut the rock volume into blocks. Step 2 was to use 3D CADD software to determine the volume, mass, weight, and center of gravity of each major block and load that information into a spreadsheet. Step 3 was to estimate the location of the reaction forces from the shale or other blocks. Finally, Step 4 was to use the preceding information to calculate the factor of safety.

After the site visit, scanning and photogrammetry, and evaluation in the office, the rock blocks at the areas of concern were found to be too complex for a spreadsheet stability assessment. As a result, the stability assessments were conducted using Itasca’s 3DEC software (2016). The assessments were conducted assuming rigid rock blocks to be compatible with the original scope (statics-based factor of safety). In a rigid block simulation in 3DEC, the blocks may slide on the joints and bedding planes, the joints and bedding planes may separate, and the blocks may move.

6.3.1 Methodology

Rock block stability was assessed using the following steps:

Use the photographs and point cloud derived from drone photogrammetry to identify joints and bedding planes in the areas of concern.

Based on the same information, extract the location, orientation, and extent of the joints and bedding planes.

Use the 3D mesh derived from the drone photogrammetry to extract the geometry of the areas of concern.

Import the rock structures (joints and bedding planes) and 3D mesh into the CADD program Rhino.

Use Griddle (Itasca, 2020) and Rhino tools to clean up the geometry and intersect the joints and bedding planes with the 3D volumes, creating rock blocks.

Use Griddle (Itasca, 2020) to discretize zones inside the rock blocks and generate a volumetric grid.

Import the volumetric grid into 3DEC and create the additional information necessary to conduct the simulation (rock properties, joint properties, etc.).

Conduct the stability simulations.

One uncertainty in the stability simulations is the support provided by the shale and siltstone layers underlying the upper sandstone. The accessible shale and siltstone layers near the Signature Blocks

Itasca Consulting Group, Inc. Page 20 www.itascacg.com were sounded using a rock hammer during the site visit. These layers were predominantly drummy, with about 80-90% of the rock surfaces sounded showing signs of being hollow and loose. The area near Turtle Rock was inaccessible but was assumed to be the same. This finding suggests that the outer segments of the shale and siltstone layers do not provide significant support to the overlying sandstone blocks. To address this uncertainty in the simulations, increasing amounts of shale and siltstone were removed from the model and block stability was assessed at each stage.

6.3.2 3DEC Software Description

Rock block stability was assessed using the Itasca software 3DEC. 3DEC is a three-dimensional numerical modeling code for advanced geotechnical analysis of soil, rock, groundwater, structural support, and masonry. 3DEC simulates the response of discontinuous media (such as jointed rock or masonry bricks) that is subject to either static or dynamic loading. The numerical formulation is based on the distinct element method (DEM) for discontinuum modeling.

The discontinuous material is represented as an assemblage of discrete blocks. The discontinuities are treated as boundary conditions between blocks; large displacements along discontinuities and rotations of blocks are allowed. Individual blocks behave (based on constitutive and joint models) as either rigid or deformable (i.e., meshed into finite difference zones) material. Continuous and discontinuous joint patterns can be generated on a statistical basis. A joint structure can be built into the model directly from the geologic mapping.

6.3.3 Signature Block Area

6.3.3.1 Block Stability

The Signature Block area modeled using 3DEC is illustrated in Figure 23. The modeling area included Blocks 1, 2, and 3 (shown in Figure 1) and covered approximately 90 ft in length and 50 ft in height. Figure 24and Figure 25 show the joints and bedding planes that form the rock blocks in the Signature Block area. The geometry was created using the procedure described in Section 6.3.1 then modeled in 3DEC.

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Figure 23 3D mesh for the Signature Block area.

Figure 24 Signature Block joints and bedding planes (left: CADD, right:

3DEC).

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Figure 25 Signature Block model (left: CADD, right: 3DEC).

The sandstone blocks that form the Signature Block are underlain by an approximately 10-foot thick layer of shale and siltstone. This weathered layer has undercut the overlaying sandstone, creating a risk for block toppling. To examine the stability of the undercut sandstone, the maximum extent of the weathered shale layer (the shale limit) was traced around the Signature Block area (Figure 26). After modeling the current site geometry developed from the point cloud, incremental amounts of the shale/siltstone layer were removed to examine how the stability of the sandstone blocks changes with a loss of underside support.

Figure 26 Signature Block plan view illustrating model extent and shale limit.

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Figure 27 shows the five different scenarios modeled for the Signature Block area, including:

current geometry based on the collected photogrammetry data;

1.0 ft offset from the shale limit;

1.5 ft offset from the shale limit;

2.0 ft offset from the shale limit; and

2.5 ft offset from the shale limit.

Each plot shows the displacement contours across the Signature Block area, as well as displacement histories at various locations along the Signature Blocks. These histories are used to determine whether the blocks in the model are stable. The history locations are shown in Figure 28.

Figure 27 Signature Block movements for varying amounts of shale/siltstone erosion.

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Figure 28 History locations for the Signature Block area.

Three blocks in the Signature Block area had the greatest predicted movement, and all three blocks are subparts of Block 1 (Figure 27). This prediction aligns with observations during the site visit.

Block 1a is defined by an open joint along the back side (southwest side) of the block. This block appears to have a small base ledge supporting the block (see Figure 30). The previous movement and small ledge show that Block 1a is a potentially hazardous block due to its geometry. Block 1b is cut by joints on all sides and is heavily undercut, presumably by a fallen block of sandstone.

Block 1c is also defined by open joints on both the northwest and southwest sides. These joints are shown in Figure 31.

6.3.3.2 Visitor Barrier Recommendations

During a project review on July 27, 2020, Itasca recommended moving and extending the barrier used to control Monument visitor access. This recommendation was based on two considerations:

The appearance of Block 1a as discussed in the preceding paragraph and illustrated in Figure 30.

The displacements measured by the Block 1 distance meter. See Section 5.2 for more information about the instrument and Figure 32 for the instrument displacements as a function of time from the time of installation to July 27, 2020.

Itasca subsequently recommended the following specific changes to the barrier. The revised barrier, shown in Figure 33:

is offset from the current rail fence by 100 ft;

takes off from the current barrier about 85 ft northeast of the current barrier end;

ends at the rail fence about 120 ft southwest of the current barrier end;

Itasca Consulting Group, Inc. Page 25 www.itascacg.com is about 405 ft long, minus the 85 ft that may be reused, requiring about 320 ft of new barrier; and the drawing in Figure 33 shows the 40-ft radius curves at the two corners in the field. These corners could be square, chamfered, or radiused as shown.

Figure 29 Blocks 1a, 1b, and 1c in Signature Block are the least stable.

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Figure 30 Photographs of Block 1 and the base of Block 1a.

Figure 31 Block movements in the Signature Block area.

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Figure 32 Measured displacements from Block 1 distance meter.

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Figure 33 Revised barrier location and dimensions.

6.3.4 Turtle Rock Area

The region of Turtle Rock modeled using 3DEC is shown in Figure 34, and the block numbers assigned to the area is shown in Figure 35. The region is about 100 ft wide and about 50 ft high.

Figure 36 illustrates the joints and bedding planes and Figure 37 shows the resulting rock blocks in both CADD and 3DEC.

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Figure 34 Turtle Rock area modeled in 3DEC.

Figure 35 Turtle Rock area block numbers.

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Figure 36 Turtle Rock joints and bedding planes (left: CADD, right: 3DEC).

Figure 37 Turtle Rock block model (left: CADD, right: 3DEC).

The shale and siltstone layers at Turtle Rock have weathered and eroded, undercutting the upper sandstone and creating overhangs. The amount of undercutting and resulting overhangs are illustrated in Figure 38. As noted in Section 6.3.1, the full extent of the shale and siltstone layers may not support the overlying sandstone. The rate of shale/siltstone erosion is also unknown. To develop some understanding of the impact of weathered shale/siltstone and rate of erosion on rock block stability, the stability analyses were conducted for the shale/siltstone limits measured and for up to 2.5 ft of additional erosion.

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Figure 38 Turtle Rock plan view illustrating model extent and shale limit.

Figure 39illustrates the Turtle Rock block movements for varying amounts of shale/siltstone erosion. From the upper left to the lower right, the images are for: 0, 0.5 ft, 1.0 ft, 1.5 ft, 2 ft, and

2.5 ft of shale/siltstone erosion. Overlaid on the block images is a plot of the displacement of selected points on each block. The figure shows that the relatively tall, thin block behind the Turtle Rock body (highlighted in red in Figure 40) is not stable for the current shale/siltstone limits. The simulations for additional shale/siltstone erosion show that the block movements accelerate. After

2.5 ft of shale/siltstone removal, the block to the right (highlighted in yellow in Figure 40) is unstable.

The Turtle Rock head and body blocks, along with the blocks under the Turtle Rock, show some movement with increasing erosion, but are stable in this rigid block analysis.

Figure 39 Turtle Rock block movements for varying amounts of shale/siltstone erosion.

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Figure 40 Turtle Rock blocks with the greatest predicted movement.

The predicted instability is compatible with observations at Turtle Rock. Figure 41 is an annotated image from the 3D mesh from photogrammetry. The red dashed line is the joint that isolates the Turtle Rock head and body from the rest of the pillar. The red-highlighted area is the face of the block predicted to be unstable. The yellow dashed lines represent the surface expression of two joints generally parallel to the red-highlighted joint. It is believed that the red joint surface was originally in line with the red joint trace, but is now offset by about 2 ft. Although not visible in this image, the two yellow-highlighted joints are open by about the same amount. Hence, the block has moved and is marginally stable.

Figure 41 Turtle Rock area block movements.

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6.3.5 Lower Rock Area

Figure 42 is a close-up aerial view of the Lower Rock area highlighted in Figure 17. The rock blocks of the Lower Rock area are from the lower sandstone layer identified in Figure 3. The nature and geometry of the rock blocks in the Lower Rock area are not compatible with the methods described in the preceding sections for the Signature Block area and Turtle Rock. The Lower Rock area instability appears to be related to the behavior of individual blocks, as those blocks are undercut due to weathering and raveling of the underlying shales and siltstones. An example of the weathering and raveling under these blocks is shown in Figure 43.

Figure 42 Aerial close-up of Lower Rock area.

Figure 43 Shale weathering under two of the Lower Rock area blocks.

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Three Lower Rock area blocks in particular appear to be the least stable (see Figure 44). This conclusion is based on the open back joints (joints behind the blocks) and that the blocks exist outboard of adjacent blocks. However, the entire Lower Rock area shows signs of ongoing rock movement, weathering, and rock breakage. A 50-ft section of the lower sandstone, just northwest of the Lower Rock area of concern, has broken away. The overall appearance of the Lower Rock area is of heavily weathered and broken rock, as shown in Figure 45.

Figure 44 Lower area rock blocks with open back joints.

Figure 45 Weathered and broken areas of the Lower Rock area.

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6.4 Other Areas

The Phase 1 scope is limited to the three areas of concern described in Section 6.1 and illustrated in Figure 17. However, other areas with potentially unstable rock blocks were observed while conducting the scope tasks. The observation of locations outside the area of concern was not comprehensive, nor was stability of these rock blocks assessed. Figure 46 and Figure 47 illustrate the location and extent of the potentially unstable blocks.

Figure 46 Potentially unstable blocks on the southeast face of the Pillar.

Figure 47 Potentially unstable blocks on the southwest face of the Pillar.

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7.0 ROCK BLOCK MONITORING

7.1 Geohazard Monitoring Background

Geohazards tend to exhibit minor displacements prior to a major or significant event. These phenomena can occur over time prior to a major block movement. Prior to a major displacement, there is often measurable deformation and observable phenomena, such as:

precipitation;

temperature change (e.g., freeze-thaw);

development of tension cracks;

settlement; and/or raveling (e.g., minor rock fall).

A geohazard monitoring program consists of the systematic detection, measurement, interpretation, and timely reporting of these measurable precursors of larger block instability.

Access to the area may continue around slopes and blocks that are exhibiting minimal signs of movement if the blocks are well monitored and risks are properly managed. If movement rates accelerate, there is a trigger action response plan (TARP) to implement further controls to limit access to the area of concern until a proper evaluation is conducted to assess the geohazard risk.

Geohazard monitoring systems have limitations that must be understood. These limitations include the following:

A monitoring system with alarm thresholds does not eliminate the need for regular review.

Automated monitoring systems are not a complete replacement for continual vigilance of BLM personnel to visually inspect and observe the unstable blocks for changing conditions.

Automated monitoring systems collect monitoring data on a specified interval. The potential for a rapid change in condition must be considered when selecting a reasonable monitoring interval with respect to the performance of each device and its telemetry that will provide sufficient warning as well as balance data management.

Technical issues such as power outages, communication failure, network outages, or equipment performance issues can affect all systems that will result in monitoring data being unavailable.

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Several items need to be considered when implementing a geohazard monitoring program as a means to evaluate and mitigate risk to the public, the infrastructure, and artifacts requiring preservation. These must first be understood and then addressed if applicable to the project site.

The primary factor is to select instrumentation that will measure the desirable metrics as appropriate for the site conditions. The key metrics typically include the measurement of displacement or strain, date and time the measurement is collected, and the ambient temperature.

Additional metrics such as velocity and acceleration can be calculated from the displacement measurement.

In addition to ensuring that the appropriate instrumentation is selected that will obtain the necessary monitoring data, there are practical considerations that should be considered:

Durable monitoring systems that have been proven to function in the field and provide precise and accurate measurements;

simplicity of installation and operation;

ease of maintenance, repairs, and re-calibration;

accessibility to equipment installed at the site; and ease of data access for presentation and analysis.

7.2 Monitoring Toolbox

Rock and soil inherently exhibit deformation due to a number of factors related to material properties and environmental conditions. There are a number of manual and automated approaches that have been developed to quantify deformation. As stated above, a key factor is selecting the most appropriate monitoring approach to capture representative movements considering the extent of geohazard risk at the site.

Direct surface monitoring methods are preferred for accessible surface locations because they are typically the least expensive option with regards to installation and maintenance. Sub-surface movement-monitoring systems are preferred where monitoring of buried or subsurface features is necessary. They are more expensive than direct surface monitoring systems because they require development of borings through the rock and soil. To allow surface monitoring of areas that are inaccessible, remote surface monitoring methods utilizing laser imaging and synthetic aperture radar have been developed. Systems that are potential solutions for tracking deformation of critical blocks at the site are discussed in the following paragraphs.

7.2.1 Crack Displacement Monitoring

As rock block movement progresses, the corresponding edges of joints or cracks that define the block will either diverge or converge. This movement can be monitored with direct surface

Itasca Consulting Group, Inc. Page 38 www.itascacg.com monitoring methods by measuring between two fixed points — one installed on the reference block and the second point installed on the block experiencing displacement. Additional local points can be included to ascertain potential directional and rotational movement. The use of pin sets would be a manual method that can be implemented as a low-cost redundant approach to verify movement detected by automated systems. This can be completed during regular visual inspections or upon servicing the instrumentation.

Automated monitoring systems can be configured to record a measurement at specified time intervals and transmit the measurement and time stamp through a base station capable of uploading to a database. This process allows for observation of movement trends over time and the opportunity to identify changes in condition prior to significant movement such that further risk mitigation measure can be implemented. Crack meters installed across the separation and attached to fixed points, similar to pin sets, can provide near real-time deformation monitoring of unstable ground. A wireline extensometer is a similar device, which monitors extension and contraction of a crack, or a series of cracks where it is safe to cross onto potentially unstable ground. However, where the crack meter is a relatively local installation, an extensometer includes an exposed wire that runs from the fixed monitoring equipment to a fixed point on the unstable block. This installation may cover several or more feet and typically is a more visible surface installation.

Where it may be unfeasible to install monitoring across the actual crack boundary, a tiltmeter instrument can be installed on the face of an unstable block. Rather than measuring the direct displacement, the tiltmeter will monitor the degree of deflection or rotation of the block.

7.2.2 Remote Displacement Monitoring

There are situations where it is advantageous to monitor deformation across multiple blocks using remote surface monitoring methods.