Attachment_3_-_PPNM_Stabilization_-_Appendix_B.pdf

APPENDIX B:

POMPEYS PILLAR NATIONAL MONUMENT ROCK

STABILIZATION - PHASE 2

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Pompeys Pillar National Monument

Rock Stabilization—Phase 2

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Pompeys Pillar National Monument Rock Stabilization—Phase 2

September 4, 2021 2-6202-01:21R23

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 2 9/15/2021 Ref. 2-6202-01:21R23 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. Phase 1 was completed in late 2020. Phase 2 started in February 2021. A site visit, including joint mapping, photography, crane access, laboratory sample collection, rock block observations, and instrumentation installation, was conducted in March 2021.

This report describes the work and findings resulting from the Phase 2. The sixteen project tasks are combined into seven report sections:

Task 01 Risk Tolerance and Risk Register

Task 02 Site Investigation and Laboratory Testing

Task 03 Rock Block Monitoring System

Tasks 04, 12 Re-Opening Recommendations

Tasks 05, 07, 08 3DEC Modeling

Task 09 Rockfall Analysis

Tasks 10, 11, 14 Remedial Measures

Task 15, 16 Report including Class C Cost Estimate

Tasks 6 and 13 were presentations to BLM. Task details are provided in nine appendices.

Major findings from the Phase 2 work include the following:

Ten threats ranked in the red region of the risk matrix, see Section 2.0

The site visit, investigation, and laboratory testing provided valuable insight into the jointing and bedding, siltstone condition, and rock strength. In particular, the siltstone layers at the Signature Block area were found to consist of eight layer of varying lithology, character, and strength (strength based on Schmidt hammer index testing).

The rock block monitoring system was installed with ten crackmeters, three distometers, and, four tiltmeters. Trigger limits were set, and automatic notifications established. As of July 2021, all instruments are within the trigger limits.

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The trigger action response plan (TARP) underwent several revisions. Currently, conditions for escalation to higher alert levels are limited to Levels 1 and 2.

Conditions are suitable for reopening the boardwalk except the lower viewing platform adjacent to Signature Block 3.

At the Signature Block area, Block 2 becomes unstable with 2.5 ft of shale removal (Cut 5) and Blocks 1 and 3 becomes unstable at 3 ft of shale removal (Cut 6).

At the Signature Block area, for a global strength reduction analysis, Block 2 becomes unstable with SRF:1.3 and Blocks 1 and 3 become unstable at SRF:1.6

Turtle Rock Block 3 is currently unstable but has not fallen likely due to interference of large asperities on the joint between Block 3 and the adjacent blocks.

The release joint at the back of Block 3 is open more than 1.5ft and if/when the asperities fail, Block 3 will continue to rotate out from the face of the bluff. The mode of failure is toppling.

Turtle Rock Block 4 is marginally stable. It is possible that additional Block 3 movement will induce movement in Blocks 1 and 4.

Turtle Rock Block 1b (the Turtle back) and the underlying block 2c is stable with an SRF likely greater than 1.5. The mode of failure is shearing of the siltstone toe followed by toppling of the sandstone blocks.

Turtle Rock Block 1a (the Turtle head) appears to be stable with minimal support, although there are uncertainties in this conclusion as discussed above. The mode of failure is loss of support from the slender pillar (block 1e) followed by toppling of the block.

Turtle Rock Blocks 2a and 2b are currently stable with an SRF greater than 1.5.

Regarding rockfall, in the Signature Block area, a few blocks of the size simulated were identified during the site visit. Until scaled, these blocks have the potential to impact the lower viewing platform. The upper viewing platform will likely not see any impact from rockfall.

The risks associated with impacts to existing walking paths are discussed above.

Regarding rockfall from the Lower Area, three blocks have the potential to become unstable. If they start rolling, there is a potential impact to the existing sidewalk at the toe of the slope. There is also a potential impact to the boardwalk from the block nearest the boardwalk.

Eleven remedial measures were identified:

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The applicability of the some of the remedial measures depends upon Phase 3 engineering development, and agency-related considerations. The latter considerations include: available budget (see Section 9.0 for the Class C cost estimate), aesthetics, service life, risk tolerance, and general approach.

The following remedial measures were found to be effective: siltstone structural stabilization, and Block 2 underpinning at the Signature Block area; and Turtle Rock head underpinning.

Details were developed for the remedial measures.

The Class C cost estimate includes costs for all remedial measures. Four remediation scenarios, which are combinations of the remedial measures, were developed. These scenarios have estimated construction costs of: $, Phase 2 recommendations are:

Remove foliage above and adjacent to the Signature Blocks, Conduct preliminary design studies for the following Signature Blocks remedial measures:

o Top of rock blocks/sandstone drainage control, include locations, materials, and methodology, o Scaling, include locations and methodology, Itasca Consulting Group, Inc. Page iv www.itascacg.com o Removal of minor rocks above Clark’s Signature to provide necessary safety for public use of the Lower Viewing Platform on the Boardwalk, o Water diversion in joints, include locations, materials, and methodology, o Shale/siltstone stabilization, include locations, materials, and methodology, o Joint treatments, include locations, materials, and methodology, o Rock bolts and cable bolts, include locations, materials, and methodology, and o Buttress and underpinning, include locations, materials, and methodology.

Conduct preliminary design studies for the Lower Rocks area:

o Rockfall controls and safety barrier placement, include locations, materials, and methodology.

Conduct preliminary design studies for the Turtle Rock area:

o Buttress and underpinning, include locations, materials, and methodology for protection and preservation of the Turtle Head.

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Table of Contents

Executive Summary ......................................................................................................................... i Table of Contents ............................................................................................................................ v List of Figures ............................................................................................................................... vii List of Tables ............................................................................................................................... viii

1.0 Introduction and Background

1.1 Areas of Concern

2.0 Task 01: Risk Tolerance and Risk Register

3.0 Task 02: Site Investigation and Laboratory Testing

4.0 Task 03: Rock Block Monitoring System

5.0 Task 04, 12: Reopening Recommendations

6.0 Task 05, 07, 08: 3DEC Modeling

6.1 Signature Block Area 3DEC Analysis

6.2 Turtle Rock Area 3DEC Analysis

7.0 Task 09: Rockfall Analysis

8.0 Task 10, 11, 14: Remedial Measures

8.1 Descriptions

8.2 Effectiveness

8.2.1 Signature Block Siltstone Stabilization

8.2.2 Turtle Rock Head Stabilization

8.3 Additional Details

9.0 Class C Cost Estimate

10.0 References

Appendix A: Risk Tolerance and Risk Register Appendix B: Site Investigation Appendix C: Signature Area 3DEC Assessment Appendix D: Turtle Rock Area 3DEC Assessment Appendix E: Rockfall Analysis Appendix F: Remedial Measures Appendix G: Rock Block Monitoring System & TARP

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Appendix H: Reopening Recommendations Appendix I: Class C Cost Estimate

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List of Figures

Figure 1 Areas of concern (oriented north up) Figure 2 Block names near the Clark signature Figure 3 Turtle Rock area block numbers Figure 4 Example risk matrix for very high probability and high impact Figure 5 Risk category numbers for 11 categories Figure 6 Cranes used for close-up site investigation activities Figure 7 Siltstone, shale, and sandstone characterization findings Figure 8 Sandstone core collection Figure 9 Subset of the Signature Block’s joint and bedding plane Figure 10 Signature blocks east and north face instrumentation Figure 11 Signature blocks top instrumentation Figure 12 Turtle Rock blocks instrumentation Figure 13 Lower Rock blocks instrumentation Figure 14 TARP levels and trigger events Figure 15 TARP levels and response plan Figure 16 3D mesh for the Signature Block area, from Phase 1 photogrammetry Figure 17 Signature Block model in 3DEC Figure 18 Signature Block plan view illustrating model extent and shale limit Figure 19 Signature Block displacement histories for incremental siltstone removal Figure 20 Signature Block velocity contours for Cuts 5 and 6 Figure 21 Signature Block displacement histories for global strength reduction Figure 22 Signature Block velocity contours for global strength reduction Figure 23 Amount of siltstone removal to cause instability Figure 24 Global SRF stability Figure 25 Turtle Rock area geometry and 3DEC blocks Figure 26 Turtle Rock global strength reduction results Figure 27 Turtle Rock rock pillar Figure 28 Turtle Rock head analysis with the rock pillar removed Figure 29 Rockfall calibration phase results Figure 30 Results for 477 blocks of realization 4 Figure 31 Risks for selected engineering projects and individual risk (Wyllie, 2018) Figure 32 Relative extent of the runout area and danger area Figure 33 Comparison of rockfall fatality risk with other risks Figure 34 Applicability of the remedial measures to the three areas of concern Figure 35 Signature Block displacement histories for global SR with 0.5 ft of shotcrete. . 35 Figure 36 Signature Block velocity contours for global SR with 0.5 ft of shotcrete

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Figure 37 Global SRF stability with 0.5 ft shotcrete Figure 38 Signature Block displacement histories for global SR with 1 ft of shotcrete Figure 39 Details of remedial measure 3, water diversion in joints Figure 40 Details of remedial measure 4, environmental protection of the siltstone Figure 41 Details of remedial measure 9, joint treatment Figure 42 Details of remedial measure 10, rockbolting Figure 43 Details of remedial measure 11, underpinning at Signature Block 2 Figure 44 Details of remedial measure 11, underpinning at Turtle Rock

List of Tables

Table 1 RBMS Trigger Limits Table 2 Average Person Risk Table 3 Descriptions and Examples of Remedial Measures Table 4 Cost Scenario 1 Table 5 Cost Scenario 2 Table 6 Cost Scenario 3 Table 7 Cost Scenario 4

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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). This report describes Phase 2 work on the project. The Phase 1 work was documented in Brose et al. (2021).

1.1 Areas of Concern

The BLM has identified three areas of concern, highlighted in Figure 1:

the Signature Block area, shown in Figure 2;

the Turtle Rock area, shown in Figure 3; and the Lower Rock area. The area highlighted in Figure 1 is larger than indicated in the project scope. The area extends from about 40 ft southwest of the boardwalk, to about 50 ft northeast of the boardwalk, and around the corner about 40 ft to the northwest.

Figure 1 Areas of concern (oriented north up).

The rock blocks at the Signature Area and Turtle Rock Area were numbered to facilitate coordination, as seen in Figure 2 and Figure 3.

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Figure 2 Block names near the Clark signature.

Figure 3 Turtle Rock area block numbers.

The sixteen project tasks are combined into seven report sections:

Task 01: Risk Tolerance and Risk Register

Task 02: Site Investigation and Laboratory Testing

Task 03: Rock Block Monitoring System

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Tasks 04, 12: Re-Opening Recommendations

Tasks 05, 07, 08: 3DEC Modeling

Task 09: Rockfall Analysis

Tasks 10, 11, 14: Remedial Measures

The following appendices provide additional project details:

Appendix A: Risk Tolerance and Risk Register

Appendix B: Site Investigation

Appendix C: Signature Area 3DEC Assessment

Appendix D: Turtle Rock Area 3DEC Assessment

Appendix E: Rockfall Analysis

Appendix F: Remedial Measures

Appendix G: Rock Block Monitoring System & TARP

Appendix H: Reopening Recommendations

Appendix I: Class C Cost Estimate

2.0 TASK 01: RISK TOLERANCE AND RISK REGISTER

BLM and Itasca staff conducted two risk assessment charettes in February 2021. The PowerPoint slides from the first charette are in Appendix A. This charette set the stage for the risk tolerance and risk register discussions that occurred internally at the BLM. In the second charette, the risk register was edited to reflect the BLM risk perceptions and tolerance. The resulting risk register is the last four pages of Appendix A.

In summary, the impact and probability of occurrence for 23 threats were assessed. The impact and probability were assessed on a five-level scale: very low, low, medium, high, and very high.

When combined, these factors produce a risk matrix such as that shown in Figure 4.

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Figure 4 Example risk matrix for very high probability and high impact.

Figure 5 Risk category numbers for 11 categories.

Ten threats were ranked in the red region of the risk matrix:

Signature Area Block 1 o Rockfall - Public Safety: Impact-High, Probability-Very High, Rating-3 o Redistribution of stress: Impact-High, Probability-High, Rating-5 o Disturbance of Block 2: Impact-High, Probability-High, Rating-5 o Education Building: Impact-Very High, Probability-Very Low, Rating-10

Signature Area Block 2 o Rockfall - Public Safety: Impact-High, Probability-High, Rating-5 o Redistribution of stress: Impact-High, Probability-High, Rating-5 o Damage to the boardwalk: Impact-Very High, Probability-High, Rating-2

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Signature Area Block 3 o Loss of the Clark Signature: Impact-Very High, Probability-Very Low, Rating-10 o Rockfall - Public Safety: Impact-Very High, Probability-Very Low, Rating-o Damage to the boardwalk: Impact-Very High, Probability-Very Low, Rating-10

These risk register rankings were used to direct the work of the later tasks.

3.0 TASK 02: SITE INVESTIGATION AND LABORATORY TESTING

A site visit to Pompeys Pillar National Monument occurred during the first week of March 2021.

The work conducted during the visit included:

siltstone characterization at the Signature Block area, especially weak siltstone at the face;

collecting cores for laboratory testing;

observing joint openness;

observing the joint under Signature Block 1a;

observing the Turtle Rock back joint orientation and extent; and observing the Turtle Rock sandstone pillar under head.

Much of this work was done from rental cranes, as shown in Figure 6.

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Figure 6 Cranes used for close-up site investigation activities.

The siltstone, shale, and sandstone layers under the Signature Block were characterized by close-up observations, sounding, and a Schmidt hammer (see Figure 7). The weakest, most friable layers were found immediately under the sandstone and from 27 inches to 47 inches below the sandstone.

Sandstone cores were collected from a previous rockfall, as shown in Figure 8. Five cores were collected for three UCS tests and 20 Brazilian tests.

Figure 7 Siltstone, shale, and sandstone characterization findings.

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Figure 8 Sandstone core collection.

The findings from the closeup observations of the Signature and Turtle Rock area were incorporated in the 3DEC models (see Section 6.0). One of the major model improvements was the joints and bedding planes included in the model. Figure 9 shows a subset of the features used in the Signature Block model, where the figure background is the point cloud developed in Phase 1.

Figure 9 Subset of the Signature Block’s joint and bedding plane.

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Additional details and findings from the site investigation work are provided in Appendix B: Site Investigation.

4.0 TASK 03: ROCK BLOCK MONITORING SYSTEM

A five-sensor rock block monitoring system (RBMS) was installed in early July 2020, during Phase 1. 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 has been automatically uploaded to a cloud-based host that is accessible via a web interface and has been periodically reviewed.

Phase 2 work included permanently procuring the Phase 1 equipment, expanding the system, developing a trigger action response plan (TARP), and managing the RBMS. The work consisted of the following steps:

Converting the current equipment rental to BLM ownership.

Updating the mounting of the existing instrumentation.

Expanding the existing RBMS, including construction and installation of additional sensors.

Installing an automated weather station.

Creating a trigger action response plan (TARP) and implementation of the TARP through

March 2021.

As of June 2021, the revised RBMS has been installed with ten crackmeters, three distometers, and four tiltmeters. Figure 10 through Figure 13 illustrate the names and locations of the instruments at: Signature blocks east and north face, Signature blocks top, Turtle Rock, and the Lower Rocks area, respectively. Trigger limits have been set, and automatic notifications established. Refer to Appendix G for additional details.

Figure 10 Signature blocks east and north face instrumentation.

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Figure 11 Signature blocks top instrumentation.

Figure 12 Turtle Rock blocks instrumentation.

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Figure 13 Lower Rock blocks instrumentation.

The trigger limits as of June 2021 are listed in Table 1. These trigger limits are based on the range of readings since the Phase 2 instrumentation system became operational in March 2021.

Table 1 RBMS Trigger Limits

# Instrument Units Upper Trigger Limit

Lower Trigger Limit

1 Sig Block 1 Top CM1 mm 2.0 -2.0

2 Sig Block 1 Top CM2 mm 2.0 -2.0

3 Sig Block 2 Top CM3 mm 2.0 -2.0

4 Sig Block 1 DM1 mm 200 10

5 Sig Block 1 DM2 mm 1200 400

6 Sig Block 2 EF CM mm 0.4 -0.3

7 Sig Block 2 NF CM mm 0.5 -0.2

8 Sig Block 2-3 NF CM mm 0.4 -0.3

9 Sig Block 2 DM mm 1.5 -4.0

10 Turtle Rock Block 1a CM mm 1.5 -1.0

11 Turtle Rock Block 1b CM mm 2.5 -2.0

12 Turtle Rock Block 2a CM mm 1.5 -1.0

13 Turtle Rock Block 2b A-axis deg 0.15 -0.15

14 Turtle Rock Block 2b B-axis deg 0.15 -0.15

15 Turtle Rock Block 3 CM mm 1.5 -1.0

16 Lower Rock Block 1 A-axis deg 0.20 -0.10

17 Lower Rock Block 1 B-axis deg 0.20 -0.10

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18 Lower Rock Block 2 A-axis deg 0.30 0.00

19 Lower Rock Block 2 B-axis deg 0.00 -0.30

20 Lower Rock Block 3 A-axis deg 0.20 -0.10

21 Lower Rock Block 3 B-axis deg 0.20 -0.10

The TARP has seven levels:

Level 0—Normal Condition - No new signs of movement Level 1—Non-Movement Condition for Review Level 2—New Movement - Low Potential of Safety Impacts Level 3—Continual Movement - Moderate Potential of Safety Impacts Level 4—Accelerating Movement - High Potential of Safety Impacts Level 5—Imminent Failure - High Risk of Safety Impacts Level 6—Active or Unexpected Failure

Figure 14 lists the trigger events to move up one or more levels based on visual observations and crackmeter, tiltmeter, and distometer readings. Level 2 displacement trigger limits have been established for all instruments. However, there is insufficient experience with the installed instrumentation to establish the trigger events for TARP Level 3 through Level 5, which are velocities and accelerations. The current Level 2 displacement triggers have been set moderately tight with respect to the instrument history since the system became operational in March 2021.

As a result, trigger events are expected.

The short-term response plan in the event of a trigger consists of the following:

Review all instrument data histories.

Conduct visual inspection of the area where the triggered instrument is located.

Conduct review meeting with BLM staff and Itasca.

During the review meeting, establish immediate actions, which may include revising the trigger limit(s), site access restrictions, new fencing or barrier locations, and other actions.

It is anticipated that as more site-specific experience is gained, the velocity and acceleration trigger limits will be developed.

Figure 15 contains broader context of the long-term TARP. The right columns of the TARP define the response plan when a specific trigger event is observed and the responsible person, or party, to complete the required actions. The response plan details decisions that key stakeholders previously have agreed upon, so that when the triggers occur, the responsible party is not determining the necessary course of action while a potential geohazard risk is occurring and time is critical. The response plan presented here contains the following components:

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Representative Timeline to Failure — This is intended to be a conceptual guide based on similar situations as a means to compare and assess geohazard risk. The actual timeline is established by a variety of site factors and is evaluated on a case-by-case basis.

Action — This defines the step(s) to take, or controls to implement, when the associated trigger event occurs.

Monitoring — The change in block condition will require increased frequency of visual inspections and RBMS data review.

Responsible Party — The person or group with ownership to complete the Action and Monitoring associated with the Ground Condition Level.

Criteria to Downgrade — The premise of a TARP is to ensure controls exist to appropriately respond when geohazard risk increases; however, as the risk decreases, there is specified criteria to allow the hazard to be downgraded in a similar manner.

Reporting — To ensure that all key stakeholders are aware and have ownership of geohazard risk, specific triggers require appropriate communication to those stakeholders.

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Figure 14 TARP levels and trigger events.

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Figure 15 TARP levels and response plan.

5.0 TASK 04, 12: REOPENING RECOMMENDATIONS

Preliminary reopening recommendations were developed in mid-March 2021, distributed to technical staff at the BLM, and were discussed in a larger group in late March. These recommendations were based on the status of the Phase 2 work at that time. Subsequent Phase 2 tasks did not necessitate changes to the recommendations, except the addition of rockfall barriers or fences below the lower rock area. The recommendations are provided in the following paragraphs.

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These recommendations broaden the definition of “re-opening” to include the potential hazard areas besides the boardwalk. These include:

Signature Area — The flat areas downslope from Blocks 1, 2, and 3.

Signature Area — The lower and upper viewing areas near the signature.

Signature Area — The boardwalk elsewhere.

Lower Rock Area — The boardwalk and sidewalk.

These recommendations are based on the following:

Phase 1 work.

The risk register and related discussions.

Phase 2 site investigation.

The installed Phase 1 and Phase 2 rock block monitoring system.

The draft trigger response action plan.

The preliminary results from the rockfall analysis.

The recommendations also assume that the risks are based on rockfall associated with small blocks (small relative to the size of blocks 1, 2, and 3, but probably not small from the BLM perspective) releasing from the upper or lower sandstone layers. The recommendations exclude the risks associated with the large-scale failure of Blocks 1, 2, or 3. The rock block monitoring system is intended to provide early warning of impending movement of the large blocks, which would initiate application of the trigger action response plan (TARP). The most-likely-to-move rock blocks in the lower rock area are also instrumented, which provides early warning of rock movement.

Conclusions for each area resulting from the work to date are detailed below:

Signature area:

o A few blocks of the size simulated were identified during the site visit. Until scaled, these blocks have the potential to impact the lower viewing platform at the signature area. The upper viewing platform will likely not see any impact from rockfall.

There is a low probability of impacts to the boardwalk or existing walking paths at the base of the pillar.

Lower area:

o Three blocks have the potential to become unstable. If they start rolling, there is a potential impact to the existing sidewalk at the toe of the slope. There is also a potential impact to the boardwalk from the block nearest the boardwalk.

o Some of these simulations assume unfavorable initial orientations and should be viewed as unlikely.

The recommendations include the following:

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The boardwalk may be re-opened except as noted below. This includes all the way to the Turtle Rock area.

The lower viewing platform by the signature should remain closed starting at the Y with the upper viewing platform.

The upper viewing platform may be re-opened.

The barriers and fence downslope of Blocks 1, 2, and 3 should remain in place.

Additional rockfall signage is required, especially downslope of the lower rock area.

Additional fencing or rockfall barriers are required downslope of the lower rock area.

6.0 TASK 05, 07, 08: 3DEC MODELING

The Phase 1 stability analysis assessments were conducted using Itasca’s 3DEC software (2016).

The assessments were conducted assuming rigid rock blocks. 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. The Phase 2 assessments were conducted with improved block geometry (due to the Phase 2 site investigation) using deformable blocks.

This section addresses three Phase 2 tasks: develop a modeling plan, update the 3DEC models using information from the Task 2 site investigation, and conduct 3DEC modeling. Section 8.2 also discusses 3DEC modeling in regard to assessing the effectiveness of the remedial measures.

6.1 Signature Block Area 3DEC Analysis

The Signature Block area modeled using 3DEC is illustrated in Figure 16. The modeling area included Blocks 1, 2, and 3 (shown in Figure 2) and covered approximately 90 ft in length and 50 ft in height. Figure 17 shows the rock blocks in the Signature Block area formed by joints and bedding planes. The geometry was created using the procedure described in the Phase 1 report (Brose et al., 2021) then modeled in 3DEC. Appendix C provides the full details of the Signature Block area 3DEC analyses, including the assessment of the effectiveness of the remedial measures.

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Figure 16 3D mesh for the Signature Block area, from Phase 1 photogrammetry.

Figure 17 Signature Block model in 3DEC.

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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 following 3DEC modeling procedure was developed:

The model was brought to equilibrium using elastic properties.

The model was changed to plastic properties (Mohr-Coulomb Constitutive Model) and brought to equilibrium again.

Instability was tracked via displacement histories and velocity in the model.

To determine the stability of blocks, two analyses were developed:

o Siltstone was incrementally removed from the model:

The maximum extent of the weathered shale layer (the shale limit) was traced around the Signature Block area (Figure 18).

Up to 6 ft of siltstone was removed in 0.5-foot increments.

o A strength reduction analysis was performed on all materials and all joints/bedding. (The strength reduction factor (SRF) was applied to cohesion, friction, and tension in the sandstone and siltstone volumes as well as all joints and bedding planes).

Figure 18 Signature Block plan view illustrating model extent and shale limit.

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Velocity contours across the Signature Block area, as well as displacement histories at various locations along the Signature Blocks, were used to determine whether the blocks in the model are stable.

For the model with incremental siltstone removal, the displacement velocities are shown in Figure

19. (Note that the model includes all the blocks and subblocks shown in Figure 17, although Figure 19 shows history results for only 16 locations (two on Block 1a, one on Block 1b, eight on Block 1c, four on Block 2 and one of Block 3.) The dashed vertical line represents when each siltstone cut was removed from the model (each cut is 0.5 ft of siltstone removal). The stage at which the block became unstable is highlighted in red. The displacement histories show that Block 2 becomes unstable with 2.5 ft of shale removal (Cut 5) and Blocks 1 and 3 become unstable at 3 ft of shale removal (Cut 6).

This instability is also seen in the velocity contours. Figure 20 shows the velocity contours at 2.5 ft of siltstone removal (Cut 5 shown on left) and 3 ft of siltstone removal (Cut 6 shown on right). At

2.5 ft of siltstone removal, Block 2 is showing continued movement, indicating instability. Once increased to 3 ft of siltstone removal, all blocks in the Signature Block area show continued movement.

Figure 19 Signature Block displacement histories for incremental siltstone removal.

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Figure 20 Signature Block velocity contours for Cuts 5 and 6.

Using a similar process for the model with global strength reduction, the displacement velocities are shown in Figure 21. The dashed vertical line represents when each strength reduction occurred.

The stage at which the block became unstable is highlighted in red. The displacement histories show that Block 2 becomes unstable with an SRF of 1.3 and Blocks 1 and 3 become unstable at an SRF of 1.6.

This instability is also seen in the velocity contours. Figure 22 shows the velocity contours at an SRF of 1.3 (shown on left) and 1.6 (shown on right). At an SRF of 1.3, Block 2 is showing continued movement, indicating instability. Once increased to an SRF of 1.6, all blocks in the Signature Block area show continued movement.

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Figure 21 Signature Block displacement histories for global strength reduction.

Figure 22 Signature Block velocity contours for global strength reduction.

A summary of the siltstone removal model and the global strength reduction model are shown in Figure 23 and Figure 24, respectively.

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Figure 23 Amount of siltstone removal to cause instability.

Figure 24 Global SRF stability.

6.2 Turtle Rock Area 3DEC Analysis

The geometry of the Turtle Rock area and the resulting 3DEC model blocks are illustrated in Figure

25. The site investigation task provided improved information about the jointing behind the Turtle Rock body, confirmed the movement of Block 3, and provided improved information about the Turtle Rock head and supporting rock. The task also reconfirmed that the character of the siltstone is much different at Turtle Rock than at the Signature Block area. Approximately 50 percent of the

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Turtle Rock siltstone beds are composed of sandstone. Therefore, siltstone material properties were increased. Horizontal ubiquitous joints were added to the siltstone to simulate low strength along horizontal layers of weak shale. Appendix D provides the full details of the Turtle Rock area 3DEC analyses, including the assessment of the effectiveness of the remedial measures.

Figure 25 Turtle Rock area geometry and 3DEC blocks.

Phase 1 3DEC analysis of the Turtle Rock area showed that Block 3 is unstable. The Phase 2 analysis confirmed this finding, as illustrated by the displacement vectors plotted in the right image of Figure 25. Phase 2 analysis also found that Block 4 is marginally stable and Blocks 1 and 2 are stable. The failure mode of Block 3 is toppling, which is confirmed by the large aperture of the joints behind the block observed at the site. Block 3 was subsequently removed from the model.

Block 4 then stabilizes. Removing Block 3 has no significant impact on the stability of Block 1 or 2.

A global strength reduction was performed to evaluate the factor of safety of the Turtle Rock area (see Figure 26). A strength reduction factor (SRF) was applied to cohesion, friction, and tension in the sandstone and siltstone volumes as well as all joints. The siltstone layer fails to support Block 2c and the Turtle Rock back block (Block 1b) at an SRF between 1.7 and 1.8. Blocks 2a and 2b are stable. The Turtle Rock Head (Block 1a) is marginally stable. It is likely stable after Block 1b is gone. Block 4 is stable.

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Figure 26 Turtle Rock global strength reduction results.

The crane access during the Task 2 site visit afforded a close-up view of the Turtle Rock head and supporting rock. Figure 27 shows two views of the rock pillar supporting the head. This rock pillar appears to be fractured, detached by a natural joint from the rock supporting the rest of the head, and with more friable layers above and below.

A partial SRF was done to evaluate the effects of ongoing deterioration of this pillar. The siltstone strength was not reduced. The model was found to be stable at an SRF greater than 3. Because the sandstone pillar appears more friable and fractured than the sandstone sampled for laboratory testing, the starting strength for this rock may be too high. The pillar geometry used in this analysis is only approximate because the photogrammetry was not able to capture detailed geometry in this area; therefore, the width may be approximately 50 percent less in the area of high compression.

Taken together, the fractured nature and uncertain geometry suggest that this analysis is unreliable.

The pillar was completely removed from the model, at which point the Turtle Rock head immediately becomes unstable, as shown in Figure 28. The failure mode is rotational toppling.

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Figure 27 Turtle Rock rock pillar.

Figure 28 Turtle Rock head analysis with the rock pillar removed.

Conclusions of the Turtle Rock analyses include the following:

Block 3 is currently unstable but has not fallen likely due to interference of large asperities on the joint between Block 3 and the adjacent blocks. The release joint at the back of Block 3 is open more than 1.5 ft, and if/when the asperities fail, Block 3 will continue to rotate out from the face of the bluff. The mode of failure is toppling.

Block 4 is marginally stable. It is possible that additional Block 3 movement will induce movement in Blocks 1 and 4.

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Block 1b (the Turtle back) and the underlying Block 2c is stable with an SRF likely greater than 1.5. The mode of failure is shearing of the siltstone toe followed by toppling of the sandstone blocks.

Block 1a (the Turtle head) appears to be stable with minimal support, although there are uncertainties in this conclusion as discussed above. The mode of failure is loss of support from the slender pillar (Block 1e) followed by toppling of the block.

Blocks 2a and 2b are currently stable with an SRF greater than 1.5.

7.0 TASK 09: ROCKFALL ANALYSIS

Rockfall analysis is the assessment of where, how far, and with what velocity and energy unstable blocks will fall, slide, bounce, and roll. The rock fall analysis was conducted using Trajec3D, a three-dimensional rigid body rock fall analysis program that can simulate the trajectory of volumetric bodies during free fall, bouncing, sliding, and rolling. The body shape is selected from a toolbar, and the size is a function of the selected geometry, mass, and rock density. The physics interaction between materials is a function of the combined properties of the fall body and the impact surface, and three parameters: coefficient of restitution (elasticity), static friction angle, and dynamic friction angle. The surface mesh will be created by importing the geometry obtained from photogrammetry (either dxf or point cloud).

Rockfall analysis was conducted only for the Signature Block area due to the following considerations:

The Signature Block area appears to be the most visited area and has adjacent segments of the boardwalk, the ground is sloped down to the flat field, and the toe of the slope is readily accessible.

In contrast, the area adjacent to and under Turtle Rock is relatively inaccessible and is essentially a vertical slope down to the flat field elevation. A rockfall assessment of the Turtle Rock area may be added to the scope if desired by the BLM.

The rockfall analysis had five well-defined steps:

Field reconnaissance of the slopes around the Signature Block area to collect information on block size, shape, distance travelled, and source location. The source location will be based on examination of the block to determine which stratigraphic unit it came from.

Calibrate the coefficient of restitution in 3DEC or Trajec3D based on the information collected in the first step. The coefficient of restitution is the main source of uncertainty in rock fall analyses. See Figure 29 for calibration phase results.

Identify block sizes and shapes that might fall from the Pillar. This work will be based on the site geology, analysis of discontinuity spacing, photogrammetry, and the 3DEC stability analyses.

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Perform forward analyses to develop a size-distance relation for rockfall runout. Multiple models will be built and run to account for different rock sizes and shapes of the potential falling rock.

If necessary, repeat the step above assuming a barrier near the toe to arrest rockfall.

At the Signature block area, several small blocks were observed near the top of Blocks 1, 2 and 3.

These blocks have the potential to fall, impacting areas below and runout to areas at the base of the pillar. The largest of these blocks has approximate dimensions of 3 ft x 2 ft x 1 ft, or approximately 6 cubic feet. The initial geometry and orientation of these blocks relative to the slope is critical to accurately predict the runout characteristics. Generic block shapes of about the same size as the blocks located in the Clark signature area were used for the initial calculations.

The initial block orientation was randomized for each drop. Four realizations were completed with 158, 224, 251, and 477 blocks respectively. The results for the fourth realization are shown in Figure 30.

Figure 29 Rockfall calibration phase results.

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Figure 30 Results for 477 blocks of realization 4.

A rockfall risk assessment was made based on the percent of time people are below the slope in the runout area. In this assessment, risk is defined as the compound relation between hazard and consequence. Hazard is the probability of runout into some area, and the consequence is expressed as the potential for fatalities. The approach used to estimate the hazard and consequence probabilities will be discussed later.

Work done by Whitman (1984) and Steffan et al. (2015) shows a relation between annual probability of failure and the number of fatalities that may occur. This relation, as adapted by Wyllie (2018) is shown in Figure 31 and establishes a reference with which to compare the risks estimated in this study.

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Figure 31 Risks for selected engineering projects and individual risk (Wyllie, 2018).

The approach used to assess the probability of a person being impacted by rockfall must consider the percentage of time that people are present in the runout area. This is analogous to the approach used by the FHWA to assess the rockfall risk to cars traveling along a highway, which is referred to as average vehicle risk (AVR). AVR is defined as the percent of time that vehicles are present in a rockfall section of highway. Modifying the AVR relation to suit people traversing through the runout area results in a relation that will be called the average person risk (APR). It is the percentage of time a person is expected to be present in the runout area, and the relation is as follows:

24 100% where ADV is the average daily visitor count, L is the length of exposure, and S is the average speed at which people walk through the area.

The average annual number of visitors to PPNM including years 2013 through 2019, is approximately 27,000 (Reffit, 2021). This value was used as a baseline assumption in the calculations resulting in an ADV of approximately 76 people per day.

The length of exposure in the Block 1 runout area is 250 ft based on the length of the trail running through the runout area. This length assumes that several rocks fall at the same time, impacting all areas of the runout area in a single event. If the event is simply a single rock, like the one that fell

Itasca Consulting Group, Inc. Page 30 www.itascacg.com in 2011, then the length of exposure is reduced from the entire runout area to a smaller area that only includes the path of the single rock. This smaller area will be referred to as the danger area.

A comparison of the extent of the runout area and the danger area is shown in Figure 32. Although the theoretical width of the danger area is simply the largest dimension of the rock block, we have assumed a slightly larger value to account for erratic movements or pieces separating from the block while rolling. Therefore, the width of the danger area is assumed to be 40 feet.

Figure 32 Relative extent of the runout area and danger area.

The last variable to consider for estimating APR is the speed of visitors through the runout area. A typical average walking speed is 3 to 4 mph (Browning et al., 2006). The typical PPNM visitor, however, will likely traverse the area at a slower speed as they stop to enjoy the views and take pictures. Taking this into consideration results in an assumed average speed through the area of 1 mph.

Using the previously mentioned assumptions combined with the APR relation results in the percentages listed in Table 2.

Table 2 Average Person Risk

Exposure APR

Runout Area 27%

Danger Area 4.3%

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Estimation of the annual probability of failure of a rockfall event must consider two elements: 1) the annual probability of a rockfall event, and 2) the probability that the runout will extend a certain distance. The last recorded rockfall event occurred 10 years ago in 2011. This resulted in a block that entered the runout area beyond the toe of the talus. There is no record of when any other rockfall in the area occurred. This single data point suggests the annual probability of an event occurring is one in ten. If an event were to occur this year, that would suggest an annual probability of two in ten. This conservative assumption of a 20% probability was used in the analysis. The second element to consider is estimating the probability that the rockfall event will reach the path where visitors are walking. This value was estimated from the rockfall simulations. The product of these two probabilities results in the annual probability of a rock landing on or crossing the trail.

Figure 33 compares the risk of fatality considering the trail at different locations with the risk of fatality in other circumstances. For example, the trail has been moved 100 ft from the toe of the talus. The fatality risk comparable with this distance assuming many rocks in the event is “home accidents”, whereas the fatality risk comparable with this distance assuming a single rock is “alcohol (light drinking)”.

Figure 33 Comparison of rockfall fatality risk with other risks.

In the signature block area, a few blocks of the size simulated were identified during the site visit.

Until scaled, these blocks have the potential to impact the lower viewing platform at the signature area. The upper viewing platform will likely not see any impact from rockfall. The risks associated with impacts to existing walking paths are discussed above.

In the lower area, three blocks have the potential to become unstable. If they start rolling, there is a potential impact to the existing sidewalk at the toe of the slope. There is also a potential impact to the boardwalk from the block nearest the boardwalk.

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Refer to Appendix E for full details of the rockfall analyses.

8.0 TASK 10, 11, 14: REMEDIAL MEASURES

8.1 Descriptions

Three tasks addressed remedial measures. In Task 10 the remedial measures were identified, in Task 11 3DEC was used to assess the effectiveness of the remedial measures, and in Task 14 additional details were developed. Appendix F contains the deliverables from all three tasks. Note that the 3DEC assessments were focused on effectiveness but were inadequate for final design.

Table 3 lists the remedial measures with descriptions, where applicable, and examples. Appendix F contains a full description.

Table 3 Descriptions and Examples of Remedial Measures

# Remedial Measure Description Where Applicable

Examples

1 Foliage control Remove foliage growing in joints and cracks

SB Kill and remove all woody foliage growing in joints and cracks

2 Top of sandstone drainage control

Control and divert water at top of blocks

SB Construct diversion structures and trenches to direct water away from the top of SB

3 Water diversion in joints

Divert water that enters joints

SB, TR, LRA Place sealing features in joints near the block tops to prevent water from going deeper into open joints

4 Shale & siltstone environmental protection

Install measures to protect the siltstone and shale from water

SB, TR, LRA Place insulation to control environmental variations

5 Shale & siltstone stabilization

Install measures to stabilize the siltstone and shale

SB, TR, LRA Place rockbolt, mesh, and shotcrete structures

6 Large Block Removal

Proactive removal of large rock blocks

SB, TR, LRA Remove Signature Block 1a, Turtle Rock Blocks 3 and 4, loose upper rock on Signature Block 2

7 Scaling Proactive removal of small rock blocks

SB, TR, LRA Remove small rock blocks on Signature Block 2 and 3

8 Rockfall controls Barriers and ditches, fencing, signage

SB Place various types of barriers and fences in critical areas

9 Joint treatments Inject various grouts to strengthen joints

SB, TR, LRA Inject grout to stabilize joints

10 Rockbolts and cable bolts

Install reinforcing to stabilize rock blocks

SB, TR, LRA Place reinforcing in east face wedge on Signature Block 1, Turtle Rock Blocks 3 and 4, Turtle Rock "chin" support

11 Buttresses and Underpinning

Install structural features that provide vertical & lateral support

SB, TR, LRA Install concrete or shotcrete beams under specific potentially unstable blocks

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The remedial measures vary in effectiveness, aesthetics, cost, risk, agency considerations, and engineering requirements.