GeotechRpt.pdf

Final Geological Engineering Report Rock Slope Investigation and Remediation Design Great Smoky Mountains National Park Blount County, Tennessee

EFLHD Reference No. HFTS-15

Schnabel Project No. 07120183.00

January 25, 2008

"We are committed to serving our clients by exceeding their expectations."

Geotechnical • Construction Monitoring • Dam Engineering • Geoscience • Environmental

656 Quince Orchard Road, Suite 700 Gaithersburg, MD 20878

Phone: (301) 417-2400 Fax: (301) 417-2730 www.schnabel-eng.com

Mr. Andre Anderson Eastern Federal Lands, Highway Division Federal Highway Administration 21400 Ridgetop Circle Sterling, Virginia 20166

Subject: Final Geological Engineering Report

Rock Slope Investigation and Remediation Little River Gorge Road Great Smoky Mountains National Park Blount County, Tennessee Schnabel Project No. 07120183.00

EFLHD Reference No. HFTS-15

Dear Mr. Anderson:

Schnabel Engineering, LLC (Schnabel) is pleased to submit this geological engineering report for the above referenced project. Our services and this report are provided in accordance with

Federal Highway Administration, Eastern Federal Lands Highway Division (EFLHD) Contract No.

DTFH71-06-D-00001 dated November 15, 2005, Task Order No. 08-001 dated October 10, 2007 and the Notice to Proceed dated October 17, 2007.

Scope of Services

Our services are provided in accordance with the EFLHD Notice to Proceed and Scope of

Work for Task Order No. 08-001. Our understanding of the work is described herein. The scope of our services includes:

1. Review background information and field reports provided to us, as well as available topographic and geologic maps;

2. Field geologic structure mapping;

3. Kinematic and limit equilibrium rock slope stability analyses;

Mr. Anderson, EFLHD

Project No. 07120183.00 2 Schnabel Engineering, LLC

4. Develop two slope stabilization design concept options (including “rock gluing” as one option) and comparative cost estimates;

5. Communicating with the COTR, PM, and NPS personnel the concept design options and comparative cost estimates; and

6. Preparation of the Final Geological Engineering Report and design plans, specifications and engineers cost estimate (PS&E) for the selected concept design option.

A Draft Geological Engineering Report, dated December 18, 2007, was submitted to

EFLHD. The draft report review meeting was held on January 8, 2008 to discuss with EFLHD and National Park Service (NPS) personnel the design concepts presented in the draft report.

Schnabel received written review comments from EFLHD on January 10, 2008 with instructions to proceed with preparing this final report and the PS&E for the Option 1 design concept

(Gluing, Bolting, Shotcreting) presented herein. This final report addresses the EFLHD review comments and issues discussed in the review meeting.

We appreciate the opportunity to be of service to you for this project. Please contact Mr.

Brian Banks at (301) 417-2400 or Mr. George Aristorenas at (703) 779-0773 if you have any questions regarding this report.

Very truly yours, SCHNABEL ENGINEERING, LLC

Prepared By:

Brian Banks, C.P.G.

Senior Associate

George Aristorenas, P.E.

Senior Associate

Reviewed By:

Eric Rehwoldt, P.E., C.P.G.

Principal

BB/GA/ER/RR/amm G:\2007\07120183\WP\DRAFT\07120183 00 FINAL Geologic Report 08_01-25.doc amarchetti

ER

amarchetti Banks amarchetti George Aristarenous

Project No. 07120183.00/January 25, 2008 1 Schnabel Engineering, LLC

Table of Contents

1.0 Executive Summary

2.0 Introduction

2.1 Background

2.2 Site Description

3.0 Geologic Descriptions

3.1 Regional Geology

3.2 Field Measurements

3.3 Findings

3.3.1 Rock Description

3.3.2 Major Discontinuities

3.3.3 Groundwater Conditions

3.3.4 Typical Rockfall Block Sizes

4.0 Assessment of Geologic Hazards

4.1 Kinematic Analysis

4.2 Limit Equilibrium Analyses

4.2.1 Planar Sliding

4.3 Assessment of Load Supported by Key Block

4.4 Assessment of the 2006 Rockfall Event

4.5 Failure Modes for Stabilization Design

5.0 Recommendations

5.1 Option 1 – Gluing, Bolting, Shotcreting

5.1.1 Rock Gluing

5.1.2 Rock Bolting

5.1.3 Spot Bolting

5.1.4 Scaling

5.1.5 Drainage Strips

5.1.6 Shotcreting

5.1.7 Construction Sequence

5.1.8 Periodic Monitoring and Maintenance

5.2 Slope Layback

6.0 Cost Estimates

6.1 Gluing, Bolting, Shotcreting

6.1.1 Rock Bolting

6.1.2 Shotcreting

6.2 Slope Layback

7.0 Limitations

Project No. 07120183.00/January 25, 2008 2 Schnabel Engineering, LLC

List of Tables

Table 1: Summary of Limit Equilibrium Analysis Results Table 2: Cost Estimate for Rock Gluing Table 3: Cost Estimate for Rock Bolting Table 4: Cost Estimate for Shotcreting Table 5: Cost Estimate for Slope Layback Table B-1: Discontinuity Survey Data Summary Table Table C-1: Reinforcement Capacities by Rock Gluing and Bolting

Appendices Appendix A: Figures

Figure 1: Site Vicinity Map Figure 2: Topographic Site Plan Figure 3: Cross Section Sketch through Big Rock Figure 4: Geologic Map Figure 5: Discontinuity Orientations Figure 6: Kinematic Analysis – NE Face of Big Rock Figure 7: Kinematic Analysis – SE Face of Big Rock Figure 8: Kinematic Analysis – Cut Slope Below Overhang Figure 9: Kinematic Analysis – Slope Layback Option 2 Figure 10: Mitigation Concept Layout on Rock Face – Option 1 Figure 11: Mitigation Concept in Cross Section – Option 1 Figure 12: Mitigation Concept in Cross Section – Option 2

Appendix B: Field Data Table B-1: Discontinuity Survey Data Summary Table (1 Sheet) Discontinuity Survey Data Sheet (5 Sheets) Descriptive Criteria for Rock Core Logging (2 Sheets)

Appendix C: Calculations

Interface Friction Angle Calculations (1 Sheet) Limit Equilibrium Analysis Results – Planar Sliding (4 Sheets) Analysis for Keyblock Loading and Reinforcement (4 Sheets) Analysis for Global and Planar Sliding Reinforcement (4 Sheets)

Appendix D: Photographs (16 Sheets)

Appendix E: Material Safety Data Sheets (13 Sheets)

Project No. 07120183.00/January 25, 2008 1 Schnabel Engineering, LLC

1.0 Executive Summary

Based on our evaluation of data from the field investigation and the project data furnished to us, we have developed the following conclusions and recommendations. This Executive

Summary has been prepared for the convenience of the users of this report and does not contain all of the information presented in this report. Therefore, the entire report should be read to assure transmission of all pertinent information. Detailed recommendations are presented in the body of this report.

1. Four major discontinuity sets (Planes 1 through 4) were identified during the geologic structure field mapping and considered in our analyses.

2. Based on our site observations analyses, and our understanding of the 2006 rockfall event, we believe there is a significant risk of a similar rockfall event in the future. Additionally, we believe there is a significant risk of local- and global-scale failure within Big Rock. Therefore, we recommend that rock slope stabilization measures be performed.

3. Based on our kinematic and limit equilibrium analyses, and consideration of the likely cause of the 2006 rockfall event, we conclude that the rock slope stabilization design must consider the following failure modes:

a. Local- and Global-scale planar sliding along Plane 3 discontinuities.

b. Global failure within Big Rock in response to Big Rock’s tendency to rotate forward and downward toward the overhang area.

4. We developed recommendations for two design concepts to reduce the potential for global- and local-scale planar sliding mechanisms indicated in our limit equilibrium analyses, and global rotation-induced failure of Big Rock. The mitigation design concepts include rock gluing, passive rock bolting, and shotcreting (Option 1), and slope layback (Option 2).

Project No. 07120183.00/January 25, 2008 2 Schnabel Engineering, LLC

5. For the Option 1, we recommend injecting PUR grout in 15 locations. The injection holes shall be about 1 ½” in diameter, and shall be generally oriented horizontal and perpendicular to the road alignment. We recommend 2 columns of glue injection holes are installed in Big Rock with a vertical spacing of 10 ft on center. The vertical spacing of glue holes below the overhang shall be about 5 ft.

The 11 injection holes recommended for Big Rock will be drilled to depths of 30 ft behind the slope faces. The 4 glue holes below the Big Rock overhang will be drilled to depths of 15 ft.

6. The Option 1 concept involves installing permanent, passive rock bolts in conjunction with rock gluing. Two types of rock bolts are proposed: horizontal rock bolts with a relatively small diameter, and diagonal rock bolts with a relatively large diameter. The small-diameter, horizontal rock bolts are recommended to reduce the local-scale rockfall hazard associated with planar sliding failure mechanisms indicated in our analyses. The large-diameter, diagonal rock bolts will be used to counteract the rotational tendency of Big Rock and reduce the potential loading increase on the key block over time. Both sets of bolts will reduce future propagation and opening of existing fractures and development of additional fractures.

7. For the horizontal rock bolts in Option 1, we recommend 1-in diameter, grade 75

(75 ksi ultimate tensile strength) bars placed within a 2.5-in diameter bore hole and grouted fully along the length. The bars should be protected from corrosion

(e.g., epoxy coated or galvanized and completely surrounded by cement grout).

Two columns of horizontal rock bolts will be installed in Big Rock including one column in each of the front faces. One additional rock bolt will be installed in the potentially unstable block at the upper front corner (see Section 5.1.3 below). We recommend the horizontal bolts are a minimum of 25 ft long.

Project No. 07120183.00/January 25, 2008 3 Schnabel Engineering, LLC

8. For the diagonal rock bolts in Option 1, we recommend 1 3/8-in diameter, grade

150 (150 ksi ultimate tensile strength) bars placed within a 4-in diameter bore hole and fully grouted. The bars should be protected from corrosion (e.g., epoxy coated or galvanized and completely surrounded by cement grout). Four diagonal rock bolts will be installed from the overhang area of Big Rock. Each of the diagonal rock bolts will extend all the way thorough Big Rock and daylight at the back (approximately 60 ft long). We recommend the bolts are spaced a minimum of 5 ft apart on center. The bolts should be inclined at about 45 degrees from horizontal.

9. In addition to pattern bolting, some larger rock blocks can be individually bolted

(“spot bolted”) to the slope face using passive rock bolts. Spot bolting is recommended for the potentially unstable rock block at the upper front corner of

Big Rock. We recommend 1 rock bolt is installed through the front corner of this rock block with a depth of 25 ft. The recommendations provided for horizontal rock bolting shall be used for this spot bolt.

10. We recommend that scaling be performed in the area below the Big Rock overhang as the first step in Option 1 to prepare the rock mass in this area for further reinforcement, and as a safety precaution to protect workers by reducing the potential for localized rockfall during reinforcement construction activities.

Scaling should be performed using hand methods. Care should be taken not to over-excavate such that the stability of the overlying Big Rock is compromised.

However, we recommend that loose material and sediments be removed.

11. Option 1 also includes installation of drainage strips in the area below the overhang to provide a pathway for water. Prior to drainage strip installation, the foliation surface and the area in general should be cleaned of loose debris. This will include hand excavation of the upper portions of the exposed fault gouge material and replacement with shotcrete (see below). Particular attention should be made to cleaning and preparing the area at the top of the exposed foliation plane from where we believe the water seepage is introduced to the foliation

Project No. 07120183.00/January 25, 2008 4 Schnabel Engineering, LLC plane. Dowels to secure the drainage board onto the rock surface shall be installed after cleaning and initial shotcreting.

12. Shotcreting is also recommended in Option 1 for the area below the Big Rock overhang, in conjunction with scaling, rock gluing, spot bolting and drainage board installation (see above), to reduce the potential for localized sliding failures in this area. We suggest the shotcreting is needed to replace upper portions of the exposed fault gouge material, and to provide a surface covering (over the drainage board) to further stabilize and protect the key block and other isolated blocks in this area against weathering effects over time. Shotcreting will occur in two stages. First, prior to drainage board installation, the upper portions of the fault gouge material will be hand excavated and replaced with shotcrete, as needed, to smoothen the surface to receive the drainage strips. After drainage strip installation, shotcrete will be placed as the surface covering in the area.

Reinforcing steel mesh shall be used within the shotcrete areas. We estimate a total volume of shotcrete of up to 10 cubic yards may be placed.

13. We recommend long-term monitoring is performed by qualified personnel to gauge the effectiveness of the Option 1 remediation efforts over time. Crack monitors should be installed across major fractures within Big Rock and read at least once every 6 months to check for rock mass movements, if any. Over time, we expect some small (i.e., local scale) rock blocks may become loose and either fall from the rock face or provide a potential rockfall hazard. Additionally, we expect the area of shotcrete below the overhang to show signs of weathering.

Therefore, monitoring efforts should include visual observations of the condition of the rock mass and shotcrete area to check for the development of loose and potentially unstable rock blocks, and damaged shotcrete. Removal of loose or potentially unstable rocks and repairs may be needed over time.

14. The Option 2 stabilization design concept involves removal of majority of Big

Rock’s mass and laying back the slope to an angle consistent with the surrounding slope, approximately 53 degrees. During slope layback, all loose and potentially

Project No. 07120183.00/January 25, 2008 5 Schnabel Engineering, LLC unstable rock blocks would need to be removed to provide a stable final slope surface. We also recommend scaling and rock gluing for the area beneath the current overhang as described above to reduce the risk for local-scale rockfall hazard in this area even after Big Rock has been removed. Additionally, excavation of the upper portions of exposed fault gouge material and replacement with shotcrete is recommended. Fault gouge should be excavated by hand rather than mechanical methods. Removal and replacement of as much exposed fault gouge as possible is recommended.

15. The method of rock excavation for Option 2 will be an important consideration in the final condition and stability of the rock face. We recommend that controlled blasting methods be implemented to reduce blast overbreak potential behind the final rock face, and to provide a relatively uniform rock face surface. Line drilling or pre-split blasting methods should be implemented, as well as control of charge weight of the blast. The use of poorly controlled blasting methods can result in instability of the final rock face and fall out of blast-damaged rock.

16. The slope layback option may result in an increased risk of small, local-scale rock blocks sliding down the slope and reaching the road. Because the geometry of the cut slope will be similar to that of the existing adjacent slopes, we expect the risk level of rockfall hazard for the cut slope will be similar to that of the existing adjacent slopes. Rockfall hazard protection would need to be considered for

Option 2. Rockfall hazard protection may include a rockfall barrier constructed at the base of the slope and/or a rockfall catchment area constructed at the shoulder of the existing roadway. A site-specific rockfall hazard study would be needed to determine the appropriate size for a possible catchment area and rockfall protection barrier.

Project No. 07120183.00/January 25, 2008 6 Schnabel Engineering, LLC

17. The estimated cost of the Option 1 and Option 2 design concepts is $222,050.00 and $124,300.00, respectively. These estimates do not include costs associated with rockfall protection (fence, wall, catchment area) at the slope, traffic control, construction administration, field observations by engineering personnel, other engineering consultation services during construction, aesthetic “touch-up” of rock anchor heads, and periodic monitoring and maintenance.

Project No. 07120183.00/January 25, 2008 7 Schnabel Engineering, LLC

2.0 Introduction

2.1 Background

In the fall of 2006, a rock fall occurred along the westbound travel lane of Little River

Gorge Road about 3.5 miles east of the Townsend Park Entrance to the Great Smoky Mountains

National Park (GRSM), Blount County, Tennessee (Appendix A - Figure 1). The rock fall was discovered after a heavy rain and wind storm and involved 1- to 7-ft wide rock blocks released from below an overhanging rock known as “Big Rock”. A site reconnaissance was conducted on

September 10, 2006 by Eastern Federal Lands (EFL) and National Parks Services (NPS) personnel who concluded that, although there is no evidence of movement, separation, or settlement of the surrounding rock mass, and the remaining rock mass appeared to be stable in the near term, long term remediation is necessary is reduce the potential of another rock fall event.

Schnabel Engineering North, LLC (Schnabel) has been asked to provide geological engineering services in support of EFL efforts to reduce the rockfall hazard potential of the slope. The purpose of the project is to characterize the rock slope instability hazards and to develop technical plans, specifications and an engineering cost estimate (PS&E) for remediation of the unstable rock slope. The rock slope remediation must preserve the aesthetics of the Little

River Gorge Road and the GRSM as much as possible. The scope of the work for this project is defined in Task Order No. 08-001 dated October 10, 2007. The project includes four phases of work:

1. Phase I – Geologic Mapping and Field Investigation. The scope for this phase includes reviewing available engineering data and reports provided by EFL personnel, and available topographic and geologic maps; and performing a field evaluation including geologic rock structure mapping to gather data for subsequent geo-engineering analysis.

2. Phase II – Draft Geologic Report and Preliminary Cost Estimates. The scope for this phase includes performing geological engineering analyses (i.e., kinematic and limit equilibrium rock slope stability analyses) to evaluate and characterize the rock slope hazards, and to develop two remediation design concepts

Project No. 07120183.00/January 25, 2008 8 Schnabel Engineering, LLC

(including polyurethane resin “rock gluing” as one option) with preliminary cost estimates. The Draft Geological Engineering Report, dated December 18, 2007, presented the results of our geological engineering assessment, remediation design concepts, and preliminary cost estimates.

3. Phase III – Draft Report Review Meeting. Schnabel personnel attended a meeting on January 8, 2008 to discuss our Draft Geological Engineering Report with the

COTR, PM, and NPS personnel.

4. Phase IV – Final Geological Engineering Report. The scope for this phase includes performing additional geological engineering analyses as necessary to develop the final Geological Engineering Report including the PS&E for the concept design option chosen.

It should be noted that core drilling, piezometer installation, and laboratory testing of rock strength characteristics was not included in our scope of services for this work. Rock mass properties were based on observations of the exposed rock slope face. Estimates of rock strength referenced herein are based on field index testing. Rockfall hazard modeling and rockfall mitigation design for excavated slopes was also excluded from our scope of services outlined above. These services may be provided as a modification to the authorized Task Order.

2.2 Site Description

Little River Gorge Road extends generally east to west between the Metcalf Bottoms picnic area and the intersection with Laurel Creek Road near the Tremont ranger station.

However, the road winds through the area, following the trend of meandering Little River. At the site, Little River Gorge Road is oriented approximately due north-south. The road separates

Little River to the east from the existing road cut slope to the west.

The subject rock slope is referred to as “Big Rock”, which outcrops along the right shoulder of the eastbound lane, to the west of the roadway. In plan view, Big Rock is roughly diamond shaped, about 30-50 feet wide on each side. Big Rock is oriented such that Big Rock’s eastern corner extends over the road. The photographs provided in Appendix D illustrate site conditions at the time of our field investigation on November 6 and 7, 2007.

Project No. 07120183.00/January 25, 2008 9 Schnabel Engineering, LLC

The side faces of Big Rock are nearly vertical. Big Rock is about 70 feet high at its front corner above the road and rises an additional 20 ft toward its back corner away from the road.

The natural slope around Big Rock rises away from the road at about a 50 degree angle on average such that only about 10 ft of Big Rock is exposed above the slope at its back corner. A topographic map is provided as Figure 2 in Appendix A. However, we observed that the southwest and northwest faces of Big Rock are poorly defined in the topographic map due to the near-vertical inclination of the rock faces.

Little River Gorge Road was constructed through the toe of Big Rock. The resulting cut slope geometry in cross section includes an approximately 15-ft high rock overhang that extends about 5 ft over the eastbound lane (Appendix A – Figure 3; Appendix D). A relatively small boulder measuring approximately 3 to 4 ft across is wedged beneath the overhang and the underlying outcrop. It supports some of the weight of Big Rock as evidenced by the results of the attempts to remove it during rockfall cleanup efforts. We understand workers wrapped a steel cable around the boulder and tried to free it by pulling with a front-end loader until the steel cable eventually snapped.

The overall slope that contains Big Rock is thought to be the natural slope for the Little

River Gorge, and is covered with thick forest vegetation typical of the region including a variety of trees, shrubs and vines. Other large outcrops in addition to Big Rock are exposed along the slope, some nearly as massive as Big Rock. Several small outcrops are exposed as near-vertical road cut slopes along the shoulder of Little River Gorge Road to the north and south of Big

Rock.

We understand that Big Rock is a local icon. This land feature is well known because of its overhang above the road and for it’s resemblance to a person’s head in profile. We understand that preservation of the appearance of Big Rock is a high priority, if it can be safely preserved.

Project No. 07120183.00/January 25, 2008 10 Schnabel Engineering, LLC

3.0 Geologic Descriptions

3.1 Regional Geology

The site is located in the Western Blue Ridge Physiographic Province, underlain predominantly by heavily folded and faulted metamorphosed sedimentary rocks. These ancient rocks were formed during multiple episodes of tectonic activity. The regional geologic structure we see today, including large-scale, northeast-trending thrust faults and folds, is the result of the most recent tectonic episode: the collision of the North American and African plates and uplift of the entire Appalachian mountain chain from about 310 to 425 million years ago. Since then, the

Atlantic Ocean was formed and the Appalachian Mountains have been eroding, revealing once deeply buried rocks.

Based on the Generalized Geologic Map of Bedrock Lithologies and Surficial Deposits in the Great Smokies National Park Region, Tennessee and North Carolina (Southworth et al., 2004; Appendix A – Figure 4), the bedrock at the site is Metcalf Phyllite of the Snowbird Group.

The map describes Metcalf Phyllite as “Lustrous, greenish-gray, chlorite-sericite phyllite and laminated metasiltstone; strongly sheared with multiple cleavages.” We characterized the rock type, based on our observations, as mylonitic phyllite, which is to say it is a strongly foliated and lineated, fine-grained, metamorphosed sedimentary rock whose crystalline texture has been modified by ductile shearing within a deep-seated shear zone. Our observations and characterization of the exposed site bedrock is consistent with the map description. The contact with the structurally underlying Cades Sandstone geologic unit occurs just to the south of the project site and is defined as a thrust fault.

3.2 Field Measurements

Mr. Brian Banks, C.P.G. and Ms. Robin Reed, C.P.G. performed geologic rock structure mapping at the site on November 6 and 7, 2007 to collect field data in support of our geological engineering analyses and remediation design development. Sketches showing the estimated shape and dimensions in cross-section were made. Measurements were made of typical rockfall block size and shape.

In addition, other data was collected to characterize the rock mass including:

Project No. 07120183.00/January 25, 2008 11 Schnabel Engineering, LLC

1. Rock description information including rock type, strength, weathering, fracture spacing, color, and foliation character. The terms used to describe the rock mass are defined in the Descriptive Criteria for Rock Core Logging included in

Appendix B.

2. Data characterizing individual discontinuities, including:

a. Location,

b. Discontinuity type,

c. Orientation (i.e., dip and dip direction),

d. Estimate of the Joint Wall Compressive Strength (JCS) based on hardness index testing,

e. Joint Roughness Coefficient (JRC, following Barton and Choubey, 1977),

f. Infilling type and coverage, and

g. Water condition.

3.3 Findings

3.3.1 Rock Description

The rock mass that comprises Big Rock above the overhang is characterized as mylonitic phyllite: weak, highly weathered, very slightly fractured, light reddish gray, and strongly foliated. The terms used to describe the rock mass are defined in the Descriptive Criteria for

Rock Core Logging included in Appendix B.

The rock mass below the Big Rock overhang is also mylonitic phyllite. However, it is highly to moderately fractured with a 0.5 to 2.0 ft wide zone of soil-like fault gouge material directly beneath the overhang. This is the area from which the recent rock fall event occurred.

The majority of the fracture surfaces are highly weathered giving an irregular, subrounded appearance to the rock block faces. Rock blocks in this zone range in size up to several feet across.

We observed several prominent fractures within Big Rock, most notably on the NE and

SE faces. Photograph No. 1 in Appendix D shows a near-horizontal fracture in the upper right portion of the face, a near vertical fracture in the upper center portion, and a block bounded by fractures at the upper left corner of the face. The central vertical fracture was observed to splay

Project No. 07120183.00/January 25, 2008 12 Schnabel Engineering, LLC in multiple directions near the middle of the rock face. Photograph No. 11 shows a prominent fracture located at the upper right corner of the SE face of Big Rock. This fracture is a continuation of the fracture observed at the upper left corner of the NE face, as shown in

Photograph No. 13. The rock block defined by this fracture comprises the upper front corner of

Big Rock. The observed fractures exhibit apertures up to 4 inches, as exhibited in Photograph

Nos. 10, 12 and 14.

The fault gauge observed in the area below the Big Rock overhang appears to terminate directly below the Big Rock overhang. Photograph Nos. 6 and 7 show the steeply dipping fault gouge zone. However, the fault gouge does not extend upward into Big Rock. We were not able to trace the zone of fault gouge beneath Big Rock.

3.3.2 Major Discontinuities

A total of 68 individual discontinuities were inspected during our rock structure survey, including 47 from Big Rock and the immediate vicinity, and an additional 21 from nearby outcrops. Appendix B includes a table (Table B-1) summarizing the discontinuity data gathered for use in our kinematic and limit equilibrium analyses and the Discontinuity Survey Data Sheets used to collect rock structure data in the field.

The orientations of these discontinuities were plotted using the computer program

Rockpack III (produced by RockWare Inc.). The Rockpack stereonet plot attached as Figure 5 shows the discontinuity orientation measurements. The stereonet plots help to visualize the clustering of orientation data. Each data cluster represents a discontinuity set, such as foliation and joint sets. The average orientations of the planes associated with the major clusters are considered to represent the average orientation of each discontinuity set. The clusters also indicate the variability in the orientation of each discontinuity set. We identified the following major discontinuity sets to consider in our analyses (refer to Table B-1 and Figure 5):

Plane 1: Joint set with an average orientation of 86°/210° (dip/dip direction). Plane

1 joints form the northeast and southwest faces of Big Rock. The JCS is estimated to be 3,000 psi. The average JRC is about 16.

Project No. 07120183.00/January 25, 2008 13 Schnabel Engineering, LLC

Plane 2: Joint set with an average orientation of 83°/127° (dip/dip direction). Plane

2 joints form the southeast and northwest faces of Big Rock. The JCS is estimated to be 3,000 psi. The average JRC is about 14.

Plane 3: Foliation with an orientation that has a consistent dip and moderately variable dip direction. For our analyses, we considered two average orientations of 44°/122° and 42°/175° (dip/dip direction). The JCS is estimated to be 3,000 psi. The average JRC is 14.

Plane 3 foliation surfaces are adversely oriented with respect to slope stability (refer to Section 4.1 – Kinematic Analysis). It is believed that sliding along a Plane 3 foliation surface contributed to the 2006 rockfall since it appears that a Plane 3 surface defines the base of the rockfall source area. Some iron staining streaks were observed along the rockfall sliding surface indicating water seepage was a contributing factor.

Plane 4: Joint set with an average orientation of 19°/052° (dip/dip direction). A

Plane 4 joint forms the major portion of the bottom face of Big Rock beneath the overhang. The JCS is estimated to be 3,000 psi. The average

JRC is about 16.

3.3.3 Groundwater Conditions

No water seepage was observed during our site investigation. However, evidence of previous water seepage (streaks of iron staining) was observed in the Plane 3 foliation surface that defines the 2006 rockfall sliding surface. The iron staining streaks suggest that water seepage is common on this surface. However, we believe that the water seepage at this location is a transient condition related to stormwater runoff in the area. During a rain event, stormwater seeps into the rockfall area. Likewise, stormwater may seep into the fractures within Big Rock.

During winter months, seepage may freeze and contribute to ice wedging.

Project No. 07120183.00/January 25, 2008 14 Schnabel Engineering, LLC

3.3.4 Typical Rockfall Block Sizes

Based on conversations with NPS personnel, the 2006 rockfall involved two rock blocks up to about 6 ft across (Photo No. 9, Appendix D) and several smaller blocks. Several rock blocks that appeared to be loose below the Big Rock overhang range in size up to about 4 ft across. We did not encounter rock blocks lying on the ground at the base of the existing rock slope.

Project No. 07120183.00/January 25, 2008 15 Schnabel Engineering, LLC

4.0 Assessment of Geologic Hazards

4.1 Kinematic Analysis

Kinematic stability analyses was performed to evaluate potential rock slope failure modes controlled by planar rock mass discontinuities, based exclusively on the geometric relationships of the discontinuities observed and measured at the site (Figure 6). Potential rock slope failure modes include sliding of rock blocks along a single planar discontinuity, sliding of wedges formed in the slope by intersecting discontinuity planes, and toppling of rock blocks.

The computer program Rockpack III for Windows (produced by RockWare Inc.) was used for the kinematic stability analyses. Inputs to the program for the analysis include:

1. Average orientation (dip magnitude and dip direction) of measured discontinuity sets within the rock mass, including foliation and joint discontinuities.

2. Slope orientation (slope magnitude and direction). The Big Rock “slope” has three orientations: both front faces of Big Rock (oblique to the road alignment), and the cut slope below the Big Rock overhang (roughly parallel to the roadway alignment). For this analysis, each of these slopes is considered to be near vertical.

3. Estimated rock mass discontinuity interface friction angle. For the kinematic analysis, an interface friction angle of 45 degrees was selected as a general condition to check for sliding potential. Note that a higher interface friction angle was used for a specific discontinuity in our limit equilibrium analysis based on the interface friction angle calculation provided in Appendix C.

A RockPack stereonet plot was constructed for each of the five rock slope configurations indicated above to illustrate the results of our kinematic analysis (Figures 6 through 8).

Project No. 07120183.00/January 25, 2008 16 Schnabel Engineering, LLC

The results of the kinematic stability analysis are summarized as follows:

1. Potential sliding failures along Plane 3 (foliation) discontinuities are indicated for the SE face of Big Rock.

2. A potential for toppling failure is indicated. Based on the kinematic analysis, toppling may occur within the NE face of Big Rock along Plane 1. However, we do not consider toppling failure as a probable failure mode for Big Rock because

Plane 1 joints were only observed to form the NE and SW faces of Big Rock, and were not observed as pervasive features within the Big Rock rock mass.

4.2 Limit Equilibrium Analyses

Where potential failure modes were identified in the kinematic analysis, more detailed limit equilibrium analyses were performed to calculate a factor of safety (FS) against planar sliding based on slope geometry, measured discontinuity orientations, estimated groundwater conditions, calculated discontinuity interface friction angles, and estimated or measured material properties of the rock mass. Limit equilibrium analyses were performed for potential planar sliding along Plane 3 identified in the SE face of Big Rock.

The limit equilibrium analyses were performed using RockPack III. The RockPack III program calculates the FS for a potential planar or wedge slide geometry as a function of driving and resisting forces.

The program was used to evaluate existing conditions (i.e., without reinforcement), as well as the force required to achieve a FS of at least 1.5. A FS of 1.5 is typically considered acceptable for permanent slopes (FHWA Publication No. TS-89-045, 1989). Therefore, for cases where the FS was found to be less than 1.5, we also calculated the horizontal force required to obtain a FS of 1.5. In these cases, the horizontal force required to obtain a FS of 1.5 was used to evaluate potential reinforcement mechanisms including rock gluing and rock anchoring.

Our limit equilibrium analyses were based on the following considerations and input parameters:

1. Slope orientation. For planar sliding along Plane 3 foliation within the SE face of

Big Rock, we considered a slope face oriented 83°/127° (dip/dip direction).

Project No. 07120183.00/January 25, 2008 17 Schnabel Engineering, LLC

2. Slope height. For planar sliding along Plane 3 foliation within the SE face of Big

Rock, we considered slope heights of 55 ft and 10 ft for the global and local slope failure conditions, respectively. For the global case, we assumed a failure surface

(i.e., plane or wedge intersection) extending through the toe of the SE face as a worst-case scenario. We assumed a 10-ft high rock block for the local case based on the unstable block observed at the top of the front corner of Big Rock.

3. Discontinuity orientations indicated by the kinematic analysis to be potential failure surfaces. Plane 3, with an average dip of 44 degrees, were considered for the planar sliding cases.

4. A discontinuity interface friction angle of 50o was used for Planes 1 and 3 based on the calculation provided in Appendix C. A higher discontinuity interface friction angle was determined in the calculation; however, we assumed 50o as a reasonable upper limit. We assumed zero cohesion in our FS analyses.

5. A tension crack was assumed to occur 15 ft from the slope crest for analysis of global planar failure in the SE face (based on fractures observed in the rock mass), and 5 ft from the slope crest for local planar failure in the SE face (based on fracture observations in the potentially unstable block at the top of the front corner of Big Rock.

6. “Dry” conditions within rock mass discontinuities were considered. Although storm events may result in short-term water seepage within rock mass discontinuities, the large apertures observed in the mapped discontinuities suggest free draining conditions that would greatly reduce the risk for buildup of groundwater uplift pressures.

7. Rock Mass unit weight of 160 pcf.

8. Earthquake loading was not included in the rock slope stability analysis.

Project No. 07120183.00/January 25, 2008 18 Schnabel Engineering, LLC

9. No other internal or external surcharges are considered in the analysis.

The RockPack III output summary sheets included in Appendix C show the input parameters and results of our FS calculations. Table 1 provides a summary of the limit equilibrium analysis results.

Table 1: Summary of Limit Equilibrium Analysis Results

Failure Mechanism

Controlling Discontinuity

Plane(s) Block

Height (ft)

Tension Crack Location -

Distance from Slope Crest

(ft)

Percent of Tension Crack

Filled with Water

Factor of Safety

Horizontal Force (lb)

Required to Achieve

FS ≥ 1.5

Global Planar Sliding (SE

Face) Plane 3 55 15 0 1.23 16,500

Local Planar Sliding (SE

Face) Plane 3 10 5 0 1.23 600

Note: Results presented above are for a 1-foot thick rock mass.

4.2.1 Planar Sliding

Limit equilibrium analysis indicates a FS of 1.23 against potential global (i.e., 70 ft high failure blocks) and local (i.e., 10 ft high failure blocks) planar sliding along Plane 3 discontinuities, assuming a dry condition for the failure surface. The bolt tension required to achieve FS greater than 1.5 for each scenario is shown in Table 1.

Based on our calculations, the rock block located at the upper front corner of Big Rock is believed to be marginally stable, with a FS of 1.23. The rock block is underlain on a Plane 3 foliation surface and bounded at the back by a tension crack as shown in Photograph Nos. 1 through 4 and 10 through 14. This rock block appears to be particularly unstable. We note that the actual geometry of this rock block is irregular compared to the simplified geometry assumed in our analysis and therefore may be more or less stable than our analysis indicates. The rock block is not confined on its side since it makes up the “nose” of Big Rock, which suggests it may be less stable than our analysis indicates. The aperture of the underlying foliation discontinuity and the associated tension crack is up to 4 inches (Photograph No. 10). The relatively large

Project No. 07120183.00/January 25, 2008 19 Schnabel Engineering, LLC aperture and occurrence of the tension crack indicates movement of this rock block has occurred in the past and further movement and eventual release is possible. Our recommended mitigation design options specifically address this potentially unstable rock block.

4.3 Assessment of Load Supported by Key Block

As illustrated in the cross sections sketch (Figure No. 3) and Photograph Nos. 2 through

6, a boulder occurs directly below the Big Rock overhang and appears to be potentially unstable.

However, as evidenced by the NPS attempt to remove the boulder by pulling with a steel cable until the cable snapped, the boulder is securely lodged between the overhang and the underlying rock. We believe the boulder may be experiencing significant loading from the overlying Big

Rock. Due to the overhanging geometry of the front of Big Rock, tensional forces are developed within the rock mass as Big Rock attempts to rotate forward and downward in the direction of the overhang. This effectively makes the boulder a key block since it is taking on some of the loading from Big Rock’s rotation.

Based on the simplified geometry of Big Rock shown in Figure No. 3, we estimated the downward compressive force being applied by the overhanging mass to the key block to be approximately 440 kips. Calculations are provided in Appendix C. While we believe a precise value of this force is almost impossible to determine and is influenced by the evolution of Big

Rock itself and its surrounding slope, we feel our estimate is reasonably conservative.

4.4 Assessment of the 2006 Rockfall Event

Photograph Nos. 3 through 8 in Appendix D show conditions at the rockfall release area below the Big Rock overhang during our site visit on November 6 and 7, 2007. Photograph No. 9 shows the two largest suspected rockfall blocks removed from the road after the event. The rockfall zone is approximately 5 ft tall by 5 ft wide by 6 ft deep, and thus represents a local failure condition. The size of the rockfall area is consistent with the combined size of the suspected rock fall blocks. Discontinuity Plane 3 (foliation) appears to have provided the release surface and defines the lower boundary of the rockfall block(s).

Based on the geometry of the release surface, we interpret that this rockfall event was controlled by planar sliding along the Plane 3 foliation surface at the bottom of the rockfall block. We believe the major contributing factor in this rockfall event was water seepage along the failure surface associated with the reported storm event, and subsequent erosion of loose fault

Project No. 07120183.00/January 25, 2008 20 Schnabel Engineering, LLC gouge in the rockfall release area, which acted to decrease the area of contact (thus sliding resistance) with the underlying sliding surface. Additionally, the water may have eroded portions of the fault gouge around the rock fall blocks to reduce confinement and stability of the blocks.

A third contributing factor may have been ice wedging that acted along the discontinuities bounding the failed rock block over time.

4.5 Failure Modes for Stabilization Design

Based on our kinematic and limit equilibrium analyses, and consideration of the likely cause of the 2006 rockfall event, we conclude that there is a risk of future rock sliding events within the rock slope area. Our analyses and observations indicate the following instability conditions and failure modes that should be considered for stabilization design:

1. Planar sliding along Plane 3 foliation in the SE face of Big Rock. Our analysis indicates a potential for global- and local-scale instabilities associated with this failure mechanism. On a local scale, this mechanism is highlighted by the existence of an unstable block observed at the upper front corner on the “nose” of

Big Rock. Our recommended mitigation design options must specifically address this potentially unstable rock block.

2. Global failure within Big Rock is in response to Big Rock’s tendency to rotate forward and downward toward the overhang area. We believe the resulting tensional forces within the rock mass are acting to propagate tension fractures observed on the NE face. The overhang is being supported to some degree by the underlying key block. Over time, as the tension fractures continue to develop and expand, the load on the key block will increase. Eventually the load on the key block may become too large for continued support and will lead to global failure within Big Rock. The remediation design must consider a method to overcome the rotational tendency of Big Rock to reduce the loading increase on the key block over time.

Project No. 07120183.00/January 25, 2008 21 Schnabel Engineering, LLC

The rock slope stabilization measures must account for these potential failure modes.

Rock slope stabilization design options addressing the three failure modes identified above are described in the following report sections.

Project No. 07120183.00/January 25, 2008 22 Schnabel Engineering, LLC

5.0 Recommendations

We have developed recommendations for two design concepts to reduce the rockfall hazard potential for global- and local-scale planar sliding failure mechanisms indicated in our limit equilibrium analyses. These design concepts were developed to increase the slope stability

FS to at least 1.5. Our recommendations consist of the following concepts:

1. Option 1 – Gluing, Bolting, Shotcreting. Based on our cost estimates (Section

6.0), this option is less expensive than Option 2 but, in our opinion, provides less long-term risk reduction than Option 2. However, provided a greater risk for future rockfall is acceptable, Option 1 remediation allows the Big Rock profile to be preserved.

2. Option 2 – Slope Layback. This option is less expensive than Option 1 and it provides a greater risk reduction for the failure mechanisms identified in Section

4.5. However, the profile of Big Rock would be drastically changed. Additionally, local-scale rockfall hazard mitigation will need to be evaluated and additional costs may be associated with excavations for a possible rockfall catchment area and rockfall barrier.

A third slope stabilization design concept that was considered included supporting the

Big Rock overhang from below using structural bracing elements extending upward from the roadway shoulder. However, we believe the two options presented below are better alternatives with respect to design and constructability. This third option was dismissed because of problems caused by geometric considerations associated with the overhang and the limited space at the edge of the travel lane. Additionally, the excavations that would be required to construct the foundations for the bracing supports at the toe of the slope may lead to further instability of Big

Rock.

5.1 Option 1 – Gluing, Bolting, Shotcreting

This rock slope stabilization design concept involves rock gluing and rock bolting to reduce the local- and global-scale rockfall hazards, and to counteract the rotational tendency of

Big Rock. Additionally, the cut slope area below the Big Rock overhang will be scaled and

Project No. 07120183.00/January 25, 2008 23 Schnabel Engineering, LLC cleaned to remove loose rock blocks and fault gouge material prior to installation of drainage strips and shotcrete. Spot bolting may also be used, if necessary, in conjunction with scaling to secure potentially unstable local-scale rock blocks that can not be removed safely by scaling. The drainage strips will transmit stormwater seepage through the area whereas the shotcrete will help stabilize the fault gouge zone and reduce the potential for rock blocks to loosen and become unstable over time.

Rock gluing, in addition to providing long term stabilization by creating a more intact rock mass, also provides a preliminary stabilization technique for locally loose blocks prior to drilling larger holes for the rock bolts which may otherwise destabilize these loose blocks.

5.1.1 Rock Gluing

Rock gluing is the process of injecting grout into the rock mass for the purpose of bonding individual blocks into a bigger, more stable, continuous mass. Rock gluing has been used in the tunneling and mining industries primarily to control water seeping into underground spaces, but as such it also provides a structural function of improving the structural integrity and slope stability of the rock mass. Rock gluing for rock stabilization is relatively new but it has been used successfully. For example, Central Federal Lands Highway Division of the Federal

Highway Administration used rock gluing to stabilize the rock mass at a tunnel entrance in

Poudre Canyon, Colorado.

Grouts used in rock gluing can be classified as either cement (i.e., suspension) grouts or chemical (i.e., solution) grouts. We recommend chemical grout for rock gluing since it is almost a pure liquid and is able to permeate narrow rock discontinuities better than cement grout. In particular, the chemical grout we recommend is called a Polyurethane Resin (PUR). The main advantages of PUR grouts include:

1. PUR grout can penetrate finer cracks than a cement grout because it is a solution grout, instead of a suspension grout, and because there is pressure generated by foaming (to force the grout into discontinuities) in addition to the grouting pressure.

2. PUR grout is available with a variety of characteristics, including viscosity, set time, permeability and strength.

Project No. 07120183.00/January 25, 2008 24 Schnabel Engineering, LLC

3. Pore volume of the rock mass is reduced, reducing the potential for opening new discontinuities.

4. Although the strength of the polyurethane grout itself can be smaller than that of a cement grout, polyurethane grouts adhere, or bond, better to the surface of rock.

5. PUR grouts form a water barrier (hydrophobic grout will even repel water), thereby reducing freeze-thaw effects in discontinuities.

6. The set time of PUR grout is shorter than that of cement grout, and it can be controlled.