Timpanogos_Cave_National_Monument,_Rockfall_Mitigation_Measures_July_2013.pdf

TIMPANOGOS CAVE NATIONAL

MONUMENT

ROCKFALL MITIGATION MEASURES

UT FLTP TICA 10(992)

GEOTECHNICAL DESIGN REPORT

Report # UT-PX-TICA-13-01

Geotechnical Services Branch

July 2013

Page i

TABLE OF CONTENTS

REPORT PAGE

1.0 INTRODUCTION

1.1 Scope of Rockfall Hazard Evaluation

1.2 Previous Reports and Investigations

2.0 GEOLOGY AND SEISMICITY

2.1 Site Geology

2.2 Regional and Local Seismic Setting

2.3 Geologic Hazards

2.4 NPS Geologic Hazard Policy

3.0 PROCEDURES AND RESULTS

3.1 Findings

4.0 ANALYSIS AND RECOMMENDATIONS

4.1 Rockfall Analysis

4.2 Rockfall Mitigation Recommendations

4.3 Rock Scaling Recommendations

4.4 Rockfall Hazard Rating System Assessment

4.5 Construction Considerations

5.0 DISCLAIMER/LIMITATIONS CLAUSE

6.0 REFERENCES

APPENDICES

Appendix A – Figures Appendix B – NPS Geologic Hazards Management Policy Appendix C – Rockfall Mitigation Methods Appendix D – Rockfall Analysis Data Appendix E – Rockfall Fence Details Appendix F - Special Contract Requirements Appendix G – Photos

1.0 INTRODUCTION

This report presents the findings of a visual reconnaissance and provides geotechnical recommendations to support the development of a rockfall hazard evaluation and associated mitigation measures within the Timpanogos Cave National Monument (TICA).

TICA is located within the American Fork Canyon along Utah State Highway (SH) 92, approximately 3 miles east of Highland, Utah in Utah County.

The main resource at the National Monument is the cave system providing access to Hanson Cave, Middle Cave, and Timpanogos Cave. To reach the cave, visitors must hike the strenuous, 1.5-mile-long cave trail, which rises steeply just over 1,000 feet. The Visitor Center and caves are open from late-May to late-September. A Project Vicinity Map and Project Location Map are presented in Appendix A.

Regular rockfall events within the park coupled with seasonal visitation emphasizes the need for proactive, systematic evaluation of natural slope hazards. For example, rock scaling efforts are conducted by Park staff on an annual basis on the readily accessible slopes within the Park. In addition, the United State Geological Survey (USGS) has conducted several rockfall hazard assessments of specific locations within the Park, as well as locations within American Fork Canyon.

To support ongoing efforts to improve the safety and performance of unstable natural slopes within the Park, this project focused on the following objectives:

1. Relate slope hazards and qualified risks with broader efforts to identify rockfall hazards within the Park;

2. Recommend a range of rockfall hazard mitigation strategies suited to Park resource management objectives and the effective use of maintenance resources, and apply these strategies toward site-specific hazard mitigation recommendations; and

3. Frame identified rockfall hazards and risks within the assest management architecture, emphasizing the need to treat specific rockfall hazard locations as assets requiring proactive, life-cycle type management.

This study provides an assessment and prioritization of rockfall hazards for unstable natural slopes adjacent to the current location of the TICA Visitor Center and along the cave access trail. Recommended mitigation measures, and strategies for effectively deploying these measures, are provided along with order-of-magnitude cost estimates for general mitigation measures.

1.1 Scope of Rockfall Hazard Evaluation

The rockfall hazard evaluation is focused on the current location of the TICA Visitor Center, located at the base of near-vertical rock cliffs comprised of highly fractured quartzite bedrock. The cliffs begin approximately 150 feet from the Visitor Center and extend to more than 600 feet in height. A talus slope is located below the cliffs and extends downslope to the Visitor Center. The talus slope is approximately 100-feet-high with an overall slope ratio of 1V:1H. Rockfall debris from the cliffs generally reach the vicinity of the Visitor Center and have occasionally impacted and penetrated the roof of the

Visitor Center. A 6-foot-high chain link fence is currently located between the toe of the talus slope and the Visitor Center. The fence has been damaged and is retaining material from numerous rockfall events.

TICA is currently completing final design plans to move the Visitor Center away from these cliffs and increase parking capacity. The proposed location for the Visitor Center is at the east end of the existing parking lot. Relocating the Visitor Center will distance the structure from the runout zone of the active talus slope; thereby, providing greater protection from rockfall impacts. Relocation plans also include minor realignment of SH 92 in the vicinity of the Visitor Center to accommodate increased parking capacity.

While the proposed site is better protected from rockfalls, the demolition and removal of the existing Visitor Center and associated structures increases the rockfall hazard to the proposed parking area. As a result, TICA has requested technical assistance with the evaluation of rockfall hazards and development of rockfall protection measures.

A cursory hazard evaluation was also conducted of natural slopes along the length of the cave trail. Lower portions of the cave trail cross beneath near vertical cliffs comprised of intensely fractured Mutual Quartzite. Rocks falling from these cliffs commonly reach the vicinity of, or impact, the Visitor Center. Upper portions of the trail abut Deseret Limestone, which at this location consist of a sequence of fractured, interbedded limestones, dolomites, and quartzites. Steep gullies in the cliffs between the cave entrance and exit have been the site of numerous rockfalls. These rockfalls resulted in one fatality in 1933 and several near misses, which has prompted the construction of suspended, weighted fences to attenuate rockfall, as well as rockfall shelters at the cave entrance and exit in 1976.

During the summer months, thousands of visitors per day access the trail and visit the cave system. Areas along the trail with known, significant rockfall hazards are marked with a red stripe. Trail users are advised to be cautious and not stop to rest in these areas.

1.2 Previous Reports and Investigations

Geotechnical personnel from CFLHD were originally consulted for technical expertise on this project in August 2009. A trip report was prepared during the initial consultation which provided preliminary geotechnical recommendations on a rockfall catchment system. The recommendations were based on a visual inspection and estimates of rock slope geometry and characteristics.

2.0 GEOLOGY AND SEISMICITY

American Fork Canyon is a deeply incised canyon that transects the Wasatch Mountain Range in central Utah. Specifically, the Canyon is within the deeply dissected Wasatch Front of northern Utah. The Wasatch Range is characterized by rugged mountain faces and narrow, east-west trending, steep sided canyons. Rapid regional uplift has caused many of the streams that drain the west side of the Wasatch Front to erode headward, carving back canyons, until they reach the mountain crests.

The Wasatch Range is an uplifted block that extends approximately 120 miles. The range is 8- to 16-miles-wide and is bounded on the west by the scarp of the seismically active Wasatch Front. The Wasatch Front rises dramatically from the valley floor and separates the geologic province of the Basin and Range to the west from the Middle Rocky Mountain province to the east. The Wasatch range is geologically complex and is characterized by the normal faults along which the Basin and Range deformation occurred.

There are many parallel ranges throughout the Timpanogos region due to the extensional tectonics pulling the crust apart in an east-west oriented pattern. Many westward flowing, high gradient, parallel streams dissect the Wasatch Front into isolated peaks separated by deep, narrow canyons.

The lower part of the America Fork Canyon is V-shaped as a result of downcutting by the American Fork River. The upper reaches of the canyon were glaciated in Pleistocene time.

American Fork Canyon trends east-west, and extends approximately 20 miles to a fault zone responsible for the formation of the Timpanogos Cave complex.

2.1 Site Geology

Currently, the Visitor Center is located at the base of near-vertical cliffs that rise over 600 feet above the floor of American Fork Canyon. A talus slope extends from the base of the cliffs to the rear of the Visitor Center. The cliffs above the Visitor Center are truncated along a north-draining side canyon and a talus slope extends along the east-facing part of the cliffs. The trail to the Timpanogos Cave complex crosses the lower portion of the east talus slope and the top of the talus slope immediately behind the Visitor Center.

The cliffs above the Visitor Center are comprised of highly fractured, Precambrian Mutual Quartzite, as are the talus slopes adjacent to the Visitor Center. Cobbles and boulders predominate on the talus slope behind the Visitor Center, where most boulders have an intermediate dimension of 6 to 12 inches, with some as large as 4 feet. The east talus slope has boulders of larger size, with most having an intermediate dimension of 1 to 2 feet, with some as large as 6 feet.

2.2 Regional and Local Seismic Setting

The Wasatch Range is tectonically active. The western flank of the range forms the eastern boundary of the Basin and Range province. This boundary is marked by a zone of active faults known as the Wasatch fault zone. The surface expression of this zone is a series of fault scarps which trend north-south along the western flank of the range. The average recurrence interval between surface-rupturing earthquakes in the central part of the fault zone, including the area near the mouth of American Fork Canyon is about 400 years (Machette and others, 1992).

Recommended seismic response parameters for design are based on the (AASHTO) LRFD Bridge Design Specifications, 5th edition, 2010, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximately 1000-year return period). The 1000-year return period uniform hazard spectrum for the project site, located at 40.444º N latitude and -111.669º W longitude, was obtained in accordance with the AASHTO ground motion maps for the probabilistic horizontal acceleration values corresponding to specific peak ground acceleration (PGA) and the spectral coefficients, namely the short- and long- period ground acceleration (Ss and S1 respectively) and corrected for the soil profile at the site. Based on visual analysis of native soils, the soils are classified as Class D according to the site class definitions specified in Table 3.10.3.1-1 of AASHTO.

A seismic hazard analysis to establish ground motions for seismic design was conducted.

The recommended spectral acceleration coefficient values for probabilistic design with a return period of 1000 years were calculated using the program provided with the AASHTO LRFD Bridge Design Manual developed by the USGS (2008) entitled “Seismic Design Parameters”, version 2.10 and are summarized in Table 1, Summary of Seismic Parameters Corrected for Class D Soils.

Horizontal Peak Ground Acceleration, (As) 0.364g Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0..862g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.465g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.22 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.25 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.92

Table 1. Summary of Seismic Parameters Corrected for Class D Soils.

Based on the long acceleration coefficient SD1 value of 0.465 g, calculated as FvS1, the site is assigned to seismic hazard Zone 3 according to Table 3.10.6-1 in AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design.

2.3 Geologic Hazards

There are a variety of geologic hazards present in the Timpanogos Cave area; however, most hazards are associated with mass wasting. The presence of highly fractured Mutual Quartzite along the high cliffs above the existing Visitor Center indicates that all areas in the general vicinity are exposed to hazards from rockfall. The presence of mature talus slopes with little or no vegetation is indicative that rockfall has been occurring for some time and that these cliffs continue to actively produce rockfall. In general, the risk of rockfall is highest near the base of the cliffs and adjacent talus slopes, and lowest in the center of the canyon floor. Rockfall events are typically most common in the winter and spring seasons, as hydraulic forces from precipitation events or snowmelt remove blocks from the cliff faces that were previously loosened as a result of physical weathering by freeze-thaw cycles and associated ice wedging. There are documented cases of rockfall occurring as a result of human and animal activities (hikers, mountain goats) in the Timpanogos Cave area. There are also documented cases of rockfall occurring as a result of seismic activity in canyons adjacent to American Fork Canyon.

A debris fan exists just to the east of the current Visitor Center. The debris fan has built up over time and is a result of recurring debris flows. The debris fan extends to the American Fork River on the north and to the existing Visitor Center parking lot on the west. The toe of this debris fan has been excavated to accommodate SH 92 and the Visitor Center parking lot, exposing debris flow deposits. The debris fan originates in the side canyon east of the existing Visitor Center. This side canyon is heavily forested, except for a narrow debris flow channel in the middle of the canyon. The debris flow channel lacks significant vegetation and organic material indicating that the channel remains active. The debris flow channel flattens as it reaches the mouth of the side canyon and disperses to feed the debris fan. The debris flow does not pose a significant hazard and has likely not been active for several decades, as evidenced by the mature forest cover atop the debris fan. This mature forest cover also provides a screen from future debris flow significantly impacting SH 92 or adjacent Park Service facilities.

The project site is located in a seismically active area. Ground motions caused by an earthquake are influenced not only by the distance from the fault planes, but also by the geology found at the site. Amplified ground motions are expected in areas underlain by alluvial materials within American Fork Canyon.

In general, saturated, loose clean sand is prone to liquefaction or lateral spreading during seismic events. Groundwater depths are relatively shallow within the canyon in areas adjacent to the American Fork River. The relative density of observed soils within the canyon indicates that these soils are generally not prone to liquefaction or lateral spreading.

Although not anticipated, localized liquefaction and lateral spreading may occur in small zones under seismic loading conditions.

American Fork Canyon is prone to sporadic flood events. These events usually occur in the summer monsoon months, between June and September. Past flood events have caused damage to Park Service facilities adjacent to the American Fork River. Road closures within the canyon have also been enacted at various times as a result of flooding and associated mudslide damage.

2.4 NPS Geologic Hazard Policy

Based on current rockfall hazard evaluation efforts by the Central Federal Lands Highway Division and previous efforts by the USGS, Timpanogos Cave National Park is undertaking proactive measures to address at-risk facilities and minimize visitor exposure to rockfall events. The following excerpt from NPS Director’s Order #50C demonstrates a clear understanding of the extent to which rockfall may be managed and/or mitigated within the Park:

“In fulfilling [it’s] mission, the NPS must balance issues of access against those of safety. Toward this goal, the NPS, “…must strive to prevent visitor injuries and fatalities within the limits of available resources.” – National Park Service Director’s Order #50C: Public Risk Management Program

Further, Director’s Order #50C states that Park Superintendents “…should strive to minimize the frequency and severity of visitor incidents by developing a range of appropriate prevention strategies and implementing risk reduction mitigation plans.” Examples of such strategies include “…conducting periodic risk assessments to identify and appropriately mitigate hazards,” and “…integrating safety measures into the design and maintenance of park facilities, as appropriate, feasible, and consistent with NPS and park mandates.” – Section 4.1 Management and Incident Reduction, Operational Policies and Procedures.

These directives, and others presented in Appendix B, not only compel the Park to undertake measures to enhance visitor safety, but require hazard mitigation strategies that are cognizant of both natural and financial resource preservation needs.

3.0 PROCEDURES AND RESULTS

The Central Federal Lands Highway Division (CFLHD) Geotechnical Section completed a field exploration program for Project UT FLTP TICA 10(992) on June 5 and 6, 2013. The scope of work for the field exploration program included a visual investigation and measurements, as well as the completion of a LiDAR scan of the slopes adjacent to the existing Visitor Center. LiDAR is a remote sensing technique that measures distance by illuminating a target with a laser and then analyzing the reflected light. The LiDAR scan was used to create a three-dimensional model of the cliffs and slopes adjacent to the existing Visitor Center. The three-dimensional model was then used in completing rockfall analyses.

3.1 Findings

In general, the visual reconnaissance at the project site concentrated on the area immediately behind the existing Visitor Center, including the active talus slope and cave access trail crossing the upper reaches of the talus slope. The Mutual Quartzite cliffs that rise above the talus slope and Visitor Center were only observed from a distance or along the cave trail where access to the cliffs could more easily be obtained.

The vertical height of the talus slope is approximately 100 feet with an overall slope ratio of 1V:1H. The majority of material on the talus slope has an intermediate dimension less than 6 inches. A 6-foot-high chain link fence is located in the breakout zone of the talus slope, a level area between the toe of the talus slope and the Visitor Center. The fence has been damaged and is retaining material from numerous rockfall events.

Wolman counts were performed on representative rock particles on the talus slope, as well as particles within the breakout zone of the talus slope (within 4 feet of the existing chain link fence behind the Visitor Center). The Wolman count is a method used to determine the size distribution of riprap rock based on a random sampling of individual particles within a matrix. While the method is typically used to size riprap sources, it works equally well on talus slopes, or other areas predominately covered with rock particles. The gradation curves as a result of these Wolman counts are presented in Figure 1.

Figure 1. Size Distribution of Rock Particles Associated With Talus Field.

An interesting observation from review of these size distribution curves is that particles within the talus slope are generally smaller than those which reach the breakout zone of the talus slope. Therefore, it can be concluded that individual rockfall particles with an intermediate dimension of 6 inches or less are generally retained within the talus slope and do not reach the breakout zone. Rockfall particles with an intermediate dimension greater than 6 inches generally have sufficient mass and energy to reach the breakout zone and cause damage to nearby structures, property, or Park users.

The existing 6-foot-high chain link fence located in the breakout zone of the talus slope provides a minimal level of rockfall retention and protection. This fence was clearly not intended to serve as a rockfall barrier and should not be relied upon as a future rockfall protection measure. While the fence is currently retaining small rocks, the fence has been penetrated several times by larger rockfall.

Observations and measurements of the cliff face indicate the presence of a predominant joint set dipping out of the face of the cliff. Dip and dip direction measurements yield a joint orientation of 50/000. There is also a fracture set which is oriented roughly 90 degrees from the predominant joint set, resulting in the release of fairly blocky rockfall particles. The size of rockfall particles is a function of the fracture spacing, which in the cliffs immediately behind the existing Visitor Center ranges from approximately 4 to 24 inches. Numerous unstable rock blocks, which pose a potential threat to Park users, were observed on slopes immediately behind the existing Visitor Center, as well as slopes adjacent to the entire cave trail. The cliffs above the side canyon to the east of the existing Visitor Center have a significantly wider fracture spacing, which yields rockfall particles with an intermediate dimension ranging between 12 and 48 inches, as observed on adjacent talus slopes.

There are several prominent features on the cliff face that contribute to the rockfall issue at TICA, specifically the concentration of rockfall in talus runout zones. A north-south trending ridgeline abuts the cliff face on its lower portion, separating the talus slopes adjacent to the existing Visitor Center. There is also a concave feature west of the ridgeline which channels rockfall through a chute and into the talus slope directly behind the existing Visitor Center. While no single rockfall source area was observed during field reconnaissance efforts, it is thought that rockfall generation is evenly and widely distributed along the cliff face. These features are labeled in Figure 2.

Figure 2. Cliff Features.

Along the cave access trail, numerous locations of unstable, overhanging blocks were observed within the Mutual Quartzite and Deseret Limestone units. Many of these unstable blocks are created due to adverse bedding and fracture plane orientation within the rock mass. In several locations, daylighting joint sets were observed immediately adjacent to or above portions of the trail. Figure 3 highlights some of the adverse joint orientations. In addition USGS studies have noted the presence of a section of limestone cliff pulling away from overlying bedding layers in the vicinity of the cave exit (Harp, Dart, and Reichenbach, 2010).

Figure 3. Adverse joint orientation of Quartzite rock mass along the cave access trail.

Based on the findings of this study, it is generally recommended to:

1. Construct a flexible rockfall fence behind the current location of the Visitor Center to protect facilities from ongoing, natural rockfall events, as well as future scaling efforts;

2. Hand-scale isolated, loose Quartzite and Limestone blocks posing imminent safety threats to Park users on slopes above the current location of the Visitor Center and along the cave access trail (the scaling effort should be commenced shortly after construction of the flexible rockfall fence is complete;

3. Complete a detailed Rockfall Hazard Rating System (RHRS) assessment of all slopes adjacent to critical Park facilities, namely the Visitor Center area and the cave access trail; and

4. Develop a systematic and prioritized approach to mitigating specific rockfall hazard locations, as identified in the RHRS assessment, which maximizes available resources using a life-cycle management system.

Following this general approach will mitigate immediate hazards, cost-effectively raise the rockfall hazard “health” of the Park’s facilities, and systematically improve the highest risk slopes in the Park with appropriately timed and measured expenditures.

4.0 ANALYSIS AND RECOMMENDATIONS

Once general corrective measures are selected for a rockfall site, the project goals are typically considered to establish the appropriate level of response and the specific design criteria. The goal of rockfall mitigation measures can range from continued maintenance, to hazard reduction, to hazard elimination. Ideally, mitigation decisions optimize life-safety, property, and mobility risk reduction with available financial resources.

The main categories of engineered rockfall mitigation measures are avoidance, stabilization, and protection. Avoidance measures relocate facilities or persons away from hazardous rockfall sites without reducing the potential for rockfall. Stabilization measures seek to inhibit the fall from occurring by anchoring rock blocks in place with rock bolting techniques or by strategically removing unstable blocks in a controlled manner with rock scaling or blasting methods. Protection measures are intended to capture falling rocks and prevent them from reaching protected areas. A more detailed presentation of available rockfall mitigation methods with associated costs is presented in Appendix C.

Due to the nature of Park Service facilities within American Fork Canyon, namely the limited footprint of existing structures and the need to maintain access to the cave trail, relocation beyond all rockfall impacts is not a feasible rockfall mitigation alternative. The Park Service has taken steps to relocate the Visitor Center to a more protected location;

however, rockfall will still pose a threat to the parking area. The potential risk of rockfall impacting the parking area will be greater than it currently is, as the existing Visitor Center serves a function as a rockfall barrier.

Mitigation measures focused on stabilization are not feasible at this site because of the geology on adjacent slopes and cliff faces. The Mutual Quartzite that comprises the cliff faces above the existing Visitor Center is intensely fractured, such that stabilization techniques (rock bolting, lashing, buttressing) would be ineffective. The source zone for rockfall generation is thought to be uniformly distributed along the cliff face, as evidenced by the wide talus fan. While there are features on the cliff face that channel falling rock, there does not appear to be one specific source zone that could readily accommodate stabilization measures in a cost effective manner. Rock scaling methods in accessible areas could have a limited effect on reducing the rockfall potential in selected areas.

Because relocation and stabilization mitigation measures were not considered feasible, rockfall protection measures were given due consideration. Protection measures are considered passive systems, as they do not actively reinforce or remove the hazard, but provide a barrier, control rockfall trajectory, reduce rockfall energy, or provide a catchment area. The most common protection measures include: catchment areas, rigid barriers, flexible fences, and drapery.

Rockfall catchment areas (ditches) are designed to catch and retain a high percentage of moving rocks before they impact adjacent facilities by dissipating their energy with the catchment zone. Using a variety of design criteria and the given geometry of the rock slopes at TICA, a catchment area approximately 30 feet wide is required to achieve a reasonable improvement in the retention level of rockfall from existing conditions. The required catchment area width is not practical without significant excavation into the talus slope and revision of the existing plans for the relocated Visitor Center and parking area.

Rigid barriers can deflect or contain rockfall depending on their location and orientation with respect to rockfall trajectory. Rigid barriers have been constructed with a variety of materials over the years, including earthen/rock berms, and structural walls (e.g., concrete gravity walls, soldier pile walls, MSE walls). A barrier designed to achieve a reasonable retention level of rockfall would either have a significant footprint, larger than could be accommodated at the site, or would be cost prohibitive because of its required height.

Aesthetic considerations could be a major drawback to the use of rigid barriers to retain rockfall.

Drapery systems are commonly used to control the velocity and energy of rockfall at or near its source. Drapery consists of wire mesh suspended down the slope from anchors at or near the slope crest. The drapery system allows rockfall to occur between the slope and the mesh, controlling its descent into a catchment area at the base of the slope. Drapery systems can be used to shorten the effective height of a slope, which reduces rockfall velocity, energy, bounce height, and runout distance. Due to the extensive nature of the rockfall source area at the project site, difficult high slope access, and extreme slope height, drapery systems are considered cost prohibitive from both an installation and continued maintenance perspective. Drapery will also create a substantial visual impact on the high slopes.

Flexible rockfall fences are also used to deflect or retain rockfall. The fence systems use a net suspended from anchored posts to intercept falling rocks. The system also relies on anchored tieback and lateral cables fitted with energy dissipation devices to contain rockfall and reduce fence damage. Flexible fences are generally cost effective solutions to rockfall issues and have versatile placement options along slopes. For these reasons, flexible rockfall fences are considered a suitable mitigation alternative for the project site.

Therefore, flexible rockfall fences will be further evaluated and analyzed to determine the minimum height and energy rating that will be most effective for this site.

4.1 Rockfall Analysis

Using the actual three dimensional slope geometry, derived from a ground-based LiDAR scan, a rockfall analysis was performed on the cliff faces and talus slopes directly behind the existing Visitor Center using CRSP-3D. The CRSP-3D analysis program is a numerical modeling tool for evaluating rockfall events and provides design criteria for rockfall fences, catchment ditches, catchment berms and other rockfall protection structures. CRSP-3D uses the Discrete Element Method for dynamic model simulation using the equations of motion for a more accurate approximation of rock and slope interaction than the methods employed by previous versions of CRSP. CRSP-3D allows the slope profile to be generated from slope survey data, including LiDAR scans, which produce much more representative slope conditions and, subsequently, a more accurate three dimensional rockfall analysis. The use of a three dimensional slope provides the ability to model possible rockfall paths on a section of slope, and has the capability to model the rotational movement of non-spherical rocks giving a more accurate portrayal of rockfall dispersion, runout distance, trajectory, bounce height, and energy along the slope.

The CRSP-3D software requires various, site specific rockfall parameters to be input, in an effort to create a more accurate simulation model. Mechanical properties of the slope, including hardness and roughness values were input into CRSP-3D, based on observations and estimations made during the field reconnaissance and as recommended in the CRSP- 3D user’s manual (Andrew et al., 2012). Various analysis points along the slope were input into the program to determine various characteristics of simulated rockfall, including velocity, bounce height, kinetic energy, and rollout distance. These characteristics are critical to the design of rockfall protection structures. Falling rock parameters, including rock density, shape, and size were input into the program and were based on field observations and measurements along the talus slope behind the existing Visitor Center. It was assumed that the rockfall source zone originates from a widely dispersed area on the cliffs above the existing Visitor Center. These input parameters are included in the rockfall analysis report in Appendix D.

The input parameters, as described above, were varied over a number of rockfall simulations, in an effort to create a parametric analysis to determine the sensitivity of the model to various input parameters. Through this calibration effort a refined model was created that closely matches observed rockfall parameters at this site. Simulation results of the refined model are presented in Appendix D.

As mentioned previously, a number of points along the slope were analyzed as to their rockfall characteristics, in an effort to determine an optimal location to place a flexible rockfall fence. Rockfall fences placed high on the slope were found to be ineffective at retaining a large percentage of rockfall due to significant bounce heights of individual rocks. Fences placed high on the slope would also be difficult to maintain and inspect.

Rockfall analyses indicated that the optimal location for a rockfall fence was near the base of the talus slope immediately behind the existing Visitor Center. A fence placed in this location would provide a suitable level of rockfall protection, as well as provide a reasonable level of access for cleanout, maintenance, and inspection. Analysis indicated a rockfall retention of 5% at the analysis point. A summary of rockfall analysis results are presented in Figures 4 and 5, with respect to an analysis point placed at an idealized rockfall flexible fence location, approximately 17 feet behind the existing Visitor Center.

Figure 4. Rockfall Bounce Height Results.

Figure 5. Rockfall Energy Results.

Based on the above results with respect to bounce height, the existing conditions on the talus slope behind the Visitor Center, including the effect of the existing chain link fence, provide a rockfall retention level of approximately 95%. This retention level is likely somewhat lower when rockfall energies are considered, as the fence is not designed to withstand impact loads from rockfall.

The level of desired rockfall retention for a given rockfall fence is based on a number of site specific risk factors, including the exposure of Park users and facilities to rockfall, as well as the likelihood of rockfall events, which considers the geology and weather conditions of the site. Cost considerations also have a significant effect on the desired level of rockfall retention. In most cases of highway rockfall protection structures, these mitigation measures must show an appreciable increase in the level of rockfall retention to justify their use and construction. It is generally cost prohibitive for rockfall protection structures to be designed to achieve a level of rockfall retention of nearly 100%.

4.2 Rockfall Mitigation Recommendations

A flexible rockfall fence placed on the talus slope behind the existing Visitor Center is the recommended rockfall mitigation measure for this site. As shown in Figure 4, the incremental increase in rockfall retention decreases significantly for fence heights greater than 10.0 feet (3.0 meters) in height. Therefore, it is recommended to install a flexible rockfall fence with a height of 10.0 feet (3.0 meters) to offer the most practical level of rockfall protection. While the initial capital cost of the recommended fence will be higher when compared with lower fence heights, the increased level of protection over the performance life of the fence will make it a worthwhile investment. The recommended rockfall fence provides a high level of risk protection from a life-safety perspective, while balancing the need for continued maintenance of the fence and catchment area.

A rockfall fence with a minimum energy rating of 500 kJ is recommended at this site. As shown in Figure 5, the maximum anticipated energy from simulated rockfall is 66 kJ.

Rockfall fence manufacturers do provide fences rated at 100 kJ; however, they do not vary significantly, cost or otherwise, from rockfall fences rated at 500 kJ. Fences rated at 500 kJ provide additional support to fence posts from cable anchors, while fence posts in systems rated at 100 kJ are unsupported, besides their foundation components. A rockfall fence rated at 500 kJ will provide protection, and a suitable level of risk reduction, from typical rockfall events at this site, but will also provide protection against anomalous rockfall events that may produce rockfall debris of significant size.

Detail drawings and a preliminary cost estimate of the recommended rockfall fence are provided in Appendix E. Special Contract Requirements (SCR’s) for the rockfall fence are presented in Appendix F. The specified rockfall fence catches the rockfall debris and can be periodically maintained by detaching the fence from the posts and removing rockfall debris into a catchment area behind the fence. A concrete barrier is recommended to separate the proposed, expanded parking area from the rockfall catchment area. Sections of this concrete barrier can be removed, as needed, and rockfall debris in the catchment area can be excavated and removed.

NPS plan sheets, presented in Appendix E, detailing the relocated Visitor Center and expanded parking area show head-in parking and a concrete sidewalk where the existing Visitor Center is located. It is recommended that the concrete sidewalk be deleted and replaced with a dedicated asphalt walking path that is literally an extension of the asphalt parking area, as shown in Appendix E. This will allow for easier access to maintain the rockfall fence and catchment area, while preventing significant and costly damage to adjacent structures during maintenance operations. It is assumed that the concrete barrier and asphalt sidewalk will not be part of the rockfall fence construction and will be included as part of the Visitor Center relocation contract.

4.3 Rock Scaling Recommendations

During the field reconnaissance effort on the slopes immediately above the Visitor Center and subsequent visual inspection of slopes adjacent to the entire cave trail, several areas immediately above and adjacent to the trail with unstable and fractured rock blocks were observed. Many of these blocks pose a threat to downslope structures, but more importantly, pose a potential threat to trail users. TICA staff currently performs scaling operations along the trail before the Park opens each spring, but these operations are limited by the lack of specialized equipment and expertise. It is recommended that TICA complete an extensive scaling effort in the very near term. This effort should include the expertise of a geotechnical engineer to identify locations or individual rock blocks that require scaling, in connection with a trained scaling crew capable of performing the identified scaling. The extensive scaling effort will likely reduce the amount of scaling required by TICA staff on an annual basis, but will also ensure that identified areas requiring scaling, currently beyond the capabilities of TICA staff, receive the level of attention that they require. These scaling efforts will ultimately contribute to an increased level of safety, as well as rockfall risk reduction, along the cave trail.

4.4 Rockfall Hazard Rating System Assessment

It is recommended to develop a systematic and prioritized approach to mitigating specific rockfall hazard locations adjacent to Park facilities and along the cave access trail, using the Rockfall Hazard Rating System assessment, which maximizes available resources using a life-cycle management system.

Oregon DOT first began developmental work on the “Rockfall Hazard Rating System” in 1984. An FHWA pooled fund study provided the research funding needed to complete exhaustive testing and refinement of the system, resulting in publication of the methodology in the early 90’s (Pierson, 1991; Pierson and Van Vickle, 1993). Since that time, the RHRS methodology and variants of the original rating system have been deployed nationwide by federal, state and municipal transportation agencies to quantify rockfall hazards and characterize associated risks. As noted in the 1993 FHWA RHRS manual (Pierson and Van Vickle, 1993), the tool provides a “…legally defensible, standardized way to prioritize the use of the limited construction funds available by numerically differentiating the apparent risks at rockfall sites.”

For the proposed RHRS assessment in TICA, a two-phase approach is recommended to scope slope hazards adjacent to Park facilities and along the cave access trail: (1) preliminary screening of natural slopes within the Park, including a simple, relative slope hazard/maintenance level rating (1-5 scale) with Park maintenance staff; and (2) RHRS rating of slopes with potential rockfall hazards.

The RHRS was developed as a process to allow agencies to actively manage the rock slopes along their highway system. The RHRS is comprised of ten rating elements (risk factors), as follows:

1. Slope Height

2. Ditch Effectiveness

3. Average Vehicle Risk

4. Percent of Design Site Distance

5. Roadway Width

6. Geologic Character (Case 1) - Case 1 refers to slopes where joints, bedding planes, or other discontinuities are the dominant features leading to rockfall.

7. Geologic Character (Case 2) - Case 2 refers to slopes where differential erosion or oversteepening is the dominant condition that leads to rockfall. Erosion features included failing debris slopes, unsupported rock units, or exposed resistant rocks either locally or throughout the slope.

8. Block Size Range/Volume of Rockfall Per Event

9. Slope, Water, and Ice Conditions

10. Rockfall History

Because the proposed RHRS assessment in TICA is not focused on slopes along a highway system, it will have to be modified slightly to reflect the specific assessment needs. This modification is easily done by deleting elements 4 and 5. Slight adjustments will also need to be made to elements 2 and 3. Element 2 will become a factor of natural slope catchment and will qualitatively evaluate the amount of rockfall that reaches the trail or adjacent facility. Element 3 becomes a factor of average trail user risk and will quantitatively measure the percentage of time a hiker will be present in the rockfall zone based on slope length, number of trail users, and average hiking speed.

The purpose of the rating system is to numerically differentiate the risks at the identified sites. Slopes are given a score for each rating element and the individual rating element scores are then totaled. Slopes with higher scores present the greater risk. Once rated, the sites can be sorted and prioritized on the basis of their scores.

Based on results of RHRS assessment in other National Parks, it is not appropriate to believe that failure to mitigate rockfall hazards on the highest rated slopes first assigns liability to the park should an accident occur at one of these locations. Engineering “standard of care” requires that available means for characterizing, assessing, managing and mitigating hazards be taken as fiscally practicable. By conducting the RHRS evaluation and then devising a prudent strategy for the expenditure of funds, addressing localized, low cost immediate hazards first, systematic modest cost hazard over large areas next, and then high cost localized hazards as possible – the overall health of natural slopes within the Park is methodically improved year-to-year, optimizing user safety per dollar spent.

4.5 Construction Considerations

Overhead Power Line: Currently, there is an overhead power line located above the talus slope behind the existing Visitor Center. This power line will require special attention during construction of the rockfall fence. There is a possibility that the power line may need to be relocated, at least temporarily during fence construction.

Construction Access: During field reconnaissance efforts, Park Service staff expressed a desire for construction of the rockfall fence to proceed prior to demolition of the existing Visitor Center. The area behind the existing Visitor Center is very confined and while compact, specialized construction equipment could be used, it would significantly add to the construction costs. It is recommended that construction of the rockfall fence proceed immediately following demolition of the existing Visitor Center, but prior to construction of the expanded parking area.

5.0 DISCLAIMER/LIMITATIONS CLAUSE

The Analysis and Recommendations section in this report include interpretations and recommendations developed by the Government in the process of preparing the design.

These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

6.0 REFERENCES

Andrew, R.D., R. Bartingdale, H. Hume, A. Rock, and R. Zhang, 2012, CRSP-3D User’s Manual, FHWA-CFL/TD-12-007, available at www.cflhd.gov, February 2012, 64 pp.

Harp, E.L., Dart, R., and Rechenbach, 2010, “Rock fall simulation at Timpanogos Cave

National Monument, American Fork Canyon, Utah, USA”, Landslides, published online January 27, 2011.

Machette, M.N., Personius, S.F., and Nelson, A.R., 1992, “Paeleoseismology of the

Wasatch Fault Zone: A summary of recent investigation, interpretations, and conclusions”, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-A, p. A1- A71.

Pierson, L.A. and R. Van Vickle, 1993,Rockfall Hazard Rating System, Federal Highway Administration Publication No. FHWA-SA-93-057, November 1993, 104 pp.

Pierson, L.A., 1991, “The Rockfall Hazard Rating System”, Oregon Department of

Transportation, Federal Highway Administration Publication No. FHWA-OR-GT- 92-05, November 1991, 16 pp.

APPENDIX A – Figures

Project Site

UT FLTP TICA 10(992)

Timpanogos Cave National Monument

Figure 1 Project Location Map

APPENDIX B – NPS Geologic Hazards Policy

APPENDIX C – Rockfall Mitigation Methods

Excerpt from DeMarco, M., 2012, “Yosemite National Park Engineered Slope Rockfall Study”, Geotechnical Report No. CA-PX-YOSE-12-01, Central Federal Lands Highway Division, FHWA, October 2012, 108 pp.

C.0 ROCKFALL MITIGATION METHODS

The goals of a successful rockfall mitigation program for engineered cut slopes should include:

o Consistently increasing slope safety in alignment with existing and future slope performance objectives for the route;

o Deploying long-life mitigation measures not requiring excessive or expensive maintenance, whenever possible;

o Maximizing the use of available construction/maintenance skills and materials within the park;

o Applying measures that best meet aesthetic, cultural and historic requirements of the roadway corridor, and balancing these requirements with asset safety; and o Consistent application of repair options park-wide for strategic expenditure allocations.

With these in mind, rockfall mitigation measures (summarized in Section C.11) should be deployed in an escalating fashion regarding effectiveness (maintainable vs. permanent solutions), installation cost, long-term maintenance cost, and visual impact to the facility. This section describes, in ascending order of complexity and cost, some of the more frequently used methods for rockfall mitigation and describes their application per a variety of slope types.

Additional information regarding application of the RHRS to selecting rockfall mitigation measures can be found in the recent rockfall engineering study completed by the FHWA Western Federal Lands Highway Division (WFLHD) for Crater Lakes National Park (Lofgren and Jenks, 2010). Detailed descriptions of slope stabilization and rockfall management systems can also be found in the CFLHD Technology Deployment Program publication entitled, “Context Sensitive Rock Slope Design Solutions” (Andrew et.al, 2011), available at www.cflhd.gov.

C.1 SLOPE SCALING

Slope scaling is the removal of loose, broken, or partially detached rock from the slope using manual methods, excavation equipment, or trim blasting techniques. Manual, or “hand-scaling”, methods commonly involve 2-3 man crews consisting of experienced scalers skilled at systematically removing loose, isolated rockfall hazards with a variety of specialized hand tools, manual and pneumatic jacks, and light blasting options. Scaling crews typically work off ropes, but may occasionally work from crane baskets or lifts, and always require spotters on the roadway to continually survey the slope for unsafe working conditions. Slope excavation, or “machine-scaling”, methods commonly involve excavators using buckets to pry rock from the slope, hydraulic hammers to bring down larger unstable rock masses, and/or heavy chains to brush large quantities of loose debris from the slope. Machine scaling uses similar crew sizes and clean-up/haul equipment as hand-scaling, and also relies on roadway spotters to ensure operation safety. Hand-scaling and machine-scaling may be done in tandem on raveling slopes where high, localized rockfall exists outside the reach of common excavators (30-40 ft). For high slopes with substantial rockfall hazard, long-reach excavators are available that extend to well over 100 ft.

Figure 1. Slope scaling and top-slope rounding using long-reach excavator, Hyampom Road, Hyampom, CA (Andrew, et.al, 2011).

Systematic scaling along a route is the most efficient and effective option for rapidly upgrading the safety and maintenance performance of engineered slopes in the park. Coupling top-slope rounding on eroding slopes and ditch improvements with slope scaling, which utilizes the same equipment and crews, may greatly mitigate pending rockfall hazards and extend the service life of slope improvements, thereby substantially deferring routine slope maintenance expenditures.

Key attributes of slope scaling include:

For slopes with moderate to low volumes of loose rock, scaling crews can quickly…