GR11-10 Final Report Part A.pdf

DE

PA

RT

MENT OF TRANSPORTATION

UNITED STATES OF AMERICA

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

WESTERN FEDERAL LANDS HIGHWAY DIVISION

VANCOUVER, WASHINGTON

WFLHD GEOTECHNICAL REPORT NO. GR11-10

PRELIMINARY ROCK SLOPE DESIGN REPORT, PART A

KETCHUM-CHALLIS HIGHWAY

SAWTOOTH NATIONAL RECREATION AREA

STANLEY, IDAHO

AUGUST 2010

Geotechnical Services

Report to:

Western Federal Lands 610 East 5th Street

Vancouver, Washington 98661-3801

PRELIMINARY ROCK SLOPE DESIGN REPORT

KETCHUM-CHALLIS HIGHWAY

SAWTOOTH NATIONAL RECREATIONAL AREA

STANLEY, IDAHO

REPORT 11-10

August 2010

Submitted by:

Cornforth Consultants, Inc.

10250 SW Greenburg Road, Suite 111

Portland, OR 97223

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Table of Contents

1. Background

2. Geologic Setting and Site Description

3. Modified RHRS Ratings

3.1. Rating Results

4. Rockfall Simulations

4.1. Rockfall Analysis Results

5. Conceptual Mitigation Measures

5.1. Temporary Roadway Protection

5.2. Scaling/Excavation

5.3. Roadside Barriers

5.4. Sculpted Shotcrete

5.5. Polyurethane Resin Grouting

5.6. Draped Mesh

5.7. Anchored Slope Mesh

5.8. Rockfall Attenuator Fences

5.9. Rock Bolts

5.10. Cable Lashing/Cable Nets

6. Summary of Conceptual Designs and Cost Estimates

6.1. Section 1 – Station 78+05 to 82+00

6.2. Section 2 – Station 82+00 to 84+00

6.3. Section 3 – Station 84+00 to 86+10

6.4. Section 4 – Station 86+10 to 88+90

6.5. Section 5 – Station 88+90 to 91+00

6.6. Section 6 – Station 91+00 to 95+00

6.7. Section 7 – Station 95+00 to 97+70

6.8. Section 8 – Station 97+70 to 100+30

6.9. Section 9 – Station 100+30 to 102+10

6.10. Section 10 – Station 102+10 to 105+00

6.11. Section 11 – Station 105+00 to 107+60

6.12. Section 12 – Station 107+60 to 111+60

6.13. Section 13 – Station 111+60 to 115+35

6.14. Section 14 – Station 115+35 to 118+70

6.15. Section 15 – Station 118+70 to 120+80

6.16. Mitigation Quantities and Cost Summary

7. Final Design Recommendations and Conclusions

List of Tables

Table 1: RHRS Upslope Rating Criteria

Table 2: RHRS Score by Section

Table 3: Section Project Ranking by Score

Table 4: Summary of RHRS Scoring

Table 5: Summary of Rockfall Simulations

Table 6: Summary of Mitigation Quantities

Table 7: Summary of Mitigation Costs

List of Appendices

Appendix A: Representative Photographs of Mitigation Measures

Appendix B: Sample Specifications

Appendix C: Annotated Preliminary Design Photographs (Included in Part B)

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1. Background

Western Federal Lands Highway Division (WFLHD) is planning to improve a 3-mile long portion of Forest Highway 26 (Idaho State Route 75) in Custer County, Idaho. The roadway is located along the Salmon River and lies within the Sawtooth National Recreation Area (NRA).

The Salmon River in this area is designated as a Wild and Scenic River corridor.

Previous work by WFLHD focused on determining an appropriate approach to improve this highway section considering realignment or major reconstruction options. The realignment option would have required three new bridges and the major reconstruction option of the existing alignment would have included rock cuts over 100 feet high and retaining walls greater than 10 feet high. After review, WFLHD and its partners, the Idaho Transportation Department (ITD) and the Forest Service, decided instead to move forward with a lesser reconstruction option. The planned improvements would include small alignment adjustments, short retaining walls to address river erosion of the roadway embankments, and rockfall mitigation measures to reduce the significant rockfall hazards within the corridor.

Specifically, a 4,300-foot-long section of Route 26 from approximately MP 205.72 to MP 206.53, referred to locally as “The Narrows,” produces significant rockfall. Our scope of work for this task order (Contract DTFH70-06-D-00002, Task Order T-10-001) is to provide conceptual rockfall mitigation design options for the rock slopes from MP 205.72 to 206.53, or approximately Station 78+00 to 120+50 by reviewing existing project information and conducting preliminary design field studies; conducting a modified Rockfall Hazard Rating System (RHRS) evaluation to quantify the rockfall hazards at each identified rockfall section;

and provide a Rockfall Mitigation Report describing the mitigation alternatives and estimated costs of each option. Lastly, Cornforth Consultants (CCI) will review the report on site with WFLHD and its partners.

2. Geologic Setting and Site Description

The overall geologic setting is dominated by the massive Idaho batholith consisting of intrusive volcanic rock units. The overburden is typically talus consisting of unconsolidated angular rock fragments in a soil matrix. Historically, the area is seismically active, with the epicenter of the 1983, 6.9 Magnitude, Borah Peak earthquake located approximately 40 miles east of the project.

Locally, the rock in the road cuts is granitic showing variable jointing and rock weathering. The joint spacing ranges from inches to 10+ feet, and the joints are closed to open and in some cases filled with soil. The majority of rockfalls originate from this unit; however, the top of the cuts generally reveal a relatively thin layer of talus, which contains rock pieces in a soil matrix. In addition, at both ends of the project, glacial till is present. Weathering and erosion can remove the support provided by the soil matrix in these materials, which can produce rockfalls. In addition, at various upslope locations, granite outcrops (some large) are present that exhibit clear evidence of past rockfalls. It appears that much of the rockfall from these outcrops are concentrated in drainages and active debris chutes. These outcrops have the potential to release

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rockfalls onto the roadway, however, due to the short field schedule and lack of traffic control, an evaluation of these outcrops was not conducted.

As an example of the rockfall hazards within this ¾-mile-long section, one block, measuring roughly 15 x 8 x 6 feet and weighing about 60 tons, was identified on May 10 as an unstable block. It fell onto the roadway later that that evening. ITD personnel spent several hours the next day breaking the block into smaller sized pieces that could be moved to clear the roadway.

Although it only fell about 35 feet, the impact energy was enough to laterally displace a large v-shaped portion of the roadway over 2 inches toward the Salmon River.

3. Modified RHRS Ratings

The slopes were delineated into 15 sections based on the geological characteristics and changes in the slope configurations. In many cases, the dividing line coincided with natural draws that bisected the rock cuts. Often, these chutes provide a high amount of rockfall activity by funneling rockfall and snow avalanches to the roadway. Where these chutes were present, they were included in the RHRS rating for the adjacent slope section.

The RHRS ratings followed the established calculations as described by FHWA publication SA- 93-057 with three modifications: 1) the average vehicle risk was not calculated due to the consistent traffic volume at each section and similar section lengths, which would have resulted in little contribution to site differentiation; 2) climate scores were not calculated due to the similar conditions at each sites; and 3) additional rating points were assigned to sites that had upslope outcrops and/or rockfall chutes as shown below on Table 1. The presence of outcrops or chutes above the rock slope was based on a review of the oblique air photos provided by WFLHD and on site observations where possible from the roadway. The station limits of the 15 delineated sites are included on Table 2.

Table 1: RHRS Upslope Rating Criteria.

Characteristic Rating No outcrops above 3

Minor outcrops above 9 Major outcrops above 27

3.1. Rating Results

The RHRS results indicate that all the delineated slope sections would be classified as ‘A’ rated slopes, indicating the presence of medium to high rockfall hazards throughout this length of roadway. All 15 sections would warrant some form of rockfall mitigation. The site scores by Section number and the relative ranking of the sites based on their Section scores are included on Tables 2 and 3 below. Review of the category scores from each site indicate that, in general, they did not obtain a large portion of their total points from geometric slope characteristics (i.e. height, sight distance). However, based on the site conditions and the rockfall history, they were generally rated high in the geologic character and rockfall history categories. The high scores are

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indicative of active rockfall conditions with a high likelihood of rockfalls reaching the roadway.

Stationing included in following thee tables is from a preliminary plan set dated May 4, 2010.

Table 2: RHRS Score by Section.

Section - Stationing RHRS Score Project Ranking

1 - Station 78+05 to 82+00 356 11 2 - Station 82+00 to 84+00 375 9 3 - Station 84+00 to 86+10 458 5 4 - Station 86+10 to 88+90 389 8 5 - Station 88+90 to 91+00 524 3 6 - Station 91+00 to 95+00 536 1 7 - Station 95+00 to 97+70 343 12 8 - Station 97+70 to 100+30 401 7 7 - Station 95+00 to 97+70 341 13 10 - Station 102+10 to 105+00 460 4 11 - Station 105+00 to 107+60 329 14 12 - Station 107+60 to 111+60 362 10 13 - Station 111+60 to 115+35 454 6 14 - Station 115+35 to 118+70 300 15 15 - Station 118+70 to 120+80 526 2

Table 3: Section Project Ranking by Score.

Project Ranking Section - Stationing RHRS Score

1 6 - Station 91+00 to 95+00 356 536 2 15 - Station 118+70 to 120+80 375 526 3 5 - Station 88+90 to 91+00 458 524 4 10 - Station 102+10 to 105+00 389 460 5 3 - Station 84+00 to 86+10 524 458 6 13 - Station 111+60 to 115+35 536 454 7 8 - Station 97+70 to 100+30 343 401 8 4 - Station 86+10 to 88+90 401 389 9 2 - Station 82+00 to 84+00 341 375

10 12 - Station 107+60 to 111+60 460 362 11 1 - Station 78+05 to 82+00 329 356 12 7 - Station 95+00 to 97+70 362 343 13 7 - Station 95+00 to 97+70 454 341 14 11 - Station 105+00 to 107+60 329 15 14 - Station 115+35 to 118+70 526 300

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The complete ratings for each section are included below on Table 4.

Table 4: Summary of RHRS Scoring Site 1 Site 2 Site 3 Site 4 Site 5 Beginning Station 78+05 82+00 84+00 86+10 88+90 Ending Station 82+00 84+00 86+10 88+90 91+00 Beginning Coordinates

N942647.4472 E1462350.870

N942872.626 E1462674.803

N943014.961 E1462815.054

N943014.961 E1462942.062

N943415.1031 E1463096.858

Ending Coordinates

N942872.626 E1462674.803

N943014.961 E1462815.054

N943014.961 E1462942.062

N943415.1031 E1463096.858

N943593.7416 E1463206.976

Slope Height & Score (ft, score)

79, 32 99, 78 110, 100 75, 27 53, 10

Ditch Effectiveness L, 27 L, 27 L, 27 L, 27 LN, 70 Sight Distance (ft) 254 428 255 592 600 Decision Sight Distance (%, score)

48, 52 49, 49 29, 100 68, 17 69, 16

Roadway Width (ft, score)

22, 62 23, 54 24, 47 25, 41 24, 47

Rock Structure DCRA, 60 DCRA, 50 DCRA, 55 DCRA, 50 CA, 81 Joint Friction Wrx, 40 Wrx, 40 Wrx, 40 Wrx, 40 Wrx, 40 Block Size/Volume (ft or yds, score)

3.5, 47 3.5, 47 3.5, 47 1000, 100 2500, 100

Rockfall History Many, 27 Occ, 15 Occ, 15 Many, 60 Many, 60 Outcrop/Chute Score

Minor, 9 Minor, 15 Major, 27 Major, 27 Chute, 100

Score - Ranking 356 - 11 375 - 9 458 - 5 389 - 8 524 - 3

Site 6 Site 7 Site 8 Site 9 Site 10 Beginning Station 91+00 95+00 97+70 100+30 102+10 Ending Station 95+00 97+70 100+30 102+10 105+00 Beginning Coordinates

N943593.7416 E1463206.976

N943901.8847 E1463461.675

N944120.0805 E1463620.659

N944316.6272 E1463790.784

N944449.9434 E1463911.693

Ending Coordinates

N943901.8847 E1463461.675

N944120.0805 E1463620.659

N944316.6272 E1463790.784

N944449.9434 E1463911.693

N944608.3092 E1464152.34

Slope Height & Score (ft, score)

83, 38 63, 16 60, 14 48, 8 96, 65

Ditch Effectiveness LN, 50 ML, 20 L, 27 LN, 65 ML, 15 Sight Distance (ft) 875 875 875 875 386 Decision Sight Distance (%, score)

100, 3 100, 3 100, 3 100, 3 44, 65

Roadway Width (ft, score)

23, 54 22, 62 25, 41 24, 47 25, 41

Rock Structure CRA, 75 DCRA, 45 DCRA, 40 CRA, 55 DCRA, 40 Joint Friction Open, 60 Gouge, 45 Gouge, 45 Wrx, 40 Wrx, 45 Block Size/Volume (ft or yds, score)

15, 100 3.5, 47 5+, 100 3, 27 4, 81

Rockfall History Many, 75 Many, 45 Many, 50 O/M, 15 Many, 27 Outcrop/Chute Score

Chute, 81 Major, 60 Chute, 81 Chute, 81 Chute, 81

Score - Ranking 536 - 1 343 - 12 401 - 7 341 - 13 460 - 4

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Table 4 (cont.) Site 11 Site 12 Site 13 Site 14 Site 15 Beginning Station 105+00 107+60 111+60 115+35 118+70 Ending Station 107+60 111+60 115+35 118+70 120+80 Beginning Coordinates

N944608.3092 E1464152.34

N944660.6434 E1464405.606

N944605.4101 E1464800.861

N944479.3458 E1465153.696

N944328.0908 E1465451.494

Ending Coordinates

N944660.6434 E1464405.606

N944605.4101 E1464800.861

N944479.3458 E1465153.696

N944328.0908 E1465451.494

N944189.825 E1465609.154

Slope Height & Score (ft, score)

68, 20 68, 20 65, 17 56, 12 160, 100

Ditch Effectiveness ML, 15 ML, 15 ML, 25 ML, 20 LN, 45 Sight Distance (ft) 500 322 500 637 475 Decision Sight Distance (%, score)

57, 32 37, 96 57, 32 73, 13 54, 38

Roadway Width (ft, score)

24, 47 24, 47 23, 54 24, 47 25, 41

Rock Structure/*Erosion Features

DRA, 27 DRA, 27 DR, 20 DCRA, 35 *Many, 50

Joint Friction/*Diff.

Erosion Rates

Wrx, 45 Gouge, 50 Gouge, 50 Gouge, 50 *Large, 81

Block Size/Volume (ft or yds, score)

3.5, 47 3, 27 12yd, 100 4, 81 4, 81

Rockfall History O/M, 15 O/M, 20 MC, 75 Many, 27 C, 81 Outcrop/Chute Score

Chute, 81 Major, 60 Chute, 81 Minor, 15 Minor, 9

Score - Ranking 329 - 14 362 - 10 454 - 6 300 - 15 526 - 2 Abbreviations: Ditch Effectiveness: M=Moderate, L=Limited; N=None Rock Structure: D=Discontinuous; C=Continuous; R=Random; A=Adverse Joint Friction: Wrx=Weathered *Case 2, Differential Weathering Type Slope

4. Rockfall Simulations

The computer program Rocfall (version 4.0) was utilized for modeling rockfall events and analyzing rockfall behavior traveling down two probable rockfall paths representing two debris chutes with upslope outcrops. The cross-sections were located near Stations 88+60 and 95+50.

These simulation models provide predictions of the bounce height, velocity, and kinetic energy of rockfalls as they pass a selected analysis point on the cross-section. This information can be used as part of the evaluation for the best location, optimal height, and required capacity of rockfall protection measures such as fences or berms. The models analyzed for this conceptual study do not replace site specific models; since, the chute geometry and likely block sizes at each site would be unique and produce different energies and bounce heights.

In rockfall modeling, the likely sources of rockfall are called initiation zones and these zones are determined in the field based on observed slope conditions. The potential rockfall sizes and shapes are also noted; however, the block size modeled is generally based on reported historical rockfall information where available. For this project, three rock sizes were modeled: 2x2x2-foot, 3x3x3-foot, and 5x5x5-foot. The blocks were modeled with an assumed unit weight of 170

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pounds per cubic foot (pcf). An initiation zone near the top of each cross section was selected for each model and 1,000 simulated rockfalls were analyzed.

4.1. Rockfall Analysis Results

Several thousand rockfall simulations were conducted for the two representative cross-sectional models in order to calibrate surface roughness and roll out distances to match observed and interpreted field conditions. Following calibration modeling, a final modeling run for each rock size was made and the results analyzed for the average, maximum, and 95th percentile for bounce heights and kinetic energies. The results have been tabulated in Table 5 below for each cross sections analyzed. The analysis point for each section was approximately 50 feet above the roadway on the natural slope, which would be a practical location to position an attenuator fence as discussed in Section 5.8 below.

Maximum bounce heights produced by the modeling ranged from 4 feet to as high as 16 feet.

The bounce heights appear to be highest for smaller rock sizes due to a generally greater effective slope roughness in comparison to the size of the modeled falling rock. The 95th percentile bounce heights were a maximum of 7 feet. Due to the expense of retaining 100% of rockfall, agencies sometimes select a goal of retaining less than the maximum predicted energies or bounce heights. As indicated in Table 5, the kinetic energy generated by the models ranged from 40 to 1,244 ft-tons with the higher energies associated with the larger rock sizes. These energies do not reflect the additional energy that could be contributed by snow avalanches and/or soil and rock debris flows, which cannot be modeled with the Rocfall software. If it is determined that these designs must also account for these conditions in addition to rockfalls, it would require further evaluation during final design.

Table 5: Summary of Rockfall Simulations Bounce Height

Distribution (ft) Kinetic Energy

Distribution (ft-tons) Area

Modeled Rock Size Avg. 95th % Max Avg. 95th % Max 2-foot cube 1.4 5.5 12.7 44 72 94 3-foot cube 0.5 1.5 5.0 106 214 258

Rockfall Chute 50’ above road

(Station 88+60 ) 5-foot cube 0.5 1.5 4.1 460 947 1,244 2-foot cube 1.6 7.0 16.2 40 58 74 3-foot cube 0.6 2.8 8.9 116 178 249

Rockfall Chute 50’ above road

(Station 95+50) 5-foot cube 0.7 3.2 8.3 548 842 1,196

5. Conceptual Mitigation Measures

The mitigation measures for each of the 15 sections were determined by observational techniques from the roadway. Access to perform a close-up evaluation of the slopes was not part of this effort for the reasons described above. Still, because the cut slopes are not overly tall, this effort provided an adequate perspective to prepare these conceptual designs. Note that all sections

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require scaling but scaling alone would not reduce or control the potential rockfall adequately to serve as a stand-alone mitigation for any section.

We understand that widening the fallout areas by making major cuts or alignment shifts towards the river, which would require taller retaining walls, are not acceptable options. Limited blast scaling is included where blocks may be too large to scale by hand but not trim blasting of the existing cuts is not recommended. Removing a thin layer of rock from the slopes using trim blasting would not be highly beneficial in reducing the rockfall potential and it would present very difficult access issues for construction equipment and create the same appearance as a new rock cut with the remnant half-cast holes showing.

We have prepared at least two mitigation options for each section, which will be reviewed on site with WFLHD and its partners to decide which to advance to final design. The selected option may include various combinations of the following measures depending on the significance of the rockfall potential, agency approval, aesthetic requirements and available budget:

1. Temporary Roadway Protection

2. Hand/Blast/Equipment Scaling,

3. Roadside Barriers,

4. Sculpted Shotcrete,

5. Polyurethane Resin Grouting (PUR),

6. Draped Slope Mesh,

7. Anchored Slope Mesh,

8. Rockfall Attenuator Fences,

9. Rock Bolts, and

10. Cable Lashing/Cable Nets.

A discussion of each of these measures with the unit costs used in preparing the cost estimates is included below. Photographic examples of several the measures that utilize mesh or cable products as they might appear are presented in Appendix A along with photographs of sculpted shotcrete test sections that were required prior to using this material in Glacier National Park.

5.1. Temporary Roadway Protection

All sections required scaling to remove loose rock. Temporary roadway protection (TRP) would be needed to retain scaled rock debris for cleanup, restrict material from entering the river, and limit material from directly impacting and possibly damaging the existing road surface or the roadway embankments. The TRP would include setting a row of Jersey barriers along the edge of pavement to restrict as much scaled rock debris as practical to the roadside ditch. These barriers could be moved and reused as work progresses at each site. Therefore, the unit cost for this would be primarily for moving the barriers so the cost would be on the order of $10 per foot of slope along the roadway. No photographs of TRP are included in Appendix A since it would not remain on site following construction.

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5.2. Hand/Blast/Equipment Scaling

The estimate of scaling hours was based on the total slope area with 200 square feet of slope scaled per hour for a high need site and 300 square feet scaled per hour for a normal need site.

Since some of the areas include tall slopes and upslope outcrops, scaling operations could require setting anchors above the slope so the scaling could be performed by workers on ropes. Due to the highly fractured nature of the bedrock, scaling would need to be closely managed to limit rock removal to critical areas. The cost estimate used for scaling was $90 per hour per scaler. If more extensive rock removal is necessary, excavation techniques using hydraulic equipment such as a trackhoe or limited blasting could be required. Due to the small quantities involved, the cost estimate used for this mitigation technique was $50 per cubic yard. No pictures of scaled slopes are included in Appendix A since the slopes would appear similar before and after this operation.

5.3. Roadside Barriers

Concrete Jersey barriers are often used on the edge of pavement to augment the effectiveness of the roadside ditch to retain rockfall for both temporary and permanent applications. The suitability of using a barrier is best determined following rockfall modeling simulations and a visual assessment of the slope. The unit price for this mitigation measure is $50 per foot, which includes foundation preparation not required for TRP. Where a 9-foot-tall Gabion basket wall is proposed as an option for Section 15, a cost of $500 per foot or $55 per square-foot-face has been used.

5.4. Sculpted Shotcrete

Shotcrete is very effective in limiting differential erosion that can lead to unsupported slope areas especially where hard rock is underlain by softer rock. By protecting the softer rock from the effects of weathering and erosion, the support provided by these weaker materials can be maintained. Due to the visual requirements on this project, the structural shotcrete used to protect slope stability could be covered with a layer of sculpted and tinted/painted shotcrete. The cost increase to add the sculpted layer is significant as listed below but the goal of this outer architectural layer is to produce a mitigation measure that closely matches the appearance (texture and color) of the adjacent native rock. Due to the distance to a batch plant, a dry mix application would be necessary. The cost for the structural layer is $180 per square yard ($20/sq.

ft.). The additional cost of the sculpted layer would be an additional $270 per square yard ($30/sq. ft.) for a total of $450 per square yard ($50/sq. ft.).

5.5. Polyurethane Resin Grouting

A developing method for rockfall mitigation is the use of polyurethane resin (PUR) injection for stabilizing fractured rock masses. This technique has been utilized in a pilot program for Central Federal Lands Highway Division and is summarized in Publication FHWA-CFL/TD-08-004.

The technique involves drilling several holes into the rock face at a 5 to 10-foot spacing that are used to inject a two-part resin into the rock, allowing the resin to seep into open fractures and subsequently bond internal rock surfaces together. This technique could be applied in highly

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fractured rock as an alternative in some cases to installing draped mesh or where the rock mass is comprised of rock pieces too small to be held in place with rock bolts alone.

The pay unit for PUR is based on the weight of PUR injected, which can vary significantly between holes depending on the nature and frequency of joints encountered within the borehole.

For estimating purposes, we have assumed 200 lbs of PUR would be injected per hole with a hole spacing of 5-foot on center, horizontally and vertically. Using an installed unit price of PUR of $15 per pound provides a cost per hole of $3,000, which translates to a cost of $120 per square-foot-face making this a very high cost alternative in comparison to other measures.

5.6. Draped Mesh

Draped mesh is a common and useful method for controlling small- to medium-sized (less than

2.5 feet in diameter) rockfall. Depending on the typical rock size to be controlled, three different types of mesh may be used. In ascending order of strength, they include triple twist gabion mesh, high strength steel mesh, and spiral or wire rope nets. Powder or PVC color coating is possible for these options in order to blend the mesh color to the color of the surrounding materials.

Examples of galvanized non-colored and colored meshes are included in Appendix A. The type of mesh required is based on the anticipated sizes of blocks being retained with the unit price for the three mesh types is $3, $6, and $10 per square foot installed for the gabion, Tecco, and wire rope nets, respectively. It is anticipated at this time that gabion mesh can be used on all slopes where proposed, which includes all the cut slopes in Sections 1 though 14.

5.7. Anchored Slope Mesh

A modification of the draped mesh is to include intermediate slope anchors to fasten the mesh to the slope on 10- to 15-foot anchor spacing. This modification is useful in directly supporting the slope faces and retaining potential rockfalls in place rather than attempting to control their decent once they fall. For this to work, it requires the use of a stronger (more costly) fabric, i.e. high-strength steel (Tecco®) mesh or a spiral/wire rope netting along with intermediate slope anchors on ten-foot centers. For conceptual design, a unit price of $25 per square foot has been used, which would cover the cost for installing the high-strength steel mesh and anchors as an option for Section 15. This technique will significantly reduce the required ditch maintenance and road patrols by holding rocks in place and allowing vegetation to reestablish.

5.8. Rockfall Attenuator Fences

The area has a number of well-defined drainages and rockfall chutes that concentrate and direct rockfalls onto the roadway. Attenuator fences serve to intercept rolling rocks, absorb much of the rockfall energy, and dramatically reduce the rockfall velocity before the rockfalls exit the attenuator system and entering the roadside ditch. By doing this, a less than adequate rock fallout area, such as is present here, can still serve to retain the slower moving rocks and keep them from entering the roadway. By allowing rolling rocks to pass underneath the attenuator system and into the ditch, maintenance can be performed at the road grade where access is best. The main

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design requirement for this type of system is to install an attenuator tall enough to intercept rolling rocks and stout enough to perform as intended. In addition, the system may need to be strengthened if debris slides and snow avalanches are anticipated, which would require additional design effort to properly size the system components for these larger loads. For conceptual design, the estimated price is $18 per square foot.

5.9. Rock Bolts

Rock bolts are used to hold potentially unstable blocks in place on the slope. They must be sized and positioned to be most effective. Based on the site conditions, key block bolting would be appropriate. By stabilizing a key block, not only is that block retained but the stability of surrounding blocks affected by that block would also be enhanced. To mitigate the visual effects of the rock bolts, the keyhole plate and the protruding portion of the bolt could be removed once the grout has set.

The field study allowed certain blocks to be identified for bolting. Based on the dimensions of the blocks, the rock bolts would need to consist of 10-, 15- or 20-foot-long bolts as color-coded on the annotated conceptual design photographs. However, the fieldwork was completed from the perspective provided at the roadway elevation. No upslope evaluation was performed so the depicted bolt locations are preliminary. Undoubtedly, additional and perhaps longer bolts may be needed once the upper portions of the cut slopes and the upslope outcrops are accessed during final design. The design photos include filled and unfilled circles representing bolt locations.

The unfilled circles indicate anticipated bolts that will be needed behind scaled areas once the scaling is complete. The unit price used for the cost estimate was $150 per foot of rock bolt.

5.10. Cable Lashing/Cable Nets

Although there was no opportunity to perform an upslope evaluation, the outcrops above the roadway were included as part of this preliminary field effort. In several instances there appeared to be outcrops that contained rock blocks that were not suitable for rock bolting but still posed a hazard to the roadway. In some instances, the blocks comprising the outcrops were fairly large, in excess of 5 feet in the longest dimension. To reduce the potential rockfall hazards associated with these blocks and the likelihood of rockfall events damaging or destroying other down slope mitigation installations, cable lashings or anchored cable nets could be used to secure blocks in place. These measures are used to resist initial displacements that could lead to block failure. The cables or cabled nets pass around the face of the block and are held tightly in place with short rock anchors.

The number of cable lashings or cable nets is not well defined due to the lack of access to the upper slope. The estimated quantities are based solely on oblique aerial photos provided by WFLHD and the limited observational opportunities available from the roadway. The conceptual cost estimate is $1,200 per lashing with an additional $15 per square foot if a cable net is included in the system. Because these installations would be restricted to upslope areas where the distance and the dark (oxidized) color of the outcrops would hide their presence, it is unlikely

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these installations would be visible from the road or river perspective. A close-up view of a cable lashing installation is included in Appendix A.

6. Summary of Conceptual Designs and Cost Estimates

The 0.75-mile-long highway segment from approximately MP 205.72 to MP 206.53 produces significant rockfall. The project goals are to improve highway safety by reducing the rockfall potential without making any significant cuts into the existing rock slope to remove unstable blocks or to enhance the effectiveness of the roadside catchment area by widening the ditch. The reduction of the rockfall potential is limited to on-slope methods, i.e. holding blocks in place (stabilization measures) or controlling the descent of blocks once they come loose (protection measures). Since the project is within the Sawtooth NRA, any measures used would need to blend into the existing slope as much as possible and the final acceptance of the mitigation methods used will require the approval of the project partners including the WFLHD, Sawtooth NRA, and ITD.

Our scope of work included delineating rockfall sections based on the methods outlined by the RHRS and providing at least two conceptual rockfall mitigation alternatives and cost estimates for each delineated section. The following subsections present a summary of the proposed rockfall mitigation options for each rock slope and include a discussion of the effectiveness of the proposed methods as well as the cost of each option. The measures proposed all have the potential to be successfully installed and completed within a compressed construction season depending on the resources and experience of the selected contractor. The preferred mitigation options are those that utilize mesh products on the slopes (protective measures) in addition to the rock bolting and cable lashing stabilization measures. Where possible, the addition of concrete barriers could be considered in lieu of or to augment the draped mesh but mesh is the preferred option since it would produce the greatest improvement for the amount invested.

The following designs are conceptual and not intended for construction at this time without additional final design fieldwork and office analyses. The measures identified for upslope outcrops as well as the upper portions of the cut slopes are less defined because no access to these areas was available during the recent fieldwork. The conceptual design options presented utilize the “on-slope” techniques routinely used for rockfall mitigation listed above. Appendix A includes selected photographic examples of these measures that can be referred to for comparing aesthetic impacts. Appendix B contains sample specifications for the proposed mitigation techniques. Appendix C included under separate cover as Part B of the report includes the mapped locations and the annotated panoramas showing the locations of the mitigation options by section. Since these panoramas were created by stitching individual photographs together, a distorted and curved appearance has resulted and they do not represent a scalable view.

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6.1. Section 1 – Station 78+05 to 82+00

Section 1 was ranked number 11 out of the 15 sites with an RHRS score of 356. The slope is 395 feet long with a maximum height on the order of 79 feet and a slope area of roughly 24,000 square feet. Both options for this section require 915 feet of rock bolts and 55 hours of scaling.

The additional Option 1 measure would include the installation of 14,275 sq. ft. of draped mesh.

In lieu of the draped mesh, Option 2 would include 2,400 sq. ft. of shotcrete and installation of a Jersey barrier along the shoulder of the road. The shotcrete and barrier in Option 2 would not provide the same level of mitigation improvement as the draped mesh and the barrier would pose additional problems for ditch maintenance and snow removal. The cost for the rockfall mitigation measures Option 1 is $191,300. The cost for Option 2 is $198,200 if plain shotcrete is used and for Option 3 is $243,200 if sculpted shotcrete is used. These options are included on Figures 1, 2, and 3 in Appendix C.

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6.2. Section 2 – Station 82+00 to 84+00

Section 2 was ranked number 9 out of the 15 sites with an RHRS score of 375. The slope is 200 feet long with a maximum height on the order of 99 feet and a slope area of roughly 17,250 square feet. Both options for this section require 695 feet of rock bolts and 58 hours of scaling.

In addition, Option 1 would include the installation of 16,250 sq. ft. of draped mesh.

Alternatively, in lieu of the draped mesh, Option 2 would include 1,480 sq. ft. of shotcrete and 200 feet of roadside Jersey barrier. The shotcrete and barrier (Option 2) would not provide the same level of mitigation improvement as the draped mesh and the barrier would add additional problems for ditch maintenance and snow removal. The cost for the rockfall mitigation measures in Option 1 is $166,800. The cost for Option 2 is $157,600 if plain shotcrete is used and for Option 3, $202,000 if sculpted shotcrete is used. These options are included on Figures 4 and 5 in Appendix C.

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6.3. Section 3 – Station 84+00 to 86+10

Section 3 was ranked number 5 out of the 15 sites with an RHRS score of 458. The slope is 210 feet long with a maximum height on the order of 110 feet and a slope area of roughly 16,250 square feet. Both options for this section require 585 feet of rock bolts, 54 hours of scaling and 6,150 sq. ft. of attenuator fence. In addition, Option 1 would include the installation of 15,200

sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 210 ft. of roadside Jersey barrier. The barrier (Option 2) would not provide the same level of mitigation improvement as the draped mesh and it would pose additional problems for ditch maintenance and snow removal. The cost for Option 1 is $267,000. The cost for Option 2 is $231,800. These options are included on Figures 6 and 7 in Appendix C.

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6.4. Section 4 – Station 86+10 to 88+90

Section 4 was ranked number 8 out of the 15 sites with an RHRS score of 389. The slope is 280 feet long with a maximum height on the order of 75 feet and a slope area of roughly 16,210 square feet. Both options for this section require 800 feet of rock bolts, 54 hours of scaling and 8,190 sq. ft. of Attenuator fence. In addition, Option 1 would include the installation of 14,810

sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 300 sq. ft. of shotcrete. Adding the shotcrete would improve the stability of a small portion of the slope and this alternative does equal the safety improvements provided by the draped mesh. The cost for the rockfall mitigation measures in Option 1 is $335,100. The cost for Option 2 is $296,700 if plain shotcrete is used and as Option 3, $305,700 if sculpted shotcrete is used. These options are shown on Figures 7, 8, 9 and 10.

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6.5. Section 5 – Station 88+90 to 91+00

Section 5 was rated number 3 out of the 15 sites with an RHRS score of 524. The slope is 210 feet long with a maximum height on the order of 53 feet and a slope area of roughly 8,550 square feet. Option 1 would include 565 feet of rock bolts, 43 hours of scaling, and 4,000 sq. ft. of Attenuator fence. Option 2 would be similar but would include the installation of 17,250 sq. ft.

of draped mesh. The additional draped mesh would provide enhanced protection from rockfalls and reduce required road patrols. The cost for Option 1 is $177,300. The cost for Option 2 with the mesh is $198,800. These options are included on Figures 10, 11 and 12.

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6.6. Section 6 – Station 91+00 to 95+00

Section 6 was rated number 1 out of the 15 sites with an RHRS score of 536. The slope is 400 feet long with a maximum height on the order of 83 feet and a slope area of roughly 29,720 square feet. Option 1 would include 1,515 feet of rock bolts, 149 hours of scaling, and 1,500 sq.

ft. of Attenuator fence. Option 2 would be similar but would include the installation of 27,720

sq. ft. of draped mesh. The additional draped mesh would provide enhanced protection from rockfalls and reduce required road patrols. The cost for Option 1 is $291,400. The cost for Option 2 with the mesh is $374,600. These options are included on Figures 12, 13, 14 and 15.

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6.7. Section 7 – Station 95+00 to 97+70

Section 7 was rated number 12 out of the 15 sites with an RHRS score of 343. The slope is 270 feet long with a maximum height on the order of 63 feet and a slope area of roughly 12,350 square feet. Option 1 would include 280 feet of rock bolts, 62 hours of scaling, and 1,500 sq. ft.

of Attenuator fence. Option 2 would be similar but would include the installation of 11,000 sq.

ft. of draped mesh. The additional draped mesh would provide enhanced protection from rockfalls and reduce required road patrols. The cost for Option 1 is $68,900. The cost for Option 2 with the mesh is $101,900. These options are included on Figures 16 and 17.

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6.8. Section 8 – Station 97+70 to 100+30

Section 8 was rated number 7 out of the 15 sites with an RHRS score of 401. The slope is 260 feet long with a maximum height on the order of 60 feet and a slope area of roughly 15,600 square feet. Option 1 would include 280 feet of rock bolts, 62 hours of scaling, and two separate Attenuator fences totaling 10,300 sq. ft. of mesh. Option 2 would be similar but would include the installation of 14,300 sq. ft. of draped mesh. The addition of the draped mesh would provide enhanced protection from rockfalls and reduce required road patrols. The cost for Option 1 is $310,000. The cost for Option 2 with the mesh is $353,000. These options are included on Figures 18, 19, 20 and 21 in Appendix C.

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6.9. Section 9 – Station 100+30 to 102+10

Section 9 was rated number 13 out of the 15 sites with an RHRS score of 341. The slope is 180 feet long with a maximum height on the order of 48 feet and a slope area of roughly 12,600 square feet. Option 1 would include 435 feet of rock bolts, 63 hours of scaling, 200 cubic yards of blast scaling, and 8,550 sq. ft. of Attenuator fence. Option 2 would be similar but would include the installation of 11,700 sq. ft. of draped mesh. A third option would replace a portion of the draped mesh (115 feet long) with injected polyurethane resin (PUR), which would reduce the draped mesh to 4,550 sq. ft. This would require 64,400 pounds of PUR. The additional draped mesh would provide enhanced protection from rockfalls and reduce required road patrols.

Option 3 (PUR) would provide the same level of improvement and further reduce maintenance requirements. The cost for Option 1 is $249,800. The cost for Option 2 with the mesh is $284,900. Due to the high cost of PUR, Option 3 would cost $1,229,500. The hazard reduction provided by the PUR does not warrant this additional investment. These options are shown on Figures 21, 22 and 23 in Appendix C.

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6.10. Section 10 – Station 102+10 to 105+00

Section 10 was rated number 4 out of the 15 sites with an RHRS score of 460. The slope is 290 feet long with a maximum height on the order of 96 feet and a slope area of roughly 27,840 square feet. Both options for this section require 515 feet of rock bolts, and 93 hours of scaling.

In addition, Option 1 would include the installation of 26,390 sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 3,300 sq. ft. of shotcrete and 290 feet of roadside Jersey barrier. The shotcrete and barrier in Option 2 would not provide the same level of mitigation improvement as the draped mesh and the barrier would pose additional maintenance issues for ditch maintenance and snow removal. The cost for the rockfall mitigation measures in Option 1 is $180,900. The cost for Option 2 is $183,200 if plain shotcrete is used and for Option 3, $281,200 if sculpted shotcrete is used. These options are shown on Figures 24, 25 and 26 in Appendix C.

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6.11. Section 11 – Station 105+00 to 107+60

Section 11 was rated number 14 out of the 15 sites with an RHRS score of 329. The slope is 260 feet long with a maximum height on the order of 68 feet and a slope area of roughly 12,950 square feet. Both options for this section would include 515 feet of rock bolts, 43 hours of scaling, and 8,075 sq. ft. of Attenuator fence. In addition, Option 1 would include the installation of 11,650 sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 6,320 sq. ft.

of shotcrete and 260 feet of roadside Jersey barrier. The shotcrete and barrier in Option 2 would not provide the same level of mitigation improvement as the draped mesh and the barrier would pose additional maintenance issues for ditch cleaning and snow removal. The cost for the rockfall mitigation measures in Option 1 is $271,800. The cost for Option 2 is $376,300 if plain shotcrete is used and for Option 3, $565,900 if sculpted shotcrete is used. These options are shown on Figures 26, 27, 28 and 29 in Appendix C.

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6.12. Section 12 – Station 107+60 to 111+60

Section 12 was rated number 10 out of the 15 sites with an RHRS score of 362. The slope is 400 feet long with a maximum height on the order of 68 feet and a slope area of roughly 27,200 square feet. Three options were developed for this section. All three options would include 525 feet of rock bolts, and 91 hours of scaling. In addition, Option 1 would include the installation of 25,200 sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 1,790 sq. ft. of shotcrete and 400 feet of roadside Jersey barrier. Option 3 would add the use of PUR to Option 2 to stabilize 7,500 sq. ft. of slope face. This would require 64,400 pounds of PUR. Option 1 provides the highest level of enhanced protection from rockfalls. Option 3 would provide more slope stability improvement then Option 2. The cost for Option 1 is $187,500. The cost for Option 2 with the roadside barrier and plain shotcrete is $167,700 and for Option 3 if sculpted shotcrete is used, $221,400. For Option 4, the addition of 60,000 pounds of PUR would add $900,000 to the cost of Option 2. The hazard reduction provided by the PUR does not warrant this additional investment. These options are shown on Figures 29, 30, 31 and 32 in Appendix C.

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6.13. Section 13 – Station 111+60 to 115+35

Section 13 was rated number 6 out of the 15 sites with an RHRS score of 454. The slope is 375 feet long with a maximum height on the order of 65 feet and a slope area of roughly 24,375 square feet. Both options for this section would include 850 feet of rock bolts and 81 hours of scaling. In addition, Option 1 would include the installation of 22,500 sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 5,815 sq. ft. of shotcrete and 375 feet of roadside Jersey barrier. The shotcrete and barrier in Option 2 would not provide the same level of mitigation improvement as the draped mesh and the barrier would pose additional maintenance issues for ditch cleaning and snow removal. The cost for the rockfall mitigation measures in Option 1 is $219,300. The cost for Option 2 is $286,800 if plain shotcrete is used and for Option 3, $461,300 if the additional sculpted shotcrete layer is applied. These options are shown on Figures 33, 34, 35 and 36 in Appendix C.

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6.14. Section 14 – Station 115+35 to 118+70

Section 14 was rated number 15 out of the 15 sites with an RHRS score of 300. The slope is 335 feet long with a maximum height on the order of 56 feet and a slope area of roughly 14,010 square feet. Three options were developed for this section. All three options would include 640 feet of rock bolts, and 47 hours of scaling. In addition, Option 1 would include the installation of 12,425 sq. ft. of draped mesh. In lieu of the draped mesh, Option 2 would include 1,500 sq. ft. of shotcrete and 335 feet of roadside Jersey barrier. Option 3 would replace the mesh and the shotcrete with PUR to stabilize 9,000 sq. ft. of slope face using 72,000 pounds of PUR. Option 1 provides the highest level of enhanced protection from rockfalls. Option 3 would provide more slope stability improvement then Option 2. The cost for Option 1 is $149,300. The cost for Option 2 with the roadside barrier and plain shotcrete is $158,700 and for Option 3, if sculpted shotcrete is used, $203,700. Option 4 using PUR would cost $1,208,700. The hazard reduction provided by the PUR does not warrant this additional investment. These options are shown on Figures 37, 38 and 39 in Appendix C.

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6.15. Section 15 – Station 118+70 to 120+80

Section 15 was rated number 2 out of the 15 sites with an RHRS score of 526. The slope is 210 feet long with a maximum height on the order of 160 feet and a slope area of roughly 20,800 square feet. This section poses a different rockfall condition that the other 14 sections. The material is composed of glacial till that contains large, some very large, boulders in a soil matrix.

As the soil supporting the boulders is eroded away by rain, snowmelt and wind action, the boulders are released and the narrow ditch at the base of the slope can not retain these events.

The resulting rockfalls often reach the road and some have crossed the road and can be seen in the river. In cases like this, two choices are available, improve the effectiveness of the fallout area or place support against the till so boulders are not released. With both approaches, the larger, poorly supported boulders that are concentrated in the mid-slope area as shown in the photograph above must be removed by scaling. To accomplish this, 60 hours of scaling would be required.

Option 1 (improving the fallout area effectiveness) would require placing a three-high gabion wall along the edge of the roadway to intercept rolling rocks. For traffic safety the gabion wall would require a jersey barrier between the wall and the travel lane. To provide adequate storage capacity and room for maintenance, this option would require a 5-foot-shift of the alignment towards the river.

Option 2 (retaining the boulders in place) would require installing an anchored high-strength steel mesh on the slope with an average anchors spacing of 10-foot-on-center. The mesh would need to extend over the brow of the existing ravel area to inhibit future expansion of the affected area.

In addition to scaling, Option 1 includes installing 220 linear feet of 9-foot tall gabion wall. The cost for this option would be on the order of $118,000 not including any cost for realigning the roadway. Option 2 would require 24,000 sq. ft. of anchored mesh. The cost for this option would be $608,000 but no realignment costs would need to be added to this amount. In addition, August 2010 26 Cornforth Consultants, Inc.

meshing the slope would likely provide an opportunity for vegetation to reestablish on the slope, which would further stabilize the till. These options are included on Figure 40.

6.16. Mitigation Quantities and Cost Summary

Th…