28_WY.PFH.4-1(4).Beartooth_Highway.Tech_Memo.Slope_Stability.2004.pdf

To: Tim Rogers, Supervisory Structural Engineer, CFLHD

From: Matt DeMarco, Geotechnical Engineer, CFLHD

Through: Scott Anderson, Lead Geotechnical Engineer, CFLHD

Subject: Slope Stability Assessment for the Beartooth Ravine Retaining Wall, WY FS 4-1(4)

In support of design efforts currently underway by PBS&J, the CFLHD geotechnical group has been requested to provide slope stability analyses and supplemental design recommendations for the proposed Beartooth Ravine retaining wall construction, Stations 41+500 to 42+000. The February 2002 URS geotechnical report describes the existing rock fill and talus slope as being highly variable along the length of the proposed wall, resulting in a marginally stable slope condition. Subsequent drilling programs along this segment by URS (September, 2002) and Kleinfelder (December, 2003) have further confirmed the variability of fill/talus conditions and depths to bedrock at anticipated wall face/foundation locations. The potential for slope instabilities and/or localized excessive settlement arising from MSE wall construction and associated surcharge loading has prompted the recommendation for a bedrock-socketed micropile with footing foundation. However, questions remain regarding the potential for slope instabilities to occur due to wall construction activities and/or additional roadway and retained fill surcharge loads not supported by the proposed wall foundation. Questions have also been raised as to whether a micropile foundation is required along the entire length of the wall – particularly along shorter wall height sections – warranting additional evaluation of wall bearing capacities, sliding, etc.

To address these questions, site-specific loading conditions (e.g., variable wall heights, wall face setbacks, roadway surcharge loads, etc.) have been evaluated based on the observed slope setting, subsurface investigation results, and recommended and published physical properties for similar fill/talus materials. The following describes the existing slope setting in the wall construction area, design assumptions considered, and results for global slope and wall stability evaluations.

Existing Slope Setting

• Three soil/rock units were generally identified by the various drilling programs: (1) man-placed fill resulting from the original roadway excavation, used as roadway subgrade and sidecast fill (gravel, cobbles, and boulders in a sandy to gravelly matrix), (2) existing slope talus (similar in composition to the man-placed fill), and (3) moderately jointed granite to granite-gneiss bedrock. Discerning between fill and talus was difficult in most of the borings, so the analyses described herein treat these materials as one unit. Fill/talus thickness

Technical Memorandum

Date: April 30, 2004 ranges from 2 to 13 m, generally following the projection of the upslope bedrock topography along the length of the proposed wall.

• Based on the September 2002 URS geotechnical report (and supplemented by the December 2003 Kleinfelder report), these units may be further characterized as follows:

o The fill/talus units exhibit a wide-range of core recoveries and Standard Penetration

Test values (SPT’s), but can generally be described as angular cobbles and boulders with intervening voids often filled with loose to medium dense sands and gravels (as determined by low SPT values). Open voids were also commonly encountered during drilling. SPT values commonly reached refusal due to the high cobble/boulder content, and coring through boulders (upwards of 1.5 m) was often required.

o Bedrock has been characterized as a strong to moderately strong granite and granite-gneiss, generally exhibiting high core recoveries (+90%) and fair-to-excellent Rock Quality Designations (RQD’s). Sufficient jointing was encountered to warrant moderate Rock Mass Ratings (fair-to-good).

• Groundwater was not encountered during drilling, and is not expected to affect slope stability during or following wall construction due to the porous nature of the fill, talus, and bedrock.

• No slope failures are evident along the proposed wall segment, and only minimal outboard lane settlement was observed within the existing roadway. However, minor shifting within the fill/talus groundmass was encountered during drilling at several locations along the outboard slope, near the proposed wall face.

Based on site observations and the results of the subsurface investigations, the existing slope is assumed to be stable over the range of fill/talus conditions and depths encountered.

However, the variability of the fill/talus soil mass, steepness of the existing slope, variable bedrock depth, and unknown talus-rock contact conditions indicate the need to minimize slope surcharge loads during wall construction through the use of deep foundations.

Proposed Wall Construction

• An MSE structure has been proposed for the entire length of the Beartooth Ravine wall. The wall extends to a maximum height of 8.5 m, is founded on a micropile-supported concrete footing (either CIP, with a width approximately 50% the wall height, or precast, segmental pile caps), utilizes soil reinforcing element lengths 70% of the wall height, and includes regularly spaced ground anchor tie-backs at the footing or through the wall face for additional lateral resistance.

• Wall footing embedment is currently planned at approximately 0.7 m below existing ground at the wall face, to the top of the micropile footing. It is anticipated that the wall face setback will range from 1.5 to 2.0 m to the bottom of the footing at all locations (assuming a nominal 1V:1.33H slope ratio).

• A temporary access road is assumed for wall construction, located immediately behind the wall footing. This added width might necessitate temporary shoring at several locations to allow for maintenance of traffic. The preferred shoring system is currently assumed to be a temporary soil nail installation.

• A footing/reinforcement design has been proposed by PBS&J (and is supported by the CFLHD geotechnical group) whereby the concrete footing (continuous cast-in-place (CIP) or segmental precast design) is constructed over a 0.4H to 0.5H footing width, but full-length reinforcements (0.7H) are then extended into the construction access road during wall erection. This approach provides for improved reinforcement pullout resistance, greater load transfer to the deep foundation, and a simpler, more efficient wall construction than employing truncated reinforcement lengths at the bottom of the wall, as originally proposed.

• It has been proposed to first construct a wire-faced MSE wall, and then follow with a precast rusticated panel facing. The precast panels sit on a CIP spread footing. This approach has several advantages. Full-height tilt-up panels serving as the integral wall face would require internal support during wall construction, complicating reinforcement and fill placement and slowing the construction process – problems avoided by bringing the wire facing up with wall reinforcement placement. Crane access was extremely limited with the original design, requiring a smaller crane and variably sized facing panels (due to crane weight restrictions).

Placing facing panels at the completion of the wall allows the standardization of panel widths, the use of a larger crane, and places the crane/panel surcharge load on the MSE structure over the deep foundation, as opposed to loading the roadway fill and/or talus slope.

• Wall face settlement, although anticipated to be minimal with well-compacted granular wall fill, was nonetheless an issue with the originally planned precast wall facing. Utilizing a wire-faced wall allows differential settlement to occur for varying wall heights prior to wall facing panel installation. With the CIP facing footing constructed following wall completion (either shortly after or at the time of facing panel installation), the final wall facing elements could be placed during the early shoulder season of ’06, allowing 7-8 months of wall settlement to occur prior to panel installation. The open wire facing also provides the opportunity to visually assess wall performance prior to final facing. Employing a micropile footing along the entire length of the retaining wall further minimizes problems associated with differential settlement.

Analysis Approach and Results

In view of the slope setting and proposed MSE wall design described above, two design questions have been raised concerning slope stability during construction and the ability to optimize the micropile foundation:

(1) Discounting the MSE wall loads (and associated surcharge loads) transferred to bedrock through the micropile foundation, do additional retained fill and traffic/construction surcharge loads create conditions for slope instabilities during and/or after wall construction?

(2) Is it possible to eliminate the use of the micropile foundation along portions of the retaining wall where shorter wall heights are planned (say, <2-4 m wall heights)?

To address these questions, slope stability analyses were conducted utilizing the FHWA program “Reinforced Slope Stability Analysis”, (ReSSa 2.0). Initial evaluations were conducted on the existing roadway and slope setting to determine appropriate fill and talus strength parameters for the marginally stable slope described in the aforementioned geotechnical reports (assumed safety factors ranging from 1.1 to 1.2). Subsequent evaluations utilized these derived strength properties to investigate changes in slope factor of safety for three different wall heights

(3-, 6-, and 9-m) without the contribution of micropile footing support. The strength parameters developed from the initial setting evaluation are given in the following table:

Talus and Fill Bedrock φ = 40o γ = 21.6 kN/m3 c = 2.5 kPa φ = 45o γ = 25.9 kN/m3 c = 240 kPa

URS recommended a lower, more conservative friction angle (35o, which is less than the existing slope angle of 38o) and zero cohesion within the fill/talus units; however, using these values in the ReSSa evaluations resulted in unrealistically low safety factors (<1.0) for the existing slope – based on existing slope angles and stability observations. The tabled strength values, resulting from the existing-slope stability assessment, appear reasonable for the materials described in the boring logs, and were used for all subsequent evaluations (tabled friction values from various authors range from 40o to 45o for granitic rocks and similar fills).

The results of the global stability evaluations for the existing and three wall height cases are summarized below:

• Backslope stabilization at the time of wall excavation, prior to wall construction, is the responsibility of the Contractor, and was not addressed in this evaluation. It is assumed that a temporary soil nail installation will be used to provide for adequate fill/talus stability to ensure maintenance of traffic on the overlying existing roadway.

• Wall excavation, including excavation equipment surcharge loads, results in a lowering of slope driving loads (soil mass unloading), thereby increasing slope stability safety factors – however, only slightly. It is possible, however, that construction activities may result in localized settling and densification of the talus rock mass.

• Wall construction, assuming no micropile foundation support, results in very small additional surcharge loads compared to the volume of fill/talus material contained within the calculated critical failure zone – for all three wall heights evaluated. The critical failure envelope lies wholly within the slope talus, and extends well downslope from the planned roadway. The wall is set back from the slope 1.5 to 2 m, depending on wall height, which results in only a small portion of the wall construction volume and traffic surcharge being added to the original slope load. As a result, the theoretical slope stability factor of safety changes very little (less than 3%) between the cases evaluated.

• The toe slope factor of safety in front of the wall increases due to relieving overlying driving loads. It is unlikely that a global stability failure would occur in front of the wall for the case without the micropile foundation, and will not be prone to slope failure when the wall loads are transferred to underlying bedrock.

• Crane pad stability at the Beartooth Ravine Bridge will be evaluated by the Contractor for the specific equipment proposed for bridge construction. It is anticipated that surcharge loads will be sufficiently high to potentially develop slope instabilities if pad loads are not transmitted to underlying bedrock (piles, tie-backs, etc.).

Based on slope stability evaluations, which consider the MSE wall loads resting on the slope talus, there are no foreseeable global stability issues associated with the proposed micropile-founded wall design. Applied slope loads will generally decrease as a result of wall construction, thereby improving the slope safety factor. Talus compaction from excavation and wall construction activities should also benefit localized slope stability along the working bench.

Although it is recommended to use the micropile foundation along the entire length of the wall to mitigate differential settlement concerns, a range of wall heights were nonetheless evaluated to determine if the shorter wall segments could be founded directly on the talus. Based on the aforementioned theoretical global stability analyses, wall heights less than 2.5m result in very little additional slope surcharge load, based on a nominal 1.5m wall face setback from the original ground line – effecting no change in calculated factors of safety. With increasing wall height, additional reinforced fill surcharge does begin to impact the global safety factor, limiting the talus-founded wall option to less than 2.5m – as determined by engineering judgment.

Bearing capacity, sliding, and overturning calculations, considering wall loads adjacent to a steep toe slope and approximate reinforcement embedment lengths of 1.0H, further indicate that theoretically stable wall designs may be constructed directly on the talus for wall heights less than 2.5m. Construction surcharge loads during wall bench excavation and micropile foundation construction will greatly assist in densifying the talus, which will further benefit stabilization of the shorter wall height segments by reducing the potential for differential settlement caused by local variations in the strength and stiffness of the fill/talus.

If we can be of any further assistance in this matter, please don’t hesitate to contact me directly at (303) 716-2193.

______________________________________________ Date_________________ Matthew J. DeMarco Geotechnical Engineer, CFLHD

References

URS Corp. Final Report, 2002, “Beartooth Highway Retaining Wall Feasibility Study and

Geotechnical Recommendations, Beartooth Highway, (U.S. 212, Park County), Wyoming,” FHWA Contract Report, URS Corp., Denver, CO, Project No. 68- FHAT0039.00, Feb. 5, 2002, 30 pp. (plus appendices).

URS Corp. Final Report, 2002, “Geotechnical Investigation, Beartooth Ravine and Fen

Mitigation Area, Beartooth Highway, U.S. 212, Park County, WY”, FHWA Contract Report, URS Corp., Denver, CO, Project No. 68-FHAT0039.00, Sept. 23, 2002, 15 pp.

(plus appendices).

Kleinfelder, Inc., Final report, 2003, “Geotechnical Investigation Report, Beartooth highway

Project, Shoshone National Forest, Park County, WY”, FHWA Contract Report, Kleinfelder, Inc., Colorado Springs, CO, Nov. 7, 2003, 15 pp.