Final_Beartooth_Geotechnical_Memo.pdf

To: Jim Kerrigan, Project Manager, CFLHD, Lakewood CO

From: Dustin Robbins, Geotechnical Engineer, CFLHD, Lakewood CO James Arthurs, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Marilyn Dodson, Lead Geotechnical Engineer, CFLHD, Lakewood CO

Subject: Updates and Revisions to Beartooth Ravine Bridge Foundation and

Retaining Wall Design

Project: WY TIG US212(9) Beartooth Highway

1.0 BACKGROUND

The Federal Highway Administration, Central Federal Lands Division (FHWA-CFLHD), in cooperation with the Wyoming Department of Transportation, Yellowstone National Park, and U.S. Forest Service, Shoshone National Forest, is proposing to construct the final phase of the Beartooth Highway Reconstruction Project, which has consisted of multiple phases of improvements over the past 20 years to U.S. 212, also known as Beartooth Highway in northern Wyoming. This final phase consists of the Beartooth Ravine Bridge and retaining walls associated with roadway improvements through steep terrain for approximately 800 feet.

Between 1998 and 2005, a series of geotechnical studies for the Beartooth Project were completed by multiple consultants, including geotechnical investigations and design recommendations for several bridges, retaining fill walls, and slope cuts along this route.

The purpose of this memorandum is threefold: 1) review available data relevant to the design and recommendations of the Ravine Bridge foundation elements and the retaining wall adjacent to the Ravine Bridge, 2) consider additional project/construction information made available since the publishing of the original recommendations for this project and modify as necessary, and 3) provide final recommendations, including updating recommendations to meet requirements of the 2014 AASHTO LRFD Bridge Design Specifications with interims through 2016.

2.0 BEARTOOTH RAVINE BRIDGE

The proposed three span bridge consists of a concrete deck over steel girders with a reverse-curve. The current bridge design includes piers, abutments, and wingwalls founded on spread footings (and ground anchors at abutment footings) supported on 300-millimeter (12-inch) diameter micropiles with estimated embedment lengths of approximately 5 to 10 meters (15 to 30 feet) in bedrock. A total length of 572 meters (1,876 feet) of micropiles has been estimated on the proposed bridge plans which includes 24 micropiles per pier and 23 micropiles per abutment.

Other micropiles were also designed for supporting the concrete cantilever retaining wall adjacent to the southwest wingwall. The boring information taken at locations near the abutments and piers are summarized in Table 1.

Technical Memorandum

Date: January 22, 2019

TABLE 1 – Summary of Geotechnical Investigation Borings at the Ravine Bridge.

FOUNDATION

LOCATION

EXPLORATION

DESIGNATION

GROUND

ELEVATION

(M)

TERMINATION

ELEVATION

(M)

EXPLORATION

DEPTH

(M)

BEDROCK

ELEVATION

(M)

Abutment 1

THB-1 (URS) 2,702.8 2,694.2 8.6 2701.8

TB-12 (KLF) 2,701.0 2,690.7 10.3 2,694.5

Pier 1* THB-2 (URS) 2,688.6 2,677.6 11.0 2,683.0 Pier 2* THB-4 (URS) 2,687.5 2,677.5 10.0 2,682.6

Abutment 2

THB-5 (URS) 2,694.2 2,682.9 11.3 2,688.1

TB-15 (KLF) 2,692.0 2,678.0 14.0 2,682.0

*The borings for Pier 1 and Pier 2 were drilled approximately 6 meters north of the proposed foundation locations.

A 2004 memorandum prepared by CFLHD titled Compilation of Geotechnical References presents a summary of the 27 geotechnical documents prepared for the project. This memorandum is included as an attachment to this memo for reference. The subsurface data summarized in the following reports contain bridge foundation information pertinent to this section of this memorandum:

URS. Final Report, Geotechnical Investigation of Selected Features: Beartooth Highway, U.S. 212, Wyoming, March 2001.

URS. Final Report, Geotechnical Investigation, Beartooth Ravine Bridge, Beartooth

Highway, U.S. 212, Wyoming, November 2002.

URS. Technical Memorandum, Addendum No. 1, Beartooth Ravine Bridge, Geotechnical

Investigation, June 2003.

Kleinfelder. Geotechnical Investigation Report, Beartooth Highway Project, December 2003.

CFLHD Technical Memorandum, Micropile Types and Ultimate Bond Stress Values for

Retaining Wall and Bridge Construction on the Beartooth Highway Project, November 2004.

2.1 BEARTOOTH RAVINE BRIDGE SITE CONDITIONS

Subsurface conditions assessed from the available geotechnical boring logs indicate that the overburden material consists of a mixture of soil fill and talus ranging in particle size from sand to boulders. The overburden ranges in thickness from 1 meter (3.5 feet) near the west abutment to nearly 6 meters (20 feet) at the east abutment. The fill/talus mixture is underlain by both slightly weathered granite and strong gneiss bedrock. The six unconfined compressive strength tests performed on the bedrock indicate compressive strengths ranging from 37 to 131 Mega Pascale’s (MPa) (5,365 to 18,995 pounds per square inch (psi)) with an average of 76 MPa (11,020 psi). RQD ranged from 37 to 100 percent.

The ground surface consists of a steep ravine with a cover of large boulders/talus. The boulders are several feet in diameter and generally appear to be large diameter talus but portions of the boulders appear to be shot rock fragments associated with the original road construction. The area around the west abutment has more soil and less talus at the ground surface. There is minimal vegetation at the area of the proposed foundations.

2.2 PREVIOUS BEARTOOTH RAVINE BRIDGE FOUNDATION RECOMMENDATIONS

The geotechnical reports conclude that due to the presence of talus and boulders in the overburden material, the bridge should be supported on either drilled shafts or micropiles.

Design recommendations for both drilled shafts and micropiles were originally provided in Allowable Stress Design (ASD) format. In addition to deep foundation design recommendations, discussions of construction methods in the boulder talus such as casing requirements and drilling methods were also discussed.

Recommendations concerning drilled shafts socketed into bedrock are summarized below:

Allowable Tip Resistance = 5MPa (100 ksf) Allowable Side Resistance = 0.5 MPa (10 ksf) ASD Factor of Safety = 2.5 for both side and tip resistance Minimum Drilled Shaft Embedment in bedrock of 3.5 meters

Recommendations concerning micropiles socketed into bedrock are summarized below:

Type A micropiles are recommended Allowable bond stress of the soil/rock bond = 0.5 MPa (10 ksf) ASD Factor of Safety = 2.5 Minimum embedment of 6 meters of at least one micropile per foundation Permanent casing required to be embedded at least 0.5 meters into bedrock

Lateral resistance for both drilled shafts and micropiles were assumed to be modeled using LPILE version 3.0 with the following LPILE parameters:

TABLE 2 - Previous LPILE Parameters by Consultants

Material P-Y Model

Subgrade Modulus

(GPa/m)/(ksi/ft)

Effective Unit Weight (kN/m3)/(pcf)

Friction Angle (deg)

Effective Cohesion (kPa)/(psi)

Compacted Embankment Fill

Sand 41/

1,813 21.3/

35 0 --

Sand/Gravel/ Cobbles

Sand 27/

1,194 11.8/

35 0 --

Bedrock Stiff Clay

543/ 24,000

11.8/

239/

0.007

2.3 REVISED BEARTOOTH RAVINE BRIDGE FOUNDATION TYPE RECOMMENDATION

Based on the latest design drawings and the technical memorandum developed by CFL in 2004, micropiles were selected as the proposed foundation system for the alignment based on the following reasons:

Efficiency of a single foundation type for multiple bridges. Prior to being divided into segments, the entire alignment was designed as one project with several bridges. There were most likely savings in design and construction of one foundation type.

Less risk in drilling micropiles than drilled shafts. It was likely assumed that drilling small diameter micropiles through boulder talus would be easier than drilling larger diameter drilled shafts.

Micropiles were also proposed for retaining wall support. Efficiency of a single foundation type for all structures was likely a consideration.

Based on discussions with the design team and on additional design and construction information available to the designers, the use of drilled shafts was re-evaluated as the preferred alternative for the revised Beartooth Ravine Bridge foundation. After a review of the previously provided geotechnical recommendations, it is recommended that drilled shafts be considered in place of micropiles for the foundations for Beartooth Ravine Bridge for the following reasons:

Since the current project is limited to the Beartooth Ravine Bridge alone and not several bridges originally considered, there is no longer any efficiency gained from the use of micropiles on multiple bridges. Additionally, a different foundation solution for the retaining wall is also being proposed, which could completely remove micropiles from the project.

Recent projects have shown the use of micropiles to often be cost prohibitive due to the limited availability of contractors.

The very large quantity of micropiles per foundation will likely increase costs above drilled shafts.

Construction techniques for rock-socketed drilled shafts have developed over the last decade and their use has become more prevalent, resulting in reductions in unit prices and lower risk with standardized repair and testing techniques.

The use of LRFD instead of the more conservative ASD resulted in larger factored resistance values than previously provided, partially due to the increased research that has been conducted on rock socketed drilled shafts in the past decade which resulted in higher resistance factors. The shafts should therefore be more economical than previously thought.

The use of a single shaft or two drilled shafts per abutment or pier will likely provide much greater lateral resistance than micropiles. The poor lateral resistance of micropiles may have been the reason that a large number of micropiles and abutment tieback anchors were proposed for each foundation.

2.4 UPDATED BEARTOOTH RAVINE BRIDGE FOUNDATION DESIGN RECOMMENDATIONS

General Discussion Based on the available geotechnical information provided by others and the discussion presented in Section 2.3 of this memo, drilled shaft foundations socketed into rock will provide suitable support for the structure provided the recommendations in this memo are followed. All of the drilled shaft axial resistance will be developed within the rock socket, and it is therefore critical that socket be constructed properly including an appropriate cleanout method that can be verified. The contractor should anticipate difficult drilling conditions and the need for temporary casing due to the presence of cobbles and boulders within the overburden material. Detailed design recommendations concerning axial resistance, lateral resistance, group effects, and construction considerations are presented in the following sections.

The design information presented in Table 3 was either interpreted from the project plans, estimated from the available geotechnical information, or provided by the structural engineers.

TABLE 3 – Summary of Bridge Information

FOUNDATION

PROPOSED TOP OF

SHAFT ELEVATION

(M/FT)

ESTIMATED

DEPTH TO ROCK

(M/FT)

FACTORED

STRENGTH LOAD

(KN/KIPS)

Abutment 1 2,696 / 8,845 1.5 / 5 1,471 / 331

Pier 1 2,680 / 8,792.5 1.5 / 5 6,873 / 1,545 Pier 2 2,678.5 / 8,787.5 1 / 3 5,826 / 1,310

Abutment 2 2,686.5 / 8,814 4.5 / 15 1,972 / 443

Geotechnical Axial Resistance The current AASHTO code requires the use of Load and Resistance Factor Design (LRFD) as opposed to the originally used ASD. To make the following updates, the nominal (unfactored) resistance values were back calculated based on the provided “allowable” values and the factor of safety. Once the nominal values were determined, the appropriate resistance factors from the current code were applied to determine the factored resistance at the strength limit state. The following table shows the updated recommendations for the axial resistance of drilled shaft foundations:

TABLE 4 - Axial Resistance of Drilled Shafts Updated to Current Code

RESISTANCE

COMPONENT

ALLOWABLE

RESISTANCE

(MPA/KSF)

NOMINAL

RESISTANCE

(MPA/KSF)

RESISTANCE

FACTOR

FACTORED

RESISTANCE

(MPA/KSF)

Side Resistance 0.5/10 1.25/25 0.55 0.68/13.75 Tip Resistance 5.0/100 12.5/250 0.50 6.25/125

In addition to the updated resistance values for LRFD, the original recommendations required a minimum embedment of 3.5 meters (11.5 feet). New code requirements and recent FHWA guidance for drilled shafts socketed into rock require a minimum embedment of at least two times the shaft diameter. It is therefore recommended that the minimum embedment be amended to the new code requirement of two times the shaft diameter.

Using the unit axial resistances updated to the current code and the minimum embedment requirements presented in AASHTO and FHWA, the following table presents the calculated axial resistance for a range of drilled shaft diameters. If greater capacities are desired, side resistances for longer rock sockets can be calculated upon request.

TABLE 5 – Calculated Axial Resistance by Shaft Diameter

SHAFT

DIAMETER

(MM)

MINIMUM

ROCK SOCKET

LENGTH (MM)

FACTORED SIDE

RESISTANCE

(KN/KIPS)

FACTORED TIP

RESISTANCE

(KN/KIPS)

TOTAL FACTORED

AXIAL RESISTANCE

(KN/KIPS)

610 1,220 1,539 / 346 1,749 / 393 3,288 / 739 762 1,524 2,402 / 540 2,729 / 614 5,131 / 1,154 915 1,830 3,463 / 779 3,935 / 885 7,399 / 1,663

1,067 2,134 4,709 / 1,059 5,351 / 1,203 10,061 / 2,262 1,220 2,440 6,157 / 1,384 6,996 / 1,573 13,153 / 2,957 1,372 2,744 7,786 / 1,750 8,848 / 1,989 16,635 / 3,740 1,525 3,050 9,620 / 2,163 10,932 / 2,458 20,551 / 4,620

The calculated axial resistances assume redundancy. If single shafts are used, then the strength limit state resistance factor should be reduced by 20 percent. The axial resistance does not require reduction due to group effects provided the shafts are spaced a minimum of three diameters, center to center.

Lateral Resistance The LPILE parameters provided by the consultants were based on the LPILE software version

3.0. Since the original reports in the early 2000’s, LPILE has produced several upgrades to the software and included additional P-Y models that may be more appropriate than those provided in the original reports. It is recommended that the LPILE parameters in Table 6 be used that have been updated to include newly available P-Y models.

TABLE 6 – Recommended LPILE Parameters

Foundation Top

Elevation (m/ft)

Bottom Elevation

(m/ft)

P-Y Model

Effective Unit

Weight (pcf/kN/m3)

Drained Friction Angle (deg)

Soil Modulus Constant

(Mpa/m/pci)

Uniaxial Compressive

Strength (Mpa/psi)

Abutment

Ground Surface

2,694.5 / 8,840

Sand (Reese)

135 / 21.2 35 61 / 225 N/A

2,694.5 / 8,840

Bottom of Foundation

Strong Rock

150 / 23.5 N/A N/A

68.9 / 10,000

Pier 1

Ground Surface

2,678.5 / 8,787.51

Sand (Reese)

135 / 21.2 35 61 / 225 N/A

2,678.5 / 8,787.51

Bottom of Foundation

Strong Rock

150 / 23.5 N/A N/A

68.9 / 10,000

Pier 2

Ground Surface

2,677.5 / 8,784.51

Sand (Reese)

135 / 21.2 35 61 / 225 N/A

2,677.5 / 8,784.51

Bottom of Foundation

Strong Rock

150 / 23.5 N/A N/A

68.9 / 10,000

Abutment

Ground Surface

2,682 / 8,799

Sand (Reese)

21.2 / 135 35 61 / 225 N/A

2,682 / 8,799

Bottom of Foundation

Strong Rock

135 / 21.2 N/A N/A

68.9 / 10,000

Note 1: The borings performed for the piers were drilled 5 to 7 meters to the north of the proposed foundation locations where the ground surface was 4 to 5 meters higher. The layer boundary at the pier locations was estimated by assuming the same thickness of overburden material is present at the pier locations that was measured at the boring locations.

The LPILE parameters provided are for single drilled shafts and do not account for the reduced lateral resistance of shafts in a group. P-multipliers are a function of the number of rows of shafts and the center to center spacing in the direction of loading. P-multipliers are specified in Table 10.7.2.4-1 in AASTHO

Construction Considerations Sloping bedrock is present at the foundation locations and therefore the estimated top of bedrock elevations will likely vary within the footprint of the foundation footing/pier cap. The contractor should determine the bedrock elevation at the location of each drilled shaft prior to drilling the shaft and fabrication of the reinforcement cage so that the minimum rock socket length of two times the shaft diameter can be achieved. The findings of these explorations should be included in the drilled shaft construction plan submittal required by the contract specifications.

In addition, to reduce the possibility for differential settlement, the pier cap/footing should not be founded on a combination of bedrock and talus or backfill soil. Where bedrock is encountered at the bottom of footing elevation, the bedrock should be overexcavated a minimum depth of 300 mm and replaced with structure backfill to create a uniform bearing pad.

Bedrock at the site will likely require rock coring techniques to excavate the rock sockets. The means and methods should be determined by the contractor and a detailed construction plan with proposed equipment and excavation techniques should be provided for review by CFLHD as required in the project specifications.

Due to the overburden material being composed of talus with cobble and boulder sized particles with only a small percentage of sand and fines, it is likely that the boreholes will require casing rather than slurry to maintain sidewall integrity. Since there are possibly significant void spaces between cobbles and boulders where concrete could flow outward when a temporary casing is removed, the contractor may need to consider adjusting the concrete slump to limit concrete losses.

Overburden talus material consists of cobbles and boulders, up to several feet in diameter in some locations. Specialized augers or other tools may be required to extract boulders and may result in reduced drilling production.

Depending on the actual depth to bedrock at the foundation locations, shoring will likely be required. Shoring within the cobbles and boulders may be difficult and the contractor should be prepared for these conditions.

Steel crosshole sonic logging tubes should be installed in all drilled shafts prior to concrete placement for drilled shaft integrity testing during construction. The recommended number of access tubes and tube spacing are dependent on the selected shaft diameter as indicated in Table

7. The crosshole sonic logging tests should be conducted in accordance with ASTM D-6760.

TABLE 7 – Recommended Number of CSL Access Tubes

SHAFT DIAMETER

(MM)

NUMBER OF ACCESS

TUBES

ACCESS TUBE SPACING

(DEGREES)

600 < D < 1,050 3 120

1,050 D < 1,500 4 90

Abutment Walls

Abutments and wing walls should be designed to resist lateral earth pressures and other applicable lateral loads in accordance with AASHTO. Lateral earth pressure is influenced by the strength of the abutment backfill, the presence or absence of water, and the ability of the abutment or wall to move in response to lateral loads. Other loads, such as live loads, construction loads, and soil compaction loads should also be considered in the design.

Unbalanced water behind an abutment or wall adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment and wing wall backfill should consist of structural backfill as specified in Section 704.04 of FP-14.

The coefficient of at-rest earth pressure should be used for design if the abutment is so restrained that it cannot be expected to rotate (deflect at the top) 0.002 times the wall height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for wall design. Active and at-rest lateral earth pressures of native materials, properly placed and compacted structural backfill, and unclassified borrow above the water table are presented in Table

10. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

TABLE 8 – Lateral Earth Pressures for Bridge Abutments

Backfill Type Assumed Backfill

Properties Case

Unfactored Equivalent Fluid Density (pcf)

Structural Backfill c = 0 psf = 34 deg.

γ = 125 pcf

Active 32

At-Rest 55

Unclassified Borrow c = 0 psf = 30 deg.

γ = 125 pcf

Active 37

At-Rest 63

It is understood that geosynthetic reinforcement of the abutment backfill may be considered to reduce lateral pressures on the abutment walls. Based on a review of the borings and the elevations obtained from the draft bridge plans, the wall bottoms for each abutment wall are expected to be founded on either boulder talus material or structural backfill. A maximum 1-foot spacing of geosynthetic material should be maintained within the backfill wedge. Earth pressure at the face of a GRS structure is considerably lower than the active earth pressure. Based on Equation 2 in FHWA (2011a), an active coefficient (Ka) of 0.3, vertical reinforcement spacing of 1 ft, the lateral earth force per unit width of wall will be less than 0.1 times the soil unit weight.

Therefore, use of an equivalent active coefficient of 0.1 for design of the wall is recommended to account for lateral earth pressures.

2.5 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the Beartooth Highway Project design are based on the (AASHTO) LRFD Bridge Design Specifications, 7th edition, 2015, 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 Beartooth Highway Project site was taken near the bridge site at 44.93757º N latitude and 109.60757º W longitudinal, 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 end project.

Ground motions caused by earthquakes are influenced not only by the distance from the fault planes, but also by the geology and soils found at the site. Subsurface profiles similar to the ones within the project, with soil layers overlaying bedrock that significantly differ in stiffness and density, will have amplified ground motions and a resonant period governed by the layer thickness and shear wave velocity of the materials. The damage potential of strong ground motions with respect to a structure is typically affected by the period of the strong earthquake motion and the resonant period of both the soil and the structure.

Seismic wave velocities obtained by refraction and tomography methods were presented in the geotechnical reports. The data indicates bedrock compression wave velocities ranging from about 3,000 m/s to 5,000 m/s with data extending to depths ranging from 10 meters to 25 meters.

Using a common version ratio for rock of 1.7, the low-end compression wave velocity of 3,000 m/s converts to a shear wave velocity of 1,765 m/s. Based on the geologic conditions (very shallow bedrock), the seismic velocity data, and the project boring logs, the average time-weighted shear wave velocity for the top 100 feet (VS100) of subsurface materials was estimated as greater than 760 m/s (2,500 feet per second). Therefore, the site is classified as Class B according to the site class definitions specified in Table 3.10.3.1-1 of AASHTO.

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 9 - Summary of Seismic Parameters Corrected for Site Class B.

TABLE 9: – Summary of Seismic Parameters Corrected for Site Class B.

SEISMIC PARAMETERS

Horizontal Peak Ground Acceleration, (As) 0.128g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.289g

Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.096g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.0

Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.0

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.0

Based on the long acceleration coefficient SD1 value of 0.096g calculated as Fv.S1, the alignment is assigned to seismic hazard Zone 1 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 as depicted in Table 4.7.4.3.1-1 in AASHTO.

3.0 RETAINING WALLS

Initial design of this segment of the U.S. Highway 212 (Beartooth Highway) began in 1998 with an initial geohazard evaluation performed by Woodward-Clyde Federal Services (WCFS, 1998).

Numerous geological and geotechnical studies followed this initial work to provide additional information for design of the proposed highway widening. One section of the roadway is located on a relatively narrow bench with steep inboard and outboard slopes adjacent. The inboard slopes are typically very high natural and cut slopes of in situ bedrock materials, while the outboard slopes are composed of fill and native silt, sand, gravel, and boulders (blast rock and talus). Some combination of cut and fill slopes is required to widen the bench for the proposed roadway section. During the project development phase, fill side retaining walls were proposed as the primary method to provide for road widening.

A cantilevered concrete wall consisting of pre-cast elements was selected as the most economic wall type. Post-tensioning of structural elements was suggested by Woodward Clyde and URS to mitigate potential subgrade movement (settlement) (WCFS, 1998; URS GWCFS, 1999; URS, 2001; and URS, 2002). Further cost analysis indicated that a Mechanically Stabilized Earth (MSE) wall with a pre-cast concrete panel facing would provide similar aesthetics at a reduced cost compared to the cantilever concrete wall (URS, 2002). Use of micropile foundations were recommended by URS to transfer loads from the spread footing foundation to bedrock.

Kleinfelder (2003) indicated that shifting of boulders in the talus materials was possible and could be a concern during construction of a micropile foundation system. PBS&J (2004) recommended adding ground anchors to the foundation system to support lateral loads on the foundation system. The PBS&J design utilized relatively conservative soil parameters, with friction angles of 35 and 34 for foundation materials (talus) and wall backfill, respectively.

A memo prepared by representatives of the geotechnical engineering group at CFL reevaluated the soil parameters used in the PBS&J design (FHWA, 2004a). Based on soil classification tests and back analysis, less conservative values were selected. Analysis based on these values indicated that the proposed walls would have negligible effect on the overall stability of the talus slopes. The memo recommended that deep foundations only be considered to address potential settlement of the wall founded on talus. Yeh and Associates (2005) presented foundation grouting and micropile alternatives to mitigate potential settlement in the talus materials.

As a result of the various proposed design approaches, in June, 2018, the CFL geotechnical team was asked to evaluate the available information (reports, boring logs, analysis, etc.) and develop recommendations to reduce the overall cost of the proposed retaining wall. These analyses and recommendations are presented in the following sections.

3.1 PREVIOUS RETAINING WALL DESIGN

Steep topography adjacent to the Beartooth Highway from approximate Sta. 41+531 to Sta.

41+983 necessitates significant cut and/or fill construction to widen the existing road. Fill-side retaining walls are proposed as the primary method to increase roadway width. The beginning of the retaining wall ties into the adjacent bridge wingwalls and then continues up station for approximately 452 meters (1,490 feet). The wall height is variable along the length of wall due to variability in the side slope geometry, and has several “high points” where the exposed wall height is 6 meters (20 feet) or greater.

Based on the geotechnical investigations as previously discussed, the wall is to be founded on a combination of side cast fill material primarily composed of blast rock and natural talus and colluvial materials. In general, these materials are very similar and can be treated as a single unit for the purposes of the retaining wall design. Due to the large size of boulders in the deposits and their young age, both units are unconsolidated and contain numerous, large voids. When these deposits are loaded, there is potential for these voids to collapse, leading to settlement of materials and structures above. Large voids at the foundation elevation could also lead to loss of wall backfill materials into the underlying strata.

3.2 PREVIOUS RETAINING WALL RECOMMENDATIONS

As previously discussed, several wall system options were explored, but the design decisions were primarily driven by potential foundation settlement. These assumptions led to the selection of a Mechanically Stabilized Earth (MSE) wall with a pre-cast concrete facing supported on micropile foundations.

PBS&J prepared design level drawings for the proposed retaining wall, dated October, 2004.

These drawings depict the MSE wall constructed on a reinforced concrete foundation. The concrete foundation ties into a system of micropiles and ground anchors that transfer the wall loads to stable ground. Current cost estimates based on these drawings were greater than the original estimates primarily due to the cost of the foundation system escalating over time.

3.3 REVISED RETAINING WALL DESIGN RECOMMENDATIONS

The CFL geotechnical team has revisited the previous work to evaluate whether any alternative wall systems may perform adequately in this situation at a lower cost than the previously designed wall. After a number of technical and constructability discussions, a Geosynthetic Reinforced Soil (GRS) wall was selected as a more feasible alternative to the previously designed MSE wall system. GRS is similar to MSE in that it consists of a reinforced, high-quality backfill, with a facing element. However, in a GRS wall, the reinforcement spacing is much closer, typically less than 12 inches, compared to a typical 18- to 24-inch spacing in MSE walls. This close reinforcement spacing forms a composite material, analogous to reinforced concrete. Concrete and soil have high compressive strengths and low tensile strengths, whereas steel and geosynthetic reinforcements have high tensile strengths. When reinforcements at close spacing are included in the soil mass, their tensile strength improves the strength of the soil and forms a composite material.

The GRS mass is both strong and relatively flexible, which will allow it to perform well under the site conditions. A rock filled welded-wire mesh facing was selected because it is also relatively flexible, and will move with the GRS mass should any settlement occur compared to a rigid cast-in-place facing that could easily crack under any settlement. A layer of geosynthetic separation fabric is included at the bottom of the wall to prevent loss of wall backfill into the bearing strata. A traffic barrier is proposed at the top of wall. In the design prepared by PBS&J, this consisted of a concrete guardrail cast integrally to a moment slab. Due to the revised wall system proposed, a post-and-beam guardrail is planned for this alternative, which will both expedite construction of the guardrail and wall, therefore reducing costs for a better technical solution. A conceptual wall section is presented in FIGURE 1 – Conceptual GRS Wall Section

Because the GRS wall is similar to an MSE wall in terms of materials used and construction methods employed, we recommend that the wall be refered to as an MSE in the contract documents. The term GRS will be used in this memo to clarify the design methodology.

FIGURE 1 – Conceptual GRS Wall Section

Due to the depth of the excavation and the wall geometry, shoring will be required. Shoring within the cobbles and boulders may be difficult and the contractor should be prepared for these conditions.

3.4 RETAINING WALL DESIGN METHODOLOGY

Design of a GRS retaining wall considers three general stability criteria: global, external, and internal. Global stability is related to the overall stability of the wall, adjacent improvements (such as roads), and slopes. External stability includes evaluation of sliding resistance, overturning resistance, and bearing resistance. Global and external stability analysis for GRS walls are evaluated in the same way as for other gravity retaining walls such as mass concrete walls, rockery walls, and MSE walls. Internal stability of the GRS wall includes checking strength of the reinforcement, overall load carrying capacity of the GRS mass, and connection strength of the facing elements. However, facing pressure in a GRS wall is considerably lower than in an MSE of the same height (FHWA, 2018).

Design analysis of the GRS wall generally followed recommendations provided in the AASHTO LRFD Bridge Design Specifications, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes (FHWA, 2009), and Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (FHWA, 2018).

The first step of design was to evaluate the global stability of the wall system and determine an appropriate overall geometry (base width and toe embedment) to provide a satisfactory factor of safety. The Slide 7.0 software package was used to evaluate global stability. Slide 7.0 uses limit-equilibrium methods to determine factors of safety for slope stability.

Next, the overall geometry was checked for external stability considerations. The external stability calculations were adapted from the equations presented in the FHWA MSE and GRS manuals (FHWA, 2009; FHWA, 2018). Finally, internal stability analysis was conducted to evaluate the appropriate geosynthetic reinforcement strength to use in the wall. Internal stability calculations were also adapted from equations presented in the FHWA manuals. Internal and external stability design calculations were performed using the MathCAD software package.

3.5 GRS RETAINING WALL DESIGN ASSUMPTIONS

Several revised assumptions were incorporated into the design. First, the material properties for the on-site soils and GRS wall backfill are critical parameters for design. Material properties presented in FHWA (2004a) were less conservative than those used in previous analyses by URS, PBS&J, and others, but appeared to be the most appropriate values based on the available boring logs. Soil and bedrock parameters used in design are summarized in Table 10.

TABLE 10 – Soil and Bedrock Parameters Used in GRS Wall Design

MATERIAL NAME

UNIT WEIGHT FRICTION ANGLE COHESION

pcf (kN/m3) degrees psf (kPa) Overburden (Fill and Talus) 140 (21.6) 40 50 (2.5)

GRS Backfill 140 (20.4) 45 0 (0) Bedrock 165 (25.9) 45 5,000 (240)

Internal stability analysis assumed a geosynthetic with a minimum ultimate tensile strength (Tult) of 4,800 pounds per foot (70 kilonewtons per meter) and strength at 2 percent strain (T2%) of 960 pounds per foot (14 kilonewtons per meter). Other geosynthetics could be used, provided they have strengths greater than the calculated loads. Multiple grades of the same type of geosynthetic could be considered to economize the design. A global reduction factor of 0.4 was applied to the ultimate strength to arrive at the factored strength used in design. This factor is recommended in FHWA (2018) for use with the woven geotextiles that are typically employed for GRS construction. If a geogrid product is used, the ultimate strength should be reduced by appropriate creep, durability, and installation damage factors, as well as the recommended AASHTO LRFD resistance factor of 0.9 for geosynthetic reinforcements.

The analyzed geometry is based on cross sections prepared by Atkins and provided to CFL on July 9, 2018. These cross sections depict the following: a roadway surface that is either crowned or super-elevated and sloping towards the cut side, a post-and-beam guardrail located 4 feet behind the proposed wall face, a 1(V):4(H) foreslope between the guardrail and wall, and steep slopes below the toe of the wall. The cross sections depict toe slopes between 1(V):0.8(H) and 1(V):1.5(H). Exposed wall heights are between about 4 and 24 feet (1.5 and 7.3 meters). There is no discernable relationship between wall height and toe slope.

Three critical sections were selected for analysis: 1) the location with the maximum exposed wall height (Sta. 41+860), 2) the location with the steepest toe slope (Sta. 41+810), and 3) a location with a steeper toe slope and greater wall height (Sta. 41+620). A minimum toe bench width of 6 feet (1.8 meters) was selected to improve global stability and bearing resistance. The total wall height at each section was then determined based on the minimum embedment required to achieve the required bench width for the toe slope ratio. The design wall heights analyzed were

1) 28 feet (8.5 meters) at Sta. 41+860, 2) 21 feet (6.3 meters) at Sta. 41+810, and 3) 28 feet (8.5 meters) at Sta. 41+620. Note that the exposed height at Sta. 41+620 is less than at Sta. 41+860;

however, the increased toe slope necessitates increased embedment, leading to a similar total wall height at the two locations.

FHWA (2009) recommends that the vehicle impact load from a post-and-beam railing system be considered in design of an MSE wall by distributing the load between the top two layers of reinforcement. This assumes a reinforcement spacing of 2 feet and implies distribution of the load to the top 5 feet of reinforcement (approximately equal to the typical installation depth of a post-and-beam system). Due to the tighter reinforcement spacing planned for the GRS wall, the guardrail load was distributed among all layers within 5 feet of the top of wall. Since the post portion of the railing will pass through these layers, the impact load will also transfer to them.

Finally, a 250 pound per square foot (12 kilonewton per square meter) surcharge load was applied across the roadway to account for the traffic live load in accordance with the AASHTO LRFD specifications.

Seismic design parameters were determined using the AASHTO design maps and an assumed Site Class C based on the provided boring logs. For the wall location (44.93757°N, 109.60757°W), a peak ground acceleration (PGA) of 0.128 and site coefficient (FPGA) of 1.2 led to a design acceleration coefficient (As) of 0.158. Per recommendations in CalTrans Report CA/UCD-SESM-08-01, all acceleration was assumed to act in the horizontal direction (kh = As, kv = 0). The Mononobe-Okabe method was used to evaluate the seismic forces acting on the GRS wall, following the method presented in FHWA (2009) for MSE retaining walls.

3.6 GRS RETAINING WALL ANALYSIS RESULTS

Results of the global stability analysis are attached to this memorandum and summarized in Table 11.

TABLE 11 – Results of Global Stability Analysis

STATION

DESIGN

HEIGHT

TOE SLOPE

RATIO

REINFORCEMENT

LENGTH

GLOBAL FACTOR

OF SAFETY

ft (m) V:H ft (m) 41+620 28 (8.5) 1 : 1.2 17.2 (5.25) 1.30 41+810 20.75 (6.33) 1 : 0.9 10.5 (3.2) 1.32 41+860 28 (8.5) 1 : 1.5 15.5 (4.75) 1.31

The global stability analysis indicates that of the sections analyzed, Sta. 41+620 is the most critical section with a reinforcement length to design height ratio of approximately 62 percent.

This ratio was used to define the minimum reinforcement length analyzed for internal and external stability. Although shorter wall sections were not analyzed directly, a minimum reinforcement length of 8 feet (2.4 meters) is recommended in accordance with the AASHTO LRFD specifications. Table 7 presents the minimum factored tensile strengths and strengths at 2 percent strain for each layer of reinforcement. Modifications to Section 714 will be provided for the final design package.

The factored bearing resistance for the GRS wall was evaluated in accordance with AASHTO LRFD Bridge Design Specification. A graph of the relationship between factored bearing resistance and base reinforcement length is presented in Figure 2 for various toe slope inclinations. As recommended by AASHTO, a resistance factor of 0.65 was applied for the strength limit state.

FIGURE 2 – Bearing resistance chart for toe slope inclinations of 30 to 45 degrees and effective base widths of 6 to 20 feet. Values of bearing resistance shown in the chart have been multiplied by a resistance factor of 0.65.

Because this design analysis is performed using non-standard geosynthetic strength parameters, a direct recommendation of reinforcement geosynthetic type per Section 714.04 of the FP-14 is not possible. The contractor should provide a wall section and materials submittal prior to initiating construction. The submittal should identify the type of geosynthetic reinforcement to be used at each elevation within the GRS wall, and include a manufacturer’s data sheet for the proposed product. The geotechnical engineer should review this submittal to ensure the design requirements are met.

TABLE 12 – Required Reinforcement Strength

DEPTH BELOW TOP OF

WALL

MINIMUM FACTORED

TENSILE STRENGTH

MINIMUM TENSILE

STRENGTH AT 2% STRAIN

(ft) (m) (lb/ft) (kN/m) (lb/ft) (kN/m)

0.75 0.23 72.79 1.06 16.38 0.24

1.50 0.46 97.65 1.43 32.92 0.48

2.25 0.69 122.93 1.79 49.71 0.73

3.00 0.91 148.64 2.17 66.73 0.97

3.75 1.14 174.67 2.55 83.94 1.22

4.50 1.37 200.87 2.93 101.25 1.48

5.25 1.60 178.93 2.61 118.60 1.73

6.00 1.83 205.17 2.99 135.93 1.98

6.75 2.06 231.33 3.38 153.21 2.24

7.50 2.29 257.39 3.76 170.44 2.49

8.25 2.51 283.33 4.13 187.60 2.74

9.00 2.74 309.15 4.51 204.69 2.99

9.75 2.97 334.86 4.89 221.72 3.24

10.50 3.20 360.46 5.26 238.68 3.48

11.25 3.43 385.95 5.63 255.59 3.73

12.00 3.66 411.36 6.00 272.44 3.98

12.75 3.89 436.68 6.37 289.25 4.22

13.50 4.11 461.93 6.74 306.01 4.47

14.25 4.34 487.11 7.11 322.73 4.71

15.00 4.57 512.23 7.48 339.42 4.95

15.75 4.80 537.29 7.84 356.08 5.20

16.50 5.03 562.30 8.21 372.71 5.44

17.25 5.26 587.27 8.57 389.31 5.68

18.00 5.49 612.19 8.93 405.89 5.92

18.75 5.72 637.07 9.30 422.44 6.17

19.50 5.94 661.92 9.66 438.98 6.41

20.25 6.17 686.74 10.02 455.49 6.65

21.00 6.40 711.52 10.38 471.99 6.89

21.75 6.63 736.28 10.75 488.47 7.13

22.50 6.86 761.01 11.11 504.94 7.37

23.25 7.09 785.71 11.47 521.40 7.61

24.00 7.32 810.40 11.83 537.84 7.85

24.75 7.54 835.06 12.19 554.27 8.09

25.50 7.77 859.71 12.55 570.69 8.33

26.25 8.00 884.33 12.91 587.09 8.57

27.00 8.23 908.94 13.27 603.49 8.81

27.75 8.46 933.54 13.62 619.88 9.05

3.7 GRS RETAINING WALL PRELIMINARY MATERIALS RECOMMENDATIONS

The following preliminary materials recommendations should be considered when preparing cost estimates and project specifications or Special Contract Requirements.

Welded-wire mesh facing should meet the requirements presented in Section 720.01(b) for the FP-14. Due to lower face pressures in GRS walls, backing mat and hardware fabric is not anticipated for this wall.

Facing rock should be a 4-inch to 5-inch nominal material.

GRS backfill should be base coarse or a similar well-graded backfill material with a maximum particle size of 1.5 inches. Section 703.05, Class C or D material is acceptable, with the adjustment of the maximum particle size. A revision to Section 704 will be provided for the backfill specification.

Geosynthetic reinforcement should meet the requirements of Section 714.04, except the strength requirements are as designated on Table 6. Revisions to Section 714 will be provided for the final design package.

Geocomposite sheet drain, Type 1 per Section 714.02.

Drainage pipe should be 6-inch diameter HDPE, PVC, or ABS pipe per Section 708.

Geotextile separation fabric should be non-woven, Class 1, Type A per Section 714.01.

Detailed materials specifications will be provided with the final design if this alternative is selected. For estimating purposes, we assume that the GRS wall will be bid as a single item based on the area of the finished face, either square meters or square feet.

The Contractor should prepare the final wall design in accordance with Section 257 of the FP-03 and project SCR and submit to CFLHD for review prior to beginning wall construction.

4.0 DISCLAIMERS/LIMITATIONS CLAUSE

Subsurface exploration was not performed as part of this scope of work. Interpretation of surface and subsurface conditions is based on the available data as discussed in the memorandum. This memorandum includes interpretations and recommendations developed by the Government in the process of preparing the design. There interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

James Arthurs Geotechnical Engineer, CFLHD

Dustin Robbins Geotechnical Engineer, CFLHD

Marilyn Dodson Lead Geotechnical Engineer, CFLHD

Attachments:

Compilation of Geotechnical References Memorandum by CFLHD, 2004

GRS Wall Global Stability Calculations

REFERENCES

FHWA, 2004a. “Tech Memo, Slope Stability Assessment for the Beartooth Ravine Retaining Wall, WY FS 4-1(4).” Dated April 30, 2004

FHWA, 2004b.“Technical Memorandum, Micropile Types and Ultimate Bond Stress Values for Retaining Wall and Bridge Construction on the Beartooth Highway Project, WY FS 4-1(4).”

Dated November 23, 2004.

FHWA, 2009. “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes.” Publication No. FHWA-NHI-10-024.

FHWA, 2018. “Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems.” Publication No. FHWA-HRT-17-080.

Kleinfelder, 2003. “Geotechnical Investigation Report, Beartooth Highway Project, Task Order No. DTFH68-03-T-00012.” Kleinfelder Project No. 15215-12, dated December 19, 2003.

PBS&J, 2004. “Beartooth Highway, FHWA Project – Wyoming HPP 4-1(4), Beartooth Ravine Wall Report.” Dated February 24, 2004.

URS, 2001.“Geotechnical Investigation of Selected Features, Beartooth Highway, U.S. 212, Wyoming.” URS Project No. 68FHA0027.00, dated April 3, 2001.

URS, 2002a. “Beartooth Highway Retaining Wall Feasibility Study and Geotechnical Recommendations, Beartooth Highway, (U.S. 212, Park County), Wyoming.” URS Project No.

68-FHAT0039.00, dated February 5, 2002

URS, 2002b. “Final Report, Geotechnical Investigation, Beartooth Ravine Bridge, Beartooth Highway, U.S. 212, Wyoming.” Dated November, 2002.

URS, 2003. “Technical Memorandum, Addendum No. 1, Beartooth Ravine Bridge, Geotechnical Investigation.” Dated June, 2003.

URS Greiner Woodward Clyde Federal Services, 1999. “Initial Geotechnical Investigation:

Beartooth Highway, U.S. 212, Wyoming.” URSGWCFS Project No. 68FHAT002200, dated October 29, 1999.

Woodward-Clyde Federal Services, 1998. “Initial Geohazards Evaluation and Geological Study:

Beartooth Highway, U.S. 212, Wyoming.” WCFS Project No. FHAT0011, dated October 27, 1998.

Yeh and Associates, 2005. “Tech Memo, Beartooth Ravine Wall (U.S. 212), Shoshone National Forest, Wyoming, Project WY HPP 4-1(4), Low Pressure Grouting Ground Improvement vesus Micropile Footing Foundation.” Dated June 6, 2005

To: Bert McCauley, Project Manager, CFLHD

From: Matt DeMarco, Geotechnical Engineer, CFLHD

Subject: Compilation of Geotechnical References, Beartooth Highway Project, WY HPP 4-1(4)

Since 1998, CFLHD has undertaken numerous geotechnical investigations along the (4) segment of the Beartooth Highway, ranging from conventional subsurface drilling and test pit investigations to state-of-the-art geophysical surveys. As a result, nearly thirty formal reports and/or Technical Memoranda have been produced covering a wide variety of topics, including bridge and retaining wall foundations, earthwork, rock slopes, material sources, and landslides, to name just a few. To assist Contractors in identifying relevant geotechnical references during construction, this memo presents a compilation of all related reports and memos, highlighting general contents of each, and conveniently tables priority references by geotechnical subject.

Each of the referenced documents, including all contract reports, is available from CFLHD.

General Category Specific Category Reference No.

Specific Project Locations and

Structures

Landslide at ~39+900 7, 9, 2 Beartooth Ravine Bridge 7, 12, 17, 19, 26 Beartooth Ravine Retaining Wall 7, 11, 19, 20, 23, 26 Beartooth Lake Outlet Bridge 7, 13, 19, 26, 22, 18 Little Bear Creek Bridges 7, 13, 26, 18 Fen Mitigation Bridge 7, 11, 19, 26 Long Lake Outlet Bridge 7, 13, 24, 26, 18 Ghost Creek and other material sources 3, 25, 24, 1

General Geotechnical

Topics

Local and regional geology/seismicity 7, 4, 2 Geotechnical hazards (general) 7, 2 Subsurface investigations – boring/test pit logs 7, 12, 13, 19, 24 Subsurface investigations – geophysical surveys 7, 5, 4 Surface and groundwater hydrology studies 15 Rock and soil laboratory testing 7, 8, 11, 12, 13, 19, 25 Material source investigations 3, 25, 24, 1 Earthwork information 7, 8, 24 Landslide/soil slope stability investigations 7, 9, 2 Rock slope investigations 7, 4, 2 Deep foundation design recommendations 7, 20, 26, 22, 18 Retaining wall design and construction 20, 23, 16, 14, 10, 6 Retaining wall and bridge aesthetic treatments 21, 6 Bridge and wall constructability 20, 11, 12, 10 Water/drainage problems 7, 15, 1

The reference numbers provided in this table refer to the complete references given in the next section. The primary geotechnical reference for the (4) project is the comprehensive study

Technical Memorandum

Date: November 24, 2004 performed by URS and reported on in the spring of 2001 (Reference 7). Reference numbers are listed in the approximate order of priority, with primary documents (first reference number in the list for a given topic) representing the latest engineering values or design summaries, or the greatest source of overall topic…