29_WY.PFH.4-1(4).Beartooth_Highway.Yeh.Tech_Memo.Talus_Grouting.2005.pdf

Technical Memorandum Beartooth Ravine Wall (U.S. 212)

Shoshone National Forest, Wyoming Project WY HPP 4-1(4)

Date: June 6, 2005

From: Ben Arndt, Yeh and Associates, Inc.

To: Bert McCauley, FHWA Project Manager

Subject: Low Pressure Grouting Ground Improvement versus Micropile Footing Foundation

This technical memorandum has been prepared to clarify the intent of our design approach for the proposed Beartooth Highway MSE walls, and to provide a comparison of improving the subgrade below the MSE walls with low pressure grouting versus a micropile footing foundation support. The memorandum will compare costs and global stability of the two design approaches.

It should be noted that we are not evaluating the previous micropile/tieback foundation design based on the following:

o It is our understanding that the previous micropile footing design supported a precast concrete wall-facing panel, which is not applicable for the current design.

A rail-anchoring slab on top of wall has been carried forward for the current design.

o The previous micropile footing design only supported the front portion of the MSE wall base.

Low Pressure Grouting Design Approach for MSE Walls Based on our review of the subsurface conditions and slope profiles of the proposed MSE wall systems, it appears that global and external stability are the controlling factors for wall design. Based on our evaluation, analysis, and experience, we presented a wall design to CFLHD that utilizes longer MSE wall reinforcement lengths and deeper embedments to satisfy AASHTO global and external stability requirements.

Furthermore, we believe most of the wall settlement will occur during construction. Post construction settlement will be minor, since MSE walls typically bridge over potentially soft or loose areas. We have proposed low-pressure, low-slump grouting as an additional subgrade improvement for two sections along the proposed MSE wall. The first section begins at Sta. 41+531.57 and extends approximately 14.4 meters to the east of the proposed bridge structure. The second section extends for approximately 49.2 meters at the west end of the proposed pull out area. The low pressure grouting was proposed for areas with higher walls, potentially loose materials with voids, deeper bedrock profiles, and steeper slope profiles in front of the wall. The grouting was proposed to reduce the potential for foundation settlement and to fill potential voids between boulders to reduce movement. The grouting was not designed to increase the global stability of the wall systems.

The grouting patterns are proposed to be a 3m by 3m spacing, for the primary and secondary holes, with specified depths, quantities, and pressures to limit the potential for grout over-runs.

Micropile Footing Foundation Support for MSE Walls We have been requested to compare our current design approach using low pressure grouting to improve the foundation of the proposed MSE walls with a micropile footing supported MSE wall foundation at the same locations.

Based on our analysis of a micropile footing, we would propose supporting the entire width of the MSE wall base on a footing. Preliminary analysis indicates the required spacing of the micropiles would range from 1.8 m by 1.8 m to 2.4 m by 2.4 m depending on the wall height. To provide adequate global stability without tiebacks, we evaluated the wall sections using a MSE reinforcement length that is 100% of the design height as proposed in the current design.

Settlement and Global Stability Comparison Based on our analysis, we believe both methods (grouting or micropiles) will limit the potential for vertical wall settlement to less than 1".

Global stability analysis for the current proposed design (without micropile support) indicates a global factor of safety greater than 1.45 at Sta. 41+520 (adjacent to the bridge) and greater than or equal to 1.30 for the remainder of the project.

Analyzing global stability of the MSE walls that are supported by micropiles and assuming MSE wall loads are transferred into the bedrock, the global factor of safety is greater than 1.6 at Sta. 41+520 and typically greater than 1.45 between Sta. 41+735 and 41+775.

Our analysis indicated the micropile supported MSE walls increase the global factor of safety by approximately 10% when compared to non-micropile supported walls.

We have defined in the Design Criteria that the minimum required global factor of safety for walls that do not support bridge abutments, critical structures, or critical utilities is

1.30. We typically refer to factors of safety when referring to wall stability, rather than analyzing risk. Risk analysis, as it impacts the traveling public, does not appear to be addressed in any current AASHTO guidelines.

Cost Comparisons The attached table is a cost comparison of the foundation considerations of the previous design, current design, and micropile supported MSE wall option. As the table illustrates the least expensive option is the current design using MSE walls with low pressure grouting in the subgrade below the MSE walls in certain wall sections. Additionally, low pressure grouting is usually performed in a shorter time period than micropile placement enabling the contractor to construct MSE in a more timely fashion and typically at a lower cost.

Using a minimum required global factor of safety of 1.30, as outlined by AASHTO and stated in the Design Criteria, we believe the current MSE design with low pressure grouting in selected areas will provide an appropriate design for the anticipated conditions. Unless the Design Criteria for the project is changed or subsurface conditions vary from what we have assumed, we do not believe a 10% increase in global stability justifies the additional cost of the micropile supported MSE walls.

If you have any questions regarding these clarifications please feel free to contact Ben Arndt at Yeh and Associates, Inc. (303) 781-9590.

Attachment: Preliminary Quantity and Cost Estimates

Technical Memorandum