31_WY.PFH.4-1(4).Beartooth_Highway.Tech_Memo.Micropile_Types.2004.pdf

To: Bert McCauley, Project Manager, CFLHD

From: Matt DeMarco, Geotechnical Engineer, CFLHD

Subject: Micropile Types and Ultimate Bond Stress Values for Retaining Wall and Bridge Construction on the Beartooth Highway Project, WY FS 4-1(4)

Micropiles have been recommended as deep foundations for all of the bridge structures along the WY FS 4-1(4) segment, including the prestigious Beartooth Ravine Bridge, and along the entire length of the Beartooth Ravine retaining wall. With the exception of the Beartooth Lake Outlet Bridge, all micropiles are assumed to be rock socketed, Type A gravity grouted installations, with casing plunge and bond lengths determined by the PBS&J structures team (lead designer for the project). Micropiles at the Outlet Bridge are assumed to require Type B pressure grouted installations to ensure adequate bond strengths and pile capacities within the deep gravel, sand, and clay units beneath bridge piers and abutments (no readily accessible bedrock at this location). Final micropile lengths will be determined for the Outlet Bridge by PBS&J and will consider such things as the minimum casing plunge required for lateral loading, bond lengths within specific soil horizons, number of micropiles desired beneath the abutment footing, etc. It is anticipated that capacities similar to conventional H-piles can be developed within the Type B micropiles at this location.

A review of recommended ultimate bond stress (UBS) values for the rock and soil units anticipated along the project indicates the following allowable bond stress (ABS) values should be used for determining pile capacities (URS, 2001; FHWA, 2000; FHWA, 1999; PTI, 1996):

Micropile

Type Rock/Soil Unit Ultimate

Bond Stress, kPa

Factor of Safety

Allowable Bond

Stress, kPa

Comments

Type A (gravity grouted)

Weathered, fractured, strong granitic bedrock

1250 – 4200

2.5

Low UBS value selected to accommodate fractured rock mass conditions, potential for settlement.

Weathered, moderately strong sandstone

520 – 1725

2.5

Moderately low sandstone unconfined compressive strengths suggest a UBS = 1000 kPa is appropriate.

Type B

(pressure grouted)

Alluvial sands, gravels, and cobbles

120 – 360

2.0 60 Assumes mostly gravel content.

Sand with gravels and silts

70 – 360

2.0 50 Assumed some gravel and coarse sands.

Clayey silts 70 – 190

2.0 35 Assumed stiff clay/silt with some sand.

Technical Memorandum

Date: November 23, 2004

Rock bond stress values are considered relatively conservative based on unconfined compressive strengths (UCS) obtained by URS (2001). PTI (1996) recommends, as an alternative, using 10% of the UCS value for the UBS for the rock unit. This would result in substantially higher ABS values for the granite and sandstone units. Also, based on the fractured, weathered nature of the rock masses within the anticipated bond zones (near the top of the rock units), a lower factor of safety could have been used (PTI recommends 2.0 for weak rock conditions).

Ultimately, however, bond strengths achieved in the field are related to many factors in addition to rock/soil properties, including the manner in which the micropile holes are drilled and prepared, how the piles are installed, the type of grout used, etc. It has, therefore, been recommended that micropile testing be conducted prior to full-scale installation to verify bond strengths and pile capacities, as well as installation procedures. This is particularly important for the Type B micropile installations at the Beartooth Lake Outlet Bridge. Soil classifications are fairly subjective at this location, cobble and boulder contents are not accurately known in the upper soil unit, clay contents are not well defined in the lower soil unit, and grout takes have not been established above or below the water table.

Additional micropile design information is available from PBS&J, including material specifications, allowable capacities, estimated tip elevations, and distribution beneath walls and bridge footings. If we can be of any further assistance in this matter, please don’t hesitate to contact me directly at (720) 963-3520.

______________________________________________ Date_________________ Matthew J. DeMarco Lead Geotechnical Engineer, CFLHD

References

FHWA, 1999, Ground Anchors and Anchored Systems – Geotechnical Engineering Circular

No.4, FHWA-IF-99-015, NTIS, Springfield, VA, 22161.

FHWA, 2000, Micropile Design and Construction Guidelines – Implementation Manual, FHWA-SA-97-070, NTIS, Springfield, VA, 22161.

PTI, 1996, Recommendations for Prestressed Rock and Soil Anchors, Post-Tensioning Institute, 1717 W. Northern Ave., Ste 114, Phoenix, AZ, 85021.

URS Corp., 2001, “Final Report – Geotechnical Investigation of Selected Features: Beartooth

Highway, U.S. 212, WY”, FHWA Contract Report, URS Corp., Denver, CO, Project No.

68-FHAT0027.00, April 3, 2001, 57 pp. (plus appendices).

References