<p><p><figure id='attachment_3531' style='max-width:679px' class='caption aligncenter'><img class="wp-image-3531 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Pre-construction photograph showing the physical constraints at a site in Wyoming where micropiles were used to stabilize a failing slope." width="679" height="453" /><figcaption class='caption-text'> Pre-construction photo, shows the physical constraints at this site. (Photograph courtesy of Hayward Baker, Inc.)</figcaption></figure></p><p><h2>Project Summary/Scope:</h2>This was a slope stabilization project. The highway was constructed in 1955. Seasonal landslide movements created ongoing maintenance and safety problems. The landslide was a block-type failure with movement occurring along a relatively weak layer at the soil-rock interface. Micropiles were selected for implementation since they were less expensive than other alternatives and could be constructed in highly variable ground and under restricted access conditions. Other advantages included the ability to maintain traffic during construction and minimal environmental impacts. Three reticulated micropile walls (Case 2) were constructed.</p><p>Subsurface Conditions: The slide mass consisted of a heterogeneous mixture of detrital sandstone, siltstone, shale and limestone with sand, silt, and clay. The slide debris consisted of clayey gravel with a varying depth. The water table varies seasonally. The effective stress cohesion (c’) was assumed to be zero and the residual friction angle (φ) was assumed to be 14 degrees.</p><p>Micropiles were battered 30 degrees from the vertical and ranged in length from 19 to 34 meters. The micropiles consisted of a high strength steel pipe, 114-mm diameter and 7.6-mm wall thickness, grouted into a 152-mm diameter drilled hole. Three sets of micropile walls were designed, each wall consisting of upslope and downslope micropiles. Each wall had ground anchors installed to increase stability. Ground anchors were installed at 30 degrees from horizontal.</p><p>Blockouts in the cap beam consisting of 150-mm diameter corrugated plastic pipe were installed at 30-degree angles to provide guides through which micropiles were drilled and grouted. Holes 150 mm in diameter were drilled using a cranesuspended, hydraulic rotary drilling rig. Tri-cone roller bits and/or down-hole-hammer drilling tools were used during drilling. Compressed air was used to flush drill cuttings. The micropiles were installed by using the high strength pipe piles as drill tooling. The piles were then withdrawn, the drill bit removed, and the pile placed to the bottom of the hole. Neat cement grout was injected from the bottom of the hole, filling the drill hole and the inside of the pipe. The ground anchors were installed after the micropiles.</p><p>Construction began in April 1997 and was completed in September 1997. Monitoring continued through June 1998.<br><h2>Complementary Technologies Used:</h2>MSE wall with geogrid reinforced backfill</p><p><img class="aligncenter wp-image-3533 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph showing Micropile drilling operation." width="272" height="471" /></p><p><img class="aligncenter wp-image-3534 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph showing reinforced concrete cap beam showing access for micropiles and anchors. " width="626" height="413" /><h2>Performance Monitoring:</h2>Instrumentation included: strain gages on the micropiles, load cells on the anchors, inclinometers, and piezometers. Ground surveying was performed to monitor ground and wall movements. The wall was monitored during and after construction. During the monitoring process all deformations were found to be within an acceptable range. The reticulated micropile wall was found to be a highly effective technique for stabilizing highway slopes.<br><h2>Project Technical Paper:</h2>Wolosick, J., Turner, J., and Hasenkamp, R. (1998). “Reticulated Minipile Walls for Landslide Stabilization-Snake River Canyon, Wyoming.” University of Wisconsin-Madison Soil Improvement Seminar, 13p.</p><p>Also see:</p><p>Sabatini, P.J., Tanyu, B., Armour, T., Groneck, P. and Keeley, J. (2005). “Micropile Design and Construction,” FHWA-NHI-05-039, Federal Highway Administration, 436p.<br><h2>Date Case History Prepared:</h2>November 2012</p></p>