Des Peres Hospital Retaining Wall - St. Louis, MO

Thursday, November 13th, 2014 by John Siwicki

Challenge

Des Peres Hospital is a 143-bed medical facility in St. Louis County, Missouri. A 13-foot tall, 12-inch thick poured concrete retaining wall provides approximately nine feet of grade separation from an upper loading dock area to parking stalls below. Heavy equipment used to resurface the loading dock area surcharged the wall, causing the wall to crack and deflect laterally. Nearly one-half the length of the 104-foot long wall was affected, with the most noticeable and maximum movement of six inches occurring at a cracked and offset control joint. Steel pipe bracing was installed diagonally from the wall face to the parking lot pavement to serve as temporary support until a permanent solution could be determined. Six soil borings completed at the project site identified residual soils over shallow shale bedrock. An initial proposal included demolishing the existing retaining wall and rebuilding it with either helical tiebacks or drilled and grouted rock anchors. This solution was very costly as it would involve demolition, temporary shoring, tiebacks or anchors, and rebuilding a new wall. It would also make the loading dock unavailable during the anticipated long construction period.

Solution

Foundation Supportworks® by Woods proposed a more cost effective and expedient solution to resupport the retaining wall using Geo-Lock™ Wall Anchors. The Geo-Lock™ Wall Anchor System consists of a steel wall plate, a steel plate anchor buried in the ground, and high-strength all-thread connecting rod. All components and hardware are hot-dip galvanized for corrosion protection. Twenty-four wall anchor locations were proposed with a center to center spacing of 4 to 5.5 feet. Anchor rod penetrations through the wall would be 2.5 to 3 feet below finished grade on the high side of the wall. With the above anchor spacing and locations on the wall, the design working load per anchor was 8.6 kips. The soil behind the deflected sections of wall was excavated to remove the lateral earth pressure so the wall could be straightened and realigned. Individual excavations were made at the proposed earth anchor locations approximately 18 feet from the front face of the wall and to a depth so the center of the plate anchor would be approximately four feet below grade. A 0.75-inch diameter all-thread rod was driven through a 1.125-inch hole in the wall, through the soil and shale bedrock, and into the pre-excavated hole. A plate anchor and wall plate were connected to opposite ends of the threaded rod. With all 24 anchors installed, the anchors were systematically tightened to straighten the wall. Drainage gravel was placed as backfill behind the realigned wall and the anchors retightened. The loading dock was unavailable for only two days. The entire project, including asphalt patching and clean up, was completed in just four days.

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