Rockfill Embankment Settlement

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Rockfill Embankment Settlement

Sugarloaf Mountain Bridge Abutment and Hoover Dam Bypass (US-93)

Anderson, Scott A. / LaFronz, Nicholas J.

The Hoover Dam Bypass project (US-93) crosses the Colorado River between Arizona and Nevada below Hoover Dam, 30 mi southeast of Las Vegas, Nevada. The Arizona approach to the river includes the seven-span, 900-ft long Sugarloaf Mountain Bridge. The east bridge abutment footing is supported on a welded-wire–reinforced, special embankment fill zone within a 130-ft high rockfill embankment. Reinforced zone backfill consists of 4-in. maximum size select granular backfill; the remainder of the embankment consists of 2-ft maximum size rock. Backfill and rockfill were obtained from project roadway cuts, and construction and acceptance of the reinforced zone and rockfill embankment were in accordance with FHWA standard specifications and project special provisions. Postconstruction settlement of the embankment was monitored for 2 years with conventional survey monuments on abutment wingwalls and the embankment and multipoint borehole extensometers (MPBX) in the roadway shoulders. The monitoring period included 3 months before placement of the closure span and deck. Roughly 1 in. of settlement beneath the reinforced fill was measured during construction, with an additional 1/2 in. of settlement within 2 years after fill placement. Postconstruction settlement of the wingwalls was approximately 1 in. The rate of settlement decreased through the 2-year period, and settlement from construction and self-weight is nearly complete. Use of the MPBX was deemed partially successful in this application.