Bureau Case History of Loess Cut Slopes in Nebraska.

摘要

Operational experience with permanent slopes in loess has led the Bureau of Reclamation to modification of past slope designs. Previously, the standard design in loess was to use 1/4:1 and 1/2:1 slopes in permanent excavations and when vertical heights exceeded 30 feet (9.1 m), 14- and 20-foot (4.3- and 6-1-m) wide benches were excavated and the slopes resumed. Lack of drainage maintenance on benches often caused surface runoff to flow across the benches and down the slopes, resulting in severe erosion. To minimize maintenance, benches were eliminated and 1/4:1 cuts up to 90 feet (27.4 m) were allowed. The first cut failed at a depth of 60 feet (18.3 m). Failures continued to occur in the 1/4:1 cut. Spalling occurred on many of the 1/4:1 slopes 25 to 30 feet (7.6 to 9.1 m) below original ground surface. Design changes were made in cuts over 35 feet (10.7 m) deep to include benches at 30-foot vertical intervals in an attempt to eliminate future failures. Failures occurred in cuts of less than 30 feet, creating further design changes. Currently, slopes of 1/4:1 are used when heights of cuts above an O&M road are less than 20 feet (6.1 m); 1/2:1 slopes are used when heights of cuts exceed 20 feet. Inclusion of a 5-foot (1.5-m) high, 1:1 slope at the toe of the 1/4:1 and 1/2:1 slope has also provided a more stable condition.