The Highvale mine, west of Edmonton, Alta., is a strip-mining operation with Upper Cretaceous soft sandstone and mudstone overlying the coal deposit. Highwalls cut in the soft rock experienced numerous failures from 1983 to 1985 that disrupted stripping and mining operations and posed a threat to safety. The performance of a 20–23 m highwall at Highvale was studied in 1987 and 1988 to gain insight to the mechanisms of failure. The study utilized surveying, slope indicators, and piezometers to determine the deformation and seepage pattern behind the highwall as it rebounded into the pit upon excavation. Translational movements were found to extend over 250 m behind the highwall along weak, probably presheared bentonitic mudstones. The sandstone and mudstone overburden was observed to extend toward the open pit. This affected the pore-pressure regime and led to a reduction in the mass strength of the sandstone and mudstone. The findings of this study contributed to a better understanding of the mechanisms of highwall failure in soft sedimentary rocks.Key words: highwall, deformations, failure, seepage, loosening, softening.