GEOTECHNICAL CHALLENGES POSED BY WEAK CLAYSTONE IN DEEP CUT SLOPES

摘要

Site characterization for a new 5.4-mile-long railroad alignment traversing mountainous terrain in western Pennsylvania indicated the presence of non-durable clay stone bedrock of the Pennsylvanian Conemaugh Group. The excavation for the new alignment would expose generally horizontally-bedded coal measure rocks exhibiting highly variable engineering properties. The sequence of competent sandstone, siltstone and shale interbedded with weak, massive claystone (Red Beds) is responsible for numerous landslides and rockfalls in the region due to undercutting by differential weathering and general instability of the weak claystone layers. However, right-of-way restrictions on this project necessitated designing cut slopes significantly steeper than traditionally utilized in this material. As a complicating factor, slickensided discontinuities, laterally discontinuous strata, and erratic topography suggested the possible occurrence of pre-existing deep bedding plane shear surface(s) that presumably developed as the result of valley stress relief and elevated pore pressures related to Pleistocene glaciation. Excavation of the proposed railroad alignment would extend to depths of up to 150 feet with the potential for reactivating ancient landslides and/or triggering new slope movements. The design approach involved fitting the new cut slopes within the available right-of-way by developing a slope protection system to prevent degradation of the claystone bedrock. Additionally, sub-horizontal drains were installed to lower groundwater levels, and an instrumentation program, consisting of inclinometers and piezometers, was carried out to monitor slope movements and water levels during construction. As expected, some slope stability failures did arise during and after construction, however these were generally minor and thus could be dealt with primarily as a maintenance issue.