Predicting the Rate of Shale Undercutting Along Highway Cuts

Abstract

Undercutting of highway cuts due to differential weathering is a common occurrence in many states. Progressive undercutting can result in frequent rock falls and dangerously unstable conditions. For timely implementation of remedial measures required to prevent undercutting-induced failures, it is essential to be able to predict the rate at which different shales weather and undercut. In order to devise a method that could be used to predict the rate of shale undercutting, fourteen sites, located in three different states, were evaluated. The sites consisted of road cuts that displayed significant amounts of undercutting. Essential to the selection of these sites was the presence of pre-split blast hole markings on the resistant undercut units. The presence of pre-split markings ensured that the exposed face of the undercut unit was the original surface exposed to weathering since the time of excavation. The amount of undercutting at each site was carefully measured at many points along the entire length of the cut. Information regarding the date of excavation of the selected road cuts was obtained from highway department records. The measured amount of undercutting at each road cut and the age of the cut were used to estimate the rate of shale undercutting since excavation. Samples of the undercutting shales were obtained from each site and tested for slake durability, Atterberg limits, clay mineralogy, and rock fabric. Correlation of laboratory test results with estimated rates of undercutting indicates that the slake durability index can be used successfully to predict the rate of shale undercutting. The rate of undercutting at the sites studied does not appear to be related to Atterberg limits or clay mineralogy, as determined by x-ray diffraction. Scanning electron microscopy suggests fabric and degree of cementation may be important factors governing the rate of shale undercutting.