JOINTING AND ROCK WEDGES OBSERVED IN A CUT SLOPE NEAR FREDERICKTOWN, MISSOURI

The design of slopes in jointed rock requires that engineers consider whether rock wedges formed by two or more joints can become unstable and slide out of the slope face. To prevent wedges or reduce the number of unstable wedges information must be gathered regarding the location, orientation, persistence, and length of discontinuities present within the rock mass. This information was collected using a scan-line survey on a cut slope near Fredericktown, Missouri. Conventional scan-line survey procedures were applied for the measurement of key features of each joint observed in the survey: dip, dip direction, persistence, location, termination type, roughness, and curvature. Digital photographs were taken of the entire cut slope to aid in the identification of actual and potential rock wedges.

This long stable cut slope allowed the inspection of the actual number of rock wedges observed in the excavation, which was the primary objective of this fieldwork. The number and size of each rock wedge in the excavation is to be recorded to create a histogram of rock wedge occurrence within the slope. These observed wedges will then be compared to predictive tools that calculate the number and size of expected wedges in a rock slope. This predictive tool requires information regarding the joint set features and will estimate a histogram of rock wedge occurrence.

Results from the conducted scan-line survey are interpreted as five distinct joint sets. Joint set properties are interpreted for each set separately to permit application of the predictive wedge analysis. In addition, histograms of the observed wedges and potential wedges are identified for the readily recognizable joint set combinations. These histograms were the principal objective of this investigation as they permit comparison of the predictive tool to the actual number of wedges observed in the slope.