Geomechanical properties of a conglomerate from Iran

This article addresses the geomechanical properties of a conglomerate that forms the geological strata along a 1.80 km length of a tunnel under construction in Iran. We present these data because there is little information extant in the literature on the geomechanical characteristics of similar materials. The conglomeratic rock is part of a thick synorogenic molasse sequence of Lower Miocene to Pliocene age. Molasse consists of tectonically undisturbed coarse-grained detritus produced by rapid erosion of mountain ranges after the final orogenic spasms. Because of this timing, the sediments have not been significantly affected by compression or shear and they are horizontally disposed. Conglomerates are common in molasse, forming thick layers in some cases. The rock we studied is a mud-supported conglomerate, comprising well- to very well-rounded limestone, marly limestone and radiolarite rock fragments in a clay matrix; according to field strength estimates, the rock is classified as a "very weak rock". Laboratory tests were performed according to ASTM standards and ISRM suggested methods on high quality core samples from tunnel-axis vertical boreholes. Results from the field and laboratory investigations showed that the rock is of very poor quality, which could lead to serious instability problems for the tunnel.