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.
From an engineering geology perspective, there are clear relationships among physical and mechanical properties of each geological stratum and its geological evolution: provenance, composition, sedimentary and diagenetic history, metamorphism and weathering. For example, for most sedimentary rocks, an increase in the age of the rock generally leads to an increase in its strength and a decrease in its porosity and permeability. However, these relationships may or may not be highly important factors in the evaluation of widely different rock types, such as intrinsically strong quartzites and intrinsically weak clay shales. Interestingly, little is reported about the geomechanical properties of conglomerates, and this is the focus of this article. Conglomerate is a terrigenous sedimentary rock type containing large, usually rounded rock fragments. Conglomerates show a wide range of strength, as low as 1 MPa for a very weak lightly clay-cemented conglomerate to over 200 MPa for a very strong, lowporosity, quartz cemented conglomerate rock. The latter conglomerate we have in mind is the Witwatersrand Conglomerate found in South African gold mines . There is little information extant in the literature on the geomechanical characteristics of weak conglomerate rocks. This motivated us to present these data on rock mechanics properties of a weak conglomerate. In this article, we highlight the properties of a conglomerate that forms the geological strata along a 1.80 km length of a tunnel under construction in Iran. The section is known as the Qomroud Long Tunnel Lot number 6 (QLT6), and the tunnel is part of the Dez- Qomroud water conveyance project which consists of four tunnels with total length of 54 kilometers, a large dam and three small dams. QLT6 is situated in the Zagros tectonically disturbed zone. The conglomeratic rock is part of a thick synorogenic molasse sequence of upper Pliocene to Pleistocene age, the Bakhtiari Formation.