soft rock, Iran

Abstract: Soft rocks are the third type of geotechnical material for which their stress/strength features are related to the overlapping spectrum between soil and rock mechanics. These materials and hard soils exhibit, at failure, a behaviour that is intermediate between that of hard soils and soft rocks. This appears to be the case for materials of sedimentary origin as well as for grouted sands that can be considered as artificial soft rocks or hard soils depending on the cementing agent, the density of the sand and the efficiency of grouting. On the other hand, a rock mass is considered to be weak when its in-situ uniaxial compressive strength is less than about one third of the in-situ stress acting upon the rock mass through which a tunnel is being excavated. The Lavarak hydro-power cavern with a 47 MW electricity potential is located about 50 km north-east of Tehran, Iran. This cavern, along with its two deep and inclined shafts, has been designed and constructed within Plio-Pleistocene soft and massive conglomerates. There are also two great reverse and thrust, high seismic potential faults (Mosha and north Tehran) in the vicinity of the cavern. The rock mass has a low shear strength (very weak) and a high deformability potential which, from in-situ tests and extensive monitoring programmes in trial chambers, it was concluded that the rock masses ' geomechanical parameters are highly stress dependant. With increasing confining pressure (to a certain extent), the deformation modulus due to compressibility and consolidation of conglomerate, will increase at least two times. Accordingly, th