A study on factors affecting the minimum rock cover for tunnel or cavern excavation in hard rock

Abstract

Excavation is often carried out in hard rock for construction of tunnels or caverns. The behaviour of the rock mass and the stability of the excavation depend on various factors, in which one of them is the amount of rock cover. It is important to study the factors affecting the adequacy of rock cover. The factors affecting the rock cover for tunnel or cavern excavation and explain their relationship to be discussed are rock mass properties, in situ stress and cavern span affect the stability for tunnel or cavern excavation. Other factors such as groundwater condition and external loads may also affect the tunnel stability. Yet, they are less important especially in the case of deep tunnel or cavern excavation. Eighteen cases were studied with sixteen cases in Hong Kong and two cases from overseas. Different aspects on the three factors were considered with the general descriptions on the rock tunnel/cavern excavation, the ground conditions, and design of rock support given in the cases. The importance of sufficient rock cover and consequences without enough rock cover will be discussed. Based on the type of rock support/reinforcement required, the extra time and cost needed when the rock cover is insufficient can be examined.