Faults are dislocations between adjacent blocks in the bedrock. Since they constitute planes and zones of weakness with little tensional strength, they are apt to influence the process and results of rock excavation. The effect of faults on rock excavation depends on the initial rock material, the material In the fault zone (fault gouge. fractured rock, mylonite, clay, water etc.), and upon the orientation of the fault plane and the rock stresses in relation to the tunnel direction. Even if a fault may cause considerable trouble to rock excavation, this is not necessarily the case.
The influence of faulting on rock excavation is a well known practical problem and has been studied on a number of sites, where faults have caused damage to tunnels and caverns in rock and obstructed excavation work (Brekke & Selmer-Olsen 1966; Brekke 1970; Brekke & Howard 1972; Martna 1983; Carlsson et al 1985; Martna & Hansen 1986). The effect of a fault depends on the initial rock material, the material in the fault such as fault gouge, fractured rock, mylonite, sand, clay etc., water pressure and upon the orientation of the fault plane and the rock stresses in relation to the direction of the tunnel.
A fault Is defined as a shear dislocation between two adjacent blocks in the bedrock. It involves often, but not always, a brittle crushing of the rock. Being normally tectonic features, their occurrence is common in areas which are or have been tectonically active and scarce In the large sedimentary platforms of the Earth. Faults occur when the stresses In the rock exceed the strength of the rock, and are associated with the different stress situations of geological regions. Faults can be classified into a number of groups (Figure 1)
Type I faults are found In areas with horizontal, tensional stresses and are often associated with volcanism. Recent examples of this type are common along the Mid-Atlantic ridge and can be observed in Iceland (Figure 2). In the East African Rift Valley and the German Rheingraben, older examples of this fault type can be found.
Types II and III are caused by high compressional stresses and are common within recent mountain belts such as the Alps, the Himalayas, the Andes and the Rocky Mountains. The Appalachians and the Scandes are examples of old mountain ranges where particularly horizontal thrust faults are common.
Type IV faults are caused by high horizontal shear stresses. The San Andreas Fault in California and the Great Glen Fault in Scotland are examples of this type.
The length of a fault and the amount of dislocation along It may vary from some millimetres to tens of kilometres and can sometimes be measured thanks to various markers in the rock such as dykes and veins (Figure 3) or sedimentary boundaries (Cf. Figure 2). Faults are often grouped into fault zones, and while the width of a single fault usually Is some tens of centimetres or less, the total width of a fault zone may amount to several kilometers.
