Abstract
This paper presents an investigation of the effect of joint spacing on the size and shape of the disturbed zone around a tunnel. A two-dimensional distinct element simulation program, UDEC, was used to model the tunnel excavation in a simply jointed rock mass, and an analytical method verified the numerical results. Rock masses were studied, ranging from intact rock to heavily jointed rock, with rock spacing less than 1/16 of the tunnel diameter. Four UDEC models were used to study the effect of joint spacing. The disturbed zone was divided into: (1) the failure zone, where loose rock blocks fall into the tunnel; (2) the open zone, where joints open; and (3) the shear zone. Numerical results were obtained for disturbed zones for different block sizes, boundary conditions, stress state, and joint orientation. An analytical study was made of the shear zones around the tunnel. The failure zone is found in the immediate vicinity of the tunnel; local block falling occurs there only when the joint spacing is small enough to create loose blocks. The open zone enlarges with reducing joint spacing and stress ratio. The shear zone around the tunnel can extend a considerable distance from the tunnel boundary. The division of the disturbed zone into these zones makes it easier to study short-term and long-term tunnel instability.
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