ABSTRACT
Figure 2 shows a circular opening of radius r0 in an infinite soil-mass subject to a hydrostatic in situ stress,σ ′0.The opening surface is subject to the outward radial pressure to the tunnel surface, pi. Considering
all the stresses on an infinitesimal element abcd of unit thickness during excavation of a circular tunnel in Figure 3, equilibrium equation can be expressed as:
If the tunnel is excavated under the groundwater table, then it acts as a drain in unsupported condition. The body force is the seepage stress, as illustrated in Figure 3:
In this state, ir and iθ are the hydraulic gradient in r and θ directions respectively, and γw is the unit weight of groundwater.
