The boundary stresses around an underground opening govern its stability. It has been studied by Hoek & Brown (1982) that the shape of underground opening does affect the boundary stresses around an underground opening. Parametric study using FLAC3D code has been carried out for two shapes of underground Opening and considering various other parameters like the tunnel depth, the in situ stress, the uniaxial crushing strength (UCS) of intact rock material and the Bieniawski's rock mass rating (RMR). The results of the study Covering induced boundary stresses, minimum safety factor and the required rock bolt length for these two shapes are presented in the paper.
As Soon as an underground excavation is made the In situ stresses within the rock mass gets disturbed and re-distribution of stresses takes place. As such, the induced stresses around an underground opening would be different from the pre-excavation stresses, i.e., the in situ stresses. A zone of disturbed stresses is formed around an opening and generally known as ‘zone of influence’. The extent of zone varies from rock to rock. For a very good and strong rock this zone is small, whereas for weak rocks it is large. In other words, if the induced stresses due to tunnelling do not exceed the in situ strength of the rock mass, the surrounding rock mass remains in an elastic state and the zone of influence is limited. However, when the induced stress is more than the strength of the rock mass, the rock fails and the condition is popularly known as the squeezing ground condition.
The boundary stresses thus govern the stability of an underground opening. Hoek & Brown (1982) have shown that the shape of underground opening affects the boundary stresses around an underground opening. To carry forward this work authors have taken up a Research Project sponsored by Ministry of Water Resources, Government of India.
The parametric study was carried out using numerical analysis code FLAC3D to obtain the influence of various shapes of underground opening on maximum boundary stress. In addition to two shapes of the underground opening, rectangular and circular, other parameters considered for the study are tunnel depth, in situ stresses, uniaxial crushing strength of intact rock material, and Bieniawski's rock mass rating. The study has been carried out under elastic conditions. The approach and the results for two shapes are given in the following paragraphs of the paper.
Theory suggests that the maximum boundary stresses are independent of opening size under elastic conditions. It also proposed that the stresses around an opening are dependent upon the magnitude of the applied stresses, i.e., the in situ stresses and the geometry or the shape of the stressed body (Hoek & Brown, 1982). Hence, along with the two shapes of underground openings the other influencing parameters selected are as follows.
Two different shapes of underground opening have been considered for the analysis in the paper as shown in Figures 1a, b.
