ABSTRACT:

The underground facilities are used for a wide range of civil and military applications (railways, highways and material storage). These underground structures are constructed in different possible ground conditions ranging from soft ground to massive jointed rock. Generally, the loads on tunnels consist of both static (overburden) and dynamic (Blast, earthquake and traffic) loads. Therefore, in the present work an attempt is made to simulate the in-situ condition through the physical modeling, to understand the tunnel deformation behavior under different loading conditions in soft rocks. The dimensions (30 cm × 20 cm × 20 cm) of the model are considered on the basis of all boundary conditions. The engineering properties of the different materials are evaluated experimentally (UCS, elastic modulus and Poisson's ratio) and the Plaster of Paris (PoP) mixed with sand and some percent of clay is selected for physical modeling, as the PoP alone shows brittle behavior. It is interpreted that the height of overburden is a critical factor in correlating degree of fracture and damage to the tunnel - lining. Series of experiments are carried out with lining materials and litho-static pressures under static loading conditions. Further, the deformation and degree of damage is determined for lined and unlined conditions. Also fracture thresholds are computed for different overburden thicknesses for safety of underground structures in weak soft rocks.

1 INTRODUCTION

Civil engineering tunnels are constructed in the full spectrum of possible ground conditions ranging from soft ground to massive unjointed rock, coupled with various states of in situ stress and hydrogeology.

Dams and hydroelectric power plants look for a better geological condition; tunnels and highways look for a better alignment escaping from weak zones, whenever possible. Even though the shape of the opening mainly depends on the purpose for which it is to be used, the safe design and construction of an underground opening requires the knowledge of the stress distribution and the displacements that occurs in and around the openings. When an opening is excavated in a medium, the in situ stress field is disturbed and new set of stresses are induced around the surrounding of the opening. Before the tunnel is excavated, the in situ stresses σν, σh1, and σh2 are uniformly distributed in the slice of rock under consideration. After removal of the rock from within the tunnel, the stresses in the immediate vicinity of the tunnel are changed and new stresses are induced. Three principal stresses σ1, σ2 and σ3 acting on a typical element of rock are shown and explained well in Figure 1.

Most of the theories, existed compute the vertical stress at a point as the stress due to the lithostatic pressure, and the horizontal stress as that stress required to completely restrain lateral deformation of an elastic body acted upon by the vertical stress. The theory predicts that the horizontal stress will be some fraction of the vertical or overburden stress dependent upon the value of Poisson's ratio for the rock. The stability of underground excavations is affected by its shape, size of opening, in situ stress, soil/rock conditions etc. Knowledge of the magnitudes and directions of these in-situ and induced stresses is an essential component of underground excavation (Goodman 1989). The horizontal stresses acting on an element of rock at a depth ‘z’ belowthe surface is much more difficult to estimate than the vertical stresses.

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Terzaghi & Richart (1952) suggested that, for a gravitationally loaded rock mass in which no lateral strain was permitted during formation of the overlying strata, the value of k is independent of depth and is given by k = v/(1 — ν), where ν is the Poisson's ratio of the rock mass. Measurements of horizontal stresses at civil and mining sites around the world show that the ratio k tends to be high at shallow depth and that it decreases at depth (Brown & Hoek 1978, Herget 1988).

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