AN EXPERIMENTAL STUDY ON CLOSURE OF AN UNDERGROUND OPENING IN JOINTED ROCK MASS

Abstract

The stresses, which exist in undisturbed rock mass, are related to the weight of overlaying strata and geological history of rock mass. This stress field is disturbed by the creation of opening and the disturbance redistributes the stresses. Some times these redistributed stresses are high enough to exceed the strength of rock mass adjacent to the tunnel boundary and lead to gradual closure of opening. The deformation of tunnel walls or the roof and the floor inside the opening after the excavation of tunnel is termed as tunnel closure. The assessment of the closure of the underground opening becomes one of the most important aspects in designing the underground structure. The factors influencing the tunnel closure in general may include, the initial in situ stress state, the mechanical properties of rock mass, the type and properties of tunnel support and the method and procedure of excavation. In the present study an attempt has been made to investigate closure of an underground opening in jointed rock mass and numerical modelling. The model material selected for the experiments was `Plaster of Paris' mixed with sand and water. Two tests were performed in plane stress state. Test-I was performed under hydrostatic stress (ay=ax) condition on the mass with orthogonal joints oriented at 00 and 90°. The jointed rock specimen size was 0.75 m x 0.75 m x 0.15 m. The specimen was made with small blocks of size 2.5 cm x 2.5 cm x 7.5 cm, and, the total number blocks consumed for making one specimen was above 1800. The test-Il was conducted under biaxial stress (a, 0.5 ay) condition on jointed mass with orthogonal joints oriented at 45°/45° with a specimen size of 0.79 m x 0.77 m x 0.15 m. The total number of blocks consumed for making the specimen was above 1900. Both the tests had same joint spacing of 25 mm for both the joint sets. The openings of 15cm and 18cm size were excavated in jointed mass for test-I and test-II respectively. The specimens were subjected to stresses in horizontal and vertical directions and corresponding closure of the opening was monitored. The experimental tests have also been modeled through Universal Distinct Element Code (UDEC 3.1) and observed failure mechanism and deformations in jointed mass. Finally, the experimentally obtained results have been compared with UDEC results.