A STUDY OF FEM ANALYSIS OF VERTICAL SHAFTS IN RIVER VALLEY PROJECTS

Abstract

Shafts are man made openings in underground or made in hillock, which may be either vertical or inclined at a steep grade (>30°) as the need be. In water resources development projects shafts are used for a number of purposes such as construction adits, service and ventillation galleries to under ground power houses, water carriers, spillways, carrying high tension cables from under ground cavities to surface (cable shaft), relieving water hammer at ends of head race or tail race pressure tunnels (surge shafts), carrying high pressure water from surge shafts or reservoirs to power station (pressure shafts), etc. Shafts are advantageous and justifiable for long tunnels of length 3 km and above. They provide additional points of attack, more favourable working sites and avoid complexities of portal construction. For many tunnels, sinking of a shaft to provide a working access is the first operation. Shafts are constructed in various shapes viz., circular, rectangular or square, elliptical horseshoe etc. Most commonly used shaft is of circular cross section. Vertical shafts are self-mucking. Inclined shafts may be self-mucking or non self-mucking. Tehri hydropower project 4 x 250 MW is presently under construction at Tehri in Uttaranchal State. It envisages one of the highest rock fill dams in the world and vast reservoir in the mountainous terrain of lesser Himalayas. The dam site is located at 1.5 km downstream of the confluence of the Bhagirathi river with its tributary Bhilangna. An Intermediate Level Outlet (ILO) at EL 700 m has been proposed to meet the objectives related to minimum irrigation releases and to control the initial filling of the reservoir. The ILO which has curvilinear alignment in plan, proposes at about 190m from the intake point, a gate chamber, having an emergency gate and a regular iii radial gate in two separate but adjoining chambers of an elliptical vertical gate shaft. The emergency gate housed in the upstream chamber of the gate shaft will remain in constant contact with the reservoir and the proposed gate will be of fixed wheel, vertical slide type emergency gate. The downstream chamber will house a regulating radial gate and shall remain empty during all probable modes of operations and closures. The shaft intake has an emergency control gate inside the intake structure, which can be approached by an access gallery from the top of the dam. The flow into the penstock may also controlled by a gate through another shaft located in the dam or abutment. Fig. I shows the geometrical details of the shaft in plait It is proposed to analyse this vertical shaft using finite element analysis. The objectives of the proposed study are To identify different loads and load combinations for checking safety. 2 - D Analysis of a shaft for stresses and deflections to check safety requirements at various depth levels. Check for requirements- of rock anchors to hold concrete lining in position. Comparison of 2D vs. 3D FEM analysis results of the shaft. The finite element method has been adopted for the analysis as it can model complex geometry, complex loading and boundary conditions. The analysis is being performed for the following loading conditions: i} Internal water pressure only ii) Minimum rock pressure + Internal water pressure iii) Maximum rock pressure + Internal water pressure iv) Maximum rock pressure after submergence + Internal water pressure + External water pressure iv v) Maximum rock pressure with concrete lining vi) Maximum rock pressure without concrete lining A two-dimensional plane stress elastic analysis and also three-dimensional analysis is carried out using software ANSYS. The software is capable of analysing all the loading conditions as mentioned above. The rock properties have been taken appropriate to the site conditions. The stresses and deformation pattern of the shaft along with the adjoining rock mass have been obtained for all the loading conditions. It has been observed that there is stress relaxation perpendicular to the excavated boundary where as a stress , concentration tangential to the excavated boundary. The zone of loosening of the rock mass is found to extend upto about 5m beyond the boundary of the opening. The normal stresses at the concrete and rock interface are tensile at some of the locations and hence anchors have been recommended in these locations. In the remaining zones nominal rock anchors for holding lining have been recommended as precautionary measures considering long-term effects. The results of 2 - D FEM analysis shall be cross-checked with 3 - D FEM analysis results which permits more realistic modelling of the given structure and its surroundings. The suitable conclusions have been drawn based on the analysis and also design has been included in the Appendix..