Design of lining for delivery tunnels in creeping hard rocks

Abstract

It is a well-known fact that the economic design of linings for hydraulic delivery tunnels calls for maximum utilization of the load-carrying capacity of the enclosing hard rocks. In present design practice, such rocks are taken to be elastic, instantaneously deformed media, and stresses in the lining are determined by the theory of elasticity. At the same time, investigations of deformation properties of hard rocks in situ show that most of such rocks, under continuous load, are capable of different degrees of continued deformation, i.e., of creep. For this reason, the actual state of stress in the lining may substantially differ from that computed from the theory of elasticity.This article integrates the results of special fietd studies of creep in hard rocks at the V. V. Kuibyshev MISI (Moscow Civil Engineering Institute) * and presents a method of stress state calculation for delivery tunnels in creepprone rocks.To determine slow deformations in various types of hard rocks, special experiments with rectangular concrete plates 1 m z in area were carried out in galleries at the sites of the Kirovsk and Andizhan dams of the Uzbek SSR (Fig. 1).Rock creep was studied at increasing and decreasing pressures on the plates. Analysis of the experimental data shows no substantial qualitative difference in the reaction of the rock surfaces in question (chlorite-sericite schists, black shales, stratified and massive sandstones) to the local static load. This makes it possible to consider the investigation results as valid for the total rock mass. We now turn to the principal deformations in these rocks upon initial loading. Figure 2 shows typical mean deformation (S) --mean pressure (p) graphs reflecting the rock surface behavior as the pressure is applied to the plate in increments. These graphs show that the enveloping function S = f(p), represented by the dashed line in Fig. 2, is generally nonlinear, at both increasing and decreasing pressures. For the rocks under study this function is substantially nonlinear as a rule in a relatively narrow range of increasing pressures, fluctuating from 2.5 to 10 kg/cm 2, depending on the rocks. At higher pressures, up to Pmax = 40 kg/cm z used in the experiments, the enveloping function can be taken as linear. The deviations from linearity are slight and irregular; they may be due to scatter of the experimental data.

Investigation of the stressed state in high-pressure tunnel linings

“…which can be presented in the form In order to obtain design functions for the creep cores of the rock in Zone I, K z (t--r), and of the concrete, K~ (t-r), an approximation of the simplest form, namely, containing one exponential function, is used, and this is quite admissible for some types of rock [5] and concrete [6]. The use of other analytical expressions for the creep core, which reflect more closeiy the deformation of rock [10] and concrete [11] with time, in principle presents no difficulties but complicates investigations to a significant degree without markedly affecting the general conclusions.…”

“…The aim of the prestressing is to preclude the development of tensile stresses in the lining-rock complex under service conditions or to confine such stresses to acceptable limits. In the author's view, based on the experimental data in [4,5], this situation depends to a considerable degree on creep of the rock; in addition a not-unimportant effect can be produced by creep of the concrete and shrinkage of the grout mix in setting.It should be noted that at present there is no well-founded design basis which would make it possible to establish, including an allowance for rock and concrete creep, the magnitude of the grout pressure which would achieve the stressed state in the concrete and the rock required to meet the service conditions. A detailed analysis of the available experience with construction carried out by this method was presented by V. S. Eristov [I].…”

“…Figures I to 3 present the published [2, 3] results of stresses measured at the internal boundary of the lining caused by grouting the surrounding rock mass under high pressure. In the author's view, based on the experimental data in [4,5], this situation depends to a considerable degree on creep of the rock; in addition a not-unimportant effect can be produced by creep of the concrete and shrinkage of the grout mix in setting. In the author's view, based on the experimental data in [4,5], this situation depends to a considerable degree on creep of the rock; in addition a not-unimportant effect can be produced by creep of the concrete and shrinkage of the grout mix in setting.…”