Use of photoelastic gauges in studying the stress state in rocks during the construction of tunnels for the Baikal-Amur railroad

Abstract

According to the technical plan for the tunneling operation, the tunnels are to be constructed parallel to an exploratory transport-drainage adit, with the adit face as a rule significantly in front of the tunnel face. This method of tunneling makes it possible to equip measuring stations with instruments in the exploratory adit ahead of the tunnel face (Fig. i), and to measure any stress change in the vicinity of the tunnel as it is being driven. The measuring stations were located in granites, and consisted of several gauges, positioned on adit side walls. The measuring stations were equipped with type FDO-2 photoelastic gauges made from K8 glass, with an external diameter of 40 mm. The gauges were arranged in 42-43-mm-diameter drillholes, at a depth of 0.5-2.5 m from the working walls, 1-1.5 m apart, in groups of twos and threes on opposite side walls of the adit. The gauge assembly and interpretation of the readings were carried out according to the methods of the Institute of Mining of the Siberian Branch of the Academy of Sciences of the USSR.* In the first stage of observations, the gauges detected the redistribution of stresses in the rock mass, induced by the tunnel face advance, thereby yielding information on the magnitude and direction of additional stresses ahead of the tunnel face. It was proposed to carry out observations on stress changes around the tunnels by means of measuring stations. Five measuring stations were installed in the Baikal tunnel, all sited on granites. While the first station was being set up, the gauges showed no alteration of stresses with time resulting from the nearby presence of a discharge breakthrough. At the remaining stations, the general nature of stress changes was observed. The results of observations from several gauges are shown in Table i, while Fig. 2 shows a graph of the change in additional stresses in the rock adjacent to the adit under the influence of the tunnel driving and the orientation of the largest of the semiprincipal stresses.