Analysis and Measurements of Behaviors of Rock around Under-ground Cavity in the Process of Excavation

Abstract

This paper deals with the results of studies on the theoretical analysis of the deformation behavior of rocks, and the actual measurements made during the underground cavity excavation at Okutataragi Pumped-Storage Hydro Project.To initiate the studies, the material properties of rock were obtained by repeated in-situ tests at the proposed location for the projects to define the initial ground stress, rock shear strength, modulus of elasticity, creep deformation ratio, retardation ratio, Poisson's ratio, and rock compressive strength. The material properties thus obtained were used to analyze the extent of rock loose zone, the rock displacement and other fundamental factors for each step of the excavation work by means of a two- or three-dimensional elastic finite element method.To compare the measured data with the analytical calculations, the rock deformeters were installed in such an arrangement as to observe changes with the time of the rock displacement at the cavity arch and side walls, and several concrete strain gauges and reinforcing bar strain meters were also buried in the arch concrete to measure the loads on the arch concrete.The comparison of the calculations and these measurements disclosed the followings;(1) The results or calculations of the analysis could be adequately utilized for designing the rock walls anchored by prestressed steel bars and for maintaining the safety or security control during the construction.(2) The three-dimensional analysis and the results of measurements confirmed that the rock strut or rock mass support method could confine the displacement to about 70 percent of the estimated values of displacement for the construction method without rock strut support. As proposed and adopted at Okutataragi Project, the rock strut was arranged to come in the center of the cavity to house the underground power station and to provide the space for assembling the power equipment, machinery and tools.(3) The horizontal displacement, which was most carefully studied, was 12 to 14mm at the measurement as against 11mm arrived at by the analysis. It follows that the analysis calculation proved to be fairly precise.(4) It is understood that the loads on the arch concrete comes primarily from the horizontal forced displacement arising out of the loose side walls, but not from the ground pressure in the upper direction as has otherwise been assumed. This was proved by the measurement of the maximum concrete compressive stress of 120 to 130kg/cm2 and the calculated estimate of 140kg/cm2 maximum.The estimations and the measurements carried out for this project have proved quite satisfactory. However, several problems have been raised for further reviews and analyses to improve the safety or security of large underground power station structures, such as;(1) Decrease of the rock strength in the loose zones.(2) A method or methods to estimate the effects of rock bolts of prestressed steel bars.(3) Development of more accurate and efficient devices or methods for measuring the initial ground pressure.(4) Improved excavation methods to prevent the rock loose zone from developing or expanding.