Effect Of Rock Bolts Research Articles

Shear resistance of rock bolts : Haas, CJ 16F, 5T, 2R Trans. Soc Min. Engrs, AIME, V260, N1, Mar. 1976, P32–40

The effectiveness of rock bolts in resisting shear displacement along preexisting fracture surfaces or slip planes was determined in the laboratory with full-scale bolts and large-scale shear test equipment. The system variables were the type of bolt, orientation of bolt relative to shear surface, and normal pressure on the interface. Results showed that, in general, both conventional bolts and resin-grouted reinforcing bars significantly increase the shear resistance along a fracture or bedding plane when the average normal compressive stress on the plane is in the order of 25 psi and when the bolt is oriented such that it tends to lengthen as the shear progresses. An increase in shear resistance is also noted at the higher normal pressure of 250 psi; however, the increase is small compared to the corresponding shear resistance without a bolt. Conventional bolts oriented such that they lose tension as shear displacement occurs added no shear resistance since the tension drops to zero at very small shear displacements.

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

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.