Jun-ichi Kodama

Kimachi sandstone does not have to fail under larger stress

Series of uniaxial compression tests combining constant strain rate and four patterns of cyclic loading were carried out on Kimachi sandstone to clarify the timing of rock failure. It was found that rock failure timing was random in triangular, reverse triangular and small amplitude triangular loading tests and all rocks failed during larger strain in large amplitude triangular loading tests. The results suggest that rock specimens did not have to fail under larger strain when the stress amplitude was smaller than a certain value and gave us clues to explain why rocks do not always fail under larger stress. The mechanism behind the experimental results was explained based on strain rate-dependent rock strength and fatigue damage.

Application of the Ultrasonic Propagation Time of a Core Sample for Stress Measurement of Underground Rocks.

Deformation rate analysis (DRA), although it has a poor theoretical basis, is a useful method for estimating rock stress in the laboratory using core samples. DRA is thought to be closely related to changes in the inner microstructure of rock samples undergoing a de formation process. Ultrasonic measurement is a useful technique for investigating structural changes in rocks. In this study, both methods were applied simultaneously to rock samples to confirm the applicability of ultrasonic measurement for the estimation of rock stress using a system of ultrasonic propagation time with accuracy in 10-2 nsec.In the experiments, core samples of sandstone were loaded up to a certain stress in order to artificially input memory of the stress into the samples, and the propagation time in the horizontal direction to the loading axis and the deformation rate were measured simultaneously. Consequently, the propagation time through the specimen was measured with high accuracy during deformation. Variations in the propagation time and lateral strain almost coincided during the loading-up process. Furthermore, the propagation time was drastically changed at the memorized stress corresponding to the deflection point of the deformation rate curve. These experimental results show that highly accurate measurement of the ultrasonic propagation time is useful, as is the DRA method using core samples, for examining changes in the microstructure inside rock samples under stress and applicable for estimating underground rock stress.

Effect of Pore Gas Pressure on Mechanical Behavior of Rocks under Confining Pressure.

The effect of pore gas pressure on the mechanical behavior of rocks has been investigated in order to confirm the validity of the effective confining pressure law among rocks. The triaxial compression tests of Noboribetsu welded tuff, Horonai sandstone (A), (B), Akabira sandstone and Akiyoshi marble were carried out with the strain rate 1.3 × 10-4/sec for confining and pore gas pressure combined with a conventional hydraulic testing machine. The stress strain curves of five kinds of rocks were determined under the condition of confining and pore gas pressure extending up to 50 MPa.The experiments show that the effective confining pressure law was valid for more porous and permeable rocks such as Noboribetsu welded tuff, Horonai sandstone (A), (B) and Akabira sandstone but Akiyoshi marble was not possible to adopt this law.The mechanical behavior of rocks whose pore are not fully saturated with gas pressure seems to largely depend on magnitude of the confining pressure acting on specimens.

Case Studies on Monitoring Rock Behavior around Longwall Face in Taiheiyo Coal Mine.

The mechanical behaviors in coal and rock masses during longwall extraction were monitored around six longwall coal panels of Taiheiyo Coal Mine by the use of hydraulic capsules, load cells etc.The following conclusions can be made from our studies.1) The abutment pressure zone, which forms in solid coal ribsides with face approach, moves deeper as a result of the enlargement of the stress relief zone.2) Monitoring the pressure changes of hydraulic capsules, it was clarified that the coal seam near the fault was severely loosened.3) Floor heave dominates as a deformation mode of roadways, and the deformation behavior of steel arches can be divided into three types according to the load-displacement curves.4) The vertical stress changes in goaf measured by load cells gradually increase with face advance. Also they are dependent on the distance from the ribside and the geometry of the old workings in upper coal seams.

Theoretical Study of Time-Dependent Strength of Granite Based on Micro-Structure Model.

In this study, time-dependent strength of granite was interpreted in terms of micro-structure model.Using Kemenys model, strength realized in monotonic loading test, creep test and cyclic loading test under uniaxial compressive condition was calculated and compared with the experimental results. In this model, the rock is assumed an elastic medium containing a column of sliding cracks and the cracks grow due to stress corrosion. It was confirmed that this model is able to precisely represent the effect of loading rate on compressive strength and the creep strength. However concerning on the fatigue behaviors, time to failure was overestimated when stress amplitude became large.Therefore, we have modified Kemenys model, where the coefficient of friction of sliding cracks was reduced gradually under cyclic loading. It was found that this modified model was able to explain the effect of stress amplitude and maximum stress, quantitatively. It was also found that the effect of loading frequency on the fatige strength was similar between the prediction and experiment.