Development of a High-stress Pressuremeter for Weak Rocks
Jyh Jong Liao
The outcrops in the central and northern regions of Western Taiwan often contain very weak and young formations. They are often poorly cemented. The rock sampling process is likely to damage the structure that originally existed in the rock masses. The strengths obtained in laboratory experiments are often scattered. Pressuremeter test is an in-situ testing method that requires no sampling. It has the potential of avoiding some of the problems that due to sample disturbance. The maximum applied stress of pressuremeter for soil tests is often below 5 MPa. A pressuremeter test in weak rocks usually needs high-pressure beyond 10 MPa to reach limit pressure. The other goal for pressuremeter tests is to determine the stress-train relations of rocks. It means that the test needs to do precise strain measurements during testing. However, using conventional pressuremeters, the volumetric strain of rocks is usually measured for calculating the radial strain, which is not accurate enough for weak rocks. To overcome the drawbacks, the author has successfully developed a new pressuremeter for testing of weak rocks. The developed pressuremeter is allowed for measuring a pressure up to 25 MPa. The pressure is induced by the expansion of the probe of pressuremeter. Either compressed air or hydraulic oil can be adopted as the expansion media. In cooperating with proportional valves and a data acquisition system, pressuremeter tests can be controlled either by stress or strain rate. Four sets of deformation gauge at the middle of the probe of the new pressuremeter are made for measuring the radial strains. The deformation gauge is composed of thin metal with four pieces of strain gages on its both surfaces. The interval between the deformation gauge is 90o. Then, using this device, the four radial deformations at the middle of the probe can be directly measured. Based on the measured radial strain, one can additionally estimate the initial shape of borehole and evaluate the homogeneity of rock mass. To avoid the effect of electric noise, all of the analogic signal outputs from measuring devices are transformed to digital signal in probe. This thesis presents the design of the high-stress pressuremeter and the calibration result of the system.
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