Permeability And Mechanics Characteristics Test Studying On Rock Or Rock Mass Under High Seepage Pressure

Because of the more high water pressure condition in engineering and rock mass seepage, there were many engineering accident examples. The rock or rock mass mechanics and permeability characteristics studies had become the front-subject which should be solved urgently. In order to study the rock and rock mass permeability, failure mechanism and mechanical properties under high seepage pressure, the Rock High Pressure Permeability Testing System had been designed and produced by author and studying team clerks. By means of the device, relatively systematical test was completed, aiming at different sample with different permeability and mechanics property. Corresponding study achievements are presented as follows:(1) Followings are the characteristics and innovations about the test machine: Because the patent product - Static Servo-Valve of Chengdu Servo Hydraulic Pressure Equipment had been used as the main controlling element, it was completed that the inlet water pressure of sample could keep stability for a long term although there was no control by computer. Because the volume change of spilling water had been accurately measured, the test time was shortened and test precision was improved. The outlet water pressure of sample could keep basically stability under the controlling system of the special pipeline filter and velocity adjust valve. Therefore, the stable water pressure was gotten at the outlet, and the simulation for high pore water pressure and low hydraulic gradient was truly achieved. Both the axial load and lateral pressure of the sample were respectively controlled by two Speed Control Valves, which made sure the test process stability and precision. Through the modification and improvement of computer controlling program, the visual monitor of test process could be enlarged infinitely by time curve or history curve. According to the changing characteristics of test data, the operation command could be modified and prolonged at will, but this would not at all influence the whole test process. The mechanics test under loading or unloading path was really achieved under controllable high permeability pressure. Aiming at the permeability test of sample, the author put forward the method of the sample waterproof under high water pressure and the test method of the square prism permeability and remodeled sample permeability under high confining pressure.(2)The detailed different stress path test operation methods to measure and calculate the permeability and mechanics characteristics of sample had been illustrated, which would be effective for the further studying.(3)The test results demonstrated that the non-Darcy flow phenomenon was ubiquitous for low permeability rocks. According to the non-linearity seepage characteristics, the Averaged Three Times Method to reckon quasi-threshold pressure gradient had been given and proved to be effective method by the examples.(4)According to the test result, whether the porosity sample permeability was high or low, the values of permeability coefficient all changed in same magnitude along with the confining pressure and axial pressure increasing or decreasing, which showed that the bigger permeability of the sample was, the higher value change value was along with the external pressure. The axial displacement of sample showed the negative value change with the external pressure increasing, which implied the sample was in the tensional state. And when the external pressure dropped, the sample showed compressed state. Furthermore, the axial displacement value could all return to the original value at each fall back phase of the external pressure, but the permeability would get lower than the original value that was tested during external pressure increased, which showed that the lateral elastic-plastic deformation of the sample was the main influence factors for the sample permeability value change during external pressure increase or decrease. But the fractured rock mass permeability change characteristics were different from the rules above. Namely, once the fission of the sample was closed by compression, its permeability could not increase any more even if external pressure was reduced.(5)According to the test result, the change scope of permeability for the same lithology sample during the complete stress-strain path was mainly controlled by confining pressure, which had been put forward for the first time. The lower value of the confining pressure was, the higher change scope of permeability was during the triaxial loading process, and vice versa. As for the same lithology sample, the permeability change characteristics had little different during loading and unloading process. As long as the confining pressure was constant, there was a limit value of the permeability change during complete stress-strain process, which was the maximal permeability coefficient, not more than the magnitude of 10^-5cm/s, and the low permeability marbles was not more than 10^-7cm/s.(6)The main external effective factors for the rock permeability were the value of external pressure and hydraulic gradient. As for the porosity medium, the hydraulic gradient influential action was more than the external pressure. Whereas, the water pressure of the sample outlet had little effect on the permeability. According to the factual water pressure difference, zero value could be given to the water pressure at the sample outlet for test studying.(7) The calculation method of shear strength under high seepage pressure was put forward: firstly, converting the measured values ofσ₁,σ₃ into the corresponding effective stress and adding the average pore water pressure to the effective stress. The average pore pressure was got from a group of samples, which are measured when the strength arrived to the peak value. Secondly, using the sum of the effective stress and the average pore water pressure to calculate the shear strength under high seepage pressure.(8) The saturation water sample test value ofσ₁,σ₃ through conventional triaxial compression could be calculated as the shear strength value according to the actual engineering high seepage pressure conditions. On the basis of checkout by actual test, it was proved that the calculation result from this method that only considered stress conversion could represent the rock or rock mass shear strength characteristics.(9) On the basis of the mechanical characteristics studying of different test path triaxial compression, the rules had been found that when the confining pressure was invariant, the shear strength and deformation characteristics accorded with Hooke elastic laws, but when the confining pressure was in loading or unloading state, the shear strength and deformation characteristics were inconformity to Hooke elastic laws. The engineering parameter value should be in accordance with the test result, which should simulate the factual stress condition.(10) On the basis of test and theory of Mohr-Coulomb, the rock or rock mass strength calculation criterion under confining pressure unloading condition had been given:σ₁+σ₃ = k (σ₁-σ₃) -b, and the corresponding strength calculation formula wasC =b/2tgφ,(?)= arcsin 1/4...