The Influence Of The Angle Between Two Loading Directions Of Granite On The Kaiser Effect Under Uniaxial Test Conditions

The method of measuring ground stress based on the Kaiser effect of rock has been widely used.It has the advantages of simple testing method,low cost and high reliability.In recent years,there have been a large number of studies using this method to measure in-situ stress.Since the direction of principal stress is difficult to determine,it is necessary to sample from multiple directions before the test.The direction of the principal stress is difficult to determine,so it is necessary to take samples in multiple directions to test,and calculate the magnitude and direction of the principal stress according to the test results.When the rock samples that have been subjected to in-situ stress in the rock mass are subjected to laboratory test loading(ie,secondary loading),if the loading direction(the axial direction of the test piece)deviates from the main direction of the in-situ stress in the rock mass by a certain angle(ie,there is a certain angle between the second and the first loading direction),whether the Kaiser effect still exist.If the specimen deviated from a certain angle can no longer reflect the magnitude of the principal stress,the whole method would be totally wrong.Because there are few related research results,and the conclusions obtained by different researchers are inconsistent,a series of uniaxial loading tests have been conducted on granites produced in Guangdong.The target stress is formed by uniaxial preloading,and then the secondary processing of the specimen is performed so that the new specimen can be tested in different direction.The main conclusions are as follows:(1)When determining the Kaiser point,a comprehensive analysis should be performed on cumulative energy count curve,cumulative ring count curve and acoustic emission energy count graph;(2)For the rocks used in the experiments in this paper,the Kaiser effect is significant from 10% to 60% of rock compressive strength;(3)When the 2 loading directions are between 15° and 20°,the samples with Kaiser effect account for 78.95% and 68.18% of the total respectively.It's obviousthat the larger the angle between two loading directions,the smaller the percentage of specimens with Kaiser effect;(4)When the 2 loading directions are between 15° and 20°,the average FR values(the ratio of the estimated stress to the pre-loaded stress)are 1.02 and 1.07.Kaiser effect still exists,and the accuracy of the reflected stress is high enough.Even when the angle is slightly larger than 20°,the Kaiser effect can still be used to measure in-situ stress,which is totally different from the conclusion of Holcomb and Costin;(5)Compared with 15°,when the 2 loading directions are at the angle of 20°,the FR value is larger,and the proportion of specimens whose Kaiser Point are hard to identify is also greater.So it can be inferred that if the angle between two loading directions is larger,the FR value would be larger,the Kaiser effect would become more ambiguous,and the accuracy of the stress it remembers would be lower.