Experimental study on the stress relief method of small-diameter compact conical-ended borehole overcoring technique

In order to measure the in-situ stress state in underground space, a small diameter conical-ended borehole strain meter for overcoring technique has been developed.The reliability of the test results of this strain meter was verified by laboratory loading and unloading experiments.First, the theory of compact conical-ended borehole overcoring technique was briefly presented.Then, a 47 mm-diameter conical-ended borehole strain meter was developed to measure the in-situ stress in underground space such as coal mine roadway and roadway chamber.A conical-ended bit was also designed and machined to drill the conical-ended hole for the tests.Resultsshow that the developed small diameter conical-ended borehole strain meter can be easily installed and glued.The strain changes during loading and unloading process can be recorded by the small diameter conical-ended borehole strain meter combined with the multi-channel strain indicator.Based on the theory of conical-ended borehole overcoring technique, the stress can be inversed using the terminal strain recorded by the strain cell and the least square method based on MATLAB code.The 24 tests results of 12 stress ratio were conducted and used for stress inversion and verification of the strain meter.The error is within 50% with average error of 18%,in which the absolute value of mean error of S₁ and S₂ are 18% and 17%,respectively.The validity of this small-diameter compact conical-ended borehole overcoring technique was verified through those experiments, which could provide a theoretical foundation and experimental verification for the application and development of the small-diameter compact conical-ended borehole overcoring technique and fill the gap in this field in China.