109 / 2021-09-30 11:17:58

Effect of Loading Rate and Time Delay on the Tangent Modulus Method (TMM) in Coal and Coal Measured Rocks

Tangent Modulus Method,TMM,Deformation rate analysis,DRA,Loading Rate,Time Dependency

Non-destructive techniques of in-situ stress measurement from oriented cored rocks have great potential to be developed as a cost effective and reliable alternative to the conventional overcoring and hydraulic fracturing methods. The tangent modulus method (TMM) is a technique that can be applied to oriented cored rocks to measure in-situ stresses. Like the deformation rate analysis (DRA) the rock specimen is subjected to two cycles of uniaxial compression and the stress-tangent modulus curve for the two cycles are obtained from the stress-strain curve. A bending point in the tangent modulus curve of the first cycle is observed which separates it from the tangent modulus curve of second cycle. The point of separation between the two curves is assumed to be the previously applied maximum stress. To understand the effect of loading conditions and the time delay, experiments were conducted on coal and coal measured rock (sandstone and limestone) specimens. The specimens were preloaded, and cyclic compression were applied under three different modes of loading, four different strain rates, and time delays of up to one week. The bending point in the stress-tangent modulus curves occurred approximately at the applied prestress levels under all three loading modes, and no effect of loading rate was observed on the bending points in TMM. However, a clear effect of time delay was observed on the TMM, which contradicts the DRA results. This could be due to the sensitivity of TMM and the range of its applicability, all of which need further investigation for the in-situ stress measurement.