Time-dependent Behaviour Of Rocks Under Triaxial Compression Condition

In underground engeneering such as mining and tunnel engineering,the surrounding rock mass will damage when the load exceeds the carrying capacity of them.This will affect the safety of construction and application of underground structures.Usually,the damage of rock mass is a progressive failure process with time elapsing,which shows time-dependent behavior significantly.Therefore,the investigation of time-dependent behavior of rock has important significance for designing and long-term stability of underground structures.Based on the engineering background,a transparent servo-control test system is developed.Taking various types of rocks as the objects of study,a series of progressive failure tests,loading-rate dependency tests and general stress relaxation tests are conducted under triaxial compression condition.The degradation characteristics of rock mechanical properties with time elapsing under different test condition are analysed,the mechanism of time-dependent behavior of rocks is also discussed.Based on the above study,the main results are as follows:1)A transparent triaxial compression servo-control test system is developed and a three dimentional digital image correlation test system with three groups of image capture units is installed.The failure progress of rocks and the strain field in the specimen surface can be observed and measured directely under triaxial compression condition.It provides an advanced method in laboratory for studying the deformation and damage evolution characteristics.2)The experimental studies on the progressive failure process of rock are carried out under different confining pressure,moisture condition and lithology.The evolution characteristics of the complete stress-strain curves and the macroscopic mechanical parameters are analyzed and compared under different test conditions.The evolution of energy and strain field during progressive failure process are discussed.Furthermore,a method for confirming the initial stress level of localization is proposed based the calculation of digital images and the effect of test condition(confining pressure and moisture condition)on the stress level is discussed.3)Using the alternative loading rate test method,the experimental studies on the loading-rate dependency of four rocks are conducted under different confining pressure and moisture condition.The loading-rate dependency of peak strength and residual strength are discussed emphasisly.A index for assess the loading rate dependency of residual strength is proposed and evaluated quantitatively.The effects of confining pressure and moisture condition on the loading rate dependency are discussed and the relation between loading rate dependency of peak strength and residual strength is analyzed.Finally,the mechanism is diacussed from the perspectively of macro-and microscope.4)The general stress relaxation phenomenon and its mechanism are explained by the series-parallel model.Based on the stress feedback control method,a series of general stress relaxation tests are carried out under different confining pressure and stress level conditioin.The evolution characteristics of general stress relaxation are analyzed and compared under different test conditions.The complete curves of general relaxation are obtained and divideded.The failure process is discussed briefly.Finally,the relation between loading-rate dependency and general stress relaxation is discussed.