Experimental Investigation on the Viscoelastic Properties of Constituents in Mudstone

In deep underground engineering, the creep behaviors of soft rocks have been widely investigated to help understand the mechanism of the time-dependent large deformation and failure of underground engineering structures. However, rocks were used to be regarded as homogeneous materials and there are limited studying results about the time-dependent properties of constituents in them to reveal their creep mechanism. In this context, the targeting nanoindentation technique (TNIT) was adopted to investigate the viscoelastic characteristics of kaolinite and quartz in a two-constituent mudstone sample. The TNIT consists of identifications of mineralogical constituents in mudstone and nanoindentation experiments on identified constituents. After conducting experiments, the unloading stages of the typical indentation curves were analyzed to calculate the hardness and elastic modulus of constituents in mudstone. And the 180 s load-holding stages with the maximum load of 50 mN were transformed to the typical creep strain-time curves for fitting analysis by using the Kelvin model, the standard viscoelastic model and the extended viscoelastic model. Fitting results show that the standard viscoelastic model can perfectly express the nanoindentation creep behaviors of both kaolinite and quartz and fitting constants are suitable to be used to calculate their creep parameters. The creep parameters of kaolinite are much smaller than that of quartz, which drives the time-dependent large deformation of the soft mudstone. At last, the standard viscoelastic model was verified on a sandstone sample.