Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.

Effect of sample size on the fluid flow through a single fractured granitoid

Most of deep geological engineered structures, such as rock caverns, nuclear waste disposal repositories, metro rail tunnels, multi-layer underground parking, are constructed within hard crystalline rocks because of their high quality and low matrix permeability. In such rocks, fluid flows mainly through fractures. Quantification of fractures along with the behavior of the fluid flow through them, at different scales, becomes quite important. Earlier studies have revealed the influence of sample size on the confining stress-permeability relationship and it has been demonstrated that permeability of the fractured rock mass decreases with an increase in sample size. However, most of the researchers have employed numerical simulations to model fluid flow through the fractureJfracture network, or laboratory investigations on intact rock samples with diameter ranging between 38 mm and 45 cm and the diameter-to-length ratio of 1：2 using different experimental methods. Also, the confining stress, 03, has been considered to be less than 30 MPa and the effect of fracture roughness has been ignored. In the present study, an extension of the previous studies on ＂laboratory simulation of flow through single fractured granite＂ was conducted, in which consistent fluid flow experiments were performed on cylindrical samples of granitoids of two different sizes （38 mm and 54 mm in diameters）, containing a ＂rough walled single fracture＂. These experiments were performed under varied confining pressure （03 - 5-40 MPa）, fluid pressure （fp ≤ 25 MPa）, and fracture roughness. The results indicate that a nonlinear relationship exists between the discharge, Q, and the effective confining pressure, σeff., and Q decreases with an increase in σeff.. Also, the effects of sample size and fracture roughness do not persist when O＇eff. ≥ 20 MPa. It is expected that such a study will be quite useful in correlating and extrapolating the laboratory scale investigations to in-situ scale and further improving theoreticalJnumerical models associated with fluid flow through rock masses.