The fractured rock surfaces were observed under different loading stages in creep shear by making use of computerized tomography (CT). Through the creep shear tests, the failure modes of the fractured rocks have been recognized based on the CT images. The study shows that the long-term shear strength of fractured rocks is mainly composed by two mechanisms: one is the anti-shear due to the interlocking of microasperities, the other is the anti-friction owing to the relative shear of macro waves on the fractured rock surfaces. The study concludes that the relationship between the peak shear strength and the normal force as follows: for the compressive shear of rocks, the shear strength of the rock would increase and approach to the critical value when the normal force becomes larger; for the tensile fractured rocks, on the other hand, the shear strength would become larger persistently when the normal force increases.
The mechanical behavior of fractured rocks has directly impact on the stability of slope, tunnel and dam. etc. The research on creep shear strength of fractured rocks can provide theoretic basis for predicting of long-term mechanical behavior of rock mass. The research of rock creep mechanism activate worldwide, since it has a great significance about long-term stability of rock mass (Sun 2007). At present, researches about the rock creep are approximately divided into three parts. Firstly, the creep mechanism of intact rocks is studied (Chen & Sun 1996). For instance, upon the experiment of tensile creep fracture in red sandstone, the creep fracture criterion is established, and combined with the rheology mechanical, further instructions about fracture parameters are concluded: presented by three threshold values of the rock, respectively, called lower limit of rheology, lower limit of destroy, upper limit of destroy. Using initial intensity factor, the rheology of the rocks can be judged among these threshold values. The natural rocks contain weak intercalation, slope instability mostly occurs, due to the existence of weak structure. The second part is focused on the creep shear character under the weak structural. Chen (Chen et al. 2009) analyzed the shear creep property of typical weak intercalation in redbed soft rock, which provided the shear strength parameters of rock creep for prospecting and design. Similar research could be found in literature (Shen & Zhang 2010), the shear creep test is carried out for the samples with greenschist discontinuities, and found the characteristic of shear creep failure is closely related to the development of the structure surface. Shen (Shen et al. 2012) carried out shear creep test on Barton's standard profiles, and the long-term strength of the discontinuities were determined by using of three methods: the transition creep law, isochronous curve and the first inflection point method. The third part investigates the crack propagation in the creep shear. In order to obtain the further details of crack development, growth and creep failure in the rocks, some researchers studied the creep of rocks with the help of industrial CT. In the 1990s, industrial CT got into the new stage of research about the rock failure characters (Yang et al. 1996). At the present stage, industrial CT is widely used to study the rock crack and defect evolution (Lei & Zhang 2003), the micromechanics of rocks under different loads (Erik 2004) and technology of CT three-dimensional re-construction of rock or mixed soil (Zhao et al. 2011). Based on the industrial CT, the researches as mentioned above focus on description of cracks in the intact rock in creep process, and find out the defect development in mechanics and geometry. As well-known, engineering rocks consist many kinds of fracture surfaces and they are under different stress. These fracture surfaces become the major factor in controlling of the long term mechanical behavior. The research on long-term mechanical behavior of fractured rocks of micro-contact and fracture model can provide theoretic basis for predicting instability of rock masses.