Abstract
In the engineering of high stress area, the measures to control the stability of surrounding rock by reducing excavation footage and excavation speed are to adjust the unloading rate of surrounding rock caused by excavation. In this study, unloading creep tests of marble under high stress conditions were carried out to study the effect of unloading rate. Research results showed that the axial and lateral instantaneous strain and creep strain of the sample increased with the increase of unloading rate; the lateral creep characteristic of marble under unloading condition was stronger than that of axial creep characteristic, and it was more obvious with the increase of unloading rate; the failure of specimens under unloading creep condition was mainly caused by the rapid increase of lateral strain, and the brittleness of rock was increasing with the increase of unloading rate. The Burgers model was used to describe the creep curves of specimens, and the variation of the parameters with the unloading rate was analyzed. The fitting results showed that the instantaneous elastic modulus E1, E'1 and the viscosity coefficients η1, η'1 all decreased with the increase of the unloading rate, which can be described by linear relationship within the unloading rate range of this experiment. Compared with the time of whole creep tests, the time for each specimen to enter the steady-state creep was similar, it was considered that the effect of unloading rate on η2/E2 and η'2/E'2 can be ignored.
Highlights
Compared to the soft rock material, it is generally considered that the hard rock material has relatively weak viscosity[1,2]
With the continuous construction of some large-scale projects, more and more practice shows that in the process of excavation or unloading of valley trenching under high ground stress, in addition to the common surface rock mass failure in a short time due to the rapid release of elastic strain energy, the deformation and failure of hard rock generally have to go through a more obvious time-dependent process
Yang[9] prepared fractured specimens from Chongqing sandstone, carried out triaxial unloading creep tests and proposed a damage creep model to establish the relationship between intact rock and fractured rock mass
Summary
Compared to the soft rock material, it is generally considered that the hard rock material has relatively weak viscosity[1,2]. Many scholars have been conducted studies on the creep characteristics of hard rock under unloading by using experiments and related theories. Such as Yan[7] carried out unloading creep tests with lateral stress unloading gradually on the marble of the Jinping Hydropower Station diversion tunnel, and pointed out that there were significant differences between the axial and lateral deformation. The unloading creep tests of hard marble under high initial confining pressure were carried out, the effects of different unloading rates on the deformation, strength and failure mode of the materials were investigated, and a creep model considering the effects of different unloading rates was established. The results can provide references for the unloading creep characteristics of hard rock and the evaluation of the time-dependent stability of hard rock projects in high stress areas
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