Clarifying the time-dependent strength deterioration characteristics of carbonaceous mudstone under dry and wet cycles is of great significance to the design of expressway cut slopes. In this work, we conducted triaxial compression creep tests on carbonaceous mudstone specimens that had undergone different numbers of dry and wet cycles to investigate their creep properties. A function was established between the steady-state viscoplastic creep rate and axial compression. The threshold stress of the steady-state viscoplastic creep rate was assumed as the long-term strength, and the long-term strength deterioration law of carbonaceous mudstone under dry and wet cycles was studied. The results showed that the transient strain, viscoelastic creep, and viscoplastic creep of carbonaceous mudstone increased with the number of dry and wet cycles, and the creep failure stress and transient elasticity modulus decreased. Based on the steady-state viscoplastic creep rate method, the long-term strength of carbonaceous mudstone after n (n = 0, 3, 6, 9) dry and wet cycles was found to be 74.25%, 64.88%, 57.56%, and 53.16% of its uniaxial compression strength, respectively. Compared with the isochronous curve method and the transition creep method, the steady-state viscoplastic creep rate method can more accurately determine the long-term rock strength. The long-term strength of carbonaceous mudstone under dry and wet cycles decays exponentially, and the long-term strength decay rate during the first three dry and wet cycles is about 215 times the average decay rate.
To investigate different responses of direct and indirect tensile strengths to loading rate, direct and indirect tension tests were performed on sandstone, rust stone, and granite specimens. Typical load curves indicate that a peak tensile stress frequently appears before the second peak stress, used to calculate the tensile strength in indirect tension tests. As expected, increase in the loading rate increases the tensile strength. In addition, the calculated tensile strengths of the indirect tension tests are frequently higher. Interestingly, the increase ratio of the tensile strength with the increase in the loading rate in indirect tension tests is higher. To verify the above results, crack propagation and stress evolution in direct and indirect tension tests were dynamically monitored using PFC 3D. For direct tension tests, specimens fail at the peak tension point, corresponding to the tensile strength. However, for indirect tension tests, minor cracks, composing of continuous microcracks, form before the peak stress and accompany with the decreased slope of the compression curve. At the peak point, tensile stresses significantly concentrate at the crack tips and further cause large-scale crack propagation. In addition, the initiation stress instead of the peak tensile stress is closer to the tensile strength, obtained from the direct tests for the same loading rate.
The crack propagation evolution of carbonaceous mudstone under the action of dry-wet cycles is an important cause of the unstable failure of this type of slope. This paper attempts to reveal the evolution mechanism of mesocrack and macrocrack propagation in carbonaceous mudstone under the action of dry-wet cycles from chemical, physical, and mechanical perspectives. Firstly, the soaking solution of carbonaceous mudstone during the dry-wet cycles was extracted for an ion concentration test to analyze the chemical reactions of carbonaceous mudstone. Then, CT scans were performed on the carbonaceous mudstone samples to study the changing pattern of mesostructure of carbonaceous mudstone during the dry-wet cycles. In the end, the mechanical properties and failure characteristics of carbonaceous mudstone after dry-wet cycles were studied by triaxial compression tests. The results showed that chemical reactions such as calcite dissolution, potassium feldspar hydrolysis, and sodium feldspar hydrolysis occurred during the dry-wet cycle of carbonaceous mudstone. Affected by the dry-wet cycles, the mesostructure of the carbonaceous mudstone gradually changed from face-face contact and edge-face contact to edge-corner contact and corner-corner contact, and the interlayer flake structure was opened and was locally curled and fractured. With the increase in the number of dry-wet cycles, the failure characteristic of carbonaceous mudstone transformed from tensile failure to shear failure, the failure surface of carbonaceous mudstone was deflected from 90° to 60°, and the crack propagation path of carbonaceous mudstone became more complicated. The chemical reaction of carbonaceous mudstone minerals during the dry-wet cycle is an important reason for the initiation and development of pores. The dry-wet cycle aggregates the propagation of mesocracks and structural disorder, transforming the uniform stress state of the rock mesostructure to the concentrated stress state, which is the important reason for the macrocrack propagation evolution of carbonaceous mudstone.
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