Abstract
Size effect has always been the focus of rock mechanics as a bridge between laboratory test and engineering site. Previously, the research conditions and objects of the rock size effect have mostly focused on cylindrical rock samples with different height-to-diameter ratios (H/Ds) under uniaxial or conventional triaxial compression, while there has been little research on the rock size effect under true triaxial compression (TTC), especially rectangular rock samples with different sizes and the same length-to-width-to-height ratio. Based on this, the deformation, strength, and failure characteristics of Beishan (BS) granite and Baihetan (BHT) basalt with different sample sizes under TTC were studied by a comparative analysis method. The size effect of deformation and failure characteristics under TTC are not obvious, including stress-strain curves, Young’s modulus, peak strains, failure angles, and macrofailure mode. However, the damage stress (σcd) and peak strength (σp) have obvious size effect; that is, the smaller the sample size is, the higher the strength is. Additionally, the relationship among the peak strength, sample size, and intermediate principal stress (σ2) is power function. In addition, by comparing the peak strength increment caused by the sample size of the two types of rocks, the σp of the fine-grained BHT basalt is more sensitive to sample size than that of the coarse-grained BS granite. Finally, by analyzing the relationship between the size of the mineral grains or clusters in the two types of hard rocks and the complexity of crack propagation in the fracture surface under TTC, it is suggested that the minimum side length of rock samples should not be less than 10 times the maximum mineral clusters (such as feldspar phenocrysts in BHT basalt). In addition, the method of estimating elastic strain is established by analyzing the relationship between the size of the rock sample σ2 and the elastic strain under TTC.
Highlights
How to combine the laboratory test results with the monitoring results of the project has been a major problem faced by rock mechanics. e size difference between indoor rock samples and engineering rock mass is the most direct obstacle to this problem, and research on the size effect is considered to be an important part of solving this problem
E relationship of the peak strength and damage stress to the sample size of the BS granite and BHT basalt under true triaxial compression (TTC) showed that the variation amplitude of the characteristic stress increment caused by the size effect in the fine-grained BHT basalt was obviously smaller than that of the medium- to coarse-grained BS granite, and the characteristic stress of the two types of rocks was BS Granite σ3 = 5 MPa
(2) e peak strength and damage stress of the BS granite and BHT basalt were significantly affected by the sample size and σ2 under TTC
Summary
How to combine the laboratory test results with the monitoring results of the project has been a major problem faced by rock mechanics. e size difference between indoor rock samples and engineering rock mass is the most direct obstacle to this problem, and research on the size effect is considered to be an important part of solving this problem. There have been few studies on rock samples with different sizes under TTC, and only the studies [21,22,23] have investigated the mechanical and failure characteristics of the same rock with different aspect ratios under true triaxial unloading conditions. These studies did not provide the strength, deformation, and failure characteristics of rock samples with a fixed aspect ratio but different sizes under TTC. By analyzing the relationship between the complexity of crack propagation and mineral particle size in the fracture surface with different sample sizes, the recommended sample sizes for analyzing crack propagation in the fracture surface are determined. e results of this study can help to understand the size effect under TTC
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