The objective of this paper is to investigate the three-dimensional strength characteristics of a porous sandstone, which is referred to as Zigong sandstone, under true triaxial stress states (σ1≥ σ2≥ σ3) by means of a Mogi-type true triaxial testing apparatus. Three series of tests (σ3 = 0, 20, and 60 MPa) are conducted. Within each series, σ2 is varied from σ2=σ3 to σ2=σ1 from test to test. For each test, σ1 is raised monotonically to failure while σ2 and σ3 are kept constant and the post-peak behavior is also captured. Deformability and strength are investigated in term of the effect of σ2. It is found that, for a constant σ3, the onset of dilatancy generally increases with the increase of σ2. Regarding peak strength, the variation of σ1 at failure versus σ2 exhibits a typical ascending-then-descending trend. A versatile deviatoric function is used to replace the one of Hoek-Brown strength criterion, which is referred to as the modified Hoek-Brown strength criterion and is further adopted to depict the strength characteristics in the three-dimensional principal stress space. Results show that the effect of σ2 on rock strength is actually a combined effect of mean stress and Lode angle.
Sandstone is one of the main formations in the earth's crust. Many human activities, such as hydrocarbon production, CO2 sequestration, and waste disposal, are intimately related to the failure of host sandstones. Understanding and charactering the mechanical responses of sandstone under different stress conditions is essential to the design and construction of many underground projects.
In the past several decades, efforts have been taken to investigate the failure mechanisms of porous sandstones based on laboratory tests. Extensive conventional triaxial tests have been conducted on cylindrical specimens of different sandstones (Wong T et al., 2012), revealing that mechanical responses of sandstones, such as failure modes, strength, deformability, and porosity evolution, are heavily dependent on the first two stress invariants related to the mean stress and the differential principal stress (Wong T et al., 1997; Grueschow E et al., 2005; Baud P et al., 2006), respectively. The effect of the third stress invariant, however, is rarely known except for the limited comparison between conventional triaxial compression (CTC: σ1> σ2= σ3) and conventional triaxial extension (CTE: σ1= σ2>σ3) (Murrell S, 1963; Mogi K, 1967). Moreover, in situ stress measurements at depths indicate a state of stress in the earth's crust that is totally non-uniform (σ1 ≠ σ2 ≠σ3) (Haimson BC, 1978; Brace WF et al., 1980; Vernik L et al., 1992). It is important to consider general states of stress to fully understand failure characteristics of sandstone.