Abstract
To study the strength, deformation, and failure patterns of columnar-jointed-rock-mass (CJRM) under unloading conditions, triaxial unloading tests using the CJRM-like material samples are carried out, and acoustic wave (AW) velocities are simultaneously recorded. Based on stress-strain curves and AW velocities under different initial confining pressures and unloading rates, the stress-strain characteristics, strength, and deformation parameters, failure modes, and variation of the AW velocity are analyzed. Test results show that the CJRM may exhibit intense volume expansion during the unloading process. With the increase of the unloading and its rate, the volume expansion becomes more serious and the failure mode becomes more complicated. By reducing the unloading (rate), a phenomenon of unloading relaxation is observed and the quality of CJRM is significantly improved. The AW velocity of CJRM shows a strong correlation with the volume strain, which verifies the effectiveness of applying AW velocity for assessing the rock quality. It is hoped that the research results may provide a reference for the construction and operation of the Baihetan Hydropower Project.
Highlights
Introduction e Baihetan HydropowerStation is located at the lower reaches of the Jinsha River. e barrage is a double-curvature arch dam with the height of 289 m. e Baihetan arch dam foundation is mainly composed of columnar-jointed basalt, which contains abundant joint surfaces and microcracks having indispensable effects on the foundation stability. e physical and mechanical property of columnarjointed basalt were conducted by many scholars [1–8]
Such a phenomenon indicates that when the unloading rate is relatively low, the failure of rock sample is a process of damage accumulation with evolution of partial failure to overall failure; the higher the unloading rate, the steeper the stress-strain curve. e increase of unloading rate accelerates the failure of CJRMlike material samples
By conducting a series of triaxial unloading tests with CJRMlike material samples, the stress-strain characteristics, strength and deformation patterns, and acoustic wave (AW) velocity characteristics during the unloading processes are studied. e main conclusions are as follows: (1) e stress-strain curve of CJRM during unloading process can be divided into two phases
Summary
Given that the joint dips of columnarjointed basalt in the Baihetan area are basically round 75°, the CJRM-like material samples used in the triaxial unloading tests in this paper are all designed with a joint dip of 75°. During the process of triaxial unloading test, the loading or unloading state can be artificially controlled by increasing or decreasing the axial and confining pressures. After the stability of confining pressure, add the axial pressure to the predetermined value (about 70% of the peak strength during triaxial compression test). When the axis pressure remains stable, unload the confining pressure at a rate of 0.2 MPa/min until the sample is broken. (3) Remove the rock sample, record the failure mode, and collect data
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