Experimental study of mechanical properties of highly-stressed rocks under true triaxial unloading conditions with different unloading directions

Abstract

During the excavation process of deep rock masses, the surrounding rock stress field forms alternating zones of stress increase (loading zones) and stress decrease (unloading zones) near the excavation surface, which can easily trigger instability and failure in high-stress rock masses. Despite numerous studies that have been conducted on rock loading and unloading failures, the understanding of rock failure mechanisms under complex true triaxial stress paths remains insufficient. This study focuses on cubic granite specimens from the Linglong gold mine in Shandong. To comprehensively investigate the strength and failure characteristics of granite samples under complex true triaxial loading and unloading paths, true triaxial loading tests under different lateral stresses were conducted, followed by further true triaxial unloading tests in different unloading directions. The experimental results reveal that with the increase of intermediate principal stress, the failure mode of granite under true triaxial loading condition changes from shear-tensile failure to tensile failure, and the true triaxial loading failure strength increases first and then decreases slowly. Under the same intermediate and minimum principal stress conditions, the true triaxial unloading failure strength of granite is smaller than its loading failure strength, and the Mogi strength formula fits well with the true triaxial unloading strength under the minimum principal stress condition. This study provides an essential theoretical basis and guidance for controlling and designing the stability of deep rock masses in engineering applications.

Keywords

• true triaxial loading and unloading
• intermediate principal stress
• unloading direction
• strength
• rock mechanics