Time Delay Mechanism of the Kaiser Effect in Sandstone Under Uniaxial Compressive Stress Conditions

The rapid and accurate measurement of in situ stress fields is one of the essential steps in geological engineering. Indoor acoustic emission tests and the Kaiser effect provide a promising prior peak stress measurement method. This paper aims to qualitatively explore the time delay mechanism of the Kaiser effect under uniaxial compressive stress. Acoustic emission tests were conducted with natural and water-soaked sandstone specimens to examine the time delay phenomenon of the Kaiser effect, and the typical Griffith fracture mechanics theory was adopted to explain the mechanism. The time delay of the Kaiser effect under uniaxial compressive stress was affected by the friction coefficient on the crack surfaces and the initial crack angle β\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta$$\end{document}. The time effect was indirectly induced by the changes in the friction coefficient on the crack surfaces, and the friction coefficient was controlled by the water content in the rock specimen. A higher initial friction coefficient led to an earlier and easier time delay of the Kaiser effect. Consequently, from the aspect of the Kaiser effect, it was suggested that when testing in situ stress, measures should be taken to maintain the original moisture contents in the rock cores after they are drilled underground. Otherwise, the in situ stress should be tested within 7 days.