Abstract
The Kaiser effect of rock has been extensively studied due to its wide application in in-situ stress measurement and rock damage quantification. The uniaxial cyclic loading and unloading (UCLU) test is commonly employed to examine the rock Kaiser effect. However, how the two critical parameters, including prescribed stress in the first loading cycle (σA) and loading strain rate (lsr), affect the appearance of the Kaiser effect lacks thorough understanding. We systematically performed UCLU tests on 75 sandstone specimens under 25 combinations of σA and lsr. σA spans from 0.5σc (σc is the uniaxial compressive strength) to 0.9σc, and lsr ranges from 10−5 s−1 to 10−3 s−1, respectively. The acoustic emission characteristics of all the rock specimens are continuously monitored over the entire tests. We find that the Kaiser effect is unanimously observed in the stable crack growth stage, corresponding to the stress levels of 0.5σc to 0.7σc because under a lower stress, the Kaiser effect is easily covered by the acoustic emissions generated by microcrack friction. The loading strain rate also heavily affects the occurrence of the Kaiser effect. When lsr does not exceed 10−4 s−1, the Felicity ratio (FR) rises quickly as lsr ascends, whereas FR increases less pronouncedly once lsr exceeds 10−4 s−1. Therefore, a relatively high loading strain rate, i.e., lsr higher than 10−4 s−1, is suggested to facilitate the appearance of the Kaiser effect.
Highlights
An accurate estimation of in-situ geo-stresses is critical for the reliable design and regular operation of underground rock-engineering projects
The friction between microcracks leads to low-energy, low-quantity acoustic emission (AE) events with high variations
The stress acting on the rock specimen in the transition stage (II) is insufficiently high to develop new microcracks, whereas some closed microcracks slip, generating some low-energy AEs
Summary
An accurate estimation of in-situ geo-stresses is critical for the reliable design and regular operation of underground rock-engineering projects. Various stress measurement approaches in rocks are present, most of which are time-consuming and uneconomic, such as hydraulic fracturing and overcoring methods [1,2,3,4,5,6]. These approaches may lack adequate accuracy [7,8,9]. The Kaiser effect is widely documented in rocks and materials under the uniaxial cyclic loading and unloading (UCLU) test [13,14]. The Kaiser effect denotes the memory of rock to the history of loading
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