Current Tectonic Stress State in an Iron Mine District, North China, Based on Overcoring, Hydraulic Fracturing, and Acoustic Emission Stress Measurements

Abstract

Full knowledge of the current tectonic stress state is crucial for assessing open-pit mine slope stability and regional tectonic evolution and geodynamic processes. Overcoring, hydraulic fracturing, and acoustic emission in situ stress measurement techniques were adopted to determine the 3D stress tensor in an iron mine district, North China, and 25 sets of stress data ranging from 56 to 490 m were measured. Accordingly, the current tectonic stress state and its relationship to regional geological tectonics were investigated. The results indicated that the stress condition seemed to favor thrust and strike-slip faulting, and the stress field was particularly controlled by the horizontal tectonic stress. A high horizontal tectonic stress considerably influenced the stability of high and steep slopes in this mine district, which requires great attention. The stress directions derived from different methods were almost similar, indicating a dominant NEE–SWW stress field direction or near-E–W direction, comparable to the direction revealed by focal mechanism solutions and other stress indicators around the mine district. According to geological structure analysis, the present-day stress field in this district generally inherited the third-stage tectonic stress field while partially retaining the characteristics of the second-stage tectonic stress field, which is the result of dynamic action and tectonic movement during different geological periods, and the maximum principal stress direction of the tectonic stress field that affects the modern tectonic activity in this area is the NEE–EW direction.

Accurate measurement techniques and prediction approaches for the in-situ rock stress

“…The overcoring (OC) method is an indirect in-situ stress measurement technique that can accurately and quantitatively determine the original rock stress vector. It is currently the only method that can obtain the magnitude and direction of the complete three-dimensional stress from a single OC test 28 , 29 . The OC method is based on measuring the response strain when the rock core is released from the stress field of the surrounding rock mass, and the original rock stress vector can be estimated by analyzing the measured strains with the knowledge of the elastic properties of rocks 6 .…”

Comparison and evaluation of overcoring and hydraulic fracturing stress measurements

“…This is of great significance to further understand the stress scale represented by these two representative methods and improve their measurement accuracy. It should be noted that the stress data of the two mines used in this study have been employed to reveal the distribution characteristics of the in-situ stress field and its interaction mechanism with regional geological structures in these two mining areas 18 , 19 , which is completely different from the purpose of this study.…”