Abstract
The filling material of the karst collapse column (KCC) is easy to be activated by mining. During this process, the mechanical properties of KCC fillings change, and its water resisting capacity constantly deteriorates and thus often leads to water inrush disaster. In this study, the samples of KCC fillings were taken on-site and then were remolded by the consolidation drainage method. The variation laws of the compressive strength, tensile strength, cohesive stress, internal friction angle, and permeability of the filling samples with respect to the consolidation pressure and moisture content were tested and analyzed. Based on an engineering example, the yield and activation and particle loss of the filling material of the KCC are analyzed. A mechanism for the lagging water inrush of KCC in the process of mining is proposed. The main results of the present study can be concluded concisely as follows. (1) The KCC fillings show obvious soft rock characteristics in the process of uniaxial compression and Brazilian split. The ratio of the uniaxial compressive strength to splitting tensile strength is between 12 : 1 and 8 : 1. The larger the consolidation pressure or the smaller the moisture content, the larger the ratio. (2) With the increase of consolidation pressure or the decrease of moisture content, the uniaxial compressive strength, elastic modulus, splitting tensile strength, cohesive stress, and internal friction angle of the filling material of the KCC increase linearly, while its permeability increases exponentially. (3) When the crack field of the surrounding rocks of the stope is connected with the KCC, its filling material will continue to yield, activate, and migrate under the fluid-solid coupling effect and finally result in the lagging water inrush from the KCC.
Highlights
As a major coal-producing area, North China has an approved production capacity of 2.82 billion tons per year, accounting for nearly 66% of China’s coal reserves [1, 2]
The shallow coal resources in North China are gradually exhausted, and coal mining is gradually extended to the deep underground [3, 4]. e deep mining of coal leads to considerable increase of the confined mining area of Ordovician limestone aquifer, which aggravates the threat of Ordovician limestone karst water disaster [5, 6]
According to the actual occurrence state of karst collapse column fillings (KCCF), we have considered various moisture contents and different consolidation pressures in the process of sample preparation (Figure 3)
Summary
As a major coal-producing area, North China has an approved production capacity of 2.82 billion tons per year, accounting for nearly 66% of China’s coal reserves [1, 2]. Researchers carried out a lot of research on the seepage characteristics and water inrush mechanism of KCC with fruitful results. Under the joint action of coal mining and highpressure Ordovician limestone water, the KCCF is constantly yielding and activated, resulting in considerable change of the internal void structure of the KCC, the gradual increase of the permeability, and the weakening of the water blocking capacity, which eventually leads to the water inrush accident of KCC. According to the actual occurrence state of KCCFs, we have considered various moisture contents and different consolidation pressures in the process of sample preparation (Figure 3). The moisture content can be chosen as 15%, 18%, 20%, 22%, and 25%, respectively, while keeping the maximum consolidation pressure constant at 3 MPa. In order to ensure the same height of samples, we reserve enough height before consolidation and cut off the surplus height of samples after consolidation, so as to ensure the same height of samples under different consolidation pressures
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