Long-Term Bearing Deformation Characteristics of Caved Gangue in Gob under Different Moisture Conditions

Abstract

In gob, the caved gangue is in a state of long-term bearing deformation with obvious time-dependent effect, even though the coal seam has been mined for a long time. As time goes on, because of the excessive deformation of caved gangue, the gob surrounding rock will easily aggravate the structural instability and cause dynamic disasters, such as mine water inrush and surface collapse. If the gob is filled with water, the bearing capacity of the caved gangue in waterlogged environment will be worse than that in dry environment, which will greatly increase the possibility of disasters. In order to study the bearing deformation characteristics of caved gangue under different moisture conditions, a testing system used for bearing deformation testing of caved gangue is developed. The typical caved gangue is chosen for samples. In view of the particle size distribution characteristics and stress state of caved gangue, the size distribution of caved gangue obeys the law of normal distribution and the load pattern is designed to step load. With the increase of axial load, the axial deformation of caved gangue increases gradually, the residual bulking coefficient decreases gradually, which are more obvious in load stage than in constant load stage. In the early constant load stage, the axial deformation of caved gangue grows rapidly, then tends to be slow and steady gradually; the relationship between strain and time meets the logarithmic relationship. In load stage, the D-value of axial strain decreases gradually along with the increase of axial load, whereas in constant load stage, it is opposite; the bearing deformation of caved gangue is consisted of two parts: instantaneous compression deformation and long-term deformation. For instantaneous compression deformation, the water acting as the lubricant promotes the compressive strain of caved gangue specimen and is conducive to rotation and slipping between each rock block. For long-term deformation, meanwhile, the water acting as the softening agent reduces the strength of the rock block and is conducive to the crushing of the rock block. The newborn rock blocks may slip and fill the gap between rock blocks, which also promotes the compressive strain of caved gangue specimen. The research results have important significance for forecasting and controlling the movement and deformation of overlying strata caused by mining.