SYT detection
SYT detection uses natural geomagnetic waves to perceive the special physical characteristics of aquifers to determine water inrush channels and water richness, (Yang et al.2010). (Fig. 5a) shows the S6 section passing through the collapse column. From northwest to southeast, there are three vertical anomalous zones with low in-situ stress values. The north end of the geological anomaly body at 100 to 120 meters is the middle in-situ stress zone. (Fig. 5b) is the S7 section, passing through the collapse column. From the section, it can be seen that the in-situ stress value at the collapse column is medium and high, and the water richness is poor. (Fig. 5c) shows the S8 section. There are two wide vertical water-rich zones from northwest to southeast, which are 300 meters and 500–600 meters, respectively, and the water richness is still poor at the collapse column.
Integrating the underground and drilling exposures and the above two geophysical exploration results, the conductivity, water richness and characteristics of the collapse column developed in the 12318 working face are as follows:the collapse column is generally weak or not rich in water in the coal-measure strata.The area affected by the collapse column and the nearby rock mass have a certain degree of water richness. Due to filling and compaction, the column body of the collapse column is less likely to be rich in water and water conduction. Due to the traction of the collapse column, the surrounding rock formations are stretched and damaged, and the fissures are relatively developed. The internal fissures are partially filled with water and show weak water richness.Ordovician ash and cold ash section are relatively developed in karst, and the water richness is relatively strong.
In the natural state, the collapse column has no conductivity in the coal-measure strata, and it is less likely to directly conduct the water from the deep Ordovician and cold ash aquifers to the coal seam. In order to analyze the impact of the collapse column on the mining of the 5#, 4# and 1# coal seams under mining conditions, FLAC is used to simulate the mining process to evaluate the safety of the fully mechanized mining face passing the collapse column.
Build a collapse column model
The simulated mining coal seams select the 5# coal, the 4# coal, and the 1# coal. The collapse column is located at the center of the model, and the top is at the top of the model. The lower ash layer may be closely hydraulically connected with the overlying Ordovician ash and cold ash water. According to the "Pansan Mine No. 2 Level Supplementary Exploration Geology Report", the water pressure of the ash is about 6.8MPa, which can be considered as a high-pressure aquifer.
The impact zone of the collapse column is dominated by fillings, and hydration reduces the strength of the fillings and also affects the evolution and expansion of the cracks in the water inrush channel,(Dong et al.2020,Yin et al.2021). The collapse column is simplified into a rectangular parallelepiped embedded in the model and the FLAC3D simulation calculation uses the Mohr-Coulomb material constitutive model. The established numerical model is shown in Fig. 6. The length (y direction), width (x direction) and height (z direction) are respectively 450m, 340m, and 200m.The mining of the working face is carried out in the order from top to bottom.
The upper boundary condition of the numerical calculation is set as the self-weight stress of the overlying strata of the coal seam, and the lower floor is simplified as the displacement boundary condition, which can move in the x and y directions, and the z direction is the fixed hinge support, ie v = 0. The boundary conditions on both sides are solid coal and rock masses, simplified as displacement boundary conditions, which can move in the z direction, and the other directions are fixed hinge bearings, that is, u = w = 0.
Affected by both mining and structure, the depth of floor failure increases, (Liang et al.2020). The collapse column not only destroys the continuity of the coal seam, but also easily forms water inrush channels after activation ,(Sun et al.2015,Yao et al.2020).Through numerical simulation, the vertical stress distribution pattern around the collapse column and the plastic zone failure characteristic diagram during the coal seam advancement process are obtained. The stress change law and reason are analyzed, and the floor failure limit depth and the thickness of the water-resistant layer are compared. Knowing whether it can be directly pushed through the collapse column helps us make a water inrush risk assessment.
Water inrush risk analysis