3.1. Roadway Deformation Characteristics
According to the laboratory test and field investigation, the deformation and failure of the 1305 auxiliary transportation roadway is mainly manifested as serious bottom heave. Considering the surrounding rock conditions and original support design, the main influencing factors of roadway floor heave are as follows:
(1) Roadway surrounding rocks are extremely unstable
The #3 coal seam of Dananhu No. 1 coal mine is a soft roof-coal-floor coal seam, and the direct bottom of the 1305 working face is mudstone and siltstone, which is cemented by mud and expanded by water. We have tested the physical and mechanical parameters of the working face floor in the laboratory and obtained that the density of the rock is about 2.06 × 103 kg/m3, the unidirectional compressive strength is 8.66 MPa, and the softening coefficient is 0.1. The roof and floor strata are of IV surrounding rock, which is extremely unstable.
(2) Significant influence of mining
After the operation of the working face, under the influence of high concentrated stress, the surrounding rock of the goaf produces serious plastic deformation. In addition, the surrounding rock is composed of mudstone and shale, which is easy to expand or decompose in the presence of water or under the influence of weathering. Soft rock has strong rheological characteristics, so the overall deformation is asymmetric, with large deformation near the goaf and small deformation near the coal pillar, which has been verified by field measurements.
(3) Damage of drainage ditch causes water seepage, deterioration, and expansion of the bottom plate
Groundwater can change the stress state and also affect the strength of the surrounding rock. The increase of pore water pressure on the structure plane reduces the normal stress on the structure plane, thus reducing the shear strength of rock mass. The lithology of 3# coal roof and floor of Dananhu No. 1 coal seam is soft, and the rock mass has low compressive strength. Therefore, in the process of loading and unloading, the ability of surrounding rock to resist deformation is greatly weakened by the influence of the V3 aquifer and groundwater seepage. Under the influence of mining, the rock structure surface of the roadway drainage ditch expands and causes damage. The wastewater, aquifer water, and working face water produced in the construction of the support structure infiltrate along the cracks of the damaged drainage ditch, resulting in the sharp reduction of the surrounding rock strength. This greatly intensifies the expansion of the floor mudstone and causes the continuous floor heave.
(4) Floor support is too little
According to the original support scheme, only a 0.2 m thick concrete layer is used for paving the roadway bottom, which is too little compared with the roadway roof. As the concrete pouring construction is carried out under the condition of no support of the floor, the soft rock floor is squeezed by two sides during the solidification of the concrete pouring, the overall solidification firmness is not strong, and a large number of defects occur in the initial stage, which cannot produce high strength inhibition to the floor heave.
The poor quality of rock mass belongs to the geological influence factors; the roadway can be relocated, but it is very difficult to deal with it after encountering. The mining influence and the weak support of the floor belong to the design influence factors, which are caused by a lack of understanding of geological conditions or the limited support technology, which can be effectively avoided; the damage of waterproof and drainage measures belongs to the construction and operation influence factors, which can be completely avoided. Therefore, after revealing the mechanism of floor heave, we should study the floor support and drainage measures in depth.
3.2. Roadway Deformation Characteristics
Based on the geological conditions and mining conditions of the 1305 auxiliary transportation roadway, a three-dimensional program was used to construct a numerical model to simulate the failure and instability of the 1305 auxiliary run through under the original support. The numerical calculation model (Figure 3
) is 50 m long, 38 m high, and 9 m thick. The 1305 auxiliary run-along trench is rectangular: 5 m wide and 3.2 m high.
At the top of the model, a 7 MPa evenly distributed load was applied to simulate the gravity effect of coal and rock mass above the roadway, which was constrained at the bottom and around, with a total number of 69,610 elements and 78,386 nodes. The physical and mechanical parameters of the coal strata are shown in Table 1
. The instability and deformation of the surrounding rock of the 1305 auxiliary haulage tunnel are investigated by calculation, and the mechanism of the roadway floor heave is revealed.
In the case of the original support (Figure 4
), due to the soft surrounding rock and large Poisson’s ratio coefficient, which is influenced by the concentrated stress generated by coal pillars in that section, the roof displacement and subsidence amount are large, the deformation amount of the coal pillar side roof and floor is large, and the floor produces asymmetric deformation. The floor heave deformation is the overall bulge, and the middle part is the most severe. The roof subsidence is 350 mm, the maximum floor heave is 735 mm, and the displacement of the two sides of the roadway is 550 mm. Therefore, obvious grooves are formed between the floor heave and the side. The deformation value of the two shoulders is 200 mm, and the maximum displacement of the roof and floor is 1021 mm, accounting for 32% of the original roadway. The roadway presents the phenomenon of anti-arch floor heave, which is “large in the middle and small on both sides”. The distribution of horizontal displacement is such that the deformation of the upper part of the roadway is smaller than that of the lower part of the roadway, the maximum deformation of the middle and lower part is 450 mm, and the displacement of both sides of the floor is 500 mm, which is 10% of the roadway width.
Since the water inflow of the two sides of the roadway is different and the softening coefficient of the surrounding rock is low, the strength of the surrounding rock of the south side and the north side of the roadway are also different, resulting in the vertical stress of the south side of the roadway being transferred to the depth of the rock mass.
The vertical stress of roadway reaches its maximum at 2–3 m of the two sides (Figure 5
). The distribution region of tensile stress on the north side is slightly larger than that on the south side, and the maximum tensile stress on the bottom plate is 0.7 MPa. The maximum horizontal stress is 21 MPa at 3.5–5 m of the roof and 3–4 m of the floor. The horizontal stress on the roof is greater than that on the floor, and the area of horizontal stress concentration is closer to the roof. The stress release phenomenon occurs in some areas of the floor due to the influence of mining, which leads to severe compression and flexure damage in this area. In the deeper region, the stress concentration is strong because the release limit is not reached. The force of the anchor rod and cable is relatively small in the roof area, and the force of the two shoulder angles increases to 150 kN. The force of the two anchors increases compared to the roof. The force of the two anchors reaches 309 kN within the range of 2 m.
Combined with field measurement, theoretical analysis, and numerical simulation, it is concluded that the deformation characteristics of the 1305 auxiliary transportation roadway are as follows: There is an obvious deflection in the middle area of the floor; both the legs and roof contract to the tunnel interior; and the floor undergoes anti-arch failure to the unrestrained side. Under the complicated conditions such as the influence of mining and unreasonable support, the high-strength extrusion of the two sides causes a large range of shear failure of the bottom slab. Finally, through comprehensive analysis, we think that the floor heave mechanism of 1305 auxiliary transportation is the extruding and flowing of floor heave.
Therefore, based on the above comprehensive analysis of the influencing factors of floor heave, we think that we should effectively deal with the floor, strengthen the support, and optimize the overall support parameters so that the “roof two sides floor” support can form a unified and effective whole.