Research On Control Of Floor Heaves With Steel Pile In Gob-side Retained Roadway

Based on the analysis of predecessors’ achievements and by comprehensive using of laboratory test, field measurement, numerical simulation and theoretical analysis, this thesis began with creep rule of post-failure rock, comparatively analyzed the evolution process of deformation and load of floor during the whole life of the roadway, revealed the mechanical causes of floor heave in various stages, determined the specific countermeasures for the floor heave control in gob-side entry retaining. On the basis of above findings, a new method termed steel pile method was innovatively proposed. Then its technical effectiveness, economic superiority, control mechanism and influencing factors, etc. were deeply studied. Main results are as follows:(1) Based on the creep test of post-failure rock with different degree of damage and increment of confining pressure, the following conclusion was obtained: ① the smaller of the damage degree, the smaller of its creep rate, the longer of its settling time. ② increasing the confining pressure can improve the stress state of the rock, and then improve its creep properties. ③ the damage form of post-failure creep is X-shaped shear failure and the block will slip significantly along the fracture plane after the failure.(2) Through the comparison between the evolution process of roadway floor deformation and the movement characteristics of overlying basic roof strata, floor heave in gob-side entry retaining was divided into four stages, namely, the first mining influence ease stage, the first mining influence severe stage, the stable stage of gob-side entry retaining and the advance influence stage of secondary recovery.(3) This thesis revealed the mechanical causes of floor heave in various stages. For the first mining influence ease stage, the first mining influence severe stage and the advance influence stage of secondary recovery, the causes can be summarized as follows: ① the floor of two ribs showed overall sinking compared with roadway floor with high abutment pressure. ② post-failure rocks behind the piles slipped into the inner side of the roadway under high abutment pressure. ③ floor rocks transport into the inner side of the roadway under the extrusion by floor strata behind the piles. For the stable stage of gob-side entry retaining, the mechanical causes of floor heave include the creep deformation of post-failure floor rock and the slippage of failure blocks along the fracture plane.(4) This thesis analyzed the technical effectiveness of the steel pile method by using numerical simulation. The results showed that steel piles can effectively decrease the accumulated value of floor heave, severity of floor heave and the stress disturbed zone. Compared with floor bolts and floor restoration, the method showed obvious advantages in controlling floor heave of gob-side retained roadway in terms of technical and economic benefits.(5) The mechanism of the floor heave control with steel piles method can be summarized as follows: ① the additional horizontal stress field and principal compressive stress arch behind the piles were formed and its strength and stiffness were raised. ② the squeezing action of rocks behind piles on floor strata was weakened, and the damage degree of floor was reduced. ③ the upward movement of the rocks in the tensile strain rising zone was restricted by the tensile force supplied by the tensile strain compression zone. ④ the damage degree of the floor entering the stable stage of gob-side entry retaining was reduced and its creep properties in this stage was improved.(6) Studies were carried out to investigate the effect of the parameters of steel piles on the floor heave control. It is found that the effectiveness of the steel piles in controlling floor heaves can be enhanced by greater installed dip angle, longer length and smaller interval of the pile.