Mechanical monitoring and analysis on shotcrete layer of soft rock main roadway supported by bolt-shotcrete

Based on the soft rock, water-trickling and hard-maintain difficulty conditions of main air-return roadway in Tarangaole Colliery, high pretension stress and intensive bolt-shotcrete support program was designed. Mechanical property of shotcrete layer was specially monitored through utilizing concrete stress meter with a type of oscillating chord after the program was carried out in the roadway. It is indicated that the interior of shotcrete layer will emerge diverse stresses in axial, radial and tangential direction, and that its exterior will emerge contact pressures when the shotcrete layer gets in touch with surrounding rock and the earlier layer. With time prolonged, internal stresses in three directions whose average values were 0.517 MPa, 0.043 MPa and -0.059 MPa respectively have been fluctuated and tended to stability. Nevertheless, two kinds of external contact pressures which were all compressive increased earlier and remained stable lately while contact pressures of layer-surrounding rock were up to 0.266 MPa and those of layer-layer were up to 0.648 MPa averagely. Spatially, along the section of roadway, all of internal stresses distributed symmetrically and increased gradually from the top of arch to the waist of wall, but stresses in the foot of wall reduced partly with bilateral symmetry. However, external contact pressures increased gradually from top to bottom and distributed unevenly along the surrounding of roadway. During the period of roadway excavating, the stress of shotcrete layer was less than its ultimate bearing capacity, and the deformation of shotcrete was limited. The two aspects show that high pretension stress and intensive bolt-shotcrete support technology meet the surrounding rock control requirement of main air-return roadway in Tarangaole Colliery.