A case of large buildings construction above oldmine goaf in Shandong Province

A case of large buildings construction site over old mine goaf in Shandong was studied. The fine exploration of the old mine goaf was carried out through investigation and mapping, high density resistivity method, magnetotelluric method, transient electromagnetic method, other geophysical exploration methods and drilling methods. The distribution position, overlap relationship, recovery rate, water accumulation and other parameters of the three-layer goafs of 4,9,10-2 coal seams were obtained, and the characteristics of the old mine goaf was revealed. The site stability of the proposed large-span data processing center and four high-rise buildings was analyzed and evaluated from two aspects: the impact of surface residual deformation on the stability of buildings and the impact of building load on the stability of goaf strata. The additional ground subsidence value caused by the activation of the old goaf in the study area was 260 mm, which exceeds the requirements of the foundation deformation during the construction and the usage of the proposed buildings. There was an overlap area between the 9 coal seam goaf fracture zones and the additional stress influence range of upper high-rise building load in the western study area. The stability of the building foundation in some areas was poor, and it was necessary to reinforce the goaf strata. The vertical section method was used to determine the horizontal governance range, and the vertical governance range was determined according to the movement characteristics of mined-out rock. Two grouting layers were divided according to the spacing of goaf in each coal seam. The spacing of grouting boreholes in the east-west direction was 25 m, and in the north-south direction was 20 m. The spacing of curtain holes was 15 m, and the curtain and grouting holes were completed once. The “pressureless flow + pressurized diffusion” grouting technology and process were used to reinforce the goaf stratum. One grouting layer adopted one grouting process and multiple grouting layers adopted upward grouting process. The grouting reinforcement effect was tested through the leakage of drilling flushing fluid, the uniaxial compression test of drilling core and the secondary grouting. There was no leakage or a small amount of leakage of drilling flushing fluid, and the uniaxial compressive strength of the stone body was above 9 MPa. The secondary grouting amount of each hole was less than 10% of the average grouting amount of the surrounding hole, the grout was effectively filled in the goaf strata. The influence of surface deformation in goaf on the stability of proposed high-rise buildings and large-span data processing center was analyzed. The stability of high-rise buildings was obviously affected by the inclination ground, and the main building foundation adopted the form of “pile + raft”,and some grouting holes under high-rise buildings were used as small piles to enhance the foundation stability. The stability of the large-span data processing center was obviously affected by the horizontal deformation of the ground. Setting the foundation connecting beam and strengthening the overall strength of the upper structure were used to enhance its ability to resist horizontal deformation. During the construction of the data processing center, the maximum ground subsidence was 5 mm, and the average subsidence speed was 0.01 mm/d. At present, the buildings in the study area have been put into use and operated stably. Large buildings were safely built and operated above three-layer coal seam gobs, which include a large-span, high standard data processing center and 22-storey complex buildings.