Study Of3D Laser Scanning, Visualization And Analysis Of The Stability Of Hazardous Cavity Under Open-Pit Mine

Hazardous cavity created by previous underground mining activities under open-pit mine constitutes a potential hazard to active mining operations; especially in open-pit mine converted from underground mine. During the process of peeling off of overlying strata, the thickness between pit bottom and roofs of the cavities is becoming thinner and thinner. Additional loading is also imposed by engineering works directly over the cavities. Moreover, ground vibrations arising from excavation with bench blasting could loosen the support of the walls and roofs of the cavities. The stress by bench blasting may exceed the strength of rock material, thus results in cover collapse. Collapse of these cavities imposes a potential hazard to workers and heavy equipment in the open-pit mine. Therefore, accurate surveys of these cavities, reasonable planning for open-pit mining based on visualization data and estimation of the stability of underground cavities under ground pressure or in the vicinity of the bench blasting are formidable problems.By considering the characteristics of underground cavity in open-pit mine, geotechnical conditions and stability of surrounding rock, the methods such as field survey, field tests,3D visualization, numerical simulation and so forth are used synthetically in the paper. The Cavity Auto-laser Scanning System(C-ALS), mining software Surpac, numerical simulation Flac3D, ANASYS and LS-DYNA are used for investigating and evaluating the stability of the cavities in Sandaozhuang open-pit mine supported by the project "detection of old mined-out area and key technology and equipment of prevention and control of water disasters" under the national11th Five-Year Science and Technology Supporting Plan of China. Cavity3D laser sanning,3D visualization of complicated cavity group under open-pit mine, numerical analysis of the stability of complicated cavity based on C-ALS and coupled Surpac-FLAC3D technology and numerical analysis of the stability of abandoned cavities in bench blasting are carried out, and the main study contents are as follows. (1) The existence of abandoned cavities created by mining activities constitutes a potential hazard to active aboveground mining operations, the safety of workers and heavy equipment. In response to this situation,3D laser scanning for complicated cavity with Cavity Auto-scanning Laser System (C-ALS) is put forward and its working principle is also introduced. Cavity under open-pit mine can be exactly located by using C-ALS, and the data of the cavity can be processed and accurately generated3D geological model of the cavity in Modelace and Voidwork software.(2)3D visualization modeling technology of complicated geological bodies which is based on Triangular Irregular Network (TIN) was introduced, and corresponding implementing scheme of3D visualization of complicated cavity group under open-pit limit on platform of Surpac software was made out. On the foundation of field surveys, collecting and sorting data, the3D visualization models of Digital Terrain Models (DTM) of open-pit surface, occurrence state of ore body and complicated gravity group of Sandaozhuang open-pit mine were generated, especially, intricate transfixion relationship in cavity group was emphatically described in3D which can be used to logically analyze jointly destructive effect.(3) Real space boundary of the irregularly cavity was obtained by laser scanning and3D block model of the surrounding rock was modelled in Surpac software based on the3D point cloud data of the cavity. The coupled Surpac-FLAC3D technology has been studied and3D model data was successfully converted into FLAC3D software. Numerical analysis model with real space shape of the cavity was created in FLAC3D, combining physical and mechanical parameters of surrounding rock based on field tests and other geological conditions, static calculation was performed. According to the numerical results, the stability of the cavity was analysed through referring to the nephogram of stress and displacement of the surrounding rock, and the accuracy and reliability of analysis results were greatly improved.(4) Based on the data of stress waves measured from bench blasting tests, the site-specific attenuation relations of PPV and principal frequency of stress waves were obtained. A numerical model is then developed based on these relations to simulate the stress wave propagation induced by bench blasting. The accuracy of the numerical prediction is verified by the field measured data. The verified numerical model is furthermore used to perform a series of simulations. They are then used to assess the roof stability of cavities in adjacent upper bench blasting together with PPV damage criterion.