Analysis And Research On Key Technologies Of Geotechnical Engineering For Restoration And Utilization Of Abandoned Mines

In recent years,with the awakening of the awareness of ecological and environmental protection,the historical problems of abandoned mines have been paid more and more attention,and the geotechnical engineering problems involved in the process of restoration and utilization of abandoned mines need to be solved.In this paper,the modes of restoration and utilization of abandoned mines around Nanjing are investigated and summarized,and the evaluation method of comprehensive benefits of restoration and utilization of abandoned mines is put forward;Based on the NIUSHOUSHAN mine restoration project in Nanjing,numerical simulation method is used to analyze and calculate the slope stability of abandoned mine under complex geological conditions,open pit water pumping operation,shallow tunnel excavation overlying slope stability,etc.the main research work and results are as follows:(1)This paper systematically reviews the status quo of restoration and utilization of abandoned mines at home and abroad,summarizes the typical modes of restoration and utilization of abandoned mines,and analyzes the main geotechnical engineering problems existing in the process of restoration and utilization of abandoned mines.(2)Based on the field investigation of typical abandoned mines(Shogunate Hill,Tangshan,Yeshan,NIUSHOUSHAN,etc.)in the surrounding areas of Nanjing,combined with the literature,this paper summarizes the status quo of the restoration of abandoned mines in the surrounding areas of Nanjing,the main restoration and utilization engineering modes and the existing geotechnical engineering problems.(3)By using the frequency statistics method,according to the field investigation results of abandoned mines in the surrounding areas of Nanjing and relevant literature specifications,the key evaluation indexes of abandoned mine restoration are selected,and the comprehensive benefit evaluation method of abandoned mine restoration and utilization engineering based on analytic hierarchy process is proposed considering the ecological,socio-economic,geological and environmental factors,Combined with entropy method to determine the comprehensive weight of the evaluation index,normalize and standardize the quantitative and qualitative indexes such as vegetation coverage rate,mine geological disaster evaluation,landform and landscape damage evaluation,and establish standardized scoring criteria,so as to improve the evaluation accuracy of comprehensive benefit evaluation method.(4)Taking the NIUSHOUSHAN restoration project in Nanjing as an example,a three-dimensional geological model of the abandoned mine is created.Based on the strength reduction method and the point safety factor method,the slope stability of the abandoned mine is analyzed.The calculation results show that the overall safety factor of NIUSHOUSHAN abandoned mine is 1.54,which is in a stable state;However,the stability coefficient of the unstable slope is less than 1.2,which is relatively dangerous;The typical dangerous section of the part with poor stability is intercepted,and the bolt +anchor cable method is used for joint reinforcement design.Based on the limit equilibrium method,the slope stability safety factor under typical working conditions(natural,rainstorm,earthquake)before and after reinforcement is calculated.The calculation results show that the slope stability factor before reinforcement is 0.83 ～ 1.095;After reinforcement,the natural condition is 1.38 ～ 1.767,greater than 1.35;The rainstorm condition is 1.353 ～ 1.684,and the earthquake condition is 1.161 ～ 1.388,which is greater than 1.15.(5)According to the characteristics of non-uniform drop of water level in the process of pumping operation of water accumulated in abandoned open pit,the formula of actual water level time curve is derived;Four working conditions of water level falling from1.92m/h,0.96m/h,0.64m/h and 0.24m/h to the end of pumping are set up to analyze the variation law of mine slope stability under each working condition.The results show that the faster the water level falling rate is,the faster the safety factor decreases,and the lower the minimum slope stability factor is;This paper analyzes the slope stability variation law of 0.5 days,1 day,2 days and 4 days of actual water level drawdown rate,and compares it with the condition of uniform drawdown under the same pumping time.The results show that when the construction period is short(less than 1 day),the water level can be approximately regarded as uniform drawdown(the difference is less than 2%);In the case of long construction period,the slope stability is better under the actual drawdown rate of water level than that under the constant drawdown rate of water level(the maximum difference is 5%).(6)The results show that under the same excavation conditions,the horizontal displacement of the slope is reduced from 63 mm in the natural state to 1.4mm,and the vertical displacement is reduced from 28 mm in the natural state to 8mm;In addition to the slope toe part of the point safety factor is 1.2 ～ 1.3,the other range of point safety factor is greater than 1.35,support reinforcement can significantly increase the local stability of the slope.