Prediction Of Surface Subsidence In Old Goaf And Study On Site Stability And Treatment Under Building Load

In order to implement the "14th Five-Year Plan" and accelerate the development of urbanization,it has become an important measure to use the site of the underlying goaf as a building foundation to solve the problem of land shortage.Based on the second-class residential and basic education land use project of Block MC00-0005-6002,Block 05,Mentougou New City,Beijing,this paper takes the geological conditions and architectural design conditions of Mentougou mining area as the background,and adopts the method of field investigation,field observation,laboratory test,theoretical analysis and numerical simulation.The law of surface residual settlement of old goaf and the safety and stability of old goaf under construction load before and after treatment are comprehensively studied and analyzed,and a reasonable and effective treatment scheme is put forward.It provides a theoretical basis for the rational utilization of the land above the old goaf in Mentougou District of Beijing and similar mining cities,and has good theoretical significance and application prospect.The main research results are as follows:(1)Through the methods of field visit,field investigation,drilling,geophysical exploration,in-situ test and laboratory test,the engineering geology of the proposed site,the occurrence form of goaf and the physical and mechanical parameters of each layer are further determined.(2)The overall trend and velocity of residual subsidence in mined-out area of small coal mines were analyzed by the previous monitoring data.On this basis,the measured data were optimized by wavelet threshold denoising method with MATLAB and SPSS software,and the combined model formed by GM(1,1)and BP neural network model in series was used to predict residual subsidence.A grey BP neural network model based on wavelet denoising is proposed.The error calculation with the original monitoring data shows that the MAPE and RMSE of the combined model are only 1.78% and 16.05 mm,respectively,which are very close to the real subsidence trend of the surface,and the fitting degree is high.(3)According to the "three down" mining specification,it is calculated that the development height of caving fracture zone in old goaf is about 42.2m,and the maximum depth of influence of building load can reach 22 m by stress control method.Therefore,it is inferred that the critical mining depth of coal seam is about 64.2m,which exceeds the actual depth of coal seam,and it is judged that there is a risk of activation in old goaf.Using Midas GTS NX finite element software numerical simulation,it is found that about 100 mm of uneven subsidence occurs between the building and the surface,and about 50 mm of subsidence occurs in the goaf roof,which further verifies that the goaf not only affects the stability of the overlying strata,but also aggravates the phenomenon of uneven subsidence of the surface and the building above.(4)Using excitation agent upgrade of cement fly ash composite grouting material on v of the proposed site of small minedout area filling governance,try with fluidity of slurry in the process of test,setting time,drainage rate and rate test,unconfined compressive strength test,microscopic test and economic comparison,finally selected solid compared to 1: 6.The watersolid ratio of 1.0 and the activator content of 1% shall be the best mixture ratio used in this project.A kind of airbag stop-grouting filling device is put forward for grouting construction,which can significantly save grouting materials and improve grouting efficiency.(5)After grouting and filling,the treatment effect and stability of goaf are evaluated by numerical simulation method.The results show that with the increase of filling rate,the stress and strain of goaf roof decrease greatly.When the filling rate reaches 95%,the settlement displacement of the surface and the building is reduced to about half of that before filling,and the grouting treatment effect is remarkable,which greatly improves the safety and stability of the goaf foundation.Finally,the quality of the treated site is tested,and the grouting filling rate and the strength of the filling body meet the technical specification requirements of goaf treatment.There are 69 figures,23 tables and 98 references in this paper.