| The contradiction between high intensity underground coal exploitation and local fragile ecological environment constrains the development of coal mining industry in Northwest China.Such an issue can be more conspicuous when mining activity runs in weakly cemented rock(WCR)strata,considering WCR physical and mechanical properties with respect to developed natural fissures,low strength,weak cementation,rich clay minerals,and easy to swell and soften with water.Facing a series of WCR-related problems regarding undetermined permeability characteristics,unclear strata water-resistance property,and poor applicability of conventional water-conservation mining theory,the thesis studies the water-resistance property variation of WCR strata in response to coal mining operated below and its control mechanism by using laboratory testing,theoretical analysis,physical simulation,computer modeling,and field measurement,mainly obtaining the following achievements.(1)WCR microstructure profile is analyzed,and WCR permeability characteristics are revealedTaking representative WCR mudstones and sandstones as samples,the porosity,pore dimension,connectivity,cementation model,and mineral compositions are identified,accompanied by their impacts on the permeability variation of WCR samples in full stress-strain process.Considering pore convergence and matrix cracking under mechanical compression,WCR damage variable D is defined,and a function describing WCR stress-damage-permeability relationship in the complete process of matrix compression and cracking is developed,effectively representing WCR hydraulic performance,that is,high primary permeability and minor increment in the residual stage.(2)WCR strata deformation and movement characteristics under hydro-mechanical coupling effect are revealed,and a criterion identifying aquiclude instability is proposedA 3D solid-fluid coupling nondestructive testing system based on servo control approach is constructed,by which WCR overburden strata caving and fracturing propagation are analyzed,WCR aquiclude stability and self-healing mechanism are revealed.Considering groundwater mobility and the change of water head acting on deformed aquiclude,a mechanical model regarding WCR aquiclude plate stability is developed,suggesting the criterion of aquiclude stress instability and strain instability.(3)WCR strata water-resistance property and groundwater regime evolution are revealed,key factors affecting water-resistance property and groundwater regime are identified and weighedImpacts of five factors regarding mining height,retreat rate,panel width,panel length,and the distance between coal seam and water body on water-resistance property and groundwater regime are analyzed,suggesting the sensitivity of the two indexes to the five factors.By BP neural network approach,an association model describing the maximum horizontal deformation of aquiclude and maximum vertical drawdown of groundwater in response to the five factors is constructed.The key factors contributing to the evolution of overburden strata water-resistance property and groundwater regime are mining height and the distance between coal seam and water body,mining height respectively,with weights of 0.43 and 0.32,0.54.(4)The correlation between the equivalent water resource carrying capacity and the other two indexes concerning equivalent mining height and equivalent water-resistance strata thickness is developedThe scientific significance of water-conservation mining based on “three equivalence theory” is analyzed,which incorporates the equivalent mining height(EMH),equivalent water-resistance strata thickness(EST),and equivalent water resource carrying capacity(ECC)as an integration.The correlation of ECC with EMH and EST is developed.Through regarding the ECC of mine sites as a constraint,a method for determining the timing and configuration of longwall panels in WCR geology is proposed,which is successively applied in longwall mining practice.This thesis includes 132 figures,44 tables,and 197 references. |
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