Water Inrush Mechanism And Risk Assessment Research Of Karst Collapse Column In North China Coal Field

Karst collapse column is a kind of special geological phenomenon in North China coal field.With the increase of mining depth and the expansion of mining area in China,karst collapse columns,in particular water-conductive karst collapse columns,gradually become serious security threats in coal mines where collapse columns exist.In order to ensure mine safety in these coal mines,relevant research aiming at internal structure,water inrush mechanism and three dimensional geomechanics model test of karst collapse column are necessary to be carried out.This dissertation takes the large water inrush accident of X7 collapse column in DongPang mine as the research object,through physical model experiment,theoretical analysis and numerical simulation method,studies the internal structure of the karst collapse column and water inrush modes of different-structure karst collapse columns,proposes the evaluation theory of water inrush risk of karst collapse column,establishes the technology system for preventing water inrush,and obtains a series of meaningful research achievements.（1）Classification of karst collapse column based on its internal structure generalized modelAccording to the existing data exposed by outcropped karst collapse columns and mine workings,this dissertation proposes the internal structure generalized model of karst collapse column.A typical karst collapse column consists of four parts:the enrockment section,the mud-breccia section,the clastic rock section and the column wall crack section,which contacts each other and shows difference in the mechanical and hydraulic characteristics at the same time.Among the four parts,the enrockment section affected by strong dynamic water pressure,usually presents mostly large and medium-sized rock blocks and rare clastic rock material with the size reflects the active level of groundwater in the early stage of karst collapse column formation,is a common water cut section in karst collapse column;The mud-breccia section above the enrockment section,its material basis for formation is the stratums existing in the coal measures strata,which are easy to decompose and can dissolve in water,such as mudstone,shale or argillaceous limestone and so on.After collapse,these stratums rapidly decompose under the influence of solid,liquid,gas three-phase power and combine with water to form mud.On the one hand,the existence of this section can balance the ground pressure,restrain surrounding rock deformation and prevent water,as the mud-breccia injected into the karst collapse first blocks the water upwelling channels of groundwater,then blocks other aquifers and the column wall cracks;On the other hand,after the hydraulic connection is blocked,the mud-breccia materials become to solidify and liquefy under the effect of gravity,gradually form similar argillaceous cementation conglomerate which function as the water-resisting section in karst collapse column;The clastic rock section locates at the top of the karst collapse column,contains both large rocks and detrital materials;The column wall crack section,as the weak zone,is located around in karst collapse column,specifically can be divided into the compression-shear fracture zone along the column wall and the tension-shear fracture zone at the top of the pillar,usually function as the channel for karst collapse column water inrush.Based on different combination feature of the internal structure,karst collapse column can be divided into eight types,e.g.the enrockment-type,and its characteristics as well as water inrush risk are analyzed.（2）Karst collapse column water inrush modes and the criterionsBy analyzing the interaction between coal-rock mass and groundwater,the induced agent for karst collapse column water inrush,and the position relationship between mining working faces and karst collapse column,this dissertation proposes five types of karst collapse column water inrush generalized model and deduces the corresponding criterion for each mode using theoretical methods such as catastrophe theory,seepage instability theory and structural mechanics,etc.Elastic rock beam（plate）mode corresponds to the condition that the mining working face locates on the top of the hidden karst collapse column,the criterion of catastrophe theory are as follows:Combined rock beam mode corresponds to the condition that a variety of lithology with thickness should not be neglected exist between the mining working face and the hidden karst collapse column,the criterion of catastrophe theory can be expressed as:Layered coal-rock mass mode applicable to the condition that the mining working faces pass through the water-resisting section of the karst collapse column,at the moment the mud-breccia section between the mining working face and the water-conductive section of the karst collapse column function as the water-resisting section,the criterion of rock mass seepage instability theory are as follows:（μh/k）²-4ρ₀（p₀-p₁）βh<0Rock wall mode is suitable for the condition that the mining working face locate by the side of the karst collapse column,the criterion of ultimate damage theory can be expressed as:（3）Geomechanics model test of large hidden collapse column water inrush of DongPang mineDongPang mine is located in Neiqiu county,Hebei province,the geological environment belongs to typical north China coal field-type,and hydrogeological condition belongs to Baiquan hydrogeological unit.There are multiple strong runoff zones in the mining area where karst collapse column is developed,results in a large water inrush accident occurred in April 2003 which led to the mine flooded.Based on the X7 collapse column which contributed to the accident in DongPang mine,applying the solid-liquid coupling model test to study the water inrush mechanism of large collapse columns,this dissertation obtains the stress and displacement distribution of mining roadway and confined water up-flowing law under combined effect of stress field and seepage field during mining.The test utilizes similarity theory to determine the scale of the corresponding modulus and the test scope;applies the fluid-structure coupling similar simulation material orthogonal experiment to develop a new type of fluid-solid coupling similar simulation material;Assembles the fluid-solid coupling analogy simulation equipment according to test requirements and carries out test arrangement based on actual working condition.The experimental results indicate that obvious stress concentration and displacement occur in a small area around the water inrush site when the water inrush happens,but the influence area is not large,thus stress behavior or large displacement is not a significant precursor of karst collapse column water inrush;whereas osmotic pressure change and water inflow are both certain advanced,can be considered as a good precursor of karst collapse column water inrush.（4）Water inrush risk assessment to karst collapse columnKarst collapse column water inrush is controlled by multiple factors,whether the water inrush occurs in the particular geological conditions should be determined.However,due to the technical and economic constraints,it is difficult to fully understand the internal structure of karst collapse column and its environmental geological conditions in practice,resulting in a number of factors affecting karst collapse column water inrush cannot present quantitative results.And for a specific project,there is no unified definition whether water inrush will happen,so how to comprehensively evaluate the water inrush risk of collapse column on basis of the existing data,is exactly a problem that cannot be evaluated accurately with a "yes" or"no".Consequently,the fuzzy comprehensive evaluation method is introduced to try to solve this problem.According to the relevant principles of fuzzy comprehensive evaluation method and the characteristics of karst collapse column water inrush,this dissertation proposes the water inrush risk evaluation factors of karst collapse column,selects five main factors and 14 indexes to build the corresponding evaluation levels;Applies multi-level analysis mathematical model and subordinate function to assess the water inrush risk of the X7 karst collapse column of DongPang mine,the evaluation result in conformity with the engineering practice,indicates the method is feasible for evaluating the water inrush risk of karst collapse column.（5）Prevention and control system research on karst collapse column water inrushBased on the internal structure of karst collapse column and its geological environment,combined with the study of different water inrush modes of karst collapse column,this dissertation proposes the water inrush prevention and control technology system of karst collapse column in north China coalfield.On basis of water inrush prevention and governance principles,establish the karst collapse column water inrush prevention and control technology system consisting of integrated exploration,water inrush precursors monitoring,water inrush water risk evaluation and prevention and control technology.