Experimental Study On Deterioration Of Macro And Meso Mechanical Properties Of Limestone Under Acid Environment And Its Application

Karst areas are widely distributed in China,and a certain number of mine roadways are located in limestone formation.For the reason that the coal seam environment is changed in mining area containing pyrite,mine water is acidic.The physical and mechanical properties of limestone are deteriorated and the stability of roadway surrounding rock are influenced by acid water.The damage rule of mechanical properties and meso structure are discussed,the influence on stability of roadways and the subtle response of key locations around the roadways are analyzed systematically.The main contents and conclusions are as follows:(1)Based on the nuclear magnetic resonance(NMR)technology,the limestone specimens are measured by NMR system before and after they were immersed in solutions of pH=3、5、7 to obtain the change rule of porosity.The results of measurement shows: Under the acid environment,the porosity of limestone specimen have the overall trend of becoming bigger,the mutative range size of porosity are bigger and the productive rate of secondary pores are higher with the stronger acidity.The deterioration degree of porosity is more serious with the initial porosity of specimen is bigger.Compared with the initial state,the porosity deterioration is expotentially deteriorated with the dereasing of pH value.(2)The immersed and dried specimens are used for triaxial compression test to obtain the deterioration rules of mechanical properties of limestone in acid environment.The results of experiment shows: the ductility of limestone is more obvious under the acidic conditions and smaller stress drop after peak is observed with the increase of pH value.Except elastic modulus exhibits a secondary decrease,mechanical parameters(uniaxial peak stress,Poisson’s ratio,internal friction angle,cohesion)exhibits a linear decrease basically with the decrease of the pH value.In addition,the effect on deformation parameters are greater than strength parameters of limestone with the deterioration of porosity under the acidic environment.And the influence degree of cohesive is greater than internal friction angle.(3)Based on the parallel bonding model which is widely used in rock materials,a series of simulation experiment are carried on to study the influence on macro mechanical properties of single microscopic parameters.On the basis of the relationship of micro and macro parameters,the micro parameters of limestone are calibrated in pH=3,5,7 by orthogonal test design.The main results shows: The calibration of deformation parameters(Young’s modulus and Poisson’s ratio)are dependent on Young’s modulus of particles and parallel-bond;the stress-strain curve of materials are mainly influenced by the joint action of the mean parallel-bond normal strength and shear strength;the essential reason of diversified distribution of cracks are the relative relationship between normal and shear strength of parallel-bond,the conjugate damage is showed when the ratio of the mean value of normal(shear)strength to deviation is around 1;the increase or decrease of the ratio causes the change from conjugate to shear damage.With the increase of friction coefficient,the secondary failure surface of samples decreases but the direction of failure surface will not change.(4)On the basis of the continuous-discrete numerical calculation method,the stability of parallel roadways and the subtle response of key locations around roadways are analyzed.The results shows: with the increasing of time under the acid environment,the plastic zone of parallel roadways is extended to run-through in middle pillar which indicates the instability of roadways.The distribution of cracks in discrete zones are corresponding to the plastic zones in overall continuous model.(5)The extended research of the stability of roadways in different lateral pressure coefficient under acid environment is carried out.The main results shows:The distribution of displacement vectors,force chains and cracks in parallel roadways are not affected by acid environment,only the size and amount of them are affected by environment condition.With the increasing of lateral pressure coefficient,the plastic zones of parallel roadways are transferred which are indicated by the amount of cracks in arch foot field A and middle pillar are decreasing and arch foot field B and roadway floor are increasing.The proportion of stretch type cracks in arch foot field A and middle pillar are increasing and the proportion of shear type cracks in arch foot B and roadway floor are increasing.