Study On Rheological Characteristics Of Samples Under Special Coal Pillar Shape Of Strip Wongawilli Mining Method

The strip-type Wongawilli new coal mining method can achieve the purpose of safe and efficient recovery of pressed coal under buildings and working face corner coal.Due to the irregularity of strip Wongawilli coal pillar,it is very important to study the rheological properties of coal and rock with different shapes.In this paper,experimental research,theoretical analysis and numerical simulation are combined.Based on the uniaxial rheological test of rock,the rheological mechanical properties of rock are discussed by using the theory of nonlinear mechanics and damage mechanics.The rheological mechanical properties of coarse yellow sandstone under the action of different empty sizes under the same bearing contact area are mainly studied.The main results are as follows :(1)The uniaxial compression test and graded rheological loading were carried out on the coal-like rock with different shapes(circular,hexagonal,quadrilateral,rhombic,triangular,irregular swallow-shaped)of the same load contact area,and the failure characteristics of the loading failure samples were obtained.The coal-like rock samples all occurred shear failure like 90 °,and the failure of the sample often occurred at the tip corner.The uniaxial compressive strength of specimens with the same contact area and different free surface is inversely proportional to the free surface area;(2)In these coal-like rock samples of different shapes,only the stress level of the conventional circular creep test is significantly lower than the instantaneous peak strength of uniaxial compression;In the coal-like classification loading test of other shapes,the final loading stress level is close to the instantaneous peak strength of coal sample under uniaxial compression;(3)From the results of rheological loading steady-state time comparison,the steady-state time ratio of coal-like rock samples with different shapes decreases with the increase of free surface area ratio,that is,the larger the free surface area is,the shorter the steady-state time is;(4)During creep loading,the final deformation of coal-like specimens with different shapes is different,and the deformation decreases with the increase of free surface area.(5)In the numerical simulation test,under the same contact surface area,the stress of rock samples with different shapes is generally concentrated in the middle position in the regular shape,and the failure often starts from the middle.In the numerical simulation of irregular shape of coal-like rock sample loading failure,there are tip,which is often the place of stress concentration,the first rupture here;(6)The instability of coal pillar has different effects on overburden surface migration in different periods.From the subsidence value and influence range,the influence of narrow coal pillar instability and small rhombic coal pillar instability on the surface is similar to that of coal pillar stability period.However,when the large rhombic coal pillar is unstable,the subsidence value increases sharply and the influence range becomes larger.