Mechanical Properties And Its Application Of Thick Frozen Shaft Wall With Water-abundant Soft Rock

With the development of China's coal mining in recent years, the key techniques of mineconstruction exist in the shaft excavation to penetrate the deep alluvium or the rich waterstrata, and the solution to the safety problem in the vertical shaft excavation when penetratingthe deep alluvium or the rich water strata. In particular, most of the shafts in such westernregions as Inner Mongolia, Gansu, Shaanxi have to penetrate the water-abundant soft rock ofCretaceous or Jurassic period. The design parameters and experience of vertical constructionin alluvium do not work in the freezing design of coal mine shaft. Therefore, it is of greattheoretical significance and engineering value to study the mechanical properties of thickfrozen shaft wall with water-abundant soft rock.On the experimental research of Hujiahe Frozen Engineering of frozen soft rock shaftwith depth of 558m undertaken by Shaanxi Binchang Group, the paper conducts a systematicresearch on the mechanical properties of frozen soft rock, the calculation model, and thedeformation characteristics of the frozen wall of vertical shaft with water-abundant soft rock,as well as the temperature field distribution characteristics of different soft rock strata bycombining theoretical analysis, numerical calculation and field survey. The paper also studiesthe mechanical properties of different rocks under different freezing and stress conditions, anda constitutive relation of frozen rock is established on the basis of the test results. Based onthe combined actions of the frozen wall and surrounding rock, a calculation model for thethickness of the deep frozen wall with soft rock under unloading conditions is established.The finite-element analysis on the deep frozen wall with soft rock is conducted, and acalculation formula for the max radial displacement of sidewall is formed. In addition, thetemperature field distribution characteristics of different frozen walls with soft rock areconcluded through field monitoring and monitoring data analysis. The study is of great importance in the safe mining of deep coal resources in northwest China, the enrichment offreezing shaft sinking technology and the frozen rock and soil mechanics theory, etc.Two kinds of typical rocks (sandstone and coal rock) are sampled from the field toconduct single-axial and tri-axial compression tests under normal temperature and differentfreezing temperature. Based on the failure process and failure mode analysis of test specimens,the stress-strain curve of freezing rock, and the variation rules of some mechanical parametersunder different temperatures, including single-axial and tri-axial compressive strength, elasticmodulus, Poisson's ratio, internal friction angle and cohesion, are concluded. Besides, acomparative analysis on the difference between mechanical properties of coal rock andsandstone under the freezing conditions is made. The paper establishes a constitutive relationof artificial freezing rock, taking account of the influence of temperature on the basis ofMohr-Coulomb criterion. The results show that:â‘ the decreasing temperature can lead tothe gradual brittle transition of test specimens, while increasing confining pressure canenhance the plasticity of test specimens. Under the same conditions, sandstone shows betterplasticity and coal rock presents obvious fragility.â‘¡the single-axial and tri-axialcompression strength of both rocks increases with decreasing temperature. Comparatively, thecompression strength of the frozen sandstone increases obviously. The confining pressure hasan extraordinary effect on tri-axial compression strength of the frozen rock.â‘¢withdecreasing temperature, the elastic modulus and shearing strength of both rocks increase invarying degrees respectively, while Poisson's ratio reduces for both. In comparison with coalrock, the increasing range of elastic modulus and the cohesion of sandstone are much obvious,while the freezing temperature has less influence on internal friction angle of rock.On the foundation of the review over both domestic and abroad traditional calculationformulas for frozen wall thickness and the external loads of frozen wall, the applicability ofthe formula in the design of the frozen wall with soft rock is analyzed, and the influence of Kvalue in heavy fluid formula on the frozen wall thickness calculation is discussed. It isconcluded that both static and dynamic theories of thick-wall cylinder with infinite lengthand the results from empirical equations suitable to epipedon can not be used in the frozenwall thickness calculation. Based on the combined action of frozen wall and surrounding rock,the calculation formula is deduced for frozen wall thickness and the external loads of frozenwall under unloading conditions. In the formula, the actual situation of shaft excavation underthe unloading conditions of frozen wall is considered, so that the negative influence ofartificial determination of coefficients is avoided when heavy fluid-ground pressure formula is adopted. Meanwhile, the height of excavation section viewed as a key element in frozen wallthickness calculation makes the calculation more scientific.The paper conducts a finite-element analysis on the deep frozen wall with soft rock withANSYS software, and the simulation calculations of the frozen wall deformation underdifferent height of excavation section, different thickness of frozen wall and averagetemperature of frozen wall are conducted. The results show that the max displacement offrozen wall increases with the increase of the average temperature of frozen wall and theheight of excavation section, and decreases with the increase of the frozen wall thickness. Onthis ground, researches on the relations between the deformation of frozen wall and thethickness of frozen wall, the average temperature of frozen wall, the height and radius ofexcavation section are conducted, and then a formula for max radial displacement of sidewallis drawn. The rationality of the formula is verified through the comparison of deformationsurvey value and calculation results.The frozen plan for the main and auxiliary shafts in Hujiahe is made, and the fieldsurveys of brine temperature, main rock temperature, and the radial temperature of frozenwall are conducted in the implementation of the plan. The paper summarizes the rules aboutthe expanding velocity, effective thickness of the deep frozen wall with soft rock strata, andthe variation rules of the average temperature with the freezing time. In addition, the paperconcludes the major freezing characteristic parameters of the frozen wall with deep soft rockfor Hujiahe shaft, which has important instructive significance on the freezing construction indeep soft rock strata.