PSI - Issue 32

M. Zhelnin et al. / Procedia Structural Integrity 32 (2021) 71–78 M. Zhelnin/ Structural Integrity Procedia 00 (2021) 000–000

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flooding of excavation by groundwater. The essence of the method is a formation of a frozen wall around a mineshaft that is planned to be constructed. During the excavation of the shaft under the protection of the frozen wall, the exposed shaft sidewall is reinforced by a shaft lining consisting of a concrete shell and cast-iron tubbing. A stress-strain states of the concrete shells are studied in [1-6]. In [1] a stability of vertical shaft at the Boulby potash mine was studied by two- and three-dimensional numerical simulations of mechanical behavior of concrete shaft lining considering backfill material and surrounding soil. Mohr-Columb criterion was employed to describe elasto-plastic strain. In [2] two-dimensional numerical simulation of a stress-strain state of a shaft lining at the Victoria mine was performed taking into account elasto-plastic strain of rock mass using Hoek-Brown criterion. In [3] damage mechanisms of a shaft lining at the Lucky Friday mine was analyzed based on numerical simulation of stress and strain distributions around the mineshaft and extensometer measurements. In [4] numerical simulation was carried out to study effect of ground settlement at the Qianyinzi coal mine on a shaft lining deformation due to a water loss of the bottom aquifer. In [5-6] a radial load acting on a lining in a vertical shaft is estimated using numerical simulation and analytical method. A number of studies are devoted to prediction of the creep process in the concrete shell of tunnels. In [7] evaluation of the creep deformation is carried out using the Voight-Kelvin model. In [8] creep behavior of a concrete tunnel lining subjected to ground pressure is described with the B4 constitutive model. In [9] a modified Nishimura model was proposed to describe non-attenuation creep curves obtained in triaxial creep tests of concrete samples used for the tunnel lining. A stress-strain state of the shaft lining significantly depends on the mechanical properties of the cement grouted soil. This soil layer around the shaft lining restrains a part of the lateral pressure from the soil mass that leads to redistribution of stress and strain fields in the shaft lining. Influence of the cement grouting on mechanical properties of soil is analyzed in [10-12]. In [10-11] experimental studies of the effect of Portland cement amount on the compressive and shear strength properties of artificially cemented sand was conducted. In [12] a method for evaluating elastic and strength properties of artificially cemented sand was developed based on a micromechanics approach. The present paper is devoted to a study of a stress-strain state of a mineshaft lining and cement grouted soil during thawing of a frozen wall based on two-dimensional numerical simulation. Numerical simulation is performed on the base of thermo-mechanical model taking into account heat absorption induced by the phase transition of ice into water and creep strain of a concrete shell. To predict creep strain, the Voigt-Kelvin model is adopted. After thawing of a frozen wall, elasto-plastic behavior of the concrete shell and cement grouted soil is evaluated using the Bresler-Pister and Drucker-Prager yield criteria respectively. An optimal thickness and elastic properties of a concrete shell are determined by numerical simulation according to the limit stress criterion. 2. Theoretical formulation The problem of a stress-strain state evolution in a circular shaft lining during thawing of the frozen wall is considered. It is supposed that the mineshaft extends up to substantial depths, so at each depth the stress-strain state is evaluated for a cross section of the mineshaft. The study of the stress-strain state of the shaft lining is performed for two geotechnical situations encountering in vertical shaft sinking. In the first situation the shaft lining is located inside the frozen wall without cement grouted soil layer as it is shown in Fig. 1(a). In Fig. 1(a) the shaft lining consists of the concrete shell and the cast-iron tubbing. Unfrozen soil adjacent to the frozen wall is subjected to the ground pressure. A creep deformation in the concrete shell is estimated from the thawing of the frozen wall to the beginning of the cement injection procedure. In the second situation plastic strains in the concrete shell induced by ground pressure and cement grouted layer are determined after thawing of the frozen wall as it is shown in Fig. 1(b).