Stiffness Determination of Backfill-Rock Interface to Numerically Investigate Backfill Stress Distributions in Mine Stopes

Numerical modeling is an effective and efficient method to investigate the stress distributions of backfill in stopes, which should be well understood in underground mining. Interface elements between backfill and rock in simulated stopes had been proved to be essential components, for which the stiffness parameters need to be assessed and assigned. However, few reports have revealed the effects of interface stiffness on backfill stress distributions, and there is not yet a clear solution to determine the interface stiffness to simulate stresses in backfilled stopes, except an empirical method for simply applying a high value suggested in FLAC manual. In this study, a new solution is first proposed to determine the normal stiffness and shear stiffness of interface elements, respectively, in numerical modeling of backfill stresses. The applicability of the solution has been verified by investigating backfill stress distributions in mine stopes of two widely used mining methods with variable stiffness values. The results show that the newly proposed method leads to totally the same backfill stress distributions with models applying the interface stiffness by the method in FLAC manual based on a “rule-of-thumb” but will save at least 20%–30% calculation time to improve modeling efficiency under the same simulation conditions and will carry much clear physical meanings corresponding to the interaction between backfill and rock walls in mine stopes. In addition, the vertical and horizontal stresses show good agreements with the analytical stresses predicted by the Marston equation under the at-rest state, which validates the reliability of the proposed solution for interface stiffness. Moreover, the plotting methods of stress distributions and the coefficient of lateral earth pressure of backfill in simulated stopes with proposed interface stiffness were discussed to further clarify the reasonable methods to investigate the backfill stresses in mine stopes, especially after considering the effects of the convergence from rock walls, which is a very significant and common phenomenon in practical mining engineering.

MODELING THE STRESS STATE OF THE BACKFILLING MASS WITH DIFFERENT PHYSICAL AND MECHANICAL PROPERTIES

Purpose. Analytical researches of the stress state of the backfilling stopes with different physical and mechanical properties using numerical modeling to determine possible zones of stability losses and predict their failure. Methods. Numerical modeling of the formation of stresses around a high stopes was carried out for the conditions of mining iron ore reserves in the depth intervals of 740-1040 m of the Pivdenno-Bilozerske deposit, where mining operations are actively carried out using the finite element method in the SolidWorks 2016 software package with reliable substantiation of the parameters of the developed geomechanical model. Results. Numerical simulation of the stress state of the backfilling mass are carried out at variable values of the modulus of its elasticity and the mining depth. It was found that with the existing actual physical and mechanical properties of the backfilling mass during the development of the Pivdenno-Bilozerske deposit, the danger of its failure is predicted at depths of more than 890 m. In the center of the filling array, the stress values change linearly, and at the junction of the roof with the side of the backfilled stopes – polynomial. It was found that an increase in the modulus of elasticity of the backfilling mass allows to reduce the compressive stresses only at the junction of the roof with the side of the backfilled stopes to a value of 800 MPa. Scientific novelty. With an increase in the depth of development, despite an increase in the elastic modulus of the fill, the values of stresses increase, which eliminates the need to increase it with a decrease in the mining depth it was found. Practical significance. The results obtained make it possible to correct the technology of formation of a backfilling mass in the primary stopes, taking into account the formation of stresses on its contour and, with an increase in the mining depth, to form a backfilling mass with viscoplastic properties.

Effect of Drainage and Consolidation on the Pore Water Pressures and Total Stresses within Backfilled Stopes and on Barricades

The pore water pressures (PWPs) and total stresses during the placement of a slurried backfill in underground mine stopes are the key parameters for the design of barricades, built to retain the backfill in the stopes. They can be affected by the drainage and consolidation of the backfill. Over the years, several studies have been reported on the pressure and stresses in backfilled stopes by accounting for the drainage and consolidation. Most of them focused on the pressure and stresses in the stopes, few specifically on the barricades. The effect of the number of draining holes commonly installed through the barricade has never been studied. In this paper, the influence of hydraulic properties and filling rate of the backfill, stope size, barricade location, and number of draining holes is systematically investigated with numerical simulations. The results show that the stresses in the backfilled stope and on the barricade largely depend on the filling rate, hydraulic conductivity, and Young’s modulus of the backfill. The draining holes can significantly decrease the PWP, but only slightly the total stresses on the barricades in short term.

Total and Effective Stresses in Backfilled Stopes during the Fill Placement on a Pervious Base for Barricade Design

Backfill is increasingly used in underground mines worldwide. Its successful application depends on the stability of the barricades built at the base of the stopes to hold the backfill in place, which in turn depends on the knowledge of the pore water pressure (PWP) and stresses during, or shortly after, the placement of the slurried backfill. Until now, self-weight consolidation is usually considered for the estimation of the PWP. There is no solution available to evaluate the total and effective stresses during, and shortly after, the filling operation. As excess PWP can simultaneously be generated (increased) and dissipated (decreased) during the backfilling operation, effective stresses can develop when the filling rate is low and/or hydraulic conductivity of the backfill is high. The arching effect has to be considered to evaluate the effective and total stresses in the backfilled stopes. In this paper, a pseudo-analytical solution is proposed to evaluate the effective and total stresses in backfilled stopes during the backfill deposition on a permeable base, by considering the self-weight consolidation and arching effect. The proposed solution is validated by numerical results obtained by Plaxis2D. A few sample applications of the proposed solution are shown.