Three-Dimensional Salt-Pillar Equations and Their Applications to Industry

Abstract

ABSTRACT Salt-pillar equations were developed over 25 years ago to assist with mine design. Pillar design is important for salt mines because the pillar size influences convergence of underground drifts with equipment clearance and headroom constraints, subsidence at the surface, and functional life of ground support. Current methodologies for salt-pillar design include using numerical-modeling techniques and salt-pillar-design equations as a method for estimating stresses and deformation rates. In this paper, three-dimensional simulations of square and rectangular salt pillars were performed, and the results were compared to the estimates of stress conditions using the salt-pillar-design equations. The results were used to investigate if the same salt-pillar-design equations are still valid following advancements in numerical-modeling software and the constitutive behavior of salt. This paper (1) reviews the salt-pillar-design equations, (2) compares the estimated stress conditions using numerical-modeling methods with the salt-pillar-design equations, and (3) discusses applications for the use of salt-pillar-design equations to salt mine design. INTRODUCTION The room and pillar size play a crucial role in the viability of an underground mine. Even for extensive mineral resource mines such as salt, pillar design influences the roof-to-floor convergence rate, stability of the salt back and nonsalt units, and accessibility period of mine workings. Unlike hard rock or coal mines, time-dependent deformational behavior of salt increases the pillar design complexity in underground salt mines. With technological advancement and laboratory creep tests on salt specimens, site-specific creep laws can be determined, and sophisticated numerical models can be developed to predict the room-and-pillar response over a specified period. Van Sambeek (1996) presented that the simple pillar-design equations for salt pillars can reproduce the results of extensive numerical models, such as stress conditions in salt pillars. The premise of the Van Sambeek salt-pillar-design equations is that the average values of the vertical stress, the two horizontal stresses (as compared to their actual distributions), and the corresponding deviatoric of those average stresses are adequate to estimate the pillar's creep behavior and structural stability. Therefore, the salt-pillar-design equations relate the stress averages to the shape of the pillar in terms of its height-to-width (H:W) and height-to-length (H:L) ratios.