Indian Journal of Science and Technology Vol. 5 No. 8 (August 2012) ISSN: 0974- 6846 Research article “Caverns” S. Nazary Moghadam et al. Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol. 3128 Stability analysis of caverns excavated in heterogeneous salt formations S. Nazary Moghadam 1 , H. Mirzabozorg 1 , A. Noorzad 2 and K. Nazokkar 3 1 Civil Engineering Department, K.N. Toosi University of Technology, No. 1346,Valiasr Street, Mirdamad Intersection 19967, P.O. Box. 15875-4416, Tehran, Iran 2 Faculty of Water and Environmental Engineering, Power and Water University of Technology, Tehran, Iran 3 Civil Engineering Department, Islamic Azad University, Central Tehran Branch, Tehran, Iran saeed_nazarimoghadam@yahoo.com, mirzabozorg@kntu.ac.ir, noorzad@pwut.ac.ir, kimianazokkar@gmail.com Abstract In this paper, the influence of the presence of non-salt interbeds in the vicinity of salt caverns on the stability of the caverns was investigated. In order to describe the time-dependent inelastic behavior of rock salt, an empirical viscoplastic creep model bounded by a diagnostic failure criterion was utilized. The creep model is then implemented in a finite element program to analyze the stability of a storage cavern excavated in a salt formation interspersed with a non-salt interbed. Finally, stress distribution in the salt formation and time-dependent closure of the cavern obtained by the finite element analyses are presented and interpreted. Keywords: Rock salt, Underground cavern, Viscoplasticity, Creep, Finite element analysis. Introduction The underground caverns excavated in salt formations have provided valuable storage capacity for petroleum products and hazardous chemical wastes. The major factor playing a vital role in a secure underground storage is the stability of the underground caverns which is dependent on the inelastic time-dependent or creep behavior of rock salt. Rock salt surrounding an underground cavern exhibits progressive time-dependent in elastic movement which may lead to significant or even full closure of the cavern, and also stress relaxation can be encountered around the cavern up to small ultimate stresses which may lead to the failure of the surrounding rock salt and instability of the salt cavern. In addition, in the case of heterogeneous salt formations which are interspersed with non-salt sedimentary interbeds, the time-dependent mechanical properties of interbeds have significant influence on the stability of the underground caverns. Since the non-salt interbeds show slower time- dependent movements than the neighboring rock salt formation, considerable shear stresses develop in the interbeds which may bring about the failure of the interbeds. Consequently, in the case of heterogeneous salt formations, the creep behavior of non-salt interbeds should be considered in the stability of the underground caverns. So far, several viscoplastic constitutive models have been employed to describe the creep behavior of rock salt. Among these constitutive models, the empirical creep model of rock salt developed for the Waste Isolation Pilot Plant known as the WIPP creep model (Herrmann et al., 1980a, 1980b) has been utilized extensively in the stability analysis of salt caverns. However, the WIPP creep model cannot describe the failure behavior of the rock salt realistically, and also it neglects the inelastic volume changes which occur during the creep of rock salt. In this regard, Cristescu (1993) presented a procedure to determine a general viscoplastic constitutive equation which describes the creep behavior together with irreversible volume changes and failure of the rock-type materials, and also he developed a constitutive model to describe the creep of rock salt. In another research, Jin and Cristescu (1998) formulated a viscoplastic constitutive model for transient creep of rock salt and they employed the model to analyze the stress distribution around a plane strain borehole excavated in rock salt using a finite element procedure. Paraschiv-Munteanu and Cristescu (2001) derived a semi-analytical procedure based on finite element discretization for the analysis of stress variation during creep of rock salt around a plane strain borehole which describes transient and steady state creep as well as failure of the rock salt. Moreover, within the same framework, investigations of Yahya et al. (2000), Peric and Crook (2004) and Heusermann et al. (2003) are worth mentioning. The objective of the present paper is to propose a procedure for stability analysis of underground caverns excavated in heterogeneous salt formations interspersed with non-salt interbeds. The WIPP creep model of rock salt is combined with the short-term failure criterion proposed by Cristescu and then the resulting elasto- viscoplastic constitutive model is employed by a finite element code to analyze the stability of the salt caverns. Also, the creep behavior of the non-salt interbeds is described by the developed constitutive model using some modifications in the material parameters. Finally, the effect of the non-salt interbed thickness on the stability of the underground cavern is investigated. Constitutive model of rock salt The time-dependent inelastic behavior of rock salt is described using the WIPP viscoplastic constitutive model. Also, as the time intervals considered in this paper are in a range that steady state creep is dominant, transient creep is neglected. In this section it is assumed that compressive stresses and strains are positive while