Modeling time-dependent behavior of gas caverns in rock salt considering creep, dilatancy and failure S. Nazary Moghadam a , H. Mirzabozorg a,⇑ , A. Noorzad b a Civil Engineering Department, K.N. Toosi University of Technology, No. 1346, Valiasr Street, Mirdamad Intersection 19967, P.O. Box. 15875-4416, Tehran, Iran b Faculty of Water and Environmental Engineering, Power and Water University of Technology, Tehran, Iran article info Article history: Received 20 January 2012 Received in revised form 12 July 2012 Accepted 1 October 2012 Keywords: Salt cavern Creep Non-linear analysis Finite element method Large deformation Viscoplasticity abstract In this paper, a numerical model of the time-dependent behavior of underground caverns excavated in rock salt is presented. An elasto-viscoplastic constitutive model is utilized to describe dilatancy, short- term failure as well as long-term failure during transient and steady state creep of rock salt. The consti- tutive model is employed by a Lagrangian finite element formulation to simulate the stress variation and ground movement during creep of rock salt around the cavern within the framework of large deforma- tions. Finally, finite element analyses are provided in order to investigate the efficiency and applicability of the proposed computational model to represent time-dependent response of gas storage caverns in rock salt. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Rock salt has increasingly attracted the attention of petroleum industry as an ideal material providing valuable storage capacity for natural gas and other hydrocarbons. Due to very low perme- ability and high ductility of rock salt, there is also motivation from nuclear industry with particular interest related to the disposal of radioactive waste. Underground storages of petroleum products are more economical than surface tank storages which enable much higher volumes and pressures to be achieved. This becomes more evident taking into account the small amount of land re- quired and security against external influences. The other advan- tage of underground storages in salt deposits is that salt caverns may be created by solution mining techniques instead of the more costly conventional excavation techniques. The above-mentioned advantages make salt deposits one of the prime alternatives for the construction of underground cavities for storage of petroleum products or radioactive wastes and hazardous chemical wastes. Time-dependent ground movement around an underground opening excavated in a rock salt formation can progress continu- ously leading to significant or even full closure of the opening, and also stress relaxation can occur around the opening up to small ultimate stresses. The consequence of this behavior is that time- dependent inelastic behavior of rock salt has to be considered in the design and the stability analysis of underground excavations in rock salt. A crucial time-dependent property of rock salt is the time-dependent deformation behavior or creep behavior defined as the inelastic deformation in time without fracturing. The creep deformation can be decomposed into two components, namely transient and steady state creep deformation. Transient creep is the creep deformation component which continuously decreases in time until a stable state is reached, while steady state creep deformation continues indefinitely under constant or nearly con- stant stress state. In general, rock salt exhibits both transient and steady state creep and it is only the time interval elapsed from the application of the load that determines which of the two com- ponents of the creep deformation is dominant. In classic models, the creep deformation is generally described using viscoplastic theory based on the existing models for metals neglecting the inelastic volume changes caused by the closing or the opening and growth of microcracks. However, experimental evidence re- veals that a typical manifestation of creep deformation of rock salt involves irreversible dilatancy and compressibility in a broad range of stress states. Hence, for a comprehensive description of the time-dependent behavior of rock salt it is important to choose a viscoplastic constitutive equation which considers irreversible compressibility–dilatancy, damage and failure as well. In order to investigate the time-dependent behavior of under- ground caverns excavated in rock salt, the constitutive modeling of rock salt is of vital importance. Constitutive models used to de- 0886-7798/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2012.10.001 ⇑ Corresponding author. Tel.: +98 21 88779623; fax: +98 21 88779475. E-mail addresses: saeed_nazarimoghadam@yahoo.com (S. Nazary Moghadam), mirzabozorg@kntu.ac.ir (H. Mirzabozorg), noorzad@pwut.ac.ir (A. Noorzad). Tunnelling and Underground Space Technology 33 (2013) 171–185 Contents lists available at SciVerse ScienceDirect Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust