ABSTRACT

A series of laboratory testing was performed to assess the effects of cyclic loading on compressive strength, elasticity and time-dependency of the Maha Sarakham rock salt. Results from the cyclic loading tests indicate that the salt compressive strength decreases with increasing number of loading cycles, which can be best represented by a power equation. The salt elastic modulus decreases slightly during the first few cycles, and tends to remain constant until failure. Axial strain-time curves compiled from loci of the maximum load of each cycle apparently show a time-dependent behavior similar to those of creep test under static loading. In the steady-state creep phase, the visco-plastic coefficients calculated from the cyclic loading test are about an order of magnitude greater than those under static loading. The salt visco-plasticity also decreases with increasing loading frequency. Surface subsidence and cavern closure simulated using parameters calibrated from cyclic loading test results are about 40% greater than those from the static loading results. This suggests that application of the property parameters obtained from the conventional static loading creep test to assess the long-term stability of storage caverns in salt with internal pressure fluctuation may not be conservative.

1 INTRODUCTION

Rock salt around storage caverns will subject to cycles of loading due to the fluctuation of cavern pressures during product injection and retrieval periods. Depending on the types of the stored products (e.g. petroleum, liquefied gas and compressed-air) and on the designed operating schemes, the injection-withdrawal durations can range from daily cycle to annual cycle, and the minimum and maximum cavern pressures can be as low as 20% and as high as 90% of the in-situ stresses at the casing shoe (cavern top). Due to the complexity of cavern ground geometry and the need to predict the future stability conditions (normally up to few decades), the stability and operating pressures of the salt caverns have commonly been analyzed and designed via numerical models equipped with time-dependent constitutive equations. A difficulty may arise at determining the representative properties of the salt under such cyclic loading states. Since the salt properties are loading path dependent (non-linear), the laboratory determined properties under static loads (as commonly practiced) may not truly represent the actual in-situ salt behavior under the cyclic loading.

The effect of cyclic loading on the elasticity and strengths of geologic materials has long been recognized (Haimson, 1974; Allemandou and Dusseault, 1996; Bagde and Petros, 2005). It has been commonly found that loading cycles can reduce the material strength and elasticity, depending on the loading amplitude and the maximum applied load in each cycle (Zhenyu and Haihong, 1990; Singh et al. 1994; Ray et al. 1999; Kodama et al., 2000). Rare investigation has however been made to identify the cyclic loading effect on the time-dependent properties and behavior of soft and creeping materials.

The objective of the presented research is to determine experimentally the effects of cyclic loading on compressive strength, elasticity, visco-elasticity and visco-plasticity of rock salt.

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