BEHAVIOUR OF GRANULAR MATERIALS DURING CYCLIC LOADING : DEM SIMULATIONS

This paper highlights the potential of Discrete Element Method (DEM) simulations to capture the cyclic behavior of the granular materials including liquefaction, post liquefaction and dynamic properties. A series of strain controlled undrained cyclic triaxial tests have been carried out on isotropically compressed assembly of spheres to understand the influence of different parameters on the cyclic behavior of granular materials. The undrained tests have been simulated by maintaining a constant volume condition of the sample throughout the cyclic triaxial tests. Post liquefaction undrained monotonic studies were carried out on samples without dissipating the excess pore water pressure developed during initial liquefaction. As observed from the results, numerical simulation using DEM has captured liquefaction, pore water pressure generation, post-liquefaction and dynamic properties similar to the experimental results. Furthermore, evolution of micromechanical parameters such as average coordination number and anisotropic coefficients during cyclic triaxial loading has also been reported and discussed. INTRODUCTION For the last few decades, the behaviour of saturated granular materials during undrained cyclic loading condition has attracted the attention of researchers worldwide. The high incidence of liquefaction during earthquakes, with its potential to damage, has made the liquefaction a prime subject of concern in geotechnical earthquake engineering. It has been reported, based on the laboratory investigations, that liquefaction and pore water pressure response during cyclic loading depends on many factors such as confining pressure, initial void ratio, initial stress condition, amount of fines, sample preparation method, over consolidation ratio, length of time under sustained pressures [1-9]. Despite the extensive research in this area, liquefaction still remains a major problem even in recent earthquakes. This is due to the fact that the mechanical behaviour of granular materials is influenced by a large number of factors, which make it difficult to establish general stress – strain relationships. The major concern during earthquake loading was to safe guard the structure from liquefaction, if the structures are resting on saturated loose sand. Often, costly ground improvement projects (e.g densification of sand) are preferred to mitigate the liquefaction hazards (e.g. earthquake induced displacements). In the last two decades, the need for evaluating earthquake-induced settlements was recognized and as a result, some analytical and empirical relationships were developed [10-12]. The important information required to evaluate the earthquake induced displacement is the undrained stress-strain response of the post liquefied sands. This will allow the designer to predict the potential resistance of liquefied sand to withstand the monotonically increasing static loads. This information is also required for the assessment of post liquefaction stress strain behaviour of soils to mitigate liquefaction hazards. Recent laboratory investigations [13-15] have achieved a qualitative understanding from laboratory investigation on the post liquefaction undrained behaviour of sandy soils. However, no attempt has been done so far to understand the post liquefaction monotonic undrained test from grain scale level. The behaviour of soils subjected to dynamic loading is governed by dynamic soil properties. The measurement of dynamic soil properties is critical task in the solution of earthquake geotechnical engineering program. Since dynamic properties are influenced by several factors, precise measurement of dynamic soil properties is somewhat a difficult task in the solution of geotechnical earthquake engineering problems [16]. Several laboratory and field techniques are available to measure the dynamic properties of soils. Some of these tests are oriented toward measurements of dynamic properties at low strain levels and some other towards large strain levels. However it is highly difficult to determine the dynamic properties for wide range of shear strain level (from low shear strain to high) using single testing apparatus. Moreover, in the recent past, many experimental investigations mostly for sandy soil have been carried out to investigate the effect of parameters such as confining pressure, amplitude of shear strain, void ratio, number of cycles, stress history, initial stress, soil type etc on the dynamic properties [17-20]. These experimental investigations are mostly confined to isotropically consolidated samples without considering the initial soil structure or fabric. Since dynamic properties are highly sensitive to all these factors it is highly essential to evaluate these properties using a numerical technique which can model granular material as an assemblage of particles for wide range of shear strain where in sample preparation problem can be avoided. To comprehensively understand the macroscopic behavior of granular materials an understanding of the micromechanical response during cyclic loading is necessary. This approach requires observations at micro level, which is very difficult to achieve by actual experiments on real granular media. This