ANALYSIS OF PILES IN PARTIALLY SATURATED SOIL

Abstract

Pile foundation is one of the most common deep foundations which is adopted when the bearing capacity of surface soil is inadequate to transfer the load from superstructure. Pile foundations are conventionally designed according to the conventional saturated soil mechanics. But this may lead to erroneous results if the surrounding soil is in a condition of partial saturation. Partially saturated condition is commonly encountered above water table level in arid and semi-arid region. In recent past, many experimental and numerical studies were conducted to predict the behaviour of piles in partially saturated soils. Saturated soil mechanics is simple as it is a two phase system. But in partially saturated soil, three material phases are available. The voids are filled with water and air. The inter-particle forces between the three phases makes the analysis of partially saturated soils complicated. The phenomenon, ‘soil suction’ is very important in the analysis of unsaturated soils. It is potential energy possessed by unit amount of water in the representative elementary volume. This helps in resisting the bending or flattening of the soil particles and their pockets. It also resists the sliding of soil particles and their pockets. Total suction is the sum of matric suction and osmotic suction. Matric suction is the difference between pore air pressure and pore water pressure. This will be always negative. Osmotic suction is due to the presence of solute in the soil. In most of the cases, osmotic suction does not have a significant effect on the total suction. So in most of the cases, matric suction is taken as the total suction. The graphical representation of the relationship between the energy and the amount of water in soils is called as Soil Water Characteristic Curve. The energy possessed by the water is represented as soil suction and the amount of waterin soil is represented either by the degree of saturation or water content. The Soil Water Characteristic Curve plays a significant role in unsaturated soil mechanics. Based on the literature review, the most common constitutive model in practice for the partially saturated soils is the Barcelona Basic Model (BBM). The Barcelona Basic Model is based on the modified Cam-Clay model. The modified Cam-Clay model is originally for saturated soil. The slope of the virgin compression line and the pre-consolidation stress are taken as constant values in iii the modified Cam-Clay model. But in partially saturated soil, these two parameters depends on the suction values. The Barcelona Basic Model takes care of these and gives more reliable results. The Mohr-Coulomb constitutive model is another constitutive model in practice for analyzing the behaviour of partially saturated soil. In this model, the effect of suction is taken care by the apparent cohesion and the unsaturated modulus of elasticity of the soil. Empirical relations are available to calculate the values of these parameters with the suction. Plaxis 2D version 8 is a Finite Element Analysis software used in the Geotechnical engineering and rock mechanics field. The analysis of piles in partially saturated soil is carried out in this software. The pile is modelled as linear-elastic, non-porous material. The effect of suction on the load carrying capacity of the pile is analyzed by two constitutive models, which are the Mohr-Coulomb model and Barcelona Basic model. Comparisons of results are done to find the effect of suction, level of groundwater table and over consolidation ratio on the load carrying capacity of the pile. This study highlights the significant influence of the suction on the behaviour of piles in partially saturated soils.