The 12 th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India Preconsolidation Pressure from Soil Index and Plasticity Properties C H Solanki Applied Mechanics Department, S V National Institute of Technology, Surat, Gujarat, India M D Desai Ex. Professor, Applied Mechanics Department, S V National Institute of Technology, Surat, Gujarat, India Key words: Soil Plasticity, Soil Compressibility, Alluvial Deposits, Pre-consolidation Pressure ABSTRACT: Some natural deposits of soils under go heavy compression in their geological history due to the weight of overlying soils. These soils are called preconsolidated or over consolidated and have been subjected to larger stresses in the past than at present. The magnitude of expected settlement depends on the magnitude of loading of subsurface soils relative to magnitude of preconsolidation pressure. Preconsolidation pressure is generally determine using consolidation test data. Consolidation test is time consuming. In this paper attempt is made to determine preconsolidation pressure using soil index and plasticity characteristics. Numbers of datasets are used for studying preconsolidation pressure with soil index and plasticity characteristics. Various methods of evaluation of preconsolidation pressure are compared for different compressibility soils. 1 Introduction The preconsolidation stress pc, is the maximum effective stress to which the soil has been exposed may result from loading . Geological evidence of past loadings should be used to estimate the order of magnitude of preconsolidation stresses before laboratory tests are performed. The casagrandes method of obtaining the preconsolidation pressure from consolidation test is based on the point of greatest curvature. The preconsolidation stress or maximum effective past pressure Pc experienced by the foundation soil is a principle factor in determining the magnitude of settlement of structure supported by the soil. Pc is maximum effective stress to reach the situ soil has been consolidated by previous loading; it is boundary between recompression and virgin consolidation pressure applied to the foundation that exceed the maximum past pressure expressed by the soil may cause substantial settlement. The ratio of pre-consolidation pressure and present overburden pressure is known as over consolidation ratio (OCR). Based on OCR soils are classified as normally consolidated, over consolidated or under consolidated. Selection of consolidation parameters such as compression index (Cc) , Recompression index (Cr) or coefficient of volume change (mv) is on the basis of OCR for computing consolidation settlement. 2 Review on various graphical methods Researchers found many graphical methods to evaluate preconsolidation pressure and these methods are usually based on the relationship of experimental void ratio (e) and effective consolidation pressure (p’) All these methods are operator dependent as they require accurate reading of logarithmic scale, drafting capability and proper judgment of selecting the points. Casagrande Method, e-logp’ (1936) is the oldest method to evaluate preconsolidation pressure. This method remains a standard method in comparision to other methods (Jose et aI,1989). IS 8009:1976 (part – 1) recommends Casgrande method to determine inter granular pressure. This method gives good results provided there is a well- defined break point in the e–log p’. It is based on the assumption that the soil experiences a change in stiffness, from a stiff response to a soft response, close to the preconsolidation stress. Schemertmann Method, e–log p’ (1955) is the adjustment of laboratory consolidation test results with an attempt to compensate for nominal sample disturbance effect. This method is basically for soft soil and not useful for stiff soil. Janbu Method (1969) propose that the consolidation stress could be determined from a plot of the constrained modulus (M = 1/ mv, where mv is the coefficient of volume compressibility) versus the axial stress in linear scale. Janbu suggested that for clays with high sensitivity and low OCR, p’c might often show up more distinctly in the stress – strain curve plotted using a linear scale.Pacheco Silva Method, e–log p’ (1970) seems very simple to draw and gives accurate results. It is very fast method and does not require any subjective interpretation results. It is not scale-dependent. It is more easy to use in soft soil where change in compressibility is not much evident. It is widely used in Brazil. Pacheco Silva uses an empirical construction from e–log p’ curve, 1475