Strength Gain of Organic Soil Deposits Subjected to Increase in the Effective Stress

Abstract

Organic soil has weak shear strength properties in its natural texture. However, a significant gain in strength can be achieved in such soils by increasing the effective stresses using preloading techniques. This research focuses on the organic soil deposits in Sri Lanka by analyzing the data from Colombo- Katunayake Expressway (CKE) project and Elevated Highway (EH) project spanning from New Kelani Bridge to Athurugiriya. As a developing country, Sri Lanka is facing a huge challenge due to its limited land available towards the infrastructure development. Therefore, construction should also be directed on organic soils despite of considering its weak shear strength properties as the soil can be modified to enhance its properties. To predict the strength gain of organic soils subjected to effective stresses, empirical correlations are used such as Skempton and Bjerrum equation (1957). As depicted in such correlations, the ratio of undrained shear strength and effective overburden pressure is observed against the Plasticity Index (PI) using field data. From the results of the analysis, it is discovered that using empirical correlations is very conservative for organic soils within the Sri Lankan context. Particularly, use of empirical correlations is very safe but beyond the economic considerations. In the extension of the research, an argument is generated to validate the use of PI to predict normalized shear ratio. To validate the argument, SPSS statistical software was used to perform the multiple regression analysis having PI, natural void ratio and organic content as the independent variables. From the statistical analysis, it was found that using only PI to predict the strength gain is not conservative for Sri Lankan organic soils subjected to effective stresses. These generated results are identical to conclusions drawn governing the inorganic soils in the state of Missouri (2011).