Correlation between Undrained Shear Strength and Index properties of Red Clay Soils in southern Ethiopia

Abstract

Undrained shear strength is one of the useful parameters in order to take engineering decisions. Most often the bearing capacity is estimated based on undrained shear strength to make conservative estimates. Some laboratory tests needed to obtain these values are expensive and time consuming, while soil properties like moisture content and Atterberg limits can be performed faster and cheaper. Many empirical formulae are available to estimate the undrained shear strength for fine grained soils like clay or silt. Determining of undrained shear strength parameters in laboratory are really tedious and time consuming. Therefore, a correlation between undrained shear strength and index properties is useful for restraint of testing number and costs. In this thesis, attempts have been made to obtain valid correlations between undrained shear strength of Red clay soils with their index properties. For this purpose, 18 samples were collected from Bule Hora town and a series of laboratory tests conducted as a primary data for this work from these two samples considered as Control Point. In the analysis part, both the MS excel spreadsheet and the SPSS software has been used for the scatter plot, correlation, and regression analysis. Attempts were made to obtain the relationships of all the parameters (water content, liquid limit, liquidity index, plastic limit, plasticity index, optimum moisture content, and maximum dry density).The analysis results show that Su has strong negative relation with LI and relatively weak positive relation with MDD. From the regression analysis it can be observed that the combination of w, LL, LI, PI and OMC gives better prediction of Su with Adjusted R2 value of 0.809. This means the developed equation can be explain, 80.9% variation of Su depends on the variation of the predictors. The outcome of this thesis may be applicable in different civil Engineering sectors, especially for preliminary investigations and prefeasibility study of Civil Engineering works such as Building, Earthdams, Earthfills, and other works that involve soil.