Development of Strength Models for Prediction of Unconfined Compressive Strength of Cement/by-Product Material Improved Soils

This paper presents the possible inclusion of pulverised fuel ash (PFA) and ground granulated blast slag (GGBS) in cement deep soil mixing for enhancement of unconfined compressive strength of weak soil materials for construction purposes. The main focus of this paper was to investigate the unconfined compressive strength (UCS) of cement, Cement/PFA and Cement/PFA/GGBS improved soils and development of mathematical and graphical models for prediction of UCS for use in design and construction. Samples of cement, blends of cement and PFA and cement/PFA/GGBS were prepared using 5%, 10%, 15% and 20% by weight of dry soil and tested for UCS after 7, 14, 28 and 56 days. A multiple regression analysis was conducted using SPSS computer programme. The results showed that soil materials with lower plasticity show higher strength development compared to those of higher plasticity for cement improvement. The study has also revealed that the inclusion of PFA and GGBS can cause reduction in amount of cement in deep soil mixing which can result to reduced cost and emission of CO2 during construction. The developed mathematical and graphical models could give reliable prediction of unconfined compressive strength for weak soil materials with initial UCS less than or equal to 25kPa and for water to binder ratio of unity based on the observed agreement between experimental and predicted data. The developed multiple regression models have also been validated using different mixtures of 6%, 8%, 12% and 16% of binders.