Improvement of Bearing Ratio of Clayey Subgrade Using Compacted Flyash Layer

Abstract

This paper presents a laboratory study on California Bearing Ratio (CBR) of flyash overlying a soft clay bed. Flyash sample (Class F) has been collected from Titagarh Thermal Power Plant situated near Kolkata. The type of soil collected locally is silty clay. Different configurations of soil and flyash overlying the soil have been maintained by varying the thickness ratio (ratio of thickness of flyash to that of clay) in the ratios of 1:2, 1:1 and 2:1. Both Standard Proctor and Modified Proctor compaction tests have been carried out to obtain respective maximum dry density and optimum moisture content to be used for CBR tests. The water contents of soil used in this study are 16, 22, 28, 34 and 40% where 16% is the optimum moisture content of the soil as obtained in Standard Proctor compaction test. Similarly water content of the soil has been kept at 12, 18, 24, 30, 36 and 42% for samples prepared using Modified Proctor energy where 12% is the corresponding optimum moisture content of soil. Improvement of CBR values of composite soil-flyash system has been observed when compacted flyash layer is placed above the compacted soil. Out of three thickness ratios tested, CBR value has been found to be the lowest at thickness ratio of 1:2 and the highest at 2:1. The improvement factor is found to be maximum for highest moulding water content irrespective of thickness ratio and compaction energy. It indicates the effective use of flyash soil composite matrix under worse affected condition of high water content. The paper highlights the nature of improvement of clayey soil when compacted flyash layer has been placed on it with different values of thickness ratio and placement moisture content and compaction energy.