Load deflection curve for organic soil mixed with different Percentage of RHA is displayed in Fig. 6. The UCS value for different proportion of RHA treated soils are listed in Table 7. Variation of UCS values for different content of RHA stabilized soil is shown in Fig. 7. As per the data revealed in the Table 7 and the Fig. 7, it can be clearly seen that the UCS value is gradually increases at initial stage due to increase of different dosages of RHA into organic soil. After reaching at the highest point, UCS value is found to decrease due to increase in RHA content. The considerable increment in UCS value is about 78.33% at 15% replacement of organic soil by RHA. The optimal RHA dosages corresponding to highest UCS value is initiated to be 15%. In previous studies, similar strength improvement was described by some researchers (e.g: Sarkar et al., 2012; Alhassan, 2008; Ramesh and Kumar, 2009; Sabat and Pradhan, 2014; Gupta and Kumar, 2016; Muntohar, 2002).
The reason of such type of behavior is due to addition of RHA, the angle of internal friction of soil increases, hence the soil gets more shear strength because of increase in angle of internal friction in addition to the cohesion of organic soil this is found as increase in UCS value. The strength decreases due to inclusion of high percentages of RHA, as the cohesion of soil decreases due to decrease in clay content.