Effect of Insitu Moisture Content in Shrink-Swell Index

Expansive soils (also called reactive soils) undergo heave and settlements due to moisture changes and could result in differential movements in light weight structures. Hence, an estimation of potential ground movement is essential for designing footings for such structures. Shrink-swell index (Iss) is commonly used as a measure of soil reactivity and it is used in Australian standard of residential footing design to calculate ground movement. The shrink-swell test estimates the strain change per unit suction of an undisturbed soil sample; assuming the strain is independent of the insitu moisture content. In this study, 14 undisturbed soil samples were collected from an expansive soil site at different times over a two-year period. The sample locations were very close to each other. The samples had different insitu moisture contents varied from 22 to 37%. Test results indicate that increase in insitu moisture within that range resulted about 50% increase in Iss. Similar increment was observed in a set of samples obtained from another expansive soil site. The impact of insitu moisture content on Iss could lead to incorrect site classification. Consequently, the results of this study indicate that if the site is tested during different times of the year, it will result in different footing recommendations.