Prediction of expansive soil based on the coefficient of linear extensibility (COLE)
Abstract
An expansive soil is any soil that is prone to large volume changes (shrinking and swelling) directly related to changing moisture conditions. The swelling capacity can cause heaving, or lifting of structures whilst shrinkage can cause differential settlement. The amounts by which the ground can shrink and/ or swell depend not only on the supply of moisture in the ground but also on the type and amount of clay minerals, internal structure and void ratios of the soil. Characterization of swelling potential is critically important to mitigate the adverse effects of expansive soils. Therefore, this study uses the coefficient of linear extensibility (COLE) as the controlling method to characterize swelling potential of expansive soils. The samples for this test were retrieved from two different locations in areas prone with expansive soils in Kibaha Township in Tanzania. Bulk density was determined on irregular soil clods and COLE was calculated as the difference between bulk density values at 1/3 bar and oven-dry condition. The coefficient of linear extensibility that ranged from 0.09 to 1.15 clearly indicated soils with high to very high shrink–swell potential. Lastly, a direct relationship existed between COLE and colloids contents with high degree of correlation (i.e. Correlation Coefficients (R) equal to 0.942 and 0.824 for samples RC5 and RB3 respectively) providing a confirmation to the presence of high to very high expansive potential of the soils.
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Introduction
Expansive soil is that which swells and shrinks under changing moisture conditions. It swells when the moisture increases and shrinks as moisture content decreases mainly during a long dry period or drought. Changing moisture conditions may be due to seasonal or interannual variations in rainfall, evapo-transpiration of vegetation, leaking water pipes or drains, landscape irrigation, cutting down or pruning trees etc. (Cheney, 1986, Biddle, 2001, Fredlund. & Hung, 2001 and Ito et al., 2014).
The swell/shrink of expansive soil usually relates to the type and amount of clay minerals present in the soil, geological formation and environmental factors (Elarabi, 2004 and Azam et al., 2013). The amount of clay mineral in soil largely depends on the type of parent material and the degree of chemical weathering. The swell/shrink behaviour of expansive soil usually causes a significant hazard to structures constructed in/on it. The pressure resulting from swelling usually causes uplift of the ground (i.e. heave), while shrinkage can cause settlement of the ground surface as well as structures on it (Freeman et al., 1991, Fityus et al., 2009 and Mokhtari & Dehghani, 2012). The fact that swelling and shrinkage are only partially, not fully recoverable, the adverse effects caused on structures are equally not reversible (Holtz & Kovacs, 1981). It is very important to recognize the existence and understand the potential problems of expansive soils prior to construction so that practical design options are identified that deal with expansive soil problems rather than waiting for costly remedial measures after construction. There are various methods to identify expansive soils both in the field and laboratory, so the choice is mainly dependent upon the type of structures to be erected and site condition, but many are also matters of choice. This study deals with the use of the coefficient of linear extensibility (COLE) to measure the shrink-swell capacity of soil clods for characterizing expansive soils.
Conclusion
The estimation of the swell potential based on the coefficient of linear extensibility (COLE) provides a means to rapidly and inexpensively quantify a soil’s shrink-swell potential. It gives the possibility to identify the soils susceptible to shrink-swell behaviour that might greatly affect the stability of the structures on them. The coefficient of linear extensibility (COLE) was computed from the bulk density of clod at suction of 33 kPa and dry density of oven-dry clod. All tested samples possessed coefficient of linear extensibility ranging from 0.9 to 1.4 in the soil horizons, and generally increasing at the surface horizons and then decreasing in the subsurface horizons with increasing depth. The trend towards increasing COLE with depth at the surface horizons is attributed to a decrease in organic matter in soil with depth while the decreasing COLE with depth in the subsurface horizons is due to decreasing clay contents because of increasing settlement of coarse fragments with increase in depth. According to Thomas et al. (2000) classifications, the under contemplation soils are classified as soils with a high to very high swelling potential. The results of this research indicate that COLE is very useful in predicting a soil’s shrink-swell potential. Once such soils have been identified, anti-swelling measures such stabilization and heat treatment may be employed to arrest the situation.