Behavior of expansive soil under water content variation and its impact to adhesion factor on friction capacity of pile foundation

In the active zone, expansive soil could change from liquid to solid state due to evaporation and from solid to liquid state due to water infiltration. This situation affected the carrying capacity of the friction pile, as well as all the soil parameters, especially the value of soil cohesion and adhesion factor between soil and pile material. In this study, a series of laboratory experiment were conducted considering variation of water content and the effect on pile adhesion factors in expansive soil. Adhesion factor measurement was performed using pile model, made from steel and concrete, in the laboratory. The results showed that, in saturated condition, the adhesion factor of concrete pile had a relatively constant value (the α value of about 0.9). This value decreased slightly sharp (at α value of about 0.6) at the time of Air Entry Value state (AEV) when soil was changed from saturated to unsaturated condition. The value of adhesion factor again increased (at α value of about 0.8) until soil water content exceeded the plastic limit, where soil was in semi solid state. After that, this value constant remained until soil water content reached the shrinkage limit. On the other hand at the same range where the adhesion factor on concrete pile showed a decline, adhesion factor on steel pile material sharply increased. This value dropped back to coincide with the adhesion factor value of a concrete pile as soil water content reached the plastic limit and soil enter into semi solid phase.