Timber Pile Foundations For Expansive Soils

This paper describes the results of a study in which two groups of small diameter timber test piles were driven to depths of between 0.5 to 3.5m into a relatively dry expansive clay soil, and subsequently monitored through changing seasons. The vertical displacements of the head of each of the piles were recorded as the soil profile heaved and fell in response to seasonal changes in ground moisture levels. These observations were found to be consistent with previous studies. It was found that piles driven to about the active depth on this site (1.7 m) are likely to experience about 22% of the adjacent ground surface movement, and that piles installed to about 2.3 m or deeper on this site will experience less than 10% of the adjacent ground surface movement. The ratio of pile movements to ground surface movements were found to be in good agreement with the results of a simplified analysis in which the soil stiffness is assumed to increase linearly with depth. It is concluded that driven, preservative treated, timber mini piles have good potential for use in constructing soil movement resistant foundations in expansive soils.