Case Study of Settlement of a Foundation on Expansive Clay Due to Moisture Demand of Trees

The depth of the active zone under a "mature" tree in an open field will stabilize somewhat, and moisture variations within the active zone will primarily occur due to seasonal moisture variations. However, the depth of the active zone will grow, and the horizontal extent of moisture variations will change due to the long term growth of trees. This effect is particularly dynamic in the urban environment where recharging of the subsoils during wet periods is often restricted by the presence of paving and buildings. Long term moisture losses due to trees caused three 9 foot (2.7 m) deep underreamed piers supporting a 4-story building to settle between 3 and 5 inches (8 to 13 cm) at a site in Southeast Houston, Texas. The total settlement of the ground surface under the floor slab was ±8 inches (20 cm). The commonly accepted depth of "seasonal moisture change" in this area of Houston is 6 ± 2 feet (1.8 ± .6 m), but the depth of the active zone was found to be ±28 feet (8.5 m) due to moisture demand of the trees.