Foundation Clay Shrinkage Caused by Large Trees

Noticable cracking began to develop in brick structures on shallow foundations 20 yr after construction on Selfridge Air Force Base, Mich. During the following 10-yr period, the cracking progressed at an increasing rate until several two-story brick houses became unfit for occupancy. The usual soil mechanics theories could not satisfy the observed nature and time relationship of the foundation settlements. The most serious structural failures occurred where large deep-rooted trees were growing in the proximity of the foundations. Soil exploration data verified that tree transpiration during the growing season extracted ground water resulting in shrinkage of the underlying clay strata. Shallow exterior footings within the subsidence area surrounding large trees suffered settlements differential with respect to other parts of the foundation. Selective tree removal was performed concurrently with the restoration of the damaged houses. During the first year after this remedial action, no additional noticeable differential settlements have occurred.