Volume 23, Issue 3 (September 2000)
Performance of Drilled Shafts with Isolation Tubes in an Expansive Environment
An experimental study that included detailed observation of four 305-mm-diameter test shafts, one reference shaft of standard design, and three test shafts with isolation tubes to mitigate skin friction in the vadose zone, is described. The shafts were loaded by naturally expanding and contracting soil over a period of 17 months. The soil at the test site was instrumented to track suction changes to permit development of a computational model predicting soil heave and shrinkage that occur during suction changes at the field site.
Maximum ground movements exceeding 35 mm were observed. Movements of only 1 to 2 mm were observed in the test shafts with isolation tubes, while movements of 4 to 5 mm were observed in the reference shaft. Tension forces in the shafts protected by the isolation tubes were minimal compared to those measured in the reference shaft.
A simple computational model was developed to predict the maximum amount of ground heave based on suction changes. A relationship among suction, total stress, and volumetric strain was obtained for the soil at the test site in the laboratory. This relationship, used as inputs to the predictive model, enabled the computation of the maximum ground heave. Such a model can be used to ascertain whether isolation tubes may be indicated.