Characterization of a smear zone around vertical drains by large-scale laboratory tests

Installation of prefabricated vertical drains using a mandrel causes distur bance of clay surrounding the drain, resulting in a "smear" zone of reduced permeability. In this paper, an attempt is made to characterize the smear zone using large-scale laboratory model tests. Two tests, simulating the ca ses of "no smear" and "with smear," were conducted. Excess pore-water press ures were monitored at seven different locations along the radial direction . In addition, undisturbed samples were collected at various locations in t he clay layer for conducting oedometer tests. The distribution of excess po re pressure due to drain installation gave a clear indication of the extent of the smear zone. The effect of reconsolidation on the properties of clay was found to be much greater than that of the remoulding of the clay. The extent of the smear zone was also confirmed from the change in permeability of the clay layer in the smear zone obtained from oedometer tests. The rad ius of the smear zone is about four times that of the mandrel, and the hori zontal permeability of the clay layer in the smear zone is approximately 1. 3 times smaller than that in the intact zone.