Model study of seepage in smear zones around vertical drains in layered soil

Physical model tests were conducted to investigate the effects of soil disturbance, commonly termed smear, around vertical drains installed to accelerate consolidation in compressible soils. A 25 mm dia. mandrel carrying a cylindrical drain was driven into 252 mm dia. models composed of alternating layers of preconsolidated kaolin and silty sand, and the effect of smear was quantified by means of radial flow permeability tests. A parametric study of the influence of sand layer thickness, clay layer thickness, mandrel driving speed and mandrel tip angle was conducted. Two tests were also conducted on clay models without sand layers. In the layered models, for which the undisturbed horizontal permeability was always much greater than the vertical permeability, the smear effect was found to be substantial. In the absence of sand layers, it was relatively modest and consistent with previous research.