Smear Effects of Vertical Drains on Soft Bangkok Clay

Abstract

Smear effects due to the installation of prefabricated vertical drains were studied in the laboratory using special equipment and in the field using a full‐scale test embankment. The prefabricated vertical drains were installed by two different sizes of mandrel. The test results showed that the smear zone can be assumed to be twice the equivalent mandrel diameter and the horizontal permeability coefficient in the smeared zone, $k_h^{\prime }\text{,}$ was approximately equal the vertical permeability coefficient in the undisturbed zone, $k_v\text{.}$ A faster settlement rate and higher amounts of compression were observed in the small mandrel area than in the large mandrel area, suggesting lesser smear effects in the former than in the latter. The total settlement prediction using the method of Skempton and Bjerrum of 1957, with stress distribution using Poulos's 1967 method, yielded reasonable agreement with the observed values. More accurate settlement‐rate predictions can be obtained using Asaoka's 1978 method when the prediction was based on settlement data having at least 60% consolidation.