Zones of smear around vertical drains

Smear zones are produced during the installation of vertical drains in that zone of soils surrounding the band drains are disturbed. Soils in smear zones are remoulded at the time of installation process and effectiveness of band drains is decreased. For example, the compressibility of surrounding soils is increased and this brings the reduction of their permeability. Basically with the reduced permeability of soils around band drains, it takes longer time to complete consolidation process. 

To prevent the formation of smear zones, raising and lowering of mandrel during drain installation must be minimized. Furthermore soil disturbance can be controlled by avoiding use of vibratory hammers that serve to drive drains into the ground. 

Fig: Smear zones in band drain