Assessment of Soil Disturbance by the Installation of Displacement Sand Drains and Prefabricated Vertical Drains

Prefabricated vertical (PV) drains are commonly used to decrease the drainage paths within soft soils in order to accelerate the time for primary consolidation. PV drains are full displacement drains of small volume that are thought to exhibit considerably less disturbance to the soil mass than displacement sand drains. Experience at three sites where excessive disturbance due to installation of PV drains is thought to have occurred, combined with available published data regarding the performance of displacement sand drains and PV drains, provides the basis for an empirical approach to assess the excessive installation disturbance effects imposed by both PV drains and displacement sand drains. The size of both the installation mandrel and the anchor are important factors in the disturbance effects. The disturbance caused by PV drains is shown to be similar to displacement sand drains when the drain spacing ratio is based on the effective diameter of the mandrel/anchor combination calculated using the mandrel/anchor combination perimeter, not the end area. With this approach, a modified drain spacing ratio (effective drain spacing / effective mandrel diameter) greater than 7 to 10 is considered necessary to reduce the excessive disturbance effects of PV drain installation.