ABSTRACT
According to the fundamental concepts of soil mechanics the placement of an external load on a low-permeable soil layer will induce excess pore water pressure, causing a consolidation process in which pore water is squeezed out of the soil, accompanied by a gradual increase in effective stress and a corresponding decrease in excess pore water pressure. The consolidation process will continue until the excess pore water pressure has dissipated, a process whose duration depends on the consolidation characteristics of the soil and the drainage paths (the longer the drainage paths, the longer the consolidation process). The idea behind the installation of vertical band drains is to reduce the length of the drainage paths and thereby reduce the time of consolidation. The use of vertical sand drains was first proposed in 1925, and patented
in 1926, by Daniel D. Moran. He also suggested the first practical application of sand drains as a means of stabilizing of mud soil beneath a roadway approach to the San Francisco Oakland Bay Bridge (Johnson, 1970). This led to some successful laboratory and field experiments followed by the installation of the first drain system in 1934. Porter (1936) described these trials and contributed to the further use and development of the system. The sand drains originally installed had generally a relatively large diam-
eter, 0.4-0.6m. Later on small-diameter sand drains came into use, for example ‘sand wicks’, 0.05m in diameter, and ‘fabridrains’ – also called ‘sand pack drains’ – 0.12m in diameter. The sand in these drains is packed into a synthetic fibre net-type tube, which prevents the drains from necking. Sand drains with a diameter of 0.18m were utilized in the oldest and bestdocumented test field existing, the one situated at Ska˚-Edeby, Sweden, established in 1957 (Hansbo, 1960). This test field is still under continuous observation.
