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Technical Paper: Blanket coverage

The efficiency of drainage blankets in surcharging of soft clays can profoundly affect the success of the vertical drain treatment by Tony Barry and Linda van der Bend report. This paper was first published in GE’s May 2006 issue.

Vertical drain treatment of soft clay deposits relies on the functionality of the horizontal drainage blanket placed at the surface.

A number of factors affect the efficiency of the blanket, including rate of discharge from the drains, the plan extent of the blanket and initial perched water from hydraulic filling and rainfall.

When designing vertical drain schemes it is generally assumed that the water pressure at the original ground surface, or at the drainage blanket level, is zero.

Much effort is spent in assessing the drain capacity and smear effects which in fact are generally of little consequence (Barry et al, 2001), whereas the capability of the drainage blanket can profoundly affect the success of the treatment.

It is helpful to look first at the effects of a poorly designed, limited functionality or inadequate drainage blanket on the behaviour of soil installed with vertical drains.

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