Field evaluation of the long-term performance of geocomposite sheet drains

Abstract

The long-term performance of geocomposite sheet drains has been monitored at three sites by measuring the effect of the drains on local groundwater hydrology during peak groundwater events. The study is on-going with the maximum record being 14 years at one location. One drain was placed 2.4 m deep in a vertical trench in soil that varies from clay and clayey gravel to silty sand. Over a three-year period, this drain has flowed as much as 100 l/min. Even during such extreme groundwater events, there has been essentially no rise in the water table just down gradient from the drain. The peak hydraulic gradient toward this drain consistently reaches 0.66–0.73 during major rainstorms and returns to a “base level” of 0.45–0.50 within 10 days after a storm peak. At a second site, a drain was installed 8 m deep in a silty sand soil behind a retaining wall. This geocomposite has flowed up to 15 l/min with no rise in the water table in the retaining wall fill. Performance of the drain has been consistently excellent over a 14-year period. Limited data suggest that the drain discharge is proportional to about 13 times the average hydraulic gradient toward the drain. At a third site, a drain was installed to a maximum depth of 2.4 m in a vertical trench in a silty sand soil. Performance of this drain is marginal because trench wall collapse during drain installation caused an uneven drain slope toward the outlet. This is a common issue where trench drains are installed at sites with a constantly high water table. The problem could be greatly mitigated by a geocomposite design in which vertical sections of the drain are completely independent units that could be quickly dropped into a trench immediately behind the excavator. Despite the construction problems, this drain is partially dewatering the road prism just down gradient from the geocomposite.