Field Evaluation of the Long-term Performance of Geocomposite Sheet Drains

Book excerpt: The long-term performance of geocomposite sheet drains were monitored at three sites by measuring the effect of the drains on site groundwater hydrology during peak groundwater events. The study is on-going with a maximum record of 14 years at one site in the southern Sierra Nevada Range, CA.‍?Over a 3-year period, the drain at one site produced as much as 100 liters per minute with no rise in the water table just down gradient from the drain. The peak hydraulic gradient toward this drain consistently reaches 0.66 to 0.73 during major rainstorms and returns to a S2base levelS3 of 0.45 to 0.50 within 10 days after a storm peak.‍?At a second site, the drain flowed up to 15 liters per minute with no water table rise in a retaining wall fill down gradient from the drain. Performance of this drain was consistently favorable over a 14-year period with no deterioration. Limited data suggest that the drain discharge (liters per minute) is proportional to about 13 times the hydraulic gradient toward the drain.‍?At a third site, drain performance is limited by construction problems. A collapsed trench wall during drain placement caused an uneven drain slope toward the outlet. This is a common problem of trench drains at sites with a high water table. The problem could be greatly mitigated by returning to one of the pioneering geocomposite drain designs, the Eljen Drain, in which vertical sections of the drain are completely independent units that can be immediately dropped into a trench behind the excavator. Despite the construction problems, the drain is partially dewatering the road prism just down gradient, as seen by approximately 1.2 meters of groundwater drawdown beneath the road.