Full-Scale Toe Drain Test Many Farms Dam

Full-scale laboratory testing was conducted on the proposed toe drain design for Many Farms Dam. The toe drain design calls for 10-inch single-wall corrugated high-density polyethylene (HDPE) drain pipe with 1/16-inch wide slotted perforations, covered with a knitted geotextile sock, and backfilled with a fine sand envelope. The geotextile sock has an apparent opening size (AOS) of #30 sieve (0.6 mm), and is placed on the pipe at the factory. The 1/16-inch slotted perforations are intended to clog with sand particles if the geotextile sock should tear. However, the slot width could potentially double if the drain pipe elongates significantly during installation. The test was conducted for 13 days, and the outflow quickly stabilized at 7.3 gallons per minute (gpm) per foot (see figure 1). The envelope loss was stabilizing at about 1,000 grams per linear foot, which was higher than expected, but believed acceptable. This loss equates to a calculated thickness loss of 0.087 inches around the pipe circumference. After testing, the pipe was carefully exhumed, and a coarser natural soil filter had formed adjacent to the geotextile sock, measuring ½ to 1 inch thick. Because of the loss of 1,000 grams of sand envelope per foot of toe drain, a coarser more broadly graded envelope material has been proposed. Traditional two-stage filters use a gravel envelope surrounded by a sand filter, and require extensive bench-cut (open-cut) excavation. Use of the geotextile sock in lieu of the gravel envelope allows construction with trenching equipment, which can offer significant cost savings. The knitted geotextile sock used in this study is the only geotextile that is preinstalled on the pipe at the factory. The knitted sock is only available with AOS of #30, which limits design options. Other geotextile products (such as monofilament woven) would have to be attached to the pipe in the field, but are available in a wide range of AOS.