Geotextiles in Embankment Dams - Association of State Dam Safety ...

Project: Many Farms DamLocation: Navajo Indian Reservation, Apache County, Arizona, USAGeosynthetic Materials Installed: Geotextile-wrapped perforated HDPE drainagepipe. The geotextile functions as a filter around the toe drain pipe which is a singlewall-corrugatedpipe with 1/16–inch wide slotted perforations. A knitted polyestergeotextile sock with an apparent opening size (AOS) of a #30 sieve (0.6 mm) coversthe pipe.Installation Date: July, 2000Summary: Geotextile wrapping around a toe drain pipe eliminated the need for asecond stage gravel filter between the filter sand and the toe drain. This designapproach allowed excavation and installation of the filter sand and toe drain pipeusing a shielded-wheel trenching machine in a vertical trench which did not need tobe dewatered.Project Details: The Many Farms Dam has an impermeable core constructed fromlocal silty soils containing highly dispersive clays. The dam embankment and itsfoundation suffered from internal erosion of the dispersive soil material.Rehabilitation of the dam included installation of a filter and drainage systemincluding a downstream toe drain.The highly dispersive clay soils at the site required the specification of a sand filterwith a fine gradation. Design of the toe drain pipe indicated that the sand was toofine and would be liable to erode through the slots in the drainage pipe. A secondaryfilter comprised of fine gravel would normally be placed between the drain pipe andthe filter sand.The dam is 2,700 feet long and at a remote location where filter sand and cleangravel must be imported at considerable expense. There was a strong desire toeconomize on the required amounts of filter sand and gravel and to use a trenchingmachine to minimize the size of the excavation and gain the productivity of themachine installation. Trenching machines can install backfill and a pipe at the sametime; however; they are limited to installing only one type of backfill material.Because a two-stage granular filter could not be efficiently placed by machine, ageotextile-wrapped drainage pipe was substituted for gravel in the re-designed toedrain as shown in Figure MF1.The design change also provided an economy with respect to the size of excavationand required amounts of costly filter material used as backfill. The economy of theexcavation is illustrated in Figure MF2.