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

Chapter 6—Construction Methods and Considerationscommon to increase the overlap distance to a range of two or three feet in order toguard against the geotextile shifting during covering operations.The frictional resistance between overlapped fabric sections is considerably less thanbetween fabric and soil. In addition, when proper overlap requirements are followed,laps can account for up to 25 percent of the total fabric cost. For the reason, sewingfabric sections together, either by pre-sewing at the factory or onsite, may bedesirable both for cost effectiveness and to maintain strength requirements.There are three basic types of seams used (Koerner, 2005a): flat or prayer type, J orDouble J, or butterfly. A stitch density of about 400 stitches per 3 feet should beused for lighter weight geotextiles. About 200 stitches per 3 feet should be utilizedfor heavier weight geotextiles. A lock type stitch should be utilized because it is lesslikely to unravel. Single or double-thread chain stitch is also utilized. Whenconstructed correctly, sewn seams can provide reliable stress transfer betweenadjacent geotextile panels.Fabric seams should be evaluated for their potential to open up under load, possiblycreating unprotected areas where soils could pipe under hydrostatic pressure or flow.Overlapping “J” type seams are preferable over lapped seams. It is recommendedthat double sewing be utilized. High strength polyester, polypropylene, or Kevlarthread should be used. Nylon thread could also be used, but some reduction in seamstrength may occur over time due to wet-dry cycles.With lighter weight geotextiles, it is possible to make seams that have a strengthequal to 80 to 90 percent of the parent material. With high weight geotextiles, it isdifficult to make seams stronger than 60 percent of the geotextile strength, orstronger than about 600 to 900 psi.Heat bonding/welding is becoming more common as new lightweight type fieldwelders are developed. These types of machines require operators that are trainedand the equipment must be maintained. Temperature control and uniformity of theheating elements are critical to ensure that the geotextiles are not burned ordamaged.6.5 CoveringOnce installed in the field, geotextile materials should be covered with the specifiedmaterials as soon as practicable. On many projects, the contractor wants to delaycover placement until all of the geotextile is placed. UV susceptible geotextilesshould be covered within 3-5 days of exposure and within 21-30 days for UV treatedand low UV susceptible polymer geotextiles. In addition, geotextiles (especially125