In an attempt to reduce minor surface instability and maintenance problems on the backslopes of a rural highway, interceptor trench drains are to be installed at the top of the slope of to intercept surface and infiltrating groundwater from the hillsides above the road. The drains are 1 to 1.5 m deep, and the drainage trench lined with a geotextile filter. A perforated drain pipe is placed in the bottom of the trench, and the trench is backfilled with coarse drainage aggregate. Sieve analyses were performed on samples of soils typical of the problem areas along the highway alignment, and the following average data (percent passing) were obtained: Design the geotextile filter for the interceptor drains.

Soils U.S. Standard Sieve No. A % in. 100 99 100 88 X in. 100 99 96 57 No. 4 68 78 10 52 65 20 34 62 21 40 61 25 100 200 20