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Grading Guide Specifications Page 5 <br />• Stabilization key excavations should be provided with subdrains. Typical subdrain details <br />are shown on Plates D-6. <br />Subdrains <br />• Subdrains may be required in canyons and swales where fill placement is proposed. Typical <br />subdrain details for canyons are shown on Plate D-3. Subdrains should be installed after <br />approval of removals and before filling, as determined by the Soils Engineer. <br />• Plastic pipe may be used for subdrains provided it is Schedule 40 or SDR 35 or equivalent. <br />Pipe should be protected against breakage, typically by placement in a square-cut <br />(backhoe) trench or as recommended by the manufacturer. <br />• Filter material for subdrains should conform to CALTRANS Specification 68-1.025 or as <br />approved by the Geotechnical Engineer for the specific site conditions. Clean 3/4-inch <br />crushed rock may be used provided it is wrapped in an acceptable filter cloth and approved <br />by the Geotechnical Engineer. Pipe diameters should be 6 inches for runs up to 500 feet <br />and 8 inches for the downstream continuations of longer runs. Four-inch diameter pipe may <br />be used in buttress and stabilization fills.