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Chapter 4 - Best Management Practices for Erosion and Sediment Control Cutoff Trench For earth fill embankments , a cutoff trench shall be excavated along the centerline of the dam . The minimum depth shall be 2 feet (60 cm) . The cutoff trench shall extend up both abutments to the riser crest elevation . The minimum bottom width shall be 4 feet (1 . 2 m) , but wide enough to allow operation of compaction equipment. The side slopes shall be no steeper than 1 : 1 . Compaction requirements shall be the same as those for the embankment. The trench shall be drained during the backfilling-compacting operations. Embankment The fill material shall be taken from approved borrow areas . It shall be clean mineral soil , free of roots , woody vegetation , oversized stones , rocks , or other objectionable material . Areas on which fill is to be placed shall be scarified prior to placement of fill . The fill material should contain sufficient moisture so that it can be formed by hand into a ball without crumbling . If water can be squeezed out of the ball , it is too wet for proper compaction . Fill material will be placed in 6 to 8 inch (15 to 20 cm) continuous layers over the entire length of the fill . Compaction shall be obtained by routing the hauling equipment over the fill so that the entire surface of the fill is traversed by at least one wheel or tread track of the equipment, or by using a compactor. The embankment shall be constructed to an elevation 10% higher than the design height to allow for settlement if compaction is obtained with hauling equipment. If compactors are used for compaction , the overbuild may be reduced to not less than 5% . Principal Spillway The riser of the principal spillway shall be securely attached to the barrel by a watertight connection . The barrel and riser shall be placed on a firm compacted soil foundation . The base of the riser shall be firmly anchored according to design criteria to prevent its floating . Pervious material such as sand , gravel or crushed stone shall not be used as backfill around the barrel or anti-seep collars . Fill material shall be placed around the pipe in 4 inch ( 10 cm) layers and compacted by hand at least to the same density as the embankment. A minimum of two feet (60 cm) of fill shall be hand-compacted over the barrel before crossing it with construction equipment. Emergency Spillway The emergency spillway should not be constructed over fill material . Design elevations, widths , entrance and exit channel slopes are critical to the successful operation of the spillway and should be adhered to closely during construction . Vegetative Stabilization The embankment and emergency spillway of the sediment basin shall be stabilized with temporary vegetation within 15 days of completion of the basin as per TEMPORARY .. SEEDING - Section 6 .65 (ES BMP 1 .65) . 4-91