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<br />e <br /> <br />" <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />.. <br /> <br />e <br /> <br />EM 1110-2-1913 <br />31 Mar 7~ <br /> <br />CHAPTER 5 <br /> <br />SEEPAGE CONTROL <br /> <br />Section I. Foundation Underseepage <br /> <br />5-1. General. Without control, under seepage in pervious foundations <br />beneath levees m~ result in (a) excessive hydrostatic pressures beneath <br />an impervious top stratum on the landside, (b) sand boils, and (c) pip- <br />ing beneath the levee itself. Under seepage problems are most acute <br />where a pervious substratum underlies a levee and extends both landward <br />and riverward of the levee and where a relatively thin top stratum <br />exists on the landside of the .levee. Principal seepage control measures <br />for foundation under seepage are (a) cutoff trenches, (b) riverside <br />impervious blankets, (c) landside seepage berms, (d) pervious toe <br />trenches, and (e) pressure relief wells. These methods will be dis- <br />cussed generally in the following paragraphs; detailed design, construc- <br />tion, and maintenance guidance is given in Appendixes B, C, and D. Turn- <br />. bull and Mansur (ref. A-5f and A-5g) have proposed control measures <br />for under seepage also. <br /> <br />5-2. Cutoffs. A cutoff beneath a levee to block seepage through per- <br />vious foundation strata is the most positive means of eliminating <br />seepage problems. Positive cutoffs may consist of excavated trenches <br />backfilled with compacted earth or slurry trenches usually located near <br />the riverside toe. Since a cutoff must penetrate approximately 95 per- <br />cent or more of the thickness of pervious strata to be effective, it is <br />not economically feasible to construct cutoffs where pervious strata are <br />of considerable thickness. For this reason cutoffs will rarely be <br />economical where they must penetrate more than 40 ft. Steel sheet <br />piling is not entirely watertight due to leakage at the interlocks but <br />can significantly reduce the possiblity of piping of sand strata in the <br />foundation. Open trench excavations can be readily made above the water <br />table, but if they must be made below the water table, well point sys- <br />tems will be required. Cutoffs made by the slurry trench method <br />(ref. A-3b(4) and A-3b(5)) can be made without a dewatering system, and <br />the cost of this type of cutoff should be favorable in many cases in <br />comparison with costs of compacted earth cutoffs. <br /> <br />5-3. Riverside Blankets. Levees are frequently situated on foundations <br />having natural covers of relatively fine-grained impervious to semi- <br />pervious soils overlying pervious sands and gravels. These surface <br />strata constitute impervious or semipervious blankets when considered in <br />connection with seepage control. If these blankets are continuous and <br />extend riverward for a considerable distance, they can effectively <br /> <br />5-1 <br /> <br />J <br />