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EM 1110-2-1913 31 Mar 78 reduce seepage flow and seepage pressures landside of the levee. Where underseepage is a problem, riverside borrow operations should be limited in depth to prevent breaching the impervious blanket. If there are limited areas where the blanket becomes thin or pinches out entirely, the blanket can be made effective by placing impervious materials in these areas. The effectiveness of the blanket depends on its thickness, length, and permeability and can be evaluated by flow-net or approximate mathematical solutions, as shown in Appendix B. Protection of the riverside blanket against erosion is important. 5-4. Landside Seepage Berms. a. General. If uplift pressures in pervious deposits underlying an impervious top stratUlll landward of a levee become greater than the effective weight of the top stratUlll, heaving and rupturing of the top stratUlll may occur, resulting in sand boils. The construction of land- side berms (where space is available) can eliminate this hazard by pro- viding (a) the additional weight needed to counteract these upward seepage forces and (b) the additional length required to reduce uplift pressures at the toe of the berm to tolerable values. Seepage berms may reinforce an existing impervious or semipervious top stratum, or, if none exists, be placed directly on pervious deposits. A berm also affords some protection against sloughing of the landside levee slope. Berms are relatively simple to construct and require very little maintenance. They frequently improve and reclaim land as areas requiring underseepage treatment are often low and wet. Berms can also serve as a source of borrow for emergency repairs to the levee. Because they require addi- tional fill material and space, they are used primarily with agricul- tural levees. Subsurface profiles must be carefully studied in select- ing berm widths. For example, where a levee is founded on a thin top stratUlll and thicker clay deposits lie a short distance landward, as shown in figure 5-1, the berm should extend far enough landward to lap the thick clay deposit, regardless of the computed required length. Otherwise, a concentration of seepage and high exit gradients may occur between the berm toe and the landward edge of the thick clay deposit. b. Types of Seepage Berms. Four types of seepage berms have been used, with selection based on available fill materials, space available landside of the levee proper, and relative costs. (1) Impervious berms. A berm constructed of impervious soils restricts the pressure relief that would otherwise occur from seepage flow through the top stratum, and consequently increases uplift pressures beneath the top stratum. However, the berm can be constructed to the 5-2 , e . i e . .. e