FLOOD04542 - Laserfiche WebLink

Home Browse Search

EM 1110-2-1913 31 Mar 78 e Here the trench is located at the landward 'luarter point of the levee, and discharge is provided through a horizontal pervious drainage layer. Unless it is deep enough, it may allow excessive seepage pressures to act at the toe. There is some advantage to a location under the levee if the trench serves also as an inspection trench and because the hori- zontal pervious drainage layer can help to control embankment seepage. $ c. Geometry. Trench geometry will depend on the volume of ex- pected underseepage, desired reduction in uplift pressure, construction practicalities, and the stability of the material in which it is being excavated. Trench widths varying from 2 to 6 ft have been used. Trench excavation can be expedited if a ditching machine can be used. However, narrow trench widths will re'luire special compaction e'luipment. One such piece of e'luipment (fig. 5-5), which is a vibrating-plate type of compactor specially made to fit on the boom of a backhoe, has apparently performed satisfactorily. . d. Backfill. The sand backfill for trenches must be designed as a filter material in accordance with criteria given in Appendix E. If a collector pipe is used, the pipe should be surrounded by about a I-ft thickness of gravel having a gradation designed to provide a stable transition between the sand backfill and the perforations or slots in the pipe. A typical section of a pervious drainage trench with collec- tor pipe is shown in figure 5-6. Placement of trench backfill must be done in such a manner as to minimize segregation. e 5-6. Pressure Relief Wells. a. General. Pressure relief wells may be installed along the lands ide toe of levees to reduce uplift pressure which may otherwise cause sand boils and piping of foundation materials. Wells accomplish this by intercepting and providing controlled outlets for seepage that would otherwise emerge uncontrolled landward of the levee. Pressure relief well systems are used where pervious strata underlying a levee are too deep or too thick to be penetrated by cutoffs or toe drains. Relief wells should ade'luately penetrate pervious strata and be spaced sufficiently close to intercept enough seepage to reduce to safe values the hydrostatic pressures acting beyond and between the wells. The wells must offer little resistance to the discharge of water while at the same time prevent the loss of any soil. They must also be capable of resisting corrosion and bacterial clogging. Relief well systems can be easily expanded if the initial installation does not provide the con- trol needed. Also, the discharge of existing wells can be increased by pumping if the need arises. A relief well system re'luires a minimum of additional real estate as compared with other seepage control . . 5-6 e