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e . . e . . e :EM 1110-2-1913 31 Mar 78 valve, rubber gasket, and plastic standpipe, as shown in figure 5-7, can be installed on each well. To safeguard against vandalism, accidental damage, and the entrance of debris, the tops of the wells should be pro- vided with a metal screen or flat gate. The elevation of the top of any protective standpipes must be used in design as the well discharge elevation. d. Well Installation. Proper methods of drilling, backfilling, and developing a relief well must be employed or the well will be of little or no use. These procedures are described in detail in Appen- dix D of this manual. Section II. Seepage Through Embankments 5-7. General. Should through seepage in an embankment emerge on the lands ide slope (fig. 5-8a), it can soften fine-grained fill in the vicinity of the landside toe, cause sloughing of the slope, or even lead to piping (internal erosion) of fine sand or silt materials. In many cases, high water stages do not act against the levee long enough for this to happen, but the possibility of a combination of high water and a period of heavy precipitation may bring this about. If lands ide sta- bility berms or berms to control under seepage are required because of foundation conditions, they may be all that is necessary to prevent seepage emergence on the slope. On the other hand, if no berms are needed, landside slopes are steep, and floodstage durations and other pertinent considerations indicate a potential p~oblem of seepage emer- gence on the slope, provisions should be incorporated in the levee sec- tion such as horizontal and/or inclined drainage layers or toe drains to prevent seepage from emerging on the lands ide slope. These require select pervious granular material and graded filter layers to ensure continued functioning, and therefore add an appreciable cost to the levee construction, unless suitable materials are available in the borrow areas with only minimal processing required. Where large quan- tities of pervious materials are available in the borrow areas, it may be more practicable to design a zoned embankment with a large lands ide pervious zone. This would provide an efficient means of through seepage control and good utilization of available materials. 5-8. Pervious Toe Drain. A pervious toe (fig. 5-8b) will provide a ready exit for seepage through the embankment and can lower the 'phreatic surface sufficiently so that no seepage will emerge on the lands ide slope. A pervious toe can also be combined with partially penetrating toe trenches, which have previously been discussed, as a method for con- trolling shallow under seepage. Such a configuration is shown in fig- ure 5-8c. 5-11