LEVEES. 83 



is unreliable and difficult to manage, and even when covered with sod is not to be 

 depended on. Where it is practicable to place a layer of clay upon the outside fairly 

 good work can be obtained, but when a better material can be had without too great 

 expense the use of sand is inadvisable. Not only is the wave action severe on sand 

 embankments, but they are liable to sink and slough as the fine particles are washed 

 out, and once sand starts to escape it is a difficult matter to arrest its movement. 

 Whole embankments will thus sink away without warning and with great rapidity. 

 Clay will resist water and erosion to a much greater extent than sand, but it does not 

 stand at grade very well. Its tendency is to settle and crack, and in this it is inferior 

 to good sand. To make a reliable embankment it must be placed in thin layers and be 

 well tamped as put in. It is very difficult to handle in wet weather and gets very 

 hard during a dry season. 



The fineness and lightness of loam renders it undesirable for levee work. Its 

 particles lack coherence, and it is even more treacherous than sand, because satura- 

 tion transforms it almost into mud. Where loam is used the embankments should 

 be given much larger dimensions than with those of clay. On the other hand, it is 

 usually very abundant, being a surface soil, and it is easily worked in dry weather, 

 packs well, and holds up to grade. 



A combination of clay and sand is highly recommended by most engineers who 

 have had experience in levee construction. It gives a bank of a permanent nature 

 and is also easily built. It prevents the cracking noticeable in clay embankments 

 and does not shrink so much, and at the same time the tough quality of the clay is 

 preserved. 



One of the most important materials used for river embankment is gravel, which 

 in many localities is easily procured. Its use is generally as a facing to prevent wash, 

 the inner portions of the levee being of a more cohesive material. 



Upon this subject Hewson says: "The lightness of a sand-bank is but a small 

 disqualification for leveeing compared with its liability to wash and leak. Its wash 

 is not confined to waves, current, and rain; but is carried on actively by the wind. 

 Sand is liable not only to run and blow away in a dry state, but also in a wet state is 

 liable to run or 'melt' like so much sugar. But while its lightness lays it open as a 

 material for levees to great objection on the ground of duration, the worst of its proper- 

 ties in such works is its liability to percolation. A bank which may be of ample section 

 to resist the total pressure brought to bear on it, when that pressure acts from the 

 outside slope against the whole weight of the bank, will yield when that pressure 

 becomes transferred from the outside of the bank to some point or plane within it. In 

 the latter case a portion only of the whole mass is engaged in the resistance of the 

 whole pressure. Now percolation of the water into the body of the work places the 

 levee under these very circumstances. 



" A thread or plane of water finding its way into the interior of an embankment 

 exerts just as much pressure against the earth on each side of it as if that thread or 



