Ch. 12] LEVEE DESIGN AND CONSTRUCTION 221 



surface down to a considerable depth, consist largely of clays and silty 

 soils which in turn overlie deep beds of sand. The clay is highly 

 plastic in some places. The Atterberg liquid limit generally is in ex- 

 cess of 50, and values greater than 100 are not uncommon. The nat- 

 ural water content of some of these fat clays ranges between 50 and 

 100 percent. It is evident therefore that primary engineering con- 

 siderations in this region are stability and settlement. 



The loess deposits found along the east wall of the Lower Missis- 

 sippi Alluvial Valley are unique in their physical properties and present 

 many unusual problems when utilized as a foundation or construction 

 material. Their marked tendency to lose shear strength when sub- 

 ject to saturation or remolding and to erode easily when in an undis- 

 turbed state introduces many problems. However, when properly 

 compacted, they possess good shear strength and are quite resistant 

 to erosion. The great difference in strength and erosion resistance of 

 loess which has been properly compacted and loess which has received 

 inadequate compaction at water contents substantially above or be- 

 low optimum is particularly striking. Loess was used in the embank- 

 ment of Arkabutla Dam with entirely satisfactory results. 



Levee Design and Construction 



The wide variation in foundation soils and in borrow materials 

 available gives rise to a multitude of problems. In the northern por- 

 tion of the valley the sandy foundations are generally stable but are 

 extremely pervious, and considerable underseepage that must be satis- 

 factorily controlled may be present. In this region it is often difficult 

 to find sufficient impervious material to cut off satisfactorily seepage 

 through the levees, and to provide an impervious blanket on the river- 

 side face. In addition, levees must often cross abandoned and filled 

 stream channels, which filling may consist largely of fat clays and silts 

 deposited in water and never exposed to any appreciable amount of 

 drying or to any overburden load. In some cases these materials are 

 not even consolidated under their own weight. Foundation stability 

 becomes a major feature in these locations, requiring the taking of 

 undisturbed samples that can be tested in the soils laboratory. 



Problems involved in levee design and construction in the central 

 portion of the valley are somewhat similar to those in the northern 

 portion except that, because the top stratum in many places is thicker 

 and the river has meandered more in the past, difficult foundation 

 problems involving stability are of more frequent occurrence. It is 

 of importance not only to determine the locations of abandoned stream 

 channels by geological investigation but also to determine the physical 



