222 tubnbull et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



properties of the materials, because the soils may be in a very weak 

 state. For example, the shearing strength of clays as determined from 

 unconfined compression tests may be as low as 0.2 ton per square foot 

 or even less. Seepage beneath the levees is also of importance at 

 many locations in the central portion of the valley, as in the northern 

 portion, and a highly specialized technique has been developed for 

 controlling sand boils which develop during flood stages of the river. 

 Much engineering talent has been devoted to these techniques, which 

 involve the proper method of bagging sand boils and the construction 

 of sublevees, protection of caving banks, and other related problems. 

 Fighting a flood brings as many and as difficult engineering problems 

 as does the problem of design and construction of a levee system. 



Since the soils in the southern portion of the valley are generally 

 clays to considerable depths, levee design and construction problems in 

 this region are more concerned with foundation stability than with 

 underseepage, although there are some notable exceptions. In this 

 region the required heights of levees are less than in the central and 

 northern portions ; this is fortunate because the strength of the founda- 

 tion soil, in general, also is less. Shear strengths below 0.2 ton per 

 square foot are by no means uncommon, and it is evident that founda- 

 tion stability becomes almost an insuperable problem in some loca- 

 tions. These problems have been solved successfully in the past 

 through determination of the properties of the soils by laboratory 

 tests and the design of berms on both landside and riverside of the 

 levee to give greater stability. In some cases construction has been 

 prosecuted in stages which continue over intervals of years so as to 

 bring the levee to grade gradually, thereby allowing the soils to con- 

 solidate and gain strength. Occasionally, levees have been built by 

 the displacement method; that is, construction is continued in a normal 

 manner and the weight of the embankment squeezes out the unstable 

 material in the foundation. As much as 200 percent of the material 

 composing the normal levee section has been required in some cases 

 before stability was reached and the levee finished to grade. 



Revetments 



The meandering of the river frequently endangers the existence of 

 the levees. In some instances this necessitates construction of a set- 

 back levee, but in places this cannot be accomplished, owing to the 

 location of a town or to the relative economy of bank protection in 

 place of levee construction. The design and construction of revetment 

 involves construction, soils, and hydraulic problems. Before a revet- 

 ment is constructed, the engineering problems involved concern the 



