226 turnbull et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



struction. A particularly interesting case which involved the com- 

 bination of excavation in very weak clays underlain by sand stratum 

 is afforded by the construction of the Algiers Lock across the river from 

 New Orleans, which is now under construction by the New Orleans 

 District, CE. This situation was further complicated by the fact 

 that gas was present in the sand stratum, and it was necessary that 

 the gas pressure be relieved. The New Orleans District accomplished 

 this by installation of pressure-relief wells which permitted the gas 

 and excess hydrostatic pressures to be dissipated. 



Structures 



The many floodgates, locks, and other drainage structures which are 

 required give rise to many foundation problems which require geo- 

 logical and engineering studies for the economical and successful 

 solution of design features. The geological study often results in re- 

 location of the structure to avoid recent channel fillings and other 

 undesirable foundation conditions. The engineering study is concerned 

 with obtaining undisturbed samples of soil where required, testing 

 samples in the laboratory, and evaluating such questions as allowable 

 bearing capacity if spread footing is used, or the length of piles neces- 

 sary if a pile foundation is required, estimate of settlement of the 

 structure, and provisions for handling seepage beneath the structure, 

 which often acquires considerable importance. Problems involved in 

 making excavations for structures have been described in the pre- 

 ceding section. 



REFERENCES 



Daly, R. (1929). Swinging sea level of the ice age: Bull. Geol. Soc. Amer., vol. 



40, No. 4, pp. 721-734. (A pioneer project.) 

 Fisk, H. N. (1944). Geological investigation of the Alluvial Valley of the lower 



Mississippi River: Mississippi River Commission, Vicksburg, Miss., 78 pages. 

 (1947). Fine-grained alluvial deposits and their effects on Mississippi 



River activity: Waterways Experiment Station, Vicksburg, Miss., 2 vols., 82 



pages. 

 Russell, R. J. (1933). Mississippi River delta: Louisiana Geol. Survey, Bull. 8, 



454 pages. 



