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



less than in the central and northern portions of the valley. However, 

 a much greater concentration of industrial facilities along its banks 

 has taken place; this in turn causes minor tendencies to meander and 

 local stretches of bank instability to become major problems. Wave 

 wash does become of considerable importance in revetments in this 

 area, as is readily apparent, because ocean-going vessels regularly 

 travel as far upstream as Baton Rouge. 



Excavations 



The making of excavations of any considerable depth in the Al- 

 luvial Valley presents varied engineering problems, as these excava- 

 tions may be made in very permeable sands or in weak, fat clays of 

 low shear strength. Furthermore the bottom of an excavation may be 

 located in impervious clays that overlie highly pervious sands of depths 

 that may exceed 100 feet and that connect directly with the Mississippi 

 River. Thus wellpoint systems are often necessary in the northern 

 and central portions of the valley to lower the ground-water level in 

 the permeable sands and to prevent loss of stability of banks and 

 foundations due to seepage forces. The high ground-water level usu- 

 ally found throughout the Alluvial Valley results in high heads which 

 must be provided for. Excavation in clays where shear strengths may 

 be less than 0.2 ton per square foot makes necessary an engineering 

 study to determine the allowable slope to which the bank may be 

 cut. Undisturbed samples of soil are regularly obtained and tested 

 in the laboratory for large excavations. Slope analyses are made by 

 the circular-arc method or the method of sliding wedges, depending on 

 which method may be most applicable. 



The presence of a pervious formation beneath the clay often makes 

 it necessary to install wellpoints or deep wells to relieve uplift pres- 

 sure against the bottom of the excavation, even though the excavation 

 is in clay out of which a negligible amount of water flows into the ex- 

 cavated area. Since the sand and gravel formation beneath the surface 

 in the Alluvial Valley is so highly pervious, even in the southern por- 

 tion, and connects with the Mississippi River, the possibility of exces- 

 sive uplift must be taken into account, even though the river may be 

 many miles away. An example of such variation in head in the per- 

 vious sand and gravel formation in the southern portion of the valley 

 near Baton Rouge is shown on Fig. 4, which shows the level in piezo- 

 meters corresponding to variations in stage in the Mississippi River, 

 which at this point is 10 miles away. The effect of these uplift pres- 

 sures must be considered in planning the excavation, and methods 

 must often be provided for their control and observation during con- 



