LEVEES. 



occur in the same locality; indeed there are some situations which are peculiarly 

 liable to them. Twenty or thirty will be found in a length of 200 or 300 feet. They 

 do not usually occur very near the base of the levee, but 40 or 50, 100 or 200 feet 

 away; indeed they are occasionally found 1000 feet distant from the levee. . . 

 When the boil is near the levee and of considerable dimensions, it is unquestionably 

 a symptom of danger. Perhaps a little more, and it would have been a crevasse. '. . . 

 When the manifestation is formidable, the remedy generally applied is to weight down 

 the weak place with an additional load of earth, which is usually laid upon a thick 

 layer of brush, to allow a partial drainage of the leak water and to prevent the contam- 

 ination of the new and dry earth by the mud of the old." 



(3) One of the most alarming experiences of the levee engineer is that which 

 comes when, with the water rising upon the outside to nearly the full height of the 

 levee, he sees the land slope sloughing away from the excess of permeability of the 

 embankment. The chief cause of sloughing may be laid to a lack of proper care in 

 construction. Had the earth, during the building of the levee, been well tamped in 

 layers, saturation would have been less liable to occur, and their rupture might have 

 been avoided. A French authority* has thus described the process: 



" In the degree that the water rises in the river, oozings of increasing extent are 

 noticed on the opposite side of the levee, and where the maximum of permeability 

 exists (taking account of the pressure and thickness), the slope on the inner side is 

 weakened and a first subsidence is produced. This subsidence, often small in itself, 

 is followed by others which come more rapidly as the permeability is increased by the 

 diminution of the thickness. Then, after a continuance of these slips, the rest of the 

 bank, having no longer a sufficient resistance, is carried away and the break enlarges 

 rapidly. It is seen that the rupture does not occur suddenly under the influence of 

 saturation ; the dike melts and is liquefied like a morsel of sugar in contact with water ; 

 it spreads out on the side exposed to the air and only gives way finally when these 

 seepages have sufficiently weakened it. The damage may, in fact, be attributed to 

 a double cause ; firstly, that the slope which too light earth takes when dry is not 

 sufficient for equilibrium when it is wet; secondly, that this earth is carried away by 

 infiltration." 



In order to prevent this seepage and sloughing several solutions have been offered 

 and tried. In France, a stanch revetment of stone paving has been placed along the 

 river side of the levee to prevent contact with the water. This is very expensive, and 

 has led to much criticism, not only on account of the cost but also because, it is claimed, 

 breaks in the paving will occur and allow the water to come against the bank, thus 

 destroying its effect. Another solution lies in giving the inner face the slope which its 

 material would take when wet, but as this does not prevent the removal of material 

 by infiltration it has been proposed to wall this inner slope with dry stone in order to 







O 





* Rivers with Free Current De Mas. 



