WET LANDS OF SOUTHERN LOUISIANA. 41 



Owing to the longer boom and narrower hull, the latter type of dredge is able to 

 leave a wider berm along the toe of the levee. It also is better able to sort the 

 material placed in the base of tbe levee; for the top layer of muck can first be 

 taken out of the canal, and then the silt underneath, while the dipper dredge 

 will usually cut up through both the silt and the muck and mix them. The 

 levee should usually be constructed in several layers, for both the base and the 

 material are likely to be so soft that subsidence will be too great if a height 

 of more than a few feet is attempted. This yielding of the base will often 

 cause the side of the canal to cave, especially if the berm be small. 



The total subsidence and shrinkage of the material in the levees often 

 amounts to 50 per cent, and in special cases is as great as SO per cent of the 

 bulk of the material, as measured in excavation. Practically all of the sub- 

 sidence and a part of the shrinkage takes place during construction, so that 

 the remaining change in height can be taken care of by maintenance. When a 

 large percentage of muck is placed in the levee the shrinkage will be great for 

 a number of years, due to the decay of the vegetable material in the muck. 



The orange-peel bucket is especially suitable for placing several layers in a 

 levee. After a canal is once cut in the soft prairie a considerable depth of soft 

 mud that makes very poor levee material will be in the bottom. The dipper 

 dredge, when working in such a canal, will place a large percentage of soft mud 

 in the levee ; while an orange-peel bucket, when dropped forcibly, will penetrate 

 the undisturbed silt below and fill with it, the soft mud running off when the 

 bucket is raised. 



If the site of the levee is along a solid ridge above ordinary water level, no 

 special precautions need be taken to prevent seepage, although all stumps and 

 logs should be removed from the site and a shallow ditch should be cut to in- 

 sure a perfect bond between the ridge and the levee. On the other hand, if the 

 levee is through very soft prairie, the material dropped from the dredge will 

 penetrate the muck and form a good bond with the underlying silt. It is on 

 the portions where the muck is thick and turfy in character that particular 

 pains must be taken. A ditch cut along the center line of the levee before the 

 dredge starts working is of no special benefit, as the material placed back in 

 the ditch by the dredge will be largely muck, although it is true that this treat- 

 ment will break the continuity of the muck and help to cut out a portion of the 

 seepage. A better plan is to wait until the first layer of material has been 

 placed by the dredge and then cut a ditch along the toe of the slope of the 

 levee opposite the dredge and refill it with impervious silt dredged from the 

 bottom of the canal. This will insure a good bonding of the material and is a 

 necessary part of the construction. At times old muck-filled bayous will be 

 encountered which must be closed with levees. In such cases the quickest, and 

 quite often the cheapest, way to insure that the levee hold its grade line is to 

 drive two rows of sheet piling- across the bayou at the proper spacing. These 

 rows should be tied together with rods and the fill made between them. 



The average unit price for dredge-built levees, where the material is meas- 

 ured in excavation, has been from 6 to 8 cents per. cubic yard, depending on the 

 amount of timber and stumps encountered. Large amounts of such work have 

 been done at an actual cost, to the owners of the dredge, of less than 3 cents 

 per cubic yard. Since it requires from one and one-half to three times as much 

 material in excavation as finally appears in the settled embankment, the con- 

 tract price would vary from 10 to 25 cents per cubic yard of such embankment. 



If the levee is to be brought to a regular cross section by hand or machine 

 work, a small additional charge should be made. In general, however, the 

 levees, as ouilt, have not been surfaced after the dredge work is finished, 



