LAND DRAINAGE BY MEANS OF PUMPS. 11 



Where good foundation can not be secured for the levee it often 

 becomes necessary to build a sublevee along the land-side toe of the 

 levee slope. This sublevee would usually start at a point on the 

 land-side slope about 8 feet below the grade line of the main levee, 

 run back with a slope of 20 to 1 for about 20 feet, and then down to 

 the surface of the land with a 3 to 1 slope. This type of construction 

 will aid very materially if there is danger of the levee material sliding, 

 it will make available a large quantity of earth for emergency work 

 on the top of the levee, and will also provide a very suitable location 

 for a road. 



CONSTRUCTION. 



Levees are built with floating dredges equipped with dipper or grab 

 bucket; with land machines usually equipped with drag-line buckets, 

 but sometimes with grab buckets; and with scrapers. Local condi- 

 tions will usually determine the type of machine to use, but the 

 majority of levees are now constructed with the land machine equipped 

 with a drag-hne bucket. If the material is put up wet, there will be 

 very little subsequent settlement, but if the material is dry or if 

 scrapers are used, the original height of the levee must exceed by at 

 least 10 per cent the height required for the permanent levee. In any 

 case the material used to form the levee should all be taken from the 

 river side of the embankment and a clean berm at least 20 feet wide 

 left between the toe of the slope and the nearest point of the borrow 

 pit. The latter must be kept shallow on the side nearest the levee, 

 with a side slope not steeper than the levee slope, so as not to under- 

 mine the embankment. In no case should the excavation cross the 

 imaginary line formed by extending the line of the outer slope of the 

 levee down below the surface of the ground. Where a current along 

 the outside slope of the levee is at all likely, transverse strips of earth 

 of a width of 15 or 20 feet should be allowed to remain across the 

 borrow pit at a spacing of about 500 feet. This will reduce the scour- 

 ing action of river currents and aid the deposition of sediment in the 

 pits. Land machines adapt themselves to this method of excava- 

 tion, while floating equipment does not. Recently hydraulic dredges 

 have been used very successfully in building levees. This method is 

 especially desirable where the foundation and material at hand are 

 both poor, a combination of conditions which often occurs. The 

 material usually placed into levees by hydraulic dredges is coarse 

 river .-.and, taken at some convenient point in the river channel. By 

 this method the base of the Levee and the surface between the levee 

 and the river Wank are Left undisturbed and the difficulty of getting 

 a land machine to work across bad foundation is avoided. Experi- 

 ence has shown SUCh levees to be satisfactory. 1 1 ydraulie-lill sand 



levee-, are also very desirable in crossing the mouths of sloughs which 



