>o6 THE IMPROVEMENT Ol- RIVERS. 



more or less during the excavation. If the masonry is to rest on piles, they should 

 be driven before the excavation is begun so as to save any delays. 



Another method is to excavate as deeply as possible without using sheet-piling, and 

 then to take sharpened planks, and set them up in a close row against "rangers," or 

 horizontal planks, which are kept in position by shores or horizontal struts, each side 

 of the excavation being braced against the opposite side. The material is then dug 

 out below the feet of the vertical planks, which are driven down by hand as far as 

 possible, when the material can be removed from the center of the excavation. 

 Leaks may be checked by using straw, weeds, excelsior, or similar materials. This 

 operation is repeated till the final depth is reached, when the masonry can be put in 

 as above described. 



In a bank of very unstable nature it is best to make the finished grade to a flat 

 slope, two or three to one, and unless the seepage is very bad, it will be found that the 

 bank will drain itself and become stable. It must of course be paved or riprapped, 

 and its foot for 150 or 200 feet below the abutment should be provided with a toe 

 made of a crib filled with riprap, or, better still, built of concrete. This should always 

 go to rock, or, if the foundation is of gravel, it should be sunk as deep as possible and 

 have a close row of long piles driven along its outer side, faced with heavy riprap. 

 The bank above this crib and around the abutment should be paved with blocks 

 with close joints, or with hand-placed riprap, and if the river is subject to high floods 

 it is well to bed the stones roughly in Portland-cement mortar, where they will be 

 most exposed to the reaction from the dam. 



Where the range of floods is very small the precautions need not be so elaborate, 

 and in such cases a protection of piles and fascines, or of woven brush and riprap, may 

 be sufficient below the abutment. 



Too much care can hardly be taken in securing this end of the dam, as several 

 cases have occurred where the river in a single flood has cut away the banks and found 

 its way around the abutment. 



In most cases it is necessary eventually to continue the protection of the banks, 

 both below the lock and below the abutment, until it has reached a length of many 

 hundred feet. As a rule, it will be sufficient for the first season to rely on the riprap 

 or paving below the abutment as just described; but if the banks are "rotten," that is, 

 if they are sandy and liable to slips, a good supply of stone should be kept on hand 

 for emergencies. In the following season the bank, where cut away by the water, 

 should be graded and riprapped, and this process should be ultimately continued as 

 far down stream as the wash and eddies from the dam show to be needful. This will 

 range from 200 to 1200 feet or more, depending on the lift of the dam and other cir- 

 cumstances. A rough practical rule may be made that for each foot of lift of a fixed 

 dam 100 feet of bank will be affected by the washing, and for each foot of lift of a 

 movable dam, 150 feet. 



Unless the bank is very hard, the riprap should never be placed on the bare soil, 

 but always on a bed of spalls or large gravel, 6 inches or more in depth. Where this 



