30 4 THE IMPROVEMENT OF RIVERS. 



built in the water, sinking as timber is added until the bed is reached. They should 

 then be weighted with stone on top to settle them. The construction may begin at 

 one or both sides of the river. Only sufficient stone is immediately filled in to hold 

 the crib in position, as it is desirable to allow the water to pass through with as great 

 freedom as possible until the dam is nearly completed. As soon as one crib is built up 

 to a height somewhat above the water stage, and is safely anchored, another is sunk 

 at its outer end until all are in place. The discharge area of the river will have been 

 contracted considerably by the time the last crib is put in, and the velocity increased, 

 so that it may be necessary to carry anchorages or wire ropes up stream to prevent 

 too great displacement. Should slight displacement occur, however, it is usually not of 

 serious import, as the timbers above water-level can all be carried through on the 

 proper line. If the dam is on rock, with therefore no danger of the river cutting 

 under the cribs, the work of connecting them by a continuous crib covering the 

 several joints should be vigorously carried forward. During this time there should 

 be on hand an abundant quantity of stone which can upon short notice be dumped 

 into the cribs in case of an approaching rise, some of which may be put in place as the 

 work progresses. When the cribs have reached their full height the filling should 

 be rapidly completed, and after that the decking may be placed. Finally, after all 

 has been done and satisfactory connection made with the lock wall and abutment at 

 the ends, the sheet-piling along the upper face must be put in place. This should 

 invariably be done with great care and should extend to the rock whenever practi- 

 cable. As this approaches completion the water above will rise and flow through the 

 valves in the lock, and if the discharge of the river exceeds the capacity of these valves 

 it will eventually rise over the crest of the dam. It is, therefore, necessary to pro- 

 ceed rapidly with the sheet-piling, and this portion of the work should be carried on 

 continuously and at several points. Where there is a considerable flow of water it 

 may be necessary to open the lock gates and depend upon the erection of the coffer- 

 dam in the head of lock to close them again with safety. This is not advisable, however, 

 unless the conditions for dry weather are favorable, as the appearance of a sudden rise 

 might render it impracticable to place the coffer in position, and the increased flow 

 passing through the lock might endanger its safety. When the sheet-piling has been 

 completed an embankment of gravel or other suitable material should be placed 

 immediately above, as has been mentioned, and protected by broken stone to pre- 

 vent its being washed over the dam. 



Where the foundation is of light material, so that there would be danger of under- 

 mining before the sheet-piling could be put in as just described, the latter should be 

 driven as soon as the foundation is above water, and gravel and brush should be kept 

 on hand to stop any under-cutting. In this case the piling is sawed off at the water 

 surface and the river flows over it, and when the dam is finished the spaces above are 

 closed by a double row of planks resting on the sheet-piles and spiked to the crib tim- 

 bers. This method is usually to be preferred in all cases where the dam does not rest 

 on bed-rock. 





