FIXED DAMS. 205 



With Coffer-dam. There are cases, as in the construction of a masonry dam, 

 in which it is necessary to perform the work inside a coffer-dam, or rather inside two 

 coffer-dams, since it is not practicable to close the whole opening at one time in a stream 

 of good discharge. The type of coffer to be used will depend upon the character of 

 foundation upon which it is to be built and the style of dam proposed. The different 

 kinds in use have been described in the chapter on " General Designs." 



If desired, the coffer may be so placed as to become a part of the completed structure, 

 or it may be left to form a protection, the down-stream arm being reduced to the proper 

 height and floored over for this purpose. The walls of the coffer are carried out from 

 either the abutment or lock, generally from the one having the poorest foundation, so 

 as to throw the increased current caused by the contraction over the best foxmdation. 

 After they have reached the middle of the river, or such a point as desired, they are 

 connected by a cross wall, extending in the direction of the stream. The inclosure 

 thus formed, when pumped out, will constitute the building ground for the dam. 

 When the dam has been completed as far as desired within this inclosure, a bulk- 

 head reaching several feet above crest level is built near its outer end, connecting the 

 two coffer-walls. This done, the latter may be removed, and the second section of 

 coffer for the opposite part of dam, which should be considerably higher than the 

 crest of dam, on the up-stream side, may be put in and pumped out, and the work 

 proceeded with. 



The discharge of the river during the latter part of the construction may be 

 passed through the lock, or through sluices left in the dam, or it may flow over the 

 unfinished surfaces of the dam, each side of the latter being built up alternately a foot 

 or two at a time, using needles or stop-planks to keep the water off the masonry being 

 placed. 



Abutment and Protection of Bank. The abutment and the protection above 

 and below it should be completed before much work is done on the dam, or the river 

 may cut into the exposed bank. The abutment must be provided with a wing wall of 

 construction similar to those for the lock, connected with the solid bank by sheet-piling 

 or by a crib sheathed inside and filled with tamped clay. The wing should be well pud- 

 dled, and where the soil is light the sheet-piling should be continued for some distance 

 back from the masonry and driven as deep as possible. The top of the abutment is 

 usually made level with the top of the lock walls. 



The construction of an abutment is frequently a difficult problem, since it may 

 have to be placed deep in the bank, and the bank may be full of seepages. One method 

 of overcoming this is to drive sheet or round piling to form a rough coffer-dam against 

 the earth, and then to excavate as rapidly as possible inside the inclosure, and build up 

 a foundation of concrete to the height that may be required. If the work is done 

 quickly the concrete will be in place before much more slipping has occurred, and 

 will permit the remainder of the masonry to be put in with more leisure. The piling 

 should be driven a little distance from the building lines, as the earth will push it in 



