WORKS FOR CREATING SLACKWATER. 141 



ing of weak timbers from this cause that walls on pile foundation have sometimes 

 cracked in two. 



Lock and dam masonry should not be built on a foundation one part of which is 

 composed of piles or grillage and another part of the natural material alone, because 

 unequal settlement will take place and crack the walls, if not at first, then later on 

 when the timber has had time to show its defects. We have seen an instance of this 

 kind in a lock wall which did not show any settlement until after the lock had been in 

 use for some years, although since the first break no additional settlement has been 

 apparent. 



Coffer-dams. A tight coffer-dam is very desirable for hydraulic foundations, 

 especially where construction is to be commenced on the bed-rock. There are two 

 general styles employed, one known as the pile coffer, the other as the crib coffer. 

 Another style, known as the box coffer, and made of plank, has sometimes been used 

 in light construction. The first is much the cheaper, but is not as suitable as the crib 

 coffer for many localities. It is composed of a row of round piles, 8 to 10 feet 

 apart, to which 10" X 10" or 12" X 12" walings are bolted. On the outside a close 

 or continuous row of sheet-piling is driven to the rock, or as far down as it is desired to go. 

 The outside is then well banked with good clay, or clay and gravel, and where neces- 

 sary riprapped" against scour. Sometimes a second row of round piles is driven inside, 

 to which the outer piles are braced, but the additional support afforded is of ques- 

 tionable value, since if the excavation draws out material from around them, as is 

 almost always the case, they have little bracing power against the water. 



Another variety of the same coffer consists of two similar rows of piles, with a 

 space of 8 or 10 feet between, and tied together with i-inch or i}-inch tie-rods and 

 struts. Walings and sheet-piling are provided for each row, placed on the inside, and 

 the vacant space is filled with clay. The outside is banked, and riprapped where 

 required. This type costs from $10 to $16 per foot run of coffer, according to cir- 

 cumstances. 



These coffer-dams require a sufficient natural bed in which to drive the piling, and 

 where this exists they have been successfully used for heads of water of 15 to 1 8 feet, 

 even in light material, and they will withstand rises excellently. They require plenty of 

 space, however, since they must be set well away from the masonry in order to pre- 

 vent loss of support during excavation by the removal of the inside supporting earth. 



The second type is the crib coffer, and consists of logs or sawed timbers, spiked 

 upon each other, with sets of ties 10 or 12 feet apart. The inside faces are planked 

 with i -inch boards to keep the filling in, and the pens are filled up with clay and rip- 

 rapped on the top, and well banked or "backed" on the outside. Sometimes, with 

 a porous material, a row of sheet-piling is driven along the outside, but if plenty of 

 backing is used this is not necessary. This style is much more expensive than the 

 pile coffer, since it requires excavation and more material, but it must be used in cast's 

 where there is no bed to hold piles, or where the contraction of the river might cause 



