LOCKS. 161 



the first signs of failure are always shown. We have seen ties split open in new cribs 

 through this pressure, and in old cribs they will be found to have become rotten at 

 the ends, while the adjoining timbers show little decay. Relief may be obtained by 

 placing blocks on each side of the ties to distribute the load, and care must be taken 

 that the foundations are also equal to the weight to be upheld. 



The tops of the cribs are usually made 6 to 10 feet wide, and the width of base 

 not less than one-half the height. As the vertical wedge of the filling always tends 

 to push the crib out at the top, the face should be battered \ inch or f inch to the 

 foot ; this may be done by setting each timber back as the building progresses, and we 

 have found it to prove very effective in retarding the forward settlement of a crib. 

 Diagonal bracing is also useful in this respect. Where high cribs are built with vertical 

 faces, and no provision made against undue weight upon the timbers, they will speedily 

 begin to settle, and in a few years may lean over 12 inches or more at the top. In 

 many cases the end of the crib next the lock wall has been bolted to the masonry, with 

 a view to retarding this settlement. The practice, however, is of very little use, and 

 results in unsightliness, as the end is held up while the rest of the crib settles, and the 

 difference in level becomes more noticeable each year. We have always found that 

 where a timber guide crib or a dam of timber cribs is of any height, a settlement com- 

 mences as soon as the work is finished, owing to the weight of the filling gradually 

 compressing the fibers of the wood, and the best that can be done is to so design the 

 work that the settlement will be equalized as far as possible. 



The tops should be made level with the coping of the chamber wall, so that boats 

 can use the cribs until the river has flooded the lock. 



For the upper entrance the cribs are usually placed on both sides, the river crib 

 being needed to keep tows from being drawn toward the dam. In this case a short 

 crib is usually sufficient on the land side, serving to keep craft from striking the wall. 

 Where the bottom is good, a cheap and effective guide may be made with piles driven 

 deeply 8 to 10 feet apart, and provided on the lock side above the upper pool 

 with a grillage of 10" X 10" timbers, spaced 10" apart vertically, and bolted or drift- 

 bolted to the piles, thus forming a continuous crib face. Some of the piles may be 

 left projecting above the top timber, to serve as check-posts. The outer end should 

 be provided with a short crib or a cluster of piles, as a buffer-post. In other cases 

 where this method would cause trouble to barges because of the water drawing toward 

 the dam through the open spaces, rectangular or triangular cribs 20 feet to 30 feet in 

 length may be employed, spaced 20 to 30 feet apart, the openings being spanned by 

 a grillage of timbers as for the piles. These types of guides will usually be found pref- 

 erable to solid cribs, as the entrance does not so easily silt up, and they are less costly 

 in establishment and repairs, while proving as satisfactory to boats. 



A plain row of single piles, or of clusters of piles, is not desirable unless the en- 

 trance is an unusually easy one, as the corners of barges catch against them, delaying 

 the maneuvers. 



