LOCKS. 157 



teract the upward pressure. It may be questioned, however, whether this method 

 offers any advantages in cheapness or efficacy over that of the inverted arch. 



An example of the force of the upward pressure occurred on one of the tributaries 

 of the Ohio, some years ago, where a lock floor, composed of piles and concrete, broke 

 up suddenly, the rupture taking place as a large hole, approximately 60 feet square, 

 at the upper end of the chamber. The lock was in use at the time, and as attempts 

 to pump out proved ineffectual the hole was temporarily filled with gravel till final 

 repairs could be made. A careful examination showed that the accident was due to 

 pressure from the upper pool, which broke under the upper miter-wall and ruptured 

 the floor. To cut off this pressure a large amount of backing was placed above and 

 outside the lock, which has been in use since that time without further repairs, the hole 

 apparently allowing only a small amount of water to pass in and out. 



Other instances are on record of floors being broken through or partly displaced, 

 and no pains should be spared to make this portion of the structure thoroughly secure. 



A space of 30 to 60 feet below the lock, when the foundation is not rock, should 

 be protected with an apron crib or heavy riprap, so that no washout will be caused by 

 the discharge from the valves. 



Where conditions permit, it would be well to provide a "sump" or drainage basin 

 in the floor, for the use of the suction when the chamber has to be pumped out, as the 

 water could then be drawn entirely off the floor, instead of remaining on it to the 

 depth of a foot or more, as is usually the case. 



Battered Chamber Walls. A very desirable method of construction is to build the 

 chamber walls with a batter on the inner face of or inch to the foot. Few exam- 

 ples of this are to be found in America, where the walls are almost invariably vertical, 

 but it is doubtful if any engineer who has had experience with both types has failed 

 to see the advantage of a battered face. On a certain tributary of the Ohio, exam- 

 ples exist of each kind, and where the vertical faces have become seamed and scarred 

 with the rubbings of craft, the battered faces show hardly a scratch after twenty years 

 of use. The reason is of course that with the latter the bottom of a barge will strike the 

 wall first, and whereas the top of the barge is protected with iron and has protruding 

 bolt-heads which will cut and scar the masonry, the bottom has only the edges of the 

 wooden planking with which to strike. The additional expense of construction for 

 battered walls is very small. 



It would be preferable, however, to continue the batter from end to end of the 

 walls, instead of making the head and tail walls plumb, as is the usual practice, since, 

 with this combination method of building, the vertical walls become badly scarred by 

 craft entering or leaving, and the corners of the walls and quoins become chipped off. 



The width of a chamber with battered walls is of course measured at the bottom 

 so as to give the full dimensions required. 



Miter-sills. These sills are usually made of 12" X 12" timbers, with their tops a 

 foot or eighteen inches above the floor. In the older locks they are frequently 16 to 



