LOCKS. 159 



frequently have to act as retaining walls also, and should be designed accordingly. 

 Sometimes they are joined to the bank by a timber crib. In this case the latter should 

 be sheathed inside and filled with tamped clay. Riprap should never be used to fill it. 



Where no drain is placed behind the chamber wall the upper wing wall can be 

 made shorter, and in some cases it has been omitted altogether, although the advisa- 

 bility of this is doubtful. The practice of putting in such drains is one which certain 

 experienced river engineers have strongly condemned, and apparently with excellent 

 reason. They are placed there for the same purpose that they are placed behind 

 retaining walls, to carry off any water that may have collected. Usually they are 

 made of riprap, and are 2 or 3 feet in width, extending from the bottom of the 

 wall to near the top, and from the lower end, where they connect with the river, to near 

 the upper end. It is thus seen that the back of the wall is open to the pool below, and 

 that the only obstacle to prevent the upper pool from flowing round is the length of 

 the upper wing wall. We have met with one example where the upper pool in high 

 water forced its way around, passing under the paving to the drain, fortunately with- 

 out causing serious damage. If the drain be omitted, the water must force its way 

 through a mass of earth two or three times longer, and the gain in safety would appear 

 to be worth more than the doubtful utility of putting in a drain which the river will 

 sooner or later choke with sediment. It may be added that the land wall in any case 

 is usually designed to support the earth under the conditions of greatest pressure, 

 that is, without any relieving drain. 



If one is put in, it should always discharge directly into the lower pool and not 

 into the tail bay, since, if the latter arrangement is adopted, it will not be possible to 

 pump out the lock. 



Paving. The space behind the land wall and between the wing walls is usually 

 paved, to prevent floods from washing out the backing. This paving may be com- 

 posed of small blocks, 10 or 12 inches deep, set on edge, or of large stones laid flat, or of 

 concrete. If the first style is used, it is a good plan to grout the joints with cement, 

 or there will be a constant growth of grass and weeds in them. Where large stones 

 are used they are difficult to reset if any sinking occurs, and for this reason are not 

 desirable. 



With a new lock the paving should never be laid till the filling has been in place 

 for one or two years, otherwise it may become badly disfigured by settlement. Tem- 

 porary protection can. be secured by the use of riprap. 



Coffer-dams. Arrangements for closing the lock at each end for repairs should 

 always be provided. A method long in use for this purpose consists in placing tim- 

 ber beams, one on the other, across the head and tail bays. . The ends of these timbers 

 rest in vertical recesses cut in the walls, and they are supported at intermediate points 

 against posts connecting with and braced to the masonry. A needle-dam of the 

 Poiree type is also used for the same purpose, and is preferable in many ways, as it is 

 simpler and more easily handled, besides requiring less timber. Where this type is 



