LOCKS. 



J 47 



Maneuvers. Boats are let into the lock when the water therein is at the level of 

 the pool from which they approach. To pass a boat through a lock up stream the upper 

 gates are closed, the lower ones opened to admit the boat and then closed again; and 

 the water is let in and fills the chamber, lifting the boat to the level of the upper pool. 

 The upper gates are then opened and the boat proceeds. If a boat desires to pass 

 down stream, the lock being full to the upper level and the gates being opened, the 

 boat enters, the upper gates are closed, the water is let out of the chamber till it is at 

 lower pool level ; the lower gates are then opened and the boat can go out. 



Calculations for Lock Walls. In determining the proportions of lock walls, 

 there are usually three main parts which govern the general dimensions. These are: 



(1) The river wall of the chamber. 



(2) The land wall of the chamber. 



(3) The mass required in the head-bay and tail-bay walls. 



There are minor portions which must also be examined, such as the wing walls, which 

 frequently act as re tain ing-walls, the miter-walls, which are usually made in arch 

 form, the upper coffer-wall or wall across the head of the lock, etc. 



River Wall of Chamber. The forces acting on the river wall are the weight of the 

 masonry and the pressures of the pools ; but in order to be secure against the most un- 

 favorable conditions, the lower pool should be assumed as drawn off. This will then leave 

 only the pressure from the upper pool acting on the wall. With fixed dams, which are 

 usually placed opposite the head walls, the maximum will occur with the chamber full, 

 but with movable dams, which are usually placed 

 toward the lower end of the lock, the maximum 

 will occur with the chamber empty, the pressure 

 being in this case on the outside of the wall. 



Where the wall is built on solid rock there 

 occurs no loss of weight by immersion, since the 

 water cannot penetrate under the foundation and 

 cause upward pressure. The full weight of the 

 masonry is thus available for stability. It is 

 the practice, however, of many engineers to take 

 no account of this in their calculations, so as to 



I 



secure an additional margin of safety, while others 



assume a mean of the two, and suppose the wall 



to lose 31! Ibs. of weight per cubic foot. Unless 



the rock is known to be dense and solid, too much 



reliance should not be placed on its impervious- p IG . j. 



ness. Where the wall is on gravel or any other porous material the full loss of weight 



of course occurs. 



The top or coping width may be made from 5 to 6 feet, as this will give sufficient 

 width for the maneuvering without excess of masonry. 





