J4 o THE IMPROVEMENT OF RIVERS. 



necessary to have a higher guard than where a long spillway has been provided; other- 

 wise, the river will "go out of lock" much sooner than it should. 



Foundations. The natural foundation is a factor of the first importance in deter- 

 mining the lift, and, therefore, the general design of the lock and dam. On a rock 

 foundation, with other conditions favorable, a lift of extreme height can be used, 

 while if the foundation be light or porous, it is a serious risk to employ other than low 

 lifts, especially with movable dams. 



Wherever rock exists within a reasonable depth, the construction should always 

 be commenced on it, as the extra expet.se will be repaid by the general stability of 

 the work. If the rock be found to be soft, it should be removed till a hard stratum is 

 reached, or be covered with a good bed of concrete wherever exposed to the action of 

 the water. 



The foundation is the most important part of the entire work, as if it is improperly 

 designed or constructed, it can rarely be remedied without rebuilding, and any defects 

 will invariably become manifest, though sometimes not for many years. The action 

 of water is so penetrating and insidious that it will attack the slightest weakness, and 

 unless the engineer can detect and secure during construction the vulnerable points 

 of the foundation and of the banks around the structure, trouble is sure to result. 

 This is especially true of foundations on seamy rock, as the head of water when the 

 cham1>er is full will force an escape through^every crevice that has been overlooked, 

 and if the rock be soft it will be steadily eaten out. F<5* this reason it may be best with 

 a doubtful foundation to cover the entire floor with concrete, and thus avoid uncer- 

 tainties. 



\Yhere the natural foundation is other than rock, the masonry may be com- 

 menced directly upon the surface, if the latter be sufficiently hard. Where this is not the 

 case, piles are usually employed, and in foundations of this character it is well to drive 

 continuous sheet-piling along the three outer sides of the lock, to prevent any escape 

 of the material by undermining. Sheet-piling must of course be driven along the 

 upper side in any case to cut off the upper pool, and where the soil is very porous a 

 second row is sometimes driven above the upper miter wall. 



Where piles are used it is the general practice in this country to cap them with 

 12" X 12" to 12" X 1 6" timbers, to which a heavy timber floor is spiked, on which 

 the masonry is commenced. It is much better, however, to excavate the soil for a 

 foot or two below the pile-heads, and commence a bed of concrete on it, several feet 

 in thickness, inclosing the piles. When this is done the weight of the wall is largely 

 carried by the surface of the natural foundation, instead of by the piles alone, and greater 

 stability secured against settlement. In the former style of foundation the entire 

 load is transmitted to the piles through the caps, and it will generally be found on 

 investigation that the pressure per square inch between the surfaces of contact is 

 greater than usually considered safe, especially as wood immersed in water and always 

 under pressure appears to become gradually soft. It is probably owing to the crush- 



