A Treatise On The Principles And Practice Of Dock Engineering

LOCK FOUNDATIONS. 247 quantity of escaping material is capable, however, of being greatly reduced by a device due to Mr. A. G. Lyster and already referred to (p. 89). We now pass on to a considération of the structural features of looks and entrances. Lock Foundations.—On the subject of foundations much that is stated in the chapter on Dock and Quay Walls is equally applicable in the present instance and need not be liere repeated. There are, however, some contin- gencies and expedients peculiarly characteristic of lock construction which call for special notice and explanation. The walls of locks differ from the ordinary type of dock walls in that they derive a considerable amount of support from the floor, especially if, as is usually the case, the latter has the form of an inverted arch or, if a flat floor, has ourved haunches tangential to the side walls, or, failing that, is sufficiently thick to admit of the existence of a virtual arch within its limits. The floor, on the other hand, without much assistance from hydrostatic pressure, has frequently to restrain the uplifting tendency induced by this lateral weight. The effect is more particularly felt in cases such as the loelc at Bremerhaven (fig. 206), where there is no artificial floor, though in the instance cited the stress is minimised by the use of bearing piles beneatli the walls. As a general rule, hard rock and stiff clay, in which there are no springs, do not call per se for any artificial covering, except such as may be judged necessary to protect their surfaces from the softening and scouring action of water. On the other hand, alluvial deposit, sand, gravel, and other inco- hesive strata, need the confinement afforded by a superimposed mass in addition to the lateral support of sheet-piles. Earth of a porous nature, moreover, is not only unsuitable for a natural floor, but is equally undesir- able as a foundation for an artificial floor, owing to its efficacy as a medium for the transmission of water pressure, on which account any covering laid upon it should be both strong and impervious. The point of perhaps the greatest importance in connection with lock foundations is that of the treatment of boils or springs, such as are often encountered in works of this class. The type of foundation most likely to cause trouble in this respect is that in which a pervious stratum lies between two others of an impervious nature, the upper of which has been pierced or is fissured by a natural fault The water-bearing stratum may then discharge copiously under considerable head, owing to a connection with some external supply located, often unsuspectedly, at some remote inland source. The following may be cited as an illustration germane to the point. The site of the Albert Lock at Hull* consists of consecutive layers of silt, peat, boulder clay, sand, boulder clay, sand, and chalk. Soon after the lower bed of clay had been laid bare in the course of excavation there occurred numerous and powerful inbursts of brackish water charged with yellow sand. The source of the trouble was primarily attributed to the * Hawkshaw on “ The Albert Doek, Hull,” Min. Proc. Inst. C.E., vol. xli.