184 DOCK ENGINEERING.
amount of projection, x. Assuming an ultimate tensile resistance, for good
concrete, of 100 Ibs. per square inch, and treating the portion o: as a canti-
lever, fracture would occur, with a uniformly distributed load,
whence, considering w as the pressure on the foundation, in tons per square
foot, and taking a factor of safety of 2,
d=^w.x.......................(36)
4. The texture and chemical composition of the foundation should be
such that it is not liable to deterioration from external influences. Certain
varieties of rock are softened and washed away by the action of water. The
writer has seen sandstone, which required the use of the pick to excavate it,
degenerate into the consistency of quicksand after a short exposure to a
running stream. Clays are very susceptible to atmospheric influences,
expanding and contracting under changes of temperature. Such strata
should be covered as rapidly as possible.
5. An unyielding foundation is, par excellence, the best, but where this
cannot be realised, the foundation must be but slightly and uniformly
compressible.
Waterftevef.
^o'Øl'
^L\3oft
I
^ i Rock
----! "' r-
Fig. 112. —Dock Wall at
Ardrossan.
rig. 110. —Section of Wall. Fig. 111. —Plan.
Herculaneum Dock, Liverpool.
The following are a few remarks on prominent varieties of earths :—
Rock, if of a good character, is the most valuable of all bases. It is
firm, durable, and unyielding. It involves, perhaps, a little more labour in
dressing to a level surface, but, in many cases, inequalities in this direction
may be met by benching in steps. Any fissures should be made good, and
unsound parts cut away. If the rock be of a soft nature, inclined to pasti-
ness, it should be well drained, and not allowed to remain long exposed.
If the site be such that the rock rises very nearly to the surface, the dock