i86
DOCK ENGINEERING.
it is evident that any perforation of the clay will allow the quicksand to
escape under the superimposed pressure.
3. A soft stratum of moderate depth overlying a hard one. In this case
it is advisable to found at the lower depth. If actual excavation of the site
be impracticable, the desired object may be attained by the use of bearing
piles, cylinders, piers, and the like.
4. A soft stratum of considerable depth. Means must be taken to
lighten the wall as far as is consistent with its stability, and to distribute
the weight over a large area. Framed timber rafts, mats of fascine work,
layers of rubble pitching, rows of logs laid horizontally—these are a few of
the methods adopted for equalising and reducing the pressure intensity over
foundations of this nature.
Artificial Foundations—Piled Foundations.—As the use of piles is of
wider application than the range of this chapter, they have been dealt with
Fig. 113. —Quay Wall at Rotterdam.
generally in a previous section (Chapter iii.). It only remains to add that,
for the purpose of dock walls, a very considerable advantage accrues to the
use of raking piles. Owing to the obliquity of the resultant pressure, there
is a considerable transverse strain upon vertical piles, whereas it is quite
feasible to drive the piles at such an inclination that this transverse strain
may be avoided, and, with it, the tendency to plough up the ground in
front. Instances of piled foundations are shown at Rotterdam (fig. 113),
Limerick (fig. 114), Sheerness (fig. 107), and Rouen (fig. 115).
Well Foundations.—-The principle of a well foundation consists in
causing a hollow shaft or cylinder to sink through a soft stratum by
excavating operations carried on from the interior, aided by weighting the
circumference, if necessary, until a firm bottom is reached, whereupon the