SLIPWAY FOUNDATIONS.
471
superimposed weight being spread over a large area. In the largest slip-
ways at present in existence, the weight of vessel and cradle does not exceed
10 tons per foot run, and in smaller slipways, it may be taken at one-fourth
less. Accordingly, where the ground is naturally very firm, little more than
mere surface dressing will be requisite, with perhaps a shallow bed of
concrete. In other instances, the site may require some dredging and
subséquent levelling with rubble filling to the under side of a concrete bed,
but in all cases of uncertain strata, bearing piles should be resorted to.
A very considérable portion of a slipway is necessarily under water, and
the construction of this section often presents some difficulty. At places
where there is a great range of tide, opportunities are afforded at low water
for getting the bulk of the work done without serious inconvenience. On
Figs. 459 and 460.—Slipway Construction.
the other band, where the tidal range is small, a temporary dam for the
exclusion of water from the site becomes a desideratum, if not a necessity.
The expense attending this mode of procedure is a deterrent to its ready
adoption. Under favourable circumstances, the work may be economically
carried out by divers in a sufficiently satisfactory manner. The following is
an account of the system as practised by Mr. John Thompson : *
“ When the portion of the site below low water had been dredged out to
the desired depth, the foundation was made by filling in broken stone of
* Lightfoot and Thompson on “ Slipways for Ships,” Min. Proc. Inst. C.E., vol. Ixxii.