Chap. XXV.
ON STONE BRIDGES.
1519
a mass of clay isthrown, or by drivingmain piles and lay-ing strong plankingacross them in a hori-zontal position, or bydriving one row ofgauging piles, and fill-ing the spaces betweenwith pile planksdriven vertically: atpage 445. is a de-scription of one of thelargest cofferdams re-quired; it is thereforeunnecessary to en-large upon it, or tomention others, the onereferred to having per-formed its duties ad-mirably.
Fig. 2782.
COFFERDAM.
iff
u,
JJJ^IXLIT
the next work, which
is done by an iron rod driven in by the pile-engine, or by an auger that will bringup the soil, as in sinking an artesian well. Soundings have been taken to the depthof 80 feet^by using a strong chest made of planks, about 18 inches square, the bottom ofwhich was shod with iron,that it might more easilyforce its way through thesoil: and M. Fauvelle, ofPerpignan , has described aneasy method of boring toany depth, either for theexamination of the soil orfor obtaining water. He ob-serves, “ if through a hollowboring-rod water be sentdown into the bore-hole asit is sunk, the water incoming up again must bringwith it all the drilled par-ticles : ” he had therefore anapparatus made composedof a hollow boring-rod ofwrought-iron tubes screwedend to end, the lower end
M
v\
A
Fig. 2783.
COFFERDAM.
being furnished with a per-forating tool suited to the strata in which it had to work; the diameter of the tool waslarger than that of the tubular rod, in order to form an annular space around it, throughwhich the water and the excavated material might rise up. The upper end of the hollowrod was connected with a force-pump by jointed or flexible tubes, which followed thedescending movement of the boring-tube for an extent of some yards; this tube may beworked in the ordinary way, with a turning handle or a jumper: the pump must be firstput in motion; a column of water is sent down to the bottom of the bore-holes throughthe interior of the tube, which, rising in the annular space between the exterior of thehollow rod and the sides of the bore-hole, creates an ascending current, which carries upthe triturated soil; the boring-tube is then worked like an ordinary boring-rod, and as thematter is acted upon by the tool at the lower end, it is immediately carried up to the topof the bore-hole by the ascending current of water: the usual necessity of drawing up theboring tube to clear it is therefore avoided, and a great portion of time is economised. Abore of 6 inches in diameter has been made to the depth of 560 feet in 140 hours atPerpignan, and this system is now universally adopted in France .
Cofferdam for building the River Wall of the new Houses of Parliament. — The excavationsfor the works were commenced on the 1st of January, 1839. The mud on the shore variedin thickness, and beneath was a bed of gravel 14 feet in depth ; under this was a stratum ofstiff clay, into which the piles were driven 2 feet, a trench being first dredged in the line