The Sun.
London, England, 1866-10-18 00:00:00

New Blackfriars Bridge.

Within the memory of living men almost every
metropolitan bridge has been built. Waterloo is a
comparatively modern structure, and even South-
wark is older by a year than that which is supposed
to be the oldest—New London-bridge. With the
exception of Putney and Battersea bridges, which
can certainly not be called metropolitan, nearly all
are of very recent date. Hungerford, Lambeth,
Chelsea, Westminster, have all, one might almost
say, been built within the last few years, and the
railway bridges are almost, comparatively speaking,
the growth of a few months. The contributions of
the latter to the adornment of the river, however,
have been very small, and, small as their ornamen-
tal aid has been, they have unfortunately been placed
in such unlucky positions as to hide or mar the
effect of something very much better than them-
selves. This is pre-eminently the case with the new
Blackfriars-bridge. Nothing can relieve its aspect
from the bizarre aspect produced by the lattice
girder bird-cage-looking railway structure beyond
it. Either from up or down the river the view of
new Blackfriars-bridge will be totally spoilt by this
juxtaposition, just as the beautiful suspension bridge
over the Conway is spoilt by the tubular girder
alongside it, and as another monstrous girder spoils
the light chain which spans the Menai Straits. So
also with the new bridge at Blackfriars, the railway
viaduct will be the death of it in an architectural
point of view. This is to be regretted, for probably
no structure of its kind in Europe can be said to
equal it either in design or magnificent solidity of
workmanship. The total length of the bridge from
end to end is to be 922 ft., and this enormous stretch
is carried on only five arches. The centre arch has a
span of no less than 1851 t., the arches on either side
of this have a stretch of 175 ft., while even those which
abut immediately on the shore have a headway of 155
feet. The centre arch will give 25 feet elevation
above high-water mark, but all the arches are'
almost flat iu the crowns, and form but a small seg-
ment of a circle. In fact, the gradient of the whole
structure is only 1 in 40, or just half that of the old
bridge, and less than a quarter of that of Holborn-
hill. The five piers which are to support these
arches are of the most massive description—each is
130 feet long by about 20 feet in width. The
piers inside two of the caissons are about half
completed. Massive granite work, which is laid on
its concrete foundations far below the bed of the
river, and, in fact, some feet into the London clay,
have already been built above low-water mark. All
the caissons for the other piers are more or leas ad-
vanced in progress, though, from the peculiar engi-
neering difficulties offered by parts of the bed of the
river, some are built much more slowly than others.
Thus on the Surrey side the London clay is reached
with comparative ease, whereas on the Middlesex
shore, even at a considerable depth below the bed of
the river, the soil is entirely composed of gravel,
and through this the water percolates with such
rapidity that almost incessant pumping is necessary
to keep the interior of the caissons free from it y;hile
the granite blocks for the foundation are being laid.
Two, however, as we have said, have been completed,
and the piers nearly built to high-water mark. The
caissons inside which the works are carried on are
the largest ever sunk in the river, and six are re-
quisite for the building of one pier. They are ordi-
nary boiler-plate chambers, of wrought iron, four
of them being rectangular, and two at each end
being pointed or triangular in shape, to form the
cut-water of each pier. It would, we venture to
think, have saved both time and money had these
caissons been sunk entire, instead of, as now, in six
divisions, for the parts where the divisions occur
have to be carefully piled between and made water-
tight before the masonry between each section can
be built up from the base of the foundation. The
plan of sinking these caissons is, however, as simple
and inexpensive as can well be devised. Guide
piles are driven to keep them in their proper places
first, and between these a cross staging is erected
at low water to support the wrought-iron cham-
ber. On this, then, is built piece by piece a
boiler—the first segment of the caisson. When it
has reached the height required to keep its upper
rim above the water at low tide it is lifted bodily
by powerful steam cranes, and lowered between the
guide piles into its place on the bed of the river. At
the next low tide, whether it be during night or day,
the workmen are engaged in screwing on plates to
compose another rim, to raise it still higher. Then
as the pressure of the water increases on its sides it
is strengthened from the interior not only with strong
piling, but with powerful wrought-iron ribs, which
are bolted all around it, and these again where the
pressure is greatest are supported by lattice girders
of wrought iron. It will readily be imagined that
these extra supports are necessary when we say that
the pressure at high tide on the side of the caissons
for one pier amounts to about 600 tons. Of course,
as fast as these great cells of wrought iron are
lowered into the river their own weight takes them
deep into its bed, and when they cease to sink of
themselves they are weighted with masses of iron
ballast till no additional pressure can get them
further down. They are then pumped out till per-
fectly free from water, when the work of dredging
out the soil commences. Generally, as this is done,
the caisson sinks a little more, till what is called the
London clay is reached. This, which is usually
called the blue clay is very hard, and more than a
foot or so into it nothing can be forced. On this
impenetrable substratum, then, when all inside the
caisson is perfectly dry, the foundations of the piers
are laid on a thick mass of solid concrete, The
centre of the pier is built of brickwork set in cement,
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tides no sign of the ironwork will be visible above
the granite. So also none of any piling which
may be used around the piers will be "drawn," but
all will be sawn by divers just above the earth. It
has been often said that these wrought-iron caissons
are liable to rust and decay, which of course they
are ; but if they all disappeared within a single
twelvemonth after being finished it would have no
more effect on the piers themselves than a change of
the weather, for these caissons are simply thin
wrought-iron chambers to keep the water out while
the massive granite foundation is being built insideit.
At the same time it may be said that this wrought
ironwork when well beneath' the water is not nearly
so liable to rust and decay as many persons suppose.
Thus, in taking down the old bridge, in the very
foundation of one of the piers, a steel trowel was found
irnbedded in the centre, and this was actually as un-
tarnished as when it came from the workshop. The
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