The Sun.
London, England, 1852-03-08 00:00:00

THE ,vjeTROPOLITAN BRIDGES.
The state of the bridges over the teem:, nee at-
tracted a good deal of attention, and not a few have
pronounced even London-bridge in danger. We are not
competent to judge of the gravity of the defects in the
latter, to which our attention has been called; but two
bridges' at least, are at this moment in a condition to
demand a costly expsnditure either to repair or to re-
new. Westminster-bridge is condemned, and a new
structure must be. raised to supply its place. The
funds for effecting this will be drawn from the whole of
the taxpayers of the country. Blackfriars-bridge be-
longs to the corporation of the city of London, and the
moneys to be expended upon this bridge will be derived
from more circumscribed sources. The bridge at West-
minster has long been under the consideration of Go-
vernment; the site has been determined on, and the
Commissioners have given notice of, their intention to
apply to Parliament for a 8111 to reconstruct this
bridge. No design has yet been adopted, but various
plans have been proposed, among the number one
40unded on the system so successfully carried out by
Mr. Hughes in the preparation of the' foundations
for the new bridge at Rochester. We lately had an
opportunity of inspecting these works, and, as
the mode of constructing these foundations appears
not only admirably contrived, but particularly appli-
cable to the repair or renewal of bridges in the metro-
polis, we propose to give a short description of the
work. The bridge is to be carried over the Medway
with three arches—a central one of 170 feet, and an
arch on either side of 140 feet span. The abutments
and the piers (above low water-mark) will be built of
stone and brick in the usual way, and the arches will be
of cast iron. The roadway is designed to be 40 feet
wide, and an opening bridge of 50 feet in width is to be
made on the Strood side for the navigation of the Med-
way. During the season of the Great Exhibition the
works in progress were visited by several of the first
engineers of the Continent, and last Thursday the con-
tractors, Messrs. Fox and Henderson, entertained a
large party of eminent men, invited to witness the sink-
hag of the last pile upon the plan devised
to meet the unexpected exigencies of the occasion.
The Strood-pier is completed up to a little above low
water level, and can only he seen at the time of the
tide. The foundation consists of massive cast iron be d
plates, covered with concrete, resting upon a series of
east iron cylinder piles, sunk through the bed of the
river into the chalk, 46 feet below the concreted surface.
These piles stand about five feet above. the bottom of the
river, and are enclosed by a curtain of cast iron plates,
serving to direct the flow of the current and to protect
the piles from injury between ordinary and extreme low
water level. This pier, as well as the other on the
Rochester side, measures over the surface of the bed
plates 70 feet 'between the points of the cutwaters, and
18 feet in width. The iron cylinder piles which sup-
port the bed plates are arranged in two rows of six in
each and one pile at either end, carrying the points of
the cutwater; they are 7 feet in diameter and are formed
of cylindrical castings 9 feet long, bolted together in
succession as the pile enters the ground. The stones,
gravel, chalk, &c., are excavated and entirely removed
from the interior of the. piles, and a concrete of gravel
and Portland cement is substituted when the pile has
reached a stratum sufficiently hard to support the weight
of the bridge.
To sink these piles 40 feet into the bed of the river,
and to effect the excavation where, st low water of ordi-
intro tides, there is 5 fleet, and a rise of upwards of 20
feet' at high tide, was, tinder the cireatestances which
presented themselves here, a problem of no common
character. In suitable soils (and this was at first
thought to be one of them) the vacuum method, which
was patented by the late Dr. Potts, would assist in solv-
ing the question, if it did not succeed for the whole of
the great depth of these foundations. An alternative
presented itself of clearing the piles from water, by
means of powerful pumps, driven by steam-engines of
from 12 to 20-horse power, and working out the excava-
tion when the bottom was laid dry. Such was the me-
thod followed on a magnificent scale by Mr. Brunel with
the greater part of the cylinder piles forming the sup-
ports for the remarkable bridge intended to carry the
South Wales Railway across the Wye, at Chepstow; and
~uch suggested itself as the resource to be called into
action, after a few trials had proved Potts's method
to he quite unavailable at Rochester. A careful
examination of the ground forming the site of the
Strood pier, during a very extraordinary low tide,
which occurred shortly after the attempts with the
vacuum method had failed, and when measures were in
progress for introducing water-pumps, brought out the
fact, not previously known, that the bottom of the river
there consisted of large rubble atones, intermixed with
old but solid timber for a considerable depth, not less
than six feet, as was then estimated, but which has
proved,to average 20 feet; and it became evident that
any effort of pumping would cause a very large profitless
outlay of money, and would result very much as would
an attempt to make the surface of a sieve dry by with-
drawing the water above it in the same way.
The principle of the diving-bell had been previously
spoken of by many, in general and indefinite terms, as
being probably useful in connexion with the system of
iron cylinder piles; and it was now seriously discussed
by the engineer and the contractors in all its forms, from
the employment of divers in the helmet and caoutchouc
dress to the use of the ordinary diving-bell, suspended
inside the cylinder pile. It was sufficiently obvious that
the cylinder pile itself might have the character of a
diving-bell given to it by securely closing the top, and
by forcing air into it until this had acquired a density
sufficient to force out the column of water; but how to
get the workmen into and out of a large iron vessel so
closed up, how to carry off the stones, earth, &c., which
had to be removed from its interior, and how to in-
troduce the concrete and and brickwork for tilling it up,
although equally obvious as conditions that must be
satisfied (and satisfied, too, in a well-ordered system to
be carried into operation as any process usual in a pro-
perly regulated manufactory) were difficulties at first
very imperfectly resolved.
Mr. John Hughes, as the engineer upon whom it de-
volved to devise and arrange the preliminary works and
mechanical means for carrying out the contract which
Fox, Henderson, and Co. had entered into, offered
some suggestions to Sir W. Cubitt and to Sir C.
For, which were favourably received, and they
assigned to him the task of making the draw-
ings and scheme 'complete, of constructing all the
necessary apparatus, and of carrying it into operation.
This. duty he performed, and has described his contriv-
ances in a paper read at the Institution of Civil Engineers
last year. A technical account of the apparatus, which
all who have seen it pronounce to be most ingenious,
well-considered, and effective, would be unsuited to our
columns, and is already recorded in the proceedings of
the Institution of Civil Engineers; but we may quote
the address of Professor Airy, when President of the
British Association, at Ipswich, in July last, as conveying
a popular idea' of it. The Astronomer Royal said—
" Considerable importance, however, is attached by en-
gineers to some of the processes lately ietroduced, espe-
cially that of thrusting down an air-tight tube or elon-
gated diving-bell, supplied with air at the proper pres-
sure, by. whicn men are enabled to perform any kind of
work under water, in almost any circumstances, and in
which men or materials may be transferred without
disturbance of the apparatus, by a contrivance bearing
the same relation to air which a common canal-lock
does to water."
Teat the working of this useful contrivance has been
eminently successful may be judged from the fact that
the 14 piles in the Strood-pier of Rochester-bridge have
been excavated 40 feet into the • bottom of the
Medway, and this was done without regard to
the period of the tide, or of the day. The
pressure of air in the pile averaged about 18
pounds per square inch, the maximum being nearly 26
pounds, or equivalent to a depth of 60 feet of water. In
the Rochester-pier 13 of the piles have been completed
to a depth of 22 feet in the bottom of the river, and the
maximum pressure during the execution was about lb
pounds to the inch. At Chepstow two of the cylinder
piles were sunk 48 feet below the bed of the river by
MreHughes's arrangement, under the direction of Mr.
Brunel, the maximum pressure being 28 pounds to
the inch.
After satisfying ourselves of the solid character of the
completed platform for the Strood-pier, we crossed over
to witness the sinking of the 14th pile for the Rochester-
pier by means of the apparatus just noticed. This pile,
standing on the bottom of the river, contained water to
the same level inside as on the outside, which was 7 or 8
feet deep at the commencement of a flood tide. The air-
pump, driven by a neat six-horse power portable steam-
engine. being set to work, the pile was tree from water
in five minutes, the workmen then passed through the
cages or air-locks and descended to the bottom of the
river and proceeded with the excavation. Bucket after
bucket was sent up through these air-locks until suffi-
cient material had been cleared away to admit of the
cylinder's further descent. The men were then recalled,
the air let off, and, as the water rose, the cylinder, by its
own weight, sank to a new footing. The process is thus
repeated until the men have been enabled to carry the
foundations to the necessary depth.
To say nothing of the complete safety under which the
work can be carried on, and the small space of the water-
wayfor this system of foundations are b
means of coffer-
occupied during its progress,the atbdeviarntages
cheapnessea i meads
compared with foundations formed
with which the superstructure can,
of rock or of
timber piling, be raised on a bed.
dams, whether of timber or iron,
ywanitdhouthtes introduc-
ingee nrityadded to these advantages the power to constructeAairrt.hH, kungobwesn,stmo
ebteboedqoufalsintokinthgetbleosaedcyttlinhdaesrtpoilceasrbrays.
are
est
foun-
dations under circumstances where they have hitherto
cboemenotihsohningghta ifmoupnrdaacttiicoanbleof; the time required for the-
cs-
taoppbleicanboolueyired, are also reduced to a certainty
sus we
pable of exact calculation.
con-
sideration, t h
judge to' what extent
It becomes now a national question of grave
tcoa
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tthergoi
metropolitan
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tvaanntba magnitude,rgenumeratediesges
Thames,
it nhse t
eitheresri t
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the public advantage.
these
new
means
may
in order
attention to these works
that the , public, as 011 as the bridge wardens, may
aide at
sevenAs acastiron arches, resting rioudien
taketpv
leeeskt
might
the the matter of more extended
gchasstteobnelfbsaWtielptesOontan public interest, let as
be used for
having the same width for its entireA new bridge of
piers of cylinder
the present bridge.
length as that be-
bridge. The present roadway would each end of the
and would serve to carry the' trafflo until the.lll
undisturbed,
outside portions of the new bridge were facesr or
shed;
ttlese:would each be 27 or -28 feet wide: the
over the old bridge would then be closed. ThoPtage
both traffic being diverted to the northern .portionin-
and ;the Westminster to the southern, would circulate'
freely while the present arches wero in course of re.
naoval. The remaining cast iron arches might then be
fixed, and the whole of the new bridge completed and
oavnedr t
thethe
opened for use, the piers would be taken away, and the
bed of the Thames be cleared from debris. All the re-
quirements imposed by the authorities who control the