Autobiography of Sir John Rennie, F.R.S. 26

Autobiography of Sir John Rennie, F.R.S. 26

The design consisted of five semi-elliptical arches, the centre being 150

feet span, the two next arches 140 feet span each, the two side or land

arches 130 feet span each; the two centre piers were 24 feet thick at

the springing, and the two side piers 22 feet each; the whole was to be

built of the best grey granite. The width of the roadway was originally

designed to be 48 feet, but was afterwards increased to 54 feet wide, at

an extra cost of 46,000_l._

 

It was intended by my father that the new bridge should be built on the

site of the old one, which was to be pulled down in the first instance,

and a temporary wooden one was to be built above it to accommodate the

traffic whilst the new bridge was building. It was considered that

as soon as the fall of 5 feet occasioned by the old bridge should be

removed, the river would be restored to its natural state, and there

would then be less difficulty and expense in making the cofferdams and

founding the piers and abutments; the old approaches to the bridge would

then be preserved, and thus a less quantity of valuable property would

be required. The wish, however, of the Corporation to preserve the old

bridge during the construction of the new one was so strong, that there

was no possibility of resisting it. I therefore yielded to their desire,

and agreed to build the new bridge immediately above the old one, and as

near as practicable to it; notwithstanding, I felt at the time that there

would be considerable difficulty, risk, and extra expense in so doing, on

account of the great depth of water in which the piers must be founded,

namely, 28 feet at low water of spring tides, and the strong current and

fall through the old bridge both during the flood and ebb, particularly

during the latter. It should be observed that the old bridge stood as it

were upon a hill, the foundations of the piers being from 28 to 30 feet

above the bottom of the river immediately above and below it, occasioned

by the great fall and scour produced by the contracted waterway; thus

it was necessary to secure the piers by large projecting starlings, and

to throw considerable quantities of stone continually round them, in

order to prevent the old bridge from being carried away. However, there

was no alternative but to build the new bridge above the old one, and

I accordingly set about taking every possible precaution in order to

prevent accident.

 

As the loose stone thrown round the piers of the old bridge was

continually washed into the holes immediately above and below, it was in

vain to attempt driving the piles for the cofferdams of the piers until

this stone was removed, which was done by dredging. The cofferdams for

the piers were elliptical in form, this shape being the best adapted for

resisting the strong current in which they were placed; they consisted of

two main rows of piles each 14 inches square, each pile being properly

hooped and shod with wrought iron, and driven 25 feet into the bed of the

river. These piles were connected together in the horizontal direction

by three rows of braces 15 inches square, namely, one at the level of

the lowest tides, another at the level of half tide, and the third at

the level of high water. At every 10 feet the two rows of piles were

fastened together with wrought-iron bolts 2½ inches diameter, which

passed through the horizontal braces or walings, as they are technically

termed, and were secured outside and inside with additional wooden cleats

15 inches square and 8 feet long, so as to cover the joints where the

main horizontal braces met; outside of these were large iron plates, and

as the bolts were screwed at each end, they could be tightened up to the

full bearing without crushing the timber. On the outside of these two

main rows of piles was a third row of the same dimensions, and driven the

same depth into the bed of the river at a distance of 6 feet in the clear

from the two main rows, and connected together with a tier of horizontal

braces, and to the two main rows of piles with bolts, cleats, and plates

of the same dimensions as those already described. When the piles had

been driven to their proper depth, and had been properly secured to each

other as above described, the joints between every pile, which had been

previously fitted to each other, were well caulked with oakum, and the

outside joints were covered with melted pitch, so as to render them

water-tight; the spaces between the three rows of piles were then filled

with strong well-puddled clay.

 

In addition to the above three rows of piles, there was a fourth row on

the inside, driven down in the form of a parallelogram, corresponding

with the exact size of the foundation of the piers, and to the same depth

as the outer piles. Every tenth pile, and those at the corners or angles,

extended up to the level of low water. Upon the heads of these piles

longitudinal and transverse braces were fixed across the inside of the

dam, at the level of low water, half tide, and high water; so that the

dam was braced internally and externally in every direction to resist the

pressure of the water, like a well-made cask. There was a powerful steam

engine, with the requisite pumps, attached to each dam, to remove any

water which might either rise from the foundation or from the outside.

Each dam was provided with a trunk secured by a valve 3 feet diameter,

laid at the level of low water, so that in the event of any unusual

pressure of water coming against the dam, these valves were opened, and

the dam was then filled with water, and all mischief was prevented. The

first pier cofferdam on the Southwark side was completed, the water

pumped out, and the earth was excavated to the depth of 30 feet, going

below low-water mark of spring tides; the bottom consisted of the stiff

London clay. Piles, consisting of Baltic fir, elm, and beech, 22 feet

long, and 12 inches diameter in the middle, properly hooped and shod with

wrought iron, were then driven 20 feet into the solid ground, or until,

with a weight of 12 cwt. falling 18 to 20 feet, they would not move above

an inch at a blow. These piles were driven 3 feet 6 inches from centre

to centre, both in the longitudinal and transverse direction. After

having driven them, their heads were cut off and accurately levelled.

The loose earth between their heads was then removed, to the depth of 12

inches, and the spaces filled in with stone bedded in concrete; all the

rows of piles were then connected together in the transverse direction

by Baltic fir sills or beams 14 inches square, well fitted to each pile

head by jagged wrought-iron spikes 20 inches long and three quarters of

an inch square, driven through the sills into the pile heads below; the

spaces between the sills were well filled in with stone and brickwork;

another row of sills was then laid in the transverse direction above

the pile heads and spiked down to the lower sills in the same manner;

the spaces between the upper sills were then filled in with stone and

brickwork. The whole surface of the foundation was covered with elm plank

6 inches thick, closely jointed together and bedded in mortar, and well

spiked down to the sills below with jagged spikes 10 inches long and half

an inch square. Upon this platform the masonry was built, each course

diminishing in length and width by a series of offsets 12 inches wide,

until they reached the shaft of the pier, when it was carried up solid to

the springing of the arches. The whole of the exterior masonry was of the

best whitish-grey granite, and the interior stone was of the best hard

Yorkshire grit stone from the quarries of Bramley Fall. The abutments

were constructed upon piles and masonry of the same character as the

piers.

 

The first stone was laid with considerable ceremony on the first pier

cofferdam from the Surrey shore by His Royal Highness the late Duke

of York. The dam was fitted up with great taste like an amphitheatre,

with seats all round, the whole being covered at top with a handsome

coloured canvas awning adorned with numerous flags of all nations. The

Lord Mayor, assisted by the Aldermen, Common Councilmen, and Committee,

with Mr. Jones, the Chairman, attended in great state, and everything

went off well. After this pier had advanced nearly to the level of high

water, one day whilst examining it, standing upon one of the cross

beams, my foot slipped, and I fell headlong into the dam upon the top

of the masonry; fortunately, my left foot caught in a nail in the beam,

and I hung by it for a few seconds. This somewhat broke and changed the

direction of my fall, and I pitched upon an inclined plank, upon which I

slid until I struck my head against a stone; my hat deadened the blow;

as it was, however, I was cut about the forehead and half stunned. The

after effects of this fall were very serious; my whole system got such

a severe shaking, that I did not recover thoroughly until nearly ten

years afterwards, and I carried on my large professional business with

the greatest difficulty. The works made satisfactory progress, and the

centres for the first and second arches from the Surrey shore were soon

fixed.

 

Each centre was composed of eight ribs, framed upon the truss principle,

resting upon a continued series of wedges in one piece, laid horizontally

and resting upon tressels or legs formed by the piles of the cofferdams,

which had been cut off and levelled for that purpose. The mode of setting

and fixing the ribs was the same as that adopted at the Waterloo and

Southwark bridges. A large lighter was constructed especially for this

purpose. In the centre was placed a strong framing, which rested upon

eight screws, four in each row, working in a strong cast-iron box, to

which levers were attached, by means of which the screws and framing

above them could be gradually raised and lowered at pleasure; at one

end of the framing there was an upright scaffold. The centres, I have

already said, consisted of eight main ribs framed together separately.

As there was no room to frame these centres near the bridge, a special

workshop and wharf were provided by the contractors at Millwall, in

the Isle of Dogs; when ready they were launched in one piece, from a

properly-prepared platform, into the river, and towed to the Southwark

end of the bridge, where the lighter, with its apparatus of powerful

sheers, crabs, and tackle, was in readiness to receive them; by these

means they were gradually hauled up an inclined plane, and then raised

upright upon the platform, supported by the frame and screws beneath,

and firmly braced to the scaffolding in the lighter; two centre frames

were thus placed upon it at one time, and adjusted by the screws to an

extra height of 2 feet, so as to allow for any deficiency in the rise of

the tide. Two ribs having been placed upon the framing, the lighter was

hauled off from the shore and placed in front of the opening in which the

centre was to be, the lighter being moored 100 yards from it, about half

an hour before high water; upon the turning of the tide it was gradually

allowed to float down with the ebb current to its place. By the time that

the lighter with the centre ribs arrived in its exact position there was

always 2 feet to spare, in order to allow for any deficiency of the tide;

as the tide fell the two ribs were securely deposited in their places

upon the framing and wedges below them. It should be observed, that upon

the wedges there was an additional framing so as to reduce the weight

of the main ribs of the centring. When the main ribs had been deposited

upon the framing wedges, they were securely braced together until the

whole number of ribs required for each centre was fixed, when they were

all braced firmly together longitudinally, transversely, and diagonally.

This mode of fixing centres for arches of any span was most successful

and economical, and I believe that my father was the inventor of it, if

it may be termed an invention. My excellent and talented friend, the

late Robert Stephenson, adopted the same method for fixing the tubes of

the Conway and Menai Straits bridges. He told me that he was not aware

that my father had proposed it before him; but in the ‘History of the