laid, ten years previously, by Mr. Stoney, consisting of a pivot well, lined with brickwork, 3 feet thick, set in cement mortar. The pivot step and the bed plate of the crane being fastened down through the same, rested on a foundation of concrete, composed, so far as the Author could judge, of about 4 parts of gravel to 1 part of cement. The bond of the brickwork was so perfect that steel wedges used in its removal generally broke the bricks. The concrete foundation, of which the contents were 2,400 cubic feet, had become one solid block. Holes 16 to 18 inches deep were drilled, and charged with about 3 inches of powder to each hole; four or more holes were fired simultaneously, by which means the concrete was torn up into large irregular blocks of from 3 to 5 tons each, with a few smaller pieces. The materials of which the concrete was composed were similar to those used in the construction of the new wall, being clean sea gravel, full of small boulders or paving stones from 1 lb. up to 5 lbs. in weight, with an admixture of gravel and sand. When broken up it was moist, but dried when exposed to the air.

So soon as a length of about 200 lineal feet of wall had been built, a cross dam was constructed between the wall and cofferdam by fitting a timber 12 inches square against the face of the wall, opposite a main pile of the cofferdam, and bolting thereto vertically, at intervals of 5 feet, horizontal framing 12 inches wide by 6 inches deep; within which 3-inch close sheet piling was driven. Raking struts were carried from the whole timber against the wall, and secured to the main piles of the dam at suitable distances. Clay puddle was tipped in behind the sheeting; and a row of short piles was driven in at the toe of the slope of the puddle, to prevent it spreading. This enabled the cofferdam to be drawn for the length thus cut off. The drawing was accomplished by a frame termed a "gallows." This, which was about 9 feet high, consisted of two uprights 12 inches square let into a frame, 12 inches wide by 6 inches thick, and stayed back thereto. Upon the uprights rested a cross piece of elm 12 inches square. From this cross piece two three-sheave blocks were suspended, through which a short link chain, inch in diameter, was reeved, with the running end passing through a snatch block, secured to a cross piece of hard wood, made fast to the foot of the gallows. The chain led to the drum of a strong double-purchase crab winch, fastened to two longitudinal runners, 12 inches square, placed as a road on the top of the dam. The gallows was moved on the runners on small iron rollers over the pile about to be drawn, and the sling chain attached to it.

On drawing the piles of the first section of the cofferdam, it was

found that where the piles and struts had been 12 feet apart, nearly the whole of the inner row of the dam had broken off at about 6 feet from the bottom, or at 18 feet below low-water level; and that where the 10-feet spacing had been adopted very few of the piles were broken. As a rule, although the piles were not shod, very few of the points were injured by the driving.

At the "return to Great Britain Quay, the cofferdam was extended in a direct line with Sir John Rogerson's Quay, 100 feet into the river; returning to the Grand Canal Dock entrances, the depth of water for more than 250 lineal feet was 14 feet at low water, so that it was considered advisable to drive a double dam for a distance of 100 lineal feet in the deepest portion of the water, with dams at right angles, and at short intervals, to the old wall of Great Britain Quay, to lessen the risk attendant upon the exclusion of so much deep water.

To prevent the dam from being pressed outwards, a foreshore of dredged material was deposited on the outside up to the level of low water, and piles and struts were placed from the inner dam landwards. Besides being in a great depth of water, it was discovered that the bed of gravel upon which the wall was founded dipped considerably at this place; consequently the piling had to be driven to 28 feet below low water, the depth at which the sound gravel bed was met with.

The greatest amount of leakage always took place at the junction of the forward cross dam against the old wall, through which the only serious "blows occurred. The footings of the old wall extended underneath the cross dam, thereby preventing the sheet piles immediately abutting against the wall from being driven farther than the footings; and as the bottom rows of old ashlar had been built with but little mortar, the water permeated through the courses for a distance of from 50 to 100 lineal feet beyond the junction of the wall and cross dam, washing away the fine sand foundation on which the wall rested.

After one or two "blows" from this cause, the Author drove a row of close sheeting 12 inches wide by 6 inches thick across the excavations, in two tiers, at the distance intended to be taken out as a length, the top tier sufficiently high to prevent the sand from being washed over. From this up to the old wall, a layer of puddle, well weighted, was placed. The "blows" were thus prevented. The excavated material was also tipped for a foreshore on the outside of the dam, which greatly assisted in equalising the pressure from the land side, and so prevented the dam from "working" with the rise and fall of the tide.

[1877-78. N.S.]

L

The total length of river wall was 2,310 lineal feet, built at the contract sum of £131,728, or at the rate of £57 108. per lineal foot, exclusive of dredging the foreshore from low-water level to the present level of the bed of the river opposite this wall, namely, 22 feet below low water.

The entire quantity of masonry was 44,500 cubic yards, consuming 85,500 tons of stone and gravel.

The timber in the cofferdam strutting, staging, &c., amounted to 156,000 cubic feet of whole and half timbers, and 21,000 cubie feet of planking. Timber not cut up and wasted on the work, when removed, was of little service, except for rough staging on works of a similar class, owing to the quantity of bolt-holes and spikes with which it had become studded.

Constant supervision had to be observed in carrying out the works, most of the materials having been conveyed overhead during the time men were working underneath. Fortunately no serious casualty occurred in connection with the works.

Considering that cofferdams occupy a place of the first importance in most temporary arrangements for the exclusion of water from the sites of docks and harbour works, the Author is of opinion that a few additional examples of what has been done, with a statement of the difficulties encountered, will be acceptable. He therefore proposes to describe two of the cofferdams previously referred to, viz., the cofferdam at the Low-water Basin, Birkenhead, and the cofferdam at the Victoria Dock Extension, Hull.

THE COFFERDAM ACROSS THE ENTRANCE TO THE LOW-WATER BASIN, BIRKENHEAD (PLATE 2).

Mr. John Ellacott, M. Inst. C.E., in his Paper on the Low-water Basin, Birkenhead,1 alludes to the destruction of this cofferdam, and the Author, who was engaged on the works for the contractor and made notes at the time, is enabled to describe its construction and to place the most prominent particulars of the work before the Institution.

This cofferdam was constructed in the years 1859-1860, and was on plan the segment of a circle whose chord was 467 feet, and versed sine 76 feet. The width at the top of the dam was 18 feet, and at the ground line 23 feet. Its extreme height from the toe of the outer row of piles to the top of the dam was 61 feet

1 Vide Minutes of Proceedings Inst. C.E., vol. xxviii., p. 525.

6 inches. The dam was formed with two rows of whole piles, battering both ways, the outer row in lengths of 35 to 40 feet, 12 inches wide by 14 inches thick, driven close as sheet piles, with toes reaching to 35 feet below Old Dock Sill; the inner row, 30 to 35 feet long, of the same width and thickness, driven also as sheet piles, with toes reaching to 30 feet below the same datum. On every fourth pile of both the outer and the inner rows was placed an upright 12 inches wide by 14 inches thick, by means of a scarf 4 feet long, secured by four 1-inch bolts, reaching to the level of 3 feet above H.W.E.S.T., the whole being secured by five tiers of wales, placed on the outside of the piles and uprights at vertical distances of about 8 feet. Through these wales were passed, at longitudinal distances of 12 feet, a series of 2-inch wrought-iron tie-rods, screwed up on each end with hexagonal nuts on cast-iron washers 9 inches square by 2 inches thick.

At the level of high water of spring tides, an inner wale, 12 inches square, was fixed on both outer and inner rows of uprights, to which diagonal horizontal struts or braces, 12 inches square, were bolted, for the purpose of keeping the uprights asunder to the proper width at the top. The whole of the interior of the dam on both the outer and the inner row of uprights, from the level of Old Dock Sill to the top, was sheeted with horizontal planking, 3 inches in thickness, to hold the puddle..

The original surface of the foreshore at the centre line of the dam was about 10 feet below Old Dock Sill, rising gradually at either end of the segment to about 7 feet below Old Dock Sill. The excavation within the dam was taken down to 18 feet below Old Dock Sill before the puddle was put in. The ground upon which the dam rested was chiefly fine sand; this, when saturated with water, became a quicksand.

The timber specified to be used was crown Memel, but it was ascertained, when piling proceeded, that for half the distance on the north side, and about 90 feet on the south, the Memel could not be driven on account of the compactness of the sand, and American elm piles were substituted.

Excavating within the dam was a difficult operation, requiring to be done as tide work in short lengths. Two sets of chain pumps capable of throwing 2,000 gallons of water per minute were fully employed in carrying the excavations down to the specified depth.

After the dam had been filled with puddle to the height of 12 feet above Old Dock Sill, the cast-iron washers began to sink into the wales and to break. To remedy this, pieces of American elm

3 feet long by 12 inches wide by 6 inches deep were placed under the washers. Then the wales exhibited signs of giving way where the tie-rods passed through. As a precaution, additional wales were put on between each of those originally in place, with an extra tie-rod driven through each additional wale at the intermediate distances, with a vertical piece of elm overlapping two wales, so as to distribute the bearing surface. The tie-rods were screwed up over these vertical chocks. This proved effectual in keeping the puddle in place.

When the puddling of the dam was near completion, the structure showed symptoms of going out towards the river, particularly near the crown of the segment. To prevent this, an extra row of piles was driven for a distance of 200 lineal feet, at 30 feet outside the dam, 8 feet apart from centre to centre, upon which a waling 14 inches square was bolted as low down as the lowest tide would permit. From this waling, raking struts 14 inches square were placed, reaching to the underside of the wale on the dam at the level of 8 feet above Old Dock Sill. It was considered expedient, during the time the dam was in place, not to remove a quantity of contiguous soil intended to be excavated, so as to prevent a "blow."

All the other portions of the work within the Low-water Basin being completed, Mr. Lyster, M. Inst. C.E., the Engineer to the Mersey Docks Board, ordered the excavation to be proceeded with; the object, as stated by Mr. Ellacott in his Paper already referred to, was "to lessen as much as possible the subsequent dredging." Wagon roads were therefore put in by the Contractor, and the work rapidly proceeded.

When about five-sixths of the excavations had been made, the water commenced bubbling up in half-a-dozen places, at about 30 feet behind the dam. Observing this, the Engineer ordered additional struts to be placed on the Basin side, as the structure commenced to warp a little, and the puddle to settle down slightly; the bubbling up of the water, however, continued, and after a day or two the nuts of the tie-rods began to fly off. It seemed evident that the dam would not much longer resist the undermining and the pressure from without. The Contractor, therefore, removed his wagons and some of the rails; but a large portion of the latter remained behind, when, on the morning of the 18th of June, 1863, the dam suddenly gave way.

The head of water, 33 feet deep, rushing in to fill the empty space of 14 acres, scoured a channel 30 feet in depth and 50 feet in width within the Basin. A large portion of the piles were broken