Construction of Corbiere lighthouse
by Imrie Bell, who built the lighthouse - 1880
The employment of concrete as a building material, though extensively adopted by the Romans, had for many centuries afterwards fallen out of use and become obsolete. On the resuscitation in modem times of this art of building, its use was almost entirely confined to the formation of a monolithic mass underground, to serve as a foundation for the stone or brickwork of the superstructure. French engineers appear to have been foremost in appreciating the value of beton, or concrete, for sea works.
During the earlier period of its employment for such works, the natural hydraulic limes were used as the cementing material, with the addition of puzzolana. Afterwards, from the valuable and exhaustive researches of Vicat, and the progressive improvements made in the manufacture of Portland cements, a vast impetus was given to the introduction of this art in the construction of buildings of all descriptions.
Concrete can be made under almost any circumstances, at a moderate cost ; it is easily formed into any shape of block that may be desired; it can frequently be deposited in moulds in the exact position it is per manently intended to occupy ; and, when carefully made, with a proper admixture of materials, which is a matter of vital importance, experience has proved that it possesses ample strength and durability under the most trying circumstances. There may be cases in which it might not be advisable to use concrete as a building material ; and an engineer requires to give carefal consideration to locality, the various kinds of materials available, cost of labour, carriage, and many other matters, before coming to a decision as to the material most suitable for the construction he has in view.
The author has been much struck by the want of attention paid to the art of producing a fair and finished surface in the exposed faces of concrete, as exemplified in many of the large engineering works in course of construction, where the exposed face has a honeycombed appearance, as well as the marks of the rough timber planks forming the frames in which the concrete has been placed. The Author has given this matter much consideration, and the result of his experience is that, in concrete building, it is easy with a little attention not only to produce a fair surface, but to form mouldings, and even tracery and ornament, and at the same time to make the facework as durable as any other part of the block.
There appear to be two reasons why but little attention has hitherto been paid to this art; first, carelessness or indifference to appearance; second, that most of those who have attempted it have done so by rendering with plaster or by grouting with liquid mortar, both objectionable and dangerous modes of effecting the object. These are at the best only a veneering liable to sudden decay, and the failure (generally occurring after wet and frosty weather) has naturally stopped a repetition of the attempt.
The plan which the author has followed in harbour walls, both above and below low water, exposed to frost, heat, storm and rain, with complete success and at an inappreciable increase of cost, is to have a smooth planed board for the face of the mould, painted over previous to commencing the work with a mucilage of soap to prevent the mortar adhering. In filling the frame, care must be taken that a finer mixture of concrete or coarse cement mortar be laid in with a trowel close to the face board, as the work proceeds, so that the mixture is carried up uniformly with that contained in the body of the work, the whole forming one homogeneous mass, and ensuring the setting process of the whole mass being carried out simultaneously, so that the face is, in fact, like the skin of an iron casting, and actually the strongest portion of the mass.
It is intended, in this paper, to give a description of the construction of the Corbiere lighthouse, which the author believes to be the first of the kind in the British Islands made of Portland cement concrete. It was erected by him as executive engineer for the States of Jersey, from the design of Sir John Coode.
La Corbiere rock, upon which the lighthouse is erected, lies off the south-western point of the island of Jersey , in latitude 49° 10' 40" north, longitude 2° 14' 60" west. It is distant from the mainland about 1,600 feet, is isolated at high water of all tides, but is accessible over a ledge of rocks, shortly after half ebb tide up to nearly half flood of each tide, when the sea is smooth — not a matter of frequent occurrence in this exposed part of the island.
This has rendered necessary, as one of the works connected with this undertaking, the construction of a causeway, to ensure the safety of the lightkeepers on their passage to and from the lighthouse. The range of tide on this coast is 32 feet at ordinary springs, and 23 feet at ordinary neaps.
The site chosen for the lighthouse being inaccessible, by land, rendered it necessary to commence operations by the formation of a road of access, extending over ½ mile in length; and along with this were constructed dwelling houses on the mainland for the lightkeepers, with the necessary storehouses and outbuildings; but there is nothing in this section of the works worthy of special notice.
The formation of the tidal causeway, over a ledge of rocks exposed to the full force of the swell of the ocean, although a subsidiary work, required much consideration. This causeway had to be built over a very irregular ledge of rocks, fall of deep pools and fissures. Owing to its submergence at each tide, the time during which operations could be carried on was limited, and they could only be proceeded with from the shore end, which retarded the rate of progress that might otherwise have been made.
It exceeds ¼ mile in length, is 6 feet in width at the top, and is formed of two side walls built of granite blocks, with a batter on the sides of 1 to 2, the height varying from 1 foot to 8 feet ; the blocks are hammer dressed on the face, and laid on level beds in cement mortar. The space between the walls is filled with Portland cement concrete, in the proportion of 8 parts of shingle and coarse sand to 1 part of cement ; the upper 8 inches being made stronger and finer, in the increased proportion of 4 parts of shingle to 1 part of cement.
The action of the sea during gales prevents any great accumulation of sea weed upon the surface of the causeway; and a sprinkling of hot lime in calm weather, afterwards brushed off with birch brooms, is found sufficient to prevent any growth.
Arrangements for the transport and landing of building material
The most important consideration connected with the arrangements for the construction of the tower was to devise a scheme for transporting the materials to the rock and landing them upon it, as this point, it was seen, would materially influence the cost of the works. The question being whether to take the material overland to some convenient point on the shore, or to transport it by sea; in either case there was no possibility of landing it upon the rook itself because the sea at this point of the coast is seldom still. When the tide commences to flow there is always a heavy swell, which upon so rough and rocky a coast renders it an impossibility to moor a barge with material close to the rock for the purpose of loading or discharging. This, therefore, entailed the necessity of landing the material at the nearest point on the mainland where a sheltered spot couid be obtained; in fact, except in calm weather, it is unsafe to approach the rock even by a rowing boat, after the causeway is covered.
In stormy weather the waves are carried to a great height ; for instance, on the completion of the lighthouse, during a storm in December 1876, a body of water was carried up the gully, and broke up the concrete platform placed 15 feet above high water spring tides, at the site of the temporary workshops and stores, and conveyed portions of it, weighing from 3 to 4 tons, over the side of the rock. The lightkeepers stated that sheets of spray during the same storm were thrown over the top of the lighthouse, or to a height of 136 feet above high water mark.
After giving the whole matter foil consideration, the author decided upon conveying all the material by sea from St Helier, One circumstance which materially influenced him in this decision was, that he had already constructed a depot and barge harbour at St Helier, for the supply of concrete for the harbour and breakwater then being constructed, at which large quantities of gravel, sand, and cement, were landed and stored, besides having at command the barges and tug boats employed upon that undertaking. Thus a constant and regular supply of material could always be depended upon; all that was required to complete the arrangement being an accessible and sheltered place near the site of the lighthouse, to which the material could be with safety transferred.
Fortunately a bight to leeward, in the lower part of the rock upon which the tower was to be erected, gave access to a bay of deep water, accessible at all states of the tide, and in close proximity to the spot which had been fixed upon for placing the workshops, stores, and concrete-mixing platform. At a distance of about 80 yards outside this spot there was a patch of rocks, uncovered at high water of neap tides, leaving a channel between, with from 10 to 14 feet depth of water at low water of spring tides, which formed a natural harbour, convenient, for the handling of barges; and a steam tug so long as they did not approach too close to the rocks on either side.
The assistance aflforded under this arrangement by the depots and plant of the St Helier harbour, in keeping up a constant supply of materials by an easy mode of carriage to the lighthouse works, made a most important saving in the cost of construction as otherwise the whole of the material, except the stone and a small portion of the sand, must have been carted a distance of rather more than 8 miles, the last ½ mile over uneven boggy ground, there being no road. This would also have entailed the necessity of unloading carts on the mainland, and reloading into trollies for transport across to the rock, which, as already explained, could only be effected at certain states of the tide, and in calm weather, and would have rendered it necessary to delay the work of construction until the causseway across the ledge was completed, thereby causing a great sacrifice of timme ; whereas by barge and steam tug from St Helier, a similar distance, the material could be landed direct at the works on the rock itself.
Under this arrangement the construction of both lighthouee and causeway was carried on simultaneously. The landing of the material from the barges on their arrival from St Helier, when moored in the deep-water channel, was carried out by an overhead rope railway from the main rock (at the foot of which were the stores and worktops) to the patch of rock on the opposite side of the channel, care being taken that sufficient headway was allowed for working the barges underneath. The rope suspended between the two rocks formed the rail or ropeway.
The carriage consisted of a single wheel with a grooved tire, which travelled on the rope, supporting a block and tackle by two side cheeks for adjusting the height, with a hook for carrying the bag or basket of material. This wheel with tackle was pulled along the ropeway by a small endless wire rope wound round a drum, worked with a small windlass by two men. An improvement was effected by having a double rope from the landing platform out to a little beyond the position where the barge was intended to be moored, where the ropes were fastened to a cross bar, and from thence continued to the patch of rook by a single rope, which, in order to give additional height was passed down to the rock over a pair of sheer legs.
A double line of rail or ropeway was thus formed, from the position which the barge would occupy when moored, to the landing place on the main rock. The main rope, secured to an eye-bolt lewised into the rock, was stretched from the apex of the sheer legs over the poisition of the barge; at this point it was secured to the centre of the cross bar, from the ends of which two ropes were fastened, and carried parallel to each other to the main rock, and firmly secured to it, after passing over the guide pulleys. At the ends of the ropes blocks and tackles were placed, to tighten or slacken them as might be required; and upon these ropeways the travelling wheels, or gins, with suspended tackle and hooks for carrying the baskets or bags of material, were run to and fro by means of the light endless wire rope, worked by the windlass.
The wire rope after two turns round the drum, passed up over a guide pulley, and was fatstened to one of the gins or travelling wheels; it was then conveyed under one of the ropeways, and round a guide pulley fixed to cross bars. Returing under the other ropeway, it was fixed to the other travelling wheel, and carried on to the shore or main rock, over a guide pulley, and round the drum; so that while the loading at the barge end was being proceeded with, the unloading at the shore end was going on at the same time. The wheels, drums, etc, had been previously employed for other purposes; by being brought into use for this work, the expense of providing new apparatus was saved. As fast as the material was landed it was removed by wheelbarrows, along roads consisting of upright poles fixed to the rock at distances suited to the length of planks — which were firmly spiked down to cross sleepers and secured to the tops of the poles — extending from the landing stage up to the cement shed and concrete-mixing platform at the depot.
At the depot there was, besides smiths' and carpenters' shops, stores, and office, a barrack for the men, fitted up with sleeping bunks all round the sides, with hammocks swung in the centre.
The platform upon which the lighthouse is erected is 9 feet high, and is formed of three courses, from which the tower, with moulded base and cap, rises. This tower is surmounted by a balcony having an iron railing, above which is the lantern provided with a dioptric illuminating apparatus of the second order, showing a fixed light extending over an arc of 250°. The dark angle, of 110°, towards the shore, is occupied by a dioptric mirror.
From seaward, between the bearings of south by east through east, to north by west, the light is white. Inshore of the eastern limits of the white light two sectors of red light are exhibited — one to the north-eastward, for marking the shoal ground of the Bigdon bank, and thence landwards; the other to the south-eastward, marking Le Vrachères and the adjacent dangers landwards, through angles of 32° and 38° respectively. The red arcs are produced by shades of flashed ruby attached to the glass of the lantern, which is 7 feet in height. The light is 135 feet above the mean level of the sea, or 119 feet above high-water of ordinary spring tides, and is visible for a distance of more than 18 miles.
The lamp is a pressure one, with the weights arranged below the body. The burner is of the Trinity House type, has three concentric wicks, consumes paraffin oil, and gives a light of about 200 standard candles. The lantern is by Messrs Chance, of the ordinary construction, except that the wall of the tower is carried a little higher than usual, and that the cast-iron pedestal is reduced by an equal amount. The inside diameter of the lantern pedestal is 10 feet. On the outside a fog-bell, weighing 5 cwt, is fixed, by means of a wrought-iron plate bracket, strengthened with angle irons, and stayed to the masonry of the tower.
In the construction of the lighthouse tower the difficulty of commencing the foundation under, or at low-water level, was not experienced, as in this case the level of the base of the platform was 100 feet above low-water; but difficulties of another kind nearly as serious had to be contended with in getting the material conveyed to this isolated height over the intervening obstacles, and so as to construct the work expeditiously and economically.
It was originally intended to build the tower with blocks made on the mainland, and carried over the tidal causeway by trollies, but this plan was given up for the reasons previously stated, and it was arranged to lay the concrete in situ. All the material had to be conveyed from the concrete-mixing floor at the level of the workshop, which was fully 50 feet below the bottom of the platform, to the top of the rock, and then hoisted up to the height of the course under construction. To accomplish this, an inclined railway, with siding on the 3-feet gauge, was laid upon longitudinal timbers, fixed firmly to uprights secured to the rock, from the concrete-mixing platform at the depot up to the site of the tower. Here a steam hoist was erected, and so arranged that when the trolly, with tipping skip was hauled up to the top of the incline, it was in position for direct lift, which was effected by unhooking the chain from the trolly and attaching it to the skip, when it was hoisted to the height required. The rod chain suspended from the traveller at the top of the scaffold, adjusted to proper length by removing or adding a link rod to suit the courses of the masonry of the tower, was then attached to the hook of the skip, the crab chain was slightly slackened, and its hook relieved, and the skip was run forward over the centre of the tower, and the material tipped on to the banker, which completed the operation.
The empty skip was drawn back into position for lowering by the counterbalance weight, the crab chain hooked to it and wound up a little in order to relieve the rod-chain, which was detached, and the skip was then lowered on to the trolly and run down the incline. During the time thus occupied another skip was being filled upon the trolly on the siding at the foot of the incline, and was ready for hauling up on the return of the empty one, an arrangement which allowed the work to go on smoothly and continuously. The scaffolding was made of timber, strutted and braced, and securely guyed with wire ropes to the rocks below.
The top of the rock was roughly quarried for the bed of the platform, leaving a core about 13 feet in diameter and 7 feet in height. The frame, a segment of one-eighth of the circle, 3 feet in height, dressed in face to a radius of 16 feet 6 inches in diameter, and a batter of 1 in 12, was fixed in place by tie-bolts lewised into the rock. The concrete, composed of 6 parts of shingle and coarse sand to 1 part of Portland cement, was then thrown in to the level of the top of the segmental frame, and the operation was repeated by shifting the frame round, until the course was complete.
The second and upper courses were laid in a similar manner. The circular platform was 29 feet 6 inches in diameter at the top, and 9 feet high. The chamfered joints were formed by fillets tacked on to the face of the frame, and when the work was stripped of the frames, the third day after depositing the concrete, the face presented an appearance equal to dressed ashlar, and as hard as a soft brick. Instead of dowels and joggles, a batten was fixed upright at each end of the frame, so that when the concrete was filled in, and the batten removed, it left a vertical slot at each joint, about 9 inches long by 4 inches wide, the full depth of course, which was filled in, and formed part of the adjoining block, when the frame was shifted horizontally round, and again filled with concrete. A concentric channel, 9 inches broad by 4 inches deep, was left in the upper surface of each course, for a similar purpose. This was filled up with the concrete forming part of the course above, so that the whole structure was bound together as one stone.
In order to give a smooth face to the work, care was taken that the Portland cement mortar, the proportions of which were 3 to 1, was laid close to the face of the mould, from 2 to 3 inches thick, and carried up with the concrete or rubble-work at the same time, to ensure equal setting and uniform colour. To prevent the mortar adhering to the face of the mould, soap, boiled with water to the consistency of cream, and applied immediately before the work was commenced, proved most satisfactory. The face moulds, which were made of pitch pine, were first washed thoroughly clean, and then thickly painted with the soap solution by a whitewasher's brush.
Upon the top of the platform the tower was carried up in the same manner as previously described — with the exception that the mould frames were made to radiate from the iron centre of a frame, firmly wedged and secured within the wall of the tower — building simultaneously two blocks, which formed opposite sectors, and completing the circular course in four shifts.
The height of the tower and platform is 44 feet; the well of the tower is 11 feet in diameter, and the thickness of the shell varies from 2 feet 6 inches, under the cavetto at the cap, to 4 feet 3 inches at the plinth mouldings ; the base is 5 feet 3 inches thick. There are two floors with three landings, as the work was designed for three floors. The girders for the floors are of rolled wrought iron, and were built into the side walls as the work proceeded. The floors are formed of irons resting on the girders, and built into the side walls; the spaces between, and for 1½ inch above the irons, are filled with fine cement concrete, in the proportion of 4 parts of shingle and sand to 1 part of cement.
In the centre of the tower a hollow cast-iron column, 13 inches in diameter and 5/8 inch thick, in four lengths, is built into the platform; the ends of each length terminate with circular flanges, which are securely bolted together. The top length is bolted to the under side of the pedestal of the optical apparatus which it supports. The partition of the watch-room is made of wrought-iron plates 5/16 inch thick. There are five windows. The openings for these and the door were made by fixing wooden centres formed to the required shape and boarded over, and the concrete filled in with the courses through which they pierced. The staircase consists of two wrought-iron spiral stringers, made of flat-iron, 7 inches long by 3/8 inch thick, with angle irons 2 inches by 3/8 inch thick, riveted to the upper and lower edges. The stringers are firmly bolted to the floor of the tower, and to the girders and cantilevers at the landings. The risers and supports are of flat bar iron, 2¼ inches deep by 7/16 inch thick, riveted to the upper angle iron of the stringers. The tread plates are of cast-iron, with grating panels. The balusters are of polished wrought-iron, 1½ inch in diameter, with bright brass caps and handrails.
The lantern pedestal is of cast iron, securely fastened to the masonry of the tower by twelve 1¼ inch holding-down bolts, 3 feet long. The inside is lagged with American yellow pine, and fitted with brass circular ventilators ; the service galleries are of cast iron; the inclined framing for supporting the roof is of wrought iron, faced with gun metal; the cupola is of copper, double lined, surmounted by a copper revolving cowl with vane. An outside ladder is fixed to the roof and a lightning conductor is carried to the foot of the tower, and down a fissure in the rocks beneath into a deep pool, which is submerged at high water of all tides.
After the removal of the inclined railway and scaffolding of the tower, the approach was formed by cutting steps out of the rock, and in some instances by forming them of concrete, protected to windward by a rough-built rubble wall in cement mortar, with stones quarried from the adjacent rocks, with which it corresponded in appearance when finished.
The survey and sections for this work were completed in February 1873; the masonry of the lighthouse and half-tide causeway was finished in November of the same year, and the light was exhibited for the first trial on 24 April, 1874.
The expenditure, including all labour, materials, plant, and tools, and freight and charges for superintendence and engineering, etc, was:
- Half-tide causeway from mainland to lighthouse, including approaches at each end, and stairs up face of rock to foot of tower - £1,606
- Lighthouse structure complete, including wrought-iron staircase, girders, floors, and general fittings - £2,976
- Lantern and apparatus fitted in place, including fog bell and clock - £2,555
- Lightkeepers' dwellings and oil stores, with wrought-iron tanks - £864
- Total cost - £8,001
The quantity of paraffin oil (which was supplied by Messrs Young and Company) consumed was 2.26 gallons in every 24 hours, calculated from quantities specially measured during a period of six months. The total cost of maintenance, including wages, oil, stores, coal, etc, is about £170 per annum, taken from an average of two years.