The 1870s harbour enlargement project
By Imrie Bell, engineer in charge of the harbour project (1880 paper for the Institution of Civil Engineers)
This account, although by its nature very technical in many respects, gives a fascinating insight into why the ambitious project to create a large, deep-water harbour for St Helier in the mid-1870s, was thwarted by winter gales and political rivalries.
The port of St Helier, in the island of Jersey, is situated in the bight of a bay closed by the headlands of Noirmont Point and La Rocque, and, lying on the south side of the island, it is protected from all winds except westerly, from north-west to south-west, during the prevalence of which winds the full swell of the Atlantic comes with all its force into the bay.
The existing harbour was designed by the late Mr James Walker, and is a tidal harbour, entirely dry at low water ; so that passenger steamers, unless they arrive at a high state of the tide, have to moor outside, and discharge the passengers into small boats in the open bay — a most inconvenient and sometimes dangerous arrangement; and this operation has to be repeated as frequently in embarking to leave the port.
Under these circumstances, the government of the island, called The States, took into consideration, in the year 1867, the necessity of providing better harbour accommodation. After considerable discussion, it was arranged to advertise in England and France for competitive designs for a harbour, with sheltered berths for vessels drawing 13 feet, lying about during all states of the tide and weather; and the result was that 42 designs were submitted, which were made over by The States to their harbour committee, with instructions to consider and report upon the whole question.
The committee, after mature deliberation, and investigation of the plans and estimates, during which they called in an independent engineer to aid them in comparing the quantities and prices with the various estimates, finally decided upon awarding the first premium to Sir John Coode, who shortly afterwards received instructions to make the necessary arrangements for carrying out the works; which were ultimately commenced under the superintendence of the author in 1872, from a modified design from the one submitted for competition.
Description of work
The site of the harbour is upon a flat shore thickly studded with high rugged rocks, composed principally of hard, close-grained syenite, extending a long distance out to sea and exposed to the full force of the Atlantic. The height of ordinary spring tides is 32 feet, and extraordinary equinoctial springs 40 feet.
The works consist of two separate arms, situated on either side of the small roads.
The western arm, forming the Hermitage Breakwater, commences from the south curtain of Elizabeth Castle, a fortress built upon a large isolated patch of rocks, distant from the mainland a little over two-thirds of a mile, and is continued over the Crow rock, which is quarried down to level of surface of breakwater, out to the Hermitage rock, which it was decided to leave intact, as this is the rock where the hermit, Saint Helier, is said to have lived in ancient times, and after whom the town of St Helier derives its name.
The breakwater is carried out from seaward side of the Hermitage rock in a southerly direction, to the Platte rock, where it terminates with an elbow 170 feet long. The total length of the breakwater is 2,665 feet, by 86 feet wide at the top, with sides battered to 1 in 12; the height at outer end is 60 feet, with 20 feet of water at low water, ordinary spring tides.
The eastern arm, forming the landing pier, with approach principally for the accommodation of steam vessels, commences from a place named Point de Pas, and is carried in a south-westerly direction for a distance of nearly 1,700 feet; it then turns in a west-north-westerly direction for about 1,700 feet, finishing with a slight cant of 300 feet, making the total length 3700 feet.
The land portion, or approach, is 48 feet wide at the level of the roadway, surmounted by a parapet wall 13 feet 6 inches above high water ordinary spring tides. The roadway of pier is 48 feet in width, provided with a double line of railway, and has besides a raised promenade 17 feet 6 inches wide, protected by a parapet wall on side facing the sea. There are three landing stages, with platforms at levels suitable to the different states of the tide, for the accommodation of steamers, and the convenience of passengers landing and embarking, which are accessible by flights of stairs.
A slipway 20 feet wide is also provided for the use of small boats and the landing of cattle; the depth of water varies from 12 to 18 feet at low water, ordinary spring tides.
Sir John Coode, in his original competitive design, estimated that the total cost would be £282,500; but of course that sum cannot be taken in reference to the works as carried out by the decision of the authorities, which plan was very much altered and enlarged from the original design, and by the time the plans were finally arranged and the works commenced, the prices of labour and material had in- creased from 20 to 30 per cent. The actual estimate of the cost of the entire structure, as made out from the working plans by Sir John Coode and the author, at the request of the authorities, showed a total amount of £379,000.
In 1877, a short time previous to the suspension of the works, the author was again instructed to report upon the actual cost of the work completed, and also to make a separate detailed estimate of the cost which the execution of the entire harbour works would entail, the result of which showed the total cost to be £395,899, which sum includes damage done to the works by the sea during the storms of three successive winters, amounting in all to £6,800, or about 1 3/4 per cent, on the estimate, thus leaving excess over original estimate to be about 2½ per cent.
These figures were based upon actual cost of work executed, and including engineering plant and all charges; so that had the whole work been allowed to proceed to its termination, it would have been completed for a sum within £400,000, certainly a moderate amount for an undertaking of such magnitude.
Sir John Coode, in his report to the insular government, recommended them to carry out the remainder of the work in solid concrete masonry, similar to that of the seaward portion of the breakwater, which had resisted the fury of all the storms so successfully.
But the extra cost of this prevented the entertainment of the proposal, and unfortunately, owing to party dissensions among the members of the States, the party under whose auspices these works were originated being in a minority, and also to financial difficulties, it was decided to proceed no further with the works under their existing form, and to take into consideration the advisability of substituting something of a less expensive description, with the result that, after three years of unprofitable discussion upon all sorts of projects, the works are now standing as they were left in 1877, when the author handed them over to the States.
Situated as these works are, exposed to the full range of the Atlantic, and in a tideway which rises to 30 and 40 feet, the history of works constructed in similar situations would lead one to anticipate great casualties. But the author is happy to have it to record that in the case of the western arm of the breakwater, after the adoption of the plan of building in solid work, no mishap of any kind occurred, and after three years' exposure in their unfinished state, they remain intact as they were left by him in 1877.
But, in the case of the eastern arm, which, having less exposure, was built of a less solid description of work, this arm of the work suffered from the effects of severe storms during three successive winters in the years 1874, 1875, and 1876. The severity of the storms of these years was curiously enough recorded by the sea itself at the opposite end of the island, at the foot of the rock upon which the author constructed the Corbière Lighthouse.
The shore between the rock on which the lighthouse stands and the mainland was very rough and rocky, upon which a half-tide causeway was built. At the foot of the rock there was a deep gorge or gulley, and during these storms a large block of rock about two tons in weight was carried by the force of the sea up the gorge and thrown over the causeway.
The breaches in the work occurred in every case where the work was unfinished or in an unprotected state. The authorities decided in the summer of 1876 to delay the construction of the landing pier until the Hermitage Breakwater was extended further out. The work at the end of the landing pier was then in two lifts or levels to suit the arrangements for carrying on the work continuously during high and low water.
The author received instructions to protect the rubble filling between the longitudinal walls of this unfinished length with a covering of concrete about two feet thick. He objected to carry out these instructions and strenuously urged that he should be allowed to construct the whole of the unfinished portion of the work out to end of foundations already laid up to surface level, and finish the extreme end with a nearly vertical scar end built with solid blocks from wall to wall, similar to what he had previously successfully carried out at the breakwater, but he was overruled, and after a lengthened correspondence he was instructed to protect the rubble filling in the slopes, and on the level between the upper and lower slopes by a concrete skin of from two to three feet thick, which was done.
During the winter gales of the same year the breach occurred at this place, evidently caused by the hydrostatic pressure from the great height of the sea bursting up the concrete skin of slope, and thus gaining an entrance into the space between the longitudinal walls, and the result was that the harbour wall, the weaker of the two, was burst out, the breach extending up to cross wall about 100 yards, where the fall of the tide prevented further damage being done.
On examination it was found that although the cross wall was intact, many of the upper blocks beyond it in the masonry of the harbour wall were forced out, leaving an entrance into the next division or compartment, which would have inevitably allowed another length of wall to be forced out had the tide not receded.
This portion of the wall was, previous to the next rise of tide, protected by a breastwork of railway metals, fixed vertically and overlapping one another, bolted and chained to the face of the wall — which acted most successfully in protecting it, and preventing further damage to the work, until the weather moderated and allowed of the wall being built up with masonry.
There appears to be no doubt that the solid section of masonry would ensure the greatest security in carrying out the work, but whether it is the only safe plan that could be adopted is a question worthy of consideration, as it entails a very greatly increased expenditure.
The author considered, after careful reflection upon the nature of the breach previously described, that a structure hearted with nibble could be made sufficiently strong to resist the power of the heaviest sea by building it in completely separate compartments, and not carrying out the longitudinal main walls in a continuous length, so that each compartment or division of the structure would be complete in itself, consisting of its four walls of masonry filled in with rubble and protected with a covering of concrete, so that in the event of one of these sections being breached by a storm, the others would remain unaffected.
Or otherwise, to build the main walls with large blocks without mortar, and fill up between with dry rubble hearting. Under either of these plans the author was confident that the works could be completed at his estimate. He also proposed an alternative plan, at the request of the Harbour Committee, for building the pier with solid masonry, on lines modified from the original design, which would have given ample accommodation for the present requirements, yet so arranged that it could be extended when required.
This plan, the author felt confident, could be completed at a cost of £450,000, including the amount already expended; but the party in power having determined that the works should be stopped, the proposal was not agreed to.
Hermitage breakwater construction
The commencement of the Hermitage breakwater was made in April 1872, upon a small piece of ground under the castle rock a few feet above high water level, containing only about 900 square yards, by the erection of a shed for the storage of cement and blacksmiths' and carpenters' shops which absorbed nearly all the ground available above high water.
During the time this was going on blasting operations were begun upon the Crow rock, and between this rock and the Castle rock the breakwater sea wall was lined out and its construction commenced.
The rock was roughly quarried into level benches, upon which the mould frames were fixed in position and securely bolted to the rock, when they were filled with concrete and covered over with stout canvas and weighted with railway metals, which were secured with chains lewised into the rock.
This was both laborious and costly work, but could not be avoided, as the work in this wall had to be pushed on as quickly as possible in order to have this portion completed above high water level before the winter's rough weather set in, as the protection of the workyard, which was then under way, depended entirely upon it.
The reclamation of the foreshore below high water level was at the same time being proceeded with. Its superficial area was over 8,000 square yards, varying in height from eight to 20 feet, the material being procured from the quarrying of the upper part of the Crow rock, which it was necessary to remove to the level of the top of the breakwater.
The large quantity of stone quarried from this rock was more than sufficient for all the requirements, temporary and permanent, of the work between the Castle and the Hermitage rocks, which included the concrete, masonry, and rubble hearting in the breakwater, the building yard and stacking depot, with the whole of the filling up of reclaimed ground up to eight feet above high water spring tides, and the building of a temporary harbour with quays, which was required in order to provide shelter to the vessels arriving wi£h cement, coals, and other materials, and to accelerate their unloading.
Upon the completion of these operations the block building floors, stores, offices, and other buildings were gone on with, along with the laying down of railways for general traffic and the foundations for the stone crushing and concrete mixing machinery. The construction of the breakwater from the Crow rock to the Hermitage was the next portion of the main work proceeded with.
The space between these rocks, 400 feet in length, was called Hell's Gate from its exposed position and the fury with which the waves rushed through it during any other than very calm weather, and in consequence much anxiety and trouble was experienced in the construction of this section of the work. It was found to be utterly impossible to build the concrete blocks in situ, though tried several times, and on some occasions with the loss of the frames which were wrenched from their position and carried bodily away, though they were bolted, chained, and lewised in every possible way.
In some instances where the frames remained rigid the water gradually forced itself through some joint, and by the continued action of the sea the whole of the cement and the finer portion of the sand was washed out, and upon the tide receding the frame was found to contain only the broken stone and large shingle.
The work was necessarily delayed for a short time until the railway could be laid to this point for the transport of the concrete blocks from the building yard on trucks which, on arrival, were lifted off by means of a pair of sheer legs and set in the position required, the legs being moved forward with the progress of the work.
This arrangement facilitated the working considerably and enabled the building to be carried on much more rapidly, though occasionally the railway was displaced and the sheer legs thrown down, but this only occurred in exceptionally boisterous weather, and was easily restored when the wind and sea moderated sufficiently to allow of the work being resumed.
The breakwater in this section consisted of two longitudinal walls connected together at short distances by cross walls bonded into them, the compartments between were filled in with rubble hearting up to about 11 inches from the top of the masonry in the walls, when the whole was covered over with concrete and finished with a barrel-shaped surface to allow the water to run off.
The continuation of the breakwater seaward from the Hermitage rock was the point where the real difficulties of the work commenced, owing to the very strong tidal currents in addition to the heavy seas that had to be contended with. The same mode of construction was continued as previously described, but the progress was much slower owing to the power of the sea, which came nearly end on and scoured out the rubble hearting, thus endangering the stability of the side walls. This was partly met by concreting over the hearting in ten feet lifts, but was not entirely satisfactory, and it was determined to build the remainder of the breakwater in solid concrete masonry with the hearting blocks composed of cement concrete in the proportion of 15 to 1, while that of the face blocks was 8 to 1 of cement.
The first idea for lifting and setting these blocks was to have a staging carried upon wrought iron piles, but the author had experienced so much trouble with various experimental appliances, which all proved unsatisfactory in their capability of withstanding the power of the sea, that it appeared to be imperative that whatever plan might ultimately be adopted the lift must be from either the top of the breakwater or from the deck of a pontoon or barge.
One plan suggested was to construct large blocks on shore and transport and place them in position by means of floating shears in a manner similar to that adopted and described by Mr B Stoney, in his very valuable paper upon harbour and marine works. But this plan, though admirably adapted to the work it was designed for by Mr Stoney, could not possibly have been used on these works, owing to the uncertainty of the sea and the strength of the currents.
After mature deliberation and numerous trials with blocks and bags of various sizes of concrete, lowered and set by means of derricks and tipping machines on barges, it was finally decided to construct the foundations with large concrete blocks weighing about 100 tons, lowered into position by means of barges fitted with steam winches, up to a height of six feet three inches above low water ordinary spring tides, and the superstructure from that level up to the top with concrete blocks weighing about 12 tons, to be set in place by an overhanging travelling crane or titan, which for practical purposes may be considered as an improvement upon the combined plans of Messrs Stoney and Parkes, and which acted most successfully up to the time that the works were suspended.
The amount of progress being pretty steady at an average rate of one foot lineal of breakwater per day, equal to 85 cubic yards, and in favourable summer weather the work proceeded easily at the rate of 127 cubic yards per day, or 1½ lineal feet, costing nearly £60 per foot, including plant and all other charges, over whole length of the breakwater, but of course a considerable portion of this was above low water.
The cost of the work executed in solid masonary with the height of breakwater at 60 feet, including 20 feet below level of low water, was, including everything, a little over £100 per lineal foot.
Workyard and depot
It is a matter of vital importance in works such as these, in order to obtain an economical as well as a successful result, that great care and attention should, at the outset, be bestowed in devising a general plan of procedure, and in organizing a proper working system, leaving details to be worked out as circumstances require from time to time, during the progress of the work.
Ss the material of which this work had to be constructed was nearly entirely concrete, the author, in devising his arrangements, considered that the central point of the system should be the concrete mixer. In deciding upon its position, three important points had to be considered: *That it should be in easy and direct communication with the temporary harbour quay where the materials arrived and were unloaded
- That its position should be so situated that the railways for conveying the materials should not interfere with those required for conveying the
concrete to building floors
- That it should be adjacent to cement shed and mechanics' shops, in order that its engine power might be used where required, and also where the coal and water supply would be available for all purposes.
The yard and depot being formed upon reclaimed ground, required special arrangements in regard to levels, as it was necessary to have the concrete mixer raised considerably above the level of the building floor for the purpose of conveying and tipping the concrete into the mould frames, and at the same time have it fixed so as to economise to the fullest extent the labour in lifting the material into the mixer, and allow of the gravel and sand being stored below the level of the railways.
These difficulties were overcome by adopting the level of the top of the breakwater as the medium level, and determining upon the levels for the machines and branch lines of railways to conform to this level, so that there should only be one lift of all the materials in their required proportions into the concrete mixer.
The main line of railway was formed practically level throughout its entire length, laid to a gauge of 3 feet, which gauge was adhered to in all the lines for the cranes and rolling stock. It commenced on the quay of temporary harbour; a siding branched off to the gravel and sand depot, and continued past the coal store, where there was a second siding for cement; it then continued along by the stone depot for supply of the stone-crushing machine, in front of mechanics' shop and stores, and curved round the end of the block building yard, continuing in a straight line and passing between the sea wall of the breakwater and the building and stacking floors, at the side of which there was left a platform for tipping the rubble stones to be used in the block building.
A crossing fixed a little beyond the Crow rock joined the line again at a short distance beyond the locomotive shed, thus completing the circle. The straight line continued along the top of the breakwater, with sidings out to where a line branched off past the buildings in which the diver's rooms and stores were situated down an incline built with rubble and cement mortar to the low water foreshore quarries, where it branched off to suit the situation of the rocks which were being quarried — the main line continuing in the straight, and lengthened as the work progressed.
The materials used were broken stone, shingle, fine gravel, and Portland cement. The stone was quarried from the rocks adjacent to the works, and consisted of a very dense syenite, 13½ feet to the ton, and traversed by broad bands of trap rock. The shingle and gravel were procured from a bay about six miles distant, and were brought by barges. The Portland cement was supplied from England by the Burham Co, the Wouldham Co, and Messrs Peters Brothers, under the usual conditions and tests as to quality.
The machinery for the stone crushing and concrete mixing was placed in the following order:
- One of Blake's patent stone crushers — size of space for reception of stone to be crushed was 15 inches by 9 inches
- The steam engine for driving the stone crusher
- A chain barrel with jib, worked by lever and friction wheels, for lifting the materials up to the hopper of the concrete mixing machine
The stone quarried above and below high water was divided into three classes:
- Picked rectangular pieces, which were hammer dressed, for the purpose of placing across the holes left in the concrete blocks for lifting bars, to ensure a solid bearing for the T heads of the bars
- The large irregular shaped pieces interspersed in the mass concrete blocks — these two classes were taken to the platform above the building floor
- The smaller pieces of stone were tipped on space in front of stone crusher, and broken into sizes suitable for the concrete mixer.
The shingle and fine gravel were unloaded at the quay from the barges into trucks by a portable 2½ ton steam crane, and taken to the store, where they were tipped. The cement was also unloaded in a similar manner from the vessels alongside the quay into waggons, which were then shunted on to the siding at the end of the cement shed, and hauled up by means of a wire rope — passed over a series of guide pulleys, and wound round the cylinder of the concrete mixer — along a line of rails fixed to the wall at back of cement shed, and tipped over and placed in tiers.
This operation did not occupy much time, and was carried out after ordinary working hours, when the concrete mixing machine was not in use.
The concrete mixing was pursued in the following manner. The tipping skip, capable of containipg a charge of about one-third of a cubic yard of concrete material, was run on a light trolly over an 18 inch gauge railway into the gravel depot, where it was supplied with its regulated quantity of gravel.
It was then pushed under the shoot from the stone crusher, where it received its proportion of broken stone, when it was wheeled round the curve, under the main roads, past the shoot of the cement hopper, where the proportion of cement was added, on to position under hoist, whence it was lifted by crane and slewed round exactly over the hopper of the concrete mixer, into which the contents were delivered, and the skip lowered again on to the trolly, taken back to the gravel depot, and the operation repeated.
There were two trollies with tipping boxes used, and passed each other half way on a loop in the railway. Upon the reception of the material in the hopper of the mixer, the man in charge immediately turned on the water, which was delivered through a perforated pipe extending across the breadth of the hopper, in a sufficient quantity, regulated by the man's experience.
The mass, after being mixed by longitudinal blades fixed upon the shaft, and turned over a series of times in the cylinder, was delivered by a shoot into side tipping trucks and run upon a line of 18-inch railway, branching out fan shape into three lines corresponding to the three rows of mould frames fixed on the building yard floor, and on the top of which the railways, consisting of short lengths of rails secured to 3-inch planks, were laid.
The road was extended along with the frames, as the latter were filled till one row of blocks was completed, the second row was then commenced, and so on. The frames could be removed within two to three days after depositing the concrete, and the blocks themselves lifted and removed on the fifth day.
The weight of these blocks varied from 7 to 12 tons each. The blocks were lifted by an overhead steam traveller or Goliath. The attachment for lifting them consisted of a sling chain with two T-headed lifting rods, which passed through the blocks. They were carried from building floor to the stacking yard, where they were carefully placed in tiers and numbered, in conformity with the different positions that they were destined to occupy in the permanent work.
In reference to the art of concrete block building, 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 the blocks, as exemplified in many of the large engineering works in course of construction in the metropolis and elsewhere, where the exposed faces of the concrete present a rough, honeycombed appear- ance, with the marks of the joints of the timber planks forming the moulds in which the blocks have been formed, or the frames inside of which they have been built in situ, in place of showing a fair and smooth surface.
The author has given this matter much consideration, and the result of his experience is that in concrete building it is perfectly easy, with a little attention, not only to produce a fair surface, but to form mouldings and panels, and even tracery and ornament, and at the same time make this face work as durable and solid as any part of the block.
There are two reasons why little attention has hitherto been paid to this art — one is carelessness or indifference to appearance, the other is that most engineers who have attempted it have done so by rendering or grouting, both most objectionable and dangerous modes of effecting the object; and which, even if successful for a time, is simply veneering, and is subject at any time to decay, the failure generally occurring after wet and frosty weather, which has naturally caused a want of confidence, and stopped a repetition.
The plan which the author has followed, and with complete success and at an inappreciable increase of cost, by which a smooth, uniform, and equal coloured face can be obtained (and if wanted the colour of the blocks might be slightly varied by different coloured sand), and which, both above and below low water, has stood successfully the test of eight years' exposure to frost and heat and storm and rain.
This plan is simply to have a smooth planed board for the face of the mould painted previous to commencing the work with a mucilage of soap, and to line inside with a finer concrete or mortar as the work proceeds, so that the mixture placed close to the face boards is carried up with that contained in the body of the block, the whole forming one homogeneous mass, and ensuring that the setting process of the whole mass shall progress simultaneously; and in fact this face, like the skin of cast-iron, is actually the strongest portion of the block
In the first section of the work, the foundation being above low water, there was not any serious obstacle to delay the prosecution of the work. The rock was blasted or quarried to a comparatively level bed; or, where it rose irregularly and, approached the vertical, the mould frame was cut and fitted to the shape of the rock, and securely bolted to it, and then filled in with the concrete to the level of the superstructure.
In the foundations below the level of low water, very great difficulty was experienced, more especially between the levels of a few feet above and below low water, owing to the very strong eddies and currents, which varied considerably with the state of the wind and tide.
The mode first tried was, after the divers in dresses had been down and cleared the rocks of seaweed, to lower concrete in bags into place by means of skips from barges, and from a tipping trolly traversing upon rails laid across the deck of a barge. When the bag inside of the trolly was filled with concrete, the trolly was started with block and tackle to give it a slight impetus, which carried it across the barge, and caused the bag to be tipped over the side of the barge.
This latter plan required careful adjustment, as upon the first trial the trolly was very nearly carried over with the bag of concrete. The surface of the bags thus deposited was very irregular, and quite unfit for the reception of the upper work. This was obviated by fixing a frame the entire breadth of the foundation, and 10 feet in length, strengthened and stiffened with railway metals.
The sides being about 18 inches deep, formed of 3-inch planks, projecting about one foot below low water, and continued with a crinoline of canvas down under water to the level of surface of the bags of concrete.
At each corner sharp-pointed iron uprights were set, formed of pieces of permanent way metals driven down to the required level, to which the sides of frame were firmly secured. The concrete, which had been previously prepared, was then thrown into this frame from barges moored in close proximity, as well as from the shore end, and the whole completed and protected with canvas on top, and weighted with rails and stones, all of which had to be done before the tide rose.
This was tolerably satisfactory work in calm weather, but with the slightest sea on it was impracticable, so that this plan proved tedious, and the rate of progress slow and consequently expensive.
After this a trial was made with a larger barge having a well in the centre through which a wrought-iron rectangular box, with folding bottom in two halves, holding a bag of concrete, about 14 tons in weight, was lowered; but there was still the disadvantage and loss of time in levelling the surface of bags at or about low water level.
During the time that these methods were in operation the author was experimenting upon lifting various sized blocks, by means of the large barge fitted with a staging carrying a double set of three sheaved blocks, with two ordinary winches, by means of which he succeeded in lifting and lowering into position blocks of over 70 tons carried below the barge, the lifting gear passing down through the well. The result proved so favourable that Sir John Coode was induced to give up entirely his idea of using staging for the foundations.
The special machinery for carrying out the work upon this plan consisted of:
- For the foundation work, a steam tug constructed by Messrs James Taylor, a steam barge, made upon the works, capable of carrying 250 tons,
fitted with powerful steam winches and Napier's patent self-holding brake, made by Messrs Chaplin in Glasgow; a barge for concrete bags, capable of carrying over 200 tons, made on the works
- For the superstructure, a titan, or steam traveller, capable of carrying and placing blocks weighing 18 tons in position, with an overhang of 50 feet.
The design was an improvement by Sir John Coode on one used on the Kurrachee works, designed by Messrs Parkes and Price, which was, as stated by Mr Parkes to the author, the first machine of the kind used for such work.
Sir John Coode's improvements consisted, first, in making the horizontal jib to radiate, which was an indispensable condition where the breadth of the work was 40 feet; second, in the engine and lowering gear being arranged at the back end of machine, so as to assist in acting as a counterbalance, there being simply a light carriage traversing between the girders forming the horizontal jib which carried the chain with sheave blocks for lifting and carrying the concrete block, instead of the engine and crab having to travel along the jib; third, in the arrangement of the tension bars overhead, which allowed of the whole being made of a lighter and consequently less expensive mode of construction. This machine was made by Stothert and Pitt of Bath, who also constructed the one for Mr Parkes.
This arrangement for the construction of the breakwater was maintained throughout, and there seemed to be nothing further required as the work was carried on continuously without any break or loss of time. The foundation was principally on rock, but where sand or clay was found it was removed by a bag dredge fixed to a barge and worked by steam, aided by divers with siebes dresses, and when the site of the foundation was thoroughly cleared, bags of concrete varying in size from 1 to 14 tons were lowered so as to form a comparatively level bed, which was then accurately made up with small broken stone and placed by the divers for the reception of the large concrete blocks.
In order to insure the surface being level throughout, a short piece of railway metal sharpened at one end was driven into the bags of concrete about 20 feet ahead of finished foundation, so as to take a firm hold. A length of rail was then lowered upright down through the well of the barge and fished on to the end of the short piece and securely held in a vertical position and driven down by a light ram from the deck of the barge to the required depth, under the direction of an assistant with a spirit level from the shore.
The long rail was then unfished and hauled up on the barge. This operation was repeated at the opposite corner at the same distance from the end of the work, when two long permanent way rails were stretched from end of completed foundation on to head of each of the short lengths on either side, which covered a length of 20 feet of breakwater. This space was then filled in with small stone to the height of the top rails, the surplus being removed by the divers drawing a rail along the tops of the longitudinal rails, leaving a uniform level bed of broken stone varying from 6 to 15 inches in thickness.
The steam barge with the concrete block 22 feet x 10 feet x from five to eight feet, suspended from the winches by two 3¾ inch round bars with T heads, was brought into position and moored by four hawsers at bow and stem to chain moorings, two at either end, in plan forming a St Andrew's cross.
The concrete block was then slowly lowered till about three feet above the level of its bed, when it was drawn close against face of block previously laid by two shore lines attached to it, and upon a signal from the divers below that all was right the brake was released and the block settled down on to its bed.
Small bags of concrete were then laid along the ends of the blocks to protect and retain the broken stone upon which the blocks rested. This, however, did not prove a satisfactory means of protection, as the perfect adjustment of the bags could not be insured, and it occurred to the author that if a bag of liquid concrete could be placed under the edge of block it would be more effectual, which was tried in the following manner:
A long bolster-shaped bag was loosely filled with concrete and lowered immediately previous to the block and laid by the divers in line of the outside of the block, so as to allow the block to bear upon eight or nine inches of the breadth of the bag, which was a few inches higher than the bed of broken stone, so that when the block was lowered it pressed upon the concrete which, in its soft state, yielded sufficiently to allow of the block bearing firmly upon the bed of broken stone, thus insuring a solid bed and at the same time securing complete protection from the danger of the broken stone being washed from under the ends of the block. This method proved most satisfactory, and was continued throughout the laying of the foundations.
The brake used on board the barge, for lowering the blocks, was Napier's patent. It acted admirably, and instantly arrested the descent of the block, with a very gentle pressure on the lever. These brakes are made either perfectly self-holding, or else aided by the addition of a small weight fixed on the handle when required to support the load.
The large foundation blocks were carried up to a height of 6 feet 3 inches above low water ordinary spring tides, and, of course, were set without mortar. Two blocks, 22 feet long, formed the entire breadth of the foundation of the breakwater, and the number of blocks in height varied according to the depth of water.
The superstructure was built in solid masonry, consisting of concrete blocks varying in weight from 5 to 14 tons each, set in Portland cement mortar by means of the titan or steam traveller, which, when not at work, was securely fixed in its place in front of the Hermitage rock by means of chains and eyebolts lewised into the rock.
Previous to commencing the work of block setting, the titan was travelled out to the end of the finished portion of the breakwater, and blocked up the truck with concrete block was then run under it, and the little carriage with lifting tackle, traversing on horizontal jib, brought over the block and the hooks, attached to the eyes of lifting bars, when the concrete block was slightly raised from the truck and run out to the end of the jib, the extreme overhang of which was 46 feet from top edge of breakwater, and lowered nearly to the level required, when it was slewed round in the direction wanted, and finally set in its bed.
In working this machine only two men were employed, one at the engine, where the levers were so arranged that the block to be set could be lifted, traversed in or out, slewed to either side of the breakwater, and lowered into place without the engineman changing his position ; the second man at out end of the jib giving the engineman whistle calls for all the different movements.
Proportion of materials
The concrete blocks were arranged under three classes, the proportions of the materials varying according to the position that the block was to occupy in the work.
- No 1 blocks, from 5 to 14 tons in weight, to be used in the face work, were made in the proportion of eight parts shingle and gravel to one part of Portland cement. They could be lifted from the building floor, and removed and stacked three days after they had been made. Cost 17s 3d per cubic yard.
- No 2 blocks, from 5 to 11 tons in weight, to be set in the hearting
of superstructure, composed of 15 parts of shingle and gravel to one part of Portland cement. These blocks could be lifted from the building floor, removed by the Hercules, and stacked six days after they had been made. Cost 14s per cubic yard
- No 3 blocks, from 70 to 100 tons in weight, for setting under low
water in the foundation courses, made in the proportion of 12 parts of shingle to one of Portland cement. These required to remain six weeks before they were strong enough to bear lifting and removal from the floor upon which they were made. Cost 12s 11d per cubic yard
This floor or platform was submerged each tide, so that each block that was commenced had to be finished and covered over with canvas and weighted with rails and stone before the return of the tide.
Eastern arm landing pier
The site for the approach to the landing pier, including the workyard and depot, had partly to be reclaimed from the sea, and the remainder had to be quarried out of the high slope facing the sea, which consisted of rock similar to that at the Hermitage but with broader bands of trap rock running through it.
The area of the foreshore reclaimed was upwards of 17,000 superficial yards, protected by a massive sea wall built of rough granite in Portland cement mortar. The two main points which had to be considered in devising the working arrangements were:
- To fix the levels of the platform for the various machines connected with the concrete block making in such manner as to work into each other with the least possible amount of manual labour, having the stores of granite and cement in the immediate vicinity
- To adjust these heights so as to enable the high level railway, which was of necessity above the level of the machines, to join in with the incline from the quarries to the landing pier.
The level of the top of the pier was adopted for the level of the workyard, and the main lines of railway with sidings commenced from the side of the Victoria Harbour, and passed through the yard, having the block building floor and stacking ground on the one side, and the machine shops and delivery shoots from the concrete mixer and stone crusher on the opposite side.
It was then continued on a curve round the approach on to the landing pier, along which it was extended as the work proceeded. For the convenience of access to the platform, a loop high level line was formed up an incline in the rear of the machine shops to the level of the top of the stone crushing machines. It then curved into the line leading from the quarries to the main line on to the landing pier.
The concrete mixing floor, with sand depot and shoot from cement store, were all on one platform under the stone crushers, which allowed of the stone being delivered through shoots, thereby saving manual labour.
The work was commenced on this side by the removal of the high rocks, by blasting, on the site of the root or shore end of the landing pier, the material being utilised in filling up the sloping foreshore to form the workyard. Along with this the construction of the sea wall, cement shed, and machine shops was proceeded with, and the opening out of quarry at South Hill, which was to supply the whole of the stone for building and hearting required for the pier works.
The gradient of the main line from the quarry was 1 in 24, with a level bench about the middle to join in to the line from the cement shed. The wagons were made side and end tipping, to carry two tons, with heavier ones for the conveyance of the large concrete blocks and heavy pieces of rock for hearting, which were constructed to carry from 10 to 16 tons.
The haulage was effected by means of locomotives, and the shunting by horses. The explosives used were Hall’s and Harvey's blasting powder and Krebb's lithofracteur. The rock-boring was mainly carried out with the Ingersoll rock drill, which worked most satisfactorily, the work performed being both cheaper and quicker than by hand.
The notes from careful observation gave the following results: Ingersoll drill, working 10 hours per day, bored a hole 2¾ of an inch diameter, and 16 feet deep, at a cost of 1s 6d per foot, including percentage for first cost of machine; while the result by manual labour for same size of hole, and working for a similar number of hours, was 4 feet 6 inches deep, at a cost of 3s 5d per foot.
A little difficulty was experienced at the outset in working the drill, which was of the ordinary chisel shape. It frequently jammed in working, especially where the rock was shaken. The author, after trials of various shaped drills, found the most suitable description to be one with cross section at cutting edge of a St George's cross, or, as it were, two chisels crossing each other vertically; the diameter of the body of the drill being kept parallel for a couple of inches from the cutting edges, which steadied the drill and kept it working in a circular hole, whereas the ordinary chisel-shaped drill would cut gradually into an oval-shaped hole and finally get jammed.
The drills were made entirely of steel, and as they decreased in length by wear, they worked in for the shorter lengths of the set.
The stone, when quarried, was separated into heaps, the picked lot being taken to the stone crusher and block-making yard, the remainder, suitable for hearting, being filled into trucks and run down the incline, along the pier, and tipped into position between the side walls.
The concrete mixer was fixed at a level sufficiently high to allow of the concrete being delivered from the shoot into small tipping trollies.
On a level with the top of the mixer was placed a platform with lines of railway, 18-inch gauge, led to the delivery shoots of the stone crushers, and to the gravel depot. The latter situated under the high level railway from which the sand and gravel were tipped.
The cement was delivered from a shoot fixed close to the receiving hopper of the concrete mixer, the water supply being obtained from a cistern placed above it, so that the whole of the materials required for the concrete were manipulated from this intermediate level; the connection of the lines of rails being made by means of a triangle instead of a turntable, was found to be more economical, both in regard to time and labour.
The concrete mixer, with cement and water supply, were at one end of the base line, and the gravel depot at the other end, with the strong crushing machines at the apex.
The process of mixing was as follows. A trolly, made to contain a charge of two-thirds of a cubic yard, with the bottom hinged so as to open in two halves, on which a tubular bottomless measure for the cement was placed upright, was first brought under the shoot of the stone crusher, where it received its proportion of broken stone, after which it was shoved along the rails to the sand depot, where it received its quantum of sand and gravel; and then run over the shoot of the concrete mixer, where the tube was filled with cement and drawn out, leaving its solid core of cement standing up through the broken stone and sand.
A cord being pulled by the attendant, which released the bottom of trolly, and the charge was deposited into the hopper of the cylindrical concrete mixer. Water was added from a perforated pipe extending across the top of hopper, and the door of hopper pulled up, and the whole was delivered into the mixer, where it was turned over till the ingredients were all thoroughly incorporated together and discharged into the trucks below.
Three men were required for the mixing — one at the crusher, one at the sand depot, and one at the concrete mixer. The quantity made was at the rate of 6 cubic yards per hour.
Concrete block making
The concrete block making pit was made on a line parallel with the shops and stores. The floor being about 3 feet 8 inches below the level of the yard, to allow the railroads to pass clear over the tops of the frames in which the blocks were made.
The stacking floor was placed adjoining and parallel to this pit, separated by a line of railway upon which the blocks were conveyed to the works; and over both floors a Hercules or steam traveller traversed on rails laid on the outside of floors, 60 feet apart. The floor of building pit provided space for three rows of frames for making the concrete blocks.
Above the level of the top of the frames was placed a pair of heavy permanent way rails, about 3 inches apart, stretching across the pit every 12 feet, and secured to the side walls, to support a light portable railway, for transporting the small tipping trollies for filling the frames. The railway was formed of light rails, fastened to narrow planks, retained in correct gauge by flat iron bars.
The planks were in 12 feet lengths, corresponding to the spaces between the heavy permanent way metals, and were laid over the row of block frames in course of construction at the time.
The line of rails from concrete mixer crossed above the building floor at right angles, having three turntables, one in line with each row of blocks, for turning the trolly on to the line where it was required.
The mould frames were in four pieces, each piece forming a side, and were framed together and screwed up tight and placed in position on the building floor. Two holes were left in each block for the purpose of allowing the T-headed lifting bars to pass through the block when it required to be lifted and removed.
These holes were formed by placing two tapered pieces of wood in the position wanted, equidistant from the centre of the block. The concrete was then tipped into the frame, and on the following day the pieces of wood were withdrawn, leaving holes of similar size in the block. At the delivery end of the concrete mixer a man was stationed, whose duty it was to attend to the delivery of the concrete into the side tipping trollies, which were made to tip on either side, and contained about two-thirds of a cubic yard.
When the trolly was filled, he closed the bottom of the mixer by means of a lever, at the same time giving a signal to the man above, who delivered a fresh charge into the mixer, while he pushed the full trolly clear of the siding, and returned with an empty trolly to the mixer, another man taking the trolly with concrete along to the turntable opposite the line of rails required, and thence on to the mould frame, into which the contents were tipped, and the empty trolly returned to the siding again ; and so on continuously.
After the moulds were filled with concrete, a straight edge was passed over the top, to strike it to a uniform level with the top of the mould frame, and the block was then marked with the date of making. The sides of frame were loosened and removed on the third day.
The blocks were lifted after the fifth day by the steam traveller, and piled one above another in the stacking yard, in such a manner that the letters painted on the blocks and divisions where they were stacked corresponded with the plan and stock book, in which the position was noted that they were permanently to occupy in the work, which prevented confusion, and enabled the foreman to assure himself of having a sufficient number of blocks of the letter required stacked in the yard in advance of his setting work.
Building of eastern arm pier
The pier consisted of two longitudinal side walls, with cross walls bonded into them at intervals, the interior being filled with granite rubble and quarry rubbish, similar to the manner of work described in the first portion of the breakwater. The work was proceeded with in two lifts, to suit high and low water levels, which allowed of its being carried on full swing, independent of the state of the tide. The lifting and placing of the blocks in position was done by means of sheer legs made with one leg shorter than the other, equal to the height of the course, the long leg being in advance of the block to be set.
The side guys from head of sheers were fitted with block and tackle, so that they could be luffed or swung to either side by means of ropes passed down the legs ; and the chain for lifting or lowering the blocks passed from the concrete blocks up through a sheave block, attached to the apex of the sheer legs, and down through a guide block fixed to the foot of sheer leg along to the winch, which was secured to the masonry of the wall and worked by manual labour.
The concrete blocks for setting were taken from the stacking yard on trucks capable of carrying 16 tons by a locomotive to the head of the incline, where they were attached to chain of steam winch, and run down the incline opposite the place where they were required, when they were lifted off the trucks and set in position. The railways down these inclines gave a good deal of trouble at first, from the effect of the sea, especially during stormy weather, but this was ultimately overcome by making the road in lengths of a rail each.
The rails were rivetted to sleepers composed of short pieces of bridge rails reversed, and the lengths fished together and bolted. The ends of the sleepers were secured in position by chains to side walls of the pier, by bolts lewised into the masonry. This was found to act satisfectorily, and it served the double purpose of securing the railway and at the same time acting as a network which kept the rubble hearting in place between the walls.
It was only during exceptional storms that trouble from this arrangement was experienced, when the rails, chains, lewis bolts, and rubble hearting were all carried away to long distances from the pier, and the whole work was delayed for two or three tides. This form of road was easily laid as the work proceeded.
The extension was made by joining on a length of rails, and packing them up in lifts of hand laid stones, the space between being made up from stone tipped from side and end tipping waggons.
The foundations were formed in a very similar manner to those described in the first portion of the Hermitage Breakwater. The rock was cleared of seaweed and other impediments, and concrete in situ brought up to a height previously determined upon to suit the various levels of benches for receiving the masonry of the superstructure.
The cross walls were formed partly with blocks and partly with concrete in situ, in heights corresponding to that of the rubble hearting, and thoroughly bonded into the masonry of the side walls, and when the work was carried up to the top course the inside rubble hearting was protected from the action of the sea by a covering of concrete from 15 to 18 inches in thickness.
During very calm weather the work was pushed on through the night. A very effectual mode of lighting the foundation work was obtained by the use of a naptha and oil lamp designed by B Lavender and made by Milne and Sons, Edinburgh, with some alterations, including a conical reflector which the author found desirable to have made.
The power of illumination was such that a newspaper could be easily read on a dark night at sixty yards distance, horizontally from the lamp, and about 20 feet below its level, the steam required for the draught of the lamp was obtained from the boiler of steam hoist used at the top of the incline by a pipe ¼ inch diameter, and the cost of the oil consumed was slightly over one penny per hour.
The arrangement of the lamp is such that the combustion of mineral oils of a heavy specific gravity produces a very luminous flame by the aid of a draught of air, produced either by steam or a very high chimney ; the former is preferable, giving the clearest light with the least smoke. The lantern is practically air tight, except underneath, where the air enters in a way designed so as to cause it to impinge both on the inside and outside of the annular flame. This lamp gives little trouble after the wick is carefully trimmed and lighted, and is well suited for night out-of-door work even during high winds.
The plant and machinery upon these works were of the most approved and hand-labour-saving nature The special plant was made in England, and cost about £10,000. The ordinary plant, which consisted of railway rolling stock, boats, barges, concrete and sand tipping machines, derricks and lifting gear, etc, was designed by the author, and made upon the works.
J C Coode was the principal assistant engineer, and E K Gibson the accountant and treasurer, with the usual staff of foremen, clerks, and time keepers, etc. The whole of the work from the commencement was carried out under the personal superintendence of the author, without contractors.
Bruce Bell – comparison of costs
A discussion of this paper by members of the Institution of Civil Engineers took place on 18 May 1880.
Bruce Bell said he had seen the place and witnessed the work, and knew the history of it, which illustrated the great mischief that opposing parties in a State can do when the one in power is bent upon upsetting the arrangements of its predecessors. This great work, after being half completed, was now at a dead stand.
The island of Jersey, and the other Channel Islands, had each their own States and their own laws, as was the case with the Isle of Man; and those States could levy taxes and carry out public works upon their own responsibility, so that it was merely a matter of borrowing money, and finding sufficient to pay the interest thereon.
Jersey was a very rich island, and, as one instance of its wealth, the value of the annual export of potatoes alone amounted to £300,000, so that it was a great discredit to them to have stopped this useful and necessary work, a work which had been well designed and well carried out. It was a pity that it should have been allowed to stop, after sinking such an amount of money; and which, had it been allowed to go on, would by this time have made St Helier the best port in the Channel Islands.
The sinking of these concrete blocks of 100 tons weight in such an exposed position, in such a tideway, was a daring feat of engineering. The roll of the sea there was severe, not to speak of the tide, which rose from 30 to 40 feet in six hours, so that it would at once be seen that there were difficulties of no mean order to contend with.
Considering that the work stood out in the open sea, and was raised to a height of 60 feet, in a depth of 10 fathoms water, the cost certainly appeared moderate; for £300 per yard run was only £150 per lineal yard of cope for each wall. He had endeavoured to compare this with the cost of similar works, but found that engineers had been chary in giving costs.
The Dover breakwater, 65 feet high, built in 7½ fathoms water, he found was stated in a paper at a discussion in the Institution of Civil Engineers to have cost £1,245 per lineal yard of breakwater, or £622 per yard cope. The breadth, however, is 80 feet, or double that of the Jersey work, so that for the same cubic contents the price may be taken at £311 per yard of cope.
The Aberdeen breakwater, also built in an exposed ocean, but only 44 feet, or two-thirds of the height, he found was stated to have cost £216 per lineal yard, or £108 per yard of cope. In similar works built without cofferdams in a protected seaway, he had an instance of a work built 14 years since by Mr Miller and himself.
The sea pier of the Albert Harbour at Greenock, which was 36 feet in height besides foundations, cost £70 per lineal yard of cope; so that the cost of these Jersey works, taking the comparative contents of exposure, contrasted favourably with works of a similar description, and reflected great credit both upon Sir John Coode, the consulting engineer and designer of the work, and upon the resident engineer who constructed it; and it was to be hoped that the parties in the island would reconcile their diflferences and set themselves to complete this great undertaking.
James Gale – Portland cement
James Gale said that the invention of Portland cement in its recent application to work of this kind, had given the engineer a new power. A piece of work like this could not have been attempted 25 years ago. He believed the first application of this cement in large concrete blocks was by B Stoney, in connection with harbour works at Dublin.
It was used, mixed with sand only, at Port Said, and since that it had been used largely in such works. In this case they had used Portland cement, with sand and broken stones, to form the blocks. The proportions were given, and he thought them very remarkable.
It must have taken a great deal of faith to mix 13 parts of broken stone and sand with one of Portland cement, for some of those large interior blocks. They seem to have done very well, and that at a very small price — not above the tenth of the price of ashlar, and cheaper than most rubble could be quarried.
There were most valuable particulars given of the cost of the concrete blocks — which he thought one of the most valuable parts of the paper — showing the remarkably small price at which this work had been executed.
This Portland cement was going to alter very materially some of the larger engineering works for the future.
There was a very similar work, described in one of the engineering newspapers, at the Isle of Man, and which he believed was also designed by Sir John Coode, which was carried out in somewhat the same way as this.
Looking at one of the diagrams of the pier, he said it appeared to him that Sir John Coode must have been very confident of his outside work when he proposed, in such an exposed situation, to build two walls and pack in loose rubble between; for one of the most destructive forces was a heavy sea striking a wall, where the air, being compressed, would blow the structure to pieces.
He had no doubt that the outer wall was practically sea and water-tight, having been so well built with those blocks of concrete. It appeared to him marvellous, for it could not have been done so well with dressed ashlar.
Bruce Bell – storm damage
Bruce Bell quite agreed with Mr Gale as to the necessity for extremely careful building outside work when hearted in such a manner. This hearting was evidently the cause of the trouble with that pier. The whole of that pier, so far as it went, was founded at low water.
He had seen some very serious cases of walls constructed with open rubble work below and close ashlar work above. They might imagine the effect of a wave 10 feet high rising within such a wall, which would, on falling, leave nothing but air in its place, so that the return wave, as there was nothing to let the air out, burst the whole thing up at once.
Mr Bell in his paper mentioned that when the injury took place to the eastern arm in that way, about 100 yards of cross wall were damaged, and that more would have been injured had not the storm given way; and that then he protected it by putting a solid breastwork of railway metals fixed upright against the face of the wall and overlapping one another firmly secured with chains and lewis bolts ; and after the weather moderated the wall was built up solid.
The reason for putting in these cross walls, he supposed, was to break it up into separate spaces, so that if one happened to be damaged there would be no harm done to the others.
The strength of these blocks at Jersey was shown by some smaller blocks that he had seen which had been raised up by the waves and thrown right over the pier, without being smashed to pieces; and they appeared to have been made of the proportions of 12 of sand and gravel to one of cement.
The old idea of concrete was, the more cement the better. In fact when D Miller read his paper, on Structures in the Sea without Cofferdams before the Institution of Civil Engineers, in 1864, one of our leading engineers said that the proper way to make concrete would be to put in plenty of cement and little or no sand.
That was now proved to have been a mistaken idea. It was found that if they mixed the sand and the cement well together, and distributed the stones properly, a first-rate concrete was made in the proportions already indicated. Concrete was composed of stones and gravel, with cementing material made of lime or cement and sand, which when mixed became mortar; and good mortar was produced by one of cement to two of sand, and that was what held the stones together; so that in talking of 12 to 1 they talked of 10 parts of stone and gravel to 1 of cement and 2 of sand.
That meant 10 parts of stone to one part of mortar; and if they could distribute the mortar so as to bind these stones together, that was all that was required to produce good concrete. A mode frequently adopted to economise cement was to make a concrete with small stones, and while this was soft, to squeeze big stones into the heart of the mass.
J M Gale – mixing concrete
J M Gale thought in mixing concrete two things were to be considered. If they wished to make a large block that would stand handling, suitable for use in making a breakwater, they must make it of such proportions that the cement would be close enough together to hold its parts fast.
But when they made concrete to keep water out they had a very different state of matters. It was known that a heap of stones taken from an ordinary stone breaking machine had somewhere between 50 and 60 per cent of void, or spaces. If that was to be made watertight these spaces must be filled with cement mortar.
He could not make a watertight concrete otherwise than by mixing one part of cement with one part of sand, and the most broken stones that he could work in was 3½ parts. Of course, in building a structure like that described in the paper, the concrete did not need to be watertight, and therefore they could put in an enormous quantity of stone, and make a block at so small a price.
Replying to the discussion, Imrie Bell, stated that in the discussion the question of proportion of the materials used in the concrete had not been very clearly described, as in the blocks described of 8 to 1 it was meant that there were 8 parts of all kinds of material to 1 part of cement — 6 of broken stone, 2 of sand, 1 of cement. These proportions, he considered, after very careful consideration, could not be reduced if first-class cement concrete, sufficient for exposed face of harbour or sea work walls, were required.
Of course these proportions could be varied if shingle containing a proportion of sand were used in the place of clean broken stone. The details of the blocks made in the proportion of 15 to 1 could be seen in the table of proportions of materials. He might, however, add that the blocks were decidedly porous, as might have been expected. The reason the author was led to adopt this proportion was simply economy.
After careful study and experiment he found that the blocks thus composed could bear lifting from the building floor six days after they were made, which proved they were amply strong for the purpose they were required — namely, to take the place of loose rubble hearting between the two side walls of breakwater when it was determined to build the whole in solid masonry.
He agreed that the heavier the block the better it would stand the action of the sea, but this only stood when the weight could be increased without increasing the exposed surface to the resistance of the sea — namely, by increasing its breadth — which for many reasons cannot always be done.
In conclusion he also stated, as an instance of the force of the sea during the gale which caused the breach previously described, that the railway metals, which were less than six inches above the surface of the breakwater, and firmly fixed down to the solid masonry, without the intervention of sleepers, were twisted and thrown out of line so much as to stop the traffic on the roads, and had to be lifted and renewed.
The sea wall in landing pier, which withstood the effect of the gale, was now standing intact, though it had been left exposed to the gales of three succeeding winters, one of which even exceeded in intensity the gale which caused the damage to the inner or harbour wall.
Sir John Coode, in designing this harbour wall, considered that, owing to the outlying rocks, and the diminished depth of water, the wave stroke to which this arm would be subjected would be of a less trying character than at the breakwater.
Having regard to the length of the inner arm and the fact of its object being simply to afford the means of the approach to the outer or landing pier portion, it was an important consideration in determining the mode of construction to arrange the structure so as to incur the least possible expenditure, more especially as this inner arm would, upon the completion of the outer portion, be entirely protected from the effects of heavy seas.
Many members of the Legislature of the Island, and other amateur engineers, suggested various plans during the construction of the works which they considered better than the one being built, one of which appeared to the author plausible, and which was brought to his notice by the late Deputy Simon, a member of the Government.
The idea was to dredge a channel or canal from the deep water in the small roads up to the existing harbour entrance. After a careful consideration of the exposed situation, combined with a range of tide of 40 feet and with strong currents, in addition to there only being about three months in the year during which dredging operations could be carried out, he was of opinion that such a scheme was not practicable; and upon further reflection his opinion remained unchanged. He further believed that the only drawback to Jersey becoming one of the most favourite and fashionable watering places was the landing and embarking of passengers at all states of the tide and weather, and this must be accomplished at St Heller as it is the capital of the island, and has all the necessary accommodations on land for the shipping frequenting the port.
There could be no doubt, if the harbour was completed with such facilities, that the prosperity of the whole island would be vastly increased, and a spurt would be given to the building trade which would favourably affect all other trades, and tend most materially to the prosperity of that most lovely little island.
Imrie Bell served his apprenticeship with Bell andMiller of Glasgow. After acting as resident engineer on theconstruction of the Meadowside graving dock for. Tod and McGregor, of Glasgow, he entered the service of the East IndianRailway Company in 1859, and was appointed resident engineer on the southern half of the large railwaybridge over the Jumna at Allahabad.
On the completion of the work in 1865, he joined the staff of the late Thomas Brassey, for whom he took charge, as contractor's engineer, of the erectionof the Sirsawa bridge over the Jumna.
Returning to Scotland in 1869, he was appointed in the following year superintendent and engineer of the Leith docks and harbour.
In 1872, on the nomination of Sir John Coode, he became executive engineer for the construction of St Helier's harbour works and breakwater and La Corbiere lighthouse.
He joined the firm of Bell and Miller at their Westminster office in 1878, moving subsequently to Glasgow, and on the death of Daniel Miller became sole partner. Retiring from business in 1898, he went to live at Croydon, where, after a short illness, he died on 21 November 1906, aged 70.
Detailed drawings from Imrie Bell's paper on the 19th century Harbour construction