THE HARBOR WORKS.

Press, Volume XI, Issue 1379, 9 April 1867, Page 2

THE HARBOR WORKS.

We publish a report of Mr Balfour, Marine Engineer, on the present state of the harbor works in Lyttelton : — Wet/lington, April 3, 1567. Sir,—-I hfivo the honor to report that, as requested in your letter of the 9th ultimo, I, on the 13th, "visited Lyttelton, and carefully examined the harbor works, being accompanied by the Provincial Engineer, from whom, and the Resident Engineer, and from Mr Wright, the contractor for the sea-wall, I received much valuable information and assistance. It would obviously be impossible to express a definite opinion on many points connected with the works without very accurate surveys, repeated at intervals, so as to extend over a considerable time, as well as a number of very accurate borings ; bat I understand that time is an important object, with a view to the immediate prosecution of further works, to accommodate shipping as soon as possible after the opening of the tunnel, and I consequently venture to make a present report with that view, premising that I am indebted to others for the greater portion of my data. 1. Sea-wall, &o—l havo examined the drawings and specifications of this work ; thoy are carefully framed, and that the details of the design are sufficiently strong is amply proved by the fact that no portion of the framing has given way, notwithstanding that it has been exposed to enormous strains, owing to the lateral motion of portions of the facing. I cannot speak decidedly as to whether the piles have been driven to a suitable depth without making fresh borings and driving several experimental piles ; the fuct that come portions of the work have not slipped at all, however, goee far to prove that the work is very nearly on a balance, and that under more favorable circumstances no slip might have occurred, in which case of course no question could have arisen as to whether the piles had been sufficiently driven. It is possible that had the timber facing been first completed, and the filling afterwards carried forwards with due care, with an ample facing of rubble, such as I understand was shown on the original design, then more favorable circumstances might have been obtained ; but as to this a decided opinion cannot possibly be formed. The fact of the piles not having been driven to the rock appears, under the circumstances, to have been fortunate, as had the points been completely jammed, and a slip of the embankment occurred, the facing must unquestionably have been overturned, if tho piles had not been broken. Were the work to be done again I should certainly recommend a different design, and would probably dispense with the timber facing entirely in the first instance, so as to allow the earth and stone bank to come to a thorough bearing before attempting anything in the shape of a wharf wall; it is however easy to be wise after an accident has shown the weak points of b. design, and I am of opinion that the Provincial Engineer was not unjustified in expecting the work to stand, more especially as portions have actually resisted the pressure of the filling and are still in their original position. The specification clearly anticipated considerable vertical settlement and provided for it, and there are several instances of engineering works in England where vertical settle__j Kα provided against, being as it were suspended in a ji<.»ai.. s uiuwmn i the most notable instance which at this moment occurs to mc is the carrying of the Liverpool and Manchester railway across the Chat Moss ; there the line was actually floated on the surface, and it has thoroughly answered the expectations of George Stephenson, the engineer who planned the work. Mr Dobson has fully explained to mc what he proposes doing to complete this work, namely, to drive one outer row of piles in a straight line, remove the present capsills, cut all the present piles to a uniform level, put on new capsills, and bring up the whole to a proper level by joisting and planking. If the work has ceased to move forward I think this would amply suffice, and indeed it is almost the only plan which could bo suggested short of what would be practically a totally new structure for completing it. The experience of the last gale on the night of the 16th ult. goes far to prove, judging from the report of tho Resident Engineer, that the already completed structure has come to a bearing, and that no further movement of any consequence need be expected; but it is impossible to predict the result of further filling. If carefully done there is a reasonable prospect of its remaining firm certainly, but it may slip, and if it once begins it will go on in spite of anything that oan bo done to prevent it until it has come to a natural bearing. As the only precaution which can be suggested, I should recommend that the remainder of the filling be kept for the present entirely separute from the present made ground. A portion may be tipped from the land side outwards, but at the same time let a road or caueeway of rubble be carried along the back of the present earthwork, and parallel thereto, but say thirty feet inland from it. This causeway may be twenty or thirty feet wide, and should be carried up so high as to be above the surface of the mud after it has settled. The filling above may be of clay, merely faced with stone to protect it from the present wash of the water. From thjs roadway tho earth should be tipped towards the land, so as to meet the other filling. I should recommend that the space between tho original filling and the new work should be left open for a considerable period as being a certain provision against slipping. After a time, when the made ground had become consolidated, the interval should be made up wich rubble. It must be always kept in mind that on such a site as is now known to exist in Lyttelton,any filling which can bo made rune a risk of slipping and damaging any works in front of it ; the proposed plan of operations seems to give the best chance not only of preventing I a slip tit all, but also, should such an accident occur, of avoiding damage to the works in front. j In face of the possibility of further motion, I should hesitate to recommend that anything should, be done to complete the timber facing before the whole of I he filling has been completed : if the outer works are really neceseary let them be undertaken, but with a full knowledge of the fuct that they may at least possibly be damaged by slips which may occur in the new filling within. One other matter which will call for very careful consideration is, the action of the sea on the under side of the road planking of tho timber facing or cribbing when finished. From the descriptions I have received of the effects of the storm of March 18, it in clear that the waves have run with great force up the sloping face of that portion of the filling which is not protected by vertical plankiug above low water level, and has washed a considerable amount of stuff away ; and this was not more than might havo been expected, though I confess 1 never anticipated that waves of so destructive a nature could have been generated in the harbor. The danifige can easily be made good by extending the planking and completing the filling at the weak point with heavy material, but the serious question arises as to how the waves comported themselves at that portion of the work when upright plunking is already in place. If they were deflected upwards with violence, as seems not improbable, they would probably have

damaged or carried away the planking had it been fixed. This will require enreful investigation, as the manner in which tho fueo work should be decked when completed will entirely depend on the expected action of the sen on the under surface of the. plunking. As this timber cribbing can only in fino weather be of any use for shipping to be alongside, I would suggest that the. accommodation of the port would be greatly increased, as well as the cribbing itself very considerably strengthened, by running out from the weakest parts two other short jetties like the lighterage ! jetty, but in the same general direction as the | screw-pile jetty ; these, it" properly constructed, would net as abutment*, and even should a further slip occur, they would not be liable to very serious injury, while vessels would, I think, be able to iio alongside them in any , weather. 2. Breakwater—l have also examined the breakwater, and had its system of construction explained to mc. The plan adopted is very suitable to the site, ami it is practically the same as that on which the National breakwaters at Portland and Alderney have been carried out; but the dillieulty to bo contended with at Lyttelton is the slow rate of progression, as works of this class are always liable to be more or less washed down during construction, and it is consequently found that the faster they are pushed on the better. At Holyhead, for instance, from 3000 to 5000 tons of material were dropped from staging every day. There can be no question that any earth or very small material will be washed out of tlte breakwater as constructed, and tend to fill up the harbor. No stone smaller than road metal should be tipped in the rubblo work at hII (and such I understand to be tho rule), and only a moderato proportion of the rubblo should be so small—no more, in fact, thau sufficient to fill up the crevices between the larger blocks. The action of a heavy sea on the mould will bo to consolidate it, and make it settle down, thus caueing unsightly work water, but not necessarily to spread it in the interior of the harbor beyond tho foot of the slope, which is usually a well delined line. If too much small material h*e been tipped, doubtless some will have been washed inside ; and I should recommend that a diver be sent down for largo samples of the bottom at various parts of the harbor within the breakwater as being the ouly satisfactory way of settling the question. There is another way, however, in which the breakwater will certainly tend to fill up the harbor, and that is by forming a pocket of still water in which the suspended silt will be deposited. This cannot be avoided, and must simply bo submitted to as inevitable. The better the harbor is made the greater will this tendency become, until indeed tho whole of the contemplated enclosure breakwaters have been completed, when the evil will be mitigated by their excluding a large proportion of the silt-bearing waters of the ebb tide. The probability is that a dredger will bo required to maintain the depth in the harbor before many years have elapsed, and it is only right that this should be kept in view. I cannot imagine the possibility of sheltering the harbor without increasing its tendency to silt up. Aβ the breakwater is now getting into deep water, and would make a very good berth for large vessels had it a wall face on the inside, I have to recommend that it be carried on in future in a different manner. I would adopt a timber framing, planked inside and outside, and filled with rubblo in the manner very successfully carried out by Abernethy, at Blyth, on the east coast of England : afterwards tipping over a sufficient amount of pretty large rubble to protect the outer face and break the force of the sea. By adopting this plan the breakwater would progress much more rapidly, berths for largo vessels would be procured at an early date, and the filling up of the harbor by the spreading of the material would be prevented The cost of the breakwater would certainly be increased but by very little more, I think, then it would cost to make a vertical facing on the inside of the rubble mound as at present contemplated. The timber work, if well executed, and good material, would probably last a number of , J l>. v- ;i r ~1.x»n.l pnvmnn-'l at a moderate cost by facing the interior by large blocks of hydraulic concrete, moulded to the proper form and deposited by a travelling crane after they have become sufficiently hard. By constructing the breakwater, as above suggested, and by continuing Norwich Quay right round to it and laying rails thereon, berths for larger vessels in direct communication with Christchurch will be obtained in a short time, and I think at less cost than by any other plan which can be suggested, while at the same time the inner jetties will be better sheltered and more convenient for steamers and small craft. I have the honor to be, Sir, Your most obedient servant, James M. Balpoub, Marine Engineer. The Secretary for Public Works.