COUNCIL PAPERS.

Globe, Volume III, Issue 253, 3 April 1875, Page 2

COUNCIL PAPERS.

CORRESPONDENCE RELATING TO THE PROPOSAL TO CONSTRUCT A GRAYING DOCK AND PATENT SLIP IN LYTTELTON. Some correspondence relating to the above subject was laid on the table of the Provincial Council yesterday. It opens with a memo, from the Secretary for Public Works to Mr Eliot, stating that the Government desire to obtain a report from him as to the best site for a graving dock within the space intended to be enclosed within the moles now being constructed from Officer’s Point to Naval Point, capable of accommodating sailing vessels and steamships of the largest size like'y to frequent these waters. Provision should also be made for a patent slip for smaller vessels. Mr Eliot replies, undertaking to report on the subject, and also on the best means of improving the navigation of the Sumner bar: his professional charge for which would be £IOO. Some correspondence then follows on the question of obtaining Sir J. Coode’s permission to employ Mr Eliot, The following is Mr Eliot’s report To his Honor the Superintendent of the Province of Canterbury. Sir, —In accordance with instructions received from the Secretary for Public Works, I have the honor to submit to you the following report upon the site for a proposed graving dock and patent slip in Lyttelton harbor. In selecting a site for a graving dock, there are various points to be considered, which in the present case may be enumerated as follows : 1. Nature of foundation. 2. Amount and nature of excavation as affecting the cost of construction. 3. Position with regard to other works already executed or proposed to be executed within the harbor, 4. Position of entrance of dock, with especial regard to affording an easy and clear approach to the longest vessels which may require to make use of it. 5. Facility of access to the dock by land. In selecting the site at Naval Point, as shown in red upon the plan accompanying this report, I have taken into full consideration all the various points above enumerated, and I am of opinion that the site, as shown, is the best that can be obtained for a graving dock within the limits of Lyttelton harbor. 1. The foundation is good, as it consists of the solid rock throughout, very nearly the

whole length of the dock; the only part of the dock which will not rest upon the solid rook being the outer end, and this part will rest upon the hard marl which everywhere forms a layer over the rock, varying from 2ft (two feet) to 6ft (six feet) in thickness. This marl is very hard and compact, and will, I think, form quite as firm a foundation as the rock itself; or, if considered necessary, a solid bearing may be obtained on the rock by slightly increasing the depth of foundation at the outer end, and thus any risk of settlement may be avoided. I think, however, such a precaution would not be required as far as I can judge from such observations as I have been able to make. 2. The amount of excavation necessary for constructing the dock will not be very great. It will be observed, from the sections on drawing No 2 accompanying this report, that the amount of excavation is s.ightly in excess of the quantity required, as filling in behind the walls : it is probable that much of the rock excavated will be available for building the walls of the dock, but if there should be more stone excavated than is required to be us p d on the work, it can be easily disposed of in constructing the retaining wall of approach road, or by tipping it on the breakwater now in course of construction from Naval Point. 3. I consider it of great importance that the whole of the north side of the harbor should be reserved for wharfage accommodation. A graving dock, built anywhere along the north side, would not only occupy space which might hereafterjbe required for jetties and wharfage, but would interfere with the communication by tramways from one wharf and another. The water is also more shoal on the north side, and consequently a greater amount of excavation would be required in the construction of a graving dock. I have therefore selected a site, not only giving the greatest depth of water, but one which cannot possibly interfere with the erection of any wharf or other work within the limits of the harbor. 4. As I understand an opening is to be left at the east end of the Naval Point Breakwater, for the passage of boats and small vessels, I do not contemplate that jetties will ever be constructed into the harbor from the Naval Point Breakwater. Indeed such jetties would be most difficult of access to vessels, especially if the jetties now being constructed from the north shore, are extended to any length. It may therefore be presumed that the entrance to the graving dock, as proposed, will always be clear and open for vessels of any length and size which may be likely to frequent the port, and that such vessels may be warped in and out of the graving dock with the greatest ease. 5. There is already a road leading down to the beach on the west side of the harbor. I would suggest that this road be continued with an easy gradient to form an approach to the graving dock, as shewn by red lines on plan. This road may be formed partly by cutting into the cliff, which is not steep, and consists of a loamy marl, and partly by an embankment. The material excavated from the cliff may be tipped to form an embankment to a little below high water mark, the embankment being protected by a retaining wall of rubble. The rubble for the wall may be obtained from the surplus material excavated from the dock, as already mentioned. As regards a site for a patent slip, I think there cannot be a better one than that adjoining the proposed site for graving dock, between the dock and the west portion of the breakwater. The same arguments used above in recommending the site which I have proposed for the graving dock, may be equally applied to the patent slip in suggesting the site as shown on the accompanying plan. From the slope at which the surface of the rock lies, it is impossible to arrange that the whole length of the slipway shall have a firm and solid bearing on the rock. It will, therefore, be necessary to form a support for the rails, for a distance of about (100 ft) one hundred feet at the outer end of the slipway. There will not be much weight upon the rails at this end, as the ship, on being grounded on the cradle, will have the greater part of its weight supported by the water, and by the time the cradle is drawn up sufficiently out of the water to bear the. full weight of the ship; the rails on which it will move will be resting on the solid rock, I would suggest that the mud or soft material, overlying the rock at the outer end of the slipway, be removed, and the space filled in with rubble, the rubble being placed in carefully, and in a compact manner, to form a suitable bearing for the sleepers which carry the rails of slipway. This slipway should be constructed, if possible, at the same time as the graving dock, as they could both be enclosed by the same cofferdam, with very little extra expense. The extra expense incurred in extending the cofferdam of the graving dock to enclose the site of slipway would be far less than the expense of employing divers to construct the slipway at a future time, and the work would be done much more effectually behind a cofferdam. If, however, it should be decided to postpone the construction of the slipway till the graving dock be completed, it would take a very short length of cofferdam hereafter to enclose it, in comparison to the length that would be required if it were placed in any position not adjoining the graving dock. I would also suggest that the same engine which is employed to pump the water out of the gravies dock may be used to work the patent slip, which is another reason for keeping the slipway close to the graving dock. The total length of slipway as arranged will be (400 ft) four hundred feet, and will take a vessel (150 ft) one hundred and fifty feet in length. Vessels longer than this will use the graving dock. The total width of platform of slipway will be (40ft) forty feet, and the gradient at which I have shown the rails to be laid is 1 in 20. The graving dock, as arranged on plan, will take a vessel 400 ft in length, and I think it improbable that any vessel will ever visit the port of a greater length than that; I have shown the entrance of dock arranged to receive an iron caisson for,the purpose of closing it, instead of gates. But this is a matter of detail to be determined hereafter, and will not materially affect the design of the dock as regards its site. As shown upon the longitudinal section, there will be a depth of 20ft at low water, spring tides, over the sill of entrance to dock; it will be necessary to dredge a channel in the mud leading to this entrance. But as I understand the whole area of the harbor is to be deepened by dredging, the draught of vessels using the dock will be limited by the depth to which the interior of the harbor is dredged,

I would suggest that the portion of beach directly to the north of the proposed site for graving dock, be reserved as a beaching gro md for sm I 1 vessels and boats. It is at present used as such, and there is not another site within the harbor which offers such facility for beaching vessels. There are two drawings accompanying this report; No 1 shows a general plan of the west side of the harbor, with the site of proposed works shown in red lines; No 2 contains a longitudinal section of site fcr patent slip, showing the surface of rock as at present excavated, and the proposed future excavation. Also a longitudinal section of site for a graving dock, and two cross sections of same. The line of rock, where shown below the mud, is in reality the surface of the hard marl which covers the rock; it was too hard to bore through with the gear which I had at my disposal, but as the object of my boring was to ascertain the nature of foundation, I am quite satisfied that the marl will afford a very good foundation, even if the solid rock should happen to be some distance below it. However, I imagine that the layer of marl over the rock is not very thick. I have purposely avoided any remarks as to details of construction in this report, and on the accompanying drawings I have not shown any more details or measurements than are necessary to explain and illustrate this report. Any further details must be reserved until the designs for the proposed works are prepared, The object of this report is simply to show that it is practicable to construct such a graving dock and patent slip, as the trade of the port requires, in the positions I have shewn, and that in my opinion these positions are the best to be obtained within the limits of the harbor. I have been requested to give an approximate estimate of the cost of constructing a graving dock and patent slip. There are, however, so many matters of detail in connection with a work of thisjdescription, that I find it,quite impossible to form even an approximate estimate without a certain amount of detail drawings, especially as the colonial prices differ so materially from those to which I have been accustomed. With the limited time at my disposal, 1 fear that any attempt I might make at an estimate of the cost of such works would be more likely to mislead than assist the Provincial Government, I may add that, if the Provincial Government should desire it, and should instruct me to do so, I will undertake, on my return to England, to prepare a set of detail working drawings for the graving dock and patent slip, with a full estimate of cost and specification for construction of same. Trusting that the suggestions and observations contained in this report may be of service to the Government, I have the honor to be, sir, Your obedient servant, Whately Eliot. Christchurch, December 2nd, 1874. From further correspondence it appears that Sir J. Coode declined to permit Mr Eliot to advise the Government as to the proper works to be executed. Mr Eliot, however, was delegated to procure all the facts and data, to enable Sir J. Coode to prepare a design, estimate, and report. The Government accordingly, at Mr Eliot’s request, agreed to his withdrawing his offer, and Sir J. Coode forwarded instructions to Mr Eliot to obtain all the necessary information to enable him to prepare plans, &c., his offer to do so for £420 having been accepted by the Provincial Government. He was requested by the Government to give an estimate of the cost of the graving dock and slip, and telegraph the required information in time for the meeting of the Provincial Council, and furnish the drawings and specifications, if possible, by June next.