saving that would have been effected in the cost of pumping would Mr. Colson. have justified the additional outlay. With reference to the concrete, described as being mixed in the proportion of 12 parts to 1 part, it should be clearly understood that it was only used in these proportions in such parts of the works where it took the place of ordinary earth filling and backing, and for this purpose it answered as well as a more expensive concrete; but it was not to be recommended for face-work, or the more important parts of a structure. He might perhaps be allowed to draw attention, as he had already done,' to this unsatisfactory way of describing the proportions in which concrete was to be mixed. The quality of the concrete really depended upon the proportions of sand to cement; for instance, 12 parts of shingle to 1 part of cement might mean a very good concrete, or a very poor one, according as the proportion of sand in the mass were large or small. If ordinary gravel was being dealt with, and the sand was in a proper proportion, sufficient to fill the interstices, or say the bulk, the proportions would be 12 parts of shingle, 4 parts of sand, and 1 part of cement. If angular or broken material was used, the proportion of sand to cement would be less, and the resulting concrete would be so much the stronger. If, on the other hand, the 12 parts of material contained a much larger portion of sand, say (or, as he had known it, even more), the actual proportions would be 6 parts of sand to 1 part of cement, and the quality of the concrete would be proportionately reduced. Therefore he thought the ordinary way of describing the proportions of concrete conveyed, like the weight of cement, no true idea of its quality. The following details gave the approximate cost of some of the more important parts of the work: Subways, including excavation, filling over arch, con-` £. 8. 60 10 24 10 Excavation, including the cost of all timbering, removal, 39 0 Approximate cost per lineal yard 124 0 1 Vide Minutes of Proceedings Inst. C.E., vol. lxii., p. 151. The approximate cost per lineal foot of the locks and docks was shown by the following statement: £. No. 12 dock.-Granite invert, Portland sub-invert.) No. 13 dock.-Granite invert, Portland stone sub-invert.) Deep dock.-Granite invert, also lower and upper altars, and concrete backing. No timber or piles in the 280 0 263 0 Taking the sectional capacity, the cost per cubic yard was: Compared with the following, the cost of these works was not Mr. Colson. unfavourable :: 1 7 0 2 14 0 Somes', Blackwall Baston, N.S. Birkenhead Malta, old dock Somerset dock, Malta Avonmouth dock (lock) about Hull docks (lock) about . . (This did not appear to include the whole of the excavation.) 1 Lectures on Marine Engineering, School of Military Engineering, 1875, 1876. J. B. Redman. 2 Deduced from Minutes of Proceedings Inst. C.E., vol. lv., p. 21. 3 Ibid., vol. xli., p. 113. With regard to the use of caissons for closing the entrances in place of gates, the advantages attending the use of the former appeared to be, first the saving of room; take for instance the locks, arranged to pass ships in or out at any time of the tide, which were also fitted and used as docks. The length now taken up by the caissons and entrances was 69 feet at the West end, and 55 feet 3 inches at the East end; had gates been adopted they must have been double at the West end to have made the lock available at all times of the tide as at present; a greater length (say about 48 feet) would, therefore, have been required; this would have taken from the effective length of the dock, or it must have been lengthened at a considerable additional cost, and at the expense of either the area of the tidal or repairing basin. The objection as to double gates would apply also to the entrances between the respective basins, and the basins and harbour, inasmuch as it might be necessary to lower the water in one basin below the level of the other, or perhaps to empty one or other of the basins. It would, therefore, be necessary to make Mr. Colson. provision for such a contingency by fitting double gates, but they would not apply to the East end of the locks and to the docks when the pressure would always be on one side; but even here a somewhat greater length of entrance would be required to accommodate the swing bridges. Another advantage was the facility with which they could be repaired as compared with gates. In the case of the ship caissons any ordinary repairs, scraping and painting, could be executed with the caisson in place, one side being done first, and then the vessel turned end for end in the groove, and the repairs done in the same way; or the caisson could be floated out of the groove and placed in dock for repairs, a spare caisson being placed in the entrance, whilst the dock was not thrown out of use even for a day. On the other hand any repairs to gates below the water line would necessitate their being lifted, which was, as a rule, a somewhat long and expensive operation, during which the dock was thrown out of use. All the entrances were crossed by railways, therefore the adoption of gates would have necessitated the addition of swing or some other arrangement of bridge, which must have been of a substantial character. The railways were now carried by the decks of the caissons, and the inconvenience of having the dock encumbered with a bridge was avoided. No doubt there was the disadvantage of having to find room for the ship caisson when removed from the entrance, and in small docks, where water space was of great value, this was, perhaps, prohibitory; this, however, was avoided by sliding the caissons into chambers specially prepared for them. With regard to the time required to manipulate the caissons, it was found that the ship or floating caissons were, under ordinary circumstances, moved clear of the entrance in twenty minutes, and were replaced in the same time. The sliding caissons were removed from the entrances, under ordinary circumstances, in six and a half minutes, although the entrance could be, and had been frequently, cleared in three minutes, and closed in the same time. No doubt a single pair of gates would cost less than a caisson, but it was an open question whether in the case of, say the East entrance to the North lock, the gates and bridge together would not exceed the cost of the caisson. This entrance was 82 feet wide, and 39 feet to the invert; the gates would measure 88 feet by 36 feet = 3,168 square feet, and if constructed of iron at £1 158.1 per square foot, would cost £5,544 the pair. 1 Vide Minutes of Proceedings Inst. C.E., vol. lv., page 72. The bridges would require to be constructed in two parts, Mr. Colson. meeting in the centre, and would be subjected to heavy loads on a small wheel base, and would, therefore, require to be substantially constructed. The weight of one wing could not be put at much less than 90 to 100 tons; this weight at £17 per ton would be = £1,700, or for the bridge complete £3,400; the total cost of both gates and bridge would therefore be approximately £5,544 + £3,400 = £8,944, against £8,500, the cost of the caisson. With regard to the West entrance the width of opening was also 82 feet, and the depth of the invert 47 feet 8 inches; the gates would, therefore, measure 88 feet by 44 feet 8 inches 3,930 square feet 8 inches, which at £1 198. would = £7,664 168., and as he had before pointed out, two sets of gates would be required to render the lock available at all times for allowing a ship to pass into or out of the repairing basin; therefore the cost of the gates would = £15,329 128.; add to this the cost of the bridge as before = £15,329 128. + £3,400 = £18,729 128. against £18,700, the cost of the caisson. There would be in addition the extra cost of the foundation for the bridge, and the bridge pits. = These estimates were of course approximate, but they were sufficiently near to show that the cost of gates and bridges together would be equal, if it did not exceed the cost of the caissons, without affording the same advantages. He had alluded to the necessity of movable bridges in connection with the gates, to carry the railways over the entrances; a double swing bridge, meeting in the centre, but without central support, was not an arrangement to be recommended for railway purposes if it could possibly be avoided. Single swing bridges would have taken up still more room on the dock side, and would have still more reduced the effective length of the docks and locks by the increased length of swing. Roller bridges might, perhaps, have been used at some of the harbour and basin entrances, when the path of the bridge would have been in a line with the quay, but it was doubtful if such an arrangement would have been a satisfactory one; they were, he believed, the least economical type of any, and the obstruction on the quay would have been very great, unless some such arrangement for lowering the bridge below the surface as that described by Mr. Giles,1 had been adopted. One of the principal conditions of dock arrangement should be that the entrances should be, as a rule, clear of all obstructions, and this ' Vide Minutes of Proceedings Inst. C.E., vol. lvii., p. 53. |