Wharfs at Hull.

The splayed wings of the entrance to the Alexandra Lock, at Hull,* are lined with timber wharfs, which are returned for a length of 300 feet up and down the River Humber. The wharfs (fig. 246) were constructed in bays, generally 10 feet in length, but 3 feet at the corners, the framing being braced both longitudinally and transversely, and covered with a 5-inch decking. The river bed in front of the wharfs had been dredged away to about 40 feet below the top of the piles, so that the piles, which were 61 feet in length and about 15 inches square, penetrated only about.

20 feet into the ground. Grooved and tongued sheet piling, 25 feet long and 8 inches thick, was driven along the front, the top being just above low water. The sheeting was driven in lengths of 6 feet at a time, all the piles in one bay being previously pitched in position so as to ensure tight contact. This sheeting held up the material at the back when the river bed was deepened in front. During construction the mud accumulated so rapidly, in the recesses behind, that whole-timber sheeting had to be driven at the back to retain it, the space enclosed between the front and back piles being excavated to enable the cross bracing to be fixed at the lowest possible level. The wharfing was constructed from a staging on piles driven by piling machines on barges. The sheet piling was driven by piling machines with telescopic leaders.

*

Hurtzig on "The Alexandra Dock, Hull," Min. Proc. Inst. C.E., vol. xcii.

CHAPTER VIII.

DOCK GATES AND CAISSONS.

DEFINITION AND RELATIVE ADVANTAGES OF GATES AND CAISSONS-METAL versus WOODEN GATES WEIGHT, COST, DURABILITY, AND STRENGTH-SINGLE-LEAF AND DOUBLE-LEAF GATES-HORIZONTAL AND VERTICAL GIRDER TYPES-STORM GATES— STRUT GATES-STRESSES IN GATES-STATICAL FORCES-METHODS OF FINDING RESULTANT PRESSURE-ZONES OF EQUAL PRESSURE-RISE OF GATES-ANALYSIS OF RESULTANT-GRAPHIC REPRESENTATION-LIMITS OF STRESS-TYPICAL EXAMPLES -VERTICAL CO-PLANAR GIRDERS-STRESS IN PANELS-EXEMPLIFICATION OF GATE CALCULATIONS—FITTINGS-EXAMPLES OF GATES AT LIVERPOOL, BIRKENHEAD, MANCHESTER, HULL, BUENOS AYRES, CALCUTTA, SOUTH SHIELDS, AND DUNKIRK— TABLE OF DOCK GATES-STRESSES IN CAISSONS-CLASSIFICATION OF CAISSONSSWINGING, TRAVERSING, SLIDING, ROLLING, FLOATING, AND SHIP CAISSONS -LOWERING PLATFORMS-EXAMPLES OF CAISSONS AT MALTA, BRUGES, BLACKWALL, CARDIFF, CALCUTTA, BELFAST, LIVERPOOL, AND GREENOCK-TABLE OF DOCK CAISSONS.

IN localities where there is considerable tidal range and where circumstances render it desirable to maintain the surface of the water set apart for the reception of shipping at a fairly constant level, it is evident that the entrance or entrances to a dock must be closed in order to impound the water, and must remain closed during those portions of each day in which the tide falls below a certain limit. This is usually effected by means of (1) gates or (2) caissons, and occasionally provision may be found for both forms of closure. Graving and repairing docks are treated in like manner, but for a different purpose, the object in this case being to exclude the external water during the time of occupancy.

Definitions. The distinctive feature of a gate is that it revolves about an axis, in most cases vertical, but occasionally horizontal, while the motion of a caisson is either rectilinear or altogether untrammelled. As with many other terms, however, employed in dock engineering, this definition is not susceptible of too rigid interpretation. There is an intermediate class of gate-caissons combining the hinge or axis of the gate with the broad beam of the caisson, and exemplified at Bristol, Dundee, Havre, and other places, though, taken on the whole, the type is rare.

The Relative Merits of Caissons and Gates, considered as two distinct, though comprehensive, classes based on the foregoing definitions, may be broadly gauged as follows:

1. Gates with vertical axes need side recesses into which they may be swung when the entrance is to be opened for the passage of vessels. This

necessitates a considerable and expensive addition to the length of the side walls, especially when the lock or entrance is of great width, as often obtains at the present day. Caissons do not occasion any increase in the length of the side walls, but, on the other hand, there must be reckoned the cost of a special chamber for sliding and rolling caissons. Ship caissons do not need a chamber, but, when out of use, they have to be berthed somewhere, and this leads to a certain amount of inconvenience in the appropriation of useful space.

2. Caissons are generally of stronger build and broader beam than gates, and they afford accommodation for the transmission of rail and road traffic across a waterway, thus discharging the functions of a bridge in addition to those peculiarly their own.

3. The first cost of a caisson is undoubtedly, in most cases, greater than that of a pair of gates, but if the cost of a swing bridge for vehicular traffic, which is a necessary adjunct in the case of gates, be also taken into consideration, the advantage will be found to lie with the caisson. This advantage is still further emphasised where a lock or passage is fitted with double gates to alternately impound or exclude water. A caisson can be constructed to act equally in both directions.

4. Caissons obviate the necessity for pointed sills and gate platforms of large area, but those of the ship type, fitting into grooves so as to be capable of acting in two directions, call for battered side walls to allow of their floating clear when manoeuvring in and out of position, and this gives the entrance an unsuitable profile for modern vessels of square amidship section and with bilge keels.

5. Floating caissons are not always manageable in boisterous weather and strong currents, and oftentimes they are only workable with difficulty. Sliding caissons, too, have to encounter the effect of wind pressure, especially if there be much clearance between their keels and the sliding ways. Neither can rolling caissons be said to be altogether exempt from the abrading or wearing effect due to the action of friction on the moving parts under lateral pressure. So that, on the whole, it may be claimed that gates are easier of movement and are more completely under control during manipulation.

Dock Gates.

Gates are sometimes distinguished as wooden gates or iron (including steel) gates, according to the nature of the bulk of the material of which they are composed. As a matter of fact, both materials enter essentially, though in varying proportions, into the construction of all gates. It would be impossible to connect the various members of a wooden gate without the aid of metal bolts, straps, and other fastenings, while iron gates depend for their watertightness (except in rare instances) on wooden posts and plates at the abutting surfaces.

EFFECTIVE WEIGHT.

303

As regards the relative advantages of wood versus iron gates, the following points may be noted :—

1. Dead Weight. For a given width of entrance, wooden gates are considerably the heavier. Greenheart is the wood now most extensively adopted in this country, but in spite of the fact that its specific gravity, though high for timber, is considerably less than that of either wrought iron or steel, being only 11 to 12 as compared with 76 and 7-8 for the metals respectively, yet it outweighs them both by reason of the excessive bulk required to offer an equal resistance to stress. This disparity in strength is still further emphasised in the case of the lighter woods, such as oak and pitch pine, considerably in vogue at Continental ports. And it must also be observed that no inconsiderable addition is made to the weight of a pair of gates by the unavoidably extensive use of metal fittings and connections. The weight of a pair of iron gates, 25 feet deep, at Dublin, for a 70-foot entrance is stated to be 90 tons. A similar pair of steel gates at Limerick are about the same weight, while a 69-foot lock at Dunkirk possesses iron gates, 24 feet deep, weighing 88 tons. As against these fairly representative values for metal gates may be set the weight, 204 tons, of the wooden gates (48 feet deep) to a 70-foot lock at Avonmouth. These gates are mainly framed in pitch pine and memel, the heelposts and mitreposts alone being of greenheart. The weight of the iron fittings, including a cast-iron roller path, amounts to 42 tons. At the south lock of Buenos Ayres Harbour, the waterway is 5 feet less in width and 13 feet less in depth, but the gates weigh as much as 206 tons, owing to their entire construction in greenheart. For entrances of greater width, wooden gates attain enormous figures, as, for example, the greenheart gates (44 feet deep) at a 90-foot passage at Liverpool, which weigh no less than 330 tons. It is quite safe to assert that a pair of metal gates of the same size would not exceed half that amount.

2. Effective Weight. Not only is the dead weight of wooden gates necessarily much in excess of that of iron gates, but the practicability of forming watertight compartments in the latter, constitutes a means of still further reducing the actual working load, since the flotation power thus obtained may be arranged so as to practically counterbalance the weight of the gates, leaving only a small margin for stability. By this means the power required for opening and closing the gates is reduced to a minimum. Even in localities where there is very great tidal range, and where anything like an exact counterbalance would be attended with much difficulty and some danger, the reduction in weight which can be safely made is far from negligible. At Dunkirk there were, some short time back, two similar entrances, 69 feet wide, one fitted with iron and the other with wooden gates. When immersed at mean sea level, the weight of the iron gates was reduced from 98 to 16 tons, to which 16 tons of water ballast was added making 32 tons in all. The wooden gates, when immersed, weighed just double this last amount. They have now been replaced by iron gates.

3. Initial Cost.-Generally speaking, gate materials may be placed as regards cost in the following order, commencing with the most expensive-Greenheart, iron, oak, and creosoted pine. The exact proportion, of course, depends on current prices. At the present time, greenheart logs of large size can hardly be obtained for less than 3s. 6d. to 4s. per cubic foot, and for great lengths, the price will run as high as 5s. or 6s. Under such circumstances, greenheart gates, for entrances ranging between 60 and 100 feet in width, may be expected to cost, under normal conditions, from 40s. to 50s. per superficial foot of gate. Oak may be priced in this country at 3s. to 4s. 6d. ; red pine at 2s. 3d. to 3s. 3d.; and pitch pine at 1s. 3d. to 2s. 3d. per cubic foot. Gates of these last named timbers will be relatively cheaper with a corresponding decrease in durability and strength. The cost of iron gates has fluctuated somewhat. In 1857 the Dublin graving dock gates cost 46s. 9d. per square foot of gate area, but the figure is a high one, and due, no doubt, to special and, possibly, local circumstances. The price of iron was certainly inordinately high about the year 1873, for the original intention of fitting the Avonmouth Lock with iron gates was abandoned in favour of wooden gates for that very reason. Iron gates constructed at Antwerp in 1873-74 cost 46s. 10d. per square foot. But in 1879, when estimates were obtained for a pair of gates at Dunkirk, the tender for ungalvanised iron had fallen to 21s. per square foot, and for galvanised iron it was only 26s. per square foot, including in both cases four coats of paint. About the same period Mr. Harrison Hayter, Past Pres. Inst. C.E., stated in the course of a discussion,* that he was in the habit of estimating the cost of wrought-iron gates at from 30s. to 40s. per square foot. Within the succeeding decade a pair of steel gates was erected at Limerick Dock entrance for 25s. 4d. per square foot. At the present time, allowing for market fluctuations, a pair of iron or steel gates might be expected to cost from 25s. to 30s. per square foot, with a slight margin in favour of steel.

On the Manchester Ship Canal, two pairs of gates were recently constructed for the same lock-one pair of greenheart and the other of steel. A statement (Table xxiv.) of their actual cost will be useful, if only as affording a basis of comparison between the two materials.†

From particulars of the cost of seventeen gates of oak for small entrances at German seaports, ranging between 25 and 45 feet in width, Messrs. Brandt and Hotopp have deduced 15s. per square foot as the average cost of such gates. They further state that "the proportion in the cost of wooden gates to that of iron or steel gates may, under present conditions, be taken as 4:5, within the limits fixed for comparison."

*Min. Proc. Inst. C. E., vol. Iv., p. 72.

+ Hunter on "Lock Gates of Greenheart and Steel," Min. Proc. Ninth Int. Nav. Cong., Düsseldorf, 1902.

Brandt and Hotopp on "Iron, Steel, and Wooden Gates," Min. Proc. Ninth Int. Nav. Cong., Düsseldorf, 1902.