Fig. 68 is the section of a cofferdam adopted at Limerick in connection with the rebuilding of a dock wall which had failed, the length being 450 feet.*

Stone Dams are similar in construction to earthwork dams, consisting of a mound of light stone rubble (such as chalk) deposited and overlaid with clay to form a watertight skin. This material is also used as filling for the interior of a cofferdam, as exemplified at Ardrossan harbour † (figs. 69 and 70).

Concrete Dams.-A somewhat novel and ingenious experiment in dam construction has been successfully tried at Liverpool. A wall was built of large concrete blocks (each containing 100 cubic feet) bedded in hydraulic mortar, with a sheet of ordinary brown paper laid between the blocks in each joint. The paper adapted itself to the surface of the bed and allowed the blocks to obtain a uniform bearing upon one another, while at the same

time it prevented any actual adhesion. The stability of the structure depended, therefore, entirely upon the resistance of the blocks to sliding friction, which proved to be ample for the purpose. The dam in question was built upon the outer sill of a lock, 100 feet wide between side walls. The sill had a straight outer face and a curved inner one for the ultimate reception of gates. The area of the sill was nearly 400 square yards, with a minimum width of 25 feet. The front of the dam was a vertical plane, the back being stepped. The total height above the sill was 42 feet, at which level a roadway was formed for traffic. High water of ordinary spring tides.

* Hall on "Limerick Dock Walls," Min. Proc. Inst. C.E., vol. ciii.

+ Robertson on "Ardrossan Harbour Extensions," Min. Proc. Inst. C.E., vol. cxx.

came up to 33 feet above the sill, but during equinoctial gales the waves frequently surged to the top of the dam and broke over the roadway.

Iron Dams usually take the form of caissons, but they are by no means common. The most striking instance of their adoption is perhaps in connection with the construction of the Thames Embankment. The caissons were of wrought-iron in half oval segments, with upright flanges at each end, so that when the halves were bolted together they formed a complete oval, 12 feet 6 inches long by 7 feet wide in the centre and 4 feet 6 inches deep. The plates were and inch thick. Angle irons were bolted round

the top of the rings, enabling them to be firmly secured to each other in the vertical position. A watertight joint was formed by a guide pile, 10 by 6 inches section, fitting into a groove between adjoining caissons. The dam was further stayed by a few surrounding piles which maintained the caissons rigid and vertical in their descent. The gross cost of this dam was £30 per lineal foot as compared with £20, the gross cost of a timber cofferdam in a similar position. Some of the iron caissons were incorporated in the permanent work at an allowance of £8 per lineal foot. With this qualifi

cation it may be added that the nett costs of the two dams were about £15 and £17 respectively.

For tidal work a dam may be conveniently contrived by sinking iron pontoons and banking them up and between with clay. The height of such a dam is necessarily small, but it materially increases the period of working within the enclosed area.

Pumps. The subject of pumping demands the most careful and earnest attention of the dock engineer, seeing that the practicability and success of his undertakings depend largely upon the efficiency of his pumping arrangements. Some evidence of this will be afforded in subsequent chapters, but the fact is almost sufficiently obvious in itself.

There are many varieties of pumps on the market, each with its own special features and capabilities. A study of the catalogues of well-known manufacturers will generally enable a satisfactory selection to be made for the particular purpose required, and the following remarks are simply appended by way of indicating such practical points as seem worthy of consideration in exercising a choice.

Valve Pumps-that is to say, lift pumps and force pumps, or any combination of these in which the action depends upon the alternate opening and closing of small valves-are only suitable for comparatively clear water. Water which is highly charged with solid matter in suspension and with floating objects is very likely to derange these delicately adjusted parts and to put the pump out of action. The gritty nature of sand causes excessive wear of the leather washers and packings, necessitating frequent renewals. Chips and gravel lodge in the valves and prevent them from closing. The jambing of the bucket packings may cause serious trouble owing to the great force frequently required to release the bucket. For drainage purposes in trench excavations, a lift pump has this advantage over a force pump, in that, if the working should by any accident become suddenly flooded, the lift pump can still discharge its function, being actuated from the summit level, whereas the machinery of a force pump is in the bottom and, consequently, would be submerged.

A very handy drainage pump for use in confined situations is the Pulsometer. It represents a rather unusual principle in pumping. The action consists in the alternate admission and exclusion of steam to and from adjoining chambers. The water is forced out of one of the two chambers by steady pressure until it sinks to the level of the discharge orifice, at which point the steam obtains a free vent, and being in contact with a large surface is so rapidly condensed as to cause a vacuum in the chamber and draw over the steam ball at the top which closes the aperture and transfers the supply to the next compartment. Meanwhile, continued condensation in the empty chamber increases the vacuum, which is filled by a fresh supply of drainage water through the lower valve leading from the suction pipe. The apparatus is compact and easily suspended by a rope or chain in any desired position.

Other appliances for dealing with small quantities of water are the simple hand-pump and the ejector. The former is of the ordinary bucket type of pump, worked by hand. The ejector is actuated by hydraulic or by steam pressure. The principle is that of forcing a small jet or current

through a nozzle in the interior of a discharge pipe of slightly greater diameter. Drainage water is drawn up from the sump, by suction, to fill the vacuum thus created.

For removing "slurry" or liquid mud, water charged with sand, gravel, cement scum, floating material, and, in fact, the general drift and debris

which find their way into a pumping well in excavations carried on under circumstances, perhaps more peculiarly characteristic of dock work than of any other branch of engineering, pumps of the strongest and simplest construction are advisable. Such, for instance, are the chain pump and the centrifugal pump.

The first of these which has demonstrated its utility from remote ages, being originally an invention of the Chinese, consists of a series of flat blades, strung at regular intervals upon two parallel endless chains. These chains hang vertically, being suspended from a revolving reel or barrel at the summit, over which they travel continuously. The descent is in the open, but on reaching the bottom the blades enter the splayed orifice of a rectangular funnel extending upwards to the point of discharge. The blades fit the interior of the funnel sufficiently closely to take the bulk of the enclosed water with them without incurring excessive friction against the sides. The pump acts admirably in lifting with absolute impartiality water, mud, pieces of brick, wood, stone, and concrete; any substance, in short, which can enter the funnel. The only thing to check its action is the intrusion of a chance wedge or plank end, transversely, between the buckets and the orifice The blades, which are of wood, are, of course, subject to a considerable amount of abrasion and have to be replaced from time to time, but repairs of this kind are easily effected. A stock of fresh blades is kept at hand, and the operation of removing a damaged blade is simply that of taking out the split keys which hold it in position on the chain.

Chain pumps with rectangular blades, 2 feet long and 6 inches wide, 141⁄2-inch centres, running at a speed of 500 feet per minute have proved capable of discharging regularly 600 tons of water per hour, which represents an efficiency of slightly less than 70 per cent. The speed may be increased to 600 or 700 feet per minute, with a corresponding greater discharge, but such speeds throw an undue strain upon the apparatus.

The action of a centrifugal pump is the revolution of a series of blades radiating from a common axis, by means of which the water is whirled round in a confined space until it acquires sufficient velocity to be projected up the discharge pipe. The blades are short, thick, and curved in form. This class of pump will "throw a good deal of extraneous material, but there is always the possibility of a fairly large object being drawn through the suction pipe and getting jambed in the blades, which are less accessible for repairs than those in a chain pump. The usual sizes of such pumps for temporary duties varies between 6 and 18 inches diameter.

Before leaving the subject, it will be well to observe that the provision. of a duplicate pumping system is a commendable arrangement. One set of pumps might easily break down at a critical moment, and even if the amount of pumping is sufficiently small to allow adequate intervals for cleaning and repairs, yet an auxiliary pump is an advisable precaution for unforeseen contingencies.