Seams of sand are very troublesome. Logs or stones under the edge of the dam are also a cause of considerable annoyance. It is sometimes best to dredge away the mud and loose soil from the site of the proposed coffer-dam; but, when this is necessary, it is usually better to construct the foundation without the use of a coffer-dam, see Art. 2 of this chapter. Coffer-dams should be used only in very shallow water, or when the bottom is clay or some material imper vious to water.
Hand Pumps. When the lift is small, water can be bailed out faster than it can be pumped by hand; but the labor is proportionally more fatiguing, and therefore bailing is not often resorted to.
The direct hand-lift foundation-pump consists of a straight tube at the bottom of which is fixed a common flap valve, and in which works a piston carrying another flap valve. The tube is either a square wooden box or a sheet-iron cylinder,—usually the latter, since it is lighter and more durable. The pump is operated by applying the power directly to the upper end of the piston-rod, the pump being held in position by wooden stays or ropes. The only advantage of the wood-box hand-lift pump is that it may be improvised on the job; and the disadvantage for foundation work of all pumps having flap valves is the danger that straw, sticks, mud, etc., will interfere with the action of the valves.
The diaphragm pump is the usual form of hand pump for foundation work. This ,pump consists of a short cast-iron cylinder having a rubber hose connected to its lower end, and being divided about midway of its height by a flexible horizontal rubber diaphragm. The central portion of the diaphragm is connected to a bent-lever handle, and there is a valve in the center of the rubber disk. The rise and fall of the center of the disk acts as a piston. A pump of this form throws a large amount of water, allows sand and gravel to pass without choking, is not easily clogged by straw, leaves, etc., and is easily unclogged. It is made in various sizes, the smallest having a capacity of 25 gallons per minute and usually costing about $20.
Steam Siphon. The steam siphon is the simplest of all pumps, since it has no movable parts whatever. It consists essen tially of a discharge pipe—open at both ends—through the side of which enters a smaller pipe having its end bent up. The lower end of the discharge pipe dips into the water; and the small pipe connects with a steam boiler. The steam, in rushing out of the small pipe,
carries with it the air in the upper end of the discharge pipe, thus tending to form a vacuum in the lower end of that pipe; the water then rises in the discharge pipe and is carried out with the steam. Although it is possible by the use of large quantities of steam to raise small quantities of water to a great height, the steam siphon is limited practically to lifting water only a few feet. Its cheapness and simplicity are recommendations in its favor, and its efficiency is not much less than that of other forms of pumps. One of the advan tages of the steam siphon is that frequently it can be improvised on the work from ordinary pipe and fittings. Several forms and sizes of steam siphons are upon the market, ranging in capacity from 5 to 200 gallons per minute, and are much better than one made from pipe. A steam siphon, or jet pump as it is usually called by the manufacturer, having a capacity of 100 to 125 gallons per minute can usually be had for something like $35. A common form of the steam siphon resembles, in external appearance, the Eads mud-pump (§ 877) represented in Fig. 94, page 438.
Pulsometer. The pulsometer is an improved form of the steam siphon. It may properly be called a steam pump which dis penses with all movable parts except the valves. The height to which it can lift water is practically unlimited. It is in very common use for pumping out coffer-dams. For an illustration showing the external appearance, see the advertising pages of any engineering newspaper.
There are several other forms of automatic vacuum pumps on the market which have substantially the same merits as the pulsometer. Centrifugal Pump. All of the preceding pumps are suitable only for handling comparatively small quantities of water, but where large amounts of water must be pumped in a short time the centrifugal pump must be used. The centrifugal pump consists of a set of blades revolving in a short cylindrical case which connects at its center with a suction (or inlet) pipe, and at its circumference with a discharge pipe. The blades being made to revolve rapidly, the air in the case is carried outward by the centrifugal force, tending to produce a vacuum in the suction pipe; the water then enters the case and is discharged likewise. The distance from the water to the pump is limited by the height to which the ordinary pressure of the air will raise the water; but.the height to which a centrifugal pump can lift the water is limited only by the velocity of the outer ends of the revolving blades. Since there are no valves in action while the pump is at work, the centrifugal pump will allow sand and large gravel—in fact almost anything that can enter between the arms— to pass. Pumps having a 6-inch to 10-inch discharge pipe are the sizes most frequently used in foundation work.
The centrifugal pump requires more labor to install and more care to operate than any form of steam siphon.