Where a sub-drain is being laid the water is frequently permitted to flow from the trench under excavation to and through this. In many if not most soils this is bad policy, since it leads to a silting up of the drain by the large amount of material washed in from the trench. It is better in most cases to leave or make a dam at the upper end of the completed trench, and place a sump-hole just ahead of this and below grade, from which the water is pumped. When a section of 20 or 30 feet has been excavated to grade another dam and sump-hole can be placed at the head of this section and the others removed, the sump-hole being filled with sand or gravel or other good material well rammed.

Where a sub-drain is started from a sump-hole, or that lower down the line is found to be too small to carry the water coming to it, a pump must be placed at this point also to remove the water from the sub-drain which is to be laid beyond it. This water is frequently raised to the sewer only, the pump being placed in a manhole and discharging the water below a temporary dam in the sewer, which prevents its flowing up the sewer onto the work.

Two or more hand-pumps are sometimes concentrated at one point when the amount of water is considerable. It would in many instances be cheaper to use a steam-pump at such a place. Piston, centrifugal, and wrecking pumps, pulsometers, and steam-siphons are the steam appliances in most common use on sewer construction. In all of these iron suction-pipes are used, from 4 to 8 or 10 inches in diameter. The piston-pump is the most economical, and adapted to widely and rapidly varying quantities of water, and if the water is fairly clean needs very little attention. It cannot, however, pump gritty water without rapid deterioration. The centrifugal pump can raise muddy or gritty water, chips, and even small stones, its first cost is less than that of a pistonpump, and it can be repaired more cheaply if damaged. It

requires a fairly constant and fixed quantity of water to keep it working, and is apt, especially when a little worn, to give trouble by losing its priming, when the rising of water in the trench before it can again be primed may give trouble. The wrecking-pump the author has found to be an excellent pump for sewerage-work. It will lift and discharge anything which can pass through its suction-pipe and is extremely simple in action. All these pumps must be firmly set over or near the trench and their position can be changed only with considerable labor. It is better to set them directly over the sump and have a suction-pipe as short and with as few joints as possible.

The pulsometer pumps muddy and gritty water, but is not economical of steam and, except in experienced hands, is apt to act in a provokingly contrary manner, particularly after some use. It has the great advantage, however, of portability, being suspended by a chain, which permits rapid changing of its position without cessation of pumping, the steam being conveyed to it through a rubber steam-hose. For pumping large quantities of water at the point where excavation is proceeding and where frequent change of location of pump and suction is necessary it is perhaps the best contrivance on the market. The steam-siphon is likewise conveniently portable, but is most extravagant of steam and is hardly practicable for raising large quantities of water.

The pulsometer and siphon are particularly adapted to raising water from the point where the work is progressing with the least interference therewith. Piston, centrifugal, and wrecking pumps are best used at a distance from the work to lift water which has flowed to them through sub-drains or the sewer, although they are often used at the work when the same sump can be used for two or three days at a time.

All suction-pipe on either steam- or hand-pumps should be provided with a strainer at the bottom, and the centrifugal

requires a foot-valve, which it is also well to supply for the other steam-pumps. If a chip or other obstacle should hold this valve open and prevent priming the suction a shovelful of stable manure dropped into the suction-pipe will in many cases enable the valve to hold its priming.

All parts of the machinery should be readily accessible, particularly any valves, and wrenches and screw-drivers, packing, oil, waste, duplicate nuts, washers, etc., should be kept constantly at hand. A cessation of pumping for 15 minutes may permit the water to drive the workmen from the trench, to soften the banks and endanger the sheathing, ruin the green masonry, stop up sub-drains, or do other serious damage. A good, intelligent, careful stationary engineer is a necessity on such work.

The water raised from the trench should not be discharged upon the ground near the sewer, unless the street has impervious pavement, as it might soak back into the trench and be pumped over and over again. It may be carried to the nearest watercourse or sewer-inlet or manhole along the gutters, in wooden troughs, or in sewer-pipe temporarily laid on the ground with joints tightly calked with oakum or clay.

It usually pays to keep the water pumped down all night, even if there is no work to be damaged by its rising, as this would again fill the surrounding ground with water, which might not drain out for several hours after pumping began the next day. It may be well to whitewash one or two sheathing-plank down to the trench bottom each evening, which will give evidence next day if the engineer has not kept the water down. A shelter should be built in front of the boiler to protect the engineer from storms.

While using a diaphragm-pump always have spare diaphragms and an extra length of suction-hose on hand.

Moving a pump and boiler often costs more indirectly in interference with the work than the immediate expense comes

to. In general every effort should be made to set the pump in such a place and manner that it need not soon be moved. Be sure to have the blocking under it solid, to prevent the suction-pipe joints from working loose or breaking.

ART. 78. HANDLING WET AND QUICKSAND TRENCHES.

If excavation is in good material and of comparatively uniform depth a sewer gang once organized should move along at a uniform rate of 300 or 400 feet a day for small pipe sewers, 25 to 200 feet for brick ones, and with little but routine work for the foreman. If genuine quicksand is encountered, however, every foot of progress must be fought for with unflagging energy, pluck, and intelligence. In ordinary wet trenches the difficulty, while not usually so great, is sometimes considerable. In both an intelligent adapting of the work to every new exigency is necessary.

Water is met with as springs in the trench or as a general exuding from all the ground. The former can easily be managed by catching the water at its point of exit and pumping it away. If it enters from the bottom of the trench it can sometimes be caught in a trough and led back and discharged into either the completed sewer or into a tub in which the suction-pipe of a pump is placed. It is absolutely useless to attempt to stop the water from coming out of the ground; the endeavor must be to handle it after it gets out. In the case of a spring in a brick-sewer trench a method often advantageous is to build into the brick-work opposite the spring a small pipe, 2 to 4 inches diameter, through which the water can enter the sewer, and to conduct it back from there to the finished sewer in a trough. This pipe can be plugged after the masonry is thoroughly set, but might better be left open to drain the ground if in a storm-sewer, or if in a combined sewer and well above the line of flow of house-sewage. This

pipe can, in many cases, be so driven into the bank at the spring that the water will flow through it and the trough be set before the brick-work is begun at that point, the trench being thus left dry.

If the water does not enter as a spring and consequently cannot be caught in this way, but if the ground is a gravel or is not readily softened by the water, an outer ring of brick may be built with quick-setting cement, and plenty of it in beds as well as joints, an occasional brick being left out to permit the water to enter the sewer-invert, over which it can flow to a sump-hole ahead or through the sewer below. If openings are not thus left in the brick-work the water will force its way through the joints. Plank should be placed over the brick-work as fast as it is laid for the masons to stand upon. This outer ring when set may be found uneven of surface, but the joints will probably be tight. The openings may then be closed by inserting a brick and calking the joints with cloth, oakum and cement, wooden wedges, tea-lead, etc., or a pipe may be inserted and the water allowed to enter it as described above. The outer ring being thus made water-tight, the inner ones can be built as usual, any depression in the outer ring being well filled with mortar. In this and in all brick-, stone-, and particularly concrete-work which water flows over while green the surface can be protected from wash by spreading rather heavy, strong brown wrapping-paper over it. Cheap wood-pulp paper is of little use. The paper when wet will cling to the masonry, remaining intact for days and even weeks.

Another plan is to dig a sump-hole 1 to 3 feet deep in the centre of the section of invert under construction, and keep the water lowered in this by a pump until the brick-work is completed and set everywhere except over the sump. the ground is very porous the water will all flow to the sump and leave the trench dry for several feet in each direction.

If