The rangers and braces can be used over and over again if they are not left in the trench; the sheathing, too, can ordinarily be used several times; but each time a set is used a few plank will probably be broken, either in driving or in drawing. As stated in connection with Table No. 21, good sheathing can ordinarily be used two to five times, taking an average of all used at the outset.

In many instances it is desirable to leave the sheathing in the trench, sometimes with and sometimes without the rangers and braces. The conditions calling for leaving in sheathing are: that drawing it may endanger the sewer, or water- or gas-pipes in the street near the trench, or adjacent buildings, or that the street-paving will be injured thereby. The danger to buildings usually exists only in connection with deep trenches in unstable soil or where a building is quite near a sewer which lies below its foundation. Water- or gasmains would be endangered if within two or three feet of, and more than that distance above the bottom of, a sewer-trench in fairly good soil. If the soil has shown a tendency to crack along the banks near the trench the sheathing should not be drawn if the street is well paved; and if water- or gas-pipe or other sewers are laid in such street the judgment of the engineer must decide at what distance they may be considered safe from disturbance if the sheathing be drawn. If the sheathing has been driven below the centre of a sewer, as must be done under some conditions, its removal would disturb the foundation of the sewer and should not be attempted. But if two or more courses of sheathing have been driven all but the lowest course may be removed if the sewer only is affected. The rangers and braces as well as the plank should usually be left in. If the banks are liable to cave with the drawing of the sheathing the trench should be filled to a distance above the sewer at least equal to its width before the top braces are knocked out heathing-plank is entirely drawn.

Before drawing sheathing the back-filling, if it is not to be rammed, should be carried to a point at least 3 feet above the bottom of the plank. The bottom set of braces and rangers may then be removed. If this gives less than 2 feet of back filling above the top of the sewer this amount should be thrown in and properly tamped. When the sewer is properly covered the remaining braces and rangers may be removed If the bank should cave

and the sheathing entirely drawn. badly on the removal of the braces it might break the sheathing, and in such a case it may be better to continue backfilling and slowly drawing the sheathing, each set of rangers being removed only as the back-filling reaches them. If there is more than one course of sheathing this plan should be followed in every case with all but the top course, unless the others are to be left in the trench, which may be cheaper in

some cases.

Drawing the sheathing is often a difficult matter if only the hands or a pick be used. A convenient plan is to use a sheathing-puller, made of iron 1 or 2 inches thick and 3 or

FIG. 18.-SHEATHING-Puller.

4 inches wide. The ring on the clamp should be so placed that the clamp will slide down the sheathing when not supported, remaining constantly horizontal. After placing this in position on a horse a simple pump-handle motion with the lever will suffice to draw the plank. A chain to be hooked tightly around a sheathing-plank may be used as a substitute for the clamp, but is not convenient for close

sheathing, which must be pried apart to admit it. Better than this sheathing puller, where excavating-machinery is being used, is to use the engine-power to draw the sheathing by fastening the clamp to a hoisting-rope.

Where a building is so situated with reference to the sewer-trench that its stability is endangered thereby the greatest care should be taken with the sheathing to prevent any material behind it from caving into or in any way entering the trench. To insure this the sheathing-plank must be tight together-in sand it may be necessary to use tongued and grooved plank—and their bottoms should be kept well below the bottom of the trench. If this is done and the bracing is strong and stiff there should be little danger, unless the material is semi-fluid, when it may be impossible to prevent a settlement of the ground and buildings, unless by freezing the soil by the Poetsch process (an exceedingly expensive one) or some similar method.

If a settlement of a portion of a building-foundation seems probable the building should be shored and jacked up. One method of accomplishing this is to make openings just above the ground-surface 6 to 10 feet apart and of a size to permit large beams-10 X 12, or 12 X 14 or 18-to be passed through them. These beams are supported at each end by jacks, which in turn rest upon blocking placed upon the ground. A careful watch is kept of these and at the least sign of settlement of the ground the jack above is screwed up an amount equal to this settlement. As a further precaution it may be advisable to shore up the walls by a sufficient number of heavy timbers, whose lower ends are supported upon platforms or grillage, wedges being placed under the foot of each and driven up when necessary to make up any settlement of the ground. The shores at their upper ends bear against beams bolted to the walls of the building, or in masonry walls are received by openings about a foot deep cut therein. Shores

alone are often employed when the building is not valuable or the danger is small.

ART. 73. LAYING SEWER-PIPE.

It will save considerable trouble in the laying of pipe if the foreman has the trench dug exactly to line and grade as ascertained by measuring and plumbing from a grade-line already set. It is better to have the bottom a little too high rather than too low.

Pipe sewer is usually laid up the grade, and the pipes are so manufactured that the specials must be laid with their bell ends pointing up. Laying the sewer-pipe in this way is more likely to produce good joints, particularly if the grade is at all steep, since if laid down grade a pipe, after being placed in position and before the next is laid, tends to slide away from the one next above it and cause a break in the inner surface of the sewer and a leaky joint. It is also much easier to lay pipe with the bell pointing ahead, and the cement joint is apt to be firmer. The only reason advanced for laying pipe down hill is that the lower end of the trench being ahead of the pipe, any ground-water will be kept drained away from the sewer construction. This is discussed in Art. 78.

For lowering into the trench pipe which does not weigh more than 100 pounds a convenient method is to use a rope of to 1 inch diameter with a hook at one end. The hook is passed through the pipe from spigot to bell and then back over the outside to the middle of the pipe and caught on the rope there, so that the pipe when suspended will be horizontal. Or the hook may pass through the pipe from bell to spigot and be simply caught over the end of the latter. The pipe is lowered over the edge of the trench by one man and received at the bottom by another if light, or by two if heavy, the hook being unfastened and pulled up. weighs more than 100 pounds two men will be

If the pipe required to

lower it, which they do

by each holding one end of a rope For pipe heavier than 200 pounds

which passes through it. it is advisable to use an ordinary three-leg derrick with light tackle-block. The pipe is then suspended by a rope or chain, with a hook at one end and a ring at the other, passed through the pipe and so hooked that it may be lowered in a horizontal position. A convenient arrangement for holding the pipe consists of a hook (Fig. 19), which should be at least two thirds the length of the pipe and very strong at the bend. The ring must come beyond the centre of gravity of the pipe to prevent its falling off the hook. By use of this a pipe can while suspended be entered into the bell FIG. 19.-PIPE-LAYING of the one previously laid and much heavy lifting by hand avoided.

Hook.

Another method of entering heavy pipe after it is in the trench is sufficiently explained by the illustration Fig. 20.

FIG. 20.-APPLIANCE FOR "ENTERING" HEAVY PIPE.

This is made of wood or iron, with a loose wheel on either side of the bar at the bottom.

Before a pipe is lowered into the trench a "bell-hole" should be dug where its bell will come, of such size that when the pipe is in position the jointer can pass his hand entirely under and around the front of the bell. It is convenient to