less than 5 feet wide, but is the better form for wide trenches. If there is more than one tier of longitudinal platforms the successive tiers should be placed alternately upon opposite sides of the trench; or if cross-platforms are used the right side of one should be vertically above the left side of the next lower, alternate platforms being vertically above each other. The number of men excavating which cast onto one platform

FIG. 12.-CROSS-STAGING IN TRENCH.

may be only two, but should increase with the difficulty of excavating, so as to keep the staging-man busy.

Where it is allowed (as it is in many cities), and the trench is over 10 feet deep, it is often economical, except in hard rock, dry sand, or quicksand, to make the excavation in alternate tunnels and open trenching, the sections of each being 8 to 20 feet long. The tunnel is usually made about 5 feet high. The amount of material to be removed and of bracing to be put in is thus reduced. But tunnelling should never be allowed under streets, except in rock, unless the tunnel is afterwards opened and back-filled as open trench, being used only to save bracing; since it is practically impossible to so compact the back filling in a tunnel as to prevent future settlement, which may not occur, however, until months or years later, when the contractor has been relieved of all

responsibility. Where the amount of traffic on a street or other conditions require it, however, a tunnel may be run under the street and a masonry lining, which may be the sewer itself, built against the outside of the excavation, so that there is no back-filling except in the form of masonry; which construction requires special tunnelling-machinery and methods. In Paris by the use of a shield a tunnel 19 feet outside diameter was run with a covering in some places of only 2 feet between it and the street-paving above, without causing any cracks in the latter. The successful tunnelling for the Boston underground railway is familiar to all. A notable instance of sewer-tunnelling is found in the sewers tunnelled through sand-rock at St. Paul, Minn., the tunnel, when lined on the bottom, constituting the sewer. Restrictions against tunnelling should not of course apply to lines whose depth is 75 feet or more, such as those passing through ridges.

There is a tendency, if a right-handed laborer always faces one way while picking, for the trench to work to his left as it descends. He should be taught to avoid this by keeping his left side to the side of the trench at which he is picking, so that both sides shall make the same angle, if any, with the vertical.

It pays to keep the picks sharpened and good shovels in the men's hands. For this purpose there should be 25 to 100 per cent more picks than laborers, to allow opportunity for sharpening them. For digging the round-pointed shovel is best, but staging-men and mortar-mixers should use squarepointed shovels. There should be a few extra shovels constantly on hand, including a few long-handled ones, but these latter should not be used for trenching except in deep trenches where the shovelling is very easy.

In soil where caving is frequent and sheathing is not used the trench should be refilled as soon as possible, since the

longer it stands the greater the probability of caving. Soils, such as clay or other heavy ground, having some cohesion will usually give warning of caving by cracking a few feet back from the edge of the trench, and should be braced as soon as such sign appears. Gravelly soils or dry sand usually give no such warning, and are particularly dangerous on this account and because they may bury and suffocate the men; while clay, coming in lumps, although it may bury and even crush them, will permit them to breathe until they can be rescued. Trenches in gravelly and sandy soil should always be sheathed.

If a boulder is met with it may be raised from the trench by a derrick or, if too large for this, may be blasted. Before blasting the earth should be removed from all sides of the boulder and the trench in the vicinity should be braced. It may sometimes be cheaper to dig a hole in the side of the bank and roll the boulder into this out of the way. In some cases, when the sewer would pass entirely under the boulder, this may be left undisturbed and tunnelled under. If it merely protrudes into the trench a portion may be removed by "feathers and wedge" or a heavy sledge.

If a water- or gas-pipe or other conduit run diagonally across a trench, or run in it, or cross one more than 8 or 10 feet wide, it should be supported in position before the earth is removed from under it. This can be done by placing across the trench at intervals of 12 feet sufficiently strong timbers or old rails, and suspending the conduit from these by chains drawn tight by driving wedges between them and the beams. Rope should not be used for this purpose, as rain causes it to contract or break in the attempt to do so. If such a pipe lies in the bank, close to or slightly protruding into the trench, the bank should be thoroughly braced just under the pipe and the pipe itself be held in place by braces. These braces should not be removed when the trench is backfilled, and if the pipe is suspended the trench should be filled

and thoroughly tamped under and around the pipe before the chains are removed. The breaking of a water-main in or near a sewer-trench is one of the most disastrous accidents which can happen to it. Small house-connection pipes crossing the trench are apt to be broken by workmen climbing over them and should be protected, as by a piece of plank or of a 2 X 4 placed across the trench just above such pipe, the ends extending 6 inches or more into the banks for support. In all cases where there is danger from water-pipe such and so many gates should be temporarily closed that the closing of only one more will entirely shut off the pressure from the threatening line of pipe, and a wrench be kept at hand for closing this.

If a drain crosses the trench the pipe should be removed and saved, and a trough substituted during construction, its ends supported in the banks. The back-filling should be carefully tamped under this and the pipe relaid in the trough.

At the first sign of quicksand the best of close sheathing should be at once put in, an experienced foreman put over this work and the best men placed upon it (see Art. 72).

The soil where a trench has previously been dug, although it were years before, is more liable to cave than that which has never been disturbed, and the sewer-trench should be kept several feet from such old trench if possible.

As a general statement it may be said that it does not pay to use any kind of machinery in excavating where the trench is less than 8 or 9 feet deep or wide, although it may be desirable or necessary to do so where for some reason the excavated material cannot be piled along the side of the trench. The advantages attending the use of machinery are: greater amount of material excavated with a given number of

men, less danger of caving of banks from the weight of earth piled upon them, less obstruction to street traffic, the convenience of having at hand means for raising boulders, lowering heavy pipes, or other material. Each of these advantages increases in force with the depth of the sewer. With several of the machines now on the market the cost of handling material increases but little with the depth. The machinery in use varies from an ordinary boom-derrick to an elaborate system of trestles, wire ropes, and buckets, which may stretch along 400 feet or more of trench.

For a large brick sewer a handy arrangement is that of two derricks with booms about 40 feet long, both placed on the same side of the trench and about 75 feet apart. Both boomand main-falls should wind upon drums driven by steampower. With this arrangement a bucket of earth can be hoisted from the excavation and, passing from one derrick to another, be deposited in the trench 125 or even 145 feet away. This plan is not adapted to narrow trenches nor to those where any considerable length of trench is to be under excavation at one time. For these one of the trestle-machines or cable-ways is preferable, the former more particularly for trenches up to 12 or 15 feet in width, the latter for wider ones and for particular cases, such as crossings of railroadtracks.

The cable-way consists essentially of a wire cable suspended over the centre of the trench, on which run one or more travellers carrying buckets; the earth being excavated at one point and cast into the buckets, which are raised and carried to the other end of the cable, where they dump the earth upon the completed sewer. It is essential to the safety of the laborers that the cable be most substantially anchored at the ends, and that it be amply strong to carry any load which can possibly come upon it. The anchorage is usually in the shape of a "dead-man," but the ordinary log placed