CHAPTER XII

CONSTRUCTION

ART. 56. TRENCHING

THE engineer must have some knowledge of construction if he is to co-operate with the contractor to secure the most effective results; in addition to which, an engineer in charge of an existing system will probably be required to personally supervise the making of repairs and even the laying of extensions.

An effort has been made to outline the more common features of sewer construction in this chapter.

The more common practice is to lay sewers up-grade. This is almost necessary with pipe sewers, since Y branches are so made that the pipe bell must point up-grade; and if laying proceeds down a steep 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 bad joint. The only reason advanced for laying a sewer downhill is that, the lower end of the trench being ahead of the pipe, any ground water will be kept drained away from the sewer to the end of the trench, where it can be pumped out. On the other hand, in laying up-grade, a sub-drain can be used (or the completed sewer itself) for removing ground water without pumping; and the trench serves to drain out the ground ahead of it and so reduce the amount to be handled during excavation.

The width of trench at the bottom should be at least 12 inches wider than the outside of the bell of the pipe to be laid, plus an allowance for sheathing, if this is to be used; and the top is generally made a little wider to allow for more or less batter to the sides of the trench. Two feet is the narrowest trench that men can work in, and 30 inches is a common minimum. For

brick or concrete sewers the bottom need be no wider than the outside width of sewer, plus sheathing. In some cases where the trench is to be comparatively shallow and dug by machinery and the banks would not stand without sheathing (as in sand or uncemented gravel), the sides of the trench are allowed to assume a natural slope, often approximating 1 to 1.

Trenches may be excavated by hand (pick and shovel) or by machinery. The former is generally preferable when only a few hundred feet of small sewer is to be laid at a stretch, or where there are boulders, rock, or frequent pipes or other obstructions lying across or in the path of the trench; also, when the ground caves badly, unless the sides are to be allowed to slope, as described above.

Machinery is especially effective and economical where wide or deep trenches are to be dug; where the ground will stand up without bracing for a depth of 8 feet or more and contains few large stones or roots, and there are no house-connection, water- or gas-pipes crossing the trench; where it is desirable to avoid piling the excavated dirt along the street; or when labor is exceedingly scarce or high priced. Machinery has been used advantageously where none of these conditions was present, but only when special local conditions outweighed other unfavorable ones.

The machinery used may be divided into five classes: (1) Simple lifting appliances, like derricks, either stationary or mounted on travellers. (2) A wire cable suspended above the center line of the trench, on which run one or more travellers carrying buckets that are first raised from the trench and then carried to one end of the cable. (3) A trestle spanning the trench, on which is supported either a track from which depend a number of travellers, each carrying a bucket, or two rails on which runs a large traveller that carries a large bucket and a man to operate it. These three are really dirt handlers rather than excavators. In each, the cable moving the carriers is operated by a stationary engine placed at the end of the trench, the whole equipment being moved ahead every few days. The fourth type consists of an excavator that both digs and elevates the

dirt, one variety working on the general principle of the ladder dredge, another employing a wheel that carries upon its periphery a number of buckets provided with cutting teeth or edges. Either ladder or wheel can be raised or lowered by the operator at will so as to give the required depth of trench. These have been used for excavating trenches up to 20 feet in depth. A steam shovel with elongated dipper handle has been used for trench excavating in a few cases. The fifth class is the drag-line scraper

which is generally applicable only where there is no sheathing or bracing, and especially when the banks are allowed to assume a natural slope. Trenching machinery is economical when the cost of running (including all labor) and of repairs, plus the rental or interest and depreciation on first cost of the machine, is less than the cost of "staging" it out (as the use of platforms is called) plus that of back-filling. If the back-filling is to be hand-tamped, this last item should not be included, since if a machine is used the material must be spread by hand after

dumping. Where the excavation can, for some reason, proceed but slowly, the use of a trenching machine is not generally economical, although it may be advisable for other reasons.

In some cases, especially where the sewer is to be laid in an important street that must be kept open for traffic, it may be advisable to require the contractor to use trenching machinery of a type that will effect this.

Obstructions in Trench. 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 to 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 back-filled; 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 houseconnection 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 2X4 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 of a water-main breaking along the trench, 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.

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 in laying out a sewer-trench it should be kept several feet from any old trench, if possible.

Sheathing. The use of sheathing is generally for the advantage of the contractor, but in some cases it is desirable that the city require its use, and in many such cases that it be left in when the trench is back-filled. The most common of such conditions is where the caving of the trench would endanger water or gas mains or other portions of public utility systems, or would undermine street railway tracks or pavement over which traffic must be maintained. In some cases tongue-and-groove sheathing should be used to prevent sand running through it and thus forming cavities behind it which would threaten the safety of other underground structures, of the pavement, or even of adjacent buildings.

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 gaspipes 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 gas-mains would be endangered if within 2 or 3 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 center 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