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Trenches in Streets

trench, earth, settlement, flooding and water

TRENCHES IN STREETS.

The opening of trenches for water, gas, and sewer pipes in the streets is perhaps the greatest cause of de struction of pavements to be found in the average city. This is especially true of the smaller cities, where wear from traffic is not excessive.

In constructing a pavement in an unpaved street an effort should always be made to lay all pipes which are likely to be needed in the street for a considerable period, in so far as they can be foreseen, before placing the pavement. Where a cut is made through a pave ment for a trench it is a matter of considerable difficulty to backfill the trench and replace the pave ment in as good condition as before it was cut, and great care is required to prevent the subsequent set tlement of the pavement over the trench. The filling of trenches over which a pavement is to be placed requires very close inspection, and frequently, neglect of such inspection causes much trouble subse quently.

The most common method of filling trenches in unpaved streets is to throw the earth in loosely, and pile the surplus earth in a ridge over the trench, leaving it for the natural settlement, when wet weather comes, to ultimately compact the earth in the trench. Usually, the settlement of such a trench will extend over a long period, and there is danger of injury to a pavement built over the trench, even after several months have elapsed and settlement seems to have taken place. Rolling will compact the earth in the top of the trench, but its effect does not reach to any considerable depth in the trench, or prevent later settlement. There are many instances in which disastrous settlements of pavements have occurred over trenches, although the material in the trenches had been considerably settled by rains and the surface rolled with a heavy roller.

Flooding. It is common practice to settle the earth in trenches by flooding with water. This is accom plished either by repeatedly filling a few inches of ,earth into the trench and then saturating with water, or by flooding the trench with a few inches of water and filling the earth into the water. It is difficult to compact the earth by flooding so that no further settle ment will take place, and it is necessary to use care that the earth be not thrown in in too large quantities at once, as when the trench is filled with scrapers or graders. When the soil is clay, subsequent settlement will take place as the clay shrinks upon drying out.

In filling sewer trenches in this manner there is usually danger of breaking the joints of the sewer in flooding the trench. Several instances have been noted in which this has occurred, and the practice should be avoided.

Tamping. The only method of effectively compact ing ordinary earth in a trench so that no danger of subsequent settlement shall exist is by placing the earth in thin layers, not more than 4 or 5 inches thick, and tamping each layer thoroughly. To accom plish this the earth must be damp enough to pack well, but not too wet.

The earth compacts into smaller space when rammed in the trench than it formerly occupied, so that when the pipe is small as compared with the size of the trench there may not ' be enough earth removed in excavating the trench to entirely refill it.