Filling Trenches - Preparation of the

FILLING TRENCHES - PREPARATION OF THE The back-filling of trenches opened to lay water and gas pipes, to make house connection to sewers, etc., so that the road surface shall be restored to its former level and remain so, is a matter of importance on both paved and un paved roads—particularly the former. The failure to re-fill the trenches properly is a source of annoyance to those who use the road and of damage to the pavement itself. It is frequently asserted by those having opportunity for knowing, that the dam age to pavements through lack of care in re-filling t: enches and re-placing the pavements is greater than the wear due to traffic. No kind of municipal work should be more rigorously inspected than the filling of a trench over which a pavement is to be laid. The nearly universal result of a neglect in this respect is that a pavement built at great expense is disfigured or damaged by settlement, the repair of which will cost many times as much as it would have cost properly to fill the trench originally.

The principal cause of failure is lack of care, but sometimes it is due to a mistake as to the proper method to be employed. A discriminating judgment is required to determine the proper method, and intelligence and faithfulness are necessary in carrying it out. There are several distinct methods used in consolidating the back-filling of trenches.

Natural Settlement.

A munon practice of those hav ing occasion to make excavations in unpaved streets is to cast back loosely the material taken out, heaping it into an unsightly and annoying ridge over the trench, and trusting to travel and the elements to restore the surface to its original level. In nearly pure sand such a ridge may in time settle to the original level, although the damage due to the ten iporaiy ridge will generally be much more than the cost of properly filling the trench in the beginning; but as a rule earth loosely put back will not attain a sufficient degree of compactness to make it a safe support for a macadam or other form of pavement. The surface may become

very compact and hard; and yet after the removal of the foot or more of soil, ordinarily necessitated by the construction of the pavement, it will be found that the earth in the trench will settle considerably under a roller run transversely over the trench. Even though the surface may support the roller, it is highly prob able that ultimately a trench which has been loosely filled will settle and cause a depression in the pavement. This is proved by the numerous depressions in pavements, and also by the fact that when trenches loosely filled are opened years afterwards, it is very common to find open cavities. The promptness with which natural settlement takes place depends upon the climatic conditions and the underdrainage. It is never safe to depend upon natural settlement to secure the proper compacting of the soil in trenches over which a pavement is to be laid, however long the time allowed for the settlement, and much less the few weeks often specified.

Flooding.

Where the water can be had cheaply, it is a common practice to attempt to consolidate the earth in the trench by flooding or puddling it. If the soil is sand or gravel and is so pervious that the trench will drain out rapidly, thorough flushing will compact the material so that no trouble will be experienced with settlement; but the flushing must be done thoroughly. It is not sufficient to fill the trench nearly full of loose material, and then turn on a gentle stream of water until the trench is full; for trenches thus filled are certain to settle later. The sand or gravel should be added in layers not more than 8 or 10 inches thick, and each layer should be flushed with a stream of water having force enough to wash the finer particles into the voids between the larger ones. Substantially the same result may be accomplished by shoveling the sand or gravel into water 8 or 10 inches deep; but this method will not be effective, if the trench is filled with a scraper or a scraping grader.