Filling Trenches - Preparation of the

However, wherever flushing is effective, tamping would be equally as good and would probably be less expensive, if the cost of the water be considered. As a rule attempting to consolidate trenches by flooding is bad practice.

Neither of the preceding methods of using water should be em ployed with clay or clayey soils, since flushing prevents rather than assists the consolidation of such soils. In other words, flushing or puddling is useful only with soils which water readily breaks down. If clay is flooded or is deposited in water, the trench is filled with a watery mud that will shrink very much as it dries out and will always be loose and porous. It is well known that a stiff-mud brick which has been moulded under exceedingly heavy pressure will shrink in drying 5 per cent, and with some clays 10 per cent; and of course the thin clay mud in a flooded trench will shrink very much more than this.

Tamping.

Except in the case of comparatively clean sand and gravel, back-filling can be thoroughly done only by tamping; and to make this method successful it is necessary (1) that the material shall be moist enough to be plastic, but neither too wet nor too dry, (2) that it shall be deposited in layers not more than 3 or 4 inches thick, and (3) that each layer shall be thoroughly rolled or tamped. To secure thorough tamping the relative numbers of tampers and shovelers is sometimes specified; but this alone is ineffectual since there is a natural tendency for the tampers to work less energetically than the shovelers, and besides more labor is required to tamp the soil around a pipe than higher up.

The amount of ramming required will vary with the character and condition of the soil. Clay and hard pan should be moistened before being tamped, while clean sand or clean gravel may be tamped dry. The tamping can be most effectively done with a compara tively small light rammer or tamper, since the effect of the blow is transmitted to a greater depth, while a bread heavy rammer consolidates the surface only. A tamper weighing 5 or 6 pounds is better than one weighing 20 or 25 pounds. the lighter one being lifted higher and giving less fatigue than the heavy one. It is important to remember that any amount of ramming will affect only a comparatively thin layer.

Obviously back-filling should not be attempted when the ma terial is frozen, since subsequent settlement is then sure to take place.

To prevent disturbing the surface of a pavement plumb ers, gas fitters, etc., are sometimes given permission to tunnel under the pavement to make their connections. This practice is never justifiable on account both of the excessive cost and of the impossibility of effectively filling the tunnel, owing to the limited space in which the work must be done. In nearly every case a depression occurs sooner or later over the tunnel Replacing All the Material. The result to be obtained in filling a trench is that the material in the trench shall have the same compactness as the soil around it; and therefore some con tend that the only proper way is to put back all the material taken out. In a majority of cases, this procedure will secure reason

ably good results; but under certain conditions it will fail. For example, the water pipe or sewer may occupy a large proportion of the volume of the trench, and consequently of necessity there will be a considerable excess of earth. Again, putting back all the earth does not insure the restoration of the original surface nor certainly prevent subsequent settlement. It has been shown that soil when taken from its natural place and compacted in an embankment will shrink from 8 to 15 per cent (see § 127), and will probably subsequently settle 2 or 3 per cent and possibly 10 to 25 per cent (see § 128). Consequently with a deep trench con taining a small pipe, it is possible to tamp the earth back so solidly as not to have enough to restore the surface; or it is possible to put all the soil back by tamping the lower portion of the trench solidly and the upper portion loosely, and still considerable set tlement take place. Therefore the specification to re-place all of the material, must have a careful and intelligent supervision to insure good results.

In the past it has not been the custom to fill trenches in such a manner as to prevent settlement; and therefore if the best re sults are to be insisted upon, the specifications should clearly reveal that fact, for contractors in bidding on work do so on the under standing that the work is to be done in at least approximately the usual manner, and any attempt to have it done in any better way, which was not clearly understood from the beginning, is likely to cause friction and irritation, and possibly finally to re suit in failure.

Re-filling with Sand or Concrete.

On account of the difficulty of getting trenches in clay or loam filled so that there will be no settlement, it has been proposed to require the trench to be filled with clean sand or gravel. It is not known that this method has ever been tried. It would probably be effective, but usually its cost would be prohibitive.

In at least a few cases trenches have been filled with a fair qual ity of natural cement concrete. The expense for the concrete was not justifiable, since it was much greater than that required thoroughly to tamp the back-filling.

Sometimes municipal authorities are lax in inspecting the filling of trenches, owing to the belief that the concrete foundation will hold up the pavement even though the material in the trench may settle; but this is bad practice, since the ordinary thickness of con crete is not designed to act as a bridge, and besides if it is thick enough to bear up over trenches it is needlessly thick elsewhere. With the usual thickness of concrete foundations, a depression is almost certain to occur if the material in the trench settles; and hence the only safe rule is to have the trenches completely and compactly filled.