Page 383 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.45

inside wheels of the scraper sink more deeply. If it becomes too steep, it is readily reduced by filling
toward the center.
If the job is shut down during any period when rain is expected, it may be wise to build the fill
up to a crown in order to allow it to drain. This involves resloping the edges on resumption of work,
and, if the work is done under exact compaction specifications, may cause confusion in the treat-
ment of the tapered layers required.
Another treatment is to preserve the trough but so grade it that all water will flow to selected
low spots. Here ditches are dug through the raised edges, and troughs of metal or wood placed to
lead the water down the slope. This is readily done in hilly country, where most of the road is on
definite gradients, but not in level country.
Such drain ditches may be made wide and gentle, so that they can be dug, backfilled, and com-
pacted by machinery, or may be hand-dug, refilled, and tamped.
If the trough shape is left without precautions, a center gully may be scoured by a heavy rain,
a pond formed in low spots, and damage done to edges by overtopping and concentrated runoff.
Scraper distance from the edge is determined by depth of spread and slope. If a slope is 1 on 2, spreads
are 6 inches deep, and compaction is one-third, each pass will be 8 inches inside the previous one.
If the edge is not firm enough to support scrapers at the proper distance to dump loads, they
should be spread farther back and dirt cast out to the edge by a grader or angling dozer.
Additional slope stakes should be set as a high fill is built up to maintain the correct width.

DRAINAGE

Drainage is an important factor in the construction of most roadways. Groundwater must be kept
far enough below the surface so as not to damage it, or weaken the subgrade directly or by sup-
plying capillary water. Water falling on the surface of the road must be conducted off it, and
runoff or streams crossing the road must be provided for.

Raising the Grade. In swamps and lowlands, the only practical method of getting the road well
above groundwater is to build a high fill. If the base course can be made entirely of rock, it will
break any contact between the water and the balance of the fill. Clean gravel or coarse sand may
serve the same purpose.
Proper quality of fill can reduce the required height substantially. However, it is often more
economical to make a higher fill of inferior material obtained from roadside ditches, as in Fig.
8.33(A), and it is often possible to lower the water level by the same operation. Draglines or exca-
vators are generally used, but dipper dredges may be preferred when much of the land is under
water, or it is intersected by numerous channels.
Either machine may work along the ditch lines, piling spoil toward the center, as in (B). The
dragline will work away from the cut, as shown, but the dredge will float in it.
If a dragline has a sufficiently long boom, it can travel on the road centerline, and dig both
ditches and pile the spoil in one pass, as in (D).
Road fill may also be obtained by ditching in dry flatlands where the road is to be raised above
floods or snowdrifts. In such circumstances elevating graders may be used as shown in Fig. 8.34,
or dozers or scrapers.

Tiling. In sloping land, it is usually more economical to lower the water table by drainage. The
standard method, Fig. 8.35, is to put shallow ditches to carry surface water through cuts (A), and,
if necessary, to place porous tile or other drains (B) 2 to 3 feet deeper in loam soils. Silt or clay
deposits may require a drain depth of as much as 7 feet, but in such a case, better results may be
obtained by a normal drain depth and by the use of a layer of pervious material under the road.
The design of subsurface drains must be carefully adapted to the requirements of the particu-
lar job. The ground may drain naturally so that no work is necessary. There may be a saturated
condition that could be relieved by providing a drain through an impervious barrier (C), or by cutting