Page 314 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

7.8 THE WORK

one-third to one-half their strength. A wall designed for that level of stress will allow creep,
whereas a design at one-tenth the strength level should avoid the problem of creep.
Poor drainage is a leading cause of retaining-wall failure. Therefore, the geogrid wall needs various
provisions for drainage, as shown in Fig. 7.5. The mortarless, interlocking Keystones drain naturally
(1). Surface runoff can be diverted away from the wall (2). Embankment flow can be directed to an
outflow pipe (3). And the groundwater that may get into the reinforced zone can be directed to
another outflow pipe (4).
Drainage. It is desirable that all areas be provided with sufficient surface slopes, proper sub-
drainage, or both, so that water will not stand anywhere and the ground will dry and firm rapidly after
saturation. Particular care may be required to subdrain any soil touching basement walls or floor.
When the soil is porous sand or gravel and the water table is low, drainage is usually automatic
and mistakes in gradient will show only briefly during rains. Impervious soils, however, demand
care in shaping so that they will drain completely, not only when the job is completed but also
after settlement of fills.
If pervious fill is placed on a relatively impervious native soil, the surface on the impervious
soil should be shaped to drain, to avoid trapping underground water in pockets. If the native soil
to be buried is pervious, it need not be graded for drainage, regardless of the type of fill, although
shaping to avoid uneven depth of fill is still advisable.
Where areas are large, rainwater flowing on the surface may constitute a serious nuisance even
if it does not erode the ground. At a price, such water can be caught in catch basins, then removed
through underground pipes. Because of the expense of such an installation it is best to have it

FIGURE 7.5 Drainage for a geogrid wall. (Courtesy of Keystone Retaining
Wall System, Inc. as published by Sweet’s Group, McGraw-Hill, Inc.)