Page 313 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.7

essential to found it below frost level on firm soil or rock. If the quality of the soil is questionable,
a wide concrete footing slab may be poured.
The thickness and strength of masonry required for a retaining wall are commonly underestimated.
Results of under-strength construction are sometimes satisfactory, but often not. For safety, wall
thickness at any point should be between one-third and two-thirds of the height of the wall above
that point, and the top should be 6 to 9 inches thick. Minimum thickness is safe when reinforced
concrete is used, when height is moderate, and the retained soil is well drained and stable.
Maximum thickness is required when a steep slope rises from the rear of the wall, and when the
ground is very unstable. Another consideration is the strength of the masonry. Reinforced concrete
is the strongest used. Plain poured concrete is considered stronger than concrete block, brick, or
mortared stone. Dry stone walls have little resistance against thrust and should be kept low.
The push from dirt behind the wall can be minimized by keeping it well drained. A layer of
gravel or other porous material should be placed along the rear face of the wall. A tile drain should
be placed beneath the foundation, and there should also be “weep” holes through the wall itself.
Ground expands when the water in it freezes, and the surface slab formed in this manner can
exert a considerable thrust. A slope or batter at the rear corner will deflect this pressure upward
so the slab will slide on the wall instead of pushing it.
1
A vertical wall often has an appearance of overhanging. A backward lean or batter of ⁄ 2 inch
for each 1 foot of height will counteract this. Such batter can be increased to any desired slope
with some increase in stability. A face slope in a dry masonry wall may permit outward move-
ment for some years before it becomes vertical or overhanging.

Geosynthetic Walls. The use of geosynthetics to build high earth retaining walls has been increas-
ing as it is a more economical choice than concrete retaining walls. A comparison is shown in Fig. 7.4.
There are two types of geosynthetics. One type is the geotextile-permeable woven or nonwoven
synthetic fabric blanket, described in Chap. 3, for stabilizing ground for vehicle passage. The
other type is called a geogrid, which is like a mesh with openings between strips of high-quality,
strong polyethylene material.
Retaining walls built with geosynthetics may be 20 to 40 feet high and can have a nearly ver-
tical face, if that part is built with decorative face blocks or open block to allow viney growth. The
geosynthetic reinforcing sheets, which are generally as deep into the fill as the wall is high, are laid
horizontally on the earth as the fill is built up. The vertical spacing between sheets is dependent
on their required strength according to the loads that will be applied on the ground above and back
of the wall. That spacing may be from 1 to 3 feet. Polymers creep at stress levels on the order of

FIGURE 7.4 Comparison of retaining walls. (Courtesy of Hoechst Celanese
Corporation.)