Page 360 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.22 THE WORK

In general, cut slopes range from vertical or even overhanging in rock and 5 on 1 in the most
stable soils down to about 1 on 6. It is the engineer’s responsibility to decide where in this wide
range the requirements of stability, safety, and the roadway budget can best be compromised.
Slides. The most serious problem associated with deep roadway cuts is that of landslides. These
may occur during the work, or at any time after completion. Dangers include loss of life or injury
among those building or using the road; destruction of excavators, trucks, cars, and other equip-
ment; and loss of use of the roadway for long periods.
The likelihood of slides increases with height and steepness of banks, but slides are caused by
internal conditions.
In rock, the cause is usually a dip of seams or joint structure that provides an inclined slide for
the cut layers, as in the right side of Fig. 8.15(A). This structure, when well lubricated by ground-
water, may produce anything from a series of minor rockfalls to a 100,000-yard roadblock.
Much the same effect is produced in unconsolidated soils that have sloping layers of pervious
and impervious material, or where a slightly pervious soil mass rests on a sloping base of clay or
rock.
The existence of such a condition may be revealed by preliminary borings, or be shown by a
line of springs as the top of the impervious layer is excavated. In this case a slide is most likely to
occur after heavy rains, when the loose soil is heavy with water, and water movement on the base
formation provides both a lubricant and pressure.
Slumping and sliding may also occur in seemingly uniform soil masses, because of water seepage
or saturation zones.
The forces involved in deep-seated, large-volume slides are usually too great to be controlled
by braces or anchors. However, most can be prevented by cutting to a flatter slope in danger areas,
and/or diverting or draining the water that starts them moving.
The likelihood of slides may be a determining factor in selecting a route. However, there are
many areas in the world where mountain roadways must be constructed for long distances along
steep soil slopes, and where money is not available for engineering investigations or drainage works.
Under such conditions a pioneer road may be cut by a dozer sidecasting down slopes and pushing
fills across canyons. The road is widened and improved by cutting down its floor.
When slides occur, the dozers simply cut new road shelves across them, repeating as often as
necessary. When fills wash out they are replaced, cutting new slots on the slopes above the road
cut where necessary.
In the course of a few years it is usually found that the larger part of the route is reasonably
stable, and that other sections stabilize with repeated working. The true problem spots that remain
can then receive detailed engineering investigation and corrective measures, at a fraction of the
cost of similar work for the entire route.

Checking. A technique is available for testing the stability of slopes with the Seismitron,
described briefly at the end of Chap. 9. The instrument’s probe or receiver may be placed on the
slope surface, but results are more accurate if it is placed in a drilled hole in the bank. It picks up
tiny sounds of ground movement called microseisms, and amplifies them so that they can be heard
in earphones.
An increasing or high frequency, say over 25 or 30 microseisms per minute, indicates danger
of a slide. A low or decreasing rate is an assurance of stability. It has been found that slopes that
failed when wet had given warning of possible failure while they were dry.
Clay, mud, or fine sand may not produce warning noises that can be recognized. Most other
formations do.

Stabilization. When the bank is high, it may be necessary to drill long holes into the toe to pre-
vent water from causing the face to slump. Since this trouble usually occurs in soil that is firm
enough to leave in high banks but not hard enough to resist percolating water, augers may be the
preferred drilling tool. Perforated metal pipe is inserted in the holes while drilling or immediately
afterward.