Page 528 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

10.34 THE WORK

Bank Slides. Most materials will rest temporarily at a steeper slope than their natural angle of
repose. Some sand and gravel may stand in vertical or overhanging banks when freshly cut, but
eventually fall or slide to slopes between 1 on 1 and 1 on 2.
The danger of undercutting high, noncaving banks is obvious. It is less apparent when a bank
caves and slides steadily when dug at the bottom, preserving a fairly uniform slope. Such a forma-
tion may gradually become too steep to be stable, without giving any indication of its condition.
A change in moisture content, a blast, thunder, or the dropping of a rock may start a slump, which
reduces the slope of the face by lowering the crest and advancing the toe into the pit.
The danger from such slides increases rapidly with bank height and steepness. In many cases,
workers have been killed and machinery buried in them.
Changes in moisture content affect both internal friction and weight, and either drying out or
becoming soaked may create or intensify unbalanced conditions.
Aside from this danger, a high, sliding bank offers the best possible bottom-loading condi-
tions. Because of the constant supply of fresh, loose material, digging is easy, and the excavator
has to move forward only at long intervals.
Damp clay will usually stand vertically when cut, but will slump or fall eventually. Vibration
from passing machinery or nearby drilling is liable to break down its structure so that it will flow.
If the movement starts at the top, a dangerous collapse may be caused.
A face of clay or silt exposed to alternate freezing and thawing or internal water pressure may
liquefy and flow out on the pit floor, eventually assuming a gradient as low as 10 percent. This
action is usually too slow to be dangerous, but should be allowed for in figuring clearances for
haul roads or in parking machinery.
High, steep rock walls should be checked for fissures running parallel to the face, which would
allow sections to fall off. These are particularly dangerous if filled with dirt that might absorb
water and exert a push.
No high face of any kind should be undercut widely without adequate bracing.
The safest way to dig high, steep banks in general is with a drag scraper. Sometimes a dragline
with a very long light boom is used to pull the crest down to the excavators.
On lower slopes, and on firm material, a dozer can be used.
Benching. It is usually good practice to limit the height of shovel cuts by taking the materials
in a series of layers or benches.
Two methods of benching a hill slope are described in Chap. 8. Pits are liable to take much
larger areas and require many more benches.
Benching may be done from the top down, or from top and bottom, as in Fig. 10.25. A bound-
ary cut is frequently carried down below the pit floor when the higher parts are exhausted.
A large number of benches may be worked at the same time, or in rotation. Each bench should
be large enough to provide ample space for shovel and trucks. If it is accessible at each end, one-
way traffic can be maintained and the need for turnaround space avoided. However, narrow roads
are often blocked by stalled vehicles, slides, rockfalls, or overbreaks.
If the bench is accessible from only one end, the shovel should work from that end so that the
width of the new cut will be available to traffic.
When layers are taken from the top down, starting at a hillcrest, or a back line which will stand
steeply, the working area may be made about as wide as desired. The excavating done on the top
widens the area available for the next cut.
When cuts are worked up from the bottom, width is largely determined by slope gradient and
face height; if the slope is 45°, a 1-foot height is required for each foot of bench width.
Top benching is preferred for steep slopes whenever immediate access can be had to the top.

LEVEL DIGGING

Material is frequently obtained by sinking the pit floor, or a part of it, in thin layers without develop-
ing a bank except at the boundaries of the excavation. The material may be piled before loading, in