Page 210 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.12 THE WORK

Burial. When the rock surface is well underground, the ditch may be refilled with dirt after the
rock has been drilled and loaded. This confines the explosion, prevents rock from scattering, muffles
the noise, and protects the ditch walls from caving. It provides a safe working area for the machine,
usually a backhoe or clamshell, that will redig the trench after the blast.
If the dirt fill is shallow, or rocky, it may be necessary to use mats or logs in addition.
Wiring must be thoroughly protected before filling on top of loaded rock. Fine dirt is hand-
shoveled over the connecting lines. If the rest of the fill is fine, a thin cushion is enough. If there
are heavy or sharp rocks that cannot be kept out, a deep protection is required.
The lead wires may be run along the rock surface beyond the area to be filled, or led up to the
surface inside air hose or other tubing.
Two caps may be placed in each hole with duplicate wiring, or Primacord can be used.
Backfill may be pushed in from the side by a dozer, or trucked from an excavator digging
another section of the ditch.

Removal. Blasted rock may be dug from the trench by machine, by hand, or by both methods.
If the ditch is narrow, with hard, irregular walls, projecting stubs may jam a bucket so fre-
quently that hand work may be cheaper. A bucket or container is frequently loaded by hand and
hoisted by a machine.
Large rocks are lifted with slings or tongs.
In general, it is good policy to blast sufficient width so as to ensure working space for a
clamshell or hoe bucket.

CAVING OF BANKS

Many soils will not stand in vertical walls, so the sides of the ditches must be either sloped back or
braced. A few soils will not stand on even a moderate slope, and these will require very heavy bracing.
Groundwater is the most important single factor in collapse of edges of cuts. It acts as a lubri-
cant that enables the soil particles to move on each other readily, and exerts pressure that moves
the particles toward the ditch. Sand faces may fall from this cause, or from the drying action of
water draining and evaporating, so that the bond between the grains is weakened.
If water is allowed to stand in a ditch, danger of caving from flow of groundwater into it and
from other dynamic forces acting in the soil is reduced. However, wave action set up by dropping
of stones or chunks of dirt may cut into the walls and undermine them.
It may happen that the upper part of a trench wall will be firm, dry material, but that a shallow
layer at the bottom is waterlogged and unstable. The sides will stand for only a short time before
becoming undermined by movement of the lower layer.
In general, caving of ditch sides is more apt to happen minutes, hours, or days after the digging
than immediately. The usual exceptions are loose, sandy, or semiliquid muds that flow into the
excavation, rather than cave or slide into it.

Stabilizing. Sloping of sides for stability is a technique chiefly used for permanent open ditches,
which will be discussed later in this chapter. It is seldom used for trenches for burial of pipes because
of the large amount of extra digging, the space required, or the area of pavement, lawn, or other
surface disturbed.
Vertical trench walls may be stabilized by bracing, draining, freezing, or chemicals. Bracing
is the most common technique, and may be required by law.

Bracing Structures. Figure 5.8(A) shows a light system of bracing or shoring used where danger
of caving is slight. Planks are placed vertically in the trench at 4-foot intervals, and pressed against
the dirt by means of push-type turnbuckles, called sheeting jacks. Bracing timbers are inserted and
the jacks removed. The planks are usually 2 or 3 inches thick, the cross braces 6 6 or larger.
Wide ditches require heavier cross braces than narrow ones.