Page 207 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.9

Ditchers can be equipped with shoes or reels to lay tile or flexible conduits immediately behind
the digging, so that shoring is not necessary.
Design, operation, and applications are discussed in Chap. 14.
Graders and Dozers. Graders can make shallow ditches with sloped sides rapidly and neatly, by
the road-building processes described in Chaps. 8 and 19.
If there is no use for the excavated soil, it is usually spread and blended beside the edges.
The bulldozer can dig a wide, shallow trench from the side, as shown in Fig. 5.6(A). The volume
of excavation required increases very rapidly with depth because of space needed for ramps.
When the practical limit for side excavation has been reached, the dozer can work in the ditch,
pushing dirt into heaps, which it then pushes to the side, as in (B) and (C).
An angling dozer can excavate by sidecasting in the same manner as a grader, but it may be
harder to keep lined up.

ROCK

Stripping. Ditches frequently contain rock that is too resistant to be dug by the available equipment.
Occasionally the line of work may be shifted, but usually it is necessary to blast.
Dirt and rotten rock are removed by conventional methods. Spoil should be piled far enough
back to allow space for the drilling equipment, and for the shovel when it returns.
After machinery has removed the soil, the rock surface should be cleaned by hand. If the trench
walls are liable to crumble and slide from drilling vibration, they should be shored up, even if
depth is shallow.
Open-cut blasting is described in Chap. 9. Trench work differs chiefly in the restricted work-
ing space, and in the fact that all shots are tight. Loading must be 50 to 100 percent heavier than
on wide faces.

Drilling. Jackhammers can be used with the operator standing on the rock, or on the surface
beside the ditch. Crawler drills are usually on the bank. Special ditching drills may be suspended
over the work by cranes.
Several drilling patterns for 3- or 4-foot widths are shown in Fig. 5.7. In each of these, blasting
is done back from an edge or face of rock exposed by digging, or by previous blasting.
The distance or length of ditch that can be blasted in a single shot depends on the near presence
of buildings, whether it is permissible to overbreak the sides, and whether delay caps are used.
The holes next to the face can throw their burden along the line of the ditch. Any holes behind
them, shot at the same time, will tend to expend more of their energy to the side, so that they will
need heavier loading. As a result, they tend to break rock outside the digging lines (overbreak)
and produce finer pieces.
In (D) there are extra edge holes, called relief holes, which are lightly loaded or empty. Their
use makes a smoother edge, reducing both overbreak and underbreak.
Delay Caps. After the blaster has decided on the area to be blasted, the number of holes included
in it may be shot at one time; or a much larger number may be fired with delay caps. The first series
will be at the face, with the other two groups following in succession.
Millisecond-delay caps give the best results, as they provide thorough breakage with minimum
concussion. They are arranged so that the wave of explosion travels back from the face, with such
short intervals between rows that each is partially confined by the force of the previous blast. This
condition is favorable to good fragmentation and reduced overbreak.
Damages. Nearby buildings, or more distant installations containing delicate apparatus, may dictate
the size and type of shots. The conservative procedure is to fire one row at a time, and muffle and
restrain the explosion with dirt and mats. Millisecond delays may permit much larger shots,
because the explosion is spread over enough time to reduce its sharpness.