Page 140 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.34 THE WORK

FIGURE 3.25 Cross-section loading.

The dynamite will probably not remove all the mud down to hardpan, but it may liquefy most
or all of that which remains, so that the fresh fill will sink through it to firm bottom.
Underfill Shooting. Figure 3.27 shows the second method. Brush, stumps, logs, and other
debris are removed from the right of way, and any heavy sod or brush is broken up by machinery
or light blasting. Fill is placed in an amount calculated to reach from firm bottom up to the desired
road grade. Charges are placed in the mud, in casings driven through the fill. Shooting is by
Primacord, or by caps in each hole, rather than by propagation.
The mud explosion is confined by the mass of fill above, and should blow out the sides, driving
out most of the mud and liquefying the remainder so that the fill will sink to hardpan.
If the fill is clay or other compact material that might tend to bridge over the blasted cavity
instead of settling in it; or if the weight is not considered to be sufficient to resist the explosion; then
deep ditches may be blasted on each side of the fill after it has been placed and the charge set off
under it afterward. Blasting these relief ditches makes it much easier for the explosion to drive the
mud from under the fill, but it also puts a thick layer of muck on top of it, which must be bladed off.
The charges under the fill must be so placed that there will be no danger of their going off by
propagation; 30 or 40 percent gelatin is recommended.

Toe Shooting. Toe shooting is illustrated in Fig. 3.28. This is the third method—fill, then blast—
done in such short sections that the muck will be driven out to the front as well as to the sides. It
has the advantage of allowing the blaster to have frequent checks on the efficiency of the work, so
that techniques of loading or filling may be altered readily. A disadvantage is that the muck blown
ahead sometimes piles up in a large hill or wave of mud, making continually larger charges neces-
sary to move it. This tendency may be reduced by making the fill with a V-shaped front, so that
most of the material will be thrown to the sides. The mud wave may be blasted away in separate
operations, but part of it will then have to be cleaned off the fill.
Toe shooting is an excellent method to widen fills, the extra material being piled on the sides
and a single line of charges set off in the mud underneath.