Page 284 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.20 THE WORK

Boulders. Large boulders also interfere with digging and are very likely to cut the drag cable if
lodged in front of the tracks. It is sometimes possible to dig deep holes in which they can be buried,
or to line them up along the edge of the pond where they should improve the appearance of the
bank. However, they are more difficult to winch out than stumps, because of difficulty in getting
a grip on them, and are an even greater nuisance in rehandling spoil. Often it is best to break them
with dynamite, or air or hand tools, into pieces small enough to bury or mix with the spoil.

Hard Digging. A small dragline has great difficulty digging hard or rocky soil. It will do its best
if the soil is not covered by water; if the bucket teeth are sharp; the boom held at a low angle; and
the shovel footing kept as low as possible.
Occasionally a swamp floor may be of cemented gravel or decomposed rock which can be bro-
ken up with a tractor-drawn ripper. Such floors, and most hardpans, can be effectively dynamit-
ed if charges can be sunk deep enough in drilled or hand-dug holes. It is sometimes sufficient to
blast a small area in which the dragline will be able to cut to depth, as it may be able to maintain
this depth through the undisturbed material around it. If the whole bottom needs to be blasted, it
will probably be cheaper to use other machines. Sometimes a single heavy blast will soften clay
throughout the whole area.
Backhoe. A backhoe has quite effective penetration, but is hampered in pond digging by the inabil-
ity to pile spoil at a distance. This limits the amount it can dig and exposes it to the danger of getting
caught in slumping piles. A very good working team is a backhoe digging the hard soil, as in Fig.
6.13, and a dragline taking it away as fast as it is dumped. Best results are obtained if they work
together, but because of the exact timing required to avoid accident, it is safer for the hoe to cut as
much as it can pile and move on, with the dragline following at a discrete distance behind. At the end
of the strip the hoe may turn and work back, building a new pile to be removed by the dragline.
Clamshell. A clamshell with a heavy bucket has good penetration but works quite slowly, and is at a
disadvantage in sticky soils because of suction holding the bucket down. If used, it may do the digging
and the casting back and spreading; or the rehandling of loosened material may be left to a dragline.

FIGURE 6.13 Loosening hard bottom with a backhoe or front
shovel.