Page 133 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.27

FIGURE 3.17 Overlapping and spacing platforms.

1
A platform 4 ⁄ 2 12 feet carries an atmospheric pressure of over 50 tons. When air can flow
under it freely, the pressure is upward as well as downward, and is not felt in lifting it. If air is
sealed from the bottom by mud, then the lift of the shovel merely serves to reduce the weight of
atmosphere and platform on the mud beneath, causing mud at the edges which carries the full
atmospheric load to squeeze in under it. A thin mud will flow quickly, a thick tenacious one may
stretch but not flow and keep the air locked away from the bottom.
If the suction is too strong to break by a direct lift on the whole platform, the pull should be concen-
trated on one side or one corner. If the mud seal is broken at one spot, it should shear off from the whole
under-surface. The platform might also be lifted and dragged at the same time. Suction presents compar-
atively little resistance to sliding, and this motion greatly increases the shearing effect on the mud seal.
Unfortunately, a dragline is apt to make a pile of mud and debris which would prevent the platform from
sliding forward, and the lattice boom is apt to twist or collapse if subjected to heavy side-strain.
A method of releasing is to catch the forward edge with the bucket teeth and hoist while pulling
back enough to keep hooked.

Root Mats. Trees growing closely in swamps usually form a mat of interlocked roots that will sup-
port crawler equipment, but will give it a rough trip. Openings may occur where the roots are lacking,
which may be crossed by means of green saplings laid across the path of the machine, as indicated in
Fig. 3.20. If it is necessary to remove trees to give the shovel space to get through, they should be cut
as close to the ground as possible, as the shovel might sink while going over the stump and get hung
up on it. Logs laid on each side of the stumps across the line of travel are protection against this.