Page 122 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.16 THE WORK

MUD

Before proceeding with the details of various types of excavation, it is in order to consider some
of the general problems. Mud is one of the most important of these.

NATURE OF MUD

Water Content. Mud is soil saturated with water to such an extent that it loses its structure and
takes on some of the properties of a liquid. Even the driest soils contain some water in very thin
films, and moderate additional amounts may give added firmness by acting as a binder. But when
the quantity of contained water is sufficient to build up water films around the grains thick enough
to serve as a lubricant so that they can move freely on each other, the soil becomes mud.

Particle Size. The quantity of water necessary to turn mineral soils into mud varies with the size,
shape, and arrangement of the particles. Small grains have much less volume in proportion to the thick-
ness of the water film they hold than large ones have, and therefore they form more fluid muds. Sharp
angular grains have projections which penetrate the film and interlock, and large grains and pebbles
develop high enough contact pressures to cut through the film. If there are enough fine particles in a
mixed soil to prevent the coarse ones from touching, the mud will have the qualities of the fines.
A fine-textured soil such as clay will also remain saturated much longer than a coarse one, as the
spaces between the grains are so small that water moves through them very slowly.
Humus. Humus, or peat, which is decayed organic material, absorbs water somewhat as a sponge
does, in large quantities, and holds it stubbornly against evaporation and drainage. When saturated,
nearly pure humus, as found in some swamps and peat beds, resembles a jelly, fibrous or smooth
in texture, and black or brown. It is the most slippery and most treacherous of the muds. It dries very
slowly, with shrinkage of 50 percent or more, to a light, fluffy soil. When mixed with inorganic
soils, as in topsoil and mucks, it greatly reduces their load-bearing qualities and makes them muddier
under wet conditions.
Making Mud. When undisturbed, inorganic soil is usually quite closely packed, with its grains fitted
together closely, and often lightly cemented by mineral deposits. When it is dug up or pushed around, the
grains are shaken away from each other into a loose structure. In this condition it can quickly absorb a
large quantity of water and become a very soft mud. As it dries, the grains settle together so that less water
is absorbed with each subsequent wetting. If it is compacted by rolling, tamping, or vibration before being
soaked, it may become even more water-resistant than in its original state.
When a firm, dry soil is covered with water, it gradually absorbs some of it and expands in volume,
but it never becomes as soft as if disturbed before wetting.
If a firm, fine-grained soil has a film of rainwater on its surface, and is passed over by a vehicle
tire, the water will be forced between the surface particles and the resulting mud will be wiped off and
pushed aside, leaving a new surface exposed to the next raindrops and wheel passage. Repetitions
of this result in a slippery road, ruts, and mudholes.
Frost. When soil freezes, the expansion of ice crystals between the particles pushes them out of
place. When the soil thaws, it is likely to become a slippery, structureless mud, often resembling
toothpaste in consistency. It ordinarily firms up fairly quickly, particularly if vibrated by a heavy
rain, but may persist for several months when upward seepage of water prevents settlement. In its
extreme condition it will not support loads, and is made more dangerous by its occurrence in places
that normally are firm, and under sod which bridges and hides it. Such places can be detected by
sounding with a crowbar, and should be avoided or treated with the same precautions as soft swamps.