Page 123 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.17

In northern winters, frost may stabilize a swamp so that it can be worked easily. Ice and frozen
earth are liable to be variable in thickness and treacherous because of snow cover and the heat of
decay of organic material. But any traveled route will gain in stability as long as freezing continues.
Freezing provides a pavementlike support for machinery, and may stabilize unfrozen mud as well.
Ice lenses at the frost line absorb capillary moisture from below. If upward flow is restricted by
coarse soil or an impervious formation, the mud will be dried and made firmer.
Mud from thawing is apt to render dirt road surfaces slippery, particularly on sunny slopes.
Early mornings such roads are often frozen hard, and work must be done, then, in cloudy weather
or at night. Tire chains are useful but may not be adequate.

Geotextile Barrier. The problem of permanent reinforcement of soft ground for embankments
and foundations is now being solved by the manufacturers of sheets of geotextile fabrics. Geotextiles
are permeable woven or nonwoven synthetic fabrics—polyolefin, polyester, or fiberglass fibers—
as recognized by the Industrial Fabrics Association. They weigh around 5 to 25 ounces per square
yard, can drain water and stabilize, separate, and reinforce soils.
The geotextile technique comprises five design considerations, as depicted in Fig. 3.11: (1) the
bearing capacity, since the existing soft ground will settle under new load and the fill on top will
have to be increased; (2) the global stability in the tensile strength of the fabric; (3) the elastic
deformation which defines the central stretching and so settlement of the reinforced soil; (4) the
anchorage length to keep the geotextile sheet from pulling out of the soil it is reinforcing; and
(5) the lateral spreading of the soil above the sheet of fabric which must be resisted by the friction
between the soil and fabric.
Geotextiles have been used to cover a peat bog, to stabilize city streets and roads in new sub-
divisions, and to cover many bigger areas. The geotextile fabric is made with many sheets seamed
together to cover the soft ground of a harbor area with sand fill laid on top, to build up to the
desired elevation for heavy industrial equipment to operate. The soft soil consolidates within 6 to
24 months so that the fill can be brought to final design height for permanent use. The runway
extensions for Washington’s National Airport were made most economically by using geotextile
fabrics on the muddy subsoils of the Potomac River.
The Mirafi 140 fabric performs three functions in soil stabilization: separation, filtration, and
reinforcement.
In building a road across a swamp, the strip is first cleared of all hard or sharp objects or bumps.
Roots, sod, and sometimes soft brush are left undisturbed.
The fabric is unrolled on the surface, with overlapping at any joints. Aggregate, usually bank
gravel, is truck-dumped and spread, preferably by light equipment.
There should be a foot or more of aggregate between the fabric and truck tires, to protect it.
Any excess can be shaved off after compaction, which preferably is done with a light vibratory
roller.
Sand. Clean sand is as troublesome as mud to two-wheel-drive vehicles. It can be stabilized
with pneumatic tire rollers and plenty of water, but the surface will loosen up as soon as it dries.
Tires spin and dig down into it rapidly, with a jerking motion that is very damaging to the drive
mechanism. All-wheel-drive vehicles ordinarily have less difficulty with it, but it consumes con-
siderable power. The general problem is one of getting traction without digging in, but there is no
danger of simply sinking, as in mud. Partially deflating the tires may help; smooth tires will do
better than those with tread, as they will not dig down as readily. Mats of brush, wire, grass, or a thin
layer of dirt may suffice to give traction.
Tracked vehicles can travel on sand without difficulty, but if equipped with grousers, care
must be taken about pulling heavy loads that may cause tracks to spin, as they will then hang up
quickly. The silica which makes up most sand is very wearing to the track parts, particularly when
particles are angular.

Quicksand. Quicksand usually is fine sand or silt through which water is moving upward
with enough pressure to prevent the grains from settling into firm contact with each other. It