Page 145 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.39

grouting procedure was used to fill the voids under the buildings and to improve the fill quality.
The single-fluid method was used because ground movement during construction is negligible
and potential damage to the foundation is minimal.
Sand Drains. Muck can be stabilized if enough water can be squeezed out of it to convert it
from a semiliquid to a solid. The weight of a heavy road fill has a squeezing effect, but the mud
and water flow together and very little compaction is obtained for a long time.
Vertical sand drains may be used to dry up the mud so that it will support a load. They consist
of columns of sand extending from hard bottom to the top of the mud, and connected at the top to
each other, and to an outlet by a sheet of sand or gravel, or other drainage systems. They are dis-
cussed in Chap. 5.
Quicklime. Where the topsoil is so soft that it is impassable by personnel with boots, lime can
be spread on the surface and worked in with farm implements such as a disc harrow pulled by a
crawler tractor. The quicklime is obtained by cooking limestone at high temperature so it becomes
calcium oxide (CaO) which reacts with water, giving off heat and expanding into hydrated lime
or calcium hydroxide [Ca(OH)2]. Therefore, the quicklime added to the soil sucks up the water,
changing the soil from a moldable, soupy mass to a crumbly solid. This makes the ground surface
more passable.
Chemicals. If digging is done in a water-bearing sand or gravel, very soft conditions and exten-
sive caving of banks may be experienced. This difficulty may be avoided by drying the area with
well points, or by other methods described in Chap. 5, but it may be more economical to prevent
water from moving through the soil. This may be done by means of cement grout, discussed above
or by other chemical treatment. The grouting material displaces water, and then hardens or gels
into a mass that prevents more water from getting between the soil particles.
One chemical treatment involves pumping a solution of silicate of soda into the ground
through drill holes, then pumping in a solution of calcium chloride that causes the silicate to form
a dense, hard mass.
American Cyanamid’s chemical grout is a thin water solution of two acrylic chemicals and one
catalyst, that is mixed with another catalyst solution just before injection into the ground. A chemical
reaction causes the mixture to form a permanent, water-impermeable gel, at a time that is control-
lable between 3 seconds and several hours. These characteristics are particularly useful in sewer
rehabilitation work.

Freezing. Any soil in which pipes may be sunk can be stabilized by freezing, but the presence
of salt, or certain other chemicals, may make it difficult.
It is accomplished by sinking a number of metal pipes, rather closely spaced, in the area to be
stabilized. Tubing containing a refrigerant, usually ammonia, is placed in these pipes and connected
to heavy-duty refrigerating apparatus. The soil water will freeze most rapidly if it is stagnant, but
even a steady flow can be checked.
This is an expensive job, mostly confined to a very fine-grained mud, or deep work which does
not respond to well points.

FILLS

Compaction. In fresh fills, the most serious mud difficulties are due to improper compaction.
Here it is sufficient to say that excellent compaction may be obtained if a fill is made in layers 6
to 18 inches in depth, and each layer is thoroughly rolled or tamped in all parts. If rollers are not
available, trucks may be used, first empty and then loaded. Such a fill usually will be incapable
of absorbing enough water to turn to deep mud. The extra expense and nuisance of making a fill
in this manner, when specifications do not call for it, may be regarded as an insurance premium
against the loss of having to stop a job for days or weeks because of a soft dump.