Page 144 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.38 THE WORK

FIGURE 3.29 Cartridge-placing tool.

under Ditching and Drainage, then excavated to firm bottom with machinery. Fill may then be
built up to road grade in thin, thoroughly compacted layers and with properly sloped sides. When it
is complete, pumping may be discontinued and the muck allowed to settle back against the slopes.
Removal of muck in any way is expensive work, except in the rare cases where it can be sold
locally for humus. The expense increases very rapidly with depth, and a point will be reached
where it is good practice to stabilize the mud rather than remove it. This is particularly true if the
area involved is wide or conditions do not permit side casting or blasting.
Grouting. One method to stabilize ground, as well as to provide seepage control and some reha-
bilitation of the ground, is by injecting a water-cement mixture which fills the voids and spaces
previously occupied by water. The mixture may include some special chemicals that react favor-
ably with the soil to form a hardened mass. Additives may include polymers to stiffen the ground.
Many of the grouting techniques were developed in Europe and have slowly been copied in North
America.
One technique used in ground made soft by excess groundwater is jet grouting, which originat-
ed in Europe but was brought to the United States in the 1980s. Jet grouting has been used not only
as a groundwater barrier but also for excavation support, as a bottom-sealing technique to prevent
pollutants from entering excavation, to combat scour beneath bridges, to stabilize slopes behind
retaining walls, and frequently to underpin existing commercial and industrial foundations.
The jet grouting method involves injecting a water and/or cement grout through jetting pipes
under extremely high pressures (generally 4,000 to 6,000 psi), forming a tight mix of grout and
native soil. This method can strengthen soft soils or form load-bearing foundations. For example,
when the historic structures in the Boston National Historical Park in Charlestown,
Massachusetts, were settling, more than 8 inches in some places, due to decades of uncontrolled
water infiltration into the loose artificial fill materials, jet grouting was used. A single-fluid jet