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138 Barry’s Advanced Construction of Buildings
Pressure grouting
In permeable soils, or soils where it is known that small cavities may be within the ground,
pressure grouting may be used to fill the voids. Holes are drilled into the ground using
mechanically driven augers. As the auger is withdrawn, cement slurry is forced down a
central tube into the bore under pressure. Pressures of up to 70,000 N/mm 2 can be exerted
by the grout on the surrounding soil. The slurry contains cementious additives, such as
PFA, microsilica, chemical grout, cement or a mixture. PFA is cheap and often used as a
bulk filler to improve the bearing capacity of the ground. As the grout enters the void, it
forces itself into the voids, cavities and fissures in the soils and rock. In weak soils, it will
displace and compact the ground as it fills the voids. As the voids are filled, the ground
becomes stiffer, more stable and water resistant. Pressure grouting can also be used around
basements and coffer dams to reduce the hydrostatic pressure on the structure. To create
a water-resistant barrier, the bores and subsequent columns of grout are placed at close
centres. PFA is generally used as the ground modification and stabilisation material,
whereas the expensive chemical mixes (resin or epoxy mixes) or those containing microsilica
are used to fill small voids and improve the ground’s water resistance.
Soil modification and recycling
With the increased use of brownfield, reclaimed and landfill sites for construction purposes,
there is a need for faster, more effective and economical methods of improving the
ground conditions. Plant has been developed that is capable of cutting into site soils
(including contaminated material), breaking up the strata, then grading and crushing the
material before mixing it with cementious additives and relaying it to provide a compacted
modified or stabilised hard standing (Figure 3.58).
Roger Bullivant has developed a soil stabilisation and modification system. The system
uses a large cutting wheel that breaks down and pulverises the soil, and in a mixing
chamber mixes the additives with the graded soil (Figure 3.59 and Photograph 3.15). In
Working direction
Unstable soil
Milling and mixing chamber
Stable or modified
soil ready for
compaction
Figure 3.58 Soil modification, stabilisation and recycling machine.