Grouting in rock fissures
The fissures may be regarded as a two-dimensional plan where the rock on each
side gives confining resistance to grout penetrating it. The further from the
injection point, the larger area to cover, and the pressure drops accordingly.
This results in range limitations. The different grouting mixtures have
different particle sizes and therefore different possibilities to penetrate the
smaller/finer/thinner fractures. The grout goes where the resistance is lowest,
larger fractures before smaller, smooth and plane fractures before rough and
winding ones, all the time seeking the lowest hydraulic resistance. Grouting
generally works very well when dealing with fractures in rock. Pregrouting is
the dominating mitigation for tunneling in Norway, where major parts of the
tunnels are in hard rock.
Grouting in soil
The pore volume in soil may be regarded as a three-dimensional medium. The
grout is spread as an expanding “spherical plane” instead of a
“spherical circumference”. For this reason, the pressure drops much faster in
soil than in rock fissures, as distance from the source increases. The pores in
soil are the voids in-between each of the grain particles. The pores are
connected through tiny passages where the grains do not have contact with each
other. The grout that penetrates where the resistance is lowest, will have
difficulty penetrating the grain skeleton evenly and fill all the voids.
Photo: Grout is orange in sample.
Photo: Grout in fines.
When the ground “does not take grout”/the grout does not penetrate, it will neither
be able to waterproof or improve the ground strength in general. In sand, silt
and clay, grout cannot be forced into the pores by pressure, even at great
depth with high confining pressure. If pressure is increased, there will be
hydraulic splitting and the grout will follow the created fissure/the widest
passage instead of evenly filling the pores.
On the other hand, artificial ground freezing penetrates all geolocical formations,
independently of permeability, providing stabilization and water-proofing.