42
Geotechnical News • December 2015
THE GROUT LINE
Application of a single GIN value
allows direct comparison of the
graphical and numeric data for indi-
vidual borehole stages, and for the
various phases of injection. It also
allows the grouting engineer to rapidly
assess and gain a feel for the prog-
ress of a single injection and / or the
progress of the grouting programme,
either by observation of the real-time
plot of the GIN curve and the evolving
GIN value during the injection, or by
visual inspection of the graphical plots
on completion of the daily injection
programme. Figure 1 gives typical
examples of the evolution of the GIN
value, within the GIN boundary curve.
Maximum injection pressure
The maximum pressure limit P
max
serves mainly to select the proper
grouting equipment, such as pump,
tubes and valves. Like the GIN itself,
it should be defined so that it complies
both with the rock mass properties and
project requirements.
If the purpose of grouting is, for
example, the impermeabilization of a
dam foundation, the maximum pres-
sure should be chosen according to
the expected future water losses and
uplift pressures after impounding. It
has to be sufficiently high in order
to avoid a fissure opening when the
reservoir is impounded. A common
value for the maximum pressure at the
borehole mouth is around 2 - 3 times
the future water pressure at that loca-
tion. Another important aspect to be
considered when selecting the proper
maximum pressure is the allowable
hydraulic gradient of the rock mass. In
this: the higher is the hydraulic gradi-
ent the higher shall be the maximum
injection pressure.
In practice, the maximum pressure can
be set in a number of ways. The most
reliable method remains certainly the
execution of grout test sections on
site in the same conditions using the
proposed mix design. Another indirect
method is to conduct hydro-fracturing
tests in the pre-injection investigation
boreholes, and to apply a factor of
safety to the measured hydro-fracture
pressure. In contrast to grouting test
sections, for hydro-fracturing tests
there is no volume constraint for the
water, which is first of all risky. Sec-
ondly, acknowledging the difference in
water and grout mix, a careful evalu-
ation of the test results by an experi-
enced person is required to be able to
extract the desired information for the
actual admissible grouting pressures.
Alternatively, an estimation may be
made with the confining overburden
and surcharge pressure, or the limit
may even be set on an empirical basis
based upon previous experience in
similar rock conditions and/or depths
of injection.
It is important to recall that the GIN
technique is actually self-regulating.
Any possible adoption of the pressure
with depth to avoid grout outflow or
Figure 1. Typical examples of the evolution of the GIN value.
