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Geotechnical News • March 2014
www.geotechnicalnews.com
THE GROUT LINE
Norwegian grouting strategies
Same or not in Sweden
The contractual arrangements should promote risk sharing
and co-operation between all parties for day-to-day deci-
sion making and provide for fair reimbursement of grouting
efforts.
Contracts is a possible step for risk management.
Both the Owner and Contractor deploy skilled personnel on
site to monitor and follow-up the grouting works to refine
the grouting strategy as necessary on an iterative basis in
response to accumulated experience.
An observational approach has been adopted for grouting
for several years.
Groundwater inflow is controlled by pre-grouting the rock
mass ahead of the face.
Same.
Unless the strategy is to conduct continuous, systematic
grouting due to known adverse conditions, at least 4 probe
holes are maintained ahead of the face to give advance infor-
mation on hydrogeological conditions.
Pre-grouting is a standard way always adopted in normal
infrastructural tunnels. No owner up to today wants to take
risk of testing another way accept in research.
Grouting is carried out ahead of the face through a 360º fan
of grout holes, usually no more than 25 m in length to limit
the scope for drillhole deviation and un-grouted ‘windows’.
Same.
Grouting intensity is varied by adjusting the number of grout
holes in the fan and hence the spacing of the grout holes at
the next planned overlapping packer position. The spacing is
dependent on previous experience of performance, complex-
ity of the hydrogeological conditions and the groundwater
inflow rate criteria.
Seldom varied. It means often 3-4 m spacing and a packer
mounted 1 m inside the borehole.
The aim is to achieve the required result in one round of
grouting.
Same.
Control holes are drilled in the pre-grouted mass to gauge the
success of the pre-grouting exercise.
Sometimes.
If seepage from the control holes indicates that the required
inflow criteria are unlikely to be achieved, then the rock mass
is treated by another round of grouting.
Sometimes.
The tunnel face is not progressed until it is determined that
the required inflow criteria are likely to be achieved.
Possibly a follow up in a measuring weir after the project is
done. After that a post-grouting campaign can be used.
Groundwater inflow criteria are usually given as rates of
inflow measured over a distance which is meaningful to the
potential far-field effects on the surrounding environment.
Typical inflow criteria are usually expressed in L/min/length
of probe hole or control hole arrays and L/min/100 m of tun-
nel length.
The inflow criteria per 100 m of tunnel length is a measure
adopted for the tunnel project. The stipulations from the
environmental courts is only is maximum drawdown and
possibly a total flow out from tunnel.
A typical ‘toolbox’ of grout types may include:
à
à
polyurethane grout for dealing with high inflows from
probe holes or grout holes (Northcroft, 2006),
à
à
OPC grout for infilling wide discontinuities,
à
à
micro-cement grout injected under high pressure
(typically at least 50 bar above the ambient ground-
water pressure) to fill typical discontinuities and fine
cracks, and
à
à
colloidal silica grout to be used in the finest of fis-
sures (Northcroft 2006; Bahadur et al., 2007).
A predetermined “Toolbox” of grouts for Sweden. Not well
established, from project to project. Often depending on the
experiences of the building leader at site. Quite common is
that the design is changed from the original documents to a
design decided on site.