Basic HTML Version
www.geotechnicalnews.com
Geotechnical News • March 2014
49
THE GROUT LINE
and the time spent on grouting after
the stipulated time. Not even unfilled
boreholes should be reimbursed for.
The risks were too high and it was
never conducted.
The most efficient way for the engi-
neer and owner according to Scandi-
navian experience is to secure control
with performance and cost related
to pre-grouting is to obtain qualified
estimates in the Bill of Quantity at
the tender stage and to re-measure
all grouting related activities during
tunnelling excavation. Such conditions
need to be reflected in the construction
contract. A guideline for compensation
units would be as follows:
• Probe drilling ahead of tunnel face
– re-measured and reimbursed by
drill meter
• Drilling for grouting – re-measured
and reimbursed by drill meter
• Grout packers - re-measured and
reimbursed by piece
• Grout materials – re-measured and
reimbursed by kg for all materials
• Grouting time – re-measured and
reimbursed by hours used for
grouting
To compensate for actual consumption
of time and materials for grouting may
sound risky for many project own-
ers. However, specifying minimum
capacities on machinery and by setting
minimum contractual production
rates the owner has tools making him
capable to control volume and cost of
pre-grouting works. Experience from
Norwegian view is as follows:
à
à
Design before contract, details
to be decided during the tunnel-
ing progress
à
à
The amount of cement governs
the sealing effect
à
à
Opening of fractures is seen as
a risk but could improve the
grouting works
à
à
Experience from Swedish point
of view is:
à
à
Design before contract, not
always
à
à
The amount of boreholes and
type of grout governs the sealing
effect
à
à
Opening of fractures often seen
as negative and as a risk
Discussion and conclusions
Sealing effect and tight enough?
The penetration length is directly
proportional to the applied pressure,
the fracture aperture and inversely
proportional to the viscosity and
yield strength. A doubled pressure
will increase the penetration length
to the double. In Figure 3 below the
penetration length is calculated using
the equations in (Gustafson and Stille,
2005) where the
grouting time is incorporated to deter-
mine the penetration length. In the
example the over pressures are 4 and 8
MPa respectively, the yield stress are
3 Pa and viscosity= 0.025 mPas. The
fracture aperture is set to 120 µm.
For infinite time the final penetration
for a pressure of 4 and 8 MPa are 80
and 160 m, respectively. For 30 min-
utes of grouting the difference is 9 and
18 m, respectively. We can see that the
penetration length is long and fast for
the first 3 minutes and after that gradu-
ally slows down.
Both the tightness (transmissivity) and
the extent (thickness) of the sealed
zone influence the ingress to a tunnel.
For instance, a very tight sealed zone
with a low extent is comparable with a
large zone with less “tightness”.
From the figure below (Figure 4) one
can see that the reduction of ingress
to the tunnel is strongly correlated to
the thickness of the sealed zone for
sealing factors of 50 or more. This
means that if the grouting can be done,
assuring that the transmissivity of the
rock mass is lowered by at least a fac-
tor of 50, the thickness of the sealed
zone can be used for prognosis of the
ingress. The thickness can be inter-
preted to a grout spread, large grout
spread mean a larger thickness of the
sealed zone.
Figure 3. A diagram showing the difference in penetra-
tion length using an overpressure of 4 and 8 MPa in a
fracture aperture of 120 µm.
Figure 4. A diagram showing the reduction of inflow to
the tunnel correlated to the sealing factor and the thick-
ness of the sealed zone. The tightness or how permeable
the sealed zone is described by the sealing factor. For a
sealing factor of 100 mean that the sealed zone has 100
times lower transmissivity than the surrounding rock.