Environmental Engineering Reference
In-Depth Information
with only water with very low (fine) cement contents being left in the upper parts of
the hole, allowing small “grout” takes to occur in these parts of the hole. Some exper-
iments carried out by Tjandrajana (1989) show this is not a problem for 2:1 water
cement ratio, but the upper 1.5 m of the hole is left largely with water after one hour
for 5 to 1 water cement ratio. The addition of plasticizers worsens the settling, because
the cement particles act alone and actually settle more quickly.
(c)
Distance grout will penetrate
. If the fractures are sufficiently open to allow penetra-
tion of the grout, the distance to which the grout will penetrate is dependent on the
fracture width, grout pressure and viscosity and the time taken in grouting. If grouting
continues for sufficient time, the limit of penetration is determined by the yield point
stress. Lombardi (1985) showed that:
Pa
C
max
R
(18.1)
max
where R
max
maximum radius of penetration (m); a
half width of the fracture (m);
grouting pressure (kPa).
Lombardi (1985) presents a method for estimating the effect of grouting time. A feel
for this can be obtained from the data in Deere and Lombardi (1985), which indicates
that for medium to thick grouts approximately 75% of maximum penetration will
occur in the first hour.
Based on this and comparison with some field data, it is considered reasonable to
conclude that the penetration of Type A and Type C cement grouts will be of the order
of those shown in Table 18.2. In the table NP indicates the grout will not penetrate the
fractures, so grouting will be ineffective.
(d)
The effectiveness of a grout curtain in reducing seepage.
Figure 18.12
shows a simpli-
fied typical embankment and foundation section with a low permeability core, high
permeability rockfill shoulders and a foundation consisting of an upper layer of per-
meable rock overlying “impermeable” rock.
Figure 18.13
presents the results of an approximate seepage analysis to show the effec-
tiveness of a grout curtain in reducing the amount of seepage beneath the embankment.
The analysis has been based on:
H
C
yield point stress (kPa); P
max
60 m all cases
w
6m all cases
h
20 m all cases
6m for concrete face plate.
Unless the grout curtain has a permeability at least 10 times lower than the ungrouted
rock the reduction in seepage is less than 50%. So for 20 Lugeon rock the grout curtain
must be less than 2 Lugeons to achieve 50% reduction in seepage. As pointed out above,
W
60 m for wide core
W
20 m for narrow core W
Table 18.2.
Approximate penetration from the
borehole of cement grout in fractures.
Fracture spacing
Lugeon value
1 m
0.50 m
0.25 m
100
20
12
4
50
12
3
2
20
3
1.5
1
10
2
1
NP
5
1
NP
NP
1
NP
NP
