Environmental Engineering Reference
In-Depth Information
Table 18.3.
Guidelines for deciding on limits of effective grouting with Type C Portland cement.
No further grouting needed when
or
20% reduction in
Water test value
Lugeon value or grout
Erodibility of
before grouting
take from previous
or All grout takes
or Grout hole
*(Lugeon)
(2)
stage
(1)
(Lugeon)
(2)
foundation
(kg cement/m)
spacing (m)
Low/non
10
20
25
1.5
High
7
15
25
1.5
Note:
(1)
For rock with joints closer than 0.5 m.
(2)
For Type A Portland cement adopt Lugeon values
20% greater.
and the discussion in Section 18.2.3 it is recommended that closure criteria should be
based on:
- Lugeon value prior to grouting the hole;
-Grout take;
-
The nature of the dam, its foundation and what is being stored in the dam.
These factors should be considered together to make decisions about whether further
grouting is required.
It is not yet possible to quantify all these effects (and may never be), mainly because of
the complexity of flow in fractured rock and the time dependent nature of cement grout
properties. It is also not possible to make rigid rules - each case should be considered on
its own merits. It is suggested that for grouting of the foundations of earth and earth and
rockfill dams with Type C Portland cement, the guidelines given in Table 18.3 be adopted.
“Erodible” foundations would include extremely to highly weathered rock and rock
with clay filled joints which might erode under seepage flows.
For grouting with Type A portland cement, Lugeon values should be increased by 20%
to account for the coarser nature of the grout particles. For microfine cement, grouting at
values half those quoted would be reasonable from a grout penetrability viewpoint, but
may not be justified by the benefit gained.
The overriding philosophy in these recommendations is that it is not possible to stop seep-
age by grouting, only to reduce it, and cement grout can only significantly reduce seepage
when it can penetrate the fractures, i.e. in relatively widely spaced open joints in moderate
to high Lugeon value rock. The objective of the grouting operation should be to locate and
fill these large fractures and thereby to avoid high and concentrated seepage flows.
Different criteria have been given for “erodible” foundations, with some misgivings. It is
better to acknowledge that, if foundations are erodible, grouting will not prevent erosion,
only reduce erosion potential. With or without grouting, filters should be provided for
erodible foundations under the downstream part of the dam to allow seepage water to
emerge in a controlled manner without erosion of the foundation. For concrete dams, the
foundation should be taken further down, to non erodible rock.
For concrete faced rockfill, gravity concrete and arch dams seepage gradients are higher
and there is some argument for using lower Lugeon values. However, the overriding con-
sideration is that the cement will not penetrate fine fractures so, at most, the values quoted
should be reduced by about 30%.
The question of whether the water is “precious” or has a high contaminants content is
not really relevant, since grouting to more restrictive Lugeon values as advocated by
Houlsby (1986) will not result in significant reduction in seepage. In these cases it will be
