Environmental Engineering Reference
In-Depth Information
2:1 or thinner or 0.5 litres/cm for WC 1:1 or thicker to fill voids of bleed water. He does
not favour the use of bentonite to give stable mixes, preferring to use low WC ratios and
bleeding of the water from the hole.
Grouts with higher WC ratio will tend to settle to lower WC ratios. Hence higher vis-
cosity results within the time of grouting and there appears to be little basis for using
higher water contents to improve penetrability. The authors feel there is merit in adding
bentonite to mixes with WC ratio greater than 1:1 (by volume) to control sedimentation.
There appears to be evidence from major grout projects that it is practical and beneficial
to do so.
18.3.6
Grout pressures
As mentioned in Section 18.1, there are two schools of thought on grout pressure:
(a) Those who limit grout pressures to below those which would lead to hydraulic
fracture;
(b) Those who believe hydraulic fracture is preferred to promote the penetration of the
grout.
The authors are advocates of avoiding hydraulic fracturing of the rock, as we are con-
cerned that the fractures opened by the grout will not all be filled by grout and the grout-
ing may worsen the situation rather than improving it. We are not concerned that seepage
still occurs through a dam foundation, provided the dam is designed to manage the seep-
age. The following discussion is based on avoiding hydraulic fracture.
As pointed out by Deere (1982), Deere and Lombardi (1985) and Lombardi (1985), the
maximum penetration distance is proportional to the pressure used for grouting. Hence it
is desirable to use as high a pressure as practicable without fracturing the rock.
The pressure which can be applied depends on the rock conditions (degree of fractur-
ing, weathering,
in situ
stresses and depth of the water table) and whether grouting is car-
ried out using a packer which is lowered down the hole at each stage (i.e. downhole with
packer grouting), or from the surface. The downhole packer method allows progressively
higher pressures.
Houlsby (1977, 1978) and WRC (1981) present graphs to allow estimation of maxi-
mum pressures at the ground surface. These are based on the assumption that the maxi-
mum pressures at the base of the stage being grouted are given by:
P
d
(18.2)
B
where P
B
pressure at base of hole in kPa;
factor depending on rock conditions;
70 for “sound” rock;
50 for “average” rock;
25 to 35 for “weak” rock; d
depth
of bottom of stage below ground surface in metres.
This allows for the weight of the overlying rock plus some spanning effect and has been
found to be satisfactory.
The tendency for rock to fracture or “jack” under grout pressures is reduced by using a
relatively low pressure to start grouting and building up with time. Since much of the
pressure in the grout is dissipated in overcoming the viscosity effects in the fracture, this
limits the pressure transmitted to outer parts of the grout penetration. Houlsby in WRC
(1981) suggests use of a starting pressure of 100 kPa (or less) for 5 minutes, then steadily
increasing the pressure over the next 25 minutes until the maximum pressure is reached.
The occurrence of fracturing can be detected by sudden loss of grout pressures at the top
