7
3. INSTALLATION
Borehole sizes are not critical but they should be
at least 100 mm (four inch
) in diameter and
not more than 30 mm
larger than the nominal size for which the spring-loaded mechanism was
designed.
3.1 Grouting requirements
For more details on grouting, refer to “Piezometers in Fully Grouted Boreholes” by Mikkelson
and Green, FMGM proceedings Oslo 2003. Copies are available from Geokon.
The general rule for grouting multilevel piezometers is to mimic the strength of the surrounding
soil. The emphasis should be on controlling the water to cement ratio. This is accomplished by
mixing the cement with the water first.
The most effective way of mixing is in a 50 to 200
gallon barrel or tub, using the drill rig pump to circulate the mix.
Any kind of bentonite powder used to make drilling mud, combined with a Type 1 or Type 2
Portland cement can be used. The exact amount of bentonite added will vary somewhat. Table 1
shows two possible mixes for strengths of 50 psi and 4 psi.
50 PSI Grout for Medium
to Hard Soils
4 PSI Grout for Soft Soils
Amount
Ratio by
Weight
Amount
Ratio by
Weight
Water
30 gallons
2.5
75 gallons
6.6
Portland
Cement
94 lb. (one sack)
1
94 lb. (one sack)
1
Bentonite
25 lb. (as required)
0.3
39 lb. (as required)
0.4
NOTES:
The 28 day compressive strength of this mix
is about 50 psi, similar to very stiff to hard
clay. The modulus is about 10,000 psi
The 28-day strength of this
mix is about four psi,
similar to very soft clay.
Table 1 - Cement/Bentonite/Water ratios
Add the measured amount of clean water to the barrel then gradually add the cement in the
correct weight ratio. Next add the bentonite powder, slowly, so clumps do not form. Keep adding
bentonite until the watery mix turns to an oily/slimy consistency. Let the grout thicken for
another five to ten minutes. Add more bentonite as required until it is a smooth, thick cream,
similar to pancake batter. It is now as heavy as it is feasible to pump.
When pumping grout (unless the tremie pipe is to be left in place,) withdraw the tremie pipe after
each batch, by an amount corresponding to the grout level in the borehole.
