Figure 2 - Typical Borehole - Geokon 3400 Series Instruction Manual

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Installation B:
The borehole is filled from the "collection zone" upwards with an impermeable bentonite grout.
(See Figure 2.)
Figure 2 - Typical Borehole Installations
Installation C:
It should be noted that since the piezometer is essentially a no flow instrument, collection zones
of appreciable size are not required. The piezometer can be placed directly in contact with most
materials, provided that the fines are not able to migrate through the filter. The latest thinking is
that it is not necessary to provide sand zones and that the piezometer can be grouted directly into
the borehole using a bentonite cement grout only. However, good results have been obtained by
placing the piezometer inside a canvas bag filled with sand before grouting.
The general rule for installing piezometers in this way is to use a bentonite grout that mimics 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 the two substances is to use a drill rig pump to circulate the mix in a 50 to 200 gallon
barrel or tub.
Any kind of bentonite powder, combined with Type I or Type II Portland cement can be used to
make drilling mud. The exact amount of bentonite needed will vary somewhat. Table 1 shows
two possible mixes for strengths of 50 psi and 4 psi.