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| INSTALLATION | GEOKON
inches below the desired piezometer tip location. The piezometer can then be
lowered into position. (Preferably, the piezometer will be encapsulated in a
canvas bag containing clean, saturated sand.) While holding the instrument in
position, (a mark on the cable is helpful) fill the borehole with clean fine sand to
a point six inches above the piezometer.
Three methods of isolating the zone to be monitored are detailed below.
INSTALLATION A
Immediately above the area filled with clean fine sand, known as the "collection
zone", the borehole should be sealed by an impermeable bentonite cement
grout mix, or with alternating layers of bentonite and sand backfill, tamped in
place for approximately one foot, followed by common backfill (see Figure 4).
If multiple piezometers are to be used in a single hole, the bentonite and sand
should be tamped in place below and above the upper piezometers, as well as at
interval between the piezometer zones. When using tamping tools special care
should be taken to ensure that the piezometer cable jackets are not cut during
installation, as this could introduce a possible pressure leak in the cable.
INSTALLATION B
The borehole is filled from the collection zone upwards with an impermeable
bentonite grout.
4:
FIGURE 4:
Typical Borehole Installations
INSTALLATION C
It should be noted that since the vibrating wire 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
