MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER
| INSTALLATION |
9
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.
Make drilling mud using any kind of bentonite powder combined with Type I or
Type II Portland cement. The exact amount of bentonite needed will vary
somewhat. Table 1 shows two possible mixes for strengths of 50 psi and 4 psi.
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. Slowly add the bentonite powder so that
clumps do not form. Keep adding bentonite until the watery mix turns to an oily/
slimy consistency. Let the grout thicken for five to 10 minutes. Add more
bentonite as required until it is a smooth, thick cream, similar to pancake batter,
which is 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.
CAUTION!
If the grout is pumped into the hole, rather than tremie piped, there
is a danger that the piezometer will be overranged and damaged. Pumping
directly into the bottom of the borehole should be avoided. It is good practice to
read the piezometer while pumping.
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
.
4.3
INSTALLATION IN FILLS AND EMBANKMENTS
GEOKON
piezometers are normally supplied with direct burial cable suitable for
placement in fills such as highway embankments and dams, both in the core
and in the surrounding materials.
For installations in non-cohesive fill materials, the piezometer may be placed
directly in the fill, or, if large aggregate sizes are present, in a saturated sand
pocket in the fill. If installed in large aggregate, additional measures may be
necessary to protect the cable from damage.
In fills such as impervious dam cores, where subatmospheric pore water
pressure may need to be measured, (as opposed to the pore air pressure), a
ceramic tip with a high air entry value is often used. This type of filter should be
carefully placed in direct contact with the compacted fill material. (See Figure 5).
Cables are normally installed inside shallow trenches with the fill material
consisting of smaller size aggregate. This fill is carefully hand compacted around
the cable. Bentonite plugs are placed at regular intervals to prevent migration of
water along the cable path. In high traffic areas and in materials that exhibit
pronounced "weaving", heavy-duty armored cable should be used.
1:
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
Note:
The 28 day compressive strength of this mix is about
50 psi,similar to very stiff/hard clay. The modulus is about
10,000 psi.
The 28 day strength of this mix is
about 4 psi, similar to very soft clay.
