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The best way to avoid the problem is to cast the cell inside the concrete if
possible. This can often be done when the initial concrete bonding layer is
spread over the surface of the ground. At this time a Model 4800-1-1P Earth
Pressure Cell with a pinch tube, is pressed into the bonding layer so that it rests
against the ground below. A weighted tripod can be used to hold the stress cell
in place until the concrete hardens. The pinch tube is arranged to protrude
above the bonding layer and, when the concrete has hardened, it is used to
pressurize the cell and ensure good contact between the cell and the
surrounding concrete. See Figure 15. The advantage of this method is its
simplicity and that it permits the ground below the concrete to be completely
compacted in the normal way.
15:
Model 4850-2 Concrete Stress Cell
Pinch Tube
Concrete Footing
or
Concrete Bonding Layer (mud mat)
Compacted Subgrade
FIGURE 15: Model 4800-1-1P Earth Pressure Cell Installation
2.2.6 INSTALLATION OF PUSH-IN PRESSURE CELLS TO MEASURE
LATERAL EARTH PRESSURES
The Model 4830 is designed to be pushed into soft soils using available drill
rods, usually AW. Unless the ground is very soft, it is recommended that a
borehole be drilled to within about two feet of the desired location, and then
push the cell the rest of the way. A few things to note and be aware of include:
TEMPERATURE EFFECTS
This pressure cell is relatively stiff due to the geometry and the need for a robust
construction for pushing into the ground. It is always advisable to obtain the pre-
installation zero pressure readings in the borehole at the borehole temperature.
It may take a significant amount of time for the sensor to come to thermal
equilibrium, but this is an important measurement and if it is not possible to take
this reading in the borehole, it may be possible to take the reading in a bucket of
water that is at the ground temperature.
PIEZOMETER SATURATION
The piezometer filter and sensor are saturated at the factory and sealed with
Mylar tape. Do not remove the tape until just before the sensor is installed in the
ground. The filter is saturated by drawing a vacuum on the sensor and then
allowing water to flow into the sensor when the vacuum is released. If the
sensor is to be installed and then removed for use at other sites, the saturation
process should be performed at each installation.
can supply the
GEOKON
necessary portable equipment to accomplish this.
OVERPRESSURE
When pushing the cell into the ground it is possible that pressures in excess of
the sensors full-scale range can be generated causing the sensor to experience a
zero shift or even permanent damage. To prevent this, readings should be taken
as the sensor is pushed. When the indicated pressure approaches 150% of full
scale the pushing operation should be terminated until the sensor output comes
back within its calibrated range.
12 | INSTALLATION | GEOKON
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