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WEAK GROUT METHOD
One way to avoid the problem is to cast the cell inside a weak grout. A method
used successfully in South Africa, by Oosthuizen et al, essentially uses the
techniques similar to the one described in Section 2.2.5. Installation of the cells
begins when the fill has reached a height of one meter above the instrument
level. The Instrument location and the cable trenches are excavated one meter
deep, the instrument pocket, with 45° sloping sides (see Figure 10).
10:
FIGURE 10: Model 4800 Earth Pressure Cell Installation
The cells (Model 4800-1-1P, complete with pinch tubes and lugs) are positioned
on a thin layer of non-shrink, sand cement grout, and are nailed in position using
the lugs on the cells provided for this purpose. The excavated pocket is then
backfilled to a depth of 300 mm with a weak concrete in 100 mm layers,
vibrated with a poker vibrator. After 24 hours, the cells are pressurized by
pinching the pinch tubes until the pressure in the cell, displayed on a connected
readout box, starts to change.
The instrument location containing the grouted cells and the cable trench is then
backfilled in 250 mm layers, using the same material as the main fill placed by
hand and compacted with pneumatic or gasoline backfill tampers, or vibratory
trench rollers. After this, standard construction filling and compaction practices
can continue.
Earth pressure cell clusters, placed according to the methods outlined above,
may be installed either in trenches, below the temporary embankment grade, or
in ramps above the temporary embankment grade. In dams, for example, it is
usually convenient to install in trenches in the impervious rolled fill core, and in
ramps in the filter zones and compacted rockfill shell zones. In earth
embankments, it is convenient to install in trenches. By doing so, adequate
MODEL 4800 SERIES VW EARTH PRESSURE CELLS | INSTALLATION | 7
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