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free to move outwards. The hydraulic line and signal cable are tied off to one of
the rebar at intervals of one meter (~ three feet).
When the rebar cage has been lowered to its proper depth, the jack is activated,
forcing the two plates out against the trench walls.
14:
FIGURE 14: Model 4820 Jackout Pressure Cell Installation
Observation of the pressure indicated by the JOPC (see Section 3 for readout
instructions) will indicate when the cell has contacted the wall. Pump up the jack
until the JOPC reading indicates a pressure roughly 70 KPa (10 psi) greater than
the slurry pressure at JOPC depth. This ensures that the cell is bearing against
the walls of the trench, and that the concrete grout pressure will not close the
jack, which could allow the reaction plates to move away from the trench walls.
Check the JOPC reading from time to time, because the pressure might bleed
away if the walls of the trench are soft and yielding. Repressurize as needed.
Leave the jack pressurized until the grout has set up.
2.2.5 INSTALLATION OF CELLS TO MEASURE EARTH PRESSURE AT THE
BASE OF FOOTINGS, FLOOR SLABS, PAVEMENTS, ETC.
Experience has shown that attempts to measure contact earth pressures on the
base of footings, floor slabs, pavements, etc., frequently meets with failure. The
problem is twofold. First, the contact stress distribution can be inherently
variable due to varying properties of the ground and varying degrees of
compaction of the ground. Thus, the contact stress at one location may not be
typical of the surrounding locations. Secondly, a cell installed as described in
Section 2.2.1 could result in the creation of an anomalous zone immediately
around the cell where there may be a different, finer grained material, under less
compaction. The material around the cell may be poorly compacted because of
the need to avoid damage to the cell.
In an earth fill, this zone of poor compaction would not be a problem, since the
earth above would move downwards to fill the voids and consolidate the
ground. However, where there is a concrete slab immediately above the cell,
this consolidation may not take place. In fact, under the influence of rainwater
and vibration, the spaces around the cell may grow, causing the cell to become
completely decoupled from the concrete above. In such a situation, the concrete
slab bridges over the gap and the loads in the concrete go around the cell
instead of through it. The cell registers only a very low pressure, which does not
change as the loads increase.
MODEL 4800 SERIES VW EARTH PRESSURE CELLS | INSTALLATION | 11
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