Table of Contents
Advertisement
14
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 a lesser degree of 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.
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.
Model 4850-2 Concrete Stress Cell
Model 4800-1-1P
Pinch Tube
Concrete Footing
or
Concrete Bonding Layer (mud mat)
Compacted Subgrade
Figure 15 - Model 4800-1-1P Earth Pressure Cell Installation
Advertisement
Table of Contents
