In an earth fill, this zone of poor compaction would not be a problem, since the earth above would simply 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 and in fact, under the influence of rain water and vibration, the spaces around the cell may grow so that the cell becomes completely de-coupled from the concrete above. In such a situation the concrete slab bridges over the gap and the loads in the concrete simply 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, if at all possible, is to cast the cell inside the concrete. This can often be done when the initial concrete bonding layer is spread over the surface of the ground. At this time Concrete Stress 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
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| Figure 2.12 - Concrete Stress Cell Installation |
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.
The advantage of this method lies in its simplicity and in that it permits the ground below the concrete to be completely compacted in the normal way.
2.13 An alternative Method for Installing Earth Pressure Cells in Fills
The method described suffers from the drawback that it is very difficult, if not impossible, to get perfect compaction of the soil around the cells without running the risk of damaging the cells.
An alternative method used successfully in South Africa essentially uses the techniques. Installation of the cells begins when the fill has reached a height of 800mm above the instrument level. The Instrument location and the cable trenches
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