Foundation, Concrete and Earthquake Engineering

Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-18

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 are excavated 500mm deep, a pocket, with 45° sloping sides, of only a further 300mm depth is required to be excavated at the instrument location. The cells, (Model 3600 complete with pinch tubes), 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 with a weak concrete (19mm aggregate), in 100mm 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 100mm layers. Each layer is compacted by a vibratory trench roller. After this, standard construction filling and compaction practices can continue.

2.14 Cable Installation

Cable placement procedures vary with individual installations. In general, however, all installations have in common the requirements that;

1. The cable must be protected from damage by angular particles of the material in which the cable is embedded.

2.  The cable must be protected from damage by compaction equipment.

3. In earth and rock embankments and backfills, the cable must be protected from stretching as a result of differential compaction of the embankment.

4. In concrete structures, the cable must be protected from damage during placement and vibration of the concrete.

5. Cables may be spliced, without affecting gage readings, nevertheless splicing should be avoided wherever possible. If necessary, waterproofing of the splice can be done.

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