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2.
INSTALLATION
2.1 PRELIMINARY TESTS
It is always wise, before installation commences, to check the cells for proper
functioning. Each cell is supplied with a calibration sheet, which shows the
relationship between readout digits and pressure, as well as the initial no load
zero reading. (Figure 19 in Section 4 shows a typical calibration sheet.)The cell
electrical leads (usually the red and black leads) are connected to a readout box
(see Section 3) and the zero reading given on the calibration sheet is compared
to the current zero reading. The two readings should not differ by more than 50
digits after due regard to corrections made for different temperatures,
barometric pressures and height above sea level and actual cell position
(whether standing up or laying down).
By pressing on the cell, it should be possible to change the readout digits,
causing them to fall as the pressure is increased.
Checks of electrical continuity can also be made using an ohmmeter. Resistance
between the gauge leads should be approximately 180 ohms, ± 5%. Check the
resistance between the two thermistor wires (usually white and green). Using
Table 5 in Appendix B, convert the resistance to temperature. Compare the
result to the current ambient temperature. Resistance between any conductor
and the shield should exceed 20 megohms. Remember to add cable resistance
when checking (22 AWG stranded copper leads are approximately 14.7Ω per
1,000 feet (48.5Ω per km), multiply by two for both directions).
2.2 PRESSURE CELL INSTALLATION
2.2.1 INSIDE FILLS AND EMBANKMENTS
Earth pressure cells are normally installed with the flat surfaces horizontal to
measure vertical stresses. However, they can be placed at other orientations,
inside the fill, to measure stresses in other directions e.g., a cell placed with the
flat surfaces vertical will measure horizontal stresses in a direction perpendicular
to the plates of the cell. They are sometimes placed at angles of 45 degrees.
Experience has shown that attempts to measure earth pressures in fills
frequently meets with failure. The problem is twofold. First, the stress
distribution in the fill can be inherently variable due to varying properties of the
ground and varying degrees of compaction of the ground. Thus, the soil stress
at one location may not be typical of the surrounding locations. Secondly, a cell
installed directly in the fill could result in the creation of an anomalous zone
immediately around the cell where there may be a different, more fine-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 expected to be a
problem since the earth above might be expected to move downwards to fill the
voids and consolidate the ground. However, under the influence of rainwater
and vibration, any spaces in the soil immediately around, and especially under,
the cell may grow, causing the cell to become completely decoupled from the
soil around it. In such situations, the internal soil stresses go around the cell
instead of through it. The cell will then register only a very low pressure, which
does not change much as the loads increase. This situation occurs frequently.
6 | INSTALLATION | GEOKON
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