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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 18 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
gage leads should be approximately 180 ohms, ± 5%. Check the resistance between the two
thermistor wires (usually white and green). Using Table 6 in Appendix B, convert the resistance
to temperature. Compare the result to the current ambient temperature. (For Model 4800HT see
Table 7 in Appendix C.) 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 Installation of Model 4800 Earth Pressure Cells 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 a lesser degree of compaction. (The material
around the cell may be poorly compacted because of the need to avoid damage to the
cell.)
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