Earth Pressure Cell Design - Geokon 4820 Instruction Manual

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Tests conducted at the University of Ohio (Ohio, USA) with several different soil types have
shown that for Geokon cells the maximum degree of over or under registration amounts to 15%
of the mean soil stress.
Other factors should be kept in mind. The inherent variability of soil properties, which give rise
to varying soil stresses at different locations, and a corresponding difficulty in getting a good
sample of the mean stress from a limited number of cell locations. In addition, the response of
the cell to its immediate surroundings depends mostly on how closely the soil mass immediately
around the cell has the same stiffness or compressibility or the same degree of compaction as the
undisturbed soil mass. Installation methods will need to pay particular attention to this
detail.

1.2 Earth Pressure Cell Design

Earth Pressure Cells are constructed from two stainless steel plates welded together around the
periphery to leave a narrow space between them. This space is completely filled with de-aired
hydraulic oil, which is connected hydraulically to a pressure transducer. The pressure transducer
converts the oil pressure into an electrical signal, which is transmitted through a signal cable to
the readout location.
In general, Geokon Earth Pressure Cells use an all welded construction; this means the space
confining the oil is entirely metal and does not require any o-rings, which tend to trap air and
,
reduce the cell stiffness. The oil is de-aired using a Nold DeAerator which materially improves
the fluid stiffness and the performance of the cell. The pressure transducer normally employed is
the Geokon Model 4500H, which is available in several different pressure ranges (see Appendix
A.1). The cable is attached to the transducer in a sealed, waterproof manner. For earth pressure
cells located inside a soil mass, the cable may be armored and provided with strain relief at the
cell to reduce the likelihood of pullout.
Located inside the vibrating wire pressure transducer housing is a thermistor for the
measurement of temperature at the cell location. In addition, a tripolar plasma surge arrestor
inside the transducer housing protects the vibrating wire pluck and read coils from electrical
transients such as may be induced by direct or indirect lightning strikes.
Alternative pressure transducers with voltage (0-100 mV, 0-5 VDC, 0-10 VDC) or current (4-20
mA) output are also available for dynamic readout capability. Consult the factory for additional
information.