Measurements
are normally carried out using the Wenner method and the data is used to arrive
at a representative soil model for the site.
Whilst the
measurements would best be carried out in representative weather conditions,
this is clearly not always possible, so allowance for seasonal effects may need
to be made in the model.
This would
normally be done by modifying the resistivity and/or depth of the surface
layer. Some typical soil resistivity values are shown in Table 8.2.
Measurements
are taken for a range of probe separations, each of which is a general
indicator of the depth to which the value applies. Measurements in a number of
directions would be taken and averaged values (excluding obvious errors) for
each separation distance would be used to derive the initial soil model.
A number of
computer programmes are commercially available and used to translate the data
into a representative soil model. It is useful to have both the average model
and the data spread, so that the error band is known, as this will influence
the subsequent calculations or suggest that the derived soil model be modified
to improve its accuracy.
It is
possible to use formulae or graphical methods to derive a two layer model. The
formula below compares the resistivity, p1 of the upper layer of depth h1 with
the lower layer of resistivity, p2:
The value ps
is the resistivity measured at a depth a. IEEE 80 includes a number of graphs
to achieve the same result, based on the work of Sunde.
It is
unusual to use formulae now, because the interactive computer programmes available
can quickly provide a model which may have a number of vertical or horizontal
interfaces. Often a three layer model is necessary to provide sufficient
accuracy.
The soil
model values are used in formula or a computer programme to calculate the earth
resistance and hazard voltages.
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