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by erosion: under a few decimetres of red soils, the alterites are thick with
less than five metres. A high density of horizontal fractures is observed in the
fissured zone (Maréchal et al., 2005). Vertical fractures with a tectonic origin
are also present. Due to the overexploitation of groundwater resources, water
levels are far below ground level and the alterites are dry while only the
fissured zone is saturated. This specificity was used to realise pumping tests
with the objective to test the four interpretation methods mentioned above
and thus to characterise the hydrodynamic properties of the fissured layer
only.
ANISOTROPY OF PERMEABILITY USING NEUMAN
METHOD AT OBSERVATION WELLS
On a bi-logarithmic plot (Fig. 1), the drawdown curves at observation wells
IFP-1/1 and IFP-1/2 during pumping tests at IFP-1 well have a complex
shape, difficult to interpret with classical methods (Theis, for example).
Drawdown curves are composed by three parts: the first one, at short times,
with strong slopes, is followed by an intermediate period during which water
level stabilisation occurs, and a third part for long times shows a new increase
in slopes.
Figure 1. Adjustment of drawdown in observation wells IFP-1/1 and IFP-1/2
using Neuman theoretical curves of types A and B.
The theory initially developed by Boulton (1970), to interpret some special
curves obtained in observation wells, takes into account the notion of
“delayed yield from storage in unconfined aquifers” (Boulton and Pontin,
1971). It was improved by Neuman (1972) who developed an analytical
solution adapted to anisotropic unconfined aquifers, where K r is the radial
permeability parallel to the aquifer extension and K z is the vertical permeability.
Neuman method considers an unconfined and infinite aquifer. When a constant
discharge rate is pumped in a complete well, the water comes for one part
from the storage in the aquifer and for the other part from gravitational
drainage at the free surface. The Neuman solution, under abacus, gives reduced
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