Armfield S12-MKII, Инструкция по эксплуатации

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Exercise G
51
The closed ring forms the upper impermeable stratum confining the aquifer (sand)
and so producing the required radial flow distribution. It will be necessary to
determine the depth of the aquifer (H) to use in equation (1) in "Water Abstraction
from a Well in a Confined Aquifer". Now if values r
1
and r
2
, s
1
and s
2
are obtained
from either the manometer pressure tappings or the water levels in the peripheral
trench and the sight tube, the value of the coefficient of permeability (k) can be found.
Q
o
must, of course, be measured from the flow leaving the well. To do this, the
outflow pipe should be diverted into a measuring cylinder to collect the outflow over a
timed period. It will be necessary to balance the inflow valves so that the well
abstraction does not lead to a falling water table while readings of s and Q
o
are being
taken. It is also important to check that a water surface is visible in the sight tube to
ensure that the aquifer remains fully saturated in the region close to the wall.
Graphs showing the level of the piezometric surface in sections along the axis of the
tank and at right angles to it can be plotted from the manometer readings. It is also
possible to prepare a plan from these graphs showing piezometric surface contour
lines.
Abstraction from a single well in a confined aquifer of rectangular plan
The apparatus is set up in exactly the same way as described in "Abstraction from a
Single Well in a Confined Aquifer with Radial Symmetry", except that the large
rectangular closed ring is used to seal off the top of the aquifer. For any shape
confined aquifer a variation of Thiem's Formula, (1) in "Water Abstraction from a Well
in a Confined Aquifer", relates the drawdown of the water surface in a well to that of
the piezometric surface above the neighbouring aquifer.
(Dupuit formula) (3)
in which s is the drawdown of the piezometric surface at a radius r from the well and
R
o
is the integration constant described below. This equation holds good only for the
area close to the well (r small) and, using a k value determined in "Abstraction from a
Single Well in a Confined Aquifer", the elevation of the piezometric surface can be
calculated using equation (3). For a confined aquifer Ro has the value where is
the diameter of the constraining cap and, in a rectangular plan aquifer of width 2L, R
o
has the value 1.27 L (Fig. G4).
Figure G4
It is suggested that values of s are computed using the measured value of Qo and
equation (3), (with the appropriate value of Ro) and a line is then drawn on a graph of