Armfield S12-MKII, Instruction Manual

Page: 61 / 67

Exercise I
57
In this experiment, the small square open ended ring is used to form the sides of the
excavation by sinking it in the sand between the two well positions and removing the
sand inside down to the lower level of the ring wall. If the sand in the catchment tank
is now saturated by admitting water via the inlet control valves, the "excavation site"
will fill with water. Now lower the water table by opening the well drain control valves
until the excavation dries out. Plot a profile along the centreline of the tank showing
the position of the water table (from the manometer readings) in relation to the wells
and excavation cross-section.
Normally, of course, more than two wells would be used and so in this case difficulty
may be experienced in getting the bottom of the excavation site completely dry.
Draining a polder or lake
This situation differs from the excavation dewatering problem in that the drainage
takes place from the floor of the polder. This means that ground water flows into the
polder, is collected in a ring ditch near the wall and pumped out from one or more
points. In this experiment the polder bank is represented by the large rectangular
open ended ring which is positioned to enclose the two wells. The sand is removed
from inside as before and a circular ditch formed in the bottom to link both of the
wells (Figure I1).
The sand is now flooded and the well control valves are opened until the polder is
drained and the inlet valves adjusted to keep the water table elsewhere at the sand
surface. It is possible to carry out this experiment without using the square ring, by
forming a natural polder bank with the sand at a stable slope (Figure I2). The position
of the water table should be determined from the manometer tubes and profiles
plotted to show this in relation to the ground surface and well positions.
Figure I1
Figure I2