Armfield Instruction Manual
38
Another perforated metal sheet is placed at the upstream overflow pipe as in
Exercise C - Draining Effect of a Tile Line
. Now the space between the two
perforated sheets is filled with sand up to about 50mm below the top of the tank.
Upstream overflow is adjusted to about 25mm below the sand surface while the
downstream overflow is adjusted to about 12mm above the tank bottom.
The upper pool is then filled with water. A small continuing input of water is
established which will be drained away by the upstream overflow. At this stage no
water should be coming into the lower pool and water levels in all standpipes
connected to the wall segment should stabilise at the elevation of water in the upper
pool. This indicates that the wall is under full hydrostatic pressure.
In the second step remove the sealing strip from underneath the wall segment and let
the water flow into the lower pool. This is equivalent to draining off the toe of a
retaining wall. Next insert dye at several points in the upstream perforated metal
sheet and let the flow lines form. At the same time watch the drop of water levels in
the standpipes. Levels should now be different in each. The pattern of the flow lines
explains why the pressure on a wall is reduced when there is a drain at its bottom.
The profile of water pressure from top to bottom of the wall is indicated by the height
of the columns of water in the standpipes.
