Environmental Engineering Reference
In-Depth Information
In fill, ring magnets are placed in the ground at predetermined intervals, in the same way
as the cross-arms are for the USBR device. In boreholes a spring loaded magnet is lowered
through the grout supporting the borehole, and is held to the walls of the hole by the spring
and the grout. The grout has a compressibility equal to, or greater than, the soil or rock sur-
rounding it. The probe is lowered down the tube and senses the position of the magnets. If
suspended by a tape, the accuracy of measurement is
1 mm. Soil Instruments Ltd. (1985)
indicate that a rod-mounted version fitted with a micrometer device to help position the
magnet can read to
0.1 mm.
The tape suspended device can be used in the same conditions as the USBR device, i.e.
in near vertical holes. The rod-mounted device can be used in any orientation, so can be
used to measure horizontal displacements, as well as vertical.
While these instruments have provided useful information on dam deformations, it is
questionable whether they should be installed in earthfill cores, because it is very difficult
to compact around the riser, and these can be a source of local collapse settlement on satu-
ration giving a potential defect in the dam from an internal erosion and piping perspective.
Figure 20.23 shows the principle of a hydraulic overflow settlement cell.
This works on a simple U tube principle. The cell is cast into the fill and connected by
a water tube to a graduated standpipe. A second drain tube allows surplus water to flow
from the cell and a third “air” tube maintains the interior of the cell at atmospheric pres-
sure. Compressed air is used to clear the air tube and drain tube, then the water tube is
Figure 20.23.
Hydraulic overflow settlement equipment (Soil Instruments Ltd., 1985).
