Environmental Engineering Reference
In-Depth Information
filled with de-aired water, pumping sufficient water to remove any bubbles in the water
tube. After allowing time to reach equilibrium, the level of water in the standpipe is the
same as that at the cell. The range of the instrument can be extended by applying small
back pressures to the standpipe.
The method is very simple and inexpensive and, according to Soil Instruments Ltd., gives
an accuracy of the order of
5 mm, depending on the back pressure being
applied.
.
Hanna (1985) indicated an accuracy of
1 mm to
2 mm. Tube lengths up to 300 m can be
used. ANCOLD (1983) gives details of such a device, which uses two cells at the same loca-
tion. These are at slightly different levels, allowing checking of whether there is air in the
lines, and rotational movement of the cell.
These devices are used in rockfill zones where it would be difficult to install cross arm
devices, but may be used in earthfill. Hanna (1985) and Soil Instruments Ltd. (1985)
describe the use of pneumatic and vibrating wire versions of the hydraulic settlement cell.
Again, in earthfill cores there may be a potential defect due to poor compaction and it
is questionable whether they should be installed.
20.4.5.2
Horizontal displacements and deformations
On some very large dams, horizontal displacements and deformations are measured using
mechanical and electronic devices embedded in the dam fill as the dam is constructed.
Their use is usually combined with hydraulic settlement gauges, so that the settled pro-
file can be determined. The mechanical instruments are installed in rockfill zones, it being
considered unwise to place them through the earth core as they may form a preferred leak-
age path.
Figure 20.24 shows a mechanical device which uses long wires, anchored to cross-arms
embedded in the dam. The wires are held at constant tension, and displacements meas-
ured at the terminal well, which is situated on the face of the dam.
Figure 20.24.
Long wire extensometer (ANCOLD, 1983).
