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| Lesotho |
| Katse Dam |
March 1998 saw another chapter written in the life of Katse Dam
in The Kingdom of Lesotho. At that time, the reservoir was fully
impounded and water issued over the spillway for the first time.
Heavy rains over three successive wet seasons had accomplished what
was only forecast as a once in ten to fifteen year phenomenon and
the reservoir was full less than twelve months after the completion
of construction works to crest elevation. During the four year construction
period Soil Instruments Limited had been involved as a nominated
subcontractor for the supply and installation of more than 850 instruments
measuring 1400 parameters, 420 of which are remotely recorded by
11 loggers relaying data to central computers in the control building.
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The main instrumentation consists of: -
- 35 inverted and hanging pendulums
installed within the curvature of the dam to accurately monitor
its rotational movement with respect to its foundation.
- 43 Vibrating Wire Extensometers installed
from the galleries of the dam and into the foundation rock. They
are also monitoring the movement of the pre-formed joint.
- 42 Vibrating Wire Jointmeters measure
the 3-directional movements of the joints in the structure. Most
are installed within the galleries but some special units, for
use in high water pressure environments, are installed on the
upstream face of the dam to monitor the movement of the pre-formed
joint.
- 48 Vibrating Wire Strain Gauges,
installed in 3-directional rosettes together with a reference
“no stress” strain gauge, monitor changes in the stress
within the concrete.
- 64 Thermometers installed in the
concrete.
- 100 Vibrating Wire Piezometers measure
the pore pressures in the foundation basalt, particularly in the
zones of high permeability.
- 22 Vibrating Wire Pressure Transducers
are installed in the pipework that feeds water to the “pre-formed
joint”.
- A total of 27 leakage weirs have
been constructed by the Main Contractor and Soil Instruments have
automated the level (flow) recording at 7 of these, by installing
special high sensitivity pressure transducers in the stilling
areas behind the weirs.
- 24 Clinometers
are installed in the lower gallery to monitor the rotational effects
of the movement of the “pre-formed joint”. They are
installed in 10 blocks above and below the joint.
- Pairs of connected fluid-filled Levelling
Vessels, installed at 4 locations in the lower 1/3rd
of the dam body, very accurately measure the rotation of the dam
over a long baseline.
- 3 water level measuring devices are installed. One pressure/level
recorder and a Rittmeyer water level measuring device register
the upstream water level. A single pressure/level recorder registers
the downstream water level.
- An array of 7 strong motion accelerographs
is installed near the left bank, mid point and right bank at the
foundation and crest levels with a single unit in the centre of
the dam body. These are linked to a dedicated central computer
which is also used as a trigger for a more frequent monitoring
regime for the other instrumentation.
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The instruments were installed progressively during construction and
monitoring commenced as each set of installations was completed. At first
monitoring was undertaken by visiting each instrument or group of instruments
with portable logging equipment. As construction progressed and as circumstances
permitted, larger groups of instruments were connected by multicore cables
to centralised loggers which could be interrogated by laptop computer.
The final stage involved the connection of these loggers by modem direct
to the central computer, where they can be automatically downloaded on
a daily basis.
With so many parameters being recorded and a number of data entry techniques
available, data management was of prime concern.
Instrument data can be manually recorded using a portable readout/logger.
Data from the instruments is automatically recorded on one of the 11
standalone loggers. These loggers can be interrogated either by a local
notebook computer or via a modem from the main computer.
The seismic control computer receives data from the remote accelerographs
and is used to trigger a change in the recording frequency of the data
stored by the software on the main computer.
The main computer produces files of data for analysis by the engineer
and also printed records of the data. The data is also regularly copied
to a backup device.
Data is required at various intervals for different types of instrument,
under a variety of circumstances: for example after a seismic event, during
rapidly changing conditions, while impounding and under normal operating
conditions. To simplify the logger monitoring regimes, only two rates
of logging have been adopted: every 2 hours or every 24 hours. A special
software package, ’DISCODER’, was developed in-house which
discards surplus data, concurrently sorts the data from the loggers and
allocates each channel to the appropriate instrument file in Excel format.
It converts raw data readings into engineering units according to a file
of constants and coefficients relevant to each instrument.
The Consultants (Lesotho Highlands Consultants) have developed their
own software to handle the post treatment phases of the data analysis.
This regroups the data from the instruments onto composite graphs according
to type, location, etc, in a format providing easy comparison and analysis
in a report published monthly for the Client's revue.
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