Katse dam, part of the first phase of the Lesotho Highlands water project, was completed earlier this year. Chris Spalton* gives more details about the extensive array of instrumentation and monitoring equipment installed at the dam
In March 1998 another chapter was written in the life of Katse dam — the 145m-high structure which forms part of the Lesotho Highlands water project in the Kingdom of Lesotho. On 10 March 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 a 10-15-year phenomenon, and the reservoir was full to crest elevation less than 12 months after completion of construction works.
During the four-year construction period soil-instruments, a UK company, 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.
The main instrumentation at the dam is deployed as follows:
•35 inverted and hanging pendulums installed within the curvature of the dam accurately monitor its rotational movement with respect to its foundation.
•43 vibrating wire extensometers are installed from the galleries of the dam and into the foundation rock. They are also monitoring the movement of the pre-formed joint.
•442 vibrating wire jointmeters measure the three-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 three directional rosettes together with a reference ‘no stress’ strain gauge, monitor changes in the stress within concrete.
•64 thermometers are 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 has automated the level (flow) recording at seven of these by installing special high-sensitivity pressure transducers in the stilling areas behind the weirs.
•24 clinometers were installed in the lower gallery and monitor the rotational effects of the movement of the pre-formed joint. They are installed in ten blocks above and below the joint.
•Pairs of connected fluid-filled levelling vessels installed at four locations in the lower third of the dam body, sometimes described as ‘horizontal pendulums’, accurately measure the rotation of the dam over a long baseline.
•Three water level measuring devices were 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 seven strong-motion accelerographs have been installed. They are near the left bank, mid point and right bank at the foundation and crest levels of the dam, 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 other instrumentation.
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 connecting these loggers by modem directly 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 a prime concern.
Instrument data can be manually recorded using a portable readout/logger. Data from the instruments are automatically recorded on one of the 11 stand-alone 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. Files of data are produced by the main computer for analysis by the engineer.
Data are required at various intervals for different types of instrument under a variety of circumstances, namely 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 two hours or every 24 hours. A special software package called Discorder 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 a Windows Excel software format. It converts raw data readings into engineering units according to a file of constants and coefficients relevant to each instrument.
The project consultant (Lesotho Highlands Consultants) has developed its own software to handle the post treatment phases of the data analysis. Instruments are automatically regrouped onto composite graphs, according to type, location, etc, in a format which allows for easy comparison and analysis in a monthly report published for the client’s review.
|Lesotho Highlands water project|
| In 1986 the Republic of South Africa and the Kingdom of Lesotho signed a treaty which paved the way for the £2B Lesotho Highlands water project. This scheme, which has been described as one of the world’s largest civil engineering projects currently in progress, aims to :
•Redirect 70m3/s of water from Lesotho to South Africa.
•Generate 72MW of hydroelectricity in Lesotho, making the country self-sufficient in power.
•Provide social and economic development in Lesotho through improved water supplies and irrigation.
In the Treaty, South Africa agreed to pay for the cost of transferring water and Lesotho agreed to pay for the power generation part of the scheme.
The water project comprises several phases (1A, 1B, II, III and IV). Katse dam forms part of phase 1A which has been completed and was inaugurated on 22 January 1998.
Katse dam, at 185m high, is Africa’s highest dam. It impounds a reservoir of 1950M m3 of water and will supply 18m3/s of water to South Africa through its 37km-long delivery tunnel. This is expected to meet the country’s water demand until 2004, when the next phase of the project should be well under way.
Phase 1B comprises the Mohale dam on the Senqunyane river, an interconnecting tunnel to the Katse reservoir, a weir on the Matsoku river to the east of Katse and an interconnecting tunnel to the Katse reservoir. Mohale dam will be 145m high, impounding 90M m3 of water. Construction is due to start later in 1998 and is scheduled to last for 48 months. Later phases are detailed in the diagram above right.