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Saturday, 21 Oct 2017

Aitik

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Aitik Tailings dam

Sweden’s largest tailings dam is at Aitik outside Gällivare in the northern part of Sweden. The tailing has a total volume of about 300 million m3. The tailings dams have a total length of about 8 km. A part of “Dam E-Fs extension” at Aitik failed in September 2000. A new dam was immediately constructed at the same place in which two fiber optic cables were installed in order to measure temperature and strain. These parameters are fundamental in dam monitoring and give information about both seepage flow and sliding at each metre along the dam. Temperature measurements have been performed twice, but strain measurements still have to be done.

 

There are two cables; one cable with multi-mode fibers for temperature measurement, and another one with single-mode fibers for strain measurements. The two cables are strapped together with tape and installed mainly parallel to the dam centre line at four different elevations. The lowest fiber is at the most downstream location as showed in the figure.

Installation of the fiber in sec 3+340 cross-section marked with “x” when going in from the cross section, and with “o” going out from the cross-section. (1) Tailings, (2) and (3) Filters, (4)Transition zone, (5) Dam toe and (6) Rock fill.
Result
The first measurement was made in June 2001 and a second one was made a half year later in November 2001. Due to seasonal variations a reversed temperature could thus be expected. The maximum or minimum temperature may however occur between the time of the measurements.

The reversed temperature profile is clearly seen in this figure, which shows the temperature in the lowest fiber, located in the filter at El 355 downstream. Similar result was obtained for El +355 upstream. At El +360 and El+365 the temperature difference was expected to be very small because the phreatic level was expected to be below the fiber. The measurements showed however that was not true all over the entire dam, because significant variation was found between chainage 3270 and 3300.

Seepage evaluations

Temperature measurements can be obtained at four elevations in the new part of the dam, providing possibilities to evaluate the seepage flow along the dam. The detection level is about 10-6 m3/(s,m) that corresponds to about half the estimated value of the hydraulic conductivity of the tailing. Regular temperature measurement should then be able to detect very small variations of seepage through the dam.

Initial estimations of the seepage flow have been done for some sections of the dam. The largest seepage flow is found in the foundation (El +355 ds) where the seepage might exceed 60E-6 m3/(s,m) over a length of about 30 m, i.e. a total flow of about 2 l/s or more (Figure 3). The seepage flow will be smaller, between 6 and 15E-6 m3/(s,m) in the remaining part of the dam (giving about 0.5 l/s in the remaining 60m). The total seepage flow in the foundation can be estimated to about 3 l/s, which is acceptably low.

The seepage through the dam at El 355us and El 360 is lower, probably less than 10-6 m3/(s,m). The seepage at El 365 should also be low.

Simulated temperature (2) with time (1) for some seepage flow Q, in the fiber at El +355 downstream, and measurements from June 11 and Nov 20, 2001 at sections 3328, 3296 and 3353.