Emma Niemeläinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Henna Punkkinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, FINLAND.
Physical monitoring: Dam and heap body movements, in situ monitoring methods
Description of the method
Dam deformation is a normal phenomenon in embankment dams and in waste rock heaps. Especially the dams made of tailings are susceptible to settle. Also external loads cause changes in dam and heap structures. (Vanden Berghe et al. 2011) However, deformation should be monitored, because excess or uneven movement can be evident of problems, such as inner erosion (ICOLD 1996). Displacements may lead to a dam/heap failure, so they must be seriously taken into account. Respectively, the deformation or foundation and abutment structures are also worth monitoring.
Deformation can be monitored on the surface with geodetic measurements. The in situ methods, such as settlement plates, inclinometers, borehole instruments, crack gauges and similar instruments, are methods, which give internal, accurately located and measured information on deformation. (Vanden Berghe et al. 2011) By using different methods at the same time it is possible to create a versatile 3D model of the change of the dam, heap or their foundation and abutments.
The different in situ method examples presented by Vanden Berghe et al. (2011) and ICOLD (2014) for measuring deformation of dams and heaps include:
Settlement gauges and plates:
- Vertical movements as settlements, heaving
- Settlements at various elevations
- Particular settlements in the foundation
Settlement gauges are installed on a known level inside the measured structure. The gauge sinks and moves freely following the material deformation. The exact location of the gauge is measured regularly, so change of the structure deformation is possible to measure as a function of time. One structure may include several gauges on each level, and the settlement of each layer should be measured separately (ICOLD 1996). Gauges can be installed also along the crest to detect the differential settlements. (Vanden Berghe et al. 2011, ICOLD 1996, ICOLD 2014.)
- Horizontal movements, slope stability, swelling
- Uneven movements of different parts of structure
- Shear strain
- Sometimes may also indicate the change of the water pressure
The inclinometer is a horizontal rod, which position inside the structure is well known and lateral profile movements are measurable. (Vanden Berghe et al. 2011, ICOLD 1996, ICOLD 2014.)
Micrometers, deformeter, dilatometers, deflectometers:
- Movements of cracks and joints
These meters are often installed around the crack or joint to measure the behaviour of these damages. These are not widely representative, but give information on small scale changes. (Vanden Berghe et al. 2011, ICOLD 1996, ICOLD 2014.)
Borehole micrometres and extensometers:
- Differential length variations and differential deflections of discontinuities
- Suitable for rock foundations
- Rod extensometers are suitable also for embankment dams, needs anchoring and protective sleeves
Extensometers and borehole micrometres are placed or grouted into soil or structure to measure the behaviour of material. They can be installed on the several directions for create larger model. (Vanden Berghe et al. 2011, ICOLD 1996, ICOLD 2014.)
- Deformations along horizontal or vertical lines (Vanden Berghe et al. 2011, ICOLD 1996, ICOLD 2014).
Appropriate method to measure the internal and external deformation; an accurate 3D model can be created. According to EC (2009), the use of inclinometers and settlement gauges is considered as a Best Available Technic (BAT) for monitoring tailings and waste rock management facilities depending on a case. These techniques are well known and widely used.
Installation of settlement plates afterwards is complicated and risky. Installation is best to be done at the construction phase, i. e. at the beginning and when the embankment has a raising phase. Tailings material may also be so weak that the reference points are impossible to set (ICOLD 1996). Without a correct calibration and a reference point the measurements are hard to interpret. The zero point may move causing a bias shift to results; respectively the depth errors are possible (Fowler [undated]).
On the other hand, the inclinometer installation is possible only when the crest has reached its highest point. At this moment, the bottom part of structure is already deformed, so it is not at initial state. (Vanden Berghe et al. 2011.)
The measuring frequency is defined in the surveillance plan which takes the mine circumstances into account. Measurement results may be collected remotely and automatically, or manually by personnel in situ. For example at Legnica-Glogow copper basin the dam inclinometers are checked once a month (EC 2009). An alarm level should be defined on the displacement rate. The alarms need to be analyzed carefully; often the inspection frequency must be temporarily increased to ensure the safety of the structure (Vanden Berghe et al. 2011). The regular, slow deformation may be normal and desired state; fast and uneven changes are evidence of concerns (ICOLD 1996).
Measuring accuracy varies from device to device. In general, measuring range of inclinometers may be 20 mm/m and precision 0,02 mm. Respectively, crack and joint gauges may have 10 mm range and ± 0,05 mm precision. Measuring range of embankment rod extensometers is 10 … 30 mm and precision ± 1 mm. Reliability and longevity are moderate or high. (ICOLD 2014.)
Deformation monitoring devices may be mounted on different parts of a dam or a heap depending on the measured structure, purposes, and expected deformation. As the devices must be often constructed inside the dam/heap, the closure phase installing is complicated and may disturb the stability of the structure. Measurements also need a stable reference point from the beginning, and long, regular surveillance period. The installation done afterwards may be useless if there are no devices installed earlier and no measuring data with which the new measurements can be compared to. (Vanden Berghe et al. 2011)
The measuring device materials must be tolerant to construction materials. Construction materials may be for example acid generating tailings, which may weaken the devices. Coatings, sleeves and sealings can be used.
European Commission (EC) 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009, European Commission. 511 p. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf
Fowler, M. [undated]. Inclinometers – the good, the bad and the future. Article on the Ninth International Symposium on Field Measurements in Geomechanics website. http://www.fmgm2015.com/__data/page/8258/M_Fowler_Dec2013_nl_article.pdf
International Commission on Large Dams (ICOLD) 1996. Monitoring of Tailing Dams. ICOLD Bulletin Preprint 104.
International Commission on Large Dams (ICOLD) 2014. Dam surveillance guide. ICOLD Bulletin Preprint 158.
Vanden Berghe, J.-F. , Ballard, J-C., Wintgens J-F. & List, B. 2011. Geotechnical Risks Related to Tailings Dam Operations. Proceedings Tailings and Mine Waste 2011, Vancouver, BC, November 6 to 9, 2011.