Soil drilling, test pits and sampling
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.
Geotechnical drillings, test pits, and soil sampling for laboratory analyses are used for dam inspection purposes when there is a need to get information concerning soil layering, characteristics and physical properties, as well as undrained shear strength of soft material of embankments and piles (EC 2009, Vanden Berghe et al. 2011). In some cases, physical properties are also monitored from water samples (e.g. turbidity from suspended solids) (ICOLD 2014). The chemical monitoring of water is presented in its own section.
Description of the methods
In cone penetration test (CPT), drilling is used to investigate stratigraphy and variation of resistance at deeper depth of soils, whereas with vane tests the tailings undrained shear strength can be measured (Vanden Berghe et al. 2011). Open test pits are useful when subsurface conditions at shallow depths are investigated or soil samples are taken for further laboratory analyses. These laboratory tests include e.g. density, water content and grain size distribution, which are indicators of the consistency and stability of the proved structure. Especially in the mine closure phase these methods must be carried out precautionary, because drilling and excavating disturbs the soil, may produce discharge routes of artesian water, or at worst may form initiative piping pathways. Other non-invasive methods for surveying internal properties of structures may be more preferable. (EC 2009, Vanden Berghe et al. 2011, ICOLD 2014)
Appropriate applications and performance
Soil drilling and test pits are well known, quick, and inexpensive methods. Layer structures, soil properties, and water conditions can be easily ascertained as nearly continuous profiles. On the other hand, these methods are invasive and thus the risk of disturbance is very high. When invasive methods are used, the risk of hydraulic fracturing and piping must be taken into account. If possible, drilling must be carried out without any contact to water and air (ICOLD 2014). It should be noticed that the waste deposits and embankments made of tailings are non-uniform and their microstratigraphy may vary in short distances, which can be seen as a variation of results. (Vanden Berghe et al. 2011, ICOLD 1996)
If the soil is loose and non-cohesive, it is challenging to take undisturbed samples for laboratory analyses. In these cases, representatively taken sample series are the best way to carry out the sampling (ICOLD 1996). The use of high-quality samplers is recommended to avoid disturbance in sampling (Vanden Berghe et al. 2011). It is very important that a sample has a representative portion of the substance to be analysed; the concentration and the form of the parameters must be the same as they were in the original environment. Sampling procedure should be systematic, and one should always keep in mind that the result of an analysis cannot be any better than the actual sample on which the analysis was originally performed (EC 2009). The more precise sampling theory and methods are widely presented in Pitard (1993).
European Commission (EC) 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009, European Commission. 551 p. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.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.
Pitard, F.F. 1993. Pierre Gy’s sampling theory and sampling practise. 2nd edition, CRC Press, New York. 488 p.
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.