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: Hydrology and seepage monitoring, tracers
Description of the method
In hydrology and seepage monitoring the behaviour of water in the deposits, retaining embankment structures and their foundations are examined. Hydrological management during the mine closure takes all the water in the air, soil and ground into account. The mine water balance changes every time when waters precipitate, infiltrate, evaporate, flow and discharge within the mine site. Also construction, deposition of materials, raising the embankments and piles, erosion and demolition affect to the water movement, phreatic surface and pore pressure inside the material.
Seepage is a flowing phenomenon that takes place in embankments and piles; the water is unevenly distributed inside the structure either horizontally and/or vertically and strives to balance. The pore pressure and the direction of the seepage flow depend on the differences between hydraulic pressures and anomalous densities of the material (EC 2009, ICOLD 2014). Seepage is a normal phenomenon in the dams constructed of soil materials and stabile seepage flow assures the dam stability by lowering the pore pressure (EC 2009). In dams made of concrete, seepage finds its paths through the less watertight parts of the dam, such as joints and concrete/rock boundaries (ICOLD 2014). An excess pore pressure causes liquefaction, while a low pore pressure stabilizes the structure (ICOLD 1996). The role of drainage structures, outlets and collection ditches is also important in the general hydrology of the deposit (ICOLD 2014).
Water conditions and seepage management require deep understanding of the hydrology of the site. The phreatic surface, the seepage flow rate and the amount of seepage may vary and depend e. g. on the permeability of the structural layers of the dam, underlying structures and boundaries, possible drainage or seepage prevention systems, water level in the pond, groundwater table, and meteorology (EC 2009, ICOLD 2014). A proper drainage system keeps the phreatic surface away from the downstream slope, which reduces the pore pressure and a risk of liquefaction (ICOLD 1996). In case of excess or instable seepage, piping, internal erosion and/or loss of fines may increase and the stability of the structure may deteriorate. Also settlements, collapses and sliding failures may develop. Tailings and tailing dams are exceptionally sensitive to internal (and external) erosion (ICOLD 1996). The hydrological balance and stability of tailings embankment or waste rock pile changes slowly every time the structure is raised, so hydrological monitoring is vital during the whole operational and after care phases.
Association of State Dam Safety Officials presents two videos concerning the failure caused by an uncontrolled seepage:
Failure Modes: Slide Failure (Windows Media Player)
Failure Modes: Piping (Windows Media Player)
Hydrology and seepage monitoring is essential in every water retaining dam and structure. According to several Finnish laws; VnA 190/2013 (Regulation of extractive wastes), VnA 319/2010 (Regulation Concerning Dam Safety) and L 494/2009 (Dam Safety Law), the health, performance and geotechnical stability of extraction waste dams must be monitored by a qualified expert using a specified monitoring program. The regularity and procedures of monitoring depend on a dam classification, which is presented in the Dam Safety Law (L 494/2009). Finnish Dam Safety guide (Isomäki et al. 2012) describes also methods which are required and/or suitable for monitoring of dams.
According to European Commission (EC 2009), ICOLD (2014), ICOLD (1996) and Johansson (1997) the monitoring properties which provide hydrological information from the dam or pile include:
- Water level in the dam pond
- Quality and quantity of seepage flow through the dam/pile itself, the foundation, the abutments and their boundaries (also tracers can be used)
- Position of the phreatic surface
- Positions of piping routes
- Dynamic pore pressure and liquefaction
- Movements and deformations of the dam/pile crest and slopes, internal movements
- Seismicity, to ensure stability of the dam and the supporting strata
- Soil mechanical properties and tailings placement procedures, as shear strength, compressibility, consolidation, grain size and density, width of the non-submerged beach
- Electrical resistivity and self-potential
- Turbidity and content of the fine graded soil of the seepage water samples
Some of these measurements are presented on their own sections, e.g. temperature measurement using optical fibres and geophysical measurements.
Instead of measuring the seepage on the absolute accurate level it is more important to observe relative changes to be able to detect the development of instability in the dam (ICOLD 2014, Johansson 1997). Monitoring and interpretation of results is important especially after storm and flood events which bring lot of water into the dam system and seepage equilibrium may change (VnA 319/2010, Isomäki et al. 2012). An increased flow with suspended particles may indicate the increased seepage and piping, while decreased flow may express drainage or clogging of a filter system (EC 2009). Piping of the dam is generally a localised, anomalous problem, which can arise and expand quickly and erode the dam’s inner structure. The monitoring of the occurrence of piping should be made versatilely and regularly using a dense monitoring net to be able to detect seepage and erosion changes as early as possible.
The methods used for monitoring an amount, level and quality of water are (e.g. EC 2009, ICOLD 1996, ICOLD 2014):
- Measure of amount and flowing speed of leaking water, sampling (water level outside the dam, bottom ditches, underdrains, collection pipes, flumes and v-dams i.e. “Thompsons dams”)
- Standpipes, groundwater observation wells, piezometers for monitoring the phreatic water table and pore pressure
- Tracer tests
- Inspections of drainage systems (outlets, pipes etc.)
Leakage monitoring can be done with the help of underdrains, collection pipes, observation wells, flumes, v-dams etc. Even leakage quantity and quality indicate the stability of the seepage; chances may lead to instability. Leakage locations are also monitored; weirs can be installed in the places where seepage path or drainage system discharges so that the changes of the flow will be recorded. (EC 2009) Also water samples can be taken for chemical analyses, turbidity/colour and temperature measurements (ICOLD 1996, FEMA 2005).
Piezometer measures a pore pressure of the water and its fluctuations. They are installed either in boreholes or preferably straight into the upstream or downstream surfaces (FEMA 2005). Piezometer types vary widely, as well as their measuring ranges: closed type piezometers have a limited vertical horizon, while in open-well type piezometers measuring range reaches nearly whole depth of the borehole (FEMA 2005).
Tracer test using e. g. colour dyes, gamma/neutron and water temperature measurements provide information about seepage paths (FEMA 2005). The tracer is injected into boreholes or released at the specific locations in the reservoir. The possible seepage locations can be observed by detecting a tracer pathway in the downstream of a dam.
The monitoring of seepage water properties on chemical point of view is not primarily included in this section. However, the indicators of seepage in water samples may be e. g. colour, turbidity, and other water chemistry “tracers”, which reveal possibly internal erosion, grain movement and unstable seepage in the dam (EC 2009).
Advantages and disadvantages of these methods are presented in Table 1.
Table 1. Advantages and disadvantages of groundwater and leakage monitoring methods (EC 2009, ICOLD 1996, ICOLD 2014, Johansson 1997, FEMA 2005).
|Groundwater wells, standpipes
||Phreatic surface, water samples
||Accurate determination of water level
||Invasive, disturbance and piping risks, information only from narrow area around the well (weak area coverage for piping detection)
|Underdrains, collection pipes
||Leaking amount, water samples
||Accurate determination of amount of the leaking water.
||Weak area coverage may not reveal piping route locations by definition.
|Flumes, weirs, v-dams
||Leaking amount, flowing speed, water samples
||Accurate determination of volume and flowing speed of the leaking water.
||Weak area coverage may not reveal piping route locations by definition.
|Piezometers, pore water pressure gauges
||Water level and pore pressure, their fluctuations; open or closed systems
||Accurate determination of water level and pore pressure, automatic monitors are possible depending on device type.
||Measurement range varies by device type, usually narrow. Invasive, risk of disturbance. Piping is a risk especially if device is mounted in a borehole.
||Piping routes, leaking amounts
||Accurate determination of piping routes.
Performance and design requirements
Measurements should be performed regularly. As discussed earlier, frequently collected data offer a valuable information e.g. when estimating anomalies. The results should always be compared to previous data. After extreme events, such as floods and storms, extra inspections and surveillance are needed. After the active closure phase, when the monitored structure is stabilized, the monitoring frequency can be reduced. (EC 2009, ICOLD 2014.)
Seepage measurement actions are usually done in the downstream slope and toe of a dam or pile and also in the downstream area outside the structure (ICOLD 2014). Changes in amount, quality and/or flowing speed, existence of wetted areas, piping and erosion, are signals of excess seepage. Automatic measurements, recordings and alarms are possible. Their reliability and longevity is high and measurements are repeatable. Flumes and weirs are not recommended for discharge less than 0.05 l/s. When using a calibrated container and stopwatch for measuring the leakage volume, their reliability is moderate. The method is limited to discharge up to 10 l/s. (ICOLD 2014) Pipes and wells may be prone to clogging, so regular inspections and maintenance are needed.
Piezometers used for soils may be either open or closed systems. The first one measures the water level with light or acoustic signal and the second one indicates the pressure with manometer or electrical gauge. Choosing the piezometer type depends on dam/pile type, conditions and surrounding materials. Closed system is more reliable, although the longevity of both systems is high. The most accurate piezometers can detect extremely small water volume changes, less than 0.001 cm3. The phreatic surface often reacts with a delay when hydrological conditions change, and thus the hydraulic conductivity of the surrounding material should be taken into account when choosing the size of the piezometer (ICOLD 1996). Results may be also over or underestimated, depending on the material properties, and thus the data interpretation must be performed by experienced personnel. Piezometers should be installed into groups for obtaining better level of redundancy. They must be watertight, corrosion resistant and regularly inspected for clogging. Piezometers may bend or sink along the deformation of the deposit. Piezometers can be used also in rocks and foundations; the system used should be closed or pressure cells installed into borehole. Automatic measuring and remote reading is possible, depending on a device. (ICOLD 1996, ICOLD 2014.)
Association of State Dam Safety Officials [no date]. Failure Modes: Piping. Video presentation: http://www.damsafety.org/news/?p=412f29c8-3fd8-4529-b5c9-8d47364c1f3e
Association of State Dam Safety Officials [no date]. Failure Modes: Slide Failure. Video presentation: http://www.damsafety.org/news/?p=412f29c8-3fd8-4529-b5c9-8d47364c1f3e
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 pp. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf
Federal Emergency Management Agency (FEMA) 2005. The National Dam Safety Program – Research Needs Workshop: Seepage through Embankment Dams. FEMA 535. 261 pp. http://www.fema.gov/media-library-data/20130726-1446-20490-1996/seepage.pdf
International Commission on Large Dams (ICOLD) 2014. Dam surveillance guide. ICOLD Bulletin Preprint 158.
International Commission on Large Dams (ICOLD) 1996. Monitoring of Tailing Dams. ICOLD Bulletin Preprint 104.
Isomäki, E., Maijala, T., Sulkakoski, M. & Torkkel, M. (eds.) 2012. Dam safety guide. Centre for Economic Development, Transport and the Environment, Reports 89/2012. 90 pp.
Johansson, S. 1997. Seepage Monitoring in Embankment Dams. Doctoral Thesis, Division of Hydraulic Engineering, Department of Civil and Environmental Engineering, Royal Institute of Technology, Stockholm, Sweden. 62 p.
L 494/2009. Patoturvallisuuslaki (Dam safety law). (In Finnish)
VnA 190/2013. Valtioneuvoston asetus kaivannaisjätteistä (Regulation of extractive wastes). (In Finnish)
VnA 319/2010. Valtioneuvoston asetus patoturvallisuudesta (Regulation Concerning Dam Safety). (In Finnish)