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: Visual inspections of dams and piles
Visual inspections are amongst the most essential methods for monitoring of mine dams, waste rock piles and their surroundings. A regular inspection must be carried out by an experienced operator who knows the mine and the behaviour of its dams/piles. The operator should follow a checklist containing the key features that may reveal signals of problems from the inspected area. If some changes or concerns are discovered during the inspection, the operator is able to react quickly and more precise monitoring and/or repairing work can be performed. Regular surveillances with accurately written notes also give useful information that helps in forming the overall picture of the dam/pile conditions in the long run (EC 2009). There are also special inspections to be performed in case of unexpected situations, when other measurement device gives an alarm signals, or after flooding or rainfall events that may affect the dam/pile stability (ICOLD 2014).
Visual inspection means a detection of visible indicators of problems in the dam/pile structure and in the surrounding areas, such as impounds and foundations. A present condition, changes, and anomalies are detected according to the checklist and reported. It is possible to check the functioning and maintenance of other measurement devices at the same time.
Main processes to be checked by visual inspection include (ICOLD 2014):
- Displacements and deformations
- Signs of wear and weathering
Settlements and potholes on the dam/pile crests and slopes, as well as turbidity and velocity of the seepage discharge water may reveal internal erosion, piping and loss of fines. Cracks, lateral movements, settlements and other deformations can be also identified as a warning signs of problems caused by excess seepage (EC 2009, Isomäki et al. 2012, ICOLD 2014). Surface erosion, wetted areas, changes in vegetation and melt areas during the winter may indicate the occurrence of seepage discharge points. The beach width and freeboard (a level of the dam crest above the pond level) must be also checked as their changes may indicate changes in the seepage amount (ICOLD 1996). The rock covering an embankment surface may be prone to cracking, erosion and weathering depending on its age, climate (sun, temperature, freeze-thawing etc.), and rock characteristics, which may decrease the dam/pile stability (ICOLD 2014).
Two different checklist examples are presented in Table 1a-b and Table 2. The content of the checklist and surveillance frequencies are always depending on a case and its needs. Date, time, and the name of an inspector need to be recorded on a document.
Table 1a: A dam checklist example (ICOLD 2014, modified).
|Dam type||Part of the dam||Changes|
|Embankments (earth dams or rock-fill dams)||Downstream face||Surfacing seepage water, turbidity|
|Cracks, local settlements, local slides|
|Dam crest||Cracks, local settlements|
|Condition of the road|
|Line of sight – check horizontal & vertical alignments|
|Upstream face (accessible section)||Vortex formation on the water surface|
|Cracks, local deformations, local slides|
|Bulging of surface sealing elements|
|Damages on the surface sealing element|
|Displacement of riprap|
|Leaking seepage water, turbidity|
|Condition of concrete|
|Clogging of drainage system|
|Contact between embankment and concrete structures or rock foundation||Relative displacements|
|Leaking seepage water|
Table 1b: A structure checklist example (ICOLD 2014, modified).
|Structure||Part of the structure||Changes|
|Cutoff walls (if accessible)||Surface||Cracks|
|Underground and abutments||Contact zones to the dam and dam surrounds||Surfacing seepage water, boils, turbidity|
|Cracks, local settlements, local landslides|
|Galleries (injection, drainage, inspection), shafts and adits||Leaking seepage water, turbidity|
|Condition (concrete, lining, rock)|
|Clogging of drainage system|
|Surroundings||Reservoir (surface)||Floating debris|
|Reservoir (banks and slopes)||Settlements and landslides|
|Cracks (as indication of slope instability)|
|Condition of infrastructure|
|Scour holes downstream|
|Displacements, or joint movement|
|Leakage along conduits, turbidity|
|Bottom outlet||Erosion, joint movement, cracks|
|Scour hole downstream|
|Electro-mechanical facilities||Power supply||Availability at the point of use|
|Gates and hoisting devices||Damages|
|Vandalism (protection measures)|
Table 2. A visual inspection example (EC 2009).
|General Items||Specific criteria||Defective?
|Roadways and access||Condition of roads and ramps|
|Damage and erosion of sides|
|Drain outlets||Flow efficiency|
|Outer perimeter||Evidence of spillage|
|Evidence of seepage|
|Presence of wet areas|
|Slurry behaviour||Slurry flowrate|
|Decant facility and access||Clarity of discharge fluid|
|Structural integrity of decant|
|Return water storage||Available capacity|
|Return water pumps|
|Tailings delivery system||Deposition position|
|Condition of pipes and valves|
|Monitoring||Damage to instruments|
|Read according to programme|
|Gates and fencing||General condition
Signage in place and legible
Special inspections are needed after unexpected situations (such as increased seepage after storms and floods, other alarming signals). These are usually more comprehensive than regular inspections and should be carried out by experienced engineers. These “ad hoc” inspections may include for example remote observation vehicles, diving, sonar sensing, and abseiling. (ICOLD 2014)
A closure and reclamation plan of Jericho Nunavut mine in Canada is presented as an example of performance of visual inspections.
The performance of visual inspections is a necessity for all kind of dams, piles, and their surrounding structures and areas. Also other surveillance methods are required to be able to get a supporting data, e.g. about non-visible internal changes. The list of advantages and disadvantages of visual inspections is presented in Table 3.
Table 3. Advantages and disadvantages of visual inspections of the dam/pile.
|Non-invasive||No depth penetration, also other surveillance methods needed (e.g. piezometers, optical methods)|
|Regular inspections give valuable information||Requires routine and an experienced operator who knows the dam/pile and its surrounding areas well|
A pre-determined checklist should be followed when performing visual inspections. The checklist takes individual dam/pile conditions and their specific properties into consideration. The surveillance frequency may vary in different cases and phases of the mine life cycle; the most important issues are checked more often, while less critical ones are checked only during wide inspections which are held less frequently. The inspection operator may be a member of normal, experienced follow-up staff, but a wider review and interpretation especially in the anomaly cases must be done by a qualified technical engineer (ICOLD 2014).
Although the frequency of inspections is case-dependent, their regularity is nevertheless important. After mine a closure, the general recommended inspection frequency for impoundment dams and waste rock piles is half-yearly (Mroueh et al. 2005, EC 2009). A periodic, wider inspection can be held once in every one or two years. The inspection frequency can be decreased when the dam or pile is well stabilized and no evidence of problems is seen (ICOLD 2014).
Note papers, binoculars, stopwatch, measuring tapes, hoses, buckets and temperature gauges are useful instruments for the inspection operator (ICOLD 2014). The inspection results are written on the checklist form, including also sketches, photographs, video tapes etc. if needed. Also the inspection conditions, such as weather, are reported. Video monitoring itself does not replace the on-site visual inspection and reporting, it is only a tool instrument. All results are recorded in a database which enables the collection of long-term observation series.
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
International Commission on Large Dams (ICOLD) 2014: Dam surveillance guide. ICOLD Bulletin Preprint 158.
Isomäki, E., Maijala, T., Sulkakoski, M. and Torkkel, M. (ed.) 2012: Patoturvallisuusopas. Elinkeino-, liikenne- ja ympäristökeskus, Raportteja 89/2012. 92 pp. (In Finnish)
Mroueh, U.-M., Mäkelä, E. and Vestola, E. 2005: Paras käyttökelpoinen tekniikka kaivosten rikastushiekka- ja sivukivialueiden sulkemisessa. (In Finnish)