CA3062528A1

CA3062528A1 - Method of excavating a tailings lagoon

Info

Publication number: CA3062528A1
Application number: CA3062528A
Authority: CA
Inventors: Richard Stanley Goodwin; Andrew Chapman; Mithila Tharanath Wanasinghe; Garry Smallwood; Stephen O'nien
Original assignee: Goodwin PLC
Current assignee: Goodwin PLC
Priority date: 2019-02-27
Filing date: 2019-11-22
Publication date: 2020-08-27
Also published as: GB2582041A; GB201914520D0; BR102019024720A2; ZA201907670B; US20200270845A1; BR102019024720B1; GB2582041B

Abstract

A method of dewatering a tailings lagoon retained by a dam comprising: excavating an excavation hole in the tailings lagoon; allowing water from surrounding tailings to enter the excavation hole; and pumping water in the excavation hole out of the excavation hole and discharging beyond a toe of the dam. Also disclosed is a method comprising: excavating a channel in a tailings lagoon from a shore of the tailings lagoon and floating a pontoon in water in the channel from the shore along the channel, wherein: excavating involves breaking down solid tailings in the tailings lagoon into a slurry using water and removing the slurry using a submersible slurry pump mounted on the pontoon.

Description

Method of Excavating a Tailings Lagoon In mining operations there is a need to store tailings, products resulting from the ore extraction process. Typically these are retained in tailings pits or tailings lagoons behind a tailings dam. Typically the tailings dam can begin as a low level dam and be built up to a greater height periodically as mining progresses to accommodate tailings as these are produced. Solid tailings are often used as part of the dam structure.
Over the last 50 years, over 1800 people have been killed as a result of tailings dam "ruptures" in different places around the world, the last two events having been in Minas Gerais Brazil.
All these tailings dam "ruptures" were the direct result of hydraulic pressure being exerted on the manmade dam which more often than not is built out of local waste mineral materials that are compacted to form the dam at the lower part of the dam/
valley.
More often than not, it is the fluidisation of the slurry behind the dam that results in the full hydraulic pressure to the depth of the tailings slurry acting on the dam especially in flat face rather than curved face dams.
Some of the "ruptures" are caused by flood rain taking the water level in the dam to above the design limit but the mine operators have not developed adequate mechanisms of removing excess rain water from the lower part of the dam. This excess of rain water that sits against the inner edge of the dam again has the effect of fluidising the settled tailings slurry and also fluidising the compacted dam itself.
Many tailings dam designs (except dams constructed from concrete) often have horizontal water drain pipes built into the bottom of the dam walls which whilst initial filling of the tailings dam with tailings these pipes are kept closed, but as the tailings slurry increases in height in the dam, these pipes are used to vent water that has seeped to the bottom of the tailings dam wall to the downstream side of the dam.
This removal of water assists in ensuring the slurry behind the dam for up to meters upstream of the dam dries out and does not become fluidised.

The most frequent problem is that these water drainage pipes at the bottom of the dam wall that vent water from the upstream side of the dam to the other side of the dam downstream become blocked or fractured and no longer are able to take the water away. This happens especially when the dam is not maintained or when production has stopped for one reason or another at the metal mine that produces the by-product tailings. It can also be a problem that the sheer quantity of rain ends up raising the water level in the dam and it overspills and fluidises the toe of the dam.
The end result of blocked dam water drainage pipes is fluidisation of the settled slurry behind the dam and even the fluidisation of the dam itself of which either or both can easily result in the rupture of the dam and catastrophic dam failure.
The present application relates to a mechanism and process that will mitigate against fluidisation of tailings material close to the dam wall on tailings pits that have been inactive for a period of time or that are active but are considered at risk of "Rupture".
The issues with tailings pits that are considered at risk of "Rupture" is that the risk of them being inherently unsafe, nobody wants to venture out onto the dam or into the tailings lagoon and even if they did, they are uncertain how to remove water and or de-fluidise the tailings especially towards the centre of the dam.
The present invention relates to a method of dewatering a tailings lagoon retained by a dam comprising: excavating an excavation hole in the tailings lagoon;
allowing water from surrounding tailings to enter the excavation hole; and pumping water in the excavation hole out of the excavation hole and discharging beyond a toe of the dam. Thus the tailings surrounding the excavation holes are dewatered and the possibility of hydrostatic pressure acting on the dam is reduced.
In an embodiment a pontoon floats in water in the excavation hole and supports equipment used for the step of pumping water.
In an embodiment before excavating the excavation hole, excavating a channel in the tailings lagoon from a shore of the tailings lagoon to a location at which the excavation hole is to be excavated and floating the pontoon from the shore to the excavation hole along the channel. This is a safe way to get the equipment necessary

2 for excavating the excavation hole and for providing water to the pontoon down the excavated channel for density control of the slurry generated from the tailings being excavated for the area near the dam.
In an embodiment excavating involves breaking down solid (i.e settled) tailings in the tailings lagoon into a slurry using water and removing the slurry using a slurry pump mounted on the pontoon. In an embodiment the excavating involves breaking down solid tailings in the tailings lagoon into a slurry using water and removing the slurry using at least two submersible slurry pumps ( or at least one submersible slurry pump and at least one water pump ( submersible, horizontal or other type )) mounted on the pontoon. This way of excavating is efficient and avoids needing to dig out tailings, which can be more dangerous and time consuming, particularly if the water content of the tailings is high.
In an embodiment during the excavating the water for breaking down the solid tailings is supplied by the pontoon preferably from a shore supply or from excess surface water on the tailings dam. This speeds up breaking down of tailings and therefore excavation.
In an embodiment the water supplied by the pontoon is at least partly water extracted from the channel, which may be supplied from a shore supply or that the channel has collected from surface water, on the pontoon and/or excavation hole by the pontoon.
In an embodiment during the excavating water is provided to the pontoon from a shore of the tailings lagoon. This speeds up excavation if water content in the tailings at the site of excavation is low.
In an embodiment water is supplied to the channel and/or excavation hole during excavation from beyond the tailings lagoon. This speeds up excavation if water content in the tailings at the site of excavation is low.
In an embodiment the amount of water contained in the slurry is controlled to be a certain minimum amount. This ensures that any pipes through which the slurry is transported from the pontoon to a discharge location can be prevented from being blocked with slurry which has too low a water content.

3 In an embodiment the pontoon is controlled remotely from a shore of the tailings lagoon and can be run unmanned for significant periods. This enhances safety.
In an embodiment dewatering is localised to tailings adjacent the dam. This is efficient as it is the tailings closest to the dam wall which must be prevented from fluidizing. If the tailings closest to the dam wall have low water content, the presence of such tailings helps hold back tailings further up the lagoon away from the dam with a higher water content.
In an embodiment the present invention provides a method comprising excavating a channel in a tailings lagoon from a shore of the tailings lagoon and floating a pontoon in water in the channel from the shore along the channel, wherein excavating involves breaking down solid tailings in the tailings lagoon into a slurry using water and removing the slurry using a slurry pump mounted on the pontoon. This method allows solidified tailings from a tailings lagoon to be broken down into a slurry for easy transport of the broken down tailings. The tailings may then be moved to a different position and/or reprocessed to extract certain materials contained in the tailings which were not previously removed. For example, new techniques may allow greater extraction of a material such as a precious metal contained in the tailings than was possible when the tailings was originally produced. Alternatively, new techniques or changes in the value of materials may mean that the tailings may be reprocessed to remove materials not previously removed from the tailings.
All of the techniques described herein relating to excavating the channel in a dewatering method apply equally to the above described method of excavating a channel.
The present invention will be described by way of example only with reference to the following drawings in which:
Figure 1 is a schematic illustration of a pontoon used in the present invention.
Figure 2 is a schematic diagram of the delivery of an un-assembled pontoon.

4 Figure 3 is a schematic cross-sectional diagram of digging a trench adjacent the tailings lagoon shore.
Figure 4 is a schematic cross-sectional diagram illustrating moving of the pontoon into position.
Figure 5 is a schematic cross-sectional diagram illustrating the pontoon loaded into the trench.
Figure 6 is a schematic cross-sectional diagram illustrating initiating excavating the channel.
Figure 7 is a schematic cross-sectional diagram illustrating excavating the channel.
Figure 8 is a schematic plan diagram of the tailings lagoon illustrating the positions of the channels.
Figure 9 is a schematic plan diagram of the tailings lagoon illustrating the positions of the channels and excavation holes.
Figure 10 is a schematic plan diagram of the tailings lagoon illustrating the de-watering of the tailings adjacent the excavation holes.
Figure 11 is a schematic cross-sectional diagram illustrating de-watering of the tailings adjacent the dam wall and the disposal of water from the excavation holes.
Below is a description of an embodiment to make dams which retain a tailings lagoon at "risk" more inherently safe by ensuring the tailings behind the dam, say 25 or more meters down and up to the surface and up to 100 meters from the dam wall is not subjected to high water content and thus the material in the dam above this level will not become fluidised and as such will stop exerting hydraulic force on the dam wall and will actually assist the dam wall hold back tailings material higher up the tailings lagoon. By de-fluidising the tailings from the surface to 25 or more meters down and from the dam to 100 or more meters upstream of the dam, it will also ensure that the dam itself does not become fluidised. The figures 25m down and up to the surface and up to 100m from the dam wall are examples and the method can be used to de-water a to a shallower level or to a deeper level and closer or further from the dam wall.
If dewatering to a greater depth using a submersible pump, a pressure compensator reservoir may need to be fitted to the pump motor housing.
To carry out this dewatering operation, two submersible slurry pump pontoons can be used with one submersible slurry pump and one water submersible pump on each pontoon. These modular section quick build pontoons can be constructed on the edge of the lagoon 100 meters upstream of the dam, one on a first (e.g the left hand looking towards the dam from below the dam wall) side of the lagoon and one on the other (e.g. right hand) side of the lagoon. These pontoons would make a channel filled with water through the tailings for themselves to float along by pumping water jetting and pumping slurry towards the upper end of the lagoon (e.g. 750meters up the lagoon) such that the pontoons end up on either side of the centre line of the dam (e.g. 75meters either side) and a distance (e.g. 100 meters) from the dam.
These pontoons would be operational 95 % or more of the time from the shore and would be controlled from a shore based control cabin linked to the pontoons with e.g.
cables or wirelessly, for example by radio signals or industrial WiFi, thus minimising the risk of operators being out in the tailings lagoon when there is a risk of dam rupture.
Each pontoon having used it's submersible slurry pump with a water submersible or other type of water pump and density control system to ensure the concentration of solids did not go above a predetermined level (say above 40 % by volume) would be used to create in their location an excavated pool (for example 30 to 50 meters in diameter) that was tapered as it went down (e.g. to a depth of up 25 meters, or if needed 50 meters with a pressure compensating reservoir fitted to the submersible pump.
This excavation hole (perhaps conical in shape) would naturally find water draining into it from the tailings. The water would convert the settled tailings from anything between 70 % solids to 40 % solids into slurry. The slurry can then be pumped out and up the lagoon using the submersible slurry pump. If there was not enough water available, which might happen if the creation of the conical hole was done in the dry season, water would be supplied additionally down the channel the pontoon has created from the shore.
Once the excavation of the two excavation holes has been completed, then the pontoon and submersible pumps would be used to pump water that flowed into the excavation holes along a pipe to the downstream side of the dam. This can be achieved with flexible pipes that are led over the top of the dam. If this process is kept running 24 hours per day then dewatering of the dam in the most critical areas would be very much improved especially if the water drain pipes from the bottom of the dam were blocked.
Also when there is rain water coming down the lagoon towards the dam it would naturally migrate to the two excavation holes via the channels excavated from the shore such that it could be pumped away by the submersible pumps. This process is designed to keep the water table in the tailings lagoon near the dam wall between 10 and 20 or more meters below where it would naturally be and thus prevent fluidisation of tailings material along the dam wall and near the centre of the dam which is its weakest point and also assist in preventing the fluidisation of the dam itself as occurred at Bento Rodrigues/Samarco.
Exactly the same techniques can be used for the purpose of reprocessing or repositioning solid (i.e. settled) tailings. That is the excavation techniques described can be used to break down solid (i.e. settled) tailings in a tailings lagoon into a slurry and remove the slurry using a slurry pump mounted on the pontoon. That slurry may then be reprocessed or repositioned. This technique can be used whilst carving a channel in a tailings lagoon or at any desired location within the tailings lagoon where an excavation hole is excavated.
The process and apparatus will now be described with reference to the figures.

Figure 1 shows a pontoon 10 which in an assembled state. The pontoon 10 may be modular so that components can be more easily transported to site. As illustrated in figures 2 - 8, one or more transport devises such as trucks can deliver components to the site where they are assembled into the pontoon 10.
The pontoon 10 comprises at least one float 20. In the example of figure 1, the pontoon 10 comprises two floats 20. These are delivered by truck 100 and placed next to each other (figure 2). Struts are used to connect the two floats 20 together and at least part of a gap between the floats 20 is covered with centre panels 25.
Various cabins may be secured to the pontoon 10, including, for example, a control cabin 31, a crew cabin 32 and/or a toilet 33. A framework 40 is assembled and secured to the pontoon 10. Pipework, flow meters are also installed onto the pontoon 10.
A submersible slurry pump 50, such as that available from Goodwin, is installed onto the framework 40. In an embodiment a submersible pump 60 is installed onto the framework 40. The submersible pump 60 may be a slurry pump or a water pump mounted in some manner to the pontoon 10. In an embodiment the two pumps 50,60 are attached at opposite ends of the pontoon 10. The submersible pumps 50,60 can be raised and lowered from the framework 40, so that the distance of extraction by the pumps 50,60 from the pontoon 10 can be varied. In another embodiment the submersible pumps 50 and or 60 are mounted in fixed position relative to the deck of the pontoon 10 and so are fixed in the horizontal position.
In the method, a trench 110 is dug into the tailings 80 of the lagoon from the shore 70. The trench may, for example, measure 15m long (10-20m long), 12m wide (5-18m wide) and 3.5m deep (1.5 to 5m deep) (figure 3). This trench 110 is filled with water, either by pumping water into it, or by allowing the trench 110 naturally to fill with water, either through water running into it from outside the lagoon or by water from tailings surrounding the trench flowing into it. The edge of the trench 110 by the shore may or may not be lined with steel piles. The pontoon 10 is then moved into the trench 110, for example by using steel or wooden rollers and/or a crane (figure 4).Or the pontoon may be built in the trench before the trench is filled with water.
The pontoon 10 is connected to the shore so that electricity, control signals and/or water can be provided to the pontoon 10, depending upon its needs. For example the pontoon 10 may have its own generator installed, or may rely on power being provided from the shore. Water can be provided to the pontoon 10 by providing water from the shore 70 into the trench 110 (and into the channel 128 and/or excavation hole 150 described below) which then flows to the pontoon 10. A
pump (e.g. pump 60) of the pontoon 10 then collects water from under the pontoon 10 for use by the pontoon 10. The flow of water may be controlled dependent upon the density of tailings in the water being consumed by the pontoon 10.

As illustrated in figure 5, water can be pumped (illustrated by 85) onto the tailings to start breakdown of the tailings, if necessary. The solid tailings 80 is broken down into slurry which fills the trench 110. The submersible slurry pump 50 then removes the slurry, for example further up the lagoon using pipes 120. The submersible water pump 60 can remove water from the trench 110 to be used by the pontoon 10 to breakdown the solid tailings 80. The submersible water pump 60 may be located higher than the slurry pump 50 as the concentration of tailings at lower depth will be lower.
The submersible slurry pump 50 may also have its own source of water, and it expels water 89 to help breakdown tailings 80 at the location at which it is extracting slurry.
For this purpose a pipe 91 provides water to the submersible slurry pump 50 and a separate pipe 120 removes the slurry, for disposal elsewhere. Material breakdown and pumping is thereby accelerated by the submersible slurry pump. A jetting ring can be fitted to the slurry pump to jet the water against the solid tailings and thereby accelerate breakdown. Jetting can also be achieved from a plurality, e.g four ( one or more ) jets, for example mounted on each corner of the pontoon 10. These jets have a dual purpose _ Hydro propulsion, and preferably directional control, for the pontoon 10 and jetting of solid tailings 80 in the lagoon.
The water pump 60 on the pontoon 10 and the piping and measuring and control system on the pontoon 10 is used for controlling the density of the slurry being pumped out by the slurry pump 50 (for example via a density control circuit).
For example the amount of water in jet 89 and/ or jet 85 is controlled. The tailings in the lagoon may have a water content of between 20 % and 70 %. If you try to pump slurries with water contents below 40 % by volume there is a high chance of the slurry pipe blocking. So control is applied to provide water at a rate which ensures the slurry being pumped has a water content above, for example 30% or 35% , preferably above 40%. The provision of water to the pontoon 10 (for example along the channel 128 or through pipes from the shore), is controlled so that the appropriate water content in the excavated slurry is present. In an embodiment there may be one or more additional optional booster pumps 250 (see figure 1) on the pontoon 10 before or after the density control circuit (after the density control circuit as illustrated). In an embodiment such booster pumps are horizontal pumps. The booster pump 250 is used to provide additional pressure head to the slurry being pumped to the upstream end of the lagoon ( away from the dam) this allows the slurry to be pumped further as it over comes the friction loss in the slurry pipeline. As can be seen from figure 1, the density control circuit can involve a water bleed valve 260. The water bleed valve 260 (a proportional valve) controls the amount of water (e.g. from the water pump 60) which is added into the pipe 120 along which the slurry pump 50 pumps slurry.
The density of the slurry in the pipe 120 can be measured at any location (either side of where a flow meter 280 is shown in the discharge pipe 120 downstream of the booster pump 250) and the water bleed valve 260 can be controlled to adjust the slurry water density accordingly. In a preferred embodiment the water content in the slurry in measured on shore. In an embodiment a current drawn by the slurry pump 50 and/or booster pump 250 is used in the control as an indication of the water content of the slurry being pumped. That is, the measurement of the density/water content of the slurry is made based on a knowledge of the current/power drawn by the slurry pump for different densities/water contents of slurry (e.g. from a predetermined look-up table or a mathematical relationship, both determined experimentally) and the actual current drawn by the slurry pump. In an embodiment a measurement of the flow rate of slurry is used in the control as an indication of the water content of the slurry being pumped. An alternative or additional way of controlling the water density in the slurry is to control the amount of water provided in water jets 85 from the pontoon 10 and/or in water jets 89 exiting the slurry pump 50. An increase in water flow in either of those jets results in an increased water content in the slurry pumped by the slurry pump 50. In an embodiment the density control circuit may additionally or alternatively raise and lower the slurry pump 50 relative to the pontoon 10. Raising the slurry pump towards the pontoon 10 results in the water content in slurry increasing because particles in the slurry tend to sink, meaning that the water content is greatest near the surface and reduces with depth.
One or more of these methods or alternative methods can be used to adjust the water content of the slurry being pumped from the pontoon, ensuring that pipes 120 leading from the pontoon 10 do not become blocked with slurry due to the density of water in the slurry being too low.
Figure 6 show the pontoon extending the trench 100 into the solid tailings 80.
This is achieved by continuing to breakdown the solid tailings at a leading edge of the pontoon 10. A water jet 87 at the trailing edge of the pontoon 10 can propel the pontoon 10 forward into the lagoon. In an embodiment individually controllable water jets directed to the sides (for example one at each corner of the pontoon 10) can be used to steer the pontoon 10 so that omnidirectional control is possible. Thus the pontoon 10 extends the trench 110 to bore a channel 128 towards a desired point in the lagoon and floats on water in the channel 128 (figure 7). The flow of water from the submersible pump 60 to breakdown the tailings (85) and to propel the pontoon (87), as well as the operation of the submersible slurry pump are controlled in any way by switching valves which may be on the pontoon 10 or located on the shore 70. In a preferred embodiment, the control is remote from a control station 200 on the shore 70, so that for normal operation no operators need to be on the pontoon 10.
Figure 8 illustrates the progress of the pontoons 10 from the left and right of the Lagoon towards the chosen location for dewatering along the channels 128 excavated by the pontoons 10.
In the case that the lagoon has a layer of water on top of the tailings 80, it may not be necessary to excavate a trench 110 and channel 128 as described above with reference to figures 3-7. For example, if the tailing 80 has one meter of more of water covering it, the pontoon can be floated out to the chosen location (such as illustrated in figure 8) at an appropriate area near the dam 5.
Once the pontoons are in position in the lagoon, for example 100m from the dam wall (e.g 200 - 20 m from the dam wall, preferably 150-50m from the dam wall) and, in the case to two pontoons 10 being used, each 75m from the lagoon centreline (e.g.
20-100m from the centreline), the pontoons 10 stop excavating the channel 128.

Thus the excavation holes are positioned in the tailings lagoon such that dewatering is localised to tailings adjacent the dam wall 5. The pontoons 10 then start to excavate holes 150 in the tailings 80 (figure 9). The excavation holes can be of any size. A size of about 30m diameter (7-75m in diameter, preferably 10-50m in diameter) and to a depth of 10 to 75m, preferably to a depth of 25-50m is thought suitable. The excavation holes 150 are excavated in the same way as the channels, using pumped water to breakdown the solid tailings (for example pumped from the end of the submersible slurry pump 50) and by lowering the submersible slurry pump 50 as the excavation hole gets deeper.

The depth of the excavation hole 150 is desirably greater than the depth of the channel 128 leading to it. In an embodiment the diameter of the excavation hole 150 is greater than the width of the channel 128 leading to it.
Once the excavation holes 150 are dug in the tailings 80 and the slurry pumping stops, water from the surrounding tailings 80 will drain into the excavation holes (illustrated as 160 in figure 10). If necessary additional water can be pumped into the excavation holes via the channels (or via a pipe) to assist pontoon 10 stability. Water drawn from the solid tailings 80 surrounding the excavation holes 150 can then safely be discharged beyond the lagoon, for example pumped by either or both pumps 50,60 via pipes 170 up and over the dam wall Sand beyond the toe of the dam ( figure 11).
In this was tailings 80 in the proximity of the dam wall 5 are de-watered, so that the tailings cannot fluidize, meaning that the dam wall 5 only has to resist the force of the tailings 80 acting on the dam wall 5 and not hydrostatic pressure. For the case where a dam is 600m long and the tailings 50m high, the average force of a fluidised tailing load is approximately 12.25MN per meter of wall length. This compares to 800kN per meter of wall in the case of the tailings having low moisture content, namely less than 7% of the water loading.
The slurry that will be discharged due to the creation of the channels 128 and/or dewatering holes 150 upstream of the dam wall 5 may optionally be captured in geotextile cloth bags to reduce turbidity (cloudiness) of any effluent water discharged downstream of the dam 5 whilst capturing the tailings to prevent further environmental contamination moving the tailings from their present location.
The dewatering holes 150 created to reduce the hydraulic pressure of the dam wall 5, once created, may optionally have installed a high volume submersible pump capable of handling dirty water, typically they would be similar to those found at the entrances to underground mines that prevent underground mines flooding during rainy seasons or flash rainfall, being able to cope with high volumes of dirty water (typically from 0% up to 15% solids) on an ad hoc basis to further reduce the possibility of any standing water anywhere on the tailings pit.
Although the invention is described using two pontoons 10, any number of pontoons can be used. For example, only one pontoon 10 may be used, or three or more may be used, depending on the desired speed and extent of dewatering and the size of the lagoon.

Claims

1. A method comprising:
excavating a channel in a tailings lagoon from a shore of the tailings lagoon and floating a pontoon in water in the channel from the shore along the channel, wherein:
excavating involves breaking down solid tailings in the tailings lagoon into a slurry using water and removing the slurry using a submersible slurry pump mounted on the pontoon.

2. The method of claim 1, wherein:
during the excavating the water for breaking down the solid tailings is supplied by the pontoon.

3. The method of claim 2, wherein:
the water supplied by the pontoon is at least partly water extracted from the channel by the pontoon.

4. The method of claim 1, 2 or 3, wherein:
during the excavating water is provided to the pontoon from a shore of the tailings lagoon.

5. The method of any of claims 1-4, wherein:
water is supplied to the channel during excavation from beyond the tailings lagoon.

6. The method of any of claims 1-5, wherein:
the amount of water contained in the slurry (density) is controlled to be a certain level.

7. The method of claim 6, wherein:
the amount of water contained in the slurry is controlled by varying the depth below the pontoon of the slurry pump.

8. The method of claim 6 or 7, wherein:
the amount of water contained in the slurry is controlled by varying the flow rate of water used in breaking down solid tailings.

9. The method of claim 6, 7 or 8, wherein:
the amount of water contained in the slurry is controlled by varying a flow rate of water provided to the slurry pump and discharged by the slurry pump.

10. The method of any of claims 6-9, wherein:
the removed slurry travels from the slurry pump to a discharge point along a pipe and the amount of water contained in the slurry in the pipe is controlled by varying the amount of water added to the pipe at a position between the slurry pump and the discharge point.

11. The method of any of claims 1-10, wherein:
the pontoon is controlled remotely from a shore of the tailings lagoon.

12. The method of any of the preceding claims, wherein:
a water jet propels the pontoon along the channel.

13. The method of any of claims 1-12, wherein a plurality of water jets on the pontoon control the pontoon directionally.

14. The method of any of claims 1-13, wherein the removed slurry is reprocessed to remove a material contained in the slurry.

15. A method of dewatering a tailings lagoon retained by a dam comprising:
excavating an excavation hole in the tailings lagoon;
allowing water from surrounding tailings to enter the excavation hole; and pumping water in the excavation hole out of the excavation hole and discharging beyond a toe of the dam.

16. The method of claim 15, wherein:
a pontoon floats in water in the excavation hole and supports equipment used for the step of pumping water.

17. The method of claim 15 or 16, wherein:
before excavating the excavation hole, the pontoon is floated along a channel in the tailings lagoon to the location at which the excavation hole is to be excavated using the method of any of claims 1 to 14.

18. The method of any of claims 15-17, wherein:
dewatering is localised to tailings adjacent the dam.