WO2024150160A1 - Drainage structure - Google Patents

Abstract

This invention relates to a method for draining soils or mine tailings. The method includes the step of inserting prefabricated elongate porous drains 16 in a pattern through a tailings or soil 18 that is to be dewatered. The drains having an upper end and a lower end, and the drains are inclined at angle more than 5 degrees, and up to 90 degrees from horizontal, and are connected into a water drainage structure 10 at their lower end, from which water can be removed, and are open to the atmosphere at their upper end.

Description

DRAINAGE STRUCTURE

BACKGROUND OF THE INVENTION

Prefabricated wick drains are commonly used in preparation of saturated soils to stabilise them prior to construction activities.

The prefabricated wick drain is inserted down into the saturated soil, and acts as an unconstrained path for the water to be released from the pore space between soil particles and migrate into the wick drain and then up to the soil surface. In this way, sufficient water is removed from the saturated soil, to stabilize the soil geotechnically. The migration of water into the wick drain is driven by the differential pressure across the membrane of the wick drain; and the water flow continues until the pore pressure on the outside of the membrane is comparable to the head of water in the wick drain.

The rate of dewatering is dictated by the differential pressure between pore pressure in the soil and the head of water present in the wick drain. The use of wick drains to dewater a saturated soil can take months or years, depending on the permeability of the soil, and the selected spacing of the wick drains.

The rate of migration of water through the soil can be described through the well-known Darcy equation where the water discharge rate is proportional to the product of the differential pressure and the hydraulic conductivity of the soil. In fine clay containing soils, this migration is slow requiring the wick drains to be inserted with spacings typically less than 10m between centres and often as low as 2m.

The pattern of wick drains inserted vertically down into the soil takes advantage of the natural anisotropy that exists with soils. Hydraulic conductivities of soils can vary by up to a factor of 10 between the vertical and horizontal planes. By regularly spacing the wick drains in the vertical plane, the water can mainly migrate horizontally along the plane of highest hydraulic permeability to a discharge point.

Prefabricated wick drains have also been used to dewater historical tailings generated by the mining industry (Tailings impoundment stabilization to mitigate mudrush risk, Amy Adams, Daniel Friedman, Ken Brouwer, Scott Davidson, Annual Meeting of International Commission on Large Dams July 3-7, 2017 Prague).

By providing permeable channels for water to flow out from the facility, the regularly spaced wick drains achieve a similar function of geotechnically stabilizing the tailings. In effect historical tailings are a form of saturated and extremely unstable soil.

In a separate development, the near horizontal sand channels, separately described in Ren (CA2812273) and Filmer (W02020/183309), perform a similar function. Water migrates from the tailings into the near horizontal sand channels, and from there by gravity to a drainage point and hence out of the facility.

In comparing these two methods, the vertical prefabricated wick drains used in construction, have an advantage of being vertically aligned, rather than almost horizontally as in the sand channels, thus benefiting from tailings anisotropy. The hydraulic conductivity of tailings is well known to be greater in the horizontal direction, typically in a ratio of around 5 to 10 (Hydrogeology and Mineral Resource Development Copyright © 2021 by Leslie Smith, Chapter 3.3.).

However, the vertical wick drains inserted from the surface of a soil or tailings facility have a major disadvantage relative to Filmer, in that there is no opportunity to drain the water from the drainage channels and allow air access into the drain. Rather each conventional vertical wick drain will fill with water, allowing for water removal from the facility only whilst the hydraulic pore pressure in the surrounding tailings exceeds the head of water accumulated in the wick drain.

Draining the water, as taught by Filmer, can lower the phreatic surface in the permeable phase and would increase the differential pressure between the pore spaces in the tailings and the wall of the wick drain. As per Darcy’s law, this increased pressure will accelerate water flows into the drain, and also allow air ingress into the soil above the phreatic surface to improve overall water recovery.

As noted previously, tailings dewatering operates on similar principles to soil consolidation.

The tailings storage facilities (TSF) retain finely divided residues and have hydraulic permeabilities that are broadly consistent with clay like soils. The primary difference is in the initial laydown of the residue, where tailings is deposited in the form of a slurry. These tailings gradually consolidate over time allowing some water to escape and be removed as surface run-off. But the consolidated tailings facility remains saturated with a substantive water content.

The ongoing slurry addition over the surface of an active TSF continuously traps this water in the tailings as the solids surface rises. It is only on cessation of operation, the top surface of the dam can ‘dry out’; but below the surface the slow vertical migration of water implies the deeper sections of the dam will remain saturated for millennia.

A tailings storage facility forms the equivalent of a highly saturated soil extending over a large area and depth. So, the same dewatering principles apply when wick drains are inserted into the tailings and the excess pore water migrates from the tailings into and through the wick drain and to the drainage point from the dam. This is described by Adams (Adams et. Al. Tailings impoundment stabilization to mitigate mudrush risk, July 3-7, 2017, 85^th Annual Meeting of International Commisssion on Large Dams). But through the history of construction, operation and closure, the mine tailings also have a unique difference, in offering three different types of problems to be solved in dewatering tailings facilities.

Firstly, there are the historical or closed TSFs which are no longer actively receiving tailings. Desaturation of the tailings would remove any ongoing risk of a dam wall failure. The installation of conventional wick drains in these historical dams, as a method of risk reduction, is complicated by the surface of the historical tailings which usually remains unstable for a number of years, and incapable of bearing the weight of heavy equipment required for wick drain insertion. This problem has been overcome in part by the use of low ground pressure equipment. An example is available from Nilex Corp.: https://www.cofra.sk/files/documents/technicke_spravy/minetailings2.pdf.

Secondly, there are active TSFs with new tailings being placed continuously. These facilities need to be dewatered both for water recovery, and for inherent structural safety. These are termed brownfield dams or TSFs. The access to the surface for inserting wick drains on these dams is even more problematic, and even if inserted, the typical wick drains used in soil consolidation would become submerged and saturated as the tailings levels rise. One can consider adapting future design and operational procedures over the remaining available dam height, but on an unstable base layer of saturated tailings.

And thirdly, there are the future tailings storage facilities, into which finely divided slurry will be placed when the mine is commissioned. In these future greenfield dams, the opportunity exists to alter the dam design, and to create a new method of operation which is effective over the full life of the dam.

Ideally, all forms of active tailings dams would be managed such that the tailings can be placed hydraulically, but these tailings would remain saturated for only a short time, with a high proportion of the contained water being recovered and recycled. Sand channels can be laid horizontally on the surface of existing tailings to transport the water from the adjacent tailings above and below, to a drainage point in brownfield and greenfield facilities. (Filmer W02020/183309). When the phreatic surface is lowered, the sand channels provide air access to the facility. But as noted previously, the horizontal sand channels described by Filmer are not able to take advantage of the anisotropy of the tailings. And these horizontal channels have a very low head for the water to be transported laterally by gravity through the sand, thus delaying the lowering of the phreatic surface. In this design, the vertical containment bunds are spaced at substantive distances apart, and hence have limited direct dewatering influence despite the anisotropy.

And for historical dams, conventional wick drains provide a solution in part, as they can be inserted vertically through the tailings at regular spacing and take advantage of the anisotropy of the tailings, but as they discharge from the surface above the tailings, they cannot enable air penetration into the tailings to fully dewater the facility, and because of the excessive head of water in the drain, the duration for dewatering is excessive.

It is an object of this invention to address the problems mentioned above by proving a drainage system:

• which can be readily installed in all forms of tailings facilities

• and which reduces the duration for dewatering of a saturated tailings

• with recovery of most of the contained water,

• whilst operating safely and productively on an unstable tailings surface.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method for draining soils or mine tailings including the step of inserting prefabricated elongate porous drains in a pattern through a tailings or soil that is to be dewatered, the drains having an upper end and a lower end, wherein: the drains are inclined at angle more than 5 degrees, preferably more 30 degrees, more preferably more than 60 degrees, more than 70 degrees, more than 80 degrees and up to 90 degrees from horizontal, and are connected into a water drainage structure at their lower end, from which water can be removed, and are open to the atmosphere at their upper end.

The mine tailings may be a greenfield tailings storage facility, comprising a sand layer or channels defining a drainage structure, and wherein the wick drains are progressively raised such that the upper end of the drain remains above the level of the tailings as it is deposited.

The mine tailings may be a brownfield tailings storage facility, comprising a sand layer or channels from which water can be pumped defining a drainage structure constructed on the existing surface of the brownfield tailings storage, and drains are inserted to connect to the drainage structure through the newly deposited tailings and remain open at the upper surface to the atmosphere.

The sand drainage structure may be inserted into pre-existing tailings by drilling into the structure and backfilling with sand, such that the water can be pumped or flow by gravity from the facility.

Centres of the drains are preferably placed more than 5m apart, and preferably more than 10m, and even more preferably more than 20m apart.

The drainage structure may include a pipe inserted through the tailings and into an underlying sand, through which water can be pumped to dewater the sand.

The drainage structure may be a pre-existing underdrain. The drainage structure may also be sand channels which have been inserted into a historical tailings. The sand in the drainage structure preferably has less than 10% by weight of particles less than 75 micron and preferably less than 5%; and has a p80 of less than 350 micron and preferably less than 250 micron.

The pattern of drains in a tailings dam may be limited to the area closer to a dam wall, to stabilise the wall.

The drains may be wick drains, or sand drains.

According to a second aspect of the invention, there is provided a soils or mine tailings drainage structure comprising elongate porous drains inserted in a pattern through the tailings or soil, the drains having an upper end and a lower end, wherein: the drains are inclined at angle at angle more than 5 degrees, preferably more 30 degrees, more preferably more than 60 degrees, more than 70 degrees, more than 80 degrees and up to 90 degrees from horizontal, and are connected into a water drainage discharge structure at their lower end from which water can be removed, and are open to the atmosphere at their upper end.

The mine tailings may be a greenfield tailings storage facility, comprising a sand layer or channels defining a drainage structure.

The mine tailings may be a brownfield tailings storage facility, comprising a sand layer or channels from which water can be pumped defining a drainage structure. These sand channels or layers may be constructed on the existing surface of the brownfield tailings storage, with the drains connecting to a drainage structure through the subsequently deposited tailings.

This structure may comprise conventional wick drains installed with their upper end connected to the drainage structure and their lower end immersed in the underlying brownfield tailings to provide a set of pathways for water to flow from the underlying tailings to the drainage point, even after additional tailings are added above. The sand drainage structure may be inserted into the soil or pre-existing tailings by drilling into the structure and backfilling with sand.

Centres of the wick drains are preferably placed more than 5m apart, and preferably more than 10m, and even more preferably more than 20m apart.

The wick drains are preferably vertical prefabricated wick drains, or sand drains.

The drainage structure may include a porous pipe inserted through the sand channels to increase the rate at which water can be pumped to dewater the sand.

The drainage structure may be a pre-existing underdrain.

Preferably, the sand in the drainage structure has less than 10% by weight of particles less than 75 micron and preferably less than 5%; and has a p80 of less than 350 micron and preferably less than 250 micron.

The pattern of drains in a tailings dam may be limited to the area closer to a dam wall, to stabilise the wall.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 A is an elevated view of a schematic diagram of a greenfield tailings storage facility;

Figure 1 B is a plan view of the tailings storage facility illustrated in Figure 1A;

Figure 2A is an elevated view of a schematic diagram of a brownfield tailings storage facility into which a sand channel has been inserted; Figure 2B is a plan view of the tailings storage facility illustrated in Figure 2A;

Figure 3A is an elevated view of a schematic diagram of a brownfield tailings storage facility with stabilisation of historical tailings;

Figure 3B is a plan view of the tailings storage facility illustrated in Figure 3A;

Figure 4A is an elevated view of a schematic diagram of a retrofitted tailings storage facility; and

Figure 4B is a plan view of the tailings storage facility illustrated in Figure 4A.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides a design and method for

• inserting a pattern of elongate prefabricated wick or sand drains through tailings,

• where each drain is inclined with a significant vertical vector,

• and is connected to a suitably sized sand drainage structure at its lower end

• and is open to the atmosphere at the upper end.

The design allows the phreatic surface within the tailings to be drawn down through the drainage structure, such as to accelerate water migration along the axis of highest permeability into a nearby drain, and from the drain removed from the facility.

By “drainage structure” is meant a permeable structure which enables transport of the water such as a volume of suitably sized permeable sand, in which water can be collected and flow freely to a discharge point and be pumped, or siphoned, or naturally flow out from the structure, at a maximum rate faster than the average flow of water into the drains.

By “connected to a drainage structure”, is meant a permeable path from the end of each drain to, or directly into, the drainage structure.

By “a significant vertical vector” is meant a slope of the drain at an angle sufficient for rapid flow of water by gravity, preferably more than 5 degrees from horizontal and even more preferably more than 30 degrees, and in some cases vertical.

By “open to the atmosphere” is meant that the drain protrudes from the surface of the soil or tailings, such that air can continuously access the upper end of the drain, thus avoiding the need for countercurrent flow of air and water within the drain.

A “wick drain” is a prefabricated wick drain comprising prefabricated geotextile filter-wrapped plastic strips with molded channels.

A “sand drain” is a continuous channel of an appropriate sand which is inserted into a historical tailings, to provide a permeable pathway for the flow of water. Such a channel may be created by drilling into the tailings, and injecting sand down the core of the drill pipe as the drill pipe is retrieved, to leave a sand channel. Alternatively a slotted pipe filled with sand may be directly embedded into the tailings, enabling water to flow through the slotted pipe and travel through the sand.

In the case of tailings deposition, where the surface is rising, the inlet of the drain is constructed such as to remain above the rising tailings surface. Drains may be constructed from prefabricated wick drain or sand or any other materials that provide an ongoing hydraulic pathway through the tailings. The terms “wick drain”, “sand drain” and “drain” are used interchangeably in describing the design of the invention, with just their method and materials of construction being different. Similarly, the invention can equally be applied to poorly drained soils or to mine tailings. These terms are also used interchangeably in describing the invention.

The invention can be present in different embodiments for application in the preparation of a construction site, or in the rehabilitation of a closed tailings dam; or in converting a brownfield tailings dam which is still active, or in a greenfield tailings dam design.

The first principle underpinning the invention is the low phreatic surface within the drain. With the water transferring rapidly through the vertical drain to the drainage point, the phreatic surface can be lowered within the drain by rapid draining. This reduction of phreatic surface increases the hydraulic gradient between any point in the surrounding tailings and the surface of the drain.

This pressure differential is quite different from the conventional prefabricated wick drain inserted from above, in which the water pressure in the drain is the equivalent of the head of water up to the discharge point of the drain on the surface.

The increased hydraulic gradient achieved by the current invention accelerates the rate of water migration into the wick drain from the surrounding tailings and also enables air to migrate from the wick drain into the surrounding tailings.

The second principle underpinning the invention is the curtain of drains with a significant vertical vector. This pattern of drains implies the migration path for water to the closest drain is predominantly in the horizontal plane. The design inherently benefits from the natural anisotropy of the tailings allowing for faster water migration from the tailings, thus accelerating the dewatering for any specific spacing of drains. This is quite different from the horizontal drains claimed by Filmer and Ren, where the predominant flow of water through the tailings to the nearest drain is in the vertical direction.

In all embodiments of the invention there exists a pattern of drains, a drainage structure, and a means of removing water from the structure.

The drainage structure is a permeable structure which enables transport of the water from the end of the wick drains to a point where this water can be permanently withdrawn from the tailings dam. The drainage structure will normally be sand, sized such that infiltration of the tailings above does not occur, thus maintaining the permeability of the drainage structure over the life of the dewatering process.

The lateral water flow through a drainage structure may be enhanced by provision of coarser material or suitably contained piping within the drainage structure, designed such that the water can enter from the surrounding sand, but the sand will not enter.

The sand utilised for the drainage structure will be selected such as to minimise the ingress of tailings over time, thus retaining its permeability. The sand will have less than 10% by weight of particles less than 75 micron and preferably less than 5%; and having a p80 of less than 350 micron and preferably less than 250 micron.

The drainage process for removing water from the drainage structure, and hence from the facility as a whole, can be one of gravity flow or pumping.

EMBODIMENTS OF THE INVENTION

Whilst the invention has the two common principles and the common drainage structure and pattern of vertical drains as structural elements to the design, there are different embodiments required for application to greenfield, brownfield, and historical facilities. The invention for all applications requires insertion of a drainage structure, to which the lower end of the wick drains can be connected. This requires insertion of the sand layer or channel in the facility.

If a greenfield tailings facility is not yet constructed, this process for creating a sand structure is relatively simple; and involves laying of a sand layer or channels at the base of the future tailings facility. The sand channels are connected to a drainage point or points, for removal of the water from the facility. Wick drains are placed such that as the tailings levels rise, water flows into the drain and hence to the drainage structure and out of the facility.

With reference to Figures 1 A and 1 B, a drainage structure 10, which in this embodiment is a layer of coarse sand, is provided, with a containment wall 12. The drainage structure 10 in this embodiment extends beneath the containment wall 12. Wick drain supports 14 support wick drains 16 which have an upper end that is open to the atmosphere and a lower end that extends into and drains into the drainage structure 10. Tailings 18 are deposited on the drainage structure, with the wick drains 16 extending through the tailings 18.

Installing the drainage structure in a brownfield facility requires a different form of embodiments.

If an existing tailings facility has a pre-existing base drain, this base drain can be repurposed to form the drainage structure. In this circumstance, the design will be similar to the greenfield embodiment.

Where no pre-existing drainage structure exists, and different approach is needed. Where the objective is only to dewater future tailings arisings, this drainage structure can be formed on the surface of the pre-existing TSF surface by laying one or more continuous bunds of sand across the existing tailings surface and connecting these bunds to a drainage tower from where water can be extracted from the facility. In this way the drainage structure is in place and will be overlaid with tailings.

Water from the overlying level of tailings can be recovered into wick drains inserted into the drainage structure and which flow into the drainage structure.

With reference to Figures 2A and 2B, a drainage structure 10, which in this embodiment is a layer of coarse sand, is laid on top of historical tailings 20, and a containment wall 12 is provided. Wick drain supports 14 support wick drains 16 which have an upper end that is open to the atmosphere and a lower end that extends into and which drain into the drainage structure 10. Tailings 18 are deposited on the drainage structure 10, with the wick drains 16 extending through the tailings 18. In this embodiment, the drainage structure 10 abuts with the containment wall 12, and the drainage structure 10 drains into drainage towers 22 which are provided with pumps for pumping water drained from the drainage structure 10.

In a further brownfield embodiment, partial dewatering of the underlying tailings can also take place by inserting conventional wick drains below the sand bunds and connected to the sand bunds. This will enable water to be partially removed from the tailings below and flow up into the drainage structure. This embodiment will geotechnically stabilise but not fully dewater the lower parts of the brownfield dam.

With reference to Figures 3A and 3B, a drainage structure 10, which in this embodiment is a layer of coarse sand, is laid on top of historical tailings 20, and a containment wall 12 is provided. Wick drain supports 14 support wick drains 16 which have an upper end that is open to the atmosphere and a lower end that extend into and drain into the drainage structure 10. Tailings 18 are deposited on the drainage structure 10, with the wick drains 16 extending through the tailings 18. In this embodiment, the drainage structure 10 abuts with the containment wall 12, and the drainage structure 10 drains into drainage towers 22 which are provided with pumps for pumping water drained from the drainage structure 10. Further, in this embodiment, conventional wick drains 24 are inserted into the historical tailings 20, and connected to the drainage structure 10.

In an embodiment for the full dewatering of historical tailings facility a drainage structure must be inserted in the base of the existing tailings facility.

Whilst not ideal, this can be achieved by injecting sand through a drilling system spatially directed to form a sand channel at the base of the tailings facility. Then then spatially connecting these sand channels to a drainage tower inserted into the tailings facility, and from which the water can be pumped or drain through an permeable dam wall.

The wick drains can then be inserted through the tailings and into the drainage structure, with angles of the drains selected to maximise coverage in the tailings whilst maintaining sufficient vertical vector for rapid dewatering of the drain.

With reference to Figures 4A and 4B, a drainage structure 10, which in this embodiment is a layer of coarse sand, is injected under historical tailings 20, and a containment wall 12 is provided. Wick drains 16 are inserted in the tailings 20 and drain into the drainage structure 10. In this embodiment, the drainage structure 10 abuts with the containment wall 12, and the drainage structure 10 drains into a drainage tower 22 which are provided with pumps for pumping water drained from the drainage structure 10.

In all these embodiments, the sand that makes up the drainage structure, whether present in a greenfield or brownfield or historical embodiment, provides the insertion point for the end of the wick or sand drains.

Where the drainage structure is submerged under a tailings layer, a prefabricated wick drain can be inserted from surface in the normal way. Alternatively, a sand drain can be inserted by drilling and backfilling with sand as the drill core as the drill bit is being withdrawn, to leave a sand channel connecting the drainage structure to the surface of the tailings.

The second requirement in all embodiments is to ensure air access into the upper end of the drain.

In the greenfield or brownfield options, where the sand drainage structure is initially exposed, prefabricated wick drains can be fixed in the sand layer. A physical support can hold a roll of wick drain above the sand surface. This roll of wick drain can then be progressively unfurled as the tailings level rises, such that its upper end of the prefabricated wick drain always remains proud of the tailings surface. As the tailings level approaches the height of the physical support, this support can be further raised, now supported by the consolidated tailings. This upper end of the drain provides ongoing air access to the drain.

Where the drain is constructed by sand, a porous piping filled with sand can be installed such as to extend above the level of the tailings, and this column can be intermittently jacked upwards as the tailings levels rise, leaving a sand column submerged in the tailings.

Thus, in all embodiments the upper end of each wick drain remains open to air.

With multiple wick drains placed in a pattern through the tailings structure, and a drainage structure below, water will flow by differential pore pressure from the tailings into the wick drain, then by gravity down the wick drain to the sand drainage structure, and then be drained.

Air will flow down the wick drain and out through the wick drain into the tailings. Hence the differential pressure between the pores of the tailings remote from the drain, and those from the aerated tailings immediately adjacent to the drain, will be greatly enhanced. This has particular application for stabilizing the containment wall of tailings dams. By placing the pattern of drains adjacent to the wall, the tailings adjacent to the wall can be rapidly consolidated. In this way tailings dams can be rendered safe, even with the use of an upstream dam design.

In cases where the lateral carrying capacity of the drainage structure is limited, or the density of the fresh tailings deposition is low, drawing down the phreatic surface can be difficult. In these cases, additional surface water can be recovered from the facility by different means, such as sand bunds connected to a drainage point.

The system of vertical wick drains provides for the transfer of water through the tailings along the plane at which hydraulic conductivity is greatest. If for example the anisotropy factor in tailings is around 10, this enhanced factor of conductivity to the nearest drain, enables the distance between drains to be greatly increased. Furthermore, the differential pore pressure is much greater than that in a conventional wick drain, further increasing water flows.

The combination of anisotropy and differential pressure enables the spacing between drains to be greater than 10m, and preferably greater than 20m and even more preferably around 30-50m.

The duration taken to dewater a saturated tailings is a function of its hydraulic conductivity and spacing between drains. However, the increased differential pressure achieved by an air-filled drain, enables the duration to be decreased to less than half and usually less than 25% that of a conventional wick drain placed at an equivalent spacing.

And the duration taken to dewater a saturated tailings using vertical drains rather than horizontal is a function of its permeability anisotropy. However, the duration can typically be reduced to less than half and usually less than 25% that of equally spaced horizontal drains.

Claims

1. A method for draining soils or mine tailings including the step of inserting elongate porous drains through a tailings or soil that is to be dewatered, the drains having an upper end and a lower end, wherein: the drains are inclined at angle more than 5 degrees and up to 90 degrees from horizontal, and are connected into a water drainage discharge structure at their lower end from which water can be removed, and are open to the atmosphere at their upper end.

2. The method claimed in claim 1 , wherein the drains are inclined at angle more than more 30 degrees from horizontal.

3. The method claimed in claim 2, wherein the drains are inclined at angle more than 60 degrees from horizontal.

4. The method claimed in claim 1 , wherein the mine tailings is a greenfield tailings storage facility, comprising a sand layer or channels defining the drainage discharge structure, and wherein the drains are progressively raised such that the upper end of the drain remains above the level of the tailings as it is deposited.

5. The method according to claim 1 , wherein the mine tailings is a brownfield tailings storage facility, comprising a sand layer or channels from which water can be pumped defining the drainage discharge structure constructed on an existing surface of the brownfield tailings storage, and drains are inserted to connect to the drainage discharge structure through the newly deposited tailings and remain open at the upper surface to the atmosphere.

6. The method according to claim 3, wherein wick drains are installed with their upper end connected to the drainage discharge structure and their lower end immersed in the underlying tailings.

7. The method according to claim 1 , wherein centres of the drains are placed more than 5m apart.

8. The method according to claim 7, wherein centres of the drains are placed more than 10m apart.

9. The method according to claim 8, wherein centres of the drains are placed more than 20m apart.

10. The method according to claim 1 , wherein the drainage discharge structure includes a pipe inserted through the tailings and into an underlying sand, through which water can be pumped to dewater the sand.

1 1 . The method according to claim 1 , wherein the drainage structure is a pre-existing underdrain.

12. The method according to claim 1 , wherein the sand in the drainage structure comprises a sand with less than 10% by weight of particles less than 75 micron and has a p80 of less than 350 micron.

13. The method according to claim 12 wherein the sand has less than 5% of particles less than 75 micron; and a p80 of less than 250 micron.

14. The method according to claim 1 , wherein the pattern of drains in a tailings dam is limited to the area closer to a dam wall, to stabilise the wall.

15. The method according to claim 1 , wherein the drains are wick drains, or sand drains.

16. A soils or mine tailings drainage structure comprising prefabricated wick or sand drains inserted in a pattern through the tailings or soil, the drains having an upper end and a lower end, wherein: the drains are inclined at angle more than 5 degrees and up to 90 degrees from horizontal, and are connected into a water drainage structure at their lower end from which water can be removed and are open to the atmosphere at their upper end.

17. The structure claimed in claim 16, wherein the drains are inclined at angle more than more 30 degrees from horizontal.

18. The structure claimed in claim 17, wherein the drains are inclined at angle more than 60 degrees from horizontal.

19. The structure according to claim 16, wherein the mine tailings is a greenfield tailings storage facility, comprising a sand layer or channels defining a drainage structure.

20. The structure according to claim 19, wherein the mine tailings is a brownfield tailings storage facility, comprising a sand layer or channels from which water can be pumped defining the drainage structure constructed on the existing surface of the brownfield tailings storage, with the drains connecting to a drainage structure through the subsequently deposited tailings

21 . The structure according to claim 20, comprising drains installed with their upper end connected to the drainage structure and their lower end immersed in the underlying brownfield tailings to provide a set of pathways for water to flow from the underlying tailings to the drainage point, even after additional tailings are added above

22. The structure according to claim 16, wherein centres of the wick drains are placed more than 5m apart.

23. The structure according to claim 22, wherein centres of the wick drains are placed more than 10m.

24. The structure according to claim 23, wherein centres of the wick drains are placed more than 20m apart.

25. The structure according to claim 16, wherein the wick drains are wick drains or sand drains.

26. The structure according to claim 16, wherein the drainage structure includes a porous pipe inserted through the sand channels to increase the rate at which water can be pumped to dewater the sand.

27. The structure according to claim 16, wherein the drainage structure is a pre-existing underdrain.

28. The structure according to claim 16, wherein the sand in the drainage structure has less than 10% by weight of particles less than 75 micron; and has a p80 of less than 350 micron.

29. The structure according to claim 28, wherein the sand in the drainage structure has less than 5% by weight of particles less than 75 micron ; and has a p80 of less than 250 micron

30. The structure according to claim 16, wherein the pattern of drains in a tailings dam is limited to the area closer to a dam wall, to stabilise the wall.