A mine water disposal pond. (Photo: Itasca Denver)

Dewatering is a vital component of safe, productive mining operations, but its role and impact are often misunderstood

By Carly Leonida, European Editor

The law of gravity dictates that the creation of an open-pit mine — essentially, a giant funnel cut into the earth through layers of rock — will channel and collect water. No matter the scale of an operation, it is inevitable that the excavation will intercept groundwater flows at some point and, depending upon the local conditions, surface water or run-off is also likely to pool in certain areas.

Effective dewatering benefits every function within an operation, from mine design and slope stability, to blasting and haulage. Yet the impact of a good strategy, prepared and executed well in advance with the consultation of experts and reviewed regularly is often underestimated.

A lack of understanding across the workforce surrounding the role of dewatering and associated infrastructure can lead to mines losing sight of dewatering targets, lack of sufficient resources and, if left unchecked, a major safety incident.

So, E&MJ asked experts from two leading consultancies to help enhance our collective understanding.

Developing an Effective Strategy

First up was Itasca International; the firm specializes in solving complex geomechanical, hydrogeological and micro seismic issues in mining.

Houmao Liu, General Manager/Principal at Itasca’s Denver office, Martin Brown, Hydrogeology Manager for Itasca Chile, and Loren Lorig, Principal for Itasca’s Minneapolis team, joined E&MJ for the discussion.

“Historically, in open pits the mine designers would come up with a design, and the hydrogeologists would say ‘okay, this is how we think it’s going to look from a hydrogeology standpoint’,” Lorig explained. “They would pass that information to the people doing the slope stability studies. We would take a look and determine which areas were meeting design acceptance criteria, and which weren’t.

“We’d pass it back to the hydrogeologists, and they would tweak the design in terms of dewatering from wells, horizontal drains, drainage tunnels, or whatever the options were. This would continue to go back and forth, and it was very time consuming.

“In recent years, there’s been a move to streamline that process and make it more rational. The idea now is that the mine comes up with a design, and the geotech people look at it and say ‘if you want to mine to this particular design, here are the targets that we need the hydrogeologists to meet.’

“The hydrogeologists then go away and come up with schemes that will meet those targets.

“This approach has the added advantage of providing targets for instrumentation and monitoring, because anything that deals with geology, geochemistry and hydrogeology is extremely uncertain.

“We never have enough information to do all of the things we would like to do; it’s just impossible to collect that much information. But if we have targets for dewatering then, as the teams mine down, they can put in piezometers, and find out if those targets are being met or not.

“If they are, fine. Maybe we can steepen the slopes. But if they’re not being met, then you have to think about putting in additional dewatering measures. Or, if that’s not possible, flattening the slopes.

“So, this notion of providing dewatering targets is something which is useful in the design and analysis part, but also in the implementation and operation of the mine, because it greatly simplifies the whole process.”

Get an Early Start

Brown explained that hydrogeological characterization should be done as early as possible in a project’s life.

“You need to have hydrogeological data at the feasibility stage in order to understand how groundwater will move when you start excavating the pit,” he told E&MJ. “And then, of course during the operations, as mines are so dynamic. Hydrogeological and geotechnical teams need to be in constant interaction to define the right dewatering targets.

“There are operations in Chile that invest US$15-20 million a year on dewatering infrastructure. To make sure resources are available to meet the defined targets, mines need to start planning as early as possible.

“Also, there are a lot of social issues associated with mining and water. What happens with the groundwater when mining activities stop? It’s something that goes on for the whole life of the mine.”

The last point is a valuable one. In many areas, water is a scarce resource, one that the mining industry competes with local communities for. It should therefore be managed responsibly and, where possible, mines should look for intersections between their own dewatering efforts and the needs of local communities. Water removed from mining operations could potentially benefit other stakeholders.

Liu explained that, ideally, the dewatering strategy should be assessed and investigated at the desktop study stage. Field investigations should be conducted for the pre-feasibility study and continue as the project proceeds. These usually include a baseline study, hydraulic testing such as packer testing, short-term pumping tests, installation of piezometers, collection of hydrogeologic data, and development of a groundwater flow model. Then, at the feasibility stage, a long-term pumping test (30-60 days) may be required, along with pilot testing of a production dewatering well.

The groundwater flow model should be continuously updated, and its predictions validated with new data to guide the dewatering program.

“One thing investors are looking for when deciding if they want to fund a project is hydrogeology and dewatering, because it involves a lot of uncertainty,” said Liu. “So, when you get to the field investigation, you should at least have a good test team and groundwater flow model, and the installation of piezometers. All this should be interfaced and supported to give the funding agency confidence, and to help them understand how it will work and what the cost would be.

“If a major dewatering effort is needed to keep the open pit dry, that would involve tighter testing to help design the right dewatering wells. Each well costs over $1 million to install, so we need to make sure that we get design the right.”

“This process involves continual learning and updating,” added Lorig. “The earlier you start to understand what’s going on, the better prepared you are to deal with surprises. Because there are always surprises. And if you wait too long to discover them, that leads to problems.”

Impacts of Excess Water

Another firm renowned for its expertise in hydrogeology and geomechanics is SRK Consulting.

Goktug Evin, Principal Hydrogeologist, and Cristian Pereira, Principal Hydrogeologist, both from SRK Consulting’s Denver team, spoke to E&MJ along with their colleague Daniel Mackie, Principal Hydrogeologist for SRK’s Vancouver office.

Evin explained: “Excess water associated with mine dewatering can have severe impacts on many aspects of a mining operation. Direct impacts could include loss of access to some or all of the working levels of the operation. Unmanaged excess water, if significant, can also be a threat to operational safety.

“Another direct impact is the capital investment and operational costs for the pumping system that is required to dewater the open pit. In most cases, once excess water is allowed in the pit, the quality of the contact water deteriorates rapidly and this raises the need for treatment, thereby increasing the overall cost of water management. Direct impacts are usually noticeable, and mine operators focus on mitigating them during mine planning or when these impacts are faced during the operation.

“In addition, there can be hidden impacts of water on the operation that can cause inefficiency throughout the life of the mine if not identified and addressed. One is the high cost associated with blasting under saturated conditions. In most cases, this leads to a high demand for explosives usage, ineffective blasting, or may require the use of special/expensive types of explosives.

“Wet hauling is also an issue, as the operator must haul the unwanted water with the ore. Trafficability is adversely impacted in such conditions. Equipment wear and tire wear are checked by the operator at the end of the day, and costs related to this type of wear are rarely associated with improper water management.

“Above all, I think the major impact of water on an open-pit operation is its impact on slope stability, which can be a game changer.”

While there are negative impacts associated with excess water, there are also potentially positive ones too. In very arid environments, water recovered via dewatering can be used to feed the processing plant and, in certain cases, with the right treatment, can even be used in local municipal applications.

Itasca’s Brown spoke to this: “There are mining companies in Chile that depend on dewatering to supply around 30% of their process water. There are environmental permitting challenges associated with that approach but, most of the time, it becomes an important source.”

A MineDW model plot showing groundwater pore pressures around a sublevel retreat mining zone. (Photo: Itasca Denver)

What, Where and Why?

Before we delve into slope stability, let’s look first at the factors that typically determine the amount and type of water ingress.

SRK’s Pereira explained: “Most of the factors that determine the amount/type of water ingress are related to the source of the water. One is groundwater storage. In this case the water levels, storability and, indirectly, the hydraulic conductivity are key. Recharge water bodies such as rivers or lakes need to be evaluated in terms of connectivity to the planned mine. Sediments, rocks units and faults that connect the source of water and the mine also need to be characterized in terms of geometry and hydraulic conductivity.”

The mining method can also conduct sources of water. For instance, block caving and the associated fracture propagation can open a direct channel connecting recharge from precipitation to the mining operations; Grasberg mine in Indonesia is a typical example.

In open pits, the depth of the excavation, slope angle, and the rate of mining will help determine how aggressive a dewatering strategy is required. Hydrogeological conditions, such as localized and regional groundwater flow, and interactions between the surface water and groundwater are also important.

The local climate, and surface runoff from precipitation (the amount of which depends upon the duration, frequency and intensity of rainfall events) is another factor; one which, depending upon the mine’s location, could change as global warming accelerates.

“Climate change has altered the rainfall patterns in the in the north of Chile where we have started to see less frequent rainfalls, but with higher intensity,” Brown explained. “So, mining operations that were designed to manage groundwater that recharges from the high Andes, slowly flowing to mining works as they progress, these have been forced to migrate to a strategy that includes surface water management as part of their dewatering strategy.

“Some areas used to have just one millimeter of rainfall a year, but now they have ten millimeters a year, every three or four years. That’s a huge change when managing water.”

Water and Slope Stability

In simple terms, the presence of water in the voids of a rock mass works against the forces that hold it together. This can promote the movement of the rock mass, with the help of gravity.

SRK’s Evin explained: “If a slope has an elevated pore pressure, the likelihood of failure is higher when compared with a slope that has reduced pore pressure. Besides safer mining, mitigated pore pressure can lead to the creation of steeper slopes, less waste rock, less hauling and a reduced mine footprint. Based on SRK’s experience obtained from large open-pit studies, financial investments focused on dissipating pore pressure can return to the operation as cost-savings with a 1:5 to 1:10 ratio.”

Pore water pressure and its role in stability has been well understood since Terzaghi developed his theory of shear strength in the early 1920s. Practitioners are well aware that, in order to keep the slope stable at the desired angle, the only parameter that can be mitigated is the pore pressure. However, there are still loose ends in current applications when it comes to fully mitigating pore pressure for slope stability and design.

“SRK has seen various examples, ranging from completely underestimating the role of pore pressure to overvaluing it, which turns the case to a ‘tail wagging the dog’ situation,” said Evin. “At the end, it’s a geotechnical problem and you should understand the rock mass first.”

Itasca’s Lorig added to this:

“In general, water and high water-pressures lead to slope instabilities through something called the effective stress principle. It can also load vertical tension cracks, which tend to push slopes out,” he explained.

“So, the ability to minimize pressures from water are absolutely critical to the designs of open-pit slopes and, by far, the most cost-effective way to do it. The other option that we have is to flatten the slopes, the cost of which would be far greater than any dewatering scheme.

“That’s why studies of pit dewatering are fundamental in most mines to their slope stability. There are mines that are situated above water levels which don’t have to worry about it at all, but those are only 5% or 10% of the world’s mines.

“Most mines have some issues with water. It may be that water pressures are low, and they can be dealt with simply by sumping into the bottom of the pit and pumping the water out. Others require some form of engineered dewatering, whether it’s wells or horizontal drains, to achieve their geotechnical stability targets.”

In depressurization, the aim is generally to reduce pore pressure as far as economically and practically possible to ensure safety around slopes and access to reserves. There are some soils, for example, very weak weathered soils, that act like sand and, if you were to somehow remove too much water, would lose apparent cohesion, also causing instability. But these are the exception rather than the norm.

Borehole drilling for field hydrogeological investigations. (Photo: Itasca Denver)

Dewatering System Design

Identification of the main factors that determine water ingress into a mine will define the dewatering strategy and monitoring system required.

“An early recognition of these factors is very important, as they will define the monitoring plan from pre-dewatering to post dewatering-conditions,” explained SRK’s Pereira.

Piezometers and flow meters are generally installed in key areas to monitor the effectiveness of dewatering.

“In Chile, most of the mines have a strategy that is based on pumping wells to try and avoid water entering the pit, which helps reducing pore pressures in pit slopes,” said Brown from Itasca. “And, in cases where rainfall is significant, surface water channels are added to divert runoff from the pits.”

How would this contrast with the dewatering strategy for a mine in a tropical setting? E&MJ wondered.

“One of the projects we worked on receives over two meters per year of rainfall,” Liu said. “People automatically assume that means there is a lot of water recharging the groundwater systems. But actually, with rainfall of that intensity, most of it becomes runoff. A lot depends upon the competency of the ore and its hydraulic conductivity.

“On the other side, at one of the projects we worked on in the Congo, groundwater recharge is a really big issue. The water just runs into sinkholes and recharges that way. We had to identify those locations and put in quite rigid dewatering wells to try and intercept the water before it got into the pit.”

Whatever strategy is selected, it’s important that dewatering performance is monitored and adjusted regularly according to the assumptions made in the conceptual model.

“The dewatering strategy should be revised every time the mine goes through a major change,” Brown explained. “For example, if a mine moves from open pit to underground operation, or if climatic conditions are expected to change significantly.

“But, if things remain the same, it’s probably necessary to revise your strategy once every two years. Especially considering the CAPEX and OPEX involved.”

Lorig added: “Another time that you would consider updating analysis and modelling would be when your instrumentation suggests a significant variation from what is predicted. Mines are continually collecting data, so it may just be a matter of getting the new data into the simulations to update the predictions.”

Liu chipped in: “It’s really site dependent. It’s up to the mine and consultant to determine the frequency of updates to the dewatering strategy.”

Data Quality and Accurate Modelling

In recent years, advanced software and modelling tools have made it possible to achieve higher resolution in site hydrogeological conditions, and improvements in computational speed and cloud services have enabled modelers to test multiple dewatering permutations in a short period of time. This is especially useful for sensitivity and uncertainty analyses that contribute to important decision making.

However, the accuracy and predictability of models still depend heavily upon the quality of the data available, the experience of the modelers, and how realistic the model is in representing site hydrogeologic condition and operations.

Itasca’s Liu explained: “There is a misconception that advanced software means we have a higher confidence level in the model result,” he said. “That’s not usually the case. Some models, even though they look very nice and have been well presented, are ill conceptualized; they do not accurately represent the hydrogeological conditions at the mine site.

“For example, if I have an open pit, the pumping well should sit a certain level below the open pit, and the model margin should be much lower than the pumping well. Instead, some models put the model margin near the pumping well or above it. So, those are the ill conceptualizations.”

Brown agreed: “It’s true that software has improved and developed a lot along the years, but the most important thing is to have the correct conceptual model. If we don’t achieve that, we don’t achieve a realistic understanding of hydrological and hydrogeological factors affecting the mining operations.”

SRK’s Pereira supported this: “Numerical tools as codes and software give us the possibility to recreate the hydrogeological conditions and predict the dewatering requirements and their potential impacts in great detail,” he explained. “Finer meshes/grids in areas of concern (mine, tailings, rivers, lakes, etc.) can simulate water levels and flows in high resolution. However, a robust conceptual model supported by field data is still the main factor for an accurate prediction.

Dewatering well located at the bottom of an open pit mine. (Photo: Itasca Chile)

Avoiding Common Pitfalls

E&MJ asked if there are any other common mistakes that mines make in their dewatering strategies and, if so, how can these be avoided?

“Common mistakes include not starting dewatering early enough, ignoring discharge quality which leads to more significant treatment requirements, or not having appropriate staff on a project during operations to interpret, manage and make the system work,” SRK’s Mackie said. “Schedule delays during mining due to higher than expected flows can also be a significant problem for mine economics.”

Pereira emphasized: “Early monitoring programs, even during the exploration phase, are key to obtaining valuable hydrogeological data. The impacts of dewatering and the prediction of inflow into the mine are more accurate with long-term transient information.

“Resource exploration, infilling or geotechnical drilling programs can all be used for groundwater data collection. This usually represents a small fraction of the total cost and will save time and money for the mine dewatering design and closure program.”

The team at Itasca stated similar findings.

“Most of the time, there’s not enough time allowed for dewatering and depressurization,” said Liu. “Starting dewatering and depressurization early will significantly reduce the associated risk.

“The second issue is inadequate monitoring data. Sufficient monitoring is a critical factor for improving dewatering performance.

“Third is conflict between production and dewatering needs. The location of dewatering infrastructure, especially within pits, is chosen to allow the mining method to be as aggressive as possible. However, these locations can sometimes conflict with the mine plan.

“It’s crucial that all teams understand the importance of dewatering to help avoid or resolve those conflicts.”

Lorig added: “One of the biggest mistakes mines make is not following through on plans to install wells, instrumentation, connect those things up and pump the water out.

“Dewatering is sometimes seen as an expense that can be nibbled away at. Mines that don’t understand or appreciate it and relegate its’ level of importance… they’re the ones that can get into trouble.

“And it’s very hard to recover from, because dewatering is something that needs to be done in advance of the mining, not in hindsight.”