Partner: Brierley Associates

In 1868 the Union Pacific Railroad reached the newly incorporated territory of Wyoming. Beneath the desolate steppe, covered in low shrubs to the horizon, the railroad company had just discovered the industrial future of Wyoming – coal.

The deposits beneath Wyoming were – and are – vast. Some seams are up to 30ft (10m) thick and today Wyoming produces 43% of the coal in the United States. But the legacy of historical coal mining is also one of unstable voids, structural cracking, and sinkholes.

For this episode we will be looking at one town in the appropriately named Carbon County of Wyoming, the town of Hanna.

The Town of Hanna

At its peak Hanna produced around 40% of the total coal in Wyoming. Given its population, which even at peak was just over 2,000, the town has a good claim to be one of the largest per capita contributors to the US industrial machine.

Today, with many local coal mines no longer in operation, it faces another great test. The town has been left with a legacy that it will be working to contain for decades to come. But in this it is not alone. It is one of many locations that are the focus of a federally funded programme to mitigate long-term geotechnical problems – problems related to ‘Abandoned Mine Lands’ (AML).

In Wyoming, the AML Division of the Department of Environmental Quality is responsible for going around the state and developing an inventory of ground damaged by coal mining activities. It also contracts geotechnical experts to assess the ground, and then, if necessary, carry out mitigation works. This work usually involves void-filling and stabilisation.

Acts of government

All the above work has largely been funded by monies made available from the Surface Mining Control and Reclamation Act (SMRCA) of 1977. This imposed a levy of 28 cents per tonne on present day coal production. This went to the Federal Government, which sent 50% back to states with AML programmes. But there was a flaw.

States with large historical coal mining, but no present-day activity, did not have access to these resources to repair the land damaged by mining. This meant that some states had advanced and well-funded programmes while others languished.

All that changed on 15 November 2021 with the passing of the Bipartisan Infrastructure Law. It reauthorised SMRCA for the coming 13 years and dropped the coal fee by 20% to 22.4 cents per tonne. However, it also provisioned direct funding from the US Treasury for AML programmes that is not tied to coal production.

This means that a lot of new regions are going to have the resources to undertake AML work that has been needed for a long time. And a lot of eyes are turning to the experience of Hanna, and the abandoned mine lands of Wyoming.

Sharing the knowledge

Some of the problems associated with AML vary from state to state. Don Newton is the administrator of the Abandoned Mine Land Division of the Department of Environmental Quality in Wyoming.

“We don’t necessarily have as big a problem as some of the other states such as Pennsylvania or West Virginia. For example, Wyoming has low sulphur coal, where Pennsylvania has much higher sulphur coal. And that is a huge problem for the surface waters,” says Newton.

When groundwater moves through historic coal mines with a high sulphur content, then emerges into local streams, it creates what is called ‘acid mine drainage’. Pennsylvania is also a state that has enormous historical mining, but less in the present.

The problems in Wyoming are typically related to subsidence, where the underground mines collapse, and then that collapse works its way up to the surface and causes damage to homes and businesses and infrastructure like roads and bridges.

The AML division has its inventory of sites that includes old gold mines 11,000ft (or 3,300m) above the tree line and it has large voids under critical infrastructure in towns. Prioritising the sites is done based on risk to people and property. Some of the remote sites are still dangerous if they are in popular leisure spots.

“Our primary purview is to reduce the risk to human health. So that’s our first and highest priority,” says Newton.

Mitigation

The first step in a new round of study is to call in a specialist consultant, in this case the geotechnical consultant and tunnel engineering company Brierley Associates. Dave Hibbard is the Subsidence Mitigation Team Leader at Brierley, and he leads the work on-site at Hanna.

“Mining took place all over Wyoming, and the geology varies enormously,” says Hibbard. “With the ancestral Rockies that exist throughout a lot of the Wyoming area, obviously, it added to a lot of structure, a lot of geologic structure in place, which adds to a lot of the dynamic challenges that we see with a lot of the mines that we investigate that have any type of faulting, or bedding dips, strike planes, and stuff of that nature.”

These are different types of split, or orientations of the geology in a particular place. So the geology is challenging.

“For subsidence to occur generally, what’s required is destabilisation of the overburden rock. So this could equate to historic underground mining, disillusion of limestone carbonates, excessive groundwater extraction within an aquifer,” says Hibbard.

In Hanna’s case, historic coal mining is the primary mechanism.

“So the risk ground from these events have led to infrastructure failure of public roads, highways, columns, utility corridors and public open space. And this is caused essentially by a failure of competent rock that manifests itself up through the surface and re consolidates and settles eventually over time.”

In other words, if there is no rock underneath that was once holding up on the surface, eventually that will fail and propagate upwards, leading to a sinkhole event. This can be very difficult to predict. One of the primary challenges of prediction is just how far below the surface these conditions exist. The shallower they are, the sooner a failure is likely. And in Hanna, everything is in place to require a lot of mitigation work.

“Hana has always been known as a very large mining town. And at one point was one of the largest mines in the US for a short period of time. So obviously, they had the full recipe for subsidence to occur. Abandoned mine, over 100 years old, lots of coal being extracted. And most of the coal extracted was from a relatively shallow depth.”

And with the entire town effectively above the mine, the Hanna was one of the primary locations in Brierley’s investigation of south-central Wyoming.

“Initially, there was a lot of data sleuthing of mining history, geology, prior mitigation and current infrastructure development,” says Hibbard.

This also included a lot of digitisation of historic mine maps, researching historic subsidence events, and ground truthing – or verifying information against real world conditions.

“During our first investigation, we requested to investigate several depressions on the Hanna Elementary School football field. These Initial investigations led to a drilling and geophysical programme, where additional research was collected to understand some of the engineering qualities of the ground supporting the infrastructure above it.”

And once they started receiving some of the lab data back, they realised that they were dealing with a lot of shallow, weak rock supporting a lot of the town’s public infrastructure.

“Lots of shallow mine workings and lots of critical public infrastructure that existed. So, after we put together all the data, it was clear to us that this was a high-risk area that affected a lot of the local residents at the time.”

At the end of the investigation, Brierley had to put together a ‘Report of Investigation’.

“Which consists of all the drilling data that we collected, consists of some geophysics that were conducted, we do a lot of remote sensing, as part of our investigation to really get a visualisation on the ground,” says Hibbard.

These days this can be done from the air via drone, and a lot of the work at Hanna has been characterised by the use of modern technology.

“Once we kind of put all that information together, we have a recommendation portion of the report that we would provide to AML. And we’ll either put it in a risk category –either low or high risk.”

And for Hanna, the risk category given was very high due to the sheer extent of the mine works and this shallow depth.

Using the tools available

Hibbard says that when dealing with underground hazards, it is essential to make use of all of the technology available. And the biggest issue is the fact that in geohazard investigations, often it is difficult to detect or see.

“Technological advances to help you get a gauge on what you’re dealing with. With Hanna in particular, not only were we dealing with open void space… but a lot of open void space. This can be 40 feet (12m), with 30 by 25ft rooms. It is a very big mine that also had flooding, so there is a huge groundwater component. And with the right topographic conditions, these reservoirs can develop artesian conditions.”

Artesian conditions mean the water will emerge at the surface without the need for pumping. Groundwater in conjunction with large, shallow mine workings is a problem and is something that must be dealt with in design, because it is beyond what normal mitigation can deal with. If uncontrolled, the results following a poorly-planned mitigation programme can be dramatic.

To stabilise the surface, it is necessary to fill these voids left behind by mine works. But filling in every void in Wyoming, or even in Hanna, would be prohibitive. Hibbard estimates that 2,000 acres (800 hectares) of the town has been undermined, so the team had to refine the mitigation area based on the risk to people and infrastructure.

“This is a fairly costly endeavour, and we generally have target areas ranging from five to 15 acres at a time. We start shallow and progress to deeper levels of the mine. This mitigates the higher risk areas first,” says Hibbard.

As for techniques, mitigation for subsidence has been going on since the 1980s. Primarily this is backfilling with a cementitious grout.

“And backfilling is pretty difficult to do, because it consists of drilling and injection of backfill material. So when you’re drilling for mine workings, you have got to plan out a series of bore holes that align with the mine maps that exist, and within the mine extent. And once you find those mine workings, target them with a pre-engineered grout, that is designed for the downhole conditions,” says Hibbard.

Back in the 1980s there was an historic mitigation programme that was conducted in Hanna, but in some respects misunderstood the problem. The teams backfilled with a sand slurry, but as it was not cementitious, it did not bind together properly, which causes problems today. Brierley has found about 50% of it still in place, with the remainder relocated through groundwater transport to other areas. Stabilisation of the sand slurry is an additional challenge.

Challenges to intervention

Even with modern technology and methods, there are risks to be aware of when preparing a mitigation strategy. A lot of the risks relate to the fact that grout is being injected under pressure.

“We inject it under pressure because we want a lateral spread underneath the ground and within the mine, that way we can influence more of a mitigation area downhole than from the surface,” says Hibbard.

Essentially it is possible to cover a 500ft (150m) radius area from one hole, injecting under pressure, depending on where they are in the mine.

“With injecting grout underneath pressure, there’s the possibility of deformation of the ground, and a lot of ground movement can occur if you’re not careful. So we have specifications for pressure criteria that that we utilise to prevent that. However, with everything as dynamic as it is, pressure can spike relatively high, or you can generate negative pressures in some cases.”

But in general ground deformation or movement is a high risk. In trying to support the surface infrastructure, a poor plan or inexperienced team could wreck it.

Then there is that artesian water issue. It could discharge to the surface through a conduit fault or even a different borehole.

“That could be millions of gallons of water that we could displace to the surface,” says Hibbard. “Controlling the groundwater is not necessarily a one-to-one – one cubic yard of grout injected does not necessarily mean one cubic yard of water displaced to the surface.

“We try to control the water to prevent an excessive amount being displaced, and if it is displaced we try to find a means of groundwater surface transport contained within an area where it’s safe for the public and has the tendency to re-percolate through the ground effectively.”

This is why it is important to be prepared for these events. In the case of groundwater, there are a number of ways of controlling it. These could be discharge wells, which allow the team to relieve groundwater pressure in a controlled manner. This prevents pressure build-up but also means that the mine isn’t completely dewatered, potentially destabilising it further.

Working in Hanna

The quality of the data is an enormous factor in a successful programme. And Hibbard says the quality of data is only as good as the initial investigation.

“Making sure that you have the geo databases, you have that the file hierarchy structures, making sure that you have all the individual components vetted for quality assurance is a huge deal in what we do. It takes a lot of time to ensure that the preservation of that data is intact, before we make recommendations. So that’s probably one of the biggest factors. And one of the biggest forms of caution.”

And a big part of what has made the work here a success is the Wyoming AML division’s desire to do each job properly, and not cut corners. This is partly down to experience, and partly to resourcing.

Newton explains, “One of the benefits that we have is we’re well-funded. And that allows us some flexibility into looking at new techniques, trying new techniques. We’ve got great engineering firms we work with, obviously Brierley being one of them. We work with a lot of engineering firms that you know, are willing to kind of push the envelope on trying new things.”

Sometimes this can mean failure, often it means success.

Newton adds, “I really think that Wyoming AML, doesn’t rest on [its] laurels – of what we’ve done in the past is the right way to do it – so we’re always looking for to learn from our neighbour, partner, our sister AML programmes around the country, because they’re doing amazing things as well.”

Wyoming has always taken the view that it should embrace new techniques and technologies, because when you are involved in AML work, there is a long time for this approach to pay off. Newton believes that the subsidence work it undertakes will probably go on forever. If the primary concern is to protect human health, infrastructure, people’s houses, you can do most of that and feel confident with it, but there is too much damage to be entirely fixed.

“We’ll never do the entire mind in any city like Rock Springs or Hanna. There’s just too much. So those surface subsidence features will propagate to the surface, and they will be happening in perpetuity.”

But Don says it is not just new technology that delivers success, it is also down to the help from the towns themselves. Normally on a project, engagement is about mollifying unhappy residents. But in Hanna and elsewhere in Wyoming, local people have been critical to the projects.

“Some of the best information we get about our sites is from local residents. That tell us: “well, did you know about this hole over here?” Or they show us a crack in a foundation or something like that. So, the local residents are key in not just keeping them happy about the work we do but having them informed about why we’re doing the work and why we continue to do the work.”

Outside of the areas where AML work is necessary, most people have not heard of these divisions. And in recent years Don, and other AML leaders have been trying to get the word out about the good work they do.

Restoring wildlife habitats, repairing open pits, putting land back into use for productive grazing… raising awareness has been essential for getting the funding renewed, and for getting money not linked to current mining production. Interested readers should check out their website: OurWorksNotDone.org

This article was based on Engineering Matters Episode #177 The Abandoned Mine Lands of Wyoming, click here to listen