Fewer than 10% of the mines in the United States are underground mines. Given a choice, we’d always choose to mine by a surface mining method, as surface mining is less expensive than underground. Unfortunately, we don’t usually have a choice! Certain commodities are found predominantly deep beneath the Earth’s surface – too deep to consider surface mining; and notable examples include gold, lead, molybdenum, platinum, potash, trona, salt, silver, and zinc. Other commodities are commonly mined on the surface, but because of their value, they are deep mined as well; and notable examples include diamonds, metallurgical & thermal coal and copper. In recent years, a third reason for going underground has emerged: the commodity is located in a suburban or urban environment, and local zoning ordinances preclude a surface mining operation. We are seeing this primarily with limestone. Finally, as time passes, the reserves that are easiest to exploit have been mined. Increasingly, we are having to mine deeper and deeper, and under more adverse conditions. The following article from the Wall Street Journal illustrates this well: "Mining a Mile Down: 175 Degrees, 600 Gallons of Water a Minute." Can you imagine mining under such conditions? It is a great engineering challenge, and we will see more and more of this!
The development of an underground mine follows a similar process to initial stages of surface mine development. A site has to be prepared, office buildings, shops, warehouses, and mineral processing facilities need to be constructed. And as with surface mine development, the timing of the infrastructure will minimize any premature upfront cash expenditures. The significant difference between surface and underground development is access to the orebody. This is usually easy to accomplish in surface mining because the orebody is close to the surface, and often it is only necessary to remove vegetation, the soil layers, and a modest amount of overburden. By contrast, accessing a deep orebody can take considerable time, effort, and money. In many cases, we can access and begin mining a surface deposit within weeks, whereas it might take several months of even a year or more to access a deep deposit.
Underground Mining (7:19)
Transcript: Underground Mining (7:19)
Hello everyone, and welcome back to Introduction to Mining Engineering. I'm Dr. Shekhar Bhattacharyya, your instructor, and today we are going to explore one of the most fascinating and technically demanding topics in our field: underground mining.
Why do we go underground? Underground mining is often the best choice when a deposit is too deep to remove economically using surface methods, when stripping ratios become too high, or when the surface area above the ore is occupied by infrastructure, urban areas, or environmentally sensitive land. Sometimes, the geometry of the ore body itself makes underground mining the most efficient choice.
How do we reach the ore body? Before we can start extracting, we need a way to get underground. There are three common approaches.
One is the shaft, which is a vertical or near-vertical entry. It is used when the ore lies deep underground. Shafts can carry miners, equipment, and sometimes ore.
Then there are declines, or ramps, which are sloping entries that allow vehicles to drive in and out, usually at gradients of 1:7 or 1:8.
Finally, there are adits, which are horizontal or near-horizontal entries driven into a hillside. They are often used when the deposit is exposed at the surface on a slope.
In coal mining, you might see a two-compartment slope: one compartment for a belt conveyor carrying coal out, and another for miners and supply vehicles.
Once underground, the mine is laid out as a network of openings with specific purposes. Drifts, crosscuts, and raises form the transportation and ventilation arteries of the mine. Pillars are blocks of ore left in place to support the roof, and stopes are the voids created when ore is removed. Levels are horizontal slices of the mine, each connected by raises or ramps.
The main underground mining methods fall into three broad categories based on how the ore and surrounding rock are supported.
The first category is unsupported methods. These rely on the natural strength of the rock with minimal added support. Examples include room-and-pillar mining, which is very common in coal, salt, and potash mining. Rooms are mined while leaving pillars of ore to support the roof naturally. Pillars may be partially removed during the final retreat stage.
Other unsupported methods include open stoping and shrinkage stoping. These are used for steep, strong ore bodies and allow gravity to help move broken ore.
The second category is supported methods, where artificial supports such as timber, rock bolts, or backfill are added. Cut-and-fill mining is a good example and is ideal for irregular ore bodies.
The third category is caving methods. These use controlled collapse to break the ore. Longwall mining in coal operations is highly mechanized and can produce very high outputs. Block caving and sublevel caving are used in large, low-grade ore bodies such as copper and diamond deposits.
What are the most common methods in the United States?
In 2023, there were 464 underground mines in the United States. Of these, 234 were coal mines, and roughly the same number were non-coal mines. Room-and-pillar mining is by far the most common method. In non-coal mines, you will find a mix of room-and-pillar, cut-and-fill, and open stoping methods, with a smaller number using shrinkage stoping or caving methods.
Keeping underground operations safe is critical. The underground environment can be dangerous, so ground control is essential. We use rock bolts, mesh, shotcrete, and steel sets to stabilize excavations based on rock quality, stress conditions, and excavation size.
Ventilation is another key aspect. It removes dust, dilutes toxic gases, and provides fresh air. Mines use main fans, auxiliary fans, and regulators, often controlled by smart systems that adjust airflow where it is needed most.
Modern technology is changing underground mining rapidly. We now have autonomous loaders and trucks, remote-controlled drilling rigs, and real-time monitoring systems for gases, rock movement, and equipment health. Digital twin models allow us to simulate operations before committing to a design, which can be very expensive. These advances improve safety, productivity, and efficiency.
Looking beyond the ore itself, underground mining generally reduces surface disturbance compared to open-pit mining. However, it can still create issues such as subsidence and groundwater impacts. Responsible mining means monitoring these effects, engaging with local communities, and planning for mine closure from day one.
Post-mining land use may involve stabilizing the ground, sealing mine openings, and sometimes repurposing infrastructure for other uses.
To wrap up, underground mining is a blend of engineering skill, careful planning, and constant attention to safety. It allows us to reach valuable resources deep underground while minimizing our footprint at the surface. As future mining engineers, your role will be to choose the right mining method, design it safely, and operate it responsibly, ensuring that we meet society's resource needs while protecting workers and the environment.
That's all for today. I'll see you again soon. Until then, keep learning and stay safe.
Learning Outcomes
At the successful completion of this module, you should be able to:
- demonstrate knowledge of the four types of openings used to access an ore body for underground mining, and specifically:
- when each would be selected, i.e., the pros and cons,
- where they would be located, and
- the materials handling options for each type of opening after the mine is in production;
- demonstrate knowledge of the process / unit operations for developing shafts, slopes, drifts, and box cuts, including the type of equipment used;
- define, and/or draw & label as appropriate, the basic terms used to describe underground mining methods and activities - specifically, the common deposit and spatial, directional, and excavation terms (see text pp 79-83 for listing and definitions);
- sketch a generalized underground metal mine, for the purpose of illustrating the basic infrastructure of such a mine;
- name the three classes of underground mining methods, and describe the distinguishing characteristic of each class;
- demonstrate an understanding of the conditions under which one method would be selected over another. Describe the “defining” or “distinguishing” characteristics of each method;
- describe and sketch the essential elements of three major unsupported methods: room and pillar, a.k.a. stope and pillar, shrinkage stoping, and sublevel stoping. Name some commodities mined by these methods;
- draw a typical room and pillar coal mine layout showing mains, submains, and panel entries;
- describe the sequence of development and production for each method; identify typical equipment utilized for the associated unit operations; additionally:
- for the room and pillar method, describe conventional, continuous, and longwall mining,
- for shrinkage stoping, describe the more popular variant known as VCR,
- for sublevel stoping, describe the variant known as bighole stoping;
- describe and sketch the essential elements of the major supported method known as cut and fill. Explain what is meant by drift and fill, undercut and fill, and overhand cut and fill;
- describe the sequence of development and production for the cut and fill method; identify typical equipment utilized for the unit operations and auxiliary operations;
- explain the concept of the historical methods of square-set stoping and stull stoping, and in what very limited circumstances they might be applied today. Describe how cut and fill mining can be used in conjunction with another primary mining method to increase the extraction ratio;
- describe and sketch the essential elements of the major caving methods known as sublevel caving and block caving, as well as another important caving method known as longwall mining (for coal and noncoal).
What is due for Module 10?
This module will take us two weeks to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
| Activity | Location | Submitting Your Work |
|---|---|---|
| Read |
| No Submission |
| Complete |
| Canvas |
Questions?
Each week an announcement is sent out in which you will have the opportunity to contribute questions about the topics you are learning about in this course. You are encouraged to engage in these discussions. The more we talk about these ideas and share our thoughts, the more we can learn from each other.