What Risks Can Faults Cause in Underground Operations?

While encountering faults in underground operations is a problem in itself, getting out of control during production would be the situation which we really want to see last. Regardless of the production method you use and the commodity you operate, we are bound to encounter a fault that will cause a discontinuity on the mass, a slip, or, in the worst case scenario, loosening of the ground. While you tried to present the closest version of your coal or ore body to reality during modeling, unfortunately realizing that it is not like that during production with great differences disrupts all the plans.

In this article, we will discuss the fault activity, possible dangers and preliminary detection of structural elements in environments where high tonnage is produced per unit time, such as “longwall, block caving, sublevel caving”, where it is not possible to observe the underground space formed during production. We will discuss what can be done, taking as an example a coal deposit (Neogene) operated by the longwall method. Our aim here is to make you think about the relationship between faults and operation method chosen, and you can examine similar scenarios specifically for your business, also an example in porphyry deposits such as Au-Cu operated by block caving method. In the near future, with the increase in metal prices, it is likely that such block production methods will become widespread in order to be able to put into operation the deep low-grade deposits, and it would be useful to foresee the problems today.

The purpose of this article and animation is to encourage thinking in order to evaluate and foresee possibilities. Many factors specific to your operation will differ from those stated here. Before making a decision, be sure to consult experts with all the data you have.

Additionally, you can explore the related articles listed below.

Let’s go back to the beginning in order, list all the sentences starting with “I wish”, analyze them, and take them into consideration for our next projects (or actually);

I wish we knew about the big “slip” in coal (Or ore) before we planned the panels,
I wish we could have a team that constantly monitors geological elements during production and keep regular records,
I wish we had taken the offset / measurement of the field periodically (weekly) from the air (drone),
I wish we had placed the geophysical (seismic) lines and directions correctly,
I wish we had taken clear and clean photographs of the drillholes / cores and saved them properly,
I wish the drill cores were not covered with dry, hard mud plaster, so that we could see them clearly while logging,
I wish we had not drilled directly on the places where we expected faults to exist,
I wish we had drilled more frequently and in the right places by evaluating all the available data,
I wish we had recorded (logged) the faults and layers accurately in the drillhole logs,
I wish we could reveal periodic fault slips by examining the regional geology in 3D with all the data,
I wish we had modeled the coal layer / mineralization and faults in 3D in the best possible way,

If you regret any one or more of these items, while watching the video, it would be useful to stop and quick look at the article sections parallel to the following items. We will also outline the activities that need to be done in order to have a better understanding of your resource body and the faulting around it as early as possible. In this way, even if we obtain little data during operation, we can make a start to think about what the data we have means and whether it is sufficient or insufficient.

Cutaway Topics

You can also follow the topics in the video with the item number. It will be useful to stop the video at the point where the relevant topic title is found, watch the episode again, and re-evaluate it in detail.

1. Coal layers and basin development - Early Phase (Extensional Basin in Neogene)

Here we can see the basin that developed for and after the coal layers (Black layers). Faulting that develops in this phase may be less effective on rock strength than relatively younger faulting equivalents in later stages. In addition, since these faults have been covered by many sedimentary rocks until today, their probability of reaching the surface is relatively less.

2. Widening - collapse (Normal Faulting) and secondary faultings

In the basin system that continues to develop, normal faults due to extensional tectonics and relatively smaller secondary faults that developed parallel to them have initiated a period within the system.

3. Late Stage - Widening - Collapse (Micro Horst-Graben)

In some regions, faults and Horst-Graben structures, which are still active to this day and have low strength in the surrounding rock, show themselves clearly at this stage.

4. Late stage faults reaching / approaching the current topographic surface

The important point here is that these faults and their secondary surfaces may have reached the topography or may have been settled at very shallow levels. Considering that late stage faulting would theoretically have played an important role in the final shape of the topography, you can observe sudden topographic rises, drainages (streams or water outlets) or landslides in the field. We will examine how important this is in “item 15”.

5. Drilling Activities (Wide range exploration drilling (Ranges over 200 m))

Generally, exploration activities are carried out by vertical drillholes by making gridlines of large masses, many square kilometers wide as the crow flies, at intervals of several hundred meters.

6. Some of the drillholes intersects the coal layer at the point where the fault slips

With the results obtained from the sparse drilling programs carried out during the preliminary exploration phase, traditional hand-drawn coal contour lines have been replaced by computer-aided 3D design programs. Thanks to this convenience, the hanging wall and footwall contour lines of coal should be quickly drawn, and the points where there is no fault should be targeted by evaluating the mass in 3D with all the data, rather than the regions where the curves become denser. Otherwise, drillings to be carried out near the fault surfaces may make the mass appear much thicker than it is at that point or, on the contrary, very thin, due to slips. In this case, it may lead to leaving the region early during production or spending extra unnecessary time in the region.

7. Raw model obtained from drillholes (Planning should not be made based on these isohypses / contour lines)

With rapid/implicit modeling, the available drilling data should be checked frequently, if possible before each drillhole plan begins. Nowadays, it is among the capabilities of every design program to quickly obtain the contour curves of hanging wall and footwall of coal using this model. Accordingly, before executing the drillhole plan, it should be ensured that drilling is done at the right point in its most up-to-date form.

8. Best points and directions for the lines of geophysical methods should be determined based on this raw model

Based on the model or isohypses obtained in the previous step, without cutting the fault surfaces obliquely, places of interest and believed to be at a serious level can be clarified using geophysical applications. In this case, the “seismic method” is the most common method that can be applied to detect deep discontinuities. It would not make any sense to evaluate geophysical applications alone. Because any data you obtain from geophysical methods will not provide a geological standard value anywhere on earth. Here you can expect the geophysical method to provide you with a contrast and show compatibility and continuity with the drillholes and field measurement data you have. Without geological data, no geophysical method will mean anything, so evaluate the correct geophysical method by applying it at the right point and direction according to the geological data you have.

9. Drillholes intersect "not only" the coal layer but also faults, logs must be completed carefully

Regardless of whether the drilling is steep or inclined, the drilling activity you start to intersects the coal layer will also intersects many fault zones on the way. In your 3D examination, you can capture these large faults from your drillhole records by holding one end of the rope and monitor their continuity. Remember, a plane passes through 3 points, the fault is a plane, you need to process the faults clearly in the drillhole logs.

10. Cores are coated with drilling mud and must be cleaned before it dries, otherwise the lithology cannot be seen

This clay-containing liquid, which we call drilling mud or fluid, is indispensable for drilling progress, is useful for getting the core, but is quite challenging in terms of visually recognizing the lithology after bringing it to the surface. If you are drilling for coal, it is inevitable that there will be a lot of clastic rock and clay in the basin, and when you add the polymer, bentonite/clay, “CMC” used for progress, you get a mixture similar to concrete plaster. When the cores are removed from the inner tube and laid out, if they are not cleaned properly, this mud, which dries more and more every minute, makes it increasingly difficult to see the cores. Because after a certain time, the brush will plaster even more rather than being of any use, and you will only be able to see the plastered cores by cutting them, which you cannot remove even with pressurized water. For this reason, it would be beneficial to ask the driller to wash the cores in the chute (temporary core chamber) or to take the core boxes to your coreyard / core shack and wash them as soon as possible. Sometimes it can be overlooked, it can be interpreted as lithology; if there is a catcher (core catcher ring) mark on the core, definitely wash the cores and boxes, there is definitely something underneath that you should see.

11. Drillhole core boxes should be photographed cleanly and clearly in their final state

As stated in the previous item in this article, the clean cores required for logging, that is, to be able to see and record the lithology data with the naked eye, must be photographed immediately before taking samples and recorded in a way that can be easily accessed later. Before the cores are photographed, they must have the appearance of polished fresh rock in order to make standard textural and mineralogical descriptions of the rock. We achieve this by moistening or wetting the cores. Too much wetting will cause localized shine in the photographs, while too little will cause them to appear mottled and different from what they are and will be misleading. There should be no flashing light in the environment you photograph, there should be no shadow of the boxes or core, and the lighting should be standard. The focal point should be wide if possible, and the middle point of the photo should be the middle of the core box. Photographs that are not taken properly will bring great regrets. If you are thinking of doing this job, doing it properly will prevent loss of time. Core photographs will be very useful in modeling and will save you from a lot of trouble for retrospective lithology matching and areas where there is a dilemma.

12. Fault locations and positions are much important in production with longwall mining, block caving and sublevel caving methods

Remember that you will not be able to observe or will have limited observation in the underground openings you create with these methods, so there will be no data that you can confirm, develop and detail while production continues. Before planning production, remember that you will encounter all the problems with the data you obtained already, evaluate all the data in the most efficient way to predict all structural elements and clarify the faults in order to predict the problems.

13. Late-stage or surface-reaching faults may cause uncontrolled production in underground openings

Late-stage faults are zones that are very likely to contain and transfer water because they approach or can reach the surface and have high permeability at the groundwater level. In addition, these fault zones are the most recent discontinuities and are often areas where low adhesion is observed in the surrounding rock due to their unconsolidated nature and recent movement. Blasting in these areas will cause block slides along the surfaces due to low adhesion, block blockages in underground openings, or problems in the blasting technique due to clay and water content. Such problems you experience may cause difficulty in controlling production.

14. The material collapsed during production may slide as blocks towards the underground opening without having the opportunity to swell

The swelling factor is very important in underground openings that are large and fast created during production. Loose coal or ore material near faults can be expected to liberate and flow faster than normal during caving. In addition, it is quite possible that the mass in the upper layers (waste rock) along the relatively younger fault surfaces mentioned above (item 13) may migrate and slide downwards as a block. Unlike ordinary cavings in production, the air or gas mixture in the underground opening / space is pressurized into the mine with sudden block movements that are impossible to observe. Even though it is unlikely, it is a potential danger for operations carried out in regions where fault zones are important for your operation. It is extremely important to record compressed air discharges with minutes and, if possible, to measure the gas content in the compressed air at the time it occurs. Even if it is extremely unlikely, the flammable gas content in this sudden compressed air may cause an explosion due to pressure in a closed area.

15. Subsidence can develop along fault zones

When you evaluate the faults and topography data that you measured underground in 3D, modeled with drillholes, and confirmed with geophysical methods, you can determine that subsidence occurs along fault zones that have developed especially in the late phase and are located close to the surface. The important point here is that the location of subsidence may not be vertical / as the crow flies above the production. For this reason, even if you do not plan vertically below residential or active use areas when making your underground production planning and panel plans, a fault with a certain dipping angle that coincides with another panel you produce may cause subsidence to occur at a point you cannot predict on the surface. No matter how large your license area is, you can take quick and periodic topography measurements using aerial vehicles (drones). With flights to be made weekly or at the time intervals you specify, you can easily detect topographical differences in a few minutes as a digital terrain model (DTM) in your 3D design program and take precautions without wasting time. It is important to constantly compare all obtained by fault measurements and drilling data that you will receive during underground production. This work follow will be extremely useful for you to make sense of possible or current delays and possible dangers in your future productions.

Subjects discussed in this article may overlap with your mineral exploration, modeling, mining operation and business development issues and may provide solutions for those. However, remember that various factors specific to your business may bring about different challenges. Therefore, seek support from expert consultants to evaluate all data together in order to convert potential into profit most efficiently.

Should you have any questions regarding the articles or consulting services, please don’t hesitate to get in touch with us.

SITE MAP

CONTACT US

Before quoting or copying from our site, you can contact info@gmrtc.com

All elements (texts, comments, videos, images) on the GMRTC website (www.gmrtc.com) are the property of GMRTC unless otherwise stated, and are published to provide insights to interested investors, professionals, and students. Any detail that may arise during your process will affect the subject matter you are interested in on this website; therefore, GMRTC (www.gmrtc.com) is not liable for any damages incurred. It is recommended that you consult experts with all your data before making any decisions based on the information provided.