Published in the December 2009 Coal USA Magazine
For those who work in the mining industry, hearing about a roof fall can be devastating. Thoughts race through the mind: Was anyone hurt? Did everyone make it out? How will the mine continue to operate? What can be done to prevent this from happening again?
Because of the serious and potentially critical nature of these incidents, it is imperative that scientists continually study the forensics of each occurrence in the most thorough manner possible.
Forensic engineering includes the investigation of materials, products, structures and components that fail or do not function as intended in a specific application. We define forensics in ground control engineering as the study of evidence discovered while examining underground roof falls to determine the probable cause, to hypothesize solutions and, finally, to design engineering safeguards to prevent recurrences.
In general terms, roof structures are similar to a spider web, in that one or more important design flaws can be present without necessarily causing a collapse or failure. However, if the combination of design deficiencies becomes more than the structure can handle, the structure can, and often does, fail.
After a roof fall, it is common practice for immediate steps to be taken to danger-off and secure the area while safe, efficient clean-up efforts are designed and executed. However, two critical actions are often overlooked: (1) determining the cause of the roof fall; and (2) determining areas that have similar symptoms and may be susceptible to the same outcome.
Primary steps for ground control forensics
Ground control forensics comprises five major steps. Each requires observational or hard data and can take anywhere from a few minutes to more than a day, depending on the complexity of the failure and the difficulty encountered in obtaining solid data that can be analyzed.
Step 1: Assess regional and local conditions that were present prior to the event.
After the area of a roof fall has been secured and appropriate safety measures implemented, the initial examinations are often the most critical. A quick assessment of the mine’s design fundamentals should be undertaken at this point.
- What were the designed widths of the entries, intersections and pillar sizes? Both inby and outby the fall area, actual measurements should be taken and projected to the unmeasured boundaries. Also important is the examination of the excavation geology and stratigraphic sequence, paying particular attention to the immediate roof and bolted horizons. A simple assessment of roof rock strengths as weak, moderate or strong is recommended.
- Are there any geological structures in the area or signs of mining-induced or horizontal stresses? Do stress fractures, slickensides, vertical or horizontal roof displacements and shear planes appear in the immediate roof? Roof cutters (as shown in figure 1) are often associated with high stresses.
- What is the depth of the excavation and the condition of the pillars? An examination of the immediate entry areas around the roof fall will permit development of answers to the same group of questions. Additional important information to be gathered at this time includes mining cycle times: how long does it take to support a cut after it is excavated? Also, what is the excavation method and cut depth? Lastly, is there a specific roof horizon that the continuous miner should stay under?
These managerial considerations can be very important when the final hypotheses are created.
Step 2: Assess post-event conditions.
This is the portion of the assessment that addresses examination of the bolting or standing support systems used in the immediate and adjacent areas of the fall. Consider:
- Are the roof bolts bent?
- Are the components intact?
- Is there a “show” of resin?
- What were the lengths, diameters and grades of the bolts used?
- How many bolts were installed to support the area, and were they installed according to the approved roof control plan?
These issues can be addressed and estimates made by examining the bolting patterns and support installations that are in the immediate section and entry. This phase of the assessment is often ignored or completed in a cursory fashion.
When conducting a comprehensive assessment, it is important that the appropriate amount of time is taken to complete these steps.
The examination of the support system is associated more closely with failure analysis. If timber, bolts, plates or other support system components appear to have contributed to the fall, then a safe attempt should be made to obtain samples or specimens.
Trained personnel can often use observational techniques to determine if the bolting systems have been installed correctly.
Step 3: Hypothesize plausible ways in which the pre-event conditions resulted in the post-event conditions.
Developing hypotheses is crucial to determining the cause of and contributing factors to a roof fall. In developing hypotheses, it is important to consider the mine design parameters, including geology (complete with weakness features and structure), the method of support and excavation, and the mining cycle times.
Mining cycle refers to the time between the excavation and support cycles. Excessive stand-times without support often lead to progressive-type roof failures and can have a dramatic impact on roof-bolting effectiveness.
During this step, at least three possible hypotheses should be created. They don’t have to be perfect or complete; they can be missing unexplained or unavailable data.
The goal during this phase is to create realistic scenarios using the information that is available to explain why the roof fall occurred. Creating these scenarios is often an interactive exercise involving operations, safety and engineering staff.
