Designing a cost effective mine stabilization program requires significant interaction between the risk manager and the Engineer who should be an expert in mine subsidence engineering. The remediation choice made by the risk manager based on options proposed by Engineer can mean easily $100,000’s to over a $1,000,000 reduction in project costs on a typical project.
When evaluating mine stabilization options, the risk manager decisions depend on the comfortability (i.e. reliability) on the Engineer’s opinions and thus their qualifications. This is important because the risk manager must rely on the asserted damage potential for the different grout options proposed by the Engineer for the lifespan of the project. Therefore, qualifications of the Engineer, risk acceptability of the risk manager and a good understanding of the damage potential all play a crucial role in determining the best decision for the project.
The process of designing the most cost effective mine stabilization plan involves a number of important steps and is summarized in the workflow chart shown in Table 1. It involves determining which mine areas under the structure are unstable and require stabilization. This typically includes subsurface mine investigation and mine stability analyses. In areas where the stability of the mine is an issue, a cost versus risk (damage) analysis is then performed considering various mine grouting options. The Engineer should provide the risk manager mine stabilization options with the relevant cost-benefit data. For example, two very important aspects of cost-benefit analysis are the grout methodology and the amount of subjacent buffer that should be grouted and stabilized around the protected structure. Different mine grouting methodologies are discussed in the article entitled Anatomy of Mine Grouting Voids. Establishing the buffer should be determined by comparing the grouted buffer width around the protected structure to the damage potential and associated costs. This is demonstrated in Figure 1 which illustrates the chance damage spectrum (CDS) and directly relates to the associated cost for a stabilization project.
After discussion with the Engineer, the risk manager then selects the cost-risk option which is most acceptable to them. Then, with the mine stabilization option selected, the Engineer optimizes the mine grouting approach and produces the plans and specifications for the project from their experience on the performance of previous mine grouting projects. Plans and specifications for mine grouting are written with a wide range of established performance-based[1] requirements.
FIGURE 1 ILLUSTRATION OF NECESSARY MINE STABILIZATION DATA TO PERFORM A COST-BENEFIT ANALYSIS
When there is an emphasis on the performance specifications there is less control on: how the grouting will be accomplished, quality of the product, owner liabilities, and claims from unanticipated work conditions.
With copyrighted plan and specification which have been through over 30 years of mine grouting experience combined with MEA’s over 40 years experience with dispute resolution proceeding. From our experience, with these plans and specifications claims are difficult to obtain even when significant loses occur by the Contractor.
[1] Performance based specifications consist of naming the product requirements without identifying procedural requirements on how to get the product. Procedural requirements control how the product can be constructed.
There are vast areas of undeveloped land which exist over underground abandoned coal mining that can be potentially used for solar farm development (see Figure 1). These land use areas can be economically feasible for this purpose even when accounting for any future land subsidence resulting from mine collapse. Therefore, prudent due diligence requires an expert analysis on how much damage could occur.
It is MEA’s experience from previous project investigations that even solar development was viable with the identified subsidence risk and predicted damage.
The economics of subsidence damage depends on the predicted number of subsidence events which would result over the lifespan of the farm and the amount of associated damage to the farm infrastructure and whether the damaged element is repairable. Therefore important elements of the economic feasibility of the farm against mine subsidence are:
The resistance of the mine structure to collapse across the project site (i.e., more resistant leads to less collapse over time).
Severity and extent of the surface subsidence across the project site.
The damage thresholds of the various farm infrastructure to those subsidence movements. For example, a significant part of this subsidence damage analysis is assessing the subsidence interaction of the tracker whose piers would be exposed to a range of horizontal and vertical movements.
FIGURE 2 A SKETCH ILLUSTRATING INDUCED TRACKER BEHAVIOR FROM A SINGLE SAG EVENT OVET AN ABANDONED MINE
This is illustrated in Figure 2 when sag subsidence is expected.
The extent and intensity of the damaged farm areas across the project site.
Moreover, based on the site specific conditions, the economics can be improved through Kaizen analysis and mitigation measures taken to reduce the expected level of damage. To understand more about mine subsidence risk see Engineering Update #14 – Establishing Mine Subsidence Risk.
A critical aspect of a site’s economic viability is the geotechnical related site risks and any ground mitigation measures to the risk(s) that needs to be taken. Therefore, in the initial stages of significant land development project, a proper initial evaluation of these geohazard factors are vital to understanding the feasibility and necessary investment into the site for the proposed construction to take place. The geotechnical risk and any needed mitigation of those risks fundamentally relates to two factors:
Nature and frequency of ground movement events over the course of the life of the project, and
The tolerance of the proposed structure to the expected ground movements.
Therefore, the assessment of the economic feasibility of the development of a site depends on the geotechnical investigator’s foresight into the critical ground conditions in the early stages of the project. And in turn, the selection of a qualified geotechnical investigator is critical to avoid significant misuse and misdirection of land development funds. At a minimum, this geotechnical investigator should be able to identify early on all the potential sources of ground movement that could have a significant
impact on site development. Secondly, this investigator should be able to prioritize their investigation and sufficiently quantify the level of importance of those risk factors have on site development. For example, investing into extensive foundation design, prior to evaluating the potential of land subsidence would be putting “the cart before the horse”. The potential subsidence damage may be found too great even when mitigation measures are taken to develop the site. Or alternatively, the subsidence damage potential is within a viable range and now the foundation design phase should proceed.
A slope failure during construction of a subdivision that occurred due to unstable slope
A sag subsidence in a farm area that prevents harvesting
Finding the appropriate geotechnical engineering expert can be difficult decision for an attorney or the less technically adept given the multiple qualifications which are necessary. Probably the most difficult aspect to assess is the engineering capability of the expert. Moreover, the more serious the case, the more effort is typically invested in selecting the geotechnical engineering expert. There are four main avenues that are taken to find a geotechnical engineering expert:
1. Expert recruitment firms,
2. Expert listing sites,
3. Word of mouth references, and
4. On-line research.
Expert recruitment firms provide an expert candidate(s) for the specified need of a case. These firms have a database of experts from which they select who they deem the most appropriate. They then contact the candidate(s) to determine interest and the expert’s knowledge and experience for that particular case. If the candidate is interested and deemed qualified, the expert is referred to the potential client for an interview. The list of experts maintained by the recruitment firm is screened by the firm to varying degrees and is a good question to ask how this list was built and maintained. These firms profit typically by adding a surcharge to the expert’s fees.
Expert listing sites provide a list of potential candidates who advertise their geotechnical engineering expert services on the site. These experts pay a periodic fee to the listing agency. Therefore, these are essential ads and require more discretion from the inquiring party, however, the charged fees are directly from the expert and thus not surcharged as above.
Word of mouth referrals have been utilized since time immemorial. Probably the best geotechnical engineering expert referrals are obtained from a recommendation(s) from a respected expert witness(s) that has worked with that expert geotechnical engineer. Such expert referrals should come from experts in an associated field, such as, a structural engineer, a civil engineer, a construction claims specialist, a mining engineer, or an engineering geologist. Given their familiarity with the field, they provide reference(s) based on the geotechnical engineering expert’s capabilities, and if they have worked with them as an expert witness, their performance in a dispute resolution setting. Another source for a referral used are colleagues that have worked with a geotechnical engineering expert witness. These references would be more based on the expert witness performance aspects and less on technical capabilities of the geotechnical engineering expert.
Another primary method used in selecting a geotechnical engineering expert witness is to perform an individual online search. Such searches result in company ads as well as typical companies which perform such services. Information which can be obtained solely based on such expert witness searches would be related to geotechnical engineering qualifications of company engineer which would likely be evaluated by the non-technical solicitor. This type of search probably provides the least amount of information regarding the individual expert witness performance in dispute resolution settings.
Expert witness qualities to look for fall into three basic categories which are the:
1. Geotechnical engineering capabilities,
2. Expert witness performance as discussed above, and
3. Expert witness integrity.
With respect to the technical capabilities, the geotechnical expert witness should be evaluated as to whether the candidate has sufficient experience in the known aspects of the subject case, but also other potential case issues, because often times, forensic investigations reveal other geotechnical issues which heretofore were unknown. Consequently, a geotechnical expert with a wider experience perspective and base should be preferred. A more specialized geotechnical engineering expert may otherwise missed unrealized aspects of the case.
With respect to geotechnical expert witness performance, the main qualities to look for are the experts verbal and written communication skills, dispute resolution experience, and educator qualities. It is also important to assess the expert geotechnical engineer’s integrity. For example, in the past has the expert performed scientifically sound investigations which hold up under cross-examination: Has this potential geotechnical expert witness been excluded from testifying as a result of a Daubert challenge? This quality becomes increasingly an issue with the strength and amount of cross-examination or resistance which is usually proportional to the size of the case.
When interviewing a potential geotechnical engineering expert, it should be done on a Zoom or similar platform. Have the geotechnical expert explain to you some of their more important cases he/she have been involved with. This is an opportunity to evaluate the expert’s communication skills and whether or not the expert can explain complex matters to conceptual understanding to you. Avoid those that appear to “give you the opinions you want” without having examined any significant project information. Where more “high profile” cases are involved, ask the geotechnical expert about the size of the cases he/she has been involved in. It is important that your expert geotechnical engineer has the experience to handle the potential level of scrutiny that he/she will be under in such cases.
Another decision which must be made is whether the expert fees are reasonable for the forensic investigation. Keep in mind that geotechnical experts, especially in larger cases, will use associates or staff to perform certain tasks. This can be a cost-effective approach when their fee rates are lower. Because of the nature of forensic investigation, it is difficult to rely on a lump sum estimate. The ultimate costs will be dependent upon the expert geotechnical engineer’s judgement of effort which is necessary given the nature of the case as it becomes apparent. Given the geotechnical engineering expert’s qualifications and experience their fee rate can be evaluated by comparing it to others with similar qualifications. This can be done by comparing rates of similar experts based on your and others with the appropriate knowledge base.
The most appropriate mine subsidence expert for your case depends on the nature of the problem to be investigated. Mine subsidence investigations can require various expertise depending upon the focus of the problem. Some of the questions that may need answers are:
• Is the damage from mine subsidence?
• Will the underground mine result in subsidence in the future?
• If subsidence were to occur, what is the range of movement you would expect?
• If subsidence does occur, how severe could the resulting damage be?
• Was the mine designed properly so that mine subsidence would not result in the future?
• How can we mitigate the subsidence risk to a tolerable level?
• Can you stabilize the mine and how much would that cost?
As you can see from the above, the subject of mine subsidence can actually involve a number of expertise depending upon focus of the investigation. Also, there is the context of the expert mine subsidence investigation: Is it being done for existing or new construction, mine design, review of a mining permit application, or is it for tort litigation? Therefore, in addition to having the technical know-how, and expert with oratorical skills may also be necessary.
Designing a cost effective mine stabilization program requires significant interaction between the risk manager and the Engineer who should be an expert in mine subsidence engineering. The remediation choice made by the risk manager based on options proposed by Engineer can mean easily $100,000’s to over a $1,000,000 reduction in project costs on a typical project.
