Wind Farms Over Mined-out Areas

There are vast areas of undeveloped land which exist over underground abandoned coal mining that can be potentially used for wind farms. These land use areas can be economically feasible for this purpose even when accounting for any future land subsidence resulting from mine collapse. This feasibility depends on how much damage could occur, if any, and whether or not the damage element was repairable and not hazardous. Therefore important elements of the economic feasibility of a wind farm against mine subsidence are:
  • The resistance of the underlying mine structure to collapse across the project site. (i.e., more resistant leads to less mine collapse potential). See EU Issue #14 for mine subsidence risk as it relates to mine collapse.
  • Severity and extent of the surface subsidence across the project site.
  • The damage thresholds of the wind farm infrastructure to those predicted subsidence movements.
  • 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. With cost-effective mitigation measures in place against mine subsidence risk, wind farms would be a viable land use over underground workings. For more information contact aosouli@meacorporation.com.

Cost Effective Mine Stabilization

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 the Engineer can mean easily $100,000’s to $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 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, a very important aspect of cost-benefit aspect is the amount of subjacent buffer of the mine that should be stabilized around the protected structure. This is quantified 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 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.
For MEA, our copyrighted plans and specifications have been through over 30 years of mine grouting experience combined with our over 40 years of experience with dispute resolution proceedings. From our experience, with these plans and specifications claims are difficult to obtain even when significant loses occur by the grouting contractor.

FIGURE 1 ILLUSTRATION OF NECESSARY MINE STABILIZATION DATA TO PERFORM A COST-BENEFIT ANALYSIS

 

Solar Farms Over Mined-Out Areas

FIGURE 1

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.

Performing a Proper Land Development Feasibility Analysis

Massive subsidence in Florida

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:

  1. Nature and frequency of ground movement events over the course of the life of the project, and
  2. 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

 

Is the Design-Build Process Good From the Owner’s Perspective?

Design-build (DB) contracts have become more popular in governmental, commercial, and institutional fields of construction. This has been heavily promoted by the construction industry to property developers as a more cost-efficient means of constructing a project. The workflows for the Conventional and Design-Build project development are illustrated in Figure 1.

FIGURE 1 WORKFLOWS FOR THE CONVENTIONAL AND DESIGN-BUILD PROJECT DEVELOPMENT

The apparent main advantages of Design-Build over conventional owner-designer-constructor projects are:
  • Owner has a one-stop shop
  • Project is more cost efficiently designed
  • Project is more efficiently managed between all disciplines
  • Project costs are more controlled
These points are discusssed below.An advantage to Design-Build projects for the Owner is that it is a one-stop shop. After the prebid design is complete and the contract has been awarded, the project development is under one roof, or one contract holder. Although the Owner will still establish the goals and scope of the project, the contract holder then is responsible for all the project administration and any related liabilities. Under DB, the owner hires a team. Therefore, the Owner’s need to provide the necessary management is reduced. However, the Owner must select the DB team based on overall qualifications and project price. Conventional project development, however, allows for designer continuity and more flexibility of the individual selection of designer and contractor of the Owner’s choice who may be better fitted for the project. For Design-Build, the selection of the designer is typically made by the contractor.Design-Build concept allows the designer and contractor to work on cost-efficient solutions. A DB project allows for construction input from an invested contractor during the design phase of the project. However, when conventional means are utilized, the Owner should encourage and allow the time and costs for its designer to sufficiently explore the associated construction costs with the appropriate contractor(s) to offset this potential advantage.Another potential advantage of DB is that the project will be more efficiently managed between all disciplines This is because it will be under the control of essentially one entity. In reality, however, some actual architectural and engineering work is needed before the DB project can be bid (see Figure 1). This consultant may also be retained by the Owner for oversight, as well as coordination of the prebid design work during the bid and the initial stages of the DB contract. After the DB contract is let, the DB design phase has to be completed with the plans and specifications issued for the construction with the contractor as the administrator and the “consultant” to the designer on constructability issues. Then, the contractor’s main work of construction begins with the designer providing mainly plans and specification clarifications, and inspection support. Given the sequence for project development, a distinct advantage of the use of DB can not be seen over conventional project development process which is simply from designer to the constructor with designer oversight and clarifications during construction (See Figure 1).Another potential advantage to Design-Build is that the project costs are more controlled. In effect, the owner may believe that since it has a DB contract that all the costs are included and thus the cost is fixed. However, because the design used to bid the job is incomplete to conceptual in nature, the scope of work is not sufficiently defined. Moreover, if the Owner refuses to pay for the conceived out-of-the-scope costs, the Owner may not have “a leg to stand on” and could result in dispute resolution proceedings. Prebid design specifications for DB contracts can be vague or misleading in many aspects. Therefore, engineering judgment has to be used for critical aspects of the project by all competitive bidders in preparing their bids for the project. Moreover, there is no standard set in DB Request for Proposal (RFP) that requires that any design needs to be done prior to bid, nor is it practical. It is unreasonable to assume the contractor would have to do the design prior to bid to resolve uncertain design criteria and unknown parameter values. Even within the course of the design process, it is not common to encounter areas of input for the design of the construction which was heretofore not recognized or not needed until at later stages in the project. Therefore, these bids are based on the “Owners” supplied information and reasonable engineering judgment. The presumption here is that DB plans and specficiations are sufficient to bid on.Prebid analyses when using engineering judgment and considering reasonable assumptions based on vague, missing, misleading, or defective design data and criteria in the DB RFP can lead to a fairly wide range of reasonable construction estimates. In conventional project development with plans and specifications issued for construction provided, a more realistic scope is identified and more competitive and less presumptive or “open book” bids for construction result. Any engineer knows that you have to have the proper input data to design something and in turn the appropriate plans and specifications to estimate the construction costs from (see Figure 2).

THE VALIDITY OF THE INPUT DATA AND THE PLANS AND SPECIFICATIONS HAS A DIRECT RELATIONSHIP TO THE CONSTRUCTION COST ESTIMATE

FIGURE 2 PROCESS OF DEVELOPING CONSTRUCTION COST ESTIMATES

 Therefore, the bids obtained from conventional means minimize the risk of the number and magnitude of design changes after the bid. If, however, DB is the direction the project is going to go, where do you draw the line in the effort made in the prebid design?CONCLUSION: For more complex and expensive construction projects there does not seem to be a distinct advantage for Design-Build over conventionally built projects unless the reduction of the contract project administrative duties is preferred by the Owner. Cost control factors are better managed through conventionally driven projects.Provide your comments below.

How to Stabilize Underground Mines

Underground mines are commonly stabilized by filling the void space created by the extracted ore with material. The reason this is done is for the protection of surface structures and landfills from damage as a result of mine collapse of the mine structure some time in the future.

FIGURE 1 PHOTOGRAPH OF INJECTING GROUT INTO A MINE VOID ABOUT 190FT BELOW THE GROUND SURFACE

The material placed in these underground mines is filled in from the ground surface, requiring the drilling of injection holes that are typically 50ft to over 300ft deep to reach the mine voids (see Figure 1).

These injection holes are typically spaced from 20 to 75 ft apart. The injected material, called grout, will be made to have various fluidities depending upon its purpose. For example, stiff grouts are used to limit the spread and are thereby used to create a grout “wall” in the mine to contain fluid grout when injected into designated areas to protect the overlying infrastructure.

These projects typically consist of subjacent mines which contain a large volume of void space from mineral extraction. It is not uncommon for quantity of the grout used to stabilize the mine to be on the order of 25,000 to 50,000 cubic yards or more for commercial and industrial projects. Therefore, the grout material costs represents a significant part of the budget of project. Therefore, investigating cost-reducing grout materials can represent significant savings to the project. Also, the deeper the mine, the greater the drilling footage and costs, and therefore, in some cases, ways to reduce the drilling footage can also be cost effective.

Various ways exist to stabilize an underground mine, which involve spot injection grouting to full saturation grouting (see Issue 24). To apply the most cost effective methodology, factors that should be considered include:

• Mine depth,

• Existing mine conditions (e.g., mine gas, flooding, roof rubble, etc.),

• Surface conditions (e.g., existing structures and utilities, site topography, etc.),

• Mine failure mechanism(s) to be mitigated, and

• Acceptable damage threshold of the protected structures(s)

Smart cost effective grout design considering the above factors can save over 50% on the cost.

Effect of Geohazards on Pipelines

PHOTO OF A BUCKLED TRANSMISSION PIPELINE FROM LAND SUBSIDENCE

When planning a pipe route or evaluating an existing one the associated geohazards along that one should be considered. Those which are most commonly considered geohazards are landsliding, land subsidence from underground mining or karst, and earthquake induced ground motions (i.e., faulting, liquefaction, lateral spreading and landsliding). This can determine whether that investigated route is viable or not. Given the long line reaches, the operator can struggle with determining which areas along the alignment contain the most critical geohazard(s). Whether along a proposed or existing route is best done by a two-phased approach. Phase 1 would identify those geohazard areas that can affect the pipeline during its lifetime, and Phase 2 would identify those geohazard areas which may potentially exceed the operator’s acceptable risk threshold.

The first step when evaluating the vulnerability of the pipeline to a geohazard is the assessment of the event (or occurrence) and severity probabilities (or in other words, what is the chance of a certain magnitude of ground motion). However, even more important is the assessment of the behavior or damage potential of the pipeline to the concerned geohazard movement that could occur during the expected operational lifetime of the line. The key overall assessment here then becomes whether the determined damage potential (occurrence and associated severity probabilities) exceeds the threshold of acceptable exposure level by the operator/owner. Evaluating the damage potential of certain site conditions many times requires numerical analysis in order to account for all the important ground movement, backfill, and pipeline conditions.

For any geohazard condition, the damage analysis should consider the primary modes of pipeline deformation, which are tensile stretching, buckling and bowing (aka upheaval buckling). These pipe deformations are a function of the nature of the ground movement the pipeline is exposed to. For example, significant tensile and compressive deformations can result from differential vertical (settlement) and lateral ground movement perpendicular to the pipeline, as well as, slippage between the ambient backfill and the line from lateral ground movement along the pipe.

PHOTO OF A TRANSMISSION PIPELINE THAT BOWED OUT OF THE GROUND FROM LAND SUBSIDENCE

Where the risk is deemed too high, there are many ways to mitigate the damage or hazard potential, these include:

• Relocating the line;

• Telemetric monitoring of pipe deformations;

• Designing for the ground movement;

• Reducing the backfill/pipe friction/adhesion against slip;

• Using restraints against upward bowing; and

• Installing stress relief joints.




 

More information on this topic can be obtained from below.

BLOG: How to Handle Geohazard Risks

Engineering UPDATE Issue #4 entitled: Improvement of Mine Support Saves Pipeline from Subsidence event

Engineering UPDATE Issue #25 entitled: Transmission Pipeline Subsidence from Mining

Engineering UPDATE Issue #44 entitled: Property Management System for Geotechnical Risks

Engineering UPDATE Issue #51 entitled: Upheaval Buckling of Pipelines