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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

How to Handle Geohazard Risks

Geohazards can be described as unexpected land movement events which can potentially result in hazardous conditions or significant damage to infrastructure. Land movement categories that are more commonly considered geohazard events are land subsidence for underlying karst or underground mining, landsliding, and earthquake motions. In addition to the movements themselves, the ramifications of these earthquake motions can be exhibited in various forms including fault displacements, land subsidence, liquefaction, lateral spreading, and landsliding.

Courtesy of the University of Missouri, 2012.

USGS air photo of the Mud Creek landslide, taken on May 27, 2017.

In handling a geohazard risk, risk evaluation is very important. The risk evaluation of a geohazard will determine whether the project will proceed, and with the go-ahead, the associated costs of the risk mitigation measures that would be taken. These costs can be significant. Therefore, having a superior understanding of the geohazard is imperative. Because of the importance of the geohazard risk assessment, which merits an extensive investigation, it should be performed by an expert experienced and specialized in the geohazard concern. In other words, the assessment falls outside the general practitioner in the associated discipline or a geotechnical engineer. For the allocated investment, the geohazard expert will provide a far superior assessment. In fact, even when the investigation budget is limited, it should be done by the geohazard specialist given their ability to extend the available project data collected.

Addressing a geohazard requires an understanding of all the geohazard-site conditions and their implications. This involves predicting the frequency of an occurrence as well as the probability spectrum of ground movement severity. Most importantly, however, is the assessment of the probability spectrum of damage potential being considered. The potential is most important to the project decision making process as this damage spectrum is evaluated against the damage threshold of the risk manager to determine the acceptable risk. Unless there are lender restrictions, commercial risk decision makers may be under greater stress to relax the acceptable risk threshold due to competitive economics. The acceptable risk results in the establishment of mitigation measures and risk protocols for effective and rapid event reactions where needed.

More information can be obtained on this topic form below.

Engineering Update #44 entitled: Property Management System for Geotechnical Risks

BLOG: How to Find a Geotechnical Engineering Expert

BLOG: What is Karst Subsidence

BLOG: What is Mine Subsidence

BLOG: What to Look for When Selecting a Geotechnical Engineering Company

BLOG: Landsliding What to Do

BLOG: What to Look for in a Mine Subsidence Expert

BLOG: Landslides

BLOG: What to Look for in a Karst Subsidence Expert

How to Find a Geotechnical Engineering Expert

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.
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  Traits to Dig for in an Engineering Expert

Retaining Wall –What Are My Options?

Retaining wall are walls which retain soil or soil banks and are used to create usable space which cannot be done by sloping the ground surface because it will fail.

There is a wide variety of ways that retaining walls are constructed. The most common ones which are used are in general order of cost are reinforced Soil Sloped Walls, Steel Sheet Pile Walls, Concrete Modular Unit Gravity Walls, Soldier Pile, and Lagging Walls, Mechanically Stabilized Earth Walls, and Cast in-Place Reinforced Concrete Wall.

From the least to the most expensive wall type there is over a 5-fold difference in the installation costs. In addition to costs, the other factors that are considered include the wall systems workability with the site and the appearance of the wall. May be considered in the selection of the wall.

Example photos of these different types of walls. In the selection of the most appropriate retaining wall type for a project, a geotechnical engineer with sufficient experience in this area should be consulted. For more information on reinforced soil slope walls see Engineering Updates 39 and 42.

Causes for Building Damage

There are a host of potential causes of building damage, however, they can be broken up into three primary categories. Those primary causes are from: environmental conditions, material/structural defects, and ground movements.

The environmental causes can be broken into mainly two subcategories: climatic and seismic (earthquakes). Climatic conditions which can cause damage to the structure are primarily in the form of temperature, wind, and precipitation. More typical examples of building damage from climatic conditions consist of temperature straining, material freeze and thaw, wind damage, and catastrophic weather (e.g. hurricanes, tornadoes, flooding).

Environmental

The second category of material or structural defects consists of flawed constructed materials or elements which result in unintended damage. The two main subcategories of this cause of damage involve improper design and defective installation. Examples of damage from improper design could involve underestimating building loads, missed load or deformation concentrations, inappropriate building elements, poor run-off drainage, and selection of improper materials such as wrong concrete type. Defective construction could involve, for example, poor honeycomb or weak concrete, missing or faulty welds, missing reinforcing steel bars, curing cracks due to improperly poured concrete, etc.

Material Defects

(Photo Credit: Gopal Mishra)

https://theconstructor.org/practical-guide/honeycombs-in-concrete-and-remedies/6889/

(Photo Credit: Caitlin McCabe)

https://www.inquirer.com/news/water-damage-new-home-construction-regulation-bill-pennsylvania-suburbs-philadelphia-20190329.html

The other primary cause is related to ground movements. The different subcategories of ground movement which most typically result in building damage include: soil settlement, soil/rock heave, landsliding, land subsidence, and earthquake shaking and associated ground failure. Land subsidence damage mainly involves sinkholes and surface depressions in karst terrain, from underground mining, and settlement from soil collapse from water saturation. Although earthquake shaking alone can result in building damage, foundation failure can also result from ground failure in the form of settlement, soil liquefaction, and landsliding (including lateral spreading). Please refer to the following blogs for additional explanation: “Causes for Building Settlement”, “Landsliding: What to Do”, “What is Karst Subsidence”, “What is Mine Subsidence”, and “Causes for Building Uplift”.

Ground Movements

If confronted with building damage, it is important to contact the appropriate experienced forensic engineer. In most cases, a qualified structural engineer would be the most appropriate initial investigator to assess the nature of the damage and provide the proper direction to determine the cause.

If MEA can assist you with your building damage problems, please contact us at 314-833-3189

Causes for Building Uplift

The most common sources of building uplift and resulting damage are expansive clay soil or rock. These soils and rocks have the ability to lift buildings when the swell pressure exhibited by these clayey materials exceeds the foundation loading. This is why flatwork, like on grade concrete slabs which are lightly loaded, can be more susceptible to uplift.
What makes the clay soil or rock have significant swell potential is its water absorption capability. The greater the absorption capability, the greater the amount of potential building uplift. The absorption strength depends on the clay mineralogy content, the density and the amount of moisture in the soil or rock. For example, a relatively dry, densely packed clay soil or rock which contained a significant amount of expansive clay particles, would have a very high swell potential.
Therefore, wetter materials such as from a possibly wet climate or surface drainage and higher groundwater tables can limit building uplift. Conversely, under drier conditions uplift can occur in such materials where access to water increased. More common examples are where surface drainage has been rerouted to, and/or landscaping has been removed (removing roots and allowing more ground absorption) from the foundation area.
In colder climates, another source of building uplift would be from freezing soils. This results when the building foundation is placed above the frost depth in the soil and subsequently the soil moisture freezes beneath the foundation, resulting in heave during cold weather.
Building heave can be mistaken for building settlements or possibly other causes. If an investigation is merited, it is recommended that a qualified geotechnical forensic engineer be consulted.
If MEA can assist you with your building uplift problems, please contact us at 314-833-3189.