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.
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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, and4. 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, and3. 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.
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 Updates39and 42.
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).
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.
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”.
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
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.
The most common causes for building settlement are from underlying deposits of compressible fill or native soils. Compressible soils which are under unchanged building foundation loading cause settlement to start immediately and taper off over time. Therefore, if the settlement is not noticed until much later in time, the presence of compressible foundation soils is not likely the culprit. One cause, which can result in building settlement at any time, would be the shrinkage of plastic clay soils. These clay soils will shrink when they “dry out” and are problematic where they are subjacent to the foundation and have significant initial moisture. Shrinkage of foundation clay soils is typically associated with added landscaping which causes water to be “sucked out” of the soils.
Another fairly common source of settlement are foundation soils that can collapse when exposed to moisture. Therefore, settlement of the structure would be noticeable after significant precipitation and is likely to occur early after and even during construction. Soils which would exhibit this behavior are loose, drier fine sands to silts. More common in colder climates, another typically early post-construction source is thawing soil. More specifically, building settlement results from thawing of frozen soils left below the foundation.
Two other more typical causes are less time dependent but are location dependent. These are building settlement from land subsidence in karst terrain and underground mining. In other words, there are only certain regions where either karst conditions and/or underground mines are present. These karst and mine subsidence events may occur at any time. These land subsidence events are discussed in blogs entitled “What is Karst Subsidence” and “What is Mine Subsidence”.
There are some causes of building settlement which are more directly identifiable. These include from underground tunneling, structures next to temporary or permanent yielding retaining walls, earthquake shaking of mainly loose fine sands which can contain some silt, and high extraction underground mining which causes immediate ground collapse.
Red herrings of building settlement, even to the professionals, can be building foundation heave, and from subtle landsliding. Landsliding is discussed in “Landsliding What to Do” and building heave will be discussed in an upcoming blog. Where the building damage is apparently from settlement but requires proper investigation a qualified geotechnical engineer expert in forensic analysis is recommended.
If MEA can assist you with your building settlement problems, please contact us at 314-833-3189.
Karst subsidence is land subsidence that is caused by cavities or voids in the underlying bedrock which collapse or from soil filling them in from above resulting in surface subsidence. Under normal circumstances, the voids or cavities were created by the flow of groundwater in fractures in soluble bedrock over a great deal of time. The most significant land subsidence effects occur over voids which have been solutioned in limestone bedrock but also result in other soluble rocks such as dolomite, gypsum, and halite. The most typical land subsidence results from groundwater draining downward into these solution voids carrying soil particles with it. This results in the ground settlement in the form of a sinkhole to a more gradual depression on the ground surface. Therefore, when downward drainage of groundwater is caused into open bedrock voids, the potential for subsidence results. Some more common triggers are: unlined surfaced drainage trenches, pumping of water wells, quarry pit dewatering and retention/detention ponds.
Figures 1 and 2 are examples of this.
FIGURE 1: SINKHOLE CAUSED BY DOWNWARD DRAINAGE FROM DEWATERING OF NEARBY QUARRY PIT
FIGURE 2: IRREGULAR DEPRESSION WHICH FORMED FROM DOWNWARD SEEPAGE OF WATER STORED IN A RETENTION POND