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.
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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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.
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
Landsliding refers to the downward shifting of the ground on a slope. The slope can naturally exist and/or be manmade. The reason for the shifting is either there has been added force to the slope or the slope has been weakened. There are a number of ways downward force can be added to the slope. Some of the more common applications which can cause landslides are:
The slope is steepened and/or heightened with fill soil
Slope becomes water logged and thus made heavier
A structure is constructed on or close to the slope
The water level in the waterway at the bottom of the slope dropped significantly causing downward seepage forces.
More typical causes for landsliding from slope weakening are: waterway bank erosion; man-made undercutting or excavations along the slope; removal of root reinforcement from vegetation; and weathering of the soil mantle. Landsliding can occur slowly to abruptly with little warning. More typical tell-tale signs of slope instability are ground cracking along the slope which is most commonly towards the upper portion of the slope; trees, poles, fences, etc. which are leaning downslope and ground surface bulging of heaving near the bottom of the slope.
Diagram of an idealized landslide showing commonly used nomenclature for its parts. Courtesy of the Utah Geological Survey.