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:
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.
FIGURE 1 WORKFLOWS FOR THE CONVENTIONAL AND DESIGN-BUILD PROJECT DEVELOPMENTThe apparent main advantages of Design-Build over conventional owner-designer-constructor projects are:
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 ESTIMATESTherefore, 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.
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.
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.
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.
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 RisksEngineering UPDATE Issue #4 entitled: Improvement of Mine Support Saves Pipeline from Subsidence eventEngineering UPDATE Issue #25 entitled: Transmission Pipeline Subsidence from MiningEngineering UPDATE Issue #44 entitled: Property Management System for Geotechnical RisksEngineering UPDATE Issue #51 entitled: Upheaval Buckling of Pipelines
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.