Level of Service or Risk A. Breaking in B. Slow aging C. Rapid deterioration D. Failed state Condition State on an entire inventory. A deterioration model, shown conceptually as an envelope of curves in figure 4, is the projection of level of service, condition or risk with respect to time. Management of risk throughout a life cycle takes advantage of the shape of this type of relationship. An example is described below. There are multiple systems in place for describing the condition of built structures, such as bridges and retaining walls. The National Bridge Inventory System (NBIS), these structures should be addressed first by an owner trying to reduce its overall risk exposure. This is often defined as “worst first” risk management. If an owner were fiscally constrained, as most are, then this approach would, at the least, get the bad apples out of the bunch. An exception that might come to mind is whether there were two classes of structures, for example, where one was discernibly worse than the other, but the costs to reduce risk for the less worse class Like new Good Fair Poor Failed Time Figure 4: Conceptual deterioration model for level of service, condition or risk established after the failure in 1967 of the Silver Bridge along U.S. Route 35 over the Ohio River, is the root of some of them. Colorado DOT has a well-documented example of a recent retaining wall inventory system developed from this root. The idea is that when a structure is evaluated and determined to be in good condition, there is little hazard of it failing to perform, and when it is found to be in poor condition there is greater hazard. The hazard is essentially an expression of the likelihood of failure to perform within a given time period (e.g., one year). If the consequence of failure is equal for all structures, then the hazard equates to the risk; however, if the consequence is different for different structures, the hazard and consequences need to be combined to solve for the risk. It is intuitive that the highest risk structures would be the most concerning and could be considered “worst,” and that 72 • DEEP FOUNDATIONS • JAN/FEB 2018 was significantly less than for the worst class. In this case, a benefit-cost analysis could be introduced to evaluate whether a fixed capital investment in reducing risk would be better spent on a few structures in the worst class or more structures in the second to worst class. This is an improvement to the approach, but it misses how an asset manager would reduce the risk from their inventory of assets, and is not fully removed from the worst first risk management approach. From the per spect ive of as set management, one cannot make a judgment on how to best reduce risk without also considering time. Recognition of a life cycle is imperative, and recognizing that the condition of an asset, which is the basis of likelihood and, thereby, risk, changes through its life cycle, as environmental and loading stressors act upon it. Since time is key to asset management, the question is less about “how bad is it” and more about “how bad could it get.” This rationale becomes the basis for risk mitigation under fixed capital expenditure, and is farthest removed from what could be termed the “worst first” approach. An asset could be in very good condition and yet be likely to deteriorate rapidly (near the end of stage B), or be in very poor condition but in a state where it really cannot get much worse (near the end of Stage C). An example close to home, as it were, would be if you let the paint deteriorate on your house to the point where the siding now needs to be restored or replaced. You may be tempted to hire someone to do this work, but it may be best (from a financial perspective) to touch up the bad paint and exposed siding and focus on your roof first. If, by providing some minor maintenance, you can avoid repeating the same process there, then this would be what you should do — preserving the shingles and tiles before their deterioration impacts the roof structure, even though the paint and siding are in the worst condition. Figure 5: Installation of lightweight concrete filled drilled shafts to mitigate embankment settlement An example is shown in figure 5 of the above approach for a geotechnical asset in the Colorado DOT inventory. Rather than fund a multi-million dollar embankment repair, the Colorado DOT placed lightweight concrete in lieu of a robust ground anchor system, which resulted in a signi f icant reduct ion in pavement
