Durability, evaluation and remediation (and 2)
09-06-2022 | Posted by Joaquín Martí
As stated in the previous post on this topic, sustainability improves when things last longer. The many challenges undergone by structures must be anticipated, so that they are prepared to withstand them. Typical problems are fires, cracking and fatigue, the vagaries of the ground, vibrations, noise, and the chemical expansion of concrete. The first half were reviewed in the earlier post, we now review the second half.
The ground is inherited from nature and, apart from some improvement measures, all we can do is to try to evaluate its properties and take them into account when designing our structures. The problem is often complicated by non-linearity, plasticity, anisotropy, strain softening, and the idiosyncrasies of cyclic response.
Principia maintains strong simulation capabilities in geotechnics. This includes pile foundations for bridges, tanks, wind turbines, even a tunnel, under static and dynamic conditions. And shallow foundations must also be studied in relation with their settlements and stability.
Mines -open pit, underground and their tailings impoundments- also motivate our studies. And the stability of earth or rockfill dams, particularly under earthquakes, and underground nuclear waste repositories, tunnels, slopes, etc. Accidents investigated in an expert witness capacity, like the Esterhazy mine in Canada or the Aznalcóllar tailings pond, pose special difficulties.
Many of our geotechnical activities are related with earthquakes: evaluation of seismic hazard and risk, study of the site effect, liquefaction potential, dynamic behaviour of shallow and deep foundations, dynamic soil-structure and soil-water-structure interaction, etc.
All sorts of ill effects are associated with vibrations: noise, discomfort, stresses, material fatigue, etc. Vibrations tend to result in decreased efficiency, higher maintenance costs, and shortened useful life. When unwanted vibrations appear, one can try to act on the source, the transmission path, or the resulting structural response.
At the source, we have dealt with vibrations in pumps for liquefied gas, the twin ties of a tied-arch bridge, a turboprop aircraft engine, etc. We acted on the transmission path on an underground military command centre, a 7th century church disturbed by traffic, an electricity generating plant, and various buildings near metro stations or railway lines.
For acting on the structure, one may modify masses, stiffnesses and/or damping. Occasionally, we use isolation systems for seismic or other vibrations, as well as control systems. We have acted on the structure of an auditorium affected by active audiences, a bridge excited by pedestrians, a water bottling facility, bridges and solar collectors excited by wind, buildings with train-induced vibrations, seismic isolation for liquefied natural gas tanks, etc.
Not all the vibrations are bad. In a large coal hopper, we used vibrations, tuned to the natural frequency of the coal arches, to break them and resume the interrupted discharge.
Noise is a related phenomenon that does not threaten the structure but may be incompatible with its function. We have studied buildings, metro stations and real estate developments near railway lines to quantify expected noise levels and propose remediation measures.
Several processes can cause long-term chemical expansion of concrete, with the alkali-silica reaction (ASR) being a relatively frequent one. We have studied the problem at several dams, been involved in three related research projects, and participated in international benchmark exercises.
The analyses are oriented to predicting their future evolution and, particularly, the safety implications. ASR is still an evolving technical field, but the experience gathered over the years makes us capable of providing reasonably accurate predictions.
Once again, improving the life of equipment and structures has many advantages and deserves our best efforts to achieve it.
