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The Champlain Towers South tragedy calls for a geotechnical and structural rethink to accommodate climate change and its effects. Designing structures to be monitored is one possible solution.
While the exact cause of the collapse of the Champlain Towers South (CTS) condominium in Surfside, Florida, on the 24 of June 2021 remains to be determined, several emerging theories point to a deterioration of the ground and the structure’s foundations due to the detrimental effects of climate change.
It's well known that the global climate is changing. Across the UK, we've experienced higher average temperatures (above the pre-industrial baseline), an increase in extreme temperature events (hot and cold), increased rainfall duration and intensity, flooding, and sea level rise. Similar phenomena are being experienced throughout the inhabited world.
Specifically for the case of the Florida coastline, sea level rise has been identified as a possible cause for degradation of the CTS’s foundations.
Sea level rise results in a saline permeation of the groundwater. However, the foundations can be exposed to additional infiltrating salts through spray, borne by storm winds, and flooding: routes which may have been exacerbated by poor structural design.
Deterioration of reinforced concrete elements under saltwater attack is well documented. The invading solution, carrying chlorine ions, is able to penetrate the concrete either through the concrete’s natural porous network or through pre-existing cracks.
Over time, the chlorine ions depassivate the reinforcement environment by reducing the pH below the point at which iron oxide can form (a mechanism which, we should note, can occur whether or not the concrete is submerged). As iron oxide has a larger specific volume than steel, the reinforced zone expands, causing cracking or spalling and creating preferential seepage pathways for the seawater, creating a vicious circle of degradation.
We mustn’t forget the ground when considering climate change degradation. Higher water tables will change ground stresses and it is also well known that salts affect clay soils electrochemically: for example, sodium ions cause clay platelets to disperse, increasing the plasticity index and, perhaps, shear strengths, depending on the parent clay mineral types and contents.
For the Surfside coastline, which comprises shallow limestone geology, another concern is karsting (where sinkholes and caves form under topography formed of soluble rocks like limestone). Although no evidence has been presented for voiding underneath the structure, an extensive ground investigation would be needed to determine whether or not it might exist.
Although the race is now on to limit the increase in global average temperatures to 1.5°C, no such targets are in place for extreme events. Critically, climate models demonstrate that the effects of these extremes will echo throughout this century, regardless of whether or not we meet 2030 and 2050 carbon reduction targets.
For engineers, this means that structures will have to resist greater loads, with greater uncertainty, than they have ever had to resist in the past
Traditionally, uncertainty in geotechnical design is accommodated by large factors of safety. However, this creates an interesting dichotomy.
On the one hand, we may improve safety by enlarging structural components or by reducing our estimates of ground strengths. On the other, we are striving to reduce the amount of raw materials we use in our structures, particularly focusing on concrete, to reduce the structure’s carbon cost.
Clearly, we cannot easily achieve both our aim to reduce the environmental impact of construction while upholding our duty of care to the profession, the planet, and clients.
Given the rapid pace of climate change and the prevalence of extreme events, we are now in the position of not, with a comfortable degree of certainty, being able to predict a building’s performance from historic data.
However, as our global climates worsen, computing technology continues to become more powerful. A possible solution to maintaining safety without specifying monstrous dimensions may therefore lie in the burgeoning discipline of structural health monitoring.
Low-cost sensors, placed throughout a structure, can monitor the condition of embedded reinforcement, accelerations within the structure (e.g. under wind loads) and the formation and propagation of cracks, amongst many other indicators of damage.
Timely intervention, targeting the worst-affected areas, may provide the Holy Grail of a cost-effective, safe solution for construction without needing to abandon coastal areas.
However, our approach to building design must adapt, just as the structure’s design must be adapted to suit its new climatic loads. Structures must be designed to be monitored, e.g. providing inspection and repair access to key elements, and a culture of maintenance must be created.
If done correctly, maintenance could create employment while reducing the need to demolish structures and, in the event of the CTS, prevent untimely tragedies.
Would you like to learn more about advances in structural health monitoring? In response to the collapse, the ASCE Library has assembled a collection of papers highlighting the importance of condition assessment of existing buildings. This collection is available for free through to 15 September 2021: https://ascelibrary.org/SHMcollection
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