The following is a guest article by Áine Ní Bhreasaíl
Freezing soil causes cracks to appear in buildings and lumps in roads. It’s not just the 9% expansion of water when it freezes to ice that that causes the ground to heave in this way, it is also the freezing and thawing of ice lenses, segregated bands of ice within the soil, which cause movement many times greater than 9%.The threat from ice lenses is significant. In 1966 20 people died when a building affected by soil freezing and thawing collapsed in Noril’sk in Russia. The extent of ice lens related damage to infrastructure and buildings in Yakutsk, also in Russia, has been so great that it was declared a natural disaster area in 1998. It’s not just Russia; areas with permanently frozen soil occupy about a quarter of the exposed land area of the Northern Hemisphere. Climate change will increase ice lens formation and increase the hazards associated with ice lenses, however it is unclear how ice lenses form in the first place.The Geotechnics Section in the Civil Engineering department here at Imperial researches all kinds of things related to the behaviour of soil and rock. Students here study the performance of dams in earthquakes, how new Crossrail tunnels will effect Victorian tube tunnels, how soils from Libya collapse as well as many other topics. Most importantly, though, they also investigate how ice lenses form using the Diamond Light Source facility in the middle of the Oxfordshire countryside!
The students used one of the X-ray beams at the Diamond Light Source to take images of ice lenses as they formed. These images were reconstructed into 3D in the same way as a CAT scanner in hospital would take images of the brain or spine. The X-rays in the Diamond Light Source are different to those found in a hospital machines because they are really bright, and are parallel to each other allowing extremely clear images free of noise. This international standard equipment allows students to resolve images down to the microscopic level, perfect for watching individual soil particles when they freeze. An example of the images recorded is seen in the picture below. On the left is a sample of glass spheres, a model soil, and on the right is a sample of Leighton Buzzard sand. Both samples have cracks that are the beginnings of ice lenses. This was the first time that the initial formation of ice lenses had ever been seen in 3D. Knowing how ice lenses form is the first step in stopping them forming in the first place and preventing the heave from freezing soil that makes buildings collapse and roads dangerous to drive on.
Áine is a PhD student in the Geotechnics section, researching the micro-mechanics of permafrost thaw, but has also spent time working with parliament as a specialist on the Energy and Climate Change Committee.