Earthquakes: how can we limit the damage caused by soil liquefaction?

Earthquakes: how can we limit the damage caused by soil liquefaction?  - Ifsttar - Isolate-img

© Perduta / Wikimedia Commons - CC BY-SA 3.0

The main goal of the ISOLATE project is to improve our understanding of and ability to prevent soil liquefaction in the event of an earthquake. A major component of this new collaborative project is use of the geotechnical centrifuge at IFSTTAR’s Nantes site. It also aims to test a new risk reduction technique that uses … bacteria.

Soil liquefaction is literally capable of causing an apartment block to slide, damaging a dam or creating a geyser in the middle of a road.  “This phenomenon is caused by an earthquake and transforms the soil from the solid to the liquid state. It occurs when a saturated sandy soil is subjected to high pressures, such as during an earthquake”, Jean-François Semblat explains. This expert in geotechnical engineering and seisomology was formerly the assistant director of the GERS department and is now a professor at ENSTA. He is also coordinating the ISOLATE project. This four-year ANR funded project got under way in 2018 and involves twenty or so researchers from several IFSTTAR laboratories (Navier, SV, SRO), the CEA at Saclay, CentraleSupelec,  and the École Nationale des Ponts et Chaussées as well as two major industrial players, EDF and Soletanche-Bachy, a contractor that specialises in foundations and soil technology. ISOLATE is the outcome of EDF’s need to better quantify and limit the risks associated with soil liquefaction under its structures, in particular in the case of storage dams. Its three goals are as follows: to gain a better understanding of the liquefaction of natural soils, to reduce the risks of it occurring, and to formulate practical recommendations for practitioners.

ISOLATE’s novel feature is that it deals with natural soils: “Most research on liquefaction is concerned with soils that are made up of sand with a uniform particle size. However, this is not representative of what is encountered in the field where a given soil contains particles of different sizes”, Jean-François Semblat points out. The team of scientists therefore intends to study liquefaction in the case of soils with different densities and particle size distributions, at different scales and for different types of structure (dikes, dams, walls…). It has a number of experimental devices at its disposal, including a resonant column and a geotechnical centrifuge which is located at the Nantes site. “This facility – which is the only one of its kind in France – is able to simulate earthquakes and reproduce the behaviour of a soil column that is in danger of liquefaction at a reduced scale of 1:50”, Jean-François Semblat explains.

Bacteria for soil strengthening

“ISOLATE will also give us an opportunity to test an innovative method for reducing the risk of soil liquefaction: biocalcification the project coordinator states next. The process, known as Biocalcis®, has been patented by Soletanche-Bachy. It consists of injecting bacteria - Sporosarcina pasteurii – into a soil which presents a liquefaction hazard, and then stimulating them with a calcifying solution. This causes the bacteria to produce bridges made of calcite (a crystallised form of calcium carbonate) – between the soil particles, forming a sort of biological cement. “The advantage of the technique is that it increases the strength of soils with minimal environmental impact. It gives us an alternative to present-day soil injection techniques which use chemical slurries”. The results of the work in the framework of the ISOLATE project will be translated into recommendations, tools, methods, practical guides, for example. These will assist those working in the sector and organisations such as the Association Française du Génie Parasismique (French Association for Earthquake Engineering) or the International Society for Soil Mechanics and Geotechnical Engineering.