Liquefaction of soils during earthquakes

Soil liquefaction most often affects soft, sandy, water-saturated soil layers. It manifests itself as a sudden loss of strength which, under unfavorable circumstances, can lead to catastrophic failure.

Earthquakes are at the origin of most disorders linked to soil liquefaction, which appear as induced phenomena in terms of seismic risk.

The least resistant soils are the most vulnerable. The risk is accentuated in the presence of strong movements.

Soil liquefaction caused by earthquakes is known to be the cause of settlement or failure of shallow and deep foundations, endangering supported structures, buildings and engineering structures. Soil liquefaction can also cause damage or destruction to earth structures (embankments, walls, dikes, dams) and port structures (quays, medians).

Lastly, low-lying slopes near bodies of water (sea, lakes) and rivers have often proved vulnerable to this phenomenon, and the scene of catastrophic failures.

This presentation of soil liquefaction is based on post-seismic field observations. It is too vague to be of use given the diversity of situations encountered, but it does cover the main aspects of the problem.

The loss of soil shear strength, seen here as the transition of soil from a solid to a liquid state, relates to the mechanical behavior of soil, and falls within the scope of the basic principles of soil mechanics. As such, the phenomenon of soil liquefaction has been reproduced in the laboratory, enabling us to interpret the mechanisms involved and the role of the factors that control them. These factors are numerous, starting with :

• soil type (sand, silt, clay) ;

• its compactness;

• physical properties ;

• site conditions, etc.

These various aspects will be discussed in the first paragraph, where it will be indicated that various definitions have been given of soil liquefaction and its effects, depending on whether we are interested in field observations, the behavior of structures or laboratory tests.

In terms of seismic risk, soil liquefaction represents a real danger for the buildings concerned. In order to reduce the vulnerability of buildings, we need to improve our knowledge of the phenomenon and develop risk assessment methods.

As such, the second paragraph aims to situate the phenomenon of soil liquefaction within the more general framework of dynamic soil behaviour. The principles that emerge from this are enshrined in geotechnical site investigation methods, which are carried out by means...