Liquefaction and earthquake
Liquefaction is an extraordinarly complex and potentially destructive phenomenon that can occur during a seismic event of significant intensity. The accelerations generated by a seismic event result in an increase in the neutral pressure of the interstitial water contained in saturated sandy soil.
Figure 1 – Comparison between stable soil on the left and liquefied soil on the right (from Encyclopaedia Britannica Inc, 2012)
The increase in neutral pressures leads to the cancellation of effective stresses and shear resistance of the soil. The temporary loss of cohesion in sandy soil causes it to behave like a fluid. This soil behavior can substantially weaken the bearing capacity of the soil, resulting in structures built on it sinking or tilting (Figure 1, 2, 3).
Figure 2-Real Cases: A) Liquefaction in the city of Urayasu, Japan; B) cars stuck in boiling muddy water (from Yasuda et al., 2013); C) effects of sand liquefaction (Niigata, 1964); Kobe event, 1995.
Undrained soil conditions
In static conditions, sandy soils are typically considered to be in a drained state, where the neutral pressure inside the soil has time to equilibrate with the surrounding conditions. However, during the phenomenon of liquefaction induced by a seismic event, the accelerations imposed on the soil create a rapid increase in neutral pressure that prevents it from dissipating within the available time frame.
Essentially, the sudden increase in neutral pressure during liquefaction, caused by the seismic forces at play, creates undrained conditions where the sandy soil loses its ability to rapidly dissipate the accumulated neutral overpressures (Fig.3).
Figure 3 – A) Stable pre-liquefaction condition, where sand grains are in contact with each other; B) Following the earthquake, cohesion between the grains is lost, and the soil exhibits liquid behavior (from Youd, 1992).
This scenario can be compared to the case where the application of an external load occurs at a much higher rate than the rate at which neutral overpressures can dissipate (e.g., penetrometer test). During the liquefaction process, despite the inherently high permeability of the sand, it will be in a state defined as ‘undrained,’ where neutral overpressures do not have the opportunity to be dissipated.
According to the current Technical Regulations, the verification against liquefaction can be omitted if at least one of the four conditions listed below is met in the excerpt from the EC7/EC8.
Methods for conducting checks
In the context of safety assessments related to liquefaction phenomena, different methodological approaches can be adopted, ranging from simplified methods to more sophisticated ones based on advanced analysis.
- In the context of verification through simplified methods ,a safety coefficient is determined. This coefficient is obtained by the ratio of cyclic resistance to liquefaction, defined as CRR (Cyclic Resistance Ratio), to the cyclic stress ratio induced by seismic action, known as CSR (Cyclic Stress Ratio).
- Advanced analysis methods are based on one-dimensional or two-dimensional analyses to determine the behavior of stress and shear deformations induced by seismic action. These approaches require the use of complex numerical calculation codes and the execution of in-situ dynamic tests and laboratory cyclic tests to define the geotechnical subsurface model.
What to do if the soil is liquefiable?
If the results of the liquefaction susceptibility analysis are unsatisfactory, it will be necessary to consider the adoption of a pile foundation system. These piles will be placed in areas with good geotechnical characteristics. Alternatively, specific interventions to improve the geotechnical properties of the foundation soil through the use of expanding resin injections will need to be planned.
Liquefaction assessment post-construction
In the case of liquefiable soil where piles are chosen as a solution, it is feasible to conduct liquefaction assessments while also considering future structural works. This process can be easily carried out with the help of the Geostru Liquiter software . In fact, as shown in Figure 5, it is evident that, after selecting the type of intervention, it is possible to recalculate the liquefaction safety factor at various depths to assess the effectiveness of the intervention, following the Priebe method.
Figure 5 – Liquiter Software Screen: Selection of the type of intervention and recalculation of the liquefaction safety factor.