Static soil liquefaction entails the sudden loss of strength of loosely packed saturated sand or silt, resulting in a sudden collapse of the sand body. Contrary to “ordinary” slope failure, in which the instable soil mass slides along a clear rupture surface while staying more or less intact, in a liquefaction flow slide the instable mass of sand (or silt) flows laminar like a viscous fluid.

Generally, for static liquefaction in an under-water slope the following conditions are required (1) the presence of a sufficiently large zone of loosely packed, non-lithified, and water-saturated sand or silt; (2) the stress state of the loosely packed sand elements should be close to the so-called metastability point (i.e. the intermediate maximum in the stress path). For this, both mean stress and shear stress should be sufficiently large, which is only the case in a sufficiently high and steep slope; and (3) the presence of a trigger, for example a (small) load change.

SLIQ2D is a 2D computer program which calculates whether in a submerged slope such metastability points as mentioned above may occur or not, based on the slope geometry, the relative density and the material properties of the sand or silt. In case the calculated metastable area is sufficiently large, this may lead to liquefaction flow slide. A clear approval criterion (e.g. the minimum size of the metastable area that will lead to a liquefaction flow slide) cannot be given. A possibility is to perform a slope stability analysis (a simple slip circle analysis or a more advanced Finite Element analysis), in which a reduced (liquefied) shear strength (e.g. a friction angle of 5 degrees) is assigned to the sand layers that are metastable according to SLIQ2D.

The first DOS-version of the program was developed by Grondmechanica Delft in 1994, commissioned by Rijkswaterstaat. In 2009 a Windows-version was released, in which it was possible to distinguish (horizontal) soil layers with different properties. That later version has been implemented in D-FlowSlide. The theoretical background of SLIQ2D can be found in the Handboek Zettingsvloeiingen. Background information on the Windows version of the model and guidance with respect to the determination of the input parameters can be found in the User Manual of SLIQ2D.

Advanced check for liquefaction: SLIQ2D

Background informations about SLIQ2D can be find in the User Manual of SLIQ2D.

Determination of the imaginary surface

In this program, the fictive slope height and the fictive slope inclination are determined using the following steps:

Step 1: Determination of the fictive height

The height of the part of the slope above water (h₁) is increased until h₂ with a factor equals to the ratio between the soil unit weight above water table and the soil below water table:
h₂ = (1 + w) . γ_s . h₁ / (γ_s – γ_w)

Step 2: Determination of the fictive slope inclination

By means of the least squares method, the submerged slope (red line in the figure below) compared to the fictive submerged slope is determined and subsequently the fictive slope inclinaison (α_R).

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