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Today we will talk about the phenomenon of soil liquefaction and the methods used by Geostru Liquiter software to calculate this risk.

In our blog, in the article “Liquefaction resistance of soil with Liquiter”, it is explained what the liquefaction phenomenon consists of and the methods used to determine the risk of liquefaction, in terms of safety factor (Fs).

How is the liquefaction resistance factor estimated?

To determine the risk of liquefaction in terms of safety factor (FS), the calculation of two variables is required:

1. CSR = Cyclic Shear Ratio and
2. CRR = Cyclic Resistance Ratio:

FS = CRR/CSR

Liquefaction occurs with values of Fs <1.

The methods, proposed in the form of simplified methods and constantly updated in recent years, are based on the calculation of the CSR and CRR parameters. These updates are included in Liquiter software.

The method based on SPT (Seed e Idriss 1971), calculates the CSR using the following formula:

CSR=τ_av/(σ’_v0)=0.65∙(a_max/g)∙(σ_v0/(σ’_v0))∙r_d

The value of rd is a reduction coefficient which is obtained from the empirical formula of Liao & Withman,1986

Rauch (1998) proposes a formula for calculating CRR in case of seismic events of magnitude 7.5:

CRR_7.5=1/[34-(N₁)₆₀] +(N₁)₆₀/135+ 50/[10∙(N₁)₆₀+45]² – 1/200

For different Magnitude (greater or less than 7.5) Seed e Idris (1982) introduced the Magnitude Scaling Factor MSF defined by the following equation:

MSF= 10^2.24/ (M_w^2.56 )

Then the liquefaction safety factor, taking into account the above considerations, can be expressed as:

FS= (CRR_7.5/CSR)MSF

Liquiter

As we have said, the Geostru Liquiter software includes the latest methods used to analyze the liquefaction of soils under seismic conditions. The required input data can be obtained from various field tests.

In summary the results provided by the program are:

• Factor of Safety against Liquefaction / Liquefaction Potential Index
• Cyclic mobility of clay
• Liquefaction of sand and clay
• CSR, variable CSR with depth from SHAKE results
• Reconsolidation Settlement, Lateral Spreading
• Residual Strength

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• On-site soil testing
• Geomechanics
• Geophysics
• Local Seismic Response