Soil liquefaction and related ground failures are commonly associated with large earthquakes. In common usage, liquefaction refers to the loss of strength in saturated, cohesion-less soils due to the build-up of pore water pressures during dynamic loading. Sladen et al. (1985) defined liquefaction as:

 

"Liquefaction is a phenomenon wherein a mass of soil loses a large percentage of its shear resistance, when subjected to monotonic, cyclic, or shock loading, and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as the reduced shear resistance"

 

Liquefaction Assessment

Evaluating the liquefaction resistance of soils is an important step in the engineering design of new structures and the retrofit of existing structures in earthquake-prone regions. The evaluation procedure widely used throughout the world is termed the simplified procedure. This simplified procedure was originally developed by Seed and Idriss (1971) using blow counts from the Standard Penetration Test (SPT) correlated with a parameter representing the seismic loading on the soil, called the Cyclic Stress Ratio (CSR). This parameter is compared to Cyclic Resistance Ratio (CRR) of the soil and if it exceeds CRR, the soil is likely to be liquefied. A safety factor against liquefaction is defined as ratio of CRR to CSR:

 

Safety Factor = CRR / CSR * Kσ * Kα

CRR = CRR7.5(ave) * MSF

Where:

CRR7.5(ave) :calculated cyclic resistance ratio (average of all selected methods at a desired depth) for an earthquake with M=7.5

MSF : Magnitude Scaling Factor

Kσ : overburden stress correction factor; only applied to the following analysis methods (see details):

               - Vancouver Task Force Report (2007)

               - NCEER (1996)

               - Cetin et al. (2004)

               - Idriss & Boulanger (2004)

each of the above-mentioned methods has its own equation for calculating Kσ

Kα : ground slope correction (is considered 1.0 in NovoLIQ)

 

Note: NovoLIQ uses an upper limit of 3.0 when calculating Factor of Safety.


Note: This theory manual is just an introduction to methods implemented in NovoLIQ and does not encompass all the technical knowledge and comments needed for soil liquefaction assessment. Therefore this document shall not be used as a reference for learning how to assess liquefaction potential. Please refer to the related books and other references for more details.

 

Note: Recently there has been technical discussions (by Dr Boulanger and Dr Idriss, 2010) about the accuracy and reliability of Cetin et al (2004) method. Therefore it is recommended that this method is used with cautious and full understanding of its risks. To obtain the full report please contact us.