LIQUEFACTION - Geotechnical Earthquake Engineering - page 160

in the liquefaction analysis in this chapter, the building loads must be included in all
liquefaction-induced settlement, bearing capacity, and stability analyses, as discussed in
Chaps. 7 through 9.
In summary, the site conditions and soil type most susceptible to liquefaction are as follows:
Site Conditions
● Site that is close to the epicenter or location of fault rupture of a major earthquake
● Site that has a groundwater table close to ground surface
Soil Type Most Susceptible to Liquefaction for Given Site Conditions
● Sand that has uniform gradation and rounded soil particles, very loose or loose density
state, recently deposited with no cementation between soil grains, and no prior preload-
ing or seismic shaking
6.4 LIQUEFACTION ANALYSIS
6.4.1 Introduction
The first step in the liquefaction analysis is to determine if the soil has the ability to liquefy dur-
ing an earthquake. As discussed in Sec. 6.3 (item number 3), the vast majority of soils that are
susceptible to liquefaction are cohesionless soils. Cohesive soils should not be considered sus-
ceptible to liquefaction unless they meet all three criteria listed in Sec. 6.3 (see item 3, soil type).
The most common type of analysis to determine the liquefaction potential is to use the
standard penetration test (SPT) (Seed et al. 1985, Stark and Olson 1995). The analysis is
based on the simplified method proposed by Seed and Idriss (1971). This method of lique-
faction analysis proposed by Seed and Idriss (1971) is often termed the simplified proce-
dure. This is the most commonly used method to evaluate the liquefaction potential of a
site. The steps are as follows:
1. Appropriate soil type: As discussed above, the first step is to determine if the soil
has the ability to liquefy during an earthquake. The soil must meet the requirements listed
in Sec. 6.3 (item 3).
2. Groundwater table: The soil must be below the groundwater table. The liquefac-
tion analysis could also be performed if it is anticipated that the groundwater table will rise
in the future, and thus the soil will eventually be below the groundwater table.
3. CSR induced by earthquake: If the soil meets the above two requirements, then the
simplified procedure can be performed. The first step in the simplified procedure is to
determine the cyclic stress ratio (CSR) that will be induced by the earthquake (Sec. 6.4.2).
A major unknown in the calculation of the CSR induced by the earthquake is the peak
horizontal ground acceleration a max that should be used in the analysis. The peak horizon-
tal ground acceleration is discussed in Sec. 5.6. Threshold values needed to produce lique-
faction are discussed in Sec. 6.3 (item 1). As previously mentioned, a liquefaction analysis
would typically not be needed for those sites having a peak ground acceleration a max less
than 0.10 g or a local magnitude M L less than 5.
4. CRR from standard penetration test: By using the standard penetration test, the
cyclic resistance ratio (CRR) of the in situ soil is then determined (Sec. 6.4.3). If the CSR
induced by the earthquake is greater than the CRR determined from the standard penetra-
tion test, then it is likely that liquefaction will occur during the earthquake, and vice versa.
5. Factor of safety (FS): The final step is to determine the factor of safety against liq-
uefaction (Sec. 6.4.4), which is defined as FS CRR/CSR.