Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8

Major Concepts

1. Three questions must be assessed when evaluating the liquefaction hazard of a given site.i) Is the soil potentially susceptible to liquefaction?ii) If potentially susceptible, will liquefaction be triggered by an earthquake event”iii) If liquefaction is triggered, what will be the consequences of liquefaction (i.e.,

what are the consequences and the potential magnitude of displacement?

2. Liquefaction susceptibility is assessed based on simple screening criteria discussed below.

3. Liquefaction triggering analyses are more detailed analysis which account for:i) The cyclic stresses induced by the earthquakeii) The soil’s resistance to liquefaction

Method used for assessing liquefaction triggering are mostly empirically based and use theStandard Penetration Test (SPT) and the Cone Penetrometer Test (CPT).

4. Methods to assess the consequences of liquefaction are presented in Lecture 10.

Definition - Liquefaction

During strong earthquake shaking, loose, saturated cohesionless soil deposits may experience asudden loss of strength and stiffness, sometimes resulting in loss of bearing capacity, largepermanent lateral displacements, and/or seismic settlement of the ground. This phenomenon iscalled soil liquefaction.

Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8

Types of Liquefaction-Induced Ground Failure

! Flow Failure

! Lateral Spread

Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8

! Ground Oscillation

! Bearing Capacity Failure

Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8

! Excessive Settlement

Excessive total settlement or differential settlement that damages a structure and reduces itfunctionality or renders it useless.

Factors Affecting Liquefaction Susceptibility

Screening Criteria - Used to determine if the soil is potentially susceptible to liquefaction. Thefive screening criteria listed below are commonly used.

• Geologic age and origin. Liquefaction potential decreases with increasing age of a soildeposit. Pre- Holocene age soil deposits generally do not liquefy, though liquefaction hasoccasionally been observed in Pleistocene-age deposits (see Table 8-1).

• Fines content and plasticity index. Liquefaction potential decreases with increasing finescontent and increasing plasticity index, PI. Data presented in Figure 8-1 (Ishihara, et al., 1989)show grain size distribution curves of soils known to have liquefied in the past. This data serves asa rough guide for liquefaction potential assessment of cohesionless soils. Soils having greater than15 percent (by weight) finer than 0.005 mm, a liquid limit greater than 35 percent, or an in-situwater content less than 0.9 times the liquid limit generally do not liquefy (Seed and ldriss, 1982).

• Saturation. Although unsaturated soils have been reported to liquefy, at least 80 to 85 percentsaturation is generally deemed to be a necessary condition for soil liquefaction. In many locations,the water table is subject to seasonal oscillations In general, it is prudent that the highestanticipated seasonal water table elevation be considered for initial screening.

• Depth below ground surface. While failures due to liquefaction of end-bearing piles restingon sand layers up to 30 in below the ground surface have been reported, shallow foundations aregenerally not affected if liquefaction occurs more fim 15 in below the ground surface.

• Soil penetration resistance. According to the data presented in Seed and ldriss (1982),liquefaction has not been observed in soil deposits having SPT (N1)60 values larger than 22.Marcuson, et al. (1990) suggest a normalized SPT value of 30 as the threshold value above whichliquefaction will not occur. However, Chinese experience, as quoted in Seed, et al. (1983),suggests that in extreme conditions liquefaction is possible in soils having normalized SPT blowcounts as high as 40. Shibata and Teparaska (1988), based on a large number of observations,conclude that no liquefaction is possible if normalized Cone Penetration Test (CPT) coneresistance, qc , is larger than 15 MPa. (Note: 150 Mpa = 15000 Kpa . 150 tsf. Also, using qc/N1

ratio of 4 for sands, this is 150/4 = 27, or an SPT (N1)60 of about 37.

Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8

If three or more of the above criteria indicate that liquefaction is not likely, the potential forliquefaction may be considered to be small enough that a formal liquefaction potential analysis isnot required. If, however, based on the above initial screening criteria, the potential forliquefaction of a cohesionless soil layer beneath the site cannot be dismissed, more rigorousanalysis of liquefaction potential is needed.

Geotechnical Earthquake EngineeringSubject: LiquefactionLecture No. 8