Threshold Load Factor for Liquefaction Triggering Evaluations
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 141, Issue 10
The -based chart developed by Andrus and Stokoe (2000) becomes less conservative when a constant cyclic shear strain, , is used for liquefaction triggering (Dobry et al. 2015), where = shear wave velocity. Specifically, natural silty sands in the Imperial Valley of California, heavily preshaken by earthquakes, have a triggering resistance for a given normalized significantly higher than uncompacted clean and silty sand fills. A higher is required for triggering in the natural sands than in the fills (0.1–0.2% versus 0.03%). Dobry et al. (2015) reached this conclusion within the usual liquefaction chart format, plotting versus normalized shear wave velocity, ( in kPa). The next logical step would be to plot directly versus . However, using is not practical for several reasons. The authors propose instead use of the threshold load factor (TLF)where CSR; and = volumetric threshold shear stress needed to start buildup of excess pore pressures. TLF measures how much bigger the earthquake demand, , is compared to the shear stress needed for the development of any excess pore pressure. The denominator in Eq. (1), , is directly related to the volumetric threshold shear strain, , which is notably constant and equal to about in most clean and silty sands of interest. That is, , where is a representative value of the modulus reduction curve in sands for a shear strain, . Also, , where = saturated soil density. In first approximation, . Finally
(1)
(2)
As , then ; this expression combined with Eqs. (1) and (2) gives , which is a one-to-one relationship between TLF and . This confirms that TLF and are two sides of the same coin. Fig. 1 shows the plot of TLF versus for an earthquake magnitude, , for the same case histories of natural sands in the Imperial Valley already plotted as CSR versus by Dobry et al. (2015). All TLF were calculated with Eq. (2) using the same parameters listed by Dobry et al. (2015). Fig. 1 is consistent with the assumption that needed for triggering in the Imperial Valley soils for is independent of . The dashed line corresponds to , computed in a similar way as used for the uncompacted fills. As expected, in the Imperial Valley is significantly greater than for uncompacted fills.
![](/cms/10.1061/(ASCE)GT.1943-5606.0001399/asset/a00d383c-4273-4849-914d-72a9d6a3b130/assets/images/large/figure1.jpg)
Implications
The authors recommend measurement of both and penetration resistance (SPT or CPT) in the field. For a given family of soils (such as the two families in Fig. 1), should be constant and independent of and penetration resistance. Mining of available liquefaction case-history large databases that include measurements may allow direct plotting of versus , with this curve replacing the current magnitude scaling factor.
References
Andrus, R. D., and Stokoe, K. H., II (2000). “Liquefaction resistance of soils from shear-wave velocity.” J. Geotech. Geoenviron. Eng., 1015–1025.
Dobry, R., Abdoun, T., Stokoe, K. H., II, Moss, R. E. S., Hatton, M., El Ganainy, H. (2015). “Liquefaction potential of recent fills versus natural sands located in high seismicity regions using shear-wave velocity.” J. Geotech. Geoenviron. Eng., 04014112.
Information & Authors
Information
Published In
Copyright
© 2015 American Society of Civil Engineers.
History
Received: May 22, 2015
Accepted: Jun 29, 2015
Published online: Jul 23, 2015
Published in print: Oct 1, 2015
Discussion open until: Dec 23, 2015
Authors
Metrics & Citations
Metrics
Citations
Download citation
If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download.