Liquefaction Software using CPT data

Twenty years ago, I had to prepare a report for a gas project, where a pipe was crossing a stream in northern Greece. The geotechnical data available were only CPTs, using the Begemann mechanical cone, where the cone resistance and the skin friction recorded every 20cm.

The soil materials encountered along the pipe route, were mainly medium dense sands and the ground water table measured at a shallow depth. Although the seismicity in the area of the project was not high, the importance level of the project was high enough to obtain quite significant design accelerations.

At that time, the typical in-situ test used as input for the liquefaction analysis, was the Standard Penetration Test. Since there were only CPT data available, I thought that I could complete the liquefaction analysis, by correlating/converting the CPT to SPT data (averaging cone resistance values for each soil layer encountered) and using the graph from Seed et al. 1985 in Figure 1.

Figure 1. SPT Clean Sand Base Curve for Magnitude 7.5 Earthquakes with data from liquefaction case histories (modified from Seed et al. 1985).

 During the process of liquefaction analysis, I felt that the bulk of the CPT data were underutilized in the analysis, and perhaps the results were overestimating or underestimating the liquefaction potential. I then started a methodical search in literature to find more about CPT & liquefaction and utilize CPT data as much as possible. One of the papers that I came across, was from Robertson & Campanella 1985. In this paper, there was a graph presenting a zone on the CPT classification chart, developed by Douglas and Olsen 1981, in which the soil material within this zone was considered susceptible to liquefaction (Zone A in Figure 2). This designated zone was developed by the work from Douglas 1982 and experience gained at the University of British Columbia (UBC).

Figure 2. Soil Classification Chart for CPT showing proposed zone of liquefiable soils (Robertson & Campanella 1985)

Instantly, I thought that this graph could be used as a screening tool in the process for the calculation of the liquefaction potential, where the soils would be only considered liquefiable, if they were within the A zone. This screening tool became later the first step in the liquefaction analysis process that was finally concluded.

 Now there was one more thing missing in order to complete the liquefaction analysis, and this was how to quantify the severity of liquefaction. How to assess if there is a risk of liquefaction at a site, where only certain points at certain random depths appear to be liquefiable. This was something that a team from Japan (Iwasaki et al. 1978) had been working on already, for more than 10 years, where a method to calculate the liquefaction potential was proposed. In 1982 Iwasaki et al., used the term Liquefaction Potential Index (LPI) to quantify the severity of liquefaction. LPI uses the F (factor of Safety) from liquefaction triggering as well as a depth-based weighting function W(z) and was defined as shown in Figure 3.

Figure 3. Equations to calculate the Liquefaction Potential Index (LPI) according to Iwasaki, et al. 1982.

The LPI parameter assumes that the severity of liquefaction manifestation is proportional to the thickness of a liquefied layer, the amount by which FS is less than 1.0, and the proximity of the layer to the ground surface. The LPI parameter assumes that each liquefying soil layer contributes to some extent to the damage potential at the ground surface.

After the whole process had been concluded, the first spreadsheet was created including all the afore mentioned checks and analyses and is given in Figure 4. Please bear in mind that at that time the input data included apart from the CPT, the ground water table, the seismic acceleration, the earthquake magnitude, and the soil parameter.

Figure 4. Liquefaction analysis results using CPT data

 About three years later, in 2003, John Ioannides (Geologismiki) developed the first version of LiqIT, a software where the liquefaction potential using SPT and CPT data could be calculated. In 2007 John and I went to USA to meet Peter Robertson. With Peter's mentoring and help as well the support from Gregg Drilling the Cliq (Geologismiki) software was born. A dedicated software to perform liquefaction analysis using CPT data. Soon, CLiq became popular in many places and now the software is well known all over the world, having more than 2,000 users over 73 countries.

Below is a figure showing the output from the Cliq ver. 3.0.3.2 software using the same data as the one in Figure 4.

Figure 5. Liquefaction analysis results using CLiq and the CPT data shown in Figure 4.

Over the years many factors have been added in the liquefaction analysis (aging factor, transition zones, weighting factor etc) but the basic concept remains the same for simplified liquefaction assessment procedures. Although I think that the simplified liquefaction assessment is good for a first screening regarding liquefaction, we should consider using a bit more advanced tools to assess liquefaction in case that the importance level of the structure is high or the consequences from liquefaction are significant. As discussed by Cubrinovski 2019, many influencing factors are always in play resulting in a unique combination of contributions, for a given site and earthquake excitation. Unweaving this complexity and identifying key factors that govern the liquefaction response and associated damage should therefore be the principal target in the engineering assessment of liquefaction.

I would like to thank George Mavridis for providing the spreadsheet to manipulate and present CPT data, Gregg Drilling for “The Trip”, Peter Robertson for mentoring me for many years, and John Ioannides for the lovely journey working together on ideas to develop software for geotechnical engineers.  

References

Seed, H. B., Tokimatsu, K., Harder, L. F. and Chung, R. M. "Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations," Journal, Geotechnical Engineering Division, ASCE, Vol. III, No. 12, December 1985

Robertson, P. K. and Campanella R. G. “Liquefaction Potential of Sands Using the CPT”, Journal of Geotechnical Engineering, Vol. Ill, No. 3, March, 1985.

Douglas, B. J., and Olsen, R. S., "Soil Classification Using Electric Cone Penetrometer," Proceedings of the ASCE Symposium on Cone Penetration Testing and Experience, Geotechnical Engineering Division, St. Louis, Mo., 1981, pp. 209-227.

Douglas, B. J., "SPT Blow count Variability Correlated to the CPT," Proceedings of the 2nd European Symposium on Penetration Testing, ESOPT II, held at Amsterdam, The Netherlands, Vol. 1, 1982, p. 41.

Iwasaki, T., Tatsuoka, F., Tokida, K., and Yasuda, S. (1978). “A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan. ”Proc., 2nd Int. Conf. on Microzonation, National Science Foundation, Washington, DC.

Iwasaki, T., Arakawa, T. & Tokida, K. 1982, Simplified Procedures for Assessing Soil Liquefaction During Earthquakes Proc. Conference on Soil Dynamics and Earthquake Engineering. Southampton, 925-939

Cubrinovski M. 2019, Some important considerations in the engineering assessment of soil liquefaction, NZGS Geomechanics Lecture 2019.