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CPT Liquefaction Assumptions: Technical Documentation (geotech_lib version 0.0.97)

Infinity Team
Updated

Assumptions

The following assumptions have been adopted in the Liquefaction Calculator module for CPTs.

Liquefaction analysis:

  • Pre-drill depth is not analysed. On the liquefaction triggering plots it is shown as a greyed-out zone. However, for the calculation of the damage indicators (e.g. Sv1D, LSN, LPI, etc) the pre-drill zone is treated as if this layer does not liquefy. If the design groundwater level is above the pre-drill level, and the pre-drill zone is of a soil type that is susceptible to liquefaction, then in the ‘Inverse Filtering’ tab in the CPT Transform Module the user can manually set the pre-drill to zero and then in the ‘customise CPT trace’ option manually enter the estimated qc and fs values for the pre-drill zone.
  • The static shear stresses are assumed to be negligible in the liquefaction analysis (i.e. the calculations assume flat ground).
  • Cone resistance (qc) and cone tip resistance (qt) are used interchangeably in the analysis. All analysis are conducted using qc.
  • Negative cone resistance (qc) and sleeve friction (fs) values are replaced with 1 kPa and 0.1 kPa respectively. If the user wants to use alternative replacement values, use the ‘customised CPT trace’ option in the ‘Inverse Filtering’ tab in the CPT Transform Module.
  • When calculating the post-liquefaction volumetric strain (εv) for free-field liquefaction induced settlement based on Fig. 3 and Appendix A in Zhang, Robertson and Brachman (2002), qc1Ncs values less than 33 and above 200 are bounded to 33 and 200 respectively.
  • When calculating the post-liquefaction volumetric strain (εv) for free-field liquefaction induced settlement based on Fig. 3 and Appendix A in Zhang, Robertson and Brachman (2002), factor of safety against liquefaction (FOSliq) values less than 1.3 and above 2.0 are bounded to 1.3 and 2.0 respectively. Linear interpolation is used for intermediary FOSliq values specified in Appendix A.
  • When calculating maximum cyclic shear strain (γmax) for estimating liquefaction induced lateral displacement based on Fig. 1 and the Appendix in Zhang, Robertson and Brachman (2004), the relative density (Dr) values less than 40% and above 90% are bounded to 40% and 90% respectively.
  • Equation 2 in Zhang et al. (2004) quotes Tatsuoka et al. (1990), which uses qc1N to estimate the relative density (Dr). However, the method developed by Tatsuoka et al. (1990) was based on clean sand. Therefore, the clean sand equivalent normalised cone resistance qc1Ncs was used instead of qc1N in this equation.
  • When Dr is estimated using Eqn 2 in Zhang et. al (2004), qc1Ncs values greater than 200 are capped at 200 in this equation.
  • Applied fill is assumed to be non-liquefiable.
  • The crust thickness is the thickness of material from the ground surface to the first point of liquefaction. However, it is only calculated if the cumulative thickness of liquefaction in the CPT is greater than 0.5 m. If the cumulative thickness is less than 0.5 m, the crust thickness is set to the total depth, from the design ground surface to the bottom of the CPT.
  • The Magnitude Scaling Factor (MSF) is applied to the CSR trace as per Eqn 2.6 in Boulanger and Idriss (2014). This means the CSR and CRR calculated and presented in the results are both adjusted to a magnitude of 7.5 for consistency. When using the custom CSR function in the liquefaction calculator, note that the CRR calculated in the liquefaction module still corresponds to a magnitude of 7.5. Therefore, the custom CSR trace should be scaled to a reference magnitude of 7.5, by applying MSF to the custom CSR trace prior to input in the liquefaction calculator.

Cyclic softening analysis:

  • The static shear stresses are assumed to be negligible in the cyclic softening analysis (i.e. the calculations assume flat ground). Kα is assumed to be 1.0 when cyclic softening assessment is carried out using Boulanger and Idriss (2007).
  • The same cyclic stress ratio (CSR) from liquefaction analysis is used for cyclic softening assessment. i.e. if the liquefaction assessment is carried out using Boulanger and Idriss (2014), and the cyclic softening assessment is carried out using Boulanger and Idriss (2007), the CSR calculated using Boulanger and Idriss (2014) is used for both liquefaction assessment and cyclic softening assessment. Boulanger and Idriss (2007) does not provide a way to estimate τpeak, therefore the CRR calculated is not empirically coupled with the method of assessing CSR from that paper, as this method is developed based on laboratory testing.
  • In Boulanger and Idriss (2007), the CRR against cyclic softening can be estimated from undrained shear strength (Su) or over-consolidation ratio (OCR). If the Su method is used, the Su profile from the CPT Transform module is used to estimate the CRR as per Equation 13 in Boulanger and Idriss (2007). If the OCR method is used, the OCR profile from the CPT Transform module is used to estimate the CRR as per Equation 15 in Boulanger and Idriss (2007).
  • The cyclic resistance ratio (CRR) for cyclic softening uses the vertical effective consolidation stress to calculate CRR in Equation 13 and 15 in Boulanger and Idriss (2007). The vertical effective consolidation stress is assumed to be the larger of the vertical effective stress during investigation and the vertical effective stress during design, at each along the depth. This is slightly conservative, because this neglects the future potential increase in undrained shear strength due to additional vertical effective stress. The undrained shear strength estimated using vertical effective stress during investigation is used throughout the analysis.
  • Cyclic softening above the design water table is neglected in the calculation. i.e. Apeiron assumes soil above the design water table is not susceptible to cyclic softening.

Residual strength ratio and residual shear strength:

  • The residual strength ratio is the ratio between the shear strength at a depth and the vertical effective stress during design, not the vertical effective stress during investigation.
  • The residual shear strength ratio cannot exceed the lesser of tan(φ) or 1.0. The friction angle φ from the CPT Transform module is used.
  • When residual strength assessment is carried out using Weber (2015) or Kramer & Wang (2015), the equivalent N1,60 profile from the CPT Transform module is used.
  • When Boulanger and Idriss (2007) is used, qc1N calculated from Robertson and Wride (1998) is used, and fines content correction is carried out as per Table 3 in Boulanger and Idriss (2007). This is because Boulanger and Idriss (2007) is an SPT-based method, but it also provdes a method to map the SPT-based relationship to a CPT-based relationship as part of the paper. The CPT-based method in the paper accepts qc1N-Sr as one of the inputs and provides a way to convert qc1N-Sr to qc1Ncs-Sr in Table 3. Apeiron adopts the proposed method in Boulanger and Idriss (2007) for the fines content correction in assessing the residual strength ratio, rather than the fines content correction method proposed in Boulanger and Idriss (2014) when the liquefaction assessment is performed.

 

Benchmarking against Cliq

We have benchmarked our calculation results against Cliq. The results are available here:

https://infinitystudiohelp.zendesk.com/hc/en-us/articles/13625448260367-CPT-Liquefaction-Calculator-benchmarking-agasint-Cliq

 

References

Liquefaction triggering, liquefaction induced settlement and lateral spreading:

  • Zhang, G., Robertson, P. K., & Brachman, R. W. (2002). Estimating liquefaction-induced ground settlements from CPT for level ground. Canadian Geotechnical Journal, 39(5), 1168-1180.
  • Zhang, G., Robertson, P. K., & Brachman, R. W. I. (2004). Estimating liquefaction-induced lateral displacements using the standard penetration test or cone penetration test. Journal of Geotechnical and Geoenvironmental Engineering, 130(8), 861-871.
  • Tatsuoka, F., Zhou, S., Sato, T., & Shibuya, S. (1990). Evaluation method of liquefaction potential and its application. Report on seismic hazards on the ground in urban areas. Ministry of Education of Japan, Tokyo, 75-109.
  • Boulanger, R. W., & Idriss, I. M. (2007). Evaluation of cyclic softening in silts and clays. Journal of geotechnical and geoenvironmental engineering, 133(6), 641-652. Boulanger, R. W., & Idriss, I. M. (2014). CPT and SPT based liquefaction triggering procedures. Report No. UCD/CGM.-14, 1, 134.
  • Robertson, P. K., & Wride, C. E. (1998). Evaluating cyclic liquefaction potential using the cone penetration test. Canadian geotechnical journal, 35(3), 442-459.
  • Boulanger, R. W., & Idriss, I. M. CPT and SPT based liquefaction triggering procedures. 2014. Center for Geotechnical Modeling, University of California at Davis, California. Report No. UCD/CGM-14/01.

Residual strength:

  • Weber, J. P. (2015). Engineering evaluation of post-liquefaction strength. University of California, Berkeley.
  • Kramer, S. L., & Wang, C. H. (2015). Empirical model for estimation of the residual strength of liquefied soil. Journal of Geotechnical and Geoenvironmental Engineering, 141(9), 04015038.

Undrained shear strength:

  • Lunne, T., Robertson, P.K., and Powell, J.J.M. (1997). Cone Penetration Testing in Geotechnical Practice.
  • Robertson, P. K. (2012, September). The James K. Mitchell Lecture: Interpretation of in-situ tests–some insights. In Proc. 4th Int. Conf. on Geotechnical and Geophysical Site Characterization–ISC (Vol. 4, pp. 3-24).
  • Robertson, P. K., & Cabal, K. L. (2015). Guide to cone penetration testing for geotechnical engineering. Signal Hill, CA: Gregg Drilling & Testing.
  • Mayne, P.W. & Peuchen, J. (2018). Evaluation of CPTU Nkt cone factor for undrained strength of clays. Cone Penetration Testing 2018 (Delft), CRC: 423–429.
  • Mayne, P. W., & Peuchen, J. (2022). Undrained shear strength of clays from piezocone tests: A database approach. In Cone penetration testing 2022 (pp. 546-551). CRC Press.

 

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