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1. Harnessing Numerical Tools to Study the Limitations of CPTs for Characterizing Complex Soil Stratigraphies for Liquefaction Assessment

   Yost, K.M. (June 2022, Proc. 12th National Conf. on Earthquake Engineering (12NCEE))

   This paper describes the use of the Material Point Method (MPM) to simulate cone penetrometer testing (CPT) in complex soil profiles. CPT-based liquefaction evaluation procedures have been shown to be inaccurate in highly interlayered soil stratigraphies. One contributing factor to this inaccuracy is that CPT measurements at discrete depths reflect the properties of all soils that fall within a zone of influence around the cone tip, not just the properties of the soil at a particular depth. Consequently, the CPT loses resolution in soil profiles with many thin, interbedded soil layers (multiple thin-layer effects) and provides inaccurate input data to liquefaction analyses. While several procedures have been proposed to correct for multiple thin-layer effects, they tend to decrease in efficacy as the thickness of soil layers decreases. Results from the MPM analyses detailed in this paper highlight limitations of (1) the CPT in characterizing complex soil stratigraphies and (2) procedures proposed to correct for multiple thin-layer effects in CPT data. 

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2. Robust Identification and Characterization of Thin Soil Layers in Cone Penetration Data by Piecewise Layer Optimization

   https://doi.org/j.compgeo.2021.104404  

   Cooper, J. (January 2022, Computers and geotechnics)

   Cone penetration testing (CPT) is a preferred method for characterizing soil profiles for evaluating seismic liquefaction triggering potential. However, CPT has limitations in characterizing highly stratified profiles because the measured tip resistance (𝑞𝑐 ) of the cone penetrometer is influenced by the properties of the soils above and below the tip. This results in measured 𝑞𝑐 values that appear ‘‘blurred’’ at sediment layer boundaries, inhibiting our ability to characterize thinly layered strata that are potentially liquefiable. Removing this ‘‘blur’’ has been previously posed as a continuous optimization problem, but in some cases this methodology has been less efficacious than desired. Thus, we propose a new approach to determine the corrected 𝑞𝑐 values (i.e. values that would be measured in a stratum absent of thin-layer effects) from measured values. This new numerical optimization algorithm searches for soil profiles with a finite number of layers which can automatically be added or removed as needed. This algorithm is provided as open-source MATLAB software. It yields corrected 𝑞𝑐 values when applied to computer-simulated and calibration chamber CPT data. We compare two versions of the new algorithm that numerically optimize different functions, one of which uses a logarithm to refine fine-scale details, but which requires longer calculation times to yield improved corrected 𝑞𝑐 profiles. 

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3. Inverse filtering procedure to correct cone penetration data for thin-layer and transition effects

   Boulanger, Ross W.; DeJong, Jason T. (June 2018, Proc., Cone Penetration Testing 2018, Hicks, Pisano, and Peuchen, eds., Delft University of Technology, The Netherlands)

   This paper presents an inverse filtering procedure for developing estimates of "true" cone penetration tip re-sistance and sleeve friction values from measured cone penetration test data in interlayered soil profiles. Re-sults of prior studies of cone penetration in layered soil profiles are utilized for developing and evaluating the inverse filtering procedure. The inverse filtering procedure has three primary components: (1) a model for how the cone penetrometer acts as a low-pass spatial filter in sampling the true distribution of soil resistance versus depth, (2) a solution procedure for iteratively determining an estimate of the true cone penetration resistance profile from the measured profile given the cone penetration filter model, and (3) a procedure for identifying sharp transition interfaces and correcting the data at those interfaces. The details of the inverse filtering pro-cedure presented herein were developed with a focus on liquefaction problems, but the concepts and frame-work should be applicable to other problems. Example applications of the inverse filtering procedure are pre-sented for four CPT soundings illustrative of a range of soil profile characteristics. The proposed procedure provides an objective, repeatable, and automatable means for correcting cone penetration test data for thin-layer and transition zone effects. 

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4. ASSESSING LIQUEFACTION IN GRAVELLY SOILS BASED ON FIELD CASE HISTORIES [ASSESSING LIQUEFACTION IN GRAVELLY SOILS BASED ON FIELD CASE HISTORIES]

   https://doi.org/10.5592/CO/2CroCEE.2023.141  

   Rollins, Kyle; Roy, Jashod (March 2023, Proceedings of the 2nd Croatian Conference on Earthquake Engineering)

   Gravelly soils have liquefied at multiple sites in at least 27 earthquakes over the past 130 years. These gravels typically contain more than 25% sand which lowers the permeability and makes them susceptible to liquefaction. Developing a reliable, cost-effective liquefaction triggering procedure for gravelly soils has been a challenge for geotechnical engineers. Typical SPT- or CPT-based correlations can be affected by large-size gravel particles and can lead to erroneous results. To deal with these problems, we have developed liquefaction triggering curves for gravelly soils based on (1) shear wave velocity (Vs) and (2) a large diameter cone penetrometer. With a cone diameter of 74 mm, the Chinese Dynamic Cone Penetration Test (DPT) is superior to smaller penetrometers and can be economically performed with conventional drilling equipment. Using logistic regression analysis, the DPT has been directly correlated to liquefaction resistance at sites where gravels did and did not liquefy in past earthquakes. Probabilistic liquefaction resistance curves were developed based on 137 data points from 10 different earthquakes in seven countries. Using a similar data set, probabilistic liquefaction triggering curves were also developed based on Vs measurements in gravelly soils. The Vs-based liquefaction triggering curves for gravels shift to the right relative to similar curves based on sands. New magnitude scaling factor (MSF) curves have also been developed specifically for gravel liquefaction which were found to be reasonably consistent with previous curves for sand. 

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5. A new shared miniature cone penetrometer for centrifuge testing

   Carey, T. (July 2018, International Conference on Physical Modeling in Geotechnics)

   Cone penetrometers (CPTs) are commonly used for characterising the soil properties of centrifuge models; CPT data is useful for interpretation and quality control. This paper describes the development and design of a new robust CPT device for centrifuge testing. The new device consists of a 6mm cone, an outer sleeve, and an inner rod that transmits cone tip forces to a load cell above the ground surface. The design eliminates the need for a custom submerged strain gauge bridge near the tip, significantly reducing cost.A direct comparison was performed between this CPT device and another similar device developed at the University of Cambridge. CPT’s were manufactured using the new design and then shipped to eight different centrifuge facilities, for quality control of similar experiments performed for LEAP (Liquefaction Experiments and Analysis Projects). All the centrifuge tests simulated a 4 m deep deposit of soil, all consisting of Ottawa F-65 sand with relative densities ranging between about 45 to 80%. The results obtained have been extremely valuable as an independent assessment of the density calculated from mass and volume measurements at different laboratories. 

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