Graduate Thesis Or Dissertation
 

Region-Specific Investigation of the Cone Penetration Test for Preliminary Classification and Estimation of Preconsolidation Stress, Undrained Shear Strength, and Cyclic Resistance of Transitional Silts

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3f462d729

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  • The Cone Penetration Test (CPT) has been widely-adopted for rapid, cost-effective exploration of the subsurface and reduction in the effort associated with drilling, sampling, and laboratory testing of soil. The adoption of the CPT has stemmed in part to the correlation of measurements to preliminary soil classification, stress history, drained and undrained shear strength parameters, and cyclic resistance for liquefaction evaluations. However, many correlations that have been developed for use with the CPT have been drawn from global databases, with the potential for improvement in accuracy following region-specific assessments. The potential for improvement through recalibration of correlations to fluvial deposits is counterbalanced by the known difficulty in characterizing soils that transition from the idealized sand-like (coarse-grained, drained shearing response) and clay-like (plastic fine-grained, undrained shear response) paradigm. This study assesses the suitability of the CPT as a means to make preliminary soil behavior type classifications, estimate preconsolidation stress and undrained shear strength, and predict cyclic resistance of the transitional silts widely-distributed throughout Southwest Washington and Western Oregon. Two sites in Western Oregon were drilled for the purposes of obtaining samples and performing advanced laboratory tests to augment stress history, undrained shear strength, and cyclic resistance data assembled from previous project sites. Then, the CPT data associated with each site was paired to the samples retrieved from adjacent boreholes in order to assess the accuracy of preliminary classifications made using the CPT-based Soil Behavior Type Index and Common Origin Q. In general, the two CPT-based preliminary soil classification methods assessed exhibited accuracies of fifty percent relative to Unified Soil Classification System (USCS) classifications, consistent with previous observations regarding the difficulty of the CPT to accurately characterize transitional silt soils. Next, region-specific CPT correlations for the preconsolidation stress, ’p, and undrained shear strength were developed in order to provide an alternative means for quantifying stress history and strength parameters in the absence of laboratory test results. The laboratory test database of monotonic undrained shear strength was also used to establish a region-specific cone factor, Nkt, and separate Stress History and Normalized Soil Engineering Parameters (SHANSEP) model for the silts of Western Oregon and which could leverage the new CPT-based ’p correlation. This study concludes with an investigation of the accuracy of CPT-based cone tip resistance to correlate to the cyclic resistance of the silt soils considering both sand-like and clay-like behavior and the assembled database of constant volume, stress-controlled cyclic direct simple shear (DSS) test data. The assessment of sand-like cyclic resistance associated with liquefaction triggering considered the overburden stress- and clean sand-corrected cone tip resistance, qc1Ncs, and the recent Common Origin Q Method, whereas the investigation into clay-like cyclic resistance associated with cyclic softening was assessed using the pore pressure-corrected cone tip resistance and the region-specific SHANSEP model considering laboratory and CPT-correlated ’p. In general, the qc1Ncs-based liquefaction triggering procedure consistently under-estimated, whereas the Q Method consistently over-estimated the measured cyclic resistance. These accuracy of these two approaches improved for non-plastic silts with larger qc1Ncs and Q. On the other hand, the cyclic resistances computed assuming clay-like behavior using the region-specific Nkt and SHANSEP models generally over-estimated the cyclic resistance, with improvements for moderate and high-plasticity silts. These results indicate that drilling, sampling, and laboratory testing of transitional silts remains necessary for accurate geotechnical characterization, and that the CPT should not be used in lieu of site-specific cyclic laboratory testing, which continues to represent the best means for quantifying the cyclic strength of transitional silt soils.
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