CPTU correlations for Norwegian clays: an update

Priscilla Paniagua,Kjell Karlsrud,Marco D'Ignazio,Tom Lunne,Jean-Sébastien L'Heureux

doi:10.3934/geosci.2019.2.82

Priscilla Paniagua, Kjell Karlsrud + Show 3 more

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https://doi.org/10.3934/geosci.2019.2.82

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Journal: AIMS Geosciences	Publication Date: Jan 1, 2019
Citations: 15	License type: cc-by

Affiliation: Norwegian Geotechnical Institute

Abstract

Geotechnical design in clay areas in Norway is mainly based on piezocone (CPTU) tests results. Strength and stiffness parameters are usually derived from CPTU parameters and empirical correlations. In order to improve geotechnical design practice (e.g. more cost-effective solutions) and to reduce risks related to the occurrence of catastrophic events (e.g. landslides, excavation failure) the Norwegian Geotechnical Institute (NGI) has recently updated its block sample database and worked on updating CPTU correlations for clays. This paper provides a short overview of NGI's block sample database consisting of 61 block samples data points collected from 17 Norwegian clay sites. Multiple regression analyses were used to evaluate possible correlations among CPTU parameters (e.g. excess pore pressure, Δu, net cone resistance, qnet, and effective cone resistance, qe), undrained shear strength (suC) and basic clay properties (e.g. overconsolidation ratio, OCR, plasticity, sensitivity). The target was to establish correlations characterized by low uncertainty. The most reliable assessment of undrained strength was obtained when using the Stress History and Normalized Soil Engineering Properties, SHANSEP, framework associated with the best estimate OCR profile extrapolated from the CPTU measurements. This well reflects the strong relation that suC has with OCR. Despite the high quality of the samples, high scatter was observed for some of the equations that compare cone factors and basic soil parameters. In addition to the natural variability of soil properties, other possible reason to justify the scatter is that even though the accuracy of CPTU probes has improved over the past decades, especially in terms of the ability to measure low values, the results can vary among the different manufacturers. Furthermore there may be several other soil parameters than the peak undrained strength that impacts the cone resistance, for instance stiffness and large strain behavior. Such factors can affect the correlation results.

Highlights

In order to improve geotechnical design practice and to reduce risks related to the occurrence of catastrophic events the Norwegian Geotechnical Institute (NGI) has recently updated its block sample database and worked on updating CPTU correlations for clays
This paper provides a short overview of NGI's block sample database consisting of 61 block samples data points collected from 17 Norwegian clay sites
A high-quality database of clays consisting of laboratory strength and consolidation test results, index parameters and CPTU parameters was evaluated, with the help of multiple regression analyses, to establish correlations among CPTU parameters, undrained shear strength from anisotropically consolidated triaxial compression consolidated undrained triaxial compression tests (CAUC) tests and some basic clay properties

Summary

Introduction

All building and construction works require reliable and proper selection of geotechnical design parameters. A thoughtful choice of the most appropriate investigation method is likely to result in improved understanding of soil behavior and, in more cost-effective and sustainable solutions for the construction, transport and energy sectors. There is a need for better understanding of the behavior of soft clays in order to improve geotechnical design, make it more innovative, and to reduce risks related to e.g. landslides and excavation failures. The Norwegian Geotechnical Institute (NGI) has carried out several studies on characterization of clays and on effect of sample quality on the choice of geotechnical design parameters. The use of high-quality block samples (Ø250 mm) over the more traditional Ø54 mm or Ø72 mm samples seemed to ensure better sample quality [1,2,3], which is mainly reflected in the higher measured undrained shear strength (suC), higher inferred preconsolidation pressure (σp') and higher measured soil stiffness (both drained and undrained)

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