Critical state characterisation of a natural low-plasticity silt from the Halden NGTS site

¹ Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya, Barcelona, Spain
² Department of Architecture, Construction and Design, Universidad del Bío-bío, Concepción, Chile
³ Dipartimento di Ingegneria Civile e Ambientale, Politecnico di Milano, Milan, Italy
⁴ CIMNE– Universitat Politècnica de Catalunya– BarcelonaTech, Barcelona, Spain
⁵ Norwegian Geotechnical Institute, Oslo, Norway
⁶ Sweco Norge AS, Trondheim, Norway

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

This paper presents a critical state characterization of a natural low-plasticity silt from the Halden NGTS site (Norway), based on a new laboratory dataset obtained from fully reconstituted loose specimens tested in both drained and undrained triaxial compression over a practical stress range. The results provide a mechanically consistent definition of the Critical State Line (CSL), overcoming limitations associated with existing laboratory data for this deposit, which are largely affected by sampling-induced densification and therefore biased towards denser states. Combining the laboratory-derived CSL with CPT-based interpretation methods, the in-situ state of the Halden silty soil is reassessed. The analysis indicates that the soil is significantly looser than previously inferred from laboratory testing on retrieved samples. This finding has important implications for both the interpretation of historical “intact” laboratory results and the general characterization of transitional silty soils. While the identified loose in-situ state is consistent with a contractive response and potential susceptibility to liquefaction, the main contribution of the study lies in demonstrating the importance of mechanically consistent CSL identification and in highlighting the impact of sampling disturbance on state assessment. Overall, the work provides practical guidance for the reliable characterization of low-plasticity silts within a critical-state framework and supports improved interpretation of both laboratory and in-situ data.