Investigation of a Coarse Silt Deposit by Varied Rate CPTU Testing

Abstract

Well-explored methods for the interpretation of CPTU and laboratory tests for clay and sand materials exist, assuming undrained and drained conditions, respectively. The determination of design parameters in silt is not straightforward due to amongst other factors the presence of an unknown degree of partial drainage during CPTU testing which complicates the interpretation and influences the results. The geotechnical community has for a long time now attempted to establish an international acknowledged interpretation framework for intermediate soils such as silts, that often neither behave undrained nor drained but experience partial drainage during standard rate CPTU tests. In addition, silts are normally not homogeneous but have a rather complicated micro-layering that lead to a scattered and complex structure. The average engineering properties cannot be predicted without extensive testing. Even though recent research draws more attention to the behaviour of these soils both in the field and in the laboratory, there still exist an absence of a comprehensive and basic engineering database of silts. The focus needs to be on the influence of the recognized partial drainage on the interpreted design parameters and methods needs to be developed on how to account for this. The aim of the present PhD study is to broaden the knowledge of silt behaviour both in the laboratory and in the field and to contribute to the improvement of a more confident interpretation framework. In particular, recommendations are needed on how to consider the influence of partial drainage on the determination of design geotechnical parameters. To study the drainage behaviour of silt, an extensive field and laboratory research program was performed at the NTNU. The work was undertaken at the established Halsen- Stjørdal test site which is located north of Trondheim and consists of a 10 m thick natural, low plastic silt deposit. Alongside carrying out several standard CPTU tests, various investigations were conducted with penetration rates varying between 0.5 mm/s and 200 mm/s to cover the whole spectrum of drainage conditions. In addition, more than 40 dissipation tests were analyzed, that provide further insight into the development of pore pressures and drainage conditions at the site. To study the material behaviour of the silt, soil samples from the silt layer were taken and analyzed in the laboratory facilities at NTNU. The laboratory analysis of the soil samples emphasize the difficulties of handling the material in the laboratory and confirm the importance of improving the methods for sampling and quality assessment and hence the confidence in the geotechnical design parameters. Recommendations are made on how to select consolidation and strength parameters in silt. Nevertheless, sample disturbance is likely to occur and may influence the results. Varied rate CPTU tests show a strong rate dependency of the measured response on the penetration rate, typically with decreasing cone resistance and increasing negative excess pore pressure for increasing penetration rates. For many silts, the drainage conditions yield partially drained conditions during standard penetration. For high rate CPTU tests, the well-known Nkt approach reveals the most promising results for the undrained shear strength. For slow rate CPTU tests, the drained strength parameters are obtained using the NTH method. If standard penetration rate CPTU results are used for the interpretation, the undrained shear strength is overestimated whereas the drained friction angle is underestimated. Consolidation analysis of the measured dissipation data confirms the rate dependency on the established soil parameters. Dissipation data and consolidation analysis confirm drained behaviour after slow penetration tests whereas the faster tests show partial consolidated behaviour. If ignored, the presence of partial consolidation influences the interpreted coefficient of consolidation and needs to be accounted for to prohibit an underestimation of this parameter. To prevent misinterpretation of geotechnical design parameters, it is necessary to carry out additional varied rate tests in deposits where partial drainage is likely to occur. The results and findings from the present PhD study on a natural silt deposit highlight the challenges when dealing with silty materials and broaden the database internationally. In particular, the research emphasizes the importance of recognizing and accounting for partial drainage when interpreting CPTU tests in silts. It is hoped that this study may contribute to improved practice for how to establish realistic strength and consolidation parameters for silts enabling safe and economical designs.