Undrained shear strength plays a fundamental role on the behaviour of clays. In overconsolidated clays, this parameter is largely influenced by test conditions, namely consolidation stress. “Prazeres Clay” is a Miocene overconsolidated formation, that can be found in a significant part of Lisbon area. Over the last decades a number of very relevant constructions have generated a large database for physical and mechanical properties of Miocene clays. Included in a broader study at the Faculty of Engineering of Porto University about Miocene clay’s physical and mechanical properties, existing data was gathered, treated and critically analysed, in order to establish a useful framework for geotechnical designers. This paper presents the results obtained for undrained shear strength, obtained from triaxial tests and Ménard Pressuremeter tests. It addresses the main difficulties associated with test’s interpretation and presents a discussion on how theoretical values relate to experimental ones. The paper proposes a range of variation for Prazeres Clay’ undrained shear strength based on a significant amount of test results, that is considered to be useful for geotechnical design. Article Highlights Undrained Shear strength is a relevant parameter for clays, and is usually derived from triaxial tests For overconsolidated clays, this parameter is highly dependent on preconsolidation stress, and on its relation to in situ stress. Based on a significant set of data, the paper presents a simple methodology for estimating this parameter

The presence of groundwater in clayey soils is known to affect its time-dependent behaviour. This paper presents some of the results of a parametric study performed in order to evaluate the influence of the construction technique and the soil/lining relative permeability, in the time-dependent behaviour of a shallow tunnel in overconsolidated clay. The excess pore pressures generated by the excavation are analysed and the evolution of the stress-paths with time around the tunnel section is presented. The movements induced on the soil surface and in depth as well as the loads acting on the support immediately after construction and in long-term conditions are analysed. A circular tunnel excavated at a depth of 20 m was considered and the analysis was performed using a two-dimensional finite-element code that included the MIT-E3 constitutive model.

In the framework of a research project led by the Faculty of Engineering -University of Porto (FEUP), an investigation and characterization campaign was carried out on a profile of the Lisbon Prazeres clay and limestone formation. The campaign comprised the use of in situ P and S-waves seismic cross-hole (CH) tests, using a borehole hammer and a sparker source, as well as mechanical tests such as SPT, PMT and SBPT. In addition, laboratory tests were carried out on undisturbed samples, using bender and ultrasonic transducers prior and during triaxial tests. The fact that two different in situ seismic sources were used allowed a comparative seismic signal analysis in time, frequency and time-frequency domains, using the Short Time Fourier Transform, STFT, and the S-Transform, ST. Following a summarized description of the developed work and obtained direct and indirect results, this paper addresses the analysis and comparison of the referred CH results using the STFT and ST approach.