The geotechnical risk of a tunnel is strongly dependent on ground properties and on the construction techniques and rate of advance. In the urban environment, design is often determined by serviceability limit states of adjacent structures and, to a lesser extent, of the tunnel itself. The observational method (OM) is an effective risk management tool in geotechnical engineering and particularly in tunnel construction. It helps with deciding in the presence of uncertainty in ground conditions. A fundamental prerequisite for its application is that only epistemic uncertainties are present, so that uncertainty reduction of the relevant parameters may be achieved by monitoring. Deterministic models are often used, limiting the versatility and significance of the OM. In this paper conceptual considerations about the implications of ground uncertainty with due account of spatial correlation are presented. A combined computational framework for random finite difference models based on MATLAB and FLAC is introduced. A case study of a tunnel in stiff clayey ground is presented to evaluate the relative importance of the magnitude, type of variability, and spatial correlation of both deformability and shear resistance parameters. A simulated application of the OM is presented with a comparative analysis of the decisions in an OM approach.

Many immersed tunnels are constructed in alluvial formations within earthquake-prone regions, making seismic resistance a critical aspect of their safety design. During an earthquake, tunnel displacements can lead to slippage between the tunnel and surrounding soil and may be further amplified by liquefaction. This phenomenon can cause severe structural damage, including tunnel flotation. This paper examines the seismic performance of immersed tunnels, starting with an overview of the deformation mechanisms affecting tunnels, including those induced by ground shaking and failure. Given its significance in large foundation deformations and its impact on tunnel integrity, liquefaction is analyzed alongside potential mitigation strategies. The seismic design process for immersed tunnels is discussed in detail, covering analytical approaches, numerical modeling techniques (such as finite element and finite difference methods), and physical modeling. Real-world examples are provided to illustrate key concepts. Finally, this paper summarizes the core factors influencing the seismic response of immersed tunnels and highlights future research directions to enhance their resilience in seismic environments.

The development of Terreiro do Paço Square began after the 1755 Lisbon earthquake, with the construction of the eastern side buildings. On the western side, an embankment had to be built first, in an area previously occupied by the river, overlying the existing soft soils. The western tower was constructed later, overlooking the Tagus River, and was not completed until the mid-nineteenth century. In 1941, a Portuguese historian reported that this tower had suffered a settlement of more than 0.5 m. Between 1956 and October 2019, the southeast and northeast corner's settlement increased by 0.15 m and 0.11 m, respectively. Interestingly, no damage was found on the external walls or the arched ceiling of the first storey, indicating a rigid body movement. More recently, the Lisbon Municipality sought to extend the Lisbon Museum by placing one of its modules in the newly vacant western tower. Using the finite elements approach and a three-dimensional numerical model, stress and strain studies were carried out with the purpose of analysing the effect of the rehabilitation works on its safety and functionality. Because of the period since the tower's completion and the permeability of the muddy soils in its foundation, a soft soil constitutive model with creep was used to simulate the ongoing settlements, with satisfactory results. This allowed for a more reliable prediction of future settlements caused by the tower's new usage. <!----> <!----> <!----> <!---->

Many immersed tunnels are constructed in alluvial formations within earthquake-prone regions, making seismic resistance a critical aspect of their safety design. During an earthquake, tunnel displacements can lead to slippage between the tunnel and surrounding soil, and may be further amplified by liquefaction. This phenomenon can cause severe structural damage, including tunnel flotation. This paper examines the seismic performance of immersed tunnels, starting with an overview of the deformation mechanisms affecting tunnels, including those induced by ground shaking and failure. Given its significance in large foundation deformations and its impact on tunnel integrity, liquefaction is analysed alongside potential mitigation strategies. The seismic design process for immersed tunnels is discussed in detail, covering analytical approaches, numerical modelling techniques (such as finite element and finite difference methods), and physical modelling. Real-world examples are provided to illustrate key concepts. Finally, the paper summarizes the core factors influencing the seismic response of immersed tunnels and highlights future research directions to enhance their resilience in seismic environments.