Tunnelling under lake Geneva

Tunnelling under lake Geneva ILF Questions: We gather that the tunnel will be located mostly within the clays and silts type (6). The material type (5) seems to have very poor geotechnical properties (NSPT  < 2 means “pulpy”) and hence it is necessary to avoid alignments within it. As it stands an accordingly thick segmental lining might be required to cope with both the water pressure and the depth of tunnelling in soft ground. In order to assess the most possible challenges and risks facing this section, we would like to have an idea of: The cross section shows the bottom level of the lake, how accurate is this level? What would be the level of accuracy in meters? It is understood that the interfaces between the ground types will need to be determined more accurately upon further site investigation. What is the level of accuracy of these interfaces in meters? We are particularly interested to know how much deeper the interface between materials (5) and (6) and between soft ground and (15) might be. Can we have some nominal values for geotechnical parameters (for (6) to (13) such as ground Young’s modulus (E), internal angle of friction, drained cohesion, coefficient of lateral earth pressure at rest (k0). The values to be provided do not need to correlate to the existing ground investigation. Instead, probable parameters are sufficient as these are not intended to be used for design but for risk definition.

Tunnelling under The Rhone

Tunnelling under The Rhone ILF Questions: In principle, our advice would be to lower the alignment at this location. Although, tunnelling is possible, the practicalities of a TBM mode change for a small length and the limited space for ground treatment would be fairly expensive and, therefore, difficult to justify. There are conventional solutions such as umbrella pipe roofs, injections ahead of the advance, freezing but also quite costly and time consuming for approx. 600 m. The geotechnical report states that ground permeability’s of 10-3m/s are possible. For this location we would like to know: How much deeper should the alignment be moved to ensure the tunnel crown has a rockhead of at least 3m? This assessment should consider the level of accuracy of the interface shown on the current section. (JS can provide estimate) Nominal values for Young’s modulus (E). The values to be provided do not need to correlate to the existing ground investigation. Instead, probable parameters are sufficient as these are not intended to be used for design but for risk definition. Possible flow rates of the groundwater regime

Tunnelling at Point E

Tunnelling at Point E (L’Arve) ILF Questions: We plan to provide alternative layout for the structures to be placed underground at point E. Similarly to the alignment under Lake Geneve, we would like to know: It is understood that the interfaces between the ground types will need to be determined more accurately upon further site investigation. What is the level of accuracy of the shown interfaces in meters? We are particularly interested to know how much deeper the interface between the quaternary deposits and the rock might be. Can we have some nominal values for geotechnical parameters such as ground Young’s modulus (E), internal angle of friction, drained cohesion, coefficient of lateral earth pressure at rest (k0)?. The values to be provided do not need to correlate to the existing ground investigation. Instead, probable parameters are sufficient as these are not intended to be used for design but for risk definition. Hydrogeologically, what are the most likely ranges of permeability of the quaternary deposits? Again, probable parameters are sufficient as these are not intended to be used for design but for risk definition.