After months of study, engineers are set this month to start modelling runs in earnest for the critical geomechanical analyses of the deep lying ‘swelling’ flysch and ‘soft’ breccia zones laying along the tunnel alignment of the Gibraltar Strait Crossing.

The TYPSA-led JV, which includes Lombardi Engineering, Geodata and Ingema, began work last year to update the preliminary design of the Crossing, which envisages around 200m of ground cover then up to 300m depth of sea over the tunnels. The main bore diameter is seen at about 10m and the service tunnel about 7.5m. The tubes will run at variable spacings, with cross passages planned every 340m.

Preliminary works

Preliminary works underway to asses the tunnelling conditions

Key to the new studies were recent findings from site investigation, particularly from 300m deep exploration drilling of the seabed along the alignment between Spain and Morocco. Following the computation runs, the JV will use the data to define excavation options and present this to the client by September, it is expected.

Length and depth – 40km between portals and up to an unprecedented 475m – were always going to be tough aspects of the proposed scheme to engineers. So, too, was the approach of looking to build the scheme – two rail tunnels and a service tunnel between – in two stages, likely to be separated by decades.

But when the SI told of the breccia, all involved knew the challenge had multiplied beyond the already known technical problem of understanding and building in the flysch that dominates the alignment. As such, and with the plan to bore through the strata at depths not attempted before by TBM, a major step in the preparation for the geomechanical analyses has been verification of new algorithms for the separate models – one for flysch, another for breccia.

Up to 85% of the alignment is in flysch in marls, sandstone and limestone. The ground presents both immediate difficulty in excavation but is also prone to significant swelling over time. Useful data on flysch has come from various sources, including observed swelling in a road tunnel in Luxembourg.

Geomechanical analyses of the flysch has, therefore, pushed for the analyses to consider two time horizons – short and long – for excavation and then spatial stability. Those horizons are also presenting different options on lining type and sequence, all of which must also fit rail operations for safety and revenues.

The breccia-filled ‘channels’ were believed to have been filled by undersea slides and the material rests as a clayer matrix, very soft, like soil. The content is variable with blocks, stones and chips from the adjacent flysch in sandstone and limestone. The problem being analysed is how to deal with the extremely low strength and deformation moduli over 3km-5km of the alignment.

Preliminary works underway to asses the tunnelling conditions Preliminary works Longitudinal section of the Gibraltar Tunnel crossing