Lisbon Metro: Blue and Green lines

Ferrovial Agroman were responsible for the construction of a 9.71m diameter tunnel under the Tagus estuary bed as part of the extension to the Lisbon metro. The geological challenges of soft-ground tunnelling beneath a major river were accentuated by the proximity of sensitive historical buildings and the limited cover between the tunnel and river bed.

Cais do Sodré & Baixa/Chiado

Works comprised the construction of 4.5km of double-track running tunnel, 3.7km of which were excavated using an Earth Pressure Balance (EPB) tunnelling machine suitable for excavation in soft ground.

The contract included the construction of two stations: Cais do Sodré was a 25m deep ‘cut and cover’ station with 1m thick diaphragm walls and a jet-grouted base slab including more than 84km of jet-grouted columns. Baixa/Chiado station consisted of two parallel underground caverns only 6m apart, each with a span of 18m. The caverns were 220m long and 140m long respectively.

Other structures included a 9.52m diameter 45m deep ventilation shaft, a drainage gallery, cross passages and access galleries.

Significant challenges were presented by the varying geological conditions which included non-cohesive soils, alluvial sediments, slimes and sandy clays; clean and clayey sands; slimy clays, marls and some calcareous sediments.

Settlement control and mitigation

The tunnel alignment ran in close proximity to the Tagus River, under an historical area with 18th century buildings and with very little cover in some of the sections.

Settlement control and mitigation was therefore of paramount importance in this area, and we undertook extensive measures including ground treatment, compensation grouting, underpinning and structural strengthening.