Diverting the sewer beneath the future tunnel of a car park in the centre of Jaén

Objective

In Calle San Clemente, next to Plaza de la Constitución in Jaén, a tunnel is planned to link the square’s car park with a new building. The sewer running along the street crossed its route. Before the tunnel could be built, the flow had to be rerouted along a new path without interrupting the service.

Difficulties

• Digging below the water table in the city centre. The chambers went down almost 5 m in a narrow street, between buildings and with the water table between 2 and 4.2 m.
• Weak ground near the surface. Beneath the fill lie marls, with a change in behaviour at about 4.8 m. The retaining walls had to be embedded below it.
• A sewer in service. Sewage had to keep flowing while the manholes were cut and reconnected.
• A precise fit. The two DN800 ductile-iron pipes had to hang beneath the slab of the future tunnel. Their socket joints are 935.6 mm in diameter.

Solution

To excavate the chambers safely, we surrounded each one with a micropile wall: 88.9 mm steel tubes drilled to 9 m and spaced 50 cm apart. Three typical sections were designed, with a strut at −1.75 m for the excavation stage and the slab as support in service. At the deeper chamber, the wall was supplemented with ground improvement by cement-grout injection.

Meanwhile, continuous over-pumping with a macerator pump carried the sewage from manhole S15 to S18. This allowed the old run to be plugged and removed without cutting the service.

Inside the walled enclosures, two reinforced-concrete chambers were cast in situ and joined by two DN800 ductile-iron pipes 7 m long. Two new PVC runs, DN1000 and DN800, connect the existing manholes to the upstream chamber.

Result

The flow now follows the new route: it enters the upstream chamber, crosses beneath the future tunnel through the two ductile-iron pipes and leaves through the downstream chamber towards manhole S17 and the existing sewer. Manholes S15 and S16 and the pipes between them were abandoned, and the tunnel’s path was left clear.