In its design-build bid, KKZ/CMA departed from the 30% baseline design in various ways. Two significant aspects involved the Río Piedras Station and the two sets of twin tunnels to the south.
"Río Piedras Station is a large 240m2 cavern 17m wide, 17m high and 150m long," explained Victor Romero, project manager for Jacobs Associates. "The crown of the cavern is less than 5m below the surface, directly beneath historic buildings. The 30% baseline design suggested a canopy of some 67 x 1m diameter interlocking steel pipes installed by microtunnelling techniques and back-filled with concrete to provide cavern excavation presupport."
Reservations about the ability of the canopy to maintain sufficient rigidity through the interlocking pipes led the design-build team to develop two alternatives – one a multi-drift NATM approach with spiling ahead of each top heading and ground improvement above the cavern, the other being the stacked drift option. Unfortunately, "the bid was being reviewed after the NATM tunnel collapse at Heathrow Airport in London and there was nervousness about the use of NATM under shallow cover for Río Pedras," said Romero. "This option was therefore withdrawn and the stacked drift option was selected."
As opposed to the microtunnel presupport, the KKZ/CMA-Jacobs stacked drift alternative is a substantial structural cavern lining. The design comprises 15 interconnecting drifts of about 3m x 3m, excavated to the full 150m length of the station cavern and back-filled with concrete to create a massive pre-installed lining for the core excavation. The method was chosen after previous experience by Kiewit of its use on the Eisenhower Tunnel through squeezing rock conditions. At Río Piedras, compensation grouting by subcontractor Soletanche-Bachy was selected to limit settlement caused by excavation of the drifts.
