The Isthmus of Panama is, for its size, one of the most influential geological structures in the world. It was formed by underwater volcanoes, a tectonically-induced rising of the seabed, and then gradual sedimentation and siltation of the result to create a more complete land bridge.

Recent scientific discoveries made during engineering works to expand the Panama Canal have pushed the date of its formation (in a tenuous state) back from a few million years, to around 10M years or even more. The visualisation of the state of this bridge between North and South America drives a lot of what scientists believe they understand about the Earth’s current glacial cycles. The glaciation of the Arctic, and the creation of the Gulf Stream are thought to be caused by this slender link. It also allowed mass migration (the "Great American Interchange") of plant and animal life between the Americas.

In modern Panama, the eponymous capital city sits on the only practicable crossing at the narrowest part of the country, and efficient movement of people through public transport is vitally important to keep the main highways as clear as possible. The country’s economy has been relatively undamaged by the global financial crisis, with up to eight per cent annual growth, and public finances were available to be levelled at the problem of residents relying on buses and cars, a problem that "would cause the collapse of vehicle traffic by 2015" according to the Panama Ministry of Economics.

Burying your problems
The solution to the traffic jams of Panama City was metro. And the City awarded the USD 1.45bn contract (which rose to USD 2bn with addendums) to construct Panama Metro Line One in 2010 to an Odebrecht-FCC consortium. The scope of the underground works was 7km of tunnels and seven underground stations, with the remainder of the 13.7km line to be constructed on viaducts with five elevated stations. Although it is a dream, and was a pet project of former Panamanian president Ricardo Martinelli to set the ball rolling to have an entire underground transport system for the city by 2035.

Critical tunnelling through difficult geology
According to Santiago Erans, Panama Metro Line One leader for FCC, The critical element for the success of the project was the tunnelled stretch of the works. Some 7.3km (a 4.2km and a 3.1km drive) of tunnel were excavated using two 9.77m by Herrenknecht EPBMs through difficult geotechnical conditions.

Rock up to 100MPa was encountered in places, and the highly fractured nature of the ground meant that the TBMs "needed their tools changing into hyperbaric form" according to Erans. In general, excavation took place in problematic areas with mixed geology, fractures and shallow ground with a changing level of rocky substratum. A general overview of geology was provided by the client during the tender process, allowing an analysis of the information to give an idea of the hydrological and geotechnical situation along the drive, a complementary study was undertaken.

Erans continues, "The complementary study expanded the stratigraphic, tectonic and hydrogeological knowledge of the terrain as well as the geotechnical characterisation of the terrain, allowing geotechnical parameters to be determined for tunnel design, stations and shafts.

"Passive seismic refraction (micro tremor) and electric tomography determined the rock and soil contact boundaries. In addition, based on the speed of the waves, the modules of dynamic deformability of rock and soil and their behaviour faced with seismic actions and the excavation possibilities could be determined. All of this allowed us to accurately define lithological distribution and complete a geotechnical and hydrological characterisation of the terrain, especially relative to its deformability, abrasiveness of the rocks and the aquifer definition and parameters of hydraulic loads."

In total 58 explorations were undertaken (34 in tunnels and 24 in stations) that covered 1856m (approximately 392m in soil and 1464m in rock). In addition 165 permeability tests were carried out and 234 pressure tests. Three shafts were set up in the station areas (with a total of 75m of perforation) for pumping tests. In terms of geophysical works, 2300m of passive seismic tests were carried out, split into 27 types, and a tomographic electric test of 400m.

With regards laboratory testing, mechanical and deformation classification and state tests were carried out which included uniaxial compression, uniaxial compression with extrensiometric bands, triaxials for rock and soil and traction resistance.

For the pump tests a diameter perforation test was carried out allowing a tube of 200mm minimum ventilation diameter to be positioned. Each pump shaft had at least four piezometrics.

Universal precast concrete segmental rings lined the tunnel and were grouted with a two component grout to achieve uniform contact and minimise movements and achieve uniform pressure, and to help reduce permeability. Segments were 1.6m-long and 350mm thick, and were in a 6 + key arrangement.

Critical path Challenges
Erans said the team knew it would be difficult to deliver the whole project in 38 months, and points out that this included all design, enabling works, auxiliary line and M&E installation, and rolling stock supply. "Right from the start we analysed in detail the activities which formed part of the project’s critical path and we studied the possibility of reducing, within what was possible, the length of some parts of the process, guaranteeing the delivery of the tasks without the possibility of delays."

There were several key decisions. "Firstly, from the detailed analysis we did we concluded that the TBM purchasing process needed to start on day one. This included a specialised team overseeing the whole process at each stage. This team had to focus on the TBM fabrication in Germany. It was also critical to properly select the team responsible for affected utilities. Several companies were contracted to detect and organise the re-locating of utilities affected by the project.

"Then the construction process for the elevated stations was changed, making it independent from the lines. This way the lines could continue to be worked on without the stations having been finished, and in addition certain parts of the design were changed, so that instead in situ construction, they were built with prefabricated elements. The goal throughout these changes was to reduce the construction time.

Erans adds that during the project, the team had to take measures to bring the project back on schedule when unforeseen complications slowed them down. "For example, we increased the number of hydromill trench cutters from three to four to accelerate the excavation of the concrete walls on the underground stations; we also increased the amount of formwork on elevated stations, and finally we increased the number of machines used in the piling of the underground station accesses and the trenches (the transition sections between tunnel and surface)."

On particular challenges working in Central America, Erans says: "Sourcing staff and equipment for a project of this scale in Panama was a challenge. Local engineering expertise in Panama was lacking due to no previous experience on projects of this type or scale. Careful management and planning allowed the consortium to source their workforce for the project (90 per cent Panamanians) and complete the design, shipping and assembly of the two TBMs and vital materials in just 14 months."

The metro is required to transport 15,000 people an hour now it is in operation. When all four lines and a tram system are completed in 2035, as per the ‘Red Master Panama Metro Plan’, it is expected to transport 40,000 people an hour. Erans concluded, "The opening to the public in April 2014 marked a significant moment in Panama’s economic and urban development and represented a shift in attitudes towards public transport investment in Central America. The Panama metro is the first system of its kind in the region."

Continuing the government’s USD 15bn programme to upgrade Panama’s infrastructure, the same consortium of Norberto Odebrecht and FCC was awarded the contract for Panama Metro Line Two on 14 May 2015.

However, this section for the east of the city is not underground. The bid of USD 1.858bn against a reference price of USD 1.819bn covers engineering design services, construction of the civil works, and installation of auxiliary works on the line, supply and installation of the railway line including rolling stock. The works will include 16 stations and 21km of elevated track and connecting stations.