Final breakthrough into the reception shaft by two Herrenknecht EPBMs marks the end of Phase I tunnelling on the Abu Hamour Southern Outfall Project in the Qatari capital, Doha.

Breakthrough celebrations in the reception shaft

The 4.47m diameter machines broke through within six days of each other following 12- and 14-month-long drives of 5km each. The newly excavated sewage tunnels, which are segmentally lined, will protect the city and its inhabitants from floods following heavy rainfall events.

Contractor Salini-Impregilo achieved advance rates of up to 165m/wk at an average depth below ground level of approximately 30m. EPB technology was selected as the optimum choice to master the cohesive soils that are characteristic of Doha’s local geology, and which contain a high proportion of rock. Herrenknecht designed a project-specific cutterhead with a large opening ratio – resulting in correspondingly efficient material excavation. High maximum thrust forces ensured high advance rates for both machines. Final breakthrough was on 29 July and marked the conclusion of Phase 1 of the tunnelling work, which began in 2008.

Launch shaft created logistical challenges

"The EPB shields and the choice of a segmental lining were perfectly suited for the project requirements in Doha," said Simone Centis, Project Manager for the contractor, Salini-Impregilo. “The partnership with Herrenknecht helped us to find the best technical answers for the often heterogeneous soil. This allowed for the highest possible efficiency during tunnelling, and facilitated project progress."

The two EPBMs were launched within two months of each other, from the same launch shaft, in the summer of 2014. This created a huge logistical challenge during the assembly process.

Mitigating flood risk

Qatar is one of the driest places on Earth. When it rains heavily in Doha this can result in dangerous flooding because the metropolis is only a few metres above sea level, and therefore only a few metres above the groundwater level. The Abu Hamour Southern Outfall Project is designed to mitigate flood risks in the future by allowing the draining off of superfluous water, lowering the high water level, and relieving the pressure on existing pumping infrastructure. Further projects to improve the drainage situation in Doha and its surrounding area are planned or already in construction, including the Inner Doha Re-sewerage Implementation Strategy (IDRIS) Project.

With its Vision 2030 programme, the Arabian Gulf State Qatar aims to offer its citizens the highest possible standard of living. To achieve this, much of the planned infrastructure involves underground development and tunnelling. Some projects are already being implemented and range from surface water flood control to traffic infrastructure. To these projects Herrenknecht is supplying 21 EPBMs to the new Doha Metro system, many of which have been delivered.

Twin flood control TBMs prior to delivery

As part of another significant infrastructure project, Herrenknecht is supplying two 4.47m diameter EPBMs to a major flood control and drainage project.

With annual rainfall of less than 100mm, Qatar is one of the driest landscapes on earth, but individual driving rain showers lead to extensive flooding in the densely populated capital city, Doha. A comprehensive drainage scheme, known as the Abu Hamour Southern Outfall Project, is being developed by Qatar's public works authority Ashghal to cover an area of 170km and ensure improved drainage for the lively southwestern section of the city.

With the tunnel construction contract awarded to Impregilo S.p.A of Italy (now merged with fellow Italian construction company Salini), the two Herrenknecht EPBMs are being used to tunnel 20m to 30m beneath one of the main streets of the city while traffic continues to flow smoothly above. The EPBMs are designed for Doha's soft limestone and, when completed, up to 16.5m³ of water/sec will flow through the main 9.5km drainage tunnel to a central pump station near the New Doha International Airport.

Site assembly of the first TBM

In early June 2014, HE Sheikh Abdul Rahman bin Khalifa Al Thani, Minister of Municipality and Urban Planning (MMUP), visited the Impregilo drainage tunnel project site and visited the second of the Herrenknecht EPBMs before it disappeared underground in July. The two tunnel boring machines are the first in Qatar to excavate a tunnel lined with a precast concrete segmentally lining.

In 2008, Herrenknecht microtunnelling technology was used to complete part of the feeder interceptors under an initial construction phase. An AVN slurry machine completed a total 4km of pipejacking. With a 3.6m o.d., it was a significant, large diameter success for pipejacking.