Digging deep to guard Doha from climate risk

Although Qatar is one of the hottest countries in the world, it also suffers from chronic flooding around Doha and faces increasing climate-related risks. At least twice a year, heavy rainfall leads to massive disruption to people’s lives and damage to homes, businesses and infrastructure. By draining rainwater and alleviating flooding in a 270km² catchment area around the capital city, the Musaimeer Pumping Station and Outfall Tunnel (MPSO) can prevent flood damage to homes, businesses and infrastructure by discharging ground and surface water offshore through the longest tunnel for this purpose in the world. It is also designed to mitigate flood risks from a 1-in-50-year storm event.

Each of the three main elements needed to complete the project involved a high level of technical complexity: the underground pumping station with 10 submersible pumps that could empty an Olympic swimming pool in two minutes; the 10.2km outfall tunnel running 15m under the seabed that is built to last 100 years; and the 19,600m² diffuser structure used to release excess rainwater offshore with minimal pressure.

270km² rainwater catchment area around the capital city

10.2km outfall tunnel running 15m below the seabed

With 90% of the asset located below ground, the logistical challenges of getting people and materials safely and efficiently into and out of the site were immense. However, this was outweighed by the environmental and social benefits. For example, the coastal site means there is zero disruption to the local population and gravity is used to channel ground water to an underground pump station at the site, which saves energy and means several existing ground-level pump stations have been decommissioned. The underground outfall tunnel also means no damaging dredging of the seabed need to take place.

Our tunnelling knowledge meant the outfall tunnel was completed 58 days early

The project ran from November 2017 to September 2022 and was not only completed on time and on budget but achieved the highest international standards for environmental and sustainable management.

Achieving compliance with the international sustainability rating scheme CEEQUAL is one of the project’s great achievements – and a first in Qatar. It sets a new standard for the protection of ecology and biodiversity on major infrastructure projects in the region and will contribute towards the nation’s sustainability commitments, as set out in the Qatar National Vision 2030.

To reach the rating’s standards, every element of the project was examined to minimise adverse environmental impacts during construction activities and the operational life of the asset. This started in the concept and design phase and followed through into rigorous site-wide environmental monitoring during construction. It required a coordinated effort and a major commitment of time and resources on the part of the client, Ashghal, the Public Works Authority of Qatar, Mott MacDonald as the project-management consultancy, and HBK-PORR as the main contractor.

Project successes

• The critical portion (outfall tunnel) was completed on budget and ahead of schedule.
• 9.4M hours were worked with no lost-time injuries to date.
• The population is protected from flood damage and urban development can be carried out at lower cost without the need for expensive foundation works.
• The project received environmental awards including the Ashghal Green Award in 2020 and the 2022 CEEQUAL (Very Good) standard.
• Adopting a sustainability strategy and measures in line with the CEEQUAL standard reduced whole-life carbon emissions by 31%, or approximately 9,500,000kg of CO2.
• In 2020, 2021 and 2022, the project received gold awards from the Royal Society for the Prevention of Accidents.
• In 2022, the project received further industry-excellence awards from the Institution of Civil Engineers (ICE) Qatar and the Middle East Economic Digest (MEED).

Tunnelling technology

Boring a 10.2km outfall tunnel straight out beneath the sea under high water pressure leaves little margin for error. To move forward, the tunnel boring machine (TBM) needed to exceed 4.5 bar of marine hydrostatic pressure. This was a constant risk during normal operations, as well as during maintenance and repairs to the TBM.

The selection of an earth pressure balance (EPB) TBM for the Musaimeer outfall tunnel was a key factor in ensuring it was completed 58 days ahead of schedule, says Gary Peach, our project manager for MPSO. These TBMs have specially designed features to allow safe and efficient excavation under challenging geological conditions, but this type of TBM had never been used under these conditions before on a Qatari project. Remarkably, EPB technology evolved from the bentonite tunnelling machine invented by one of Mott MacDonald’s tunnelling experts, John Bartlett, in 1964.

To operate it effectively through different conditions required careful calculation of the optimal settings, such as the cutterhead rotation speed and advance rate thrust. If the ground was too dry, the TBM might grind to a halt. Adding too much water to lubricate the excavated material might compromise the structural integrity of the seabed. “The geological conditions pushed the parameters of the TBM to the limit,” says Gary. “However, adjusting the cutting speed of the TBM to cope with different ground conditions was one way that we successfully managed to recover time on the programme.”

Khalid Saif Al-Khayareen, manager of the drainage-networks projects department at Qatar’s Public Works Authority, continues: “Ensuring the use of an appropriate TBM was a key factor in successfully completing the outfall tunnel, considering the challenging ground conditions and the 10km length. Mott MacDonald was thoroughly engaged during the design, selection and procurement process of the earth pressure balance TBM.”

Critical overview ensures success

To meet the programme’s ambitious schedule, the riser shaft of the diffuser had to be constructed offshore before the TBM had finished work on the outfall tunnel. The riser was constructed by drilling through limestone strata riddled with fissures, cavities and pockets of loose material. It was essential that the riser shaft was dry, to enable safe connection with the tunnel. We carried out design review to assure that the complex drilling and riser construction process was completed without water leakage. Our design advice was also critical in ensuring that the tunnel remained aligned throughout its 10.2km journey, so that it arrived exactly on target at its intended point of connection with the diffuser.

100% of excavated tunnel spoil was reused for reclamation

The constant quest for environmental excellence on the MPSO project, led by our sustainability team, also helped to drive innovation and create value for the client and community. Using an earth pressure balance TBM on the MPSO project enables excavated material from the tunnel to potentially be reused rather than sent to landfill.

In total, 506,589t or 100% of total excavated tunnel spoil, was reused for a regional coastal reclamation project with a total cost saving of QAR 11,257,370 for the Al Khor Municipality. The use of biodegradable chemicals added some extra cost but ensured that the excavated spoil was not contaminated and could be recycled. Transporting the tunnel spoil directly to the Al Khor municipality without any intermediate stockpiling also helped to minimise CO2 emissions.

Inlet works initial excavation around existing tunnel connection shaft

During the construction phase, the project team actively pursued measures to reduce water use, such as the reuse of treated TBM water removed from the tunnel for dust suppression and cleaning activities. All water use on the project, including the reuse of reclaimed and wastewater, was monitored and reported weekly and monthly for environmental compliance.

Programme efficiency

The use of building information modelling (BIM) to capture, record and share project data was another first for the client and the main contractor. This meant that the client could be sent programme management updates at the end of every 10-hour shift, providing early warnings of deviations from the schedule.

Using BIM for clash detection was critical in keeping the project on track. For example, by developing multiple models showing the precise location of the proposed pipework for each new utility, it was possible to identify potential clashes between the designs. This avoided the need for remedial work to relocate pipes and holes on site, and eliminated unnecessary costs.

Our design review team used BIM to identify the best system for transporting people and materials along the 10.2km outfall tunnel on a single-track rail line – a one-hour journey along the full length of the tunnel. Adding crossing points – where the track splits to let oncoming trains pass one another – is a way to increase flexibility, but it also adds delays. After modelling for one, two and three crossing points, a single, moveable crossing point at between 800m to 3km behind the TBM was selected as the best solution. This gave the optimal balance between time and flexibility.

Completed TBM concrete lined tunnel with supply train travelling along it

Then, Covid-19 social distancing rules threatened to undermine this careful planning. With only six people able to travel in each passenger train instead of 18, three times more journeys along the tunnel would be needed, creating higher risks and requiring more maintenance.

Again, an efficient transport solution was required. After careful analysis by our team, it was decided that a passenger car would be added to the materials train, and that space for materials would be added to the passenger train. Although this was a logistical challenge, strong programme management helped to keep everything moving safely and in sequence.

“Mott MacDonald’s design review and construction supervision team played an instrumental role in the smooth and on-time delivery of the project, even with the pandemic restrictions,” says Khalid Al-Khayareen.

190t of steel saved on the pump station roof using prefabrication

Our team also focused on creating a legacy by adopting innovative techniques and tools to drive efficiency and sustainability, even though it took time to train people and get them comfortable with new ways of working.

For example, the construction of the pump station was the first time that the contractor had used prefabricated construction elements – following the principles of design for manufacture and assembly (DfMA) – on a project. Using prefabrication in the roof structure reduced the amount of steel used by 190t, saving a total cost of QAR 2,500,000.

Storage of tunnel concrete lining segments at the precast factory

“The project team used all possible techniques to accelerate the construction programme, such as extensive precast-element production and large-span, reinforced concrete precast beams used for both the pumping station and above-ground buildings,” says Khalid.

Part of the legacy of the project is that more than 100 Mott MacDonald and Ashghal staff have been trained, and continue to be trained, in the use of the latest BIM management tools, such as the international construction operations building information exchange standard, known as COBie.

Safety first

The safety record of the MPSO project – 9.2M hours worked with no lost-time injuries – is testament to the rigorous inspection system followed during construction activities. Despite Covid-19, daily checks by quality, safety, mechanical, electrical and plumbing engineers never stopped.

9.2M hours worked with no lost-time injuries

“Our safety department is the eyes and ears of the project, providing a world of information that is used to analyse data trends and decide when intervention is needed,” says Gary Peach.

The team used an inspection checklist covering more than 100 items, which were selected to ensure that maintenance was being carried out to the level expected.

100 items on the inspection checklist

Being visible on site and cooperating closely with the contractor was key to ensuring that the highest standards of safety were met, and problems were picked up and addressed quickly. The ability to share analysis of health and safety data trends with the client and contractor helped to create a common sense of purpose.

Ensuring environmental safety was another key priority. Adopting a rigorous material selection process helped to ensure that the materials used on the project are of high quality and have no detrimental impact on the environment or people’s health.

In 2020, 2021 and 2022, the project received gold awards from the Royal Society for the Prevention of Accidents.

A sustainable future

The delivery of this unique infrastructure project pushed boundaries on many fronts – but a relentless focus on sustainability is a major part of the story.

“The prioritisation of environmental and sustainability management helped the project not only achieve CEEQUAL certification, but also produced value far beyond its immediate benefits,” says Dila Ersenkal, our Middle East unit sustainability and social outcomes lead.

For example, the durability of the materials and the selection of equipment with low maintenance requirements and a long design life will provide long term value for Qatar. Meanwhile, the selection of local goods and service providers adds economic benefits while promoting sustainable consumption and production.

The above-ground portion of the completed project (90% of the asset is underground)

As well as protecting communities from flooding today, MPSO also removes a barrier to sustainable urban development in southern Doha. The new hydraulic infrastructure is designed to keep ground water below –3m, which will reduce the need for expensive foundation works and lower the cost of urbanisation.

Finally, by spreading knowledge about how to implement sustainable projects through training, published papers and presentations, the MPSO team is influencing behaviour in the infrastructure sector and creating a path for future projects to follow.

Close cooperation

The MPSO project involved a huge commitment to monitor, document, analyse and report to stakeholders on key risks and mitigation measures.

“Overall, the project received more than 40 audits by the client covering health and safety, quality, environment and sustainability, as well as overarching project management,” says Dila Ersenkal.

Ensuring the protection of the marine environment was one priority area for stakeholders. Before work could begin, several environmental permits had to be obtained from the Ministry of Environment and Climate Change, as well as from Kahramaa, an independent water and electricity regulator. Key stakeholders were then given regular updates on the environmental impact of the works.

First, we carried out a survey of seawater quality around the site to establish a baseline. Then we compared operational data collected throughout the project with the baseline environmental data to identify long-term trends.

We regularly sampled and analysed seawater monthly while conducting continual online monitoring of its quality by installing oceanographic buoys. The results of this monitoring exercise were submitted monthly to the authorities by the MPSO project team.

Throughout the project, daily, weekly, and monthly monitoring of groundwater quality was also carried out to ensure no ill effects from construction activities.

In recognition of the high standards of environmental and sustainability management on the project, it has received several environmental awards, including the Ashghal Green Award in 2020 and the 2022 CEEQUAL (Very Good), the highest score ever in Qatar.