Tunnel Breakthrough Celebrated on Abu Dhabi Sewerage Services Company’s Strategic Tunnel Enhancement Program

By the CH2M HILL Access Water team

At the end of last year a tunneling milestone was reached on Abu Dhabi Sewerage Services Company’s (ADSSC) Strategic Tunnel Enhancement Program (STEP) in the Emirate of Abu Dhabi. On December 18 a ceremony was held to celebrate the 5th tunnel boring machine (TBM) breakthrough on the deepest section of the 41 kilometer long STEP deep sewer tunnel. A “breakthrough” signals the completion of tunnel excavation between two deep shafts. Check out the video below to see some footage of the TBM breaking through!

ADSSC is the service provider for sewerage services and owns and operates the sewerage network and treatment plants throughout the Emirate of Abu Dhabi. To meet growing sewerage infrastructure demands, ADSSC has developed a comprehensive plan to accommodate the projected sewage flows far into the future. The cornerstone of the plan is STEP, for which CH2M HILL is the Program Manager and Impregilo S.p.A. is one of the deep tunnel contractors.

ADSSC, CH2M HILL and Impregilo S.p.A. staff were joined by His Excellency Sheikh Sultan Bin Tahnoon Al Nahyan to witness the TBM breaking through after six months of mining a 4.3 kilometer section of the tunnel beneath the Abu Dhabi desert, at depths of around 80 metres. The Herrenknecht earth pressure balance (EPB) TBM typically progressed at rates of 200 metres per week to install the 6.0 metre internal diameter precast segmental lining; the finished tunnel will have a 250 millimetre thick in-situ concrete, with a high-density polythylene membrane, corrosion protection lining on the intrados. The 360 ton, 6.96 metre diameter, 110 metre long TBM tunneled through mixed ground conditions of interbeddded dolomitic siltstone, claystone, mudstone and gypsum, with around 80 metres head of saline groundwater, without incident. Over 70 skilled tunnel workers were employed on the TBM drive.

The breakthrough marked the completion of over 25 kilometers of mined tunnel! A total of eight TBMs will have been used to excavate the entire STEP deep sewer tunnel with a further 16 kilometers of tunnel still being mined by three EPB TBMs, with around 9 kilometers remaining at the time of writing.

ADSSC Program Management Department Manager Shahzad H. Orakzai commented, “The [breakthrough] ceremony was very well received by the attending guests and the Chief Guest was impressed beyond our expectations. The ADSSC Chairman was personally congratulated by His Highness the Crown Prince and asked me to pass on the congratulatory messages from His Highness the Crown Prince to CH2M HILL.”

Mr. Orakzai continued, “His Highness the Crown Prince praised the professionalism of the team. He further praised the hard work and dedication of the team in achieving impressive results while delivering such a challenging and a world-class project in the UAE. He was equally impressed with the skills transfer approach wherein young Emirati engineers are being engaged in delivery of this programme and are being mentored to become highly-trained and well-informed professionals for other future projects in the UAE. We look forward to CH2M HILL’s continued value-added engagement to the Program.”

The AED5.7-billion STEP is one of the longest gravity-driven sewerage tunnels in the world. STEP features three key components: 41 kilometers of deep sewer tunnel, 43 kilometers of smaller diameter link sewers, and a pumping station adjacent to the Al Wathba Independent Sewerage Treatment Plants (ISTPs).

STEP will operate by using link sewers to intercept the flows from existing sewers upstream of the existing pumping stations, and convey the flows by gravity into the deep tunnel. The deep tunnel will convey the flows, again by gravity, to the new main pumping station where the flows will be pumped out of the deep tunnel into the Al Wathba ISTPs. The link sewers and the deep tunnel will replace the existing main collector system and eliminate the need of up to 34 existing pumping stations, which are nearing capacity and require extensive maintenance.

When completed, STEP will be recognized as an efficient, cost-effective and sustainable solution to meet the long-term wastewater collection and conveyance needs of Abu Dhabi Island and mainland. The system is designed for an ultimate capacity of 1.7 million cubic metres of wastewater per day by 2030.

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