One of the most challenging underwater tunnel projects in the world is tracking two months ahead of schedule nearly a year after achieving a major milestone.
The Istanbul Strait Road Tube Crossing, scheduled to open by the end of the year, will provide a new connection between the Asian side of Istanbul, Turkey, and the city’s European side.
The centerpiece of the new link is a 3.4-mile double-deck road tunnel bored under the historic Bosphorus Strait, 360 ft below sea level. The top deck will carry eastbound traffic and the bottom deck westbound vehicles. The $1.25 billion build-operate-transfer project also includes shorter twin tunnels built using the New Austrian Tunneling Method (NATM), as well as cut-and-cover sections.
The project has several specific goals:
- Provide connections to a wide network of highways in the Istanbul area, on both sides of the Strait;
- Increase capacity across the Bosphorus by 130,000 vehicles (cars and minivans) a day;
- Reduce commuting time for motorists by as much as 60 minutes in each direction; and
- Provide a blueprint for funding future infrastructure projects in the Republic of Turkey.
Sanja Zlatanic, senior vice president at HNTB and chair of the firm's national tunnel practice, emphasized the sustainable environmental benefits of the project. “By reducing driving time for motorists, the new tunnel will significantly reduce carbon usage in the Istanbul area,” she said. “The overall effect will be a positive for the Asian economy, in particular.”
HNTB is playing a key role in the project as independent design verifier. The project concessionaire, selected by the Ministry of Transport, Maritime Affairs and Communications, and the Director General of Infrastructure Investments, is ATAŞ (Avrasya Tüneli İşletme İnşaat ve Yatırım A.Ş.), a joint venture of the distinguished Turkish firm Yapı Merkezi, and SK Engineering and Construction Co. Ltd., a highly experienced firm from Korea. The concessionaire will build and operate the facility for a concession period of 26 years. Each of the joint venture firms are well-known for their successes in large-scale infrastructure and transportation projects in their respective market sectors. With a total investment of $1.245 billion, of which about $300 million is in equity, ATAŞ is committed to start the tunnel operation within 48 months from the financial closure.
Eliminating the risk
In August 2015, the project achieved its most significant milestone when the tunnel-boring machine (TBM)—44.94 ft in diameter—completed a successful breakthrough at the European end of the project. The machine has the highest cutterhead power per square foot of excavation area in the world, is second with 12 bar design pressure and is sixth in the world in excavation area.
In summarizing the progress to date, Yapi Merkezi and SK E&C JV (the contractor) noted that all “risky activities which involve various tunneling works were successfully completed ahead of their schedule.” In addition to the TBM tunnel, this included TBM transition boxes at each end of the subsea tunnel to launch and retrieve the TBM and twin tunnels in Asia, each 0.6 miles in length, that were conventionally mined using a sequential excavation method under a densely populated district and connected to the transition box at the Asian side and further to the subsea tunnel.
“The TBM performed well despite very challenging geological and hydrological conditions,” Zlatanic emphasized. “The geology encountered by the TBM generally consisted of Trakya bedrock on both sides and alluvial sediments at the bottom of the channel. The bedrock, sedimentary in nature, is composed of inter-layered siltstones and sandstones, and it is systematically intruded by numerous volcanic/igneous dikes of diabase, andesite or dacite.”
In fact, workers excavated 28 dike zones with a frequency of around 295 ft and thicknesses that varied from 3.28 ft to 393.7 ft according to a publication from Yapi Merkezi. Overall, the TBM work required 476 days, with three shifts of working seven days, 24 hours a day; work advanced at an average of just under 23 ft per day.
This publication also noted that during excavation, hyperbaric repaid-maintenance operations were needed four times over a period of 47 days because of the geological conditions. One of the operations occurred near the deepest point and under the highest water pressure any TBM has ever encountered: a level of 10.8 bar pressure. The massive tunnel required more than 15,000 precast segments with high impermeability that were 600 millimeters thick. The segments were connected to each other using more than 30,000 bolts.
Despite these challenges, the TBM machine broke through at the European transition box within 24 millimeters of the ideal finishing location. The Asian twin tunnels, which took 445 days to complete, had excavated faces that met each other within 2 mm.
Confirm and verify
As independent design verifier, HNTB has been ensuring that the project has benefited from good risk-management practices. That has included evaluation of major design components—including design assumptions and methodology for both temporary and permanent structures—excavation methods, sequencing and support of excavation for the tunnel and approaches, technical specifications, and quality and durability of materials selected.
The review process encompassed detailed reviews and independent analyses and checks for category 3 structures (in compliance with European standards) and all other critical elements of the project. Specifically, this pertained to:
- Tunnel structures more than 10 ft in equivalent diameter;
- Structures with high structural redundancy;
- Structures containing unconventional, innovative or complex design aspects;
- Bridges with spans exceeding 164 ft and/or with skews greater than 45°;
- Difficult foundation problems; and
- Retaining walls with an effective retained height of 46 ft or greater.
To ensure quality performance and mitigate potential risks, HNTB’s verification role also was expanded to cover all other major elements and disciplines including verification of the tunnel alignment; the tunnel’s major mechanical and electrical systems and services including power, lighting, hydraulics, tunnel ventilation, fire life safety, communications, architectural and space planning, traffic control, and system integrations; and facility operation.
“Our goal was to verify compliance with requirements of the Turkish Ministry of Transportation and with design codes and standards,” Zlatanic said. “Our reviews also included potential value engineering ideas and ways to improve construction cost and/or schedule.”
Zlatanic also added that, “In light of the project’s accelerated schedule, the IDV role is critical in reviewing the design for correctness, identifying potential risk issues, verifying corrective actions and minimizing potential that any critical issue may ‘fall through the cracks.’”
Specific challenges encountered not only included the high hydrostatic pressure, but also the reality of boring the large subsea tunnel in a seismically active region. Less than a mile south of the tunnel, the North Anatolian fault is capable of producing earthquakes registering 7 on the Richter magnitude scale. To decrease seismic stresses, the contractor designed and installed two flexible seismic joints/segments—the first time the technique has been used in TBM tunneling under similar conditions.
“The contractor implemented tight, very organized reviews and procedures—including a careful quality control process—as well as making sure the right machine and operator were selected,” Zlatanic noted. HNTB has reviewed all design specifications, involving more than a dozen local subcontractors; developed independent calculations for every major project element; and identified cost and scheduled savings that could be implemented in project design and construction.
“The contractor would take our recommendations and implement them if the ideas met stringent schedule requirements and sufficient time was available to implement quality procedures,” she explained. “Time presented the biggest challenge. To stay on schedule, HNTB ensured that comprehensive and timely reviews occurred and were well-coordinated.”
Just about final
With the TBM tunnel breakthrough complete, the project has moved into its final phase. That consists primarily of road-widening work on the Asian side to complement similar work completed on the European side in phase 1.
Zlatanic said the project will be open by the end of 2016. The city, the Turkish Ministry of Transportation and the city municipal agencies have expressed interest in building additional tunnels under the Bosphorus to connect the two continents. An even larger tunnel—at 60 ft in diameter, this would be the world’s largest—is being planned for just north of the current project.
Still, another tunnel is anticipated to replace an existing bridge that crosses the Golden Horn, the primary inlet from the Bosphorus.
“We expect to hear details soon for conceptual solutions to the 60-ft tunnel,” Zlatanic said. “And we have done some conceptual assessment work in preparation for what we anticipate will be an announcement soon about the Golden Horn tunnel. In all cases, we undertake careful planning to be able to manage the area’s complex geological, hydrological and seismic conditions, and suggest tunnel solutions that would maintain the integrity of the historic buildings on both sides of the Bosphorus and preserve Istanbul’s remarkable vistas.”