Celebrating Marmaray Crossing: Vital link on the modern silk road

Arup’s Mohammed Tabarra celebrates the international success of the Marmary Tunnel one year one – a project dreamed of since 1860 but only possible because of engineering advances now.

Marmaray Tunnel Crossovers

Connecting East to West across the Bosphorus Strait has been an historic ambition dating back to the Ottoman Empire in 1860. Shifting tectonic plates and construction across one of the busiest shipping lanes in the world has meant that until recently, this century-and-a-half old dream could not become a reality.

"The Marmaray Tunnel is described as a vital link on the modern Silk Road, which will provide seamless rail transport from Turkey to China for a large volume of freight trains."

In October last year,the Marmaray Tunnel officially opened, becoming the deepest undersea tunnel and one of the largest transport projects in the world. The opening coincided with the 90th anniversary of the modern Turkish republic and is part of a much broader national infrastructure plan which includes a third bridge across the Bosphorus and the creation of the largest airport in the world and the third airport for Istanbul.

More from Arup

Once fully operational, the tracks underneath the Bosphorus Sea are expected to connect more than 1.5 million people each day between Asia and Europe within approximately four minutes. It will place Istanbul at the hub of travel between East and West, offering huge economic benefits, as well as forming the first tunnel connecting two continents. 

The 8.5km Marmaray crossing was by no means a simple engineering feat. It includes 1.5 kilometre of immersed tube which sit on the ocean floor, enabling trains to travel directly underneath the Bosphorus. This immersed tube is accessed by two bore tunnels from Kazlıçeşme on the European side and Ayrılıkçeşme on the Asian side of Istanbul.

The route of the tunnel also created unique cultural challenges. Comprised of three underground stations, Uskudar, Sirkeci and Yenikapi, the tunnel passes by Istanbul old city centre which has a strong archaeological heritage from numerous old civilisations, so careful planning and sensitivity to how tunnelling could affect ancient foundations was needed.

 Ensuring history and modern technology work in harmony  

This combination of cultural and engineering challenges meant that experience across multiple disciplines was essential. Arup’s experience in other complex geotechnical and tunnelling projects including High Speed 1 and the Hong Kong-Zhuhai-Macau bridge, enabled us to provide a full peer review of all tunnel ventilation and smoke control system design reports, drawings, specifications, risk analysis management system and test procedures, from preliminary design up to installation, commissioning and handover.

A key challenge was providing the necessary ventilation for the tunnel while taking into consideration the precious artefacts and archaeological sites dotted across the route. 

The Marmaray Tunnel is described as a vital link on the modern Silk Road, which will provide seamless rail transport from Turkey to China for a large volume of freight trains. While this will provide tremendous trade opportunities between East and West, the impact of a freight train fire compared to a passenger train created engineering challenges. 

Freight trains typically have a much larger fire load of around 100MW, compared to passenger trains which might be closer to 20MW.  This means that the ventilation fans need to be significantly larger than for most tunnels, creating the need for more excavation.

 Soon after work began at the middle station, Sirkica, the digging uncovered a 6,000 year old archaeological site approximately five metres below ground. These findings revealed thousands of artefacts which helped archaeologists trace Istanbul's history back more than 2,000 years further than ever before and it was essential that these treasures were excavated, which delayed Sirkeci Station for two years.

As a result, the fan plants at Sirkeci Station would be unable to operate on time, and engineers had to revise and adapt the tunnel ventilation strategy to deliver a safe service during the initial operating phase, so that the building process could still progress.

A full analysis including station smoke test procedures resulted in ventilation zones becoming longer and there was a restriction on the number of trains which could be in the tunnel at one time. This approach has meant that the project was not been delayed further, nor had safety been compromised.

What we now see today is collaboration between Japanese and Turkish contractors, designers from Singapore, specialists from the USA, rail safety officers from Germany and design verification Engineers from UK, making the Marmaray Crossing a truly international effort. However, it was the flexibility and readjustment of ventilation plans which enabled such a successful and futuristic project to continue whist protecting Turkey’s ancient heritage.

With rapid urbanisation, the demand for new infrastructure in previously impossible locations and busy urban areas will continue to increase. The Marmaray Tunnel is a good example of the analysis and balance required to mitigate risk, ensure sensitivity to existing structures and local communities and enable the kind of ambitious growth of countries like Turkey. 

Client: Turkish Ministry of Transport

Contractor: Taisei Gama Nurol JV

Key designers:  CPG (Singapore) + AECOM (USA)

 Cost: $2.5 billion


Mohammed Tabarra is associate director, Arup