France-based Bouygues Construction, through its two subsidiaries, Dragages Hong Kong and Bouygues Travaux Publics, has secured a €1.15bn tunnel construction contract in Hong Kong.

Under the contract, which is considered to be the largest design-build contract ever awarded in Hong Kong, Bouygues will build a 4.2km-long undersea road tunnel.

The project comprises the construction of an undersea twin-tube tunnel, each tube with two traffic lanes and 14m in diameter.

The tunnel will connect the New Territories, north of Hong Kong, to Lantau Island, where the international airport is located.

It will be bored 50m below sea level using two tunnel boring machines (TBMs), which are rotary drilling machines for excavating and building the structure.

The two tunnel tubes will be connected every 100m by 42 cross passages.

Bouygues will use ground-freezing technology to provide a watertight environment to enable these passages to be bored.

Maintenance operations, primarily with regard to the cutting heads of the TBMs, will be executed by teams of divers who will stay in a hyperbaric base camp for four weeks at a time to help deal with any issues that may arise during the process.

The project also includes the building of two cut-and-cover approach tunnels of 530m and 670m respectively.

The programme will be executed in line with environmental standards with regard to marine ecology, water quality, noise impact and waste management.

Bouygues said that the two ventilation buildings, powered by wind energy, have been designed to qualify for BEAM Plus Gold rating, and will be built to the north and south of the tunnel.

"The project comprises the construction of an undersea twin-tube tunnel, each tube with two traffic lanes and 14m in diameter."

Two technologies have also been developed by the Bouygues Construction Research & Development Department to be used on the project, Mobydic and Snake.

These systems reduce the need for manual operations in hyperbaric conditions.

Mobydic is a system of sensors equipped in the disc cutters in the heads of the TBMs to make it possible to permanently monitor the state of wear of the cutters while enabling real-time geological mapping of rock faces.

Snake is a remote-controlled exploration arm with a high-pressure jet, which will clean the TBM heads and eliminate clogging to allow them to be inspected.

Bouygues Construction deputy chief executive officer Philippe Bonnave said: "This contract is a major event for Bouygues Construction, which in the space of a year has won the two largest construction contracts ever awarded in Hong Kong: this one, and the Hong Kong-Zhuhai-Macao bridge."

Scheduled to be completed by the end of 2018, the project will employ more than 1,000 employees at peak periods.