World’s biggest jacked tunnels save time and money

The tunnel jacking for Boston’s Central Artery project – known as the Big Dig – is by far the largest, most complex project of its kind in the world. Requiring a quantum leap in scale – well over ten times the size of any jacked tunnels attempted in the USA before – it has involved major innovation. Recognised as the most challenging component of the Central Artery Project – which takes much of the city’s highway network underground, removing heavy traffic congestion and pollution – the tunnel jacking has delivered a low maintenance, robust construction while adding substantial environmental advantages and contributed to over US$300 million in construction savings.

Tunnelling under a live railway
Creating Boston’s I-90/I-93 interstate interchange meant constructing multi-lane highway tunnels under the approach to South Station. This complex network of seven interconnecting railtracks carries over 40 000 commuters and 400 train movements per day.

Before our team’s involvement the design concept being developed required five phased relocations of the railway – an approach unacceptable to the railway authorities. Apart from moving the tracks, each phase would have involved re-establishing the extensive control systems which included sensitive buried fibre optics. All this would have been very time consuming and a major safety issue. Our alternative enabled three full-size interstate highway tunnels – totalling over 240m in length with single elements weighing up to 30,000t – to be jacked under an operating railway with no track relocations, train speed restrictions or interruptions to the service and with an excellent safety record.

This alternative to traditional cut and cover methods also reduced excavation considerably, bringing added environmental benefit by minimising transportation of heavily contaminated materials through the city. Another major advantage was that the project allowed overhead electrification to proceed concurrently with tunnel construction, thus advancing a safer and more efficient railway service for commuters.

Experts from Mott MacDonald and our North American company Hatch Mott MacDonald introduced the alternative of tunnel jacking at the bid stage and won the design contract as a specialist tunnelling sub-consultant to the local consultants JV, Maguire-Harris. This was on a cost plus fixed fee basis and the Mott MacDonald involvement was extended throughout the construction phase providing value engineering and site supervision services. The Standard Federal fixed price construction contract for the intersection was awarded to the JV of Slattery-Skanska, Interbeton, White and Perini for US$400m, with the tunnel jacking element being US$140m. Specialist tunnel jacking expertise was provided by Skanska, Edmund Nuttall and John Ropkins Ltd.

Site constraints
The jacked tunnel site is located at the intersection of the I-90 and I-93 interstate highways. This was the most complex contract in the entire Central Artery Project with severe spatial and operational constraints. The space for construction was limited to an area of just 300m by 700m enclosed between the interstate highways, the railway system and a waterway. The limitations were significantly eased by the introduction of tunnel jacking since the railway no longer needed the phased relocations. However, available construction space was still considerably less than the normal needs for tunnel jacking. This was overcome by the unique use of combined jacking pits and global ground freezing.

Global ground freezing
The ground freezing for the jacked tunnels was the largest ever undertaken beneath an operating railway. It greatly simplified the breaching of the headwalls of the jacking pits and created a safer working environment at the tunnel face. It also allowed the unique temporary ‘parking’ of a 30,000t box unit beneath the railway which successfully addressed the spatial constraints of the jacking pits. In addition, frozen ground presented a safer and easier way to deal with the numerous obstructions encountered in the tunnel face. The ground conditions beneath the railway presented the most challenging array of mixed face conditions including two centuries of uncontrolled fill and buried obstacles overlying compressible strata, which became major obstacles during the tunnelling works. These included old masonry foundations, reinforced concrete, a buried trackway and forests of timber piles.

With the major benefits, ground freezing brought its own challenges. Excavation at the tunnel face was safer but this required development of enhanced performance for the British Webster roadheaders. Further innovations were introduced to cope with the large ground movements created by the freezing. These included a system of hydraulic jacks to allow pressure relief to the headwalls and a heating control system in the ground and tunnel walls to avoid the tunnels becoming locked in the ground.

Special anti-drag system
A special new anti-drag system (ADS) was developed which utilised hundreds of steel ropes above and beneath the tunnel units. Its key role was to de-couple the jacked tunnel boxes from the ground and infrastructure above. This prevented the tendency of the ground to move laterally with the tunnel as it is jacked forward. The ADS – the highest capacity, most extensive such system ever used – also provided improved alignment control with additional sets of steel ropes beneath the tunnels. Trains safely moved uninterrupted overhead as the tunnel sections were installed below the tracks at a rate of 1-2m per day.