Up and under - tunnelling in small spaces
With cities getting ever more crowded, a sleek Japanese tunnelling method could enable the construction of safe, energy-efficient tunnels in the tightest of spaces.
Makoto Kanai, Vice President of Japanese construction company Obayashi, based in Tokyo, visited the British Tunnelling Society at the Institution of Civil Engineers in London in September, to explain how the ultra rapid under pass (URUP) technique allows speedy construction of road and railway underpasses in urban areas.
Central to the process are Obayashi’s circular tunnel boring machines (TBM) – also known as tunnel shields – coming in a range of diameters from 2.1 to 13.6 metres, both in oval and rectangular shapes.
Kanai said that the method harks back to the work of Sir Marc Isambard Brunel, who developed the first tunnelling shield and used it to excavate the Thames Tunnel, still in use as part of the East London overground railway line. However, where Brunel’s method required vertical entry and exit shafts for the shield, which were made by driving an iron ring into the ground using the weight of the brick walls of the shaft being assembled on top of it, the URUP method involves launching the tunnelling shield directly into the ground at a more acute angle to the surface. This means that the entire length of an underpass can be constructed continuously from end to end with a single shield machine.
Developed for rapid assembly, the URUP method could reduce the area of excavation by as much as 82% compared to cut and cover, which involves the total excavation of the underpass then backfilling the road over the tunnel roof, requiring a long construction period and a significant amount of road space.
Requiring less space for the construction of the tunnel well leads to material savings, and can also cut build times by as much as two-thirds, claimed Kanai. In addition, less construction work means fewer road works, less traffic congestion and consequently less noise and CO2 pollution. ‘This is a most economic and durable tunnel with small settlement values and good build quality,’ said Kanai. He added that the absence of a shaft also reduces construction noise, with no need for vibration-generating equipment such as piledrivers and excavators.
Another divergence from Brunel’s method, in which the shield would be propelled forward using jacks, allowing bricklayers to assemble supporting walls in the gap behind, is that URUP operators do not add material from the tail shield. ‘In Japan my opinion is not a popular one - many Japanese engineers believe injection from the tail shield is good but I do not agree,’ said Kanai. ‘If you have many probes and pipes, or pressure from activity, you can damage the tail plate and the lining. That’s why I like injection from inside.’
Though he conceded that there remained room for improvement concerning tunnel face stability and ground movement control, Kanai said that, ‘Confidence in the [URUP] technology allows traffic overhead during work at a distance of 1.2m’.
The tunnels also have bending rigidity, with each part of the segment made watertight by a hydrophilic polyurethane sealing material, and durable because of the addition of steel fibre – ‘We wanted more toughness [in the event of] an earthquake,’ he said.
Kanai added that with proper earth face pressure, admixture injection rate and attitude control, the URUP method can be used for other types of construction, such as ramp tunnel construction using circular shields, as well as underpass construction using rectangular shields. The concept has been used successfully on a number of projects in Japan, including a metropolitan expressway and a ring road tunnel in Tokyo, but Kanai hinted that Obayashi would welcome working in the UK, having already completed four tunnel jobs in mainland Europe.