There is a clear movement around the world to consider larger TBM tunnels as a cost effective option in new infrastructure projects, and as a tunnel engineer, I have seen this option taken up with increasing popularity in Asia. Improved TBM technology and extensive past experience is helping tunnellers to achieve safe and successful excavation of ever larger tunnels.
Although in some cases the ground conditions would permit traditional excavation methods for tunnelling with wider spans, TBMs can frequently offer a more efficient solution to support the ground quickly and safely and allow faster progress rates. The need for larger TBMs is motivated by the demand for tunnels to accommodate larger infrastructure, such as more traffic lanes for a road or multiple railway tracks. Large TBM tunnels also allow opportunities for multiple functionalities such as combined highway and railway elements, or for other purposes such as the SMART tunnel in Kuala Lumpur, Malaysia which accommodates both highway and stormwater transfer facilities. The increased capacities of these larger tunnels enable more efficient and value-added use of urban areas, by relocating some functions to underground space. This is particularly important for cities where land supply is scarce.
Simultaneously, the increased size of these tunnels presents new technical challenges to both designers and contractors, such as increased pressure due to soft ground or weak rock, which can place a higher load on the tunnel lining. Higher strength concrete might then be used but this would then need added measures for fire prevention.
Another key challenge for large tunnels through poor ground with rich underground water content is the greater risks of lining uplifting, stepping and ovalisating due to buoyancy during segment erection. For a larger diameter tunnel, it is usually to increase numbers of segments per ring in order to reduce size and weight for ease of delivery and handling within the TBM during erection. More segments per ring allows greater flexibility and lining deformation, but can also increase the risks affecting the lining integrity.
For functional purposes, large tunnels often require the provision of internal structures and these can have adverse impacts on the structural integrity for the lining. Take the example of TBM road tunnel which may need a ventilation slab with supports at the tunnel shoulder level, potentially resulting in unacceptable local loading of the lining. The design of the segments clearly needs careful consideration to address this. For larger TBM tunnels the solution may need to allow for significant internally induced forces.
Another challenge for designers to consider is the provision of structural openings to accommodate associated plants and equipment. The presence of these openings can have a substantial impact on stress distribution within the segmental lining of large TBM tunnels. Consequently, strengthening work is typically required around the openings for both temporary and permanent cases in order to maintain the overall integrity of the tunnel structure. Advanced analysis techniques can be used to optimise the structural designs in these areas to ensure cost effective solutions.
Atkins is now involved in the delivery of three of the largest TBM tunnels in Hong Kong: all over 14m in diameter. One of these is the world’s largest TBM, at over 17m in diameter, this is for the Tuen Mun - Chek Lap Kok Link (TMCLK) project. This tunnel will accommodate Hong Kong’s deepest and longest sub-sea road tunnels connecting the Hong Kong - Zhuhai - Macau Bridge Boundary Crossing Facilities to the main route northwards towards the mainland of China. We are also working on a highway tunnel which is being excavated by Earth Pressure Balance Machine (EPBM) TBM of over 14m diameter. The tunnel is located at the North-Eastern New Territories of Hong Kong and serves the Liantang cross boundary connection. This TBM is currently the largest EPBM type TBM in Hong Kong, and is now driving through extremely complex geological ground conditions with high groundwater levels, beneath hilly topography.
These examples show that large TBM tunnels can be used to deliver major elements of infrastructure in a cost effective manner through challenging conditions. Whether they are located within urban or rural areas, they enable us to increase land values, minimise congestion and associated pollution, and reduce environmental impacts and disruption due to construction.
Through careful planning and design, and utilising the latest analytical techniques, our approach to large TBMs has been able to deliver optimal and sustainable designs, benefitting both our clients and society in general. Our experienced tunnellers understand the challenges and solutions for large TBM tunnels across a whole range of ground conditions and for multiple different uses. We are constantly researching new advances in tunneling techniques to ensure that we can help deliver the right solutions for our clients.