Soft Ground Tunnels

Primary Support Design – Soft Ground Tunnels

For designing and building tunnels in soft ground and very weak rock, we follow the Sequential Excavation Method (SEM), also known as the New Austrian Tunnelling Method (NATM). The primary advantage of this method is that the support of the excavation is carried out in stages using shotcrete and utilising the strength of the surrounding ground. Depending on the actual ground conditions encountered during construction, several support measures are used on an as needed basis. Support measures may include:

• Pre-support measures such as grouted spiles or canopy tubes
• Face stabilisation measures such as support buttress or face bolts
• Lining stability measures such as temporary invert or mini piles

This approach provides a high degree of flexibility during construction and makes it possible to construct large tunnels in relatively poor ground conditions. Therefore, it is ideally suited for the construction of complex tunnel arrangements. It does require a representative of the designer during construction to ensure the adequate application of the various measures.

Our Expertise:

• State-of-the-art soil mechanics
• Construction sequence for varying tunnel spans in difficult ground conditions
• Primary shotcrete lining design
• Face stability assessment using both analytical and advanced numerical analyses
• Design of ground improvement requirements
• Analysis and specification of dewatering requirements