Soft ground tunnelling course

MODULE 1. General design

The course starts with a recap on general design methodology within the context of soft ground to ensure that all participants have the correct foundation for this course. A risk-based design is essential for safe designs and this is implicit or explicit in all design work. The boundaries of soft ground (vs hard rock) will be outlined. A wide range of factors influence a design such as functional requirements like waterproofing, fire protection and durability as well structural performance. Sustainability will be examined in detail.

MODULE 2. Soil-structure interaction

The biggest influence on any tunnel is the ground. A thorough understanding of the type of ground and the hydrogeology is required in order to identify the key mechanisms of ground behaviour. This governs the choice of construction method and from the two arise the design calculations. By focusing on the fundamentals of ground behaviour, the course will show how to develop safe and cost-effective solutions in any situation. Settlement is a key concern for soft ground tunnelling. Prediction, monitoring and mitigation of settlement will be discussed.

MODULE 3. Construction methods

Having understood the mechanisms present, a construction method can be chosen to manage these risks effectively. This has an influence on the design so this topic appears before design. Common construction techniques in soft ground tunnelling include: cut & cover; TBMs with segmental linings; sprayed concrete lined tunnels (SCL/NATM). Immersed tube tunnels and pipejacking are less common so they will only be discussed briefly. In addition ground treatment is sometimes used to reduce water ingress or to improve the ground.

MODULE 4. Design methods

There are three type of (structural) design methods – empirical, analytical and numerical method. Each one will be examined in detail, considering the common tools used for soft ground cases. Guidance will be given on how and when to use each method, as well as their benefits and limitations. Special structures such as portals, junctions and shafts will be considered, along with special design cases such as seismic loading and fire.

MODULE 5. Sprayed concrete linings (SCL/NATM)

This module will look in detail at SCL tunnelling in soft ground, beginning with mix design and material technology, then advancing to the properties of sprayed concrete and material behaviour. Construction methods, including the excavation sequence, have a large influence on design so this will be explored before explaining the common analytical and numerical design methods. Construction management and monitoring play a key role in successful SCL tunnelling.