The high stakes of soft soil tunnelling. From choosing the right TBM, to picking a confident path beneath the feet of a city.

This article was first published in the August 2024 edition of Geomechanics and Tunneling Journal. 

As cities grow, and infrastructure projects within them become ever more ambitious, there is an increasing demand for tunnelling. 

This means designers and engineers are facing a growing number of challenges, especially in tunnelling safely and efficiently through soft ground, where managing settlements and ensuring stability are critical.

At the same time, tunnelling teams are expected to do more with less, including working with tighter budgets, meeting shorter deadlines, and acknowledging the demand for lower margins of error.

The job is not getting any simpler… But there are ways of reducing risk and uncertainty, and boosting the accuracy of your tunnelling decisions.

Why urban tunnelling can be an unsettling experience

Densely populated urban areas raise a special set of problems for tunnel engineers and designers. There’s the proximity of existing infrastructure, utility networks, and buildings. There will be strict environmental considerations, such as controlling noise, vibration, and dust to mitigate impacts on residents and businesses. Then the sheer logistical complexity of operating tunnel boring machines (TBMs) in confined spaces with limited access, can prove testing to say the least.

Even the choice of TBMs poses a unique set of questions and complex decisions. Which type will deliver the most effective results in the circumstances and soil conditions – Earth Pressure Balance or Slurry Shields – and how do you arrive at the correct choice?

How numerical modelling makes the best choice clearer

Accurate, insightful numerical modelling has become profoundly important in meeting these goals, and essential in simulating the behaviour of soft soils under various loading and drainage conditions. Through advanced simulation methods and Seequent’s unique connected geotechnical workflow, engineers can ensure the safe, efficient, and sustainable development of underground infrastructure in cities.

It allows engineers to create detailed tunnel geometries and construction sequences, accommodating the specific requirements of both types of TBM. It facilitates the modelling of complex tunnel shapes and the incorporation of different lining types, ensuring accurate simulations of the excavation process in a timely manner.

Key features include the PLAXIS Tunnel Designer, a powerful tool specifically created It comes with an extensive material library, and an automation and scripting interface, all perfectly integrated in a collaborative and connected workflow.

PLAXIS can therefore play a pivotal role in predicting ground movements, assessing tunnel performance, and optimising support strategies.

Understanding the problems…

A substantial portion of tunnels pass through soft ground, which often exhibits high compressibility and low shear strength, increasing the risk of ground settlement and tunnel deformation. Accurate numerical models engineers ensure the stability and integrity of the tunnel throughout the construction process and its operational life.

The presence of nearby structures, such as buildings and other tunnels, complicates tunnel projects. The excavation process can induce ground movements that affect the stability and integrity of these structures. Numerical modelling plays a crucial role in assessing the potential impacts and designing mitigation measures to protect existing infrastructure. By simulating the interaction between the tunnel and the surrounding ground, engineers can predict settlement patterns, identify critical zones, and implement strategies to minimise adverse effects, such as adjusting excavation techniques or installing protective barriers.

…and making the right TBM choice

– EPB Machines are designed to control ground pressure at the face of the tunnel. They use the excavated material itself to form a plug, balancing the pressure and minimising settlement. The primary advantage of EPB machines is their ability to handle a variety of soil types, especially those with high plasticity and low permeability.

– Slurry Shield Machines use a pressurised slurry (usually bentonite) to support the tunnel face. This method is particularly effective in highly permeable soils, such as sands and gravels, where maintaining face pressure is critical. Slurry shields are adept at managing water ingress and maintaining face stability. The downside is the complexity and cost associated with slurry treatment and separation processes, as well as the need for extensive surface infrastructure to handle the slurry.

PLAXIS plays a key role in predicting ground movement and assessing tunnel performance in a record setting subsea traffic tunnel.

In the Tuen Mun-Check Lap Kok Link, Golder Associates Hong Kong Ltd, constructed one of the world’s largest subsea TBM tunnels, at a groundbreaking 55m below the surface and a 4.1 km length. Faced with the hugely complex soil conditions of newly reclaimed Hong Kong land, they knew that traditional geotechnical methods would not provide the ground movement control that would achieve the required 120-year service life.

Using PLAXIS Golder Associates produced a design that met the demanding TBM face slurry pressure required to maintain stability. They also achieved optimisations that shaved two months off construction time, reduced CO2 emissions by 1,500 tons and saved USD 16 million.