Infrastructure often suffers severe damage due to both natural geotechnical hazards, such as floods or earthquakes, and man-made ones such as underground construction work and excavations.

Civil engineers and disaster risk management teams have extensively studied methods to prevent these risks and are still looking for more effective ways of avoiding large-scale deformations associated with these hazards.

The advent of computer-aided simulations has provided researchers with particle-based methods such as moving particle simulation (MPS), which is a valuable tool for independent deformation analysis even in larger regions. While the method has gained popularity over the last few years, it has yet to be applied when predicting ground behavior during design or construction work.

By bringing together small-scale model experimentation and a computer-aided engineering (CAE) analysis through MPS, a team of researchers from Shibaura Institute of Technology led by Professor Shinya Inazumi from the College of Engineering has investigated a few mysteries around earth pressure balance (EPB) shield tunneling.

Their recent study was published in Tunnelling and Underground Space Technology on 21 August 2024.

EPB is a widely used method for creating tunnels that utilizes the excavated muddy soil to provide support for the tunnel face, which is done by using foam, slurry, or other additives to plasticize the excavated material to ensure it is impermeable to water and is easily transportable.

The team recognized that despite being a popular technique, not much is known about how the plasticity of muddy soil adjusted by mixing excavated soil with plasticizing additives like bentonite solution affects the earth pressure inside a tunneling chamber. Insights into these factors can not only significantly increase the chances of avoiding ground deformations but also ensure efficient sediment management during the tunneling process.

"Urban centers are increasingly becoming reliant on underground infrastructures, therefore we wanted a prediction tool that can improve the resilience of urban infrastructure while lowering the costs associated with delays and structural damage due to unstable tunnel operations by ensuring efficient management of soil plasticity," Prof. Inazumi states.

He also highlighted that since the research lab associated with the study aligns itself with the UN's sustainable development goals, they also explored the environmental footprint arising from large volumes of excavated material and the use of chemical additives such as bentonite in search of ways to improve the sustainability of construction projects.

The experimental setup consisted of a sealable soil tank simulating a chamber and descending and ascending stages of an agitation blade model that was achieved by installing a twin-pair earth pressure gauge in a shield tunneling machine.

This system along with the calculations by a computer-aided analysis system based on a moving particle simulation (MPS) was able to precisely simulate the tunneling process, which included measuring the variations in earth pressure in response to plasticity variation induced by the agitation of muddy soil.

The researchers found earth pressure a trustworthy, reliable indicator for analyzing soil's plasticity and correlating factors such as vane shear strength and slump value which together impact the stability of the tunnel and the operation of machinery.

Backed by MPS, the CAE analysis system proposed by the team precisely reflects the experimental data, confirming its suitability for assessing and visualizing the plasticity and fluidity of muddy soil during tunneling.

Evaluating the value of earth pressure in actual field conditions by analyzing the plasticity state of the muddy soil in different soil conditions can be both time-consuming and an expensive affair. The small-scale model experiment, when combined with the computation power demonstrated in this study can be a valuable asset for optimizing EPB shield tunneling operations and improving sediment management strategies.

The technology opens new possibilities for innovative strategies that can significantly improve the safety and efficiency of underground civil construction works especially in urban environments.

"The results of this study can directly influence the construction of subway systems, underground utilities, and roads in densely populated urban areas by enabling controlled operations that cause less disturbance to the surrounding ground.

"We also hope that our proposed strategy is applied to optimize the environmental impact of the tunneling process and improve safety protocols in areas prone to earthquakes or other geotechnical hazards," concludes Prof. Inazumi.

More information: Assessment of plasticity of muddy soil for earth pressure balance shield tunneling, Tunnelling and Underground Space Technology (2024). DOI: 10.1016/j.tust.2024.106044