soil conditioning in the application of EPB Tunneling

The advancement of mechanized tunneling technology has significantly transformed the construction of underground infrastructure, particularly with the widespread application of Earth Pressure Balance (EPB) shield machines. The EPB machines are widely used due to their ability to maintain consistency and to control the pressure at the face of the tunnel. The machine can guarantee the ground stability, minimize the risk of subsidence and avoid the inflow of the water and soil particularly in the soft ground conditions. Moreover, the machine is adaptive with the various types of soil allowing for precise adjustments while excavating, making it well-suited to be opted in tunnel projects in urban area. These machines rely on the controlled manipulation of excavated soil at the tunnel face to maintain stability and ensure efficient progress through a variety of ground conditions. Central to the performance of EPB tunneling is the process of soil conditioning, which involves the treatment of in-situ soil using additives to modify its properties for optimal pressure transfer and spoil handling. Soil conditioning enhances the plasticity, flowability, and cohesiveness of the excavated material, enabling it to behave like a pressurized fluid within the working chamber. This modified soil acts as a supporting medium, counterbalancing the earth and water pressures at the tunnel face while also facilitating smoother transportation through the screw and belt conveyors system. The choice of conditioning agent plays a critical role in adapting the soil behavior to the operational requirements of EPB tunneling. In the choice of conditioning agents, it is fundamental to correctly their choice and assess effectiveness of various soil conditioning agents used in EPB shield machines. Drawing from the insights provided in the reviewed literature, the tests aim to assess the advantages and limitations of each conditioning method in relation to different soil types and conditions in which will be excavated the tunnel. Understanding these conditioning techniques is essential not only for achieving efficient excavation but also for minimizing environmental impact, reducing mechanical wear, and improving overall tunnel stability. This work contributes to the ongoing efforts in optimizing EPB tunneling technology. It was performed an extensive test campaign on challenging soil types obtained from a real case study. Due to the limited quantities of soil available a first test campaign using miniflow cone test was performed. Then each of these soils was conditioned using foam, bentonite slurry and/or water absorbing polymer and then tested for slump test. The study also integrates experimental investigations using a modified vane test and bulk density measurement, offering a deeper insight into the behavior of conditioned soils under controlled settings. Finally, it has been performed a screw conveyor extraction test to assess the behaviour of conditioned soil under pressurized conditions. The purpose was to determine, for a challenging type of soil, the optimal combination of foam, water, slurry, and polymer to reach the soil consistency which provides the proper pressure control at tunnel face, smooth flow via the screw conveyor beside reducing the mechanical wear while utilizing the EPB machine.