Quantitative interrelations of conditioning and recycling indices of high-saturation clay soils for EPB shield tunnelling

It is widely recognized that the improvement effects of the foam on the soil properties during earth pressure balance shield (EPBS) tunnelling can be well characterized by some conditioning evaluation indices such as shear strength, flowability, and compressibility. The number of EPBS has increased significantly worldwide in the past few decades. Consequently, recycling the EPBS muck wastes for other applications becomes important, and in this context, estimating the recycling potential of foam-conditioned soils is of great practical significance. Although numerous studies have proved dewaterability to be one of the most important indices to assess the reusability of clay muck wastes, the dewatering behaviors of foam-conditioned clay soils and their quantitative interrelations with common conditioning evaluation indices are still not properly understood. To understand the comprehensive effects of foams on high-saturation clay soil that is comparable to natural clays of real tunnels, a series of laboratory experiments were carried out, including undrained shear tests, flowability tests, and vacuum filtration tests. The concept of specific resistance was introduced to characterize the dewaterability, taking the effects of filtration time and pressure into consideration. The moisture migration mechanisms at both the conditioning and dewatering stages were analyzed using low-field nuclear magnetic resonance tests. It was found that increasing the degree of saturation and the foam injection ratio could lead to a significant enhancement in the flowability (vertical and horizontal slumps), compressibility, and dewaterability, but a reduction in the undrained shear strength. The injected foams caused the moisture to migrate from micropores to mesopores during conditioning, but from mesopores to micropores and macropores during dewatering. Accordingly, quantitative expressions were established to determine the interrelations between conditioning and dewatering indices. The specific resistance was also demonstrated to be an effective alternative variable to characterize the conditioning performance of foams for high-saturation clay soils. Finally, a new application diagram of different evaluation indices, which can assist engineers in effectively assessing the foam’s effects on clay soils, was provided to contribute valuable knowledge to EPBS tunnelling practices.