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<p>The tunnelling performance of earth pressure balance (EPB) shield machines strongly relates to the shear behaviour of foam-conditioned muck. However, the role of foam bubbles in modifying the mechanical properties of natural soils was rarely revealed due to the inapplicability of traditional triaxial testing equipment on highly fluidized muck materials. In this study, a series of unconsolidated undrained triaxial compression tests were conducted on foam-conditioned gravelly clay with varying foam injection ratios (<i>FIR</i>s) applying a self-developed triaxial testing apparatus. Due to the high compressibility and potential fracture of foam bubbles, the foam-conditioned gravelly clay exhibited obvious volume contraction during both confining pressure application and shearing. This volume contraction effectively served as "drainage paths". The fracture and compression of foam bubbles during shearing led to an increase in the proportion of "soil–soil" and "soil–foam" contacts and a decrease of "foam–foam" contacts. Since the former two contacts were less compressible than the latter one, the foam-conditioned gravelly clay demonstrated strain hardening behaviour. The presence of foam bubbles enhanced excess pore pressure in foam-conditioned gravelly clay during shearing. In addition, also because of the higher compressibility of "foam–foam" contacts, achieving the stabilization of excess pore pressure proved challenging. The volume contraction, the strain hardening, and the enhancement of excess pore pressure became more pronounced with higher confining pressure and greater <i>FIR</i>. Furthermore, the lubrication effects provided by water molecules and foam bubbles rendered the internal friction angle negligible, making the shear strength of gravelly clay primarily cohesive. The cohesion strength decreased progressively with increasing <i>FIR</i>. Additionally, the muck without foam conditioning tended to reach the critical state at a nearly constant deviatoric stress regardless of confining pressure, whereas the foam-conditioned muck reached the critical state at higher deviatoric stresses as confining pressure increased. This study provides valuable insights into the triaxial shearing behaviour of highly fluid foam-conditioned muck. Meanwhile, this new apparatus has the potential to be applied in future investigations of the mechanical behaviour of marine soft soils, thereby facilitating advancements in marine resource development.</p>

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Shear behaviour of foam-conditioned gravelly clay: insights from unconsolidated undrained triaxial compression tests

  • 期刊论文

  • 发表日期：

    2026年4月24日

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  • Hemei Sun,
  • Shuying Wang,
  • Zhunlin Ni,
  • Tongming Qu,
  • Jiazheng Zhong,
  • Hanbiao Zhu

摘要

The tunnelling performance of earth pressure balance (EPB) shield machines strongly relates to the shear behaviour of foam-conditioned muck. However, the role of foam bubbles in modifying the mechanical properties of natural soils was rarely revealed due to the inapplicability of traditional triaxial testing equipment on highly fluidized muck materials. In this study, a series of unconsolidated undrained triaxial compression tests were conducted on foam-conditioned gravelly clay with varying foam injection ratios (FIRs) applying a self-developed triaxial testing apparatus. Due to the high compressibility and potential fracture of foam bubbles, the foam-conditioned gravelly clay exhibited obvious volume contraction during both confining pressure application and shearing. This volume contraction effectively served as "drainage paths". The fracture and compression of foam bubbles during shearing led to an increase in the proportion of "soil–soil" and "soil–foam" contacts and a decrease of "foam–foam" contacts. Since the former two contacts were less compressible than the latter one, the foam-conditioned gravelly clay demonstrated strain hardening behaviour. The presence of foam bubbles enhanced excess pore pressure in foam-conditioned gravelly clay during shearing. In addition, also because of the higher compressibility of "foam–foam" contacts, achieving the stabilization of excess pore pressure proved challenging. The volume contraction, the strain hardening, and the enhancement of excess pore pressure became more pronounced with higher confining pressure and greater FIR. Furthermore, the lubrication effects provided by water molecules and foam bubbles rendered the internal friction angle negligible, making the shear strength of gravelly clay primarily cohesive. The cohesion strength decreased progressively with increasing FIR. Additionally, the muck without foam conditioning tended to reach the critical state at a nearly constant deviatoric stress regardless of confining pressure, whereas the foam-conditioned muck reached the critical state at higher deviatoric stresses as confining pressure increased. This study provides valuable insights into the triaxial shearing behaviour of highly fluid foam-conditioned muck. Meanwhile, this new apparatus has the potential to be applied in future investigations of the mechanical behaviour of marine soft soils, thereby facilitating advancements in marine resource development.