Permeability Characteristics of Poorly Graded Sand Conditioned with Foam in Different Conditioning States

Abstract Investigating the permeability characteristics of sand-foam mixtures in different conditioning states is important not only in earth pressure balance (EPB) shield tunneling and standstill but also for posttunneling soil handling. Slump and permeability tests were carried out on poorly graded sand that was conditioned with different foam injection ratios (FIRs) and water contents. According to the slump values and the apparent states of the conditioned sand in the slump tests, the sand conditioning was classified into five cases: insufficient, suitable without any water or foam bleeding, suitable but with water bleeding, excessive with likely foam bleeding, and excessive with water bleeding. With suitable conditioning, the permeability in the conditioned sand generally exhibited three consecutive stages during the tests: the stable stage, fast growth stage and slow growth stage. An increasing FIR and water content enhanced the antipermeability effect (the ability of preventing water from flowing in porous medium), but a considerable increase in water content (≥10 %) weakened the antipermeability effect. Finally, a simplified model was proposed to explain the rheology. The foam significantly reduced the total energy required for the sand particles to flow. In addition, the permeability characteristics of the conditioned sand found in the tests were explained. The low initial permeability coefficient and its duration depended on the ability of the water to penetrate the plugging structures formed by the foam.