Observations on Sand Boils from Simple Model Tests

Sand boils are a long-recognized phenomenon associated with soil liquefaction. Deep liquefied sand is acknowledged to be able to burst through overlying impermeable layers. However, the exact conditions required to produce sand boils are unclear, as is their relative importance to engineering structures. To investigate the phenomenon further, a series of simple model tests were performed. The models consisted of small sand samples containing one or more layers of consolidated silt, visible in profile, which are then vibrated. Visual monitoring was taken of structural settlement and the influence of multiple layering, and the ensuing water films and sand boils observed. It was found that relatively heavy foundations over relatively thin silt layers were susceptible to sand boils. If the silt layer was too thick to be unduly strained by the foundation then the occurrence of water films was suppressed. Buried silt layers were seen to experience a hitherto unobserved phenomenon whereby silt was carried up towards the soil surface with the pore fluid during shaking. It is concluded that the relative importance of sand boils and associated layered-soil phenomena is an area requiring further investigation.