Mechanism of sand eruption from liquefied ground through gap of pavement and subsurface cavities

Abstract

Significant number of subsurface cavities was found in the liquefied ground after the 2011 Great East Japan earthquake. Using the results of the radar exploration conducted in Urayasu-city, Shinkiba-area and Narashino-city, all of those suffered from damage by the ground liquefaction, characteristics of subsurface cavities are investigated. It was found that cavities tended to form near man-holes and joints in pavement. Size and shape of the cavities are larger and thinner compared to those of cavities observed in the non-liquefied ground. A series of model tests was conducted in order to understand the mechanism of sand eruption and underground cavity formation when liquefaction occurs. Liquefaction and sand boiling was simulated in the model test by the upward seepage flow. With the increase in the hydraulic gradient, sand grains initially moved horizontally and then vertically, causing disturbance and loosening in the ground. The flow rate at the gap to cause sand eruption increased with the increase in the grain size. The hydraulic gradient causing sand eruption was much higher than the critical hydraulic gradient in all the test cases.