Fluidization and defluidization induced by an underground leaking pipe under different conditions

Soil fluidization induced by underground pipe leakage represents one of the primary causes of frequent ground collapses in urban areas. This study conducted experimental model tests to characterize the fluidization and defluidization processes resulting from water jetting caused by a buried leaking pipe under varying conditions of sand particle size, overlying sand bed height, and pipe leaking slot size. Under different overlying sand bed heights (50–70 cm) and pipe slot widths (6–8 mm), the critical fluidization flow rate remained constant. The head loss in the sand-water system consisted mainly of orifice head loss, seepage head loss, and sand particle vortex head loss. The orifice head loss was independent of sand bed height but showed a negative correlation with slot width. The vortex head loss was influenced by cavity height, and empirical equations were proposed to describe it. In addition, the arching effect was analyzed quantitatively by introducing two indicators: average intensity and sustained intensity, which represented the magnitude and duration of the arching effect, respectively. Greater sand bed heights and larger slot sizes resulted in a higher average intensity of the arching effect. Finally, the application of a Froude number for identifying the inception of fluidization was validated based on the experimental results.