A large amount of wast soil is usually generated from construction sites when building subways or stations. There has been a strong demand for effective utilization of the wast soil from a social point of view.
      Conventionally, the wast soil from construction sites has been fluidized and used as back-filling materials for cut and cover tunnels among other purposes. Its brittleness, however, has prevented its extensive usage. For the purpose of improving quality and toughness, the Railway Technical Research Institute (RTRI) has focused attention on polypropylene (PP for brevity) fabric, used for packing belt material and reliably available with constant quality, and developed fluidized soil with PP fabric mixed.
      In an endurance test with a train load applied repeatedly, it was verified that a combination of PP fabric and fluidized soil with an unconfined compression strength q_u of 2000 kPa could ensure durability comparable to that of fluidized soil with strength q_u = 6000 kPa used for inverts (the bottom part of tunnel lining concrete) in a shield tunnel (Fig. 1). It was also found that mixing PP fabric increased the initial rigidity, and improved a fracture form from a cleavage fracture due to brittleness beyond the allowable maximum strength (one of weaknesses of fluidized soil) to a shear fracture (Fig. 2). Since the amount of cement required for fluidized soil mixed with PP fabric is less than that of conventional fluidized soil, material costs can be decreased by about 40% (Table 1).