Pillar-reinforcement technology beneath existing structures: Small-scale model tests

More than 90% of the nation's flood damage now occurs in the downtown areas of metropolitan cities, where underground space is highly developed. To prevent and/or minimize such damage, we propose the installation of underground rainwater detention caverns, built beneath existing structures. For achieving flood control, multiple caverns would be used, rather than one larger cavern, which could have stability problems. Because of stress concentration in the pillars between two adjacent caverns, pillar stability is considered the Achilles' heel of a multiple-cavern design concept. Therefore, a new pillar-reinforcement technology for improved pillar stability is proposed. In this technology, reinforcing materials, comprising a steel bar and PC strands, are installed with the application of pressurized grouting, thus applying prestress to the PC strands and anchor body. This method has the advantage of utilizing full strength mobilized in situ ground, while minimizing the necessity to construct precast concrete structures. Using pressurized grouting provides an increase in ground strength, and even more importantly, it reduces stress concentrations in the pillars. Applying prestress further increases ground strength because of the increase in internal pressure. In this study, numerical analyses and small-scale model tests were performed to verify the ground reinforcement effects. First, the stress changes in a pillar were determined via the small-scale model tests at each construction stage. In particular, changes in the major and minor principal stresses were measured at the sidewalls, using a p-q diagram, to assess the safety of pillars. The influence of pressurized grouting on the pillar behavior was ascertained. The zone affected by the prestress level was verified by varying the prestress during the smallscale model tests. Further, experimental results were compared with the results of numerical analyses; reasonably good correlation was observed.