Since the Hyogo-Ken Nanbu earthquake in 1995, earthquake-resistance reinforcement work for pillars and piers on existing viaducts/bridges has been carried out. On the other hand, such work on the existing foundations that support these structure will require huge amounts of time and money because the reinforcement targets are situated underground. Accordingly, the RTRI developed a simple and economical construction method to reinforce levels of earthquake resistance and proposed a related design method.

The proposed construction technique involves placing sheet piles around the existing foundations and combining these sheet piles and the foundations with reinforcing footing (Fig. 1). This is simpler and more economical than conventional reinforcing pile work or reinforcement techniques involving soil improvement. In one example, both construction costs and the construction time could be roughly halved compared with conventional reinforcing pile work using the cast-in-place concrete pile method.

From a model experiment using a 1/10-model of a full-scale pile foundation, the RTRI confirmed that the horizontal rigidity of the model foundation increased in the order of 20% due to the enhanced horizontal ground resistance provided by the sheet piles, and that the sectional force generated on the existing pile was roughly halved (Fig. 2), thus achieving improved earthquake resistance.

The RTRI also proposed a design method using a skeleton structure analysis model that embodies this reinforcement effectiveness, and prepared the Earthquake-Resistance Reinforcement Design Manual to enable its application to practical design.

In some existing foundations of JR and private railway companies, earthquake-resistance reinforcements using this method have already been implemented.