The damage to retaining walls due to liquefaction of the backfill can be effectively reduced by the distributive installation of piles into the backfill. In this study, the author focused on the effect of rigidity of the piles on the effectiveness of this countermeasure. Through the 3-D numerical analyses and brief model tests, it is confirmed that the installation of piles, even those with low rigidity, can effectively reduce the damage to the retaining walls.
Many retaining walls, including quay walls, were damaged by the 2011 Tohoku Earthquake off the Pacific coast of Japan; Fig. 1 shows a retaining wall damaged during this earthquake. Generally, the backfill behind the walls predominantly consists of loose sand, which has a high risk of liquefaction during earthquakes. The liquefaction increases the lateral earth pressure of the retained soil, resulting in the deformation and collapse of retaining walls. Therefore, the reinforcement of retaining walls against large earthquakes, such as the Nankai Trough Earthquake predicted to occur in the near future (e.g. Ogata, 2015), is an urgent priority.
Several effective countermeasures against liquefaction have been proposed and researched widely. For example, the soil improvement method (e.g. Gaafer et al., 2015), which involves solidifying the soil by injecting grout, can prevent liquefaction in the backfill. However, this method requires high construction cost if the backfill is widespread and thick.
Another example is the deterrent pile method (Hamada and Higuchi, 2010), where steel piles are distributively installed in the backfill such that they are not connected with quay walls shown as Fig. 2. It has been verified through centrifuge model tests that these piles prevent soil from flowing laterally and inhibit the increasing excess pore water pressure of soil near the piles. However, the installation of steel piles incurs high material cost.
As many retaining walls require countermeasures and the existing methods are too expensive for implementation, the author proposes the use of a low-rigidity material as the deterrent piles in Fig. 2.
