The recent utilization of reclaimed land provides large-area,man-made island,e.g.,those used for airports,has necessitated their construction in relatively deep seas,which naturally requires them to be surrounded by seawalls that are directly exposed to strong waves since no protective berakwaters are present, Consequently, failures frequently occur during and after construction. The settlement of reclaimed land behind the seawalls is considered to be responsible for most,it does lead to land-usage problems and expensivelong-term maintenance requirements. As the failure mechanism has not yet been clarified,this led to the present study which investigates settlement failures by reproducing them using a series of model experiments in a large wave flume. In addition, experiments were performed to elucidate possible countermeasures.
　By combining our experimental results with those from field surveys, various mechanisms were found to cause leakage of reclaimed soil, with this loss of soil su
bsequently leading to seawall failure by settlement. The main results are described as follows.
1) As the possibility of settlement increases if the sheet fails to stop reclaimed soil leakage, it must be secured against direct wave action during the construction period.
2) Measurements showed that a high pore water pressure on the stone backfill will substantially increase leakage through the damaged sheet. In comparison with the toe pressure at the front of the vertical wall,backfill pore water pressures were about 20% lower,and quickly damped out into the reclaimed soil (sand). However,if all the soil surrounding the backfill is under saturated conditions, the backfill pore water pressures are much greater. A new method is proposed in which a pressure relief gravel filter situated on the top of the stone backfill is used to effectively reduce the pore pressure.
3) Boiling occurs in the reclaimed soil when the soil above the vackfill has a small thickness and is saturated, which may result when wave overtopping is large and/or tides are high.
4) High impulsive pressures occur on the joint plate connecting two caissons. These pressures are generated by water plunging into the gap between the walls, which can crack the plate and in turn lead to damaging the geotextile sheet. Beased on the experimental results, methods are described to (i) estimate the pressure and (ii) practically reduce the pressure.