The ancient Qiantang River seawall

built in Ming and Qing dynasties

is an important barrier in defending the estuary area of Qiantang River against floods and tides. To prevent strong hydrodynamic loads scouring the foundation and lowering the stability of the seawall

the apron structure is built at the front of the ancient seawall. The apron in the Haining region is mostly built by laying unbonded strip stones. To investigate the failure mechanism of such apron

wave and flow flume model tests are performed. It is shown that such apron is vulnerable to the impact by strong tidal bores where the covering stones are missing. The soil under the apron is vulnerable to the erosion by ebb tide seepage and longitudinal current

which leads to the sinking of the apron. As a cover layer

the apron has a good performance in protecting the soil

and can prevent the soil from instantaneous and residual liquefaction.