SEAWALL SUBSIDENCE IN SHANGHAI: CHARACTERISTICS AND DRIVING MECHANISMS

Shanghai has long been suffered from land subsidence since 1920s. Accumulated subsidence imposes a huge threat to the coastland, which could get high risk of inundation by storm surges under the combined impact of rapid sea level rise. Using high precision-leveling of 2009 to 2015 along the seawall, together with the historical GPS data and leveling covering the entire city since 1980s, we attempt to describe the vertical displacement of the seawall, and try to interpret the natural and anthropogenic mechanisms that induce the pattern of the displacement. Results displayed subsidence rates of the seawall exhibit significant spatial variability.87% of seawalls have experienced overall subsidence in the past 7 years. After analyzing the possible correlation between sediment rates and groundwater level, Quaternary sediments, thickness of clayey and compressible Holocene deposit as well as anthropogenic activities, we come to the conclusion that, consolidation of late Holocene deposit and the external loading are the primary factors responsible for the observed displacement variability, and groundwater level and land reclamation on the tidal flat also have significant impact on the rapid subsidence. Conversely, tectonic subsidence and differential post-construction settlement could not be the triggering factors that made the settlement in the coastal zone. Due to the significant impact of external loading from engineering construction on the seawall, continuous monitoring is urgently needed during the process of the projection construction.