Recent Characteristics Of Land Subsidence In Shanghai And Its Effect On Performance Of Key Municipal Facilities

Located at the mouth of the Yangtze River,Shanghai has a Quaternary sediment with thickness of 150-350 m.It contains five major confined aquifers and 11 engineering geological layers that are 100 m in depth and above.Land subsidence induced by long-term groundwater exploitation and recharge and engineering construction activities is one of the major geological disasters in Shanghai,causing uneven deformation of key municipal infrastructures such as metros,elevated roads,and cross-river bridges,which seriously threatens the safety of urban lifelines.Based on the data obtained from the author's long-term work involved in on-site monitoring,recent characteristics of Shanghai land subsidence and its influencing factors were studied using in-situ tests and numerical simulations.Through analyzing the long-term performance of metros,elevated roads,and cross-river bridges,the impact of regional land subsidence on the operation safety of key municipal infrastructures.The main contents are as follows:(1)The new spatial-temporal distribution characteristics of land subsidence in Shanghai were obtained: the land subsidence was generally slow but the uneven settlement was significant.The deep soil expanded and the shallow layer was not uniformly compressed.Before and after 1999,the confined aquifer water levels experienced a process from decline to increase.Except for the fourth aquifer,the deformation of other confined aquifers was consistent with the rise and fall of the water level,and the fourth confined aquifer was continuously compressed after 1999.After the expansion began in 2009,the overall land subsidence slowed.With the change of the deep soil from compression to expansion,all of land subsidence in Shanghai was controlled within 6mm.However,due to the influence of geotechnical engineering activities,shallow soil layers continue to be compressed,and uneven settlement in the central city area is more serious.In the differential settlement induced by excavation engineering,the excavation unloading affects the soil layer within 1 to 3 times the excavation depth range,and the maximum impact range of decompression and precipitation exceeds 10 times the excavation depth.(2)Based on the comprehensive analysis of the longitudinal bending deformation characteristics and development trend of the metro structure,it is proposed to evaluate the safe operation status using the curvature radius and the relative deformation.Due to the cumulative effect of land subsidence,the metros built in different periods has the phenomena of fullline settlement,local uplift,partial uplift and full-line uplift.The cumulative settlement or uplift amount is closely related to the completion time.With the characteristics of the expansion of the deep soil and the overall mitigation of land subsidence,the cumulative amount of subsidence will gradually decrease,and the overall metros will rise,but the amount of floating will be small.Due to the continuous non-uniform compression of the shallow layer,the metro structure is completely deformed in the longitudinal direction.The use of a single index in the radius of curvature or relative deformation may be misjudged for severity,and it is therefore recommended that a comprehensive analysis of the two indicators be used.(3)The law of urban road deformation with strata in different geological zones and different basic forms was obtained.Affected by the regional land subsidence,the Shanghai Central Line sank during the entire land subsidence,and some of the elevated roads swelled after the expansion of the deep soil.The deformation of the Central Link is closely related to its basic form and the geological environment in which it is located.The settlement of the ground road is close to the settlement of the adjacent ground.The deformation trend of the elevated road is consistent with the trend of compression and expansion of the soil below the elevated pile end.Due to the piercing effect of the tip,the elevated settlement is slightly greater than the compression of the soil below the tip.The elevated bulge is mainly affected by the deformation of the deep strata,and the settlement of the ground road is affected by the deep strata deformation and the engineering construction activities.(4)On the basis of obtaining long-term settlement characteristics of cross-river bridges,the reasons for the long-term differential settlement of cross-river bridges caused by groundwater mining and irrigation in the area were clarified.During the deep strata compression,the settlement of the bridge pier increases linearly,and the settlement of the Puxi pier is significantly larger than that of the Pudong pier,with the maximum differential settlement reaching 225 mm.After the new characteristics of land subsidence appeared,the settlement of the bridge piers was relatively small,but it still showed that the settlement of Puxi piers was greater than the settlement of Pudong piers.The regional difference of deep groundwater is the main reason for the uneven settlement of bridge piers caused by inhomogeneous deformation induced by deep strata.