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Land subsidence severely threatens most of the coastal plains around the world where high productive industrial and agricultural activities and urban centers are concentrated. Coastal subsidence damages infrastructures and exacerbates the effect of the sea‐level rise at regional scale. Although it is a well‐known process, there is still much more to be improved on the monitoring, mapping and modeling of ground movements, as well as the understanding of controlling mechanisms. The International Geoscience Programme recently approved an international project (IGCP 663) aiming to bring together worldwide researchers to share expertise on subsidence processes typically occurring in coastal areas and cities, including basic research, monitoring and observation, modelling and management. In this paper, we provide the research communities and potential stakeholders with the basic information to join the participating teams in developing this project. Specifically, major advances on coastal subsidence studies and information on well‐known and new case studies of land subsidence in China, Italy, The Netherlands, Indonesia, Vietnam and Thailand are highlighted and summarized. Meanwhile, the networking, dissemination, annual meeting and field trip are briefly introduced. [ABSTRACT FROM AUTHOR]

The article presents a research report conducted on the Land Subsidence with its monitoring, impact and mechanism in the coastal cities. It states that land subsidence is a geohazard which is consisted in the lower ground surface. The geohazard is a geological state which takes place because of natural and humaninduced processes occurring in the deep subsoil.

Land subsidence can be observed with time-series of Interferometric Synthetic Aperture Radar (InSAR) data. However, existing approaches only reveal subsidence signals that are multi-scale mixed, which is not conducive to the systematic analysis of subsidence of different mechanisms. A deformation signal decomposition (DSD) method based on spectral analysis is used to decompose the deformation extracted by time-series InSAR into three classes of deformation signals. They refer to large-scale deformation related to geological settings, medium-scale deformation caused more by group excavation, and small-scale deformation along linear infrastructures. TerraSAR-X datasets for Shanghai spanning April 2013 to September 2020, and Sentinel-1A datasets spanning January 2016 to September 2020 are used in this study. The results were cross-verified between the TerraSAR-X and Sentinel-1A datasets, and validated against levelling measurements. Subsidence signals caused by different mechanisms were automatically decomposed, which facilitates a systematic analysis for targeted diagnosis of land subsidence signals. A detailed analysis was conducted jointly at three scales of surface displacement, geological conditions, major construction activities, and subsidence mechanisms. It indicated that construction activities were the leading cause of land subsidence, and suggests that local authorities that wish to mitigate surface subsidence may benefit from primarily considering this process. [ABSTRACT FROM AUTHOR]

Excessive groundwater withdrawal has caused severe land subsidence worldwide. The pore water pressure and the deformation of pumped hydrostratigraphic units are complex. A fully coupled three‐dimensional numerical simulation was carried out for different pumping plans in this paper. When groundwater is pumped from a confined aquifer, the great compaction occurs in the pumped aquifer and its upper and lower adjacent aquitard units. Land subsidence is smaller and the area affected by land subsidence is greater when groundwater is pumped from the deeper confined aquifer. The pore water pressure in the pumped confined aquifer changes immediately with pumpage. In the adjacent aquitard units, however, the pore water pressure increases in the early pumping time and decreases in the early recharging time. The decrease in the pore water pressure vertically spreads from the interface between aquitard and pumped aquifer to the other surface of the aquitard. The pumped aquifer compacts and rebounds immediately with pumping and non‐pumping or recharging actions, while the compaction and rebounding of the aquitard units clearly lag behind. The compaction of the adjacent aquitard unit first occurs near the interface between aquitard and pumped aquifer units, and the compaction zone spreads outward as the pumping goes on. The aquitards may expand vertically within some zones. Due to the inelastic deformation of soil skeleton, different pumping plans result in different land subsidence. For the same net pumpage, maximal land subsidence and horizontal displacement are the smallest for constant discharge and the greatest for recharge‐discharge cycle. Article impact statement: Analysis of pore water pressure and deformation has the value of understanding the mechanisms of pumping‐induced land subsidence. [ABSTRACT FROM AUTHOR]

Shanghai has the largest newly reclaimed area in China's coastal regions, and dredger fills are universally distributed in reclaimed lands there. Multi-scale geotechnical properties of dredger fills in region-scale (>1 km2), site-scale (1 m2–1 km2), mesoscale (1 cm2–1 m2), and microscale (1 µm2–1 cm2) were studied mainly through cone penetration investigation, borehole investigation, and microscopic inspections. Dredger impact craters, soil structures, and soil textures were investigated qualitatively and quantitatively to identify the dredger fills. Ultimate land subsidence since 1 July 2018 and land subsidence velocity characterized by the land subsidence from 1 July 2018 to 1 January 2019 were evaluated in Shanghai's typical reclaimed regions. Afterwards, Bayesian network including sixteen variables was proposed for land subsidence risk assessment. Surface subsidence, land subsidence velocity, and comprehensive land subsidence were the most sensitive to data point location and average initial void ratio of underlying strata. These three risk variables were also highly correlated to the thickness of the dredger fill layer and the reclamation time. This finding indicates that variances of the four independent variables (i.e., data point location, average initial void of underlying strata, land reclamation time, and thickness of dredger fill layer) significantly decide variances of the three risk levels. [ABSTRACT FROM AUTHOR]

Severe land subsidence caused by groundwater exploitation is one of the anthropogenic environmental problems. Even when groundwater exploitation has been limited or prohibited in some areas, it will continue in some water-deficient areas for a long time. Therefore, it is a challenge to reasonably exploit groundwater and not to cause severe land subsidence. A physical model test was conducted in this paper to simulate the extraction and recharge of groundwater, and to observe the soil deformation characteristics and the change of the pore water pressure. Based on the test results, the mechanism of land subsidence was investigated and different pumping modes were involved including the single-well pumping, single-well pumping and recharging, and double-well pumping and recharging tests. The tests indicated that the sand layer and clay layer had different pore water pressure change and deformation characteristics. In the sand layer, the pore water pressure decreased when pumping and increased when pumping stopped. In the clay layer, however, pore water pressure first increased rapidly and then decreased when pumping, and it decreased rapidly and then increased after pumping stopped. The variation of pore water pressure in the clay layer was consistent with the Mandel's effect and was related to the distance to the pumped layer. The further the distance was, the longer the Mandel effect lasted. The sand layer mainly exhibited elastic deformation and the clay layer almost exhibited plastic deformation. The clay layer showed clear hysteresis. The hysteresis time was increased with the distance from the pumped sand layer. In addition, the results indicated that the three pumping modes could reduce land subsidence. With regard to the pumpage and land subsidence, the double-well pumping and recharging mode was better than the other two. With pumping or pumping and recharging together, the increasing rate of vertical displacement became smaller and gradually reached a stable state. In general, these three modes were reasonable for groundwater exploitation, and could effectively control land subsidence. [ABSTRACT FROM AUTHOR]

Partial expansion was observed in stratified subsidence during foundation pit dewatering. However, the phenomenon was suspected to be an error because the compression of layers is known to occur when subsidence occurs. A slice of the subsidence cone induced by drawdown was selected as the prototype. Model tests were performed to investigate the phenomenon. The underlying confined aquifer was generated as a movable rigid plate with a hinge at one end. The overlying layers were simulated with remolded materials collected from a construction site. Model tests performed under the conceptual model indicated that partial expansion occurred in stratified settlements under coordination deformation and consolidation conditions. During foundation pit dewatering, rapid drawdown resulted in rapid subsidence in the dewatered confined aquifer. The rapidly subsiding confined aquifer top was the bottom deformation boundary of the overlying layers. Non-coordination deformation was observed at the top and bottom of the subsiding overlying layers. The subsidence of overlying layers was larger at the bottom than at the top. The layers expanded and became thicker. The phenomenon was verified using numerical simulation method based on finite difference method. Compared with numerical simulation results, the boundary effect of the physical tests was obvious in the observation point close to the movable endpoint. The tensile stress of the overlying soil layers induced by the underlying settlement of dewatered confined aquifer contributed to the expansion phenomenon. The partial expansion of overlying soil layers was defined as inversed rebound. The inversed rebound was induced by inversed coordination deformation. Compression was induced by the consolidation in the overlying soil layers because of drainage. Partial expansion occurred when the expansion exceeded the compression. Considering the inversed rebound, traditional layer-wise summation method for calculating subsidence should be revised and improved. [ABSTRACT FROM AUTHOR]

Summary: Shanghai is one of the cities suffering from land subsidence in China. Land subsidence has caused serious financial losses. Thus, artificial recharge measures have been adopted to compensate the drawdown in shallow, confined aquifers and thereby control land subsidence. In this study, a multi-cycle recharge–recovery field experiment was performed to investigate the response of a shallow, confined aquifer to artificial recharge through a well. In the experiment, a series of recharge–recovery cycles with different recharge volumes and durations, with and without artificial pressure, were performed. The water levels monitored in the recharge and observation wells indicated the response of the aquifer to the multi-cycle recharge–recovery process. Meanwhile, a finite-difference method (FDM) numerical model was established, and its parameters were obtained via a reversed numerical analysis on the experimental data. The responses of the shallow, confined aquifer to the multi-cycle recharge–recovery process were simulated in detail using the model. The calculation results showed that the water level dropped significantly when the recharge ended. Moreover, the efficiency of a multi-cycle recharge was found to be higher than that of a concentrated one under the same recharge volume and time. The relationship between recharge frequency and efficiency, expressed as H =0.29498 f 0.40163 and R 2 =0.97264, respectively, was obtained through the FDM numerical simulation. In the recharge intervals, the optimal recharge efficiency was achieved when the water level rose to 40% of the peak. [ABSTRACT FROM AUTHOR]

Land reclamation has been increasingly employed in many coastal cities to resolve issues associated with land scarcity and natural hazards. Especially, land subsidence is a non-negligible environmental geological problem in reclamation areas, which is essentially caused by soil consolidation. However, spatial-scale evaluation on the average degree of consolidation (ADC) of soil layers and the effects of soil consolidation on land subsidence have rarely been reported. This study aims to carry out the integrated analysis on soil consolidation and subsidence mechanism in Chongming East Shoal (CES) reclamation area, Shanghai, at spatial-, macro-, and micro-scale so that appropriate guides can be provided to resist the potential environmental hazards. The interferometric synthetic aperture radar (InSAR) technique was utilized to retrieve the settlement curves of the selected onshore (Ra) and offshore (Rb) areas. Then, the hyperbolic (HP) model and three-point modified exponential (TME) model were combined applied to predict the ultimate settlement and to determine the range of ADC rather than a single pattern. With two boreholes Ba and Bb set within Ra and Rb, conventional tests, MIP test, and SEM test were conducted on the collected undisturbed soil to clarify the geological features of exposed soil layers and the micro-scale pore and structure characteristics of representative compression layer. The preliminary results showed that the ADC in Rb (93.1–94.1%) was considerably higher than that in Ra (60.8–78.7%); the clay layer was distinguished as the representative compression layer; on micro-scale, the poor permeability conditions contributed to the low consolidation efficiency and slight subsidence in Rb, although there was more compression space. During urbanization, the offshore area may suffer from potential subsidence when it is subjected to an increasing ground load, which requires special attention. [ABSTRACT FROM AUTHOR]

• CT was applied to various types of soil that were collected in a reclamation area. • The coefficient of variation-based results showed the soil heterogeneity. • The averaged Hu was correlated with the physical properties and compressibility. • The averaged Hu values reflected the relative amount of soil particles and water. Nowadays, land reclamation by dredged soil has become an effective solution to the growing demand for land resources in coastal areas. However, the widespread dredger fill (artificial deposits in reclamation area) can compress the underlying soil layers, causing different degrees of land subsidence and posing a threat to infrastructure in relation to agriculture. Therefore, it is necessary to facilitate an understanding of the physical properties and compressibility of soil in the reclamation area. X-ray computed tomography (CT), as a non-destructive method, can provide insight into the structure of soil. In this paper, 44 undisturbed soil samples comprised of dredger fill, sandy silt, muddy clay, clay, and silty clay, were collected from ten 55 m deep boreholes in Chongming East Shoal reclamation area in Shanghai. CT and conventional tests were conducted on all soils. The coefficient of variation of the scanning-slice-related standard deviation was used to evaluate the soil heterogeneity, which reflects the spatial variability of the arrangement of soil particles. The results indicated that the slice-related coefficient of variation had a reliable capacity to evaluate the soil heterogeneity; the averaged Hounsfield (AHu) values of the soil sample had a significant statistical correlation with the conventional-test-derived physical properties (wet bulk density, dry bulk density, gravimetric water content, and porosity). The AHu values essentially reflected the relative amounts of soil particles and water in saturated soil. The granulometric compositions that can lead to denser arrangements of soil particles tended to have higher Hu values. A linear relationship was also found between AHu values and the compression index, which may help to predict the compressibility of the soil. As a rule, these findings open new avenues for research into the physical properties and compressibility evaluation of varied types of soil. [ABSTRACT FROM AUTHOR]