| Land reclamation by dredged soil has been increasingly employed in Shanghai,a coastal port city,to resolve issues associated with land scarcity and marine pollution because of economic and social development and the growing human population.Shanghai has successfully implemented several reclamation projects in Chongming East Shoal,Pudong Side Shoal,and Hengsha East Shoal,which have facilitated the acquisition of substantial land resources.Under the premise of reasonable planning and comprehensive utilization,these reclaimed “new lands” can promote the sustainable development of Shanghai's economy and society,accelerating the process of urbanization and industrialization.However,hydraulic reclamation-derived dredger fill can produce a large degree of deformation during sedimentation and can further compress the underlying soft soil layers deposited in marine and terrestrial environments,resulting in soil consolidation and deformation.The accumulation of compression and deformation in multiple soil layers and on the ground is a form of land subsidence that is an unavoidable environmental geological phenomenon in reclamation areas,which poses a series of engineering geological hazards.Land subsidence not only threatens the safe use of soil foundations and buildings(structures),especially linear engineering,but also destroys the human living environment and weakens the capacity of coastal areas to resist marine hazards,such as floods,storms,and sea-level rise.Therefore,the monitoring and prediction of land subsidence in reclamation areas are important for clarifying the evolutionary processes that might occur over longer periods.Furthermore,such efforts can contribute to explaining the mechanism of land subsidence from the perspective of engineering geological properties of multiple soil layers so that appropriate measures can be taken to prevent land subsidence and appropriately plan engineering construction.Here,Chongming East Shoal,a typical multi-phase reclamation area in Shanghai,was studied.Preliminary engineering geological data and in-situ monitoring data are lacking in this area compared with core urban areas,as research on land subsidence in Chongming East Shoal is still in its infancy.Consequently,the current distribution and consolidation mechanism of regional land subsidence are poorly understood,exacerbating efforts to effectively avoid land subsidence-derived environmental hazards or dangers.Below,the specific approaches used to study land subsidence in Chongming East Shoal are described.(1)Field investigation.Ten boreholes,each with depths of 55 m,along the eastwest long axis and across the main cofferdams were made.The main soil layers that can be affected by engineering construction were exposed to clarify the stratum structure of the typical profiles,which revealed that they were consistent with the expansion direction of Chongming East Shoal from west to east.(2)Laboratory test.This approach aimed to explore the differential engineering geological properties of plain fill,dredger fill,sandy silt,muddy clayey soil,clay,and silty clay at various scales to clarify the engineering geological properties of dredger fill after 20-year self-weight consolidation and locate the main compression layers under the current stress conditions.Furthermore,Computed Tomography(CT),Mercury Intrusion Porosimetry(MIP),and Scanning Electron Microscopy(SEM)were used to explore the mesostructure,micro-structure,and micro-pore characteristics of each soil layer.(3)Monitoring and analysis of land subsidence.Seventy Sentinel-1A SAR images were collected from the beginning of 2015 to the end of 2019 and were processed by advanced remote sensing technology—the Small Baseline Subset Interferometric Synthetic Aperture Radar(SBAS-In SAR)—to derive regional land deformation information.The subsidence pattern of multi-phase reclamation areas from west to east was then explored,and the representative region of interests(ROIs)were selected to estimate the average consolidation degree of the multi-layer soil.(4)The mechanism of the differential settlement phenomenon was explored by evaluating the stratum structure,physical and compositional characteristics,chemical and compression features,permeability,and microstructural characteristics of soil in multi-phase reclamation areas.(5)Finally,the relationship between representative engineering geological parameters and land subsidence velocity was explored by a BP(error backpropagation)Neural Network.The results show that:(1)Due to the influence of soil structure,dredger fill is characterized by the property of over consolidated.On the contrary,the clay and silty clay layer are under consolidated,contributing considerable to the land subsidence.From top to bottom,the widespread soil layers in the study area include silty soil(dredger fill and sandy silt)and clayey soil(muddy clayey soil,clay and silty clay).Dredger fill has undergone self-weight consolidation for more than 20 years,and was also affected by surface evaporation and human activities,resulting in soil shrinkage and enhanced soil structure.Therefore,the dredger fill shows abnormal over consolidated characteristics.Meanwhile,the shallow sandy silt was also characterized by over consolidated and muddy clayey soil was close to normal consolidation.At present,the above three soil layers contribute a small amount of land subsidence.Both clay and silty clay were under consolidated,but the clay layer was considered as the representative compression layer in the study area,which contributes a lot to the land subsidence.(2)In SAR-derived results shows that the subsidence velocity was slow in late reclaimed area.In this study,the SBAS-In SAR-derived average subsidence velocity within the main reclamation areas showed opposite pattern with that in common reclamation area where the subsidence velocity of offshore reclamation area should be fast than the onshore reclamation area due to late reclamation projects.On this basis,the ranges of average consolidation degree of soil in the selected offshore and onshore lands were estimated by using hyperbolic method and three-point modified exponential curve method.The results show that under the current stress state,the consolidation process of the offshore soil layers has been basically completed,while that of offshore soil layers develops rapidly.This method can facilitate an understanding of the development of regional land subsidence and the degree of soil consolidation.Meanwhile,it has good applicability and strong timeliness for consolidation evaluation under the condition of poor in-situ monitoring information.(3)The poor permeability and consolidation condition in the offshore reclamation area was the internal control factor that caused the slow land subsidence.The mechanism of differential subsidence was explored by comparing the consolidation characteristics of multi-phase reclamation area.The results show that,with the shortening of the land forming time,the thickness of the soil layer increases,the clay content increases,and the compressibility increases.The bound water film thickens,the micro heterogeneity increases,the micro pore complexity increases,and the clay agglomeration weakens.The clay particles are often dispersed in the pores to form a flocculation structure,resulting in the drainage channel silting and permeability coefficient drops.Therefore,the consolidation efficiency is low and corresponding subsidence velocity is slow.(4)Based on BP Neural Network,the relationship model between subsidence velocity and multi-scale engineering geological parameters of soil was established.Sixteen groups of clay samples collected from clay layer were taken as the research object.Ten representative multi-scale engineering geological parameters,including clay layer thickness,clay content,permeability coefficient,cation exchange capacity,water content and rebound index;fractal dimension of micro pore morphology,average equivalent diameter of structural unit,and standard deviation of directional frequency and average shape factor were selected as independent variables.The In SAR-derived subsidence velocities within multi-phase reclamation areas were selected as dependent variables to build the model.The results show that when the macro and micro multiscale parameters were considered together during analysis,the prediction accuracy of soil engineering geological parameters on land subsidence velocity was higher and the correlation was closer.Further work on more abundant laboratory tests,together with a longer moitoring period and a higher precision,is desirable to enrich and improve these preliminary results. |
PDF Full Text Request