| With the continuous advancement of the Belt and Road initiative and the Ocean Power Strategy,large-scale coastal and ocean engineering construction such as large-scale reclamation of land from the sea,large docks,and cross-sea connection channels is rapidly heating up.Large-scale coastal and marine projects need to build cofferdams for soil retaining,water stop,and wave prevention,and create a dry construction environment for the construction of permanent infrastructure.However,due to the intensified plate tectonic movement and frequent extreme climates,the extraordinary increase in water and super-high hydrodynamic impacts caused by sea-source earthquakes and storm surges will pose a huge threat to coastal protection projects.The cantilever double-row steel sheet pile(DSSP)cofferdam composite structure has the advantages of good overall dynamic performance,strong deformation resistance,small footprint,good engineering applicability,convenient and controllable construction,etc.,and is increasingly used in large-scale coastal and marine Engineering.The DSSP cofferdam involves the mechanical coupling of pile-fill-tie rod-seabed complex structure,and is impacted by multiple external forces such as ultra-high water level,wave dynamics,pumping,and excavation.The force transmission mechanism is unclear,the failure mode is unknown,the cofferdam deformation has a three-dimensional space effect,and the stability criterion is lacking.The theoretical research and calculation methods are lagging behind the engineering practice,bring greater uncertainty to the construction of large-scale coastal and marine engineering infrastructure.Therefore,it is of great significance to carry out the research on the static and dynamic mechanism and calculation method of the double-row steel sheet pile cofferdam in the coastal zone.The static and dynamic characteristics of the DSSP cofferdam in the coastal zone are studied via the combination of methods of case investigation,centrifuge model test,large-scale flume model test,fluid dynamics numerical simulation,finite element dynamic time-history numerical simulation,3D design and visualization analysis based on BIM technology,and large-span 3D finite element numerical simulation.The main innovation results are as follows:(1)Case studies of the double-row steel sheet pile cofferdam structure in the coastal zone was carried out.It was discovered that steel sheet piles have been successfully applied in coastal silt formations,sandy formations,and lithological formations.The adaptability of DSSP in under special engineering geological conditions such as lithological strata,high-permeability sandy layers and deep soft soil layers was analyzed.The insertion depth of the lithologic stratum is limited,and it is difficult to stop the water from the pile end.The width of the cofferdam should be increased and the bottom seepage prevention should be done.The DSSP in soft soil zones needs to reserve a large deformation space,and the soil inside the cofferdam needs to strengthen preventing the pile skirting failure.(2)Based on the centrifuge model tests,the force and deformation behavior of the DSSP cofferdam in compaction,pumping and excavation stages are analyzed.It is found that the bending moment of the double-row steel sheet piles is symmetrically distributed during compaction.As the packing density decreases,the bending moment increases significantly,and the upper bending moment increases more significantly.After pumping,the bending moment of the land side pile is distributed in an S shape,and the bending moment of the sea side pile is distributed in a C shape under the action of the sea side water load.The landside excavation causes further deformation of the landside steel sheet piles,which are transferred to the seaside steel sheet piles through tie rods and backfill soil,giving full play to the overall structural performance of the two rows of steel sheet piles,backfill soil and tie rods under unbalanced earth pressure of both sides of the cofferdam.The settlement of the cofferdam mainly occurs during the compaction process,the lower the density,the greater the settlement.The increase in the width of the cofferdam has the most obvious effect on controlling the horizontal displacement of the cofferdam,and the decrease in the height of the tie rod has an obvious effect on controlling the deformation of the cofferdam and the bending moment of the steel sheet pile.(3)Numerical simulation via finite element model of the double-row steel sheet pile cofferdam was carried out with verification of the centrifuge model tests.It was found that the cofferdam formed by the two rows of steel sheet piles,tie rods,fill soil and foundation produced coordinated deformation as a whole,which bore the unbalanced water and soil pressures on both sides.The weight of the soil fill between the piles has obvious effects on the deformation of the cofferdam backfill and the seabed.The compression modulus of the backfill mainly affects the deformation of the backfill part.The internal friction angle mainly affects the horizontal displacement of the weir during the pumping and excavation stage.Increasing the width of the cofferdam and reducing the buried depth of the tie rods can effectively reduce the deformation of the cofferdam and the bending moment of the steel sheet pile.The excavation depth and excavation distance on the land side of the cofferdam have a significant impact on the overall stability of the cofferdam.The excavation distance and the width of the cofferdam should maintain an appropriate proportional relationship,and the excavation depth and insertion depth should also maintain an appropriate proportional relationship.At the same time,the soft soil under the excavation surface needs to be reinforced in the weir.(4)Based on the underwater quasi-distributed optical fiber testing technology,a large-scale water flume model test under the wave dynamic action of a DSSP cofferdam was carried out.It was found that under high water level and large waves,the combined action of fluctuating pressure,surging waves,and seepage scouring would cause the DSSP cofferdam without wave wall and no anti-scouring and weir top anti-seepage measures a continuous failure mode of "surging waveserosion-leakage-tilting-failure".Increasing the seaside steel sheet pile wave prevention,setting the inner and outer side of the cofferdam to prevent erosion and gravel bags,increasing the seepage prevention of the top of the weir and strengthening the lock and water stop have significant effects on improving the overall stability of the double-row steel sheet pile cofferdam.When the pile tip is located in the soft clay formation,the cofferdam has large settlement,larger waves,and large structural deformation,but the cofferdam exhibits stronger toughness.Even when the structure yielded,it still maintained a certain water-retaining height and didn’t collapse.(5)A new dynamic time history analysis method is established for a wave-structure-rock-soil coupling analysis by combining the fluid dynamics calculation software XFlow and the finite element software Plaxis Dynamics.The simulation and experiment are basically the same in cofferdam deformation,steel sheet pile stress and failure mode.Comparing the pseudo-static method and the cyclic load method,the wave dynamic time-history spectrum method proposed in this paper not only considers the dynamic time-history effects of waves,but also involves the influence of the increase in hydrodynamic pressure caused by wave superposition and breaking.The wave time-history spectrum method is coupled with the rock-soil finite element dynamic calculation method to obtain the deformation instability law of the continuous failure of the DSSP cofferdam.Different from the hydrostatic pressure,the wave power is transmitted from the outer row of piles to the inner row of piles through the backfill and tie rods.At the same time,Plaxflow was used to simulate the dynamic seepage effect of waves,and the pore water pressure agreeed with the test.It was found that the DSSPs had a good barrier effect on the wave dynamic water pressure,and the greater the hydrodynamic pressure,the more obvious the barrier effect.(6)Case study on monitoring data of the DSSP cofferdam of the China Shipbuilding Industry Corporation Changxing Dockyard was analyzed.It is found that the deformation of the cofferdam has obvious three-dimensional spatial effects.Based on BIM technology,the 3D geological model,cofferdam structure model reconstruction and 3D monitoring data visualization analysis were carried out.A three-dimensional finite element numerical simulation considering the spatial effect was carried out.The simulated macro-weir deformation law is more consistent with the engineering monitoring data.The maximum deformation error of the steel sheet pile is only 4.6%～10.6%,and the simulation accuracy is higher than that of traditional methods.The three-dimensional finite element numerical simulation effectively reflects the influence of the spatial effect in the length direction of the cofferdam.It is found that the deformation of the large-span DSSP cofferdam is the accumulation of influence from the water pressure difference on both sides of the two-dimensional section,wave dynamics,soil excavation and three-dimensional longitudinal deformation.The three-dimensional space effect is most obvious in the middle of the long straight side of the cofferdam. |
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