Theoretical Study On Preloading Consolidation Combining With Dynamic Compaction In Treating Reclaimed Dredged Soil Ground And Its Application In Practical Engineering

With the fast development of economy and growth of population,the shortage of land resources has become more and more severe in China.Land reclamation helps lot in gaining land resources from the ocean,thus satisfying the urgent demand for land to some extent.Dredged soil is cheap and accessible and it has been widely used in land reclamation but it is also of low strength,high water content and void ratio,and easy to deform largely,posing threat to building on the backfill foundation.So it is important to improve the ground condition after filling dredged soil.Preloading consolidation(PC)with prefabricated vertical drains(PVD)and dynamic compaction(DC)and often combined to strengthen the backfill dredged soil ground,since they help with the drainage of pore water,decrease of void ratio and increase of effective stress.However,research on PC with PVD combining with DC in treating dredged soil is still limited.In this dissertation,an analytical model of consolidation of backfill dredged soil foundation with single PVD was developed first.Both the upper and the lower drainage boundaries of the PVD in this study are impeded,which is more practical compared with existing analytical models.Furthermore,the impeded drainage boundaries in this study can be degenerated into pervious or impervious by changing the value of the coefficients in the expressions of impeded drainage conditions,which suggests the impeded upper and lower boundaries in this study is more general compared with existing analytical models.The consolidation problem was solved applying separation of variable method.The influences of drainage boundary conditions,well-resistance effect and properties of dredged soil on excess pore water pressure and degree of consolidation were investigated then.Main conclusions can be drawn as follows:the method in this study was verified against existing research.When the drainage condition of one of the PVD’s boundary is good,the effect of the other boundary’s drainage condition on degree of consolidation is not significant.However,when the drainage condition of one of the PVD’s boundary is bad,the effect of the other boundary’s drainage condition on degree of consolidation is significant.The influence of the coefficient of variation of well-resistance effect with time is significant at value of 10-7 s-1 order of magnitude.The effect of permeability coefficient of dredged soil on degree of consolidation is only obvious when it is smaller than 5×10-10 m/s.The influence of smear effect is significant throughout the whole process of consolidation.A preloading consolidation model with PVD for backfill dredged soil was developed based on ABAQUS platform considering the material non-linearity of backfill dredged soil and vertical drains with time dependent well resistance.The 3D problem was converted into a plane strain problem by transforming the PVD into sand walls.The influence and spacing of PVDs,loading interval,height of preloading and total loading on the settlement,pore pressure and variation of void ratio with time and depth were investigated.Conclusions can be drawn as follows:the influence of inserted length of PVD on consolidation of backfill dredged soil is significant.The PVDs are suggested to penetrate the soil layers with low permeability coefficients.The decrease of spacing of PVDs helps shorten the drainage paths and accelerates the consolidation procedure,while the ultimate degree of consolidated is not affected.Uniform height of preload and loading interval per step is better for the safety of reclaimed ground.If the total loading increases,the loading interval and number of loading step should also increase.After preloading consolidation,dynamic compaction model for backfill dredged soil was developed based on ABAQUS platform.The values of parameters of dredged soil inherit from those after preloading consolidation.The process of dynamic compaction was modeled by an impact load acting on the reclaimed ground.Infinite element absorbing boundaries are applied.The criterion for defining improvement region and stopping tamping were determined according to reported researches and codes.The influences of DC energy level,tamping height,hammer diameter,tamping times and sand cushion thickness on the crater depth and improvement region were comprehensively investigated-Main conclusions were drawn as follows:DC energy level of 1400 kJ can satisfy the demand of construction as well as guarantee the effect of DC.The crater depth due to heavy hammer and low tamping height is larger compared with that due to light hammer and high tamping height.The influence of hammer diameter is insignificant within the range of code requirement.The thickness of sand cushion is suggested to be 1.5 m to 1.75 m for backfill dredged soil.Some instructions were provided for the design and construction of PC combining with DC procedures on backfill dredged soil ground according to the theoretical researches mentioned above and related codes in this filed.The instructions were then applied to the preliminary design of a berth project in Putou district,Meizhou Harbor in Fujian Province.Finite element model was also developed to test the performance of the instructions and help with the improvement of the design procedure.Finally,risk and investment management analysis was conducted on the PC and DC method.The advantages on risk control and investment management were confirmed.Based on a berth project in Putou district,Meizhou Harbor in Fujian Province,the Value Engineering(VE)was introduced to evaluate the values of several possible schemes of ground treatment and control the cost of the project,thus confirming that the scheme investigated in this dissertation is the best scheme.Then the PC and DC scheme was applied to this project,and some instructions were given to the preliminary design of the project.Finite element model was also developed to test the performance of the instructions and help with the improvement of the design procedure.Finally,risk identification and assessment were conducted on the PC and DC scheme from the perspective of riks management and control.Some measures on risk management and control were proposed.