Consolidation Theory And Settlement Prediction Of Improved Vacuum Preloading Method For Ultra-Soft Hydraulic Reclamation Foundation

With the development of economic,the conflict of the rare of land source is standing out as part of the urbanization process.Therefore,newly dredger fills from a harbor basin or a sea channel are hydraulically placed in seawater for land reclamation in recent years.Sediments dredged from harbor basins or sea channels are generally used to reclaim ocean,lake,or river sediments.This newly dredged soil is commonly new hydraulic mud that contains a large amount of fine soil particles and has a high moisture content.These properties indicate that the dredged soil is ultra-soft and highly compressible,especially for those ultra-soft foundations with an extremely high moisture content and high content of drift mud.With high initial moisture contents,high compressibility,low permeability,low shear strength,rheological,and unstable soil structure,the improvement of the ultra-soft foundations usually has a very limited effect while using the traditional vacuum preloading method.To understand how to achieve the goal of rapid consolidation on those ultra-soft foundations with an extremely high moisture content and high content of drift mud,solve the facing problems of long construction period and poor consolidation effect,becomes the most urgency and important research and development direction in vacuum preloading method.In this paper,a series of research was developed to consolidate newly dredged fills with an extremely high moisture content and high content of drift mud,that are hydraulically placed in seawater for land reclamation in coastal region of Tianjin City,China.The issue of long construction period,poor consolidation effect and the twice design and application of plastic drainage plate in ultra-soft foundations was conducted and studied with laboratory model tests,real field tests and theoretical analysis.The main contents and achievements are as follows:(1)A series of model tests was conducted in the laboratory to study the improved consolidation theory of the vacuum preloading method with inherent factor(soil particle grading components)and external factor(vacuum loading method)on the newly dredged fills with an extremely high moisture content and high content of drift mud in coastal region of Tianjin City.The results show that the soil will have a smaller void ratio and more drainage paths when it has a closer radial distance with the PVDs with a same soil particle grading components;With a same vacuum loading method,soil with a high clay content will avoid the clogging of PVDs and have a better consolidation effect.(2)A series of model tests was conducted in the laboratory to study the improved consolidation method of the vacuum preloading based on the study of improved consolidation theory with inherent and external factor.The problems of long construction period and poor consolidation effect with vacuum preloading method,were studied with the improved gas injection and pressure charging vacuum pressure method,synchronous and alternate vacuum preloading method,and the membrane-less vacuum preloading method.The results show that the improved gas injection and pressure charging vacuum pressure method could shorten the consolidation period by increasing the drainage paths in the soil;the synchronous and alternate vacuum preloading method could improve the foundations much more even by changing the motion rules of the soil particles into alternating motion between those two sets of PVDs,which can decrease or prevent clogging;the membrane-less vacuum preloading method could improve the cost-effectiveness and achieve the same treatment effect with a traditional vacuum preloading method.(3)Four groups of real field tests were conducted to consolidate newly dredged fills that are hydraulically placed in seawater for land reclamation in the Lin Gang Industrial Zone of Tianjin City,China,including sand-less vacuum preloading technique,combined technique of vacuum preloading and pneumatic fracturing,improved gas injection and pressure charging vacuum pressure technique and efficient novel moisture separator technique.The real field tests results show that multiple vacuum preloading method and improved gas injection and pressure charging vacuum pressure method is the best method during the improvement of newly dredged fills with an extremely high moisture content and high content of drift mud in coastal region of Tianjin City.(4)A full-scale engineering project on a very soft-clay ground was studied to present the improved method of vacuum preloading method.With the results of the dissipation of pore water pressure,the settlement curves and all the intensity indexes of the treatment soil,a conclusion was proposed that the improved method and an efficient and energy-saving moisture separator are efficient and cost-effective for improving sites with low bearing capacity and high compressibility soils,and could protect the environment as well.These results provide theoretical basis for practice in the future engineering projects.(5)Newly dredged fills that are hydraulically placed in seawater for land reclamation and the natural deposition soils in the coastal region of Tianjin City are taken as research object,to study the consolidation and secondary consolidation properties by mechanical properties tests.The coefficient of consolidation was calculated with Asaoka back analysis method and the site measurement results;Methods of curve-fitting were used to calculate the final settlement results,including hyperbolic method,Fxponential curve method and Asaoka method.Considering the properties of the newly dredged fills with an extremely high moisture content and high content of drift mud,per the measured results,the author assumes that hyperbolic method is more suitable for the settlement prediction and analysis in the coastal region of Tianjin City.