Rheology Property And Non-linear Reheological Constitutive Model Study Of Recently Reclaimed Soft Clay In Tianjin Binhai New Area

With the rapid development of The Tianjin Binhai New Area, requirement for amount for land have been increasing, and the scale of reclamation projects has become larger and larger. As a result, many structures have to be constructed on the soft clay foundation. This kind of soil foundation is usually formed by hydraulic filling of the dredge and improved by the vacuum preloading method. When a relatively large load is applied on the foundation, not only the elastic and plastic displacement will take place, the time-dependency displacement, i.e., the rheological displacement, will also develop. This phenomenon can be observed during the construction stage when the degree of consolidation is low, which may induce serious engineering problems and cause huge economic lost. It has been shown that the rheological problem is concerned with many respects of geotechnical engineering, such as (1)Bearing capacity: the ove-loaded may cause a failure zone in the subsoil much larger than that is estimated by the conventional bearing capacity calculation theories; (2)Slope stability: a temporary load far away from the most dangerous slip surface of a soft clay slope may lead to large scale failure of the slope; (3) Earth pressure: the weigh of the piling up of some construction material, which is out or the rang of the area to influence the earth pressure to a retaining structure according to the conventional earth pressure theory, may induce excessive displacement of the retaining wall. This phenomenon can not be reasonably explained with the conventional geotechnical theories. Besides, the rheological displacement is developing within a relatively short period of time after loading, making it different from the so called creep that is concerned with long term displacement with load unchanged. In this thesis the engineering geology characteristics and rheological properties of the soft clay are tested in the laboratory; the mechanism of rheological displacement of the soft clay is discussed; the constitutive relations that can describe the rheological properties of the soil is established and the achievements have been applied to solve practical engineering problems. The content of the thesis covers the following aspects:1.The systematic rheological tests on the undisturbed soft clay samples taken from the construction sites in the Tianjin Binhai New Area are conducted. In order to carry out such kind of special test, an apparatus is made by refitting a conventional triaxial apparatus and adding a special loading facility. The strengths of the soil sample are tested with the laboratory vane apparatus made by the ELE company. Based on the test results, the rules governing the displacement developing with time for the soft clay are established, and the strength reduction factor is found.2. To understand the essence of the plastic rheological properties of soft clay, the comprehensive knowledge of several subjects should be involved. In the thesis, the concepts in solid mechanics, fluid mechanics and soil mechanics are introduced for analyzing the mechanism of plastic rheological phenomenon.3. The non-linear rheological constitutive model is established. Based on laboratory test data and the basic theory of continuum medium mechanics, modern fitting methods and numerical simulation technique are used to establish nonlinear rheological constitutive model for the recently reclaimed soft clay foundations in the Binhai New Area.4. The theoretical model is accomplished numerically making use of the existing commercial software. According to the established constitutive model, the ABAQUS user material subroutine UMAT is programmed. The powerful processing capabilities and non-linear solution techniques in ABAQUS are employed to solve practical engineering problems.5. Applications in case studies. The established calculation tool has been used to analyze the failure of some practical engineering cases in soft clay areas in Tianjin Binhai New Area, including bearing capacity failure and slope failure. The calculated results coincide well with those observed in the fields.