Depositional History,Mechanical Properties And Constitutive Modeling Of Ningbo Soft Clay

There is a very close relationship between the mechanical properties of soft clay and its depositional history and environment.A series of physical and chemical tests are done on shallow soft clay of Ningbo plain.Combining with previous research results,the sea level changes in Ningbo plain and depositional history of Ningbo soft clay are made clear and verified.Unconfined compression test,routine consolidation test and the triaxial undrain shear test are done on undisturbed and remoulded clay of each layer of Upper Ningbo clay.UNIFIED model is modified according to problem happening when using it to modeling the triaxial undrain test of Ningbo soft clay.Then modified model is used to modeling the result of routine consolidation test and triaxial undrain shear test of Ningbo soft clay.The main conclusions obtained are as follows:1)The Holocene sea-level changes in Ningbo plain can be divided into four stages,corresponding to the deposition of 4^th clay?including the 4-1and the 4-2 clay?,sand layer,the 3^rd clay and the 2^nd clay?including the 2-2a and 2-2b clay?.4^th clay was formed during the first Transgression as coastal neritic facies.Sand layer was formed during the first sea regression as estuary sedimentary facies.3^rd clay was formed during the second Transgression as estuary littoral facies.The 2^nd clay was formed during the second regression period as coastal lagoon facies.2)Physical and chemical tests are done on Ningbo soft clays,it is pointed out that Upper Ningbo soft clays are in soft plastic or plastic flow state.Upper Ningbo soft clays belong to low liquid limit clay except some samples from the 2^nd layer,which belong to high liquid limit clay.The grading curves of Upper Ningbo clays are more placid than Shanghai 4^thh clay except the 3^rd clay.In addition,clay content of Ningbo clays are higher then Shanghai clay.pH value and ORP value of Upper Ningbo clays'pore solution were negatively correlated with the overall.Excluding the sand layer whose depth is 20m,salt content in pore solution shows regular change that increased first and then decreased with the the increase of the depth.The amount of salt content in sand layer is far lower than the neighbor layers.pH value,ORP value and salt content of pore solution have greatly proved the sea level changes and depositional history of Ningbo plain.3)Routine consolidation test and triaxial undrain test are done on Ningbo undisturbed and remolded clays.Routine consolidation test results show that consolidation curve inflection points of the 2-2a layer and the 4-1layer are not very obvious,and consolidation curve of the 2-2b layer and the3-2b layer have obvious inflection point.Comparing the q/p'in traxial undrain test of undisturbed and remolded clay,it is found that the strength of 2-2a,2-2b and 4-1 undisturbed clays are all higher than remoulded ones.It shows that 2-2a,2-2b and 4-1 clays have a certain structure,and the structure of 2-2b is strongest,and the strength of 2-2a,2-2b and 4-1 clays were negatively correlated with plastic index.4)UNIFIED constitutive is modified in this paper.The reason why UNIFIED constitutive can not fit 2-2b clay's triaxial undrain shear test well is that dissipation of clay's structure in consolidation test and triaxial undrain test is different,while UNIFIED constitutive cannot reflect the different effect on structure by plastic shear strain and the plastic volumetric strain.The constitutive is modified Aiming at the defects,parameter A which means the ratio of plastic shear strain and plastic volumetric strain is added into the expression of structure develop function.Results of modeling the routine consolidation test and triaxial undrain shear test of Ningbo 2-2a,2-2b and 4-1 clays show that modified constitutive can reflect the mechanical properties of Ningbo clays.The ratios of plastic shear strain and plastic volumetric strain——A of Ningbo 2-2a,2-2b and 4-1 clays are small,which are between 0.03⁰.05.It shows that plastic volumetric strain has greater influence on structure of Ningbo clay than plastic shear strain.