MICROSCOPIC STUDY ON SHANGHAI SATURATED SOFT CLAY’S DYNAMIC BEHAVIOR By Tang Yi-qun, Zhang Xi, Zhao Shu-kai Tongji University, China

INTRODUCTION Constructing three-dimensional traffic network primary portion subway multitudinous population dense buildings narrow streets crowded vehicles SHANGHAI CITYSHANGHAI CITY

INTRODUCTION According to the No.1 and No.2 subway lines, the depth of the subway is almost located in or across the stratum of saturated soft clay the axes settlement is greater in some areas of Shanghai city No.1 subway line it is concerned with the deformation, damage of soil microstructure and the residual deformation of saturated soft clay under subway-included vibration loading

INTRODUCTION The clay particle array is very complex and random The inner structure results in primary deformation and displacement of the soil under the vibrational loading The structure elements are in dynamic equilibrium within the process of the soil deformation and the soil microstructure has particular change characteristics in various space and time

INTRODUCTION Base: the studies on physics mechanics characteristic of the saturated soft clay under vibrational loading Goal: the study on the microstructure deformation and damage mechanism of saturated soft clay the cyclic triaxial test the scanning electronic microscope Using

PREPARATION OF SOIL SAMPLES There are two types of soil samples: the undisturbed sample the failure sample by cyclic triaxial test

PREPARATION OF SOIL SAMPLES  The preparation process of soil samples for microstructure study is as follows: 1.The soil sample is borrowed according to the requirements. When the sample contains water, it is broken off. The selected portion represents the characteristics of soil microstructures.

PREPARATION OF SOIL SAMPLES 2.According to direction of observation, the soil sample is broken into reasonable size for microscope with a flat section. 3.The previous prepared sample is dried with drying oven. 4.The dried sample is put in depositing equipment and deposited a layer of electric material.

OBSERVED RESULTS Contact state of the basic elements Figure1. Contact Status of Vertical Surface (5000 Times) There are many flaky or platy polymers in the vertical surface. In most cases their contact status is surface-surface, edge-surface, and so on (Fig.1).

OBSERVED RESULTS Contact state of the basic elements Figure2. Contact Status of Horizontal Surface (5000 Times) Because there are many granular polymers in the horizontal surface, it is difficult to distinguish the surface and edge of the granular polymers. Their contact is direct and inlaid. And some basic elements contact with each other by the binding material. (Fig.2)

OBSERVED RESULTS  It is defined as the basic microstructure elements, which have obvious physical boundary under a certain times. The elements are divided into grade Ⅰ and grade Ⅱ :  Grade Ⅰ element has greater origin cohesion. And basically it is microscopic polymer that is difficult to be divided.  Grade Ⅱ element is always composed of grade Ⅰ microscopic polymers, flaky or platy polymers, or granular polymers. Shape and size of the basic microstructure elements

OBSERVED RESULTS Shape and size of the basic microstructure elements Figure3(a). Horizontal Surface of Undisturbed Soil(1000 Times) Figure4(a). Horizontal Surface of Disturbed Soil (1000 Times) From figures we can obviously observe the shape and size of the basic elements. They are undisturbed and disturbed soil elements after cyclic triaxial test in the horizontal and vertical surface.

OBSERVED RESULTS Shape and size of the basic microstructure elements Figure3(b). Vertical Surface of Undisturbed Soil(1000 Times) Figure4(b). Vertical Surface of Disturbed Soil (1000 Times) After comparison between the two figures, the soil particles after vibration are more broken and definite directional. Because the soil particles is rearranged after vibration.

OBSERVED RESULTS  From above pictures we find out that some of elements directly contact with each other. It proves that there is a material that directly contacts elements, which is likely electric charge, water film, colloidal film and so on around the elements.  Some of elements contact with each other by the binding material. And the connected material and connected force must be further investigated with chemical analysis or else. However it is beyond the study contents of this paper. Connection pattern of the basic elements

OBSERVED RESULTS Shape and size of pore Figure3(a). Horizontal Surface of Undisturbed Soil(1000 Times) Figure4(a). Horizontal Surface of Disturbed Soil (1000 Times) Under the scanning electronic microscope (the SEM) we can observe the pore between two basic elements and the big pore between several elements. The shape and size of pore depend on the boundary condition and the arrangement condition of the elements. After contrasting figures, we can observe that the pore space obviously diminish after cyclic loading vibration, i.e. the soil particle has a compacted tendency.

ANALYZING MECHANISM OF DEFORMATION inertia force (shearing force) saturated soft clay multiporous loosened honeycombing structure high void ratio high moisture content under vibrational loading

ANALYZING MECHANISM OF DEFORMATION Ⅰ Figure 5. Dynamic Shear Stress-strain curve Here comes the shearing stress-strain curve under cyclic load by using the cyclic triaxial test, which adopts sine wave load at a frequency of 2.0 Hz(Fig.5). The process of soil deformation is divided into following three stages (Fig.5):

ANALYZING MECHANISM OF DEFORMATION  The first stage: bearing less shearing stress, the deformation could generally restitute after unloading. The soil microstructure is undamaged, and the state of soil particle is similar to Fig.3. The structure of soil particles has no obvious change. The microstructure of the soil particle will be brought back to a former condition, so the residual deformation is not engendered. The state of the soil is elastic. and this is the quasi-elastic stage.

ANALYZING MECHANISM OF DEFORMATION Ⅱ Figure 5. Dynamic Shear Stress-strain curve

ANALYZING MECHANISM OF DEFORMATION  The second stage: As the shearing stress over a certain value, the soil particle starts to slip and the soil microstructure starts to deform. It is the process of energy accumulation. When the energy of the shearing stress is accumulated to a certain value, which is greater than the binding energy between portions of the soil particles, the soil particle slips and the soil microstructure are partly damaged. So the soil residual deformation is engendered. The stage reflects the soil elastic- plasticity. This is the elastic-plastic stage.

ANALYZING MECHANISM OF DEFORMATION Ⅲ Figure 5. Dynamic Shear Stress-strain curve

ANALYZING MECHANISM OF DEFORMATION  The third stage: As the shearing stress continuously increases (the stress ratio > the critical stress ratio), the soil microstructure is completely destroyed. It results in the decrease of the soil strength, and the strain tends to infinity, and the soil is softened. This is defined the softening stage.  According to the above results it could be concluded that the deformation mechanisms of soil microstructure vary in different stress stages. This result can prove the results of the cyclic triaxial test.

ANALYZING MECHANISM OF DEFORMATION  It is obvious that the soil around tunnel should be in the first stage (the quasi- elastic stage). The soil particle is slipped, and the soil microstructure is damaged beyond this stage. It will lead to the soil residual deformation and result in excessive settlement along tunnel axes, which probably influence the normal running of subway.

Conclusion  After the operation of subway, the axes settlement is greater. This problem is caused by quite a few factors. However, to a great extent, it is concerned with the residual deformation of saturated soft clay under subway-included vibration loading. And the residual deformation is engendered as a result of the variation of pore water pressure, deformation and damage of soil microstructure. Its generating and development are represented as a process from quantitative change to qualitative change.  The soil microstructure is investigated using the SEM. There are many flaky or platy polymers, whose contact status is surface-surface, edge- surface, edge- edge, and edge-corner, in the vertical surface; there are many granular polymers, whose surface and edge are difficultly distinguished, in the horizontal surface. Their contact is direct, and inlaid. Some of them contact with each other by binding material. Furthermore, observe that the shape, size, connected pattern of the soil particle, and the shape, size of the pore.  The process of soil deformation under cyclic loading is divided into three stages: the quasi-elastic stage, the elastic-plastic stage and the softening stage. The three stages are explained from the point of view of form characteristic. After the operation of subway tunnel, the soil deformation is controlled in the first stage by monitoring and controlling the shearing strain in order that the subway train can normally run.

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