| Soft soil distributes widely in Shenzhen bay, with the rapid development of area economy more and more large scale projects, such as express highway, airdrome, loading field, dock, railway, hydraulic engineering and so on, are built on this kind of soft soil foundation in recent years, which leads to stricter and newer requirements on soft soil foundation engineering. The main engineering problem of soft soil foundation is deformation, namely, settlement. The water content of soft soil in Shenzhen bay is as high as 60%-125%, which is much higher than its liquid limit 39%-55%. Its void ratio is larger than 1.0, strength (Cu) is 10-30kPa, degree of sensitivity (St) is about 4-8, compression coefficient (av) is 1.5-2.0MPa-1, and its coefficient of consolidation (Cv) is 10-3-10-4cm2/s. So the soft soil has a large consolidation deformation, and sometimes the deformation may exceed 40% of the thickness of compression layer. Therefore it is important to assess the amount of consolidation deformation and the consolidation time of soft soil exactly in selecting foundation treating scheme reasonably and evaluating on treating effect of soft soil foundation. At present, preloading draining consolidation method is used to treat soft soil foundation in many projects in Shenzhen bay. The results show that there is a large settlement amount of soft soil after preloading draining through sand drain and plastic drain board, and it's a long time to a stable settlement, what's more, there is obvious difference between theoretical calculation value and observed settlement value both in the amount of consolidation settlement and the prediction of settlement progress, which prolongs the course of preloading and postpone the latter work. So the accuracy of consolidation settlement calculation must be enhanced. On the other hand, it is necessary to understand consolidation mechanism and to establish a reasonable model. The property of soft soil in Shenzhen bay is studied in this paper associated with the project of "Study on structural character and structural mechanical model of marine soft soil in China"supported by NNSF (40172092) and the project of "Foundation treating engineering of Shenzhen-Hongkong western corridor"in Guangdong province. Finally, on the basis of the combination of macromechanics behavior with quantitative analysis of microstructure, the consolidation deformation mechanism of structural soft soil is analyzed and the consolidation model of structural soft soil is established by means of settlement and pore pressure observation in situ. We have got some recognizes: Firstly, according to the study of the structure, the change of structure and the property of consolidation of soft soil in Shenzhen bay, several new understandings are achieved: ①the soft soil in Shenzhen bay is a kind of structural soil, and has natural structural strength. The original structure of soft soil is flocculated structure in which structure skeleton is formed depending on the electrostatic connection between edge and face or edge and edge of structure units. The strength of this kind of structural connection is stronger than bond water connection between face and face of structure units and weaker than cementation connection in typical structural soil (laterite, loess). Under the loading this sort of structure is easy to be destroyed, and the relationship between stress and strain of soft soil is different from that of typical structural soil which presents strain softening property after the peak value, while there is a stress platform or slight stress drop near the structure yield stress of structural soft soil and the curve of stress and strain is the type of stress hardening on whole.②on the basis of compression character of structural soft soil, the true compression curve should be divided into four parts. The inflection point of compression curve σk should be structure yield stress that is the sum of pre-consolidation pressure σc and structural strength. q. Structural strength has great influence on the consolidation property of soft soil.③final settlement calculation equation and evaluation method of remolding degree is given in this thesis taking the structural character and remolding influence into account. Example calculation proves that it is necessary to take structural character and remolding influence into account in consolidation settlement calculation of soft soil.④at the time when primary consolidation finishes tp which is determined by means of Casagrande method on ε-logt curve, the pore pressure doesn't dissipate completely. So tp is not the cut-off point of conventional primary and secondary consolidation, in fact, it illustrates the drainage of different kind of pore water in the progress of consolidation. On the other hand, the degree of consolidation calculated from actual measured pore pressure is lower than that calculated from settlement obviously, which shows that the alteration of coefficient of permeability k is larger than that of coefficient of compression av.⑤the curve of consolidation deformation and time presents typical "S"both in laboratory instant loading and in situ multi loading progress. With the damage of structure strength and the drainage of different pore water in the course of consolidation, according to the change features of water content, the progress of consolidation deformation along with time can be divided into three stages: σ'<σk, deformation mainly is primary consolidation, the curve is flatter, and the consolidation time mainly depends on the time of water drainage. Although structure units gather under the function of effective stress, obvious relative slip does not occur, and the structure is relative intact. When as σ'>σk, structure is destroyed, the amount of consolidation increases, void ratio decreases rapidly, free water in pore drains out, andpart of weak bond water drains out evening. The deviation between actual measured settlement and the calculation value according to conventional consolidation theory increases because the primary consolidation and secondary consolidation proceed at the same time; when there is a distinguished point on curve the consolidation time reaches tp, pores in soil are very small, the void ratio decreases slowly, and the pore water in soil is bond water. At this stage, structural units form aggregates in terms of the connection between face and face, drainage of bond water and sliding of structural units compose of the major deformation, which makes secondary consolidation become the main forms of consolidation. Secondly, the change of pore pressure during consolidation is studied. The results show that: ①under the condition of step loading, the features of pore pressure changing at different stage in the consolidation progress vary accordingly due to the influence of structural character, which is closely related to the deformation capacity of soft soil skeleton. With the loss of structural strength, soil volume is compressed gradually, and the curve of accumulated pore pressure and accumulated load, the increment of pore and load show phased change. Before the damage of structural strength, the coefficient of pore pressure A<1; at the stage of structure damage, A=1; and after the damage of structure, A<1. The feature of pore pressure changing in consolidation progress of soft soil also illustrates the difference in the relationship of stress and strain between structural soft soil and typical structural soil. ②the feature of water content changing in consolidation progress shows that the free water in pore is drained out at the early stage of primary consolidation, part of weak bond water may be drained out at the latter stage, and the grains of soft soil are mainly connected by bond water film. It also suggests that the effect of secondary consolidation is obvious at the latter stage of primary consolidation, and the deviation between actual measured settlement at the latter stage of primary consolidation and the value calculated from conventional consolidation theory increase. Thirdly, quantitative study of structure changing feature of soft soil in consolidation deformation progress is done by means of mercury intrusion method and SEM, the relationship between the parameters of microstructure and the amount of consolidation deformation is obtained. The results as follows: ①different types of pores behave differently to pressure. Pores of which diameters are larger than 1000? account more than 75% in original soft soil, and there is a large amount of free water in pores; when p is 100 kPa the content of pores larger than 1000? decreases to about 15%, pores larger than 1000? while smaller than1500? occupy 50%, and the content of pores smaller than 1000? is 35%, under this pressure the type of water in pores is bond-free water; when p is 200 kPa the content of pores larger than 1000? turns to be 5%, pores which size are between 1000?-625? occupy 30%, content of pores smaller than 625? is 65%, and most of pores are filled with bond water.②during consolidation progress, with the lossing ofstructural strength and changing type of water in pores, the microstructure of soft soil exhibits obvious phasic feature. At the early stage of consolidation, the effective stress is low; the microstructure is comparable stable due to the existence of structural strength, and the changing of directing property, size and shape of structural units and pores is slight; when effective stress increases to structure yield stress the structural strength lose gradually, so the microstructure is strongly adjusting, causing the increase of average diameter, the decrease of circularity and average shape factor, the obvious directing arrangement in the direction vertical to pressure of structural units, along with the decrease of average diameter, more complicated shape and less evident changing in directing property of pores. At this time, the original structure formed in natural sediment is destroyed, and new micro structure comes into being under the changing stress; with the continuous increase of effective stress, new micro structure adjust accordingly, and the parameters of structure units and pores change slightly. Finally, according to the actual condition in soft foundation treating projects, the structural viscoelastic consolidation model of vertical drains foundation of soft soil under time dependent loading is established, and corresponding finite element program is edited. The relationship between consolidation deformation and time is calculated by means of elastic model, viscoelastic model and structural viscoelastic model built in this thesis. The calculating results show that the difference between the settlement value calculated form elastic consolidation theory and the actual measured value increases gradually with the development of consolidation, that the value calculated from viscoelastic theory is close to the actual measured value in the latter stage of consolidation, while the former is lower than the latter in the early consolidation stage, and that the value calculated from structural viscoelastic consolidation theory is more accordant to the actual measured value. In elastic consolidation theory the pore pressure dissipates rapidly, which leads to a lower calculation value than actual measured value; in viscoelastic consolidation theory the calculation value of pore pressure is higher than actual measured one, especially after the damage of structure; although there is some difference between the value of pore pressure calculated from structural viscoelastic consolidation theory and actual measured value, the results are much closer to the true than the other two theories. The influence of structural character, parameters of viscoelastic property E1 ,η1 , coefficient of permeability k and the range interval D on consolidation character of vertical drains foundation is studied. A large number of calculation results illustrate that ①the structural character mainly play a role in the early stage of consolidation. The early consolidation stage of soft soil turns fast before the damage of structural strength. Primary and secondary consolidation will proceed at the same time after the damage of structural strength, and the consolidation features have connection to parameters of viscoelastic property E1,η1. In viscoelastic model E1 mainly have influence on the latter stage of...
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