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Involving several areas of geological engineering, geotechnical engineering and tunnel engineering, this book describes the soft soil deformation characteristics and dynamic responses induced by subway vibration load. Based on field monitoring and laboratory testing data, with both comprehensive micro-and macroanalysis, the authors present dynamic characteristics and deformation settlement of saturated soft clay surrounding subway tunnels using dynamic and static methodology. Mechanism of deformation, failure in microstructure of soft clay soil, dynamic response, macro deformation and settlement are all discussed and analyzed thoroughly and systematically. Some of the research findings in this book have been widely applied by large subway companies and will have broader application prospects in future. All the above make this book a valuable reference not only for technical engineers working in subway design or construction but also for advanced graduate students. Prof. Yiqun Tang works at the Department of Geotechnical Engineering, Tongji University, Shanghai, China.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Recently, subway has become the major part of urban transportation. Rapid development of economy in China has spurred and accelerated the urban traffic congestion problem. With lots of people migrating into large and medium cities, the previous existing urban transportation hardly meets the demand of the population expansion and urban sprawl. The subway system brings free-flow traffic at all times and realizes multidimensional space traffic to make faster modernization of urban development. At present, many large modern cities have interlaced the heartland by subways, such as Shanghai, Nanjing, and Shenzhen. A large number of medium cities are constructing subway systems. Even some small cities are applying for subway construction to alleviate the traffic jam. Hereby, the subway construction has a really great prospective.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 2. Field Tests

Subjected to the saturated soft clay surrounding tunnel areas of the Jing’an Temple Station-Jiangsu Road Station in Shanghai Line 2, continuous dynamic field monitoring was conducted by numbers of earth pressure and pore pressure transducers at different locations and depths. Some significant conclusions were obtained as follows:

1.

During the subway train passing over, two dynamic response frequencies were generated in surrounding saturated soft clay. According to the measured dynamic response frequencies in soil and corresponding natural soil frequency, some appropriate measurements should be considered to avoid similar or same frequencies between these two to induce resonance, which will result in great damage to the subway tunnel.

2.

In the direction perpendicular to the subway tunnel axis, the traffic load attenuated with increasing distance. The dynamic response attenuation with distance can be predicted in a formula by statistical analysis. The range of influence and the dynamic response amplitude were also obtained.

3.

The dynamic response amplitude varied linearly with the depth. And the values in different rush hour stages were various, largest in the morning rush hours, secondary in the evening, and least at noon. It indicated that the passage flow was largest in the morning. During this stage, the soil was in a worst situation and the dynamic response values in this period should be chosen as the reference in the subway tunnel design and construction.

4.

The pore pressure of saturated soft clay had very distinct response characteristics with the subway traffic load, and closer to the tunnel, the response is more sensitive.

5.

The dynamic response of pore pressure in saturated soft clay had hysteresis effect. Besides the distance between measuring point and subway track, it also has some relation to the loading direction.

6.

The pore pressure dissipation in saturated soft clay was much slower than the increasing time of pore pressure when subjected to the subway traffic load.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 3. Laboratory Tests

Through a series of laboratory tests under cyclic loads on saturated soft clay in Shanghai and Nanjing, the pore water pressure, soil dynamic strength, dynamic stress-strain relation, and dynamic elastic modulus were discussed deeply, taking vibration loading frequency, amplitude, and vibration loading cycles into account. The dynamic properties and microstructure deformation damage mechanism were analyzed. Some important conclusions were drawn as below:

1.

The development of pore water pressure can be divided into three stages: fast ramping stage, slowly growth stage, and stabilization stage. The logistic model was proposed to fit the pore water pressure growth curves and coefficients of correlation that were all above 0.99. At the fast ramping stage, the growth rate was not always a constant. The pore water pressure increased sharply, slowed down in a short time, and then got into stabilization. After regression analysis, the growth rate of pore water pressure obeyed in the ExpDecay2-type curve. In addition, the excess pore water pressure declined abruptly once the vibration loading ceased and finally recovered at a marginal higher value compared to static pore water (hydrostatic) pressure.

2.

The deformation characteristics of saturated soft clay under cyclic loads were mainly influenced by cyclic stress ratio, confining pressure, vibration loading frequency, and vibration cycle number. There existed a critical cyclic stress ratio, relevant to the soil properties, applied load form and consolidation pressure, etc. When the applied dynamic load was smaller than the critical cyclic stress, the dynamic stress increased with vibration loading time. While the growth rate and amplitude gradually fell into a stable constant, to the contrary, the deformation augmented fast and finally got into failure. Under low loading frequency, the influence of frequency was not very conspicuous and can be neglected. While in the high-frequency circumstance, the soil behaved more elastic properties. The main reason may be that when high-frequency vibration load is applied, the pore water pressure could not dissipate in time, and with longer loading cycle duration, the pore water pressure dissipated fully and the creep effects on soil particles were more obvious. The soil characteristics are more plasticity and viscosity.

3.

As for the subway tunnel, the deformation in the soil deep around the interior wall of the subway tunnel was relatively small, while the axial strain curve of soils just beneath the subway tunnel was in a very short resilience stage and got into the plastic deformation stage very quickly, resulting in large lateral deformation. Hence, even if there may be no large deformation occurring around the subway tunnel during a long operation time, significant differential settlement is probably to arise as time lasts.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 4. Research of Microstructure

As a matter of fact, the macroscopic deformation of saturated soft clay under subway loading is a reconstruction process of microstructure. All the structure elements including particles and pores change under subway loading. According to the above analysis, the rearrangement state of microstructure under subway loading is related to the vibration duration and stress level. Based on SEM and MIP tests, fractal theory analysis, and correlation analysis of pore microstructure parameters and macroscopic characteristics, some conclusions can be drawn:

1.

The saturated mucky clay of the fourth layer in Shanghai mainly has flocculated structure and honeycomb-flocculation structure. Clay mineral particles are major illite, less chlorite, and tiny montmorillonite. The shape of structure elements are mainly laminar (see Fig. 4.11), while the aggregates are flocculated in flower or feather shape. The connections of structure elements are mainly edge to face and edge to edge which forms an open pore structure with high void ratio.

2.

The saturated mucky clay of Shanghai has primary pore structure of large inter-aggregate pores (radiuses are over 50 nm); it has pore throat effect in the process of mercury intrusion.

3.

The variation features of pore structure parameters with different depth can be indicated by the cyclic stress ratio (CSR). With the increase of depth, the CSR grows while the specific surface area decreases at first and slightly rebounds later. The mean distribution radius, retention factor, and porosity have the opposite trend.

4.

The microscopic test results indicate that packing model is not suitable for explaining the mechanism of deformation in saturated soft clay under low stress conditions.

5.

Based on the fractal theory, the study on clay pore distribution shows that Menger sponge random model is not appropriate to describe the fractal characteristic of mucky clay in the whole size range. But the model based on thermodynamic relations is suitable. The relationship between fractal dimension calculated by thermodynamic relations model and CSR was demonstrated as well as explanations of the variation features.

6.

The correlation analysis of pore microstructure parameters and macroscopic deformation (macroscopic force) shows that a critical value of subway loading plays an important role in the microstructure variations of mucky clay near subway tunnel.

7.

During a long time since the first operation of subway, soil structure elements of soil layers near subway tunnel and at the bottom of track tend to be compacted. However, the pressure applied on soil at the bottom of subway tunnel is quite large as the trend is alleviated. The rearrangement of soil elements is the main reason of deformation. Therefore, the accumulation of deformation will continue for a long time.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 5. Finite Element Modeling

This chapter primarily conducted the finite element analysis of the dynamic response of soil around the tunnel under dynamic load induced by the subway train; the main work and conclusions are summarized as below:

1.

This chapter analyzed the mechanism of the generation of dynamic load induced by subway train and simulated it by programming in ANSYS.

2.

According to the field investigation and data from the boreholes, we developed a 3D finite element numerical model by inputting the relevant data.

3.

For obtaining accurate results without too large computational domain, artificial boundary was applied to simulate the soil body in the discretization of this model.

4.

The dynamic response of the soil in the model was consistent to the data obtained from the field test. And the results were proved effective and reliable.

5.

According to the calculation results,

σ

z

propagates further in vertical direction, while

σ

x

propagates further in the horizontal direction. Furthermore, the influence range of the dynamic load was determined by this model.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 6. Settlement Prediction of Soils Surrounding Subway Tunnel

This chapter made a comprehensive analysis of the influence factors on subway settlement and proposed a prediction model on the basis of gray system theory:

1.

In soft soil area, ground settlement caused by tunnel excavation is controlled by many factors such as grouting, volume of excavated earth, and the soil pressure and advancing speed. For controlling the influence of tunnel excavation on the surrounding environment, it is necessary to take all the key factors into consideration to make appropriate construction projects.

2.

Grouting volume and slurry consistency are crucial factors to control ground settlement. Considering the heave effect on the tunnel axis, the volume of grouting should be between 2.5 and 2.8 m

3

when the slurry consistency is between 9 and 11.

3.

The non-isochronous nonhomogeneous exponential gray prediction model NNGM (1, 1) was improved based on a traditional model GM (1, 1). We reconstructed the whitenization differential equation of GM (1, 1) to expand its application and predict nonhomogeneous exponential data series. To meet the requirements of gray model of the raw data, non-isochronous data sequence were transformed into the isochronous by use of cubic spline interpolation and then transformed back by the same method. In addition, we analyzed the long-term settlement of the subway tunnel in the aspect of effective stress in the last part of this chapter.

4.

Gray prediction model is a reasonable and potential approach to simulate and predict cumulative plastic deformation of soils under cyclic loads and long-term settlement of subway tunnel.

5.

Compared with gray prediction theory, the effective stress method is more like an empirical predicted model for predicting the settlement of subway. It is more suitable to make a simple qualitative analysis and evaluation for deformation and settlement. The gray model could be used to predict the long-term settlement.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

Chapter 7. Conclusions and Prospects

In this monograph, we took Shanghai muddy clay as the study object and analyzed the characteristics of soil dynamic response under subway loadings based on field monitoring of earth pressure and pore water pressure within soils. Benefiting from the field test, some basic parameters were obtained for the laboratory tests. A series of laboratory tests were conducted with a dynamic triaxial apparatus with unidirectional excitation combined with a multifunctional dynamic triaxial test system. The research focused on four main aspects: (1) the increasing law of pore water pressure of soft clay under subway load, (2) the relationship between dynamic stress and strain, (3) the law of strength degradation under dynamic loading, and (4) creep behavior under dynamic loading. In addition to the experimental study above, 3D finite element model was established to simulate dynamic response of soil surrounding the subway tunnel when the subway trains goes through. Besides the macroscopical study of the research object, the microstructure of soil was studied as well based on mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The former one provides access to quantitatively analyze the microstructure of soft clay before and after vibration (such as variations of pore size distribution, pore quantity, and other microcosmic parameters), while the images of SEM provided visible and qualitative analysis of soil structure.

Yiqun Tang, Jie Zhou, Xingwei Ren, Qi Yang

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