Jianxiu Wang

Controlling subsidence caused by de-watering in a deep foundation pit

In China, more and more underground structures are being built close to buildings of architectural merit. When installing deep foundations, three ways of controlling seepage are suggested. In Mode III, an installed curtain extends partly into the confined aquifer so that the hydraulic connection between the excavated section and the water in the host material is partly isolated. The combined effects of pumping, curtain efficiency and recharge are discussed. Based on the engineering geological conditions of the deep foundation pit of the Yishan Road Station of Metro Line 9, Shanghai, China, in situ pumping and recharge tests were carried out. The combined effects are similar to the results obtained using a three-dimensional finite difference method (FDM) numerical simulation.RésuméEn Chine, des structures souterraines de plus en plus nombreuses sont construites à proximité de bâtiments présentant un intérêt architectural. Trois moyens de contrôler les écoulements lors de la réalisation des fondations profondes sont suggérés. Dans le Mode III, un rideau d’injection est réalisé, pénétrant partiellement dans l’aquifère captif de sorte que la connexion hydraulique entre le volume excavé et l’eau de la formation hôte est partiellement coupée. Les effets combinés du pompage, du rideau d’injection et d’une recharge hydraulique sont discutés. Considérant les conditions géologiques et géotechniques de l’excavation profonde de la station Yishan Road de la ligne 9 à Shanghai (Chine), des essais de pompages in situ et des essais de recharge hydraulique ont été réalisés. Les effets obtenus sont semblables aux résultats issus d’une simulation numérique basée sur une méthode tridimensionnelle aux différences finies.

Using Bayesian networks in analyzing powerful earthquake disaster chains

Substantial economic losses, building damage, and loss of life have been caused by secondary disasters that result from strong earthquakes. Earthquake disaster chains occur when secondary disasters take place in sequence. In this paper, we summarize 23 common earthquake disaster chains, whose structures include the serial, parallel, and parallel–serial (dendroid disaster chain) types. Evaluating the probability of powerful earthquake disaster chains is urgently needed for effective disaster prediction and emergency management. To this end, we introduce Bayesian networks (BNs) to assess powerful earthquake disaster chains. The structural graph of a powerful earthquake disaster chain is presented, and the proposed BN modeling method is provided and discussed. BN model of the earthquake–landslides–barrier lakes–floods disaster chain is established. The use of BN shows that such a model enables the effective analysis of earthquake disaster chains. Probability inference reveals that population density, loose debris volume, flooded areas, and landslide dam stability are the most critical links that lead to loss of life in earthquake disaster chains.

A Thermodynamics-Based Model on the Internal Erosion of Earth Structures

The present paper describes a model of internal erosion of earth structures, based on rigorous thermodynamic principles and the theory of porous media. A particular focus of this paper is concerned with the initial stage of internal erosion, when the pore volume forms a continuous network, without the formation of macroscopic cavities or channels. The continuum approach is applicable in this case. The soil skeleton saturated by a pore fluid is treated as the superposition of three continua in interaction, with independent velocity fields. The pore fluid itself consists of a mixture of water and eroded particles. The erosion kinetics is based on the shear stress developed at the solid–fluid interface. The applicability of the model is illustrated by numerical simulations based on the finite element method. These simulations show how the phenomenon of piping can progressively arise, and preferentially in regions where hydraulic gradients are critical. Effects of mechanical degradations due to internal erosion are at the same time demonstrated.

Inference of creep mechanism in underground soil loss of karst conduits I. Conceptual model

Karst is widely distributed in the southwest of China, especially in Guizhou Province. The phenomenon of desertification in these areas is very serious. And soil erosion is the key link in the process of desertification. Through field monitoring, underground soil leakage is derived to the main mode of soil loss in this area. Shear strength tests and creep experiments were carried out with the aim of analyzing the creep mechanism in underground soil loss. It is shown that the water content can lead to the great influence on the shear strength of the brown clay. This variation has been combined with creep characteristics besides the structural geology, hydrology condition and brown clay distribution circumstance (field observation). A conceptual creep model of the brown clay sliding along the karst conduits has been unveiled to show the detailed inference of the creep mechanism in the underground soil loss: geology and hydrology control the development of the karst conduit system; and penetration of water induces the weakening of the shear strength of the surface soil and accelerates the creeping and sliding of the brown clay along the karst conduit system. This understanding of the creep mechanism has significant implications for the future management of the soil erosion in the karst area.

Land subsidence caused by internal soil erosion owing to pumping confined aquifer groundwater during the deep foundation construction in Shanghai

Land subsidence is presented in many factors in different areas with urbanization. Internal soil erosion, owing to pumping confined groundwater during the deep foundation pit construction, has contributed to land subsidence. Four governing equations are presented to describe the process of internal soil erosion based on the mathematical–physical model. The finite element computation results, based on practical deep foundation pit engineering consisted of 8 layers of soil of Shanghai area, demonstrate that internal soil erosion will cause the increment of land subsidence and deformation and is related to the hydraulic gradient and the characters of the soils.

Quantitative analysis of the microstructure of Shanghai muddy clay before and after freezing

Artificial freezing is becoming a common method of enhancing soil quality when undertaking large scale underground structures in Shanghai. This paper presents the variation in the frozen strength of Shanghai muddy clay with temperature. SEM studies were undertaken to analyse the shape and size of the pores in the frozen material compared with the undisturbed clay. The results show that compared with undisturbed soil, the shape and the orientation of the pores in samples frozen at −10, −15 and −20°C did not significantly change after the compression testing, although the pore volume increased with decreasing temperature. The relationship between microstructure and moisture migration is discussed.RésuméLa congélation des sols devient une technique usuelle pour améliorer la résistance des sols pour des travaux souterrains importants réalisés à Shanghai. L’article présente l’évolution, en fonction de la température, de la résistance de sols argileux gelés. Des études au microscope électronique à balayage ont été réalisées afin d’étudier la forme et la taille des pores d’échantillons congelés, par comparaison avec des échantillons intacts. Il apparaît que la forme et l’orientation des pores d’échantillons congelés à des températures de −10, −15 et −20°C ne changent pas de façon significative après les essais de compression, bien que le volume des pores augmente quand la température décroît. Les relations entre la microstructure et les phénomènes de migration des molécules d’eau sont discutées.

Field experiments and numerical simulations of whirlpool foundation pit dewatering

A whirlpool foundation pit is a small-diameter, deep circular pit. Because of its depth and small diameter, a large drawdown is required, and a limited number of wells can be installed inside the pit. During excavation, partially penetrating wells inside and outside the foundation pit have to be installed to lower the water level when the aquifer is too thick. However, partially penetrating wells near partially penetrating curtains cannot be treated by analytical methods. Therefore, it is necessary to use numerical methods to predict dewatering during excavation. Field experiments were performed on whirlpool foundation pit 1880 of Baosteel Group, Shanghai, China, to obtain pumping rates and drawdown, pumping with a single well and two wells in the confined aquifer. The results indicate that the drawdown inside the pit induced by pumping wells outside the foundation pit was small, whereas it was large for pumping wells inside the pit. The pumping wells inside and outside the pit had to be combined to lower the water level. A three-dimensional numerical model was developed to simulate the dewatering process. The hydraulic conductivities of the confined aquifers were inversed by using the pumping tests. Operation schedules were simulated with the corrected model for different combinations of wells inside and outside the pit. The results suggest that different schedules and operation conditions affect drawdown. The monitored results during dewatering indicate that the simulation and field measurements were in agreement. The results can be applied to similar situations.

Laboratory model tests on water inrush in foundation pit bottom

Numerous deep foundation pits were constructed in China in the course of urbanization. Water inrush is one of the most important causes of foundation pit accidents. At present, few images and detail courses of the water inrush have been recorded and reported. The evidences for the hypothesis of water inrush calculation are not enough. In this study, model tests were performed to verify the water inrush course in foundation pit bottom. The water inrush modes of foundation pits were analyzed for the aquitards that included clay and silty clay in Shanghai, China. The deformation and failure characteristics of the layers under different water pressures were obtained. The deformation course without water inrush was divided into three stages: continuous deformation, progressive deformation, and equilibrium stages. The deformation course with water inrush was divided into four stages: continuous deformation, progressive deformation, shear failure, and water inrush and sinking stages. The course included creep deformation and micro-cracks development before failure. It was not an instantaneous phenomenon but a time-depending one. The pore water pressure was the response of aquitard to boundary water pressure, which indicated the seepage in low-permeable aquitard and the development of micro-cracks. The water inrush formula was verified and tested using the model test results. The limit equilibrium method had larger safe reserves, and the prestressed homogeneous continuous-beam method was relatively accurately.

Study of soil structures strength and stiffness loss based on thermodynamics and continuum mechanics

Based on the theory of damage mechanics and porous medium continuum mechanics, a solid skeleton strength degradation model for internal erosion of earth structure is presented in this paper. Saturated soil is considered as a mixture composed of matrix skeleton and porous space, the latter being filled by fluid which consists of a mixture of water and eroded particles. With these eroded particles being carried out with fluid, porous space in earth enlarges, and the effective stress area of solid skeleton decreases, which leads the effective stress acting on the solid skeleton to increase. This process also involves earth structure stiffness loss and strength degradation. This paper presents a constitutive model and sets up the relations between the strain and the internal erosion damage variable using thermodynamics and damage mechanics.

The influence of water content on soil erosion in the desertification area of Guizhou, China

Rocky desertification of karst in Southwest China has seriously influenced the normal life of local people. Soil erosion is the key process for inducing rocky desertification. This paper analyzes the stability and variation of shear strength under different of water contents of the brownish clay soil in carbonate rock area Chenqi of Puding in Guizhou province. Results show that the cracking of brownish clay soil aggregates occurs in the early soaked stage. The lower the initial water content of soil aggregates, the faster and more completely large particles disintegrate. Stability of soil aggregates is stronger with higher water content, but the entire shear strength decreases significantly; it will collapse entirely in the rainy season. Therefore, for desertification prevention and control in carbonate rock areas, greater attention should be given to effective protection measures during the rainy periods after long-term dry weather. The control during this period is essential in the whole process of desertification mitigation.