This study investigates the long-term settlement of Nanjing Metro Line 10 in China. A maximum settlement of 240mm was measured within 5.75years after the track system was constructed. Four settlement ...troughs were observed along the longitudinal direction of the railway line. The settlement of the tunnel developed rapidly in the first two years and then the rate of settlement gradually decreased. The excessive settlement and large differential settlement of the tunnel were primarily caused by the deep and non-uniform distribution of the underlying soft soil. The long-term tunnel settlement mainly consists of two components, including recompression settlement following construction and secondary consolidation settlement of the foundation. Each component of settlement was analyzed and estimated in this study. The computed results correspond well with the measured data, and the recompression deformation and the secondary consolidation deformation represents approximately 78.0–100% and 0–22.0% of the total tunnel settlement, respectively. The measured data and calculated results revealed that the lengths of the cement mixing piles used to improve the tunnel foundation are insufficient to reduce the long-term settlement. Piles entirely penetrating the soft silty clay layer are recommended.
•The long-term settlement of Nanjing Metro Line 10 in China is documented.•The tunnel settlement is mainly induced by the soft silty clay underneath.•The components of the long-term tunnel settlement are analyzed.
Arching effect, which is a common phenomenon in any system involving soil–structure interaction, has been found to be inevitably affected by various factors, including loading conditions. This study ...investigated the evolution of arching effect induced by cyclic loading by conducting a series of tests using a trapdoor apparatus. The test box was instrumented to control the displacement of the moving gate and to record the variation of vertical stress distribution by using a set of dynamic load cells. Digital images were captured during tests and processed using particle image velocimetry (PIV) to determine the displacement field and hence to examine the variation of geometric features of arch and particle movements. The evolution process of arching effect, from the initial formation to the finial collapse, was identified. Depending on the analysis for the geometry appearance, displacement region, and variation of cyclic stresses, both stable and collapsed arches were observed. By increasing the amplitude of cyclic loading step by step, critical loading amplitude corresponding to the threshold of collapse of the arching effect was determined. Based on the results, the effects of trapdoor displacement, cyclic loading frequency, and filling height on arching effect are discussed.
Abstract
Energy piles have been popular globally with functions of both pile foundation and ground source heat pumps. Although several researches have been devoted to the probabilistic design and ...assessment of energy piles, the corresponding procedures are too complicated for engineers. As a simple variant of the reliability-based design method, the load and resistance factor design (LRFD) approach for the geotechnical design of energy piles is presented in this study. Firstly, the load-transfer model for energy piles is developed to investigate the effect of cyclic thermal loading on the pile settlement. Then, the LRFD procedures based on first-order reliability method and target reliability method are implemented into two different constrained nonlinear optimization problems, respectively. The proposed LRFD model for energy piles is demonstrated through an example pile and a series of parametric analyses.
•A theoretical TBM torque model was developed in rock–soil interface mixed ground.•A new prediction index, MTPI, was proposed to evaluate the cutability of mixed face.•TBM performance prediction ...method based on MTPI and torque capacity was proposed.•The predictive capability of the proposed method was validated by field data.
Tunnelling in rock–soil interface (RSI) mixed ground often results in high cutterhead torque and torque fluctuation. Therefore, choosing a reasonable penetration rate to reduce the torque and improve tunnelling efficiency is paramount. In this study, considering the interaction between the cutterhead and RSI, an excavation torque model was first established based on the CSM model for a disc cutter and a new three-dimensional limit analysis model for a scraper. Further, considering the friction at the cutterhead in RSI, a total torque model was developed. Based on the study of excavation torque change characteristics when the cutterhead located in RSI, a simplified relationship among torque, face composition, penetration and rock strength was established. Subsequently, a new prediction index, Mixed-face Torque Penetration Index (MTPI), which includes the torque exerted by the TBM in the calculated penetration rate, was proposed to evaluate rock-soil mixed face cutability. This index allows for the inclusion of a TBM–rock mass and TBM–soil interaction factor. The methodology based on MTPI and TBM torque capacity to predict the penetration rate of the machine in different geological conditions and TBM specifications was proposed. Finally, the predictive capability of the proposed torque model and penetration prediction model were validated by the good agreement between the predictions and field measurement data from the Guangzhou Metro Line 21 tunnel in China. This study can match the tunneling parameters with the geological conditions within the limitation of TBM and provide guidance for TBM operation or evaluate whether the TBM satisfies the project schedule.
It is known that a lot of uncertainties are involved in geotechnical design of energy piles. In this paper, a Bayesian updating framework is presented to characterize those uncertainties. The ...load-transfer model is developed to predict the thermomechanical response of energy piles. Considering the cross-case variability of the uncertainty in the axial strains of pile, the global model bias is firstly calibrated by establishing a comprehensive database consisting of 12 energy pile cases. Furthermore, the uncertainty in input parameters is considered in the Bayesian updating of model bias in a specific case. The variability of the uncertain parameters is effectively reduced after updating. The coefficient of variation of prediction is decreased from 0.34 to 0.13. The present framework can well quantify uncertain factors and improve the accuracy and reliability of the prediction model.
This paper investigates the seismic properties of a cross transfer subway station under a horizontal earthquake wave in soft soil stratum. A 3-D simulation analysis model is established considering ...the dimensions and soil layers of a real structure. The HS-small model is employed to simulate the non-linear stiffness of soft soil. The dynamic responses of a single station case and a cross transfer station case are explored, followed by the direction of the input earthquake wave, the type of soil reinforcement in the overlapping area, had on the dynamic response were also analyzed. The simulation results show that the subway station has better seismic response than the single station, and soil reinforcement can increase the dynamic response of the different components in the overlapping area. As soil reinforcement stiffness increases, the dynamic response increases in both the upper and bottom stations at varying degrees. The findings from this paper provide a reference to the seismic design of cross transfer stations in similar ground.
•Propose a method to identify the settlement risk section along metro lines.•Extend Artificial Neural Network–Cellular Automata on uncertainty analysis.•Present a detailed case study for ...verification.
“Metro economy” has led to intensive land development along the metro lines. However, engineering activities associated with land development inevitably disturb the service environment, causing severe settlement of adjacent metro tunnel structures in soft soil areas. As a reference point for pre-treating this type of tunnel settlement, a method is proposed to classify settlement risk sections along metro lines based on a land-use development simulation and corresponding uncertainty analysis. First, the land-use development along the metro line was simulated by Artificial Neural Network–Cellular Automata (ANN-CA). Second, the land-use development process was considered a random event rather than a deterministic prediction as in a typical ANN-CA, with its probability quantified based on the cells’ conversion probability. The classification of the surrounding land-use development probability was used to allocate the settlement risk sections of metro lines. This method was applied to the Han–You section of the Nanjing Metro Line 2. The predicted settlement risk sections corresponded suitably with the actual settlement troughs, demonstrating the effectiveness of this method. Thus, this method provides a novel consideration for the pre-treatment of metro tunnel settlement from the perspective of interactions between the metro line and surrounding land development.
In the classical conception of the arching mechanism, the well-known trapdoor problem assumes distinctive modes (e.g., either active or passive mode) in the original geostatic overburden pressure. ...However, in various geotechnical applications, underground inclusions or structures may experience alternant upward and downward movements induced by construction activities or environmental changes, leading the ground to be initial loss or compaction state. If the initial disturbed state occurs, the estimation of vertical load on structures is improper based on classical trapdoor mechanism. In this study, a new conception, alternant active and passive trapdoor problem was introduced. Using a comprehensive measured system, such as particle image velocity technique and various sensors, alternant active and passive trapdoor tests were conducted to explore the mechanical behavior and kinematic mechanism of the backfill subjected to continuous loading and unloading. The development of arching with trapdoor displacement was visualized and quantified in different modes and states. Then, this problem was treated in a statical analysis based upon the visualized kinematic mechanisms. Solutions for the trapdoor load in the typical development states of arching were calculated by limit equilibrium method. Finally, two simplified ground reaction curves representing the progressive development of arching were proposed to facilitate in engineering applications. The simplified method was verified by the trapdoor test results.
Tunneling in mixed ground often results in severe torque fluctuations and a low advance rate. Therefore, choosing a reasonable set of parameters for accurate advance rate prediction is paramount to ...reduce cutter wear and improve tunneling efficiency. However, since the geological parameters in mixed ground conditions are diverse and uncertain, the prediction of the advance rate (AR) of EPB shield tunneling is significantly more difficult than that in homogeneous ground (i.e., full-face hard-rock ground). In addition, the operating parameters of the EPB shield tunneling can be subjective and suboptimal, and each of them has some intricate influence on AR. In this paper, an optimized back-propagation neural network by genetic algorithm (BPNN-GA) was proposed for reasonable operating parameter selection and accurate AR prediction, and four typical machine learning methods were used for comparison. Five processing strategies with different input parameters were also proposed and compared to determine the optimum selection of geological parameters in mixed ground conditions. The proposed models with strategies were adopted in the case study of the Nanjing Metro Line S6 project, and a total of 1188 rings of datasets were used for this study. The results showed that the proposed modified BPNN with the genetic algorithm could be effectively implemented for the AR prediction. It concluded that Strategy B—i.e., using the composite ratio and the geological parameters of each layer as input—was the best strategy in mixed ground conditions for advance rate prediction. Hence, a high correlation between measured and predicted AR was observed in this study with a correlation coefficient (R2) of 0.920.
Over soft soil area the geosynthetic-reinforced and pile-supported subgrade was often used to reduce the post-construction settlement. But after operation it was found that the settlement sometimes ...was bigger than required. For settlement evaluation the stress concentration ratio between pile and soil is very important and the arching effect is often considered to solve this problem. There are some methods to analyze the sharing of fill load, but the dynamic train loads were often ignored. In this paper the finite element was used to simulate the stress concentration ratio during fill stage construction and dynamic load which were compared with the in-situ test. The results during fill stage also were compared with EBGEO and BS8006. They showed that the reliability of current methods is related to some factors including fill height, the stiffness ratio of pile and soil. There is a critical filling height whether the dynamic load decrease arching effect or not. According to FE results, the critical height is 3m. When the filling height is smaller than 3m, the stress concentration ratio should be re-evaluated by the reduction factor.
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