Tunnel deformation and stress response under the bilateral foundation pit construction: a case study

Deformation responses of the foundation pit construction of the urban metro station: A case study in Xiamen

Collapse risk analysis is of great significance for ensuring construction safety in foundation pits. This study proposes a comprehensive methodology for dynamic risk analysis of foundation pit collapse during construction based on a fuzzy Bayesian network (FBN) and a fuzzy analytical hierarchy process (FAHP). Firstly, the potential risk factors contributing to foundation pit collapse are identified based on the results of statistical analysis of foundation pit collapse cases, expert inquiry, and fault tree analysis. Then, a FAHP and improved expert elicitation considering a confidence index are adopted to elicit the probability parameters of the BN. On this basis, quantitative risk reasoning and sensitivity analysis of foundation pit collapse are achieved by means of fuzzy Bayesian inference. Finally, an actual deep foundation pit in a metro station was used to illustrate a specific application of this approach, and the results were in accordance with the field observations and numerical simulation results. The proposed approach can provide effective decision-making support for planners and engineers, which is vital to the prevention and control of the occurrence of the foundation pit collapse accidents.

/pdf/collapse-risk-analysis-of-deep-foundation-pits-in-metro-2zkgcnszet.pdf

Collapse Risk Analysis of Deep Foundation Pits in Metro Stations Using a Fuzzy Bayesian Network and a Fuzzy AHP

A comprehensive understanding of deformation characteristics and control measures for large-scale foundation pits in soft soil regions is of increasing importance to their risk assessment, design, and construction. In this study, the influences of the thickness of soft soil on the deformation of foundation pits are discussed based on statistical data from 24 foundation pits in floodplain areas of Nanjing, China. The mean maximum ground surface settlement around the pit in floodplain areas of Nanjing was 0.38%H, and the maximum ground surface settlement was observed at 0.48H. The maximum influence scope of ground surface settlement was about 3.5H, and the average maximum horizontal displacement of supporting structures was 0.23%H. On this basis, a model of ground surface settlement around the pit according to excavation depth for foundation pits in floodplain areas is derived. In addition, the influence scope around the pit and relevant control measures are proposed for the protection of surrounding underground structures and roads.

Deformation characteristics and control for foundation pits in floodplain areas of Nanjing, China

So far, there have been a large number of diaphragm walls in the Yangtze River Delta as engineering examples of deep foundation pit maintenance structures in subway stations, but there is a lack of systematic research and summary on the deformation characteristics of ground connecting walls. This study aimed to clarify the deformation law of the diaphragm wall during the excavation of a deep foundation pit in a soft soil region. Based on the monitoring data of the diaphragm wall of the deep foundation pit of the Hangzhou metro station, the monitoring data of the deep foundation pits of 15 subway stations in Shanghai and Ningbo cities around Hangzhou were considered. Grouping and classification methods were used to analyze the similarities and differences in the deformation characteristics of the diaphragm wall in the three regions. The results indicate the following: the maximum lateral deformation of the diaphragm wall in Hangzhou increases linearly with the relative depth of the maximum lateral deformation. The maximum lateral deformation of the foundation pit in Hangzhou is 0.072% H∼0.459% H, with a mean of 0.173% H. The wall deformation in Hangzhou varies significantly with the depth of the foundation pit, but the influence of the depth of the foundation pit on the wall deformation is considerably less than that in Shanghai and Ningbo. The corresponding position of the maximum lateral deformation in the excavation depth increases linearly with the excavation depth of the foundation pit, and the corresponding position of the lateral deformation of the diaphragm wall in Shanghai is more affected by the excavation depth of the foundation pit. The lateral deformation of the diaphragm wall increases rapidly in the range of 0 H–0.5 H, and the maximum lateral deformation occurs at 0.5 H–1.1 H.

/pdf/deformation-law-of-the-diaphragm-wall-during-deep-foundation-31mp9ur1xq.pdf

Deformation Law of the Diaphragm Wall during Deep Foundation Pit Construction on Lake and Sea Soft Soil in the Yangtze River Delta

Performance of a foundation pit supported by bored piles and steel struts: A case study

Numerical simulation and simplified analytical method to evaluate the displacement of adjacent tunnels caused by excavation

This study presented a numerical investigation of the tensile membrane effect in the geogrid reinforced piled embankment under dynamic loading. It has been found that the maximum sag of the geogrid was attained at the center of two adjacent piles, while localization of tensile force was observed at the corners of the pile caps. Under the dynamic loading, the sag and the strain of the geogrid increased when compared with the static loading, an increase of around 45.9% and 24% was, respectively, yielded for the sag and the strain of the geogrid. This increase of tension in the geogrid may mainly result from the degradation of the soil arching effect under dynamic loading. A parameter study was performed, and it showed that the tensile force in the geogrid increases with the rise of the embankment height, the clear pile spacing, and the tensile stiffness of the geogrid. However, the increase of the friction angle of the embankment fill led to a decrease in the tension in the geogrid, which may benefit from the improvement of the soil arching developed in the embankment fill. It was found that the tensile behavior of the geogrid was the most sensitive to the pile spacing. When pile spacing increased from 2.0 m to 3.0 m, the tensile force in the geogrid increased by about 248%.

/pdf/finite-element-analysis-on-the-tensile-membrane-effect-in-1cftynsp.pdf

Finite-Element Analysis on the Tensile Membrane Effect in Geogrid Reinforced Piled Embankment under Dynamic Loading

Piles have been widely used to improve the bearing capacity of the soft foundation. The existing research obtains significant findings on the load transfer mechanism for rigid piled embankments. However, limited studies have been focused on the deep cement mixing (DCM) piled embankment. To grasp the load transfer characteristics of DCM piled embankments, a three-dimensional numerical simulation was conducted in this study, which was validated by the measurements from the field case. It was found that the effect of soil arching was reduced compared with the rigid piled embankment. This induced approximately 61.5% larger vertical stress transferred to the subsoil surface and approximately 83–150% larger settlement of the embankment in DCM piled foundation system. To further understand the working mechanism of this system, the factors which influence the load transfer mechanism were investigated. It is found that the area replacement ratio is the most influential factor affecting the settlement at the top of the embankment, whereas the elastic modulus of the DCM pile influences most the vertical stress and the earth pressure coefficient. The cyclic load with vehicle speeds of 90 km/h will lead to approximately 34% growth of embankment settlement and about 11% reduction in the maximum earth pressure coefficient. Based on the numerical simulation results, the analytical equation of the normalized vertical stress acting on the subsoil surface for the DCM piled foundation was proposed and validated by two field cases, with the difference in the range of 13.8~16.7%.

/pdf/three-dimensional-analysis-of-load-transfer-mechanism-for-3f19gr3b.pdf

Three-Dimensional Analysis of Load Transfer Mechanism for Deep Cement Mixing Piled Embankment under Static and Cyclic Load

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Papers

Performance of a foundation pit supported by bored piles and steel struts: A case study

Numerical simulation and simplified analytical method to evaluate the displacement of adjacent tunnels caused by excavation

Finite-Element Analysis on the Tensile Membrane Effect in Geogrid Reinforced Piled Embankment under Dynamic Loading

Three-Dimensional Analysis of Load Transfer Mechanism for Deep Cement Mixing Piled Embankment under Static and Cyclic Load