APPLICATION OF NUMERICAL ANALYSES FOR DEEP EXCAVATIONS IN SOFT GROUND

• Main Authors: DAO, SY DAN, 陶士旦
• Other Authors: BIN-CHEN BENSON HSIUNG
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/8m9544

博士 === 國立高雄應用科技大學 === 土木工程與防災科技研究所 === 103 === Accompanying with the rapid growth of urban areas, the deep excavations have been popularly adopted for construction of basements of high-rise buildings and underground transport networks in recent years and are often located very close to existing buildings and services, especially in large cities. Therefore, in order to protect the adjacent properties, thoroughly understanding the behaviors of deep excavations and responses of adjacent buildings during the process of excavation construction is necessary and essential. The behaviors of deep excavations and responses of adjacent buildings during the process of excavation construction can be predicted by using empirical or numerical methods. Nowadays, with the development of constitutive soil models and computer technology, finite element analyses have been commonly used to evaluate the performance of deep excavations with high accuracy. The main objective of this research is to explore the behaviors of deep excavations in soft ground by using numerical analyses based on cases selected from Kaohsiung, Taiwan and Central Ha Noi, Vietnam. The commercial programs of PLAXIS 2D (2009) and PLAXIS 3D (2013) were used as numerical tools for two-dimensional and three-dimensional finite element analyses, respectively, in this study. A case history of deep excavation in thick layers of sand, which is well documented in Kaohsiung City, Taiwan, and the other deep excavation in Central Ha Noi, Vietnam, which belongs to Ha Noi MRT system, were adopted as bases for numerical analyses in this study. The input parameters of the numerical analyses were obtained or estimated from the test data, empirical correlations or back analyses. The results of previous studies relating to the simulating process of the numerical analyses were also consulted in this report. In this research, a simple method for evaluation of surface settlements is first developed. In said method, additional two parameters, “α” for the ratio of the width of primary strain zone behind the retaining wall to the excavation depth and "β" for ratio of Young’s modulus of each soil layer in the small strain zone to that in the primary strain zone, are defined. The results derived from this research indicated that said simple method interpreted from this research using a basic constitutive model of soil can reasonably predict the movements. It is thus suggested that values of β from 3 to 5 shall be taken once the simple method intends to be adopted for predicting ground surface settlements caused by deep excavations in sands, but change of  can only give minor impact on the movements. Three-dimensional effect of deep excavations in sands is explored in this study, and the ratio called “Plane Strain Ratio (PSR)” for said excavations is determined using numerical analyses. It is first indicated that PSR increases gradually with the increase of distance from the corner. However, the maximum PSR value cannot reach 1.0 for excavation width (B) larger than 20 m. It can only be 0.78 for case of B of 100 m. It is again found that PSR is often larger than in sand rather than in clay. Difference in ground condition is expected to be the reason for that, but further verification is necessary to be delivered later. In three evaluated constitutive soil models, i.e. Mohr-Coulomb model, Hardening soil model and Hardening soil model with small strain stiffness, selected for this research, the Hardening soil model with small strain stiffness is a comparatively better model for predicting both wall deflections and ground surface settlements caused by deep excavations. Furthermore, the variations of input parameters E', and 0.7 are to significantly influence the predictions of the maximum lateral and vertical displacements. The findings derived from this study are expected to help to explore behaviours of the excavations in Central Ha Noi. Similar to the previous finding, the HSS model is a preferable model for predicting both wall deflections and ground settlements induced by deep excavations in Central Ha Noi. Building footing type, building position and building footing stiffness all affect the building deformation parameters. In this study, the most unfavorable distance from the building to the retaining wall found is 5 m. However, further works regarding testing and interpretation of soil properties and analyses of observed behavior of deep excavations in Ha Noi shall be undertaken.