Numerical analysis of lateral movements and strut forces in deep cement mixing walls with top-down construction in soft clay

This article presents the observed and simulated lateral movements and strut forces induced in deep cement mixing walls under deep excavation using top-down construction techniques in soft Bangkok clay. The walls are supported laterally by permanent concrete slabs and temporary struts. A three-dimensional numerical model is first calibrated with observed data from a case study. Then, a parametric study is performed to compare this construction method with the bottom-up method and investigate the influence of the DCM wall thickness on lateral movements and strut forces of the wall.     

Deep Cement Mixed Walls with Steel Inclusions for Excavation Support

Deep cement mixed (DCM) walls are widely used in supporting excavations in many parts of the world. In this paper, a case study of an excavation supported by a DCM wall with steel inclusions is analysed using a three-dimensional finite element model and based on the coupled theory of nonlinear porous media. The DCM wall is constructed with wide flange steel inclusions. The stress–strain behaviour of the DCM wall section is simulated using an extended version of the Mohr–Coulomb model, which considers the strain-softening behaviour of DCM columns beyond yield. The computed lateral deformations are compared with the field measurements to validate the numerical modelling procedure. Using the same case study, the internal stability of the wall against bending and shear failure modes is investigated. In addition, the lateral pressure distribution along the wall length is investigated because in practice design is carried out considering a uniform pressure distribution assuming rigid wall movements. A parametric study was carried out to investigate the viability of DCM walls in supporting excavations by varying the spacing between steel inclusions, wall thickness and initial lateral earth pressure. Based on the results of the parametric study, guidelines are proposed to select the most efficient geometric arrangement of steel inclusions within DCM walls.     

Three-dimensional numerical analysis of deep excavations with cross walls

Previous plane strain analysis of a case history has shown that cross walls in an excavation can effectively reduce movements induced by deep excavation. This study performed three-dimensional numerical analyses for 4 deep excavation cases with different installations of cross walls, including different excavation depths, cross wall intervals and cross wall depths. Both the observed and computed wall deflections for the 4 cases were compared with those of the same excavations that were assumed with no cross walls installed to demonstrate the effectiveness of cross walls in reducing lateral wall deflections. The results show that the cross wall also had a corner effect similar to that of the diaphragm wall. The deflection of the diaphragm wall was smallest at the location of the cross wall installed and then increased with the increasing distance from the cross wall, up to the midpoint between two cross walls. Many factors such as in situ soil properties, diaphragm wall properties, construction procedure, cross wall depth and so on may affect the amount of reduction in lateral wall deflections due to the installation of cross walls. Under the same condition, the amount of reduction was highly dependent on the depth of cross walls, distance to the cross walls and the cross wall interval.     

基坑支护的混凝土芯搅拌桩有限元分析

介绍了一种新型的基坑支护形式,即混凝土芯水泥土搅拌桩,它是一种融合了复合地基和桩基础优点的地基处理方法,是一种结合围护挡土结构和防渗止水帷幕功能于一体的支扩结构.笔者还采用有限元方法分析了基坑支护中的混凝土芯水泥土搅拌桩的位移大小分布和方向分布、应力大小分布和方向分布、塑性区的分布和临空面的水平位移.分析结果表明,采用门型结构的混凝土芯水泥土搅拌桩进行支护,既安全可靠,又经济可行.     

土钉墙数值模拟在干船坞坞墙中应用探讨

从减少基坑开挖过程中坞墙位移和控制坞壁渗水角度出发,利用数值模拟分析,探讨在深基坑开挖中得到广泛应用的普通土钉墙和用于船坞坞室基坑开挖中的复合土钉墙技术,将联合劲性水泥土搅拌桩的复合土钉墙用于干船坞的坞墙结构。从工程探讨角度,研究土钉联合劲性水泥土搅拌桩、预应力锚杆的复合土钉墙在干船坞这一特定条件下的应用问题,并通过分析模拟结果,为干船坞坞墙设计施工过程中安全问题提出一些注意点     

Simplified approach to estimate the maximum wall deflection for deep excavations with cross walls in clay under the undrained condition

Previous studies have shown that use of cross walls in deep excavations can reduce the wall deflection to a very small amount. However, design of cross walls is costly because the deflection behavior of the diaphragm wall with cross walls is in nature three dimensional. The objective of this study was to establish a simplified approach used as a first approximation to design cross walls such that the lateral wall deflection can satisfy a design criterion. A series of parametric studies using a three-dimensional numerical method was performed to obtain the influence factors on wall deflections, including excavation geometry, wall system stiffness, axial stiffness of strut, axial stiffness of the cross wall, normalized undrained shear strength of clay and the cross wall depth. Then, a simplified formula for predicting the wall deflection for excavations without and with cross walls was established using multivariate regression analysis, respectively. The formulas were validated through 36 excavation cases without cross walls and 12 cases with cross walls. The simplified formulas can be used to develop a spreadsheet that estimates the cross wall sizes and intervals based on the entered excavation geometry, material properties of retaining-strut system, in situ undrained shear strength and tolerable wall deflection. The estimated cross wall sizes and intervals should be verified by an appropriate full numerical analysis.     

A parametric study of wall deflections in deep excavations with the installation of cross walls

Several case studies have revealed that the installation of cross walls in excavations can effectively reduce the amount of wall deflection and ground settlement. However, the behaviour of the diaphragm wall due to the installation of the cross walls is still unclear. This study performed a series of 3D numerical studies of wall deflections for deep excavations with cross walls and studied the effects on the wall deflection of several parameters, including the number of cross walls, the distance to the cross wall, the cross wall interval, the cross wall height and the cross wall embedment. The results presented in this study can be used as a first approximation for cases in which cross walls are designed to reduce the wall deflection induced by deep excavation.     

Finite-element modeling of a complex deep excavation in Shanghai

The excavation of the north square underground shopping center of Shanghai South Railway Station is a complex deep excavation using the top-down construction method. The excavation has a considerable size and is close to the operating Metro Lines. In order to predict the performance of the excavation more accurately, 3D finite-element analyses are conducted to simulate the construction of this complex excavation. The effects of the anisotropic soil stiffness, the adjacent excavation, and zone excavation on the wall deformation are investigated. It is shown that the numerical simulation with anisotropic soil stiffness yields a more reasonable prediction of the wall deflection than the case with isotropic soil stiffness. The deformation of the shared diaphragm wall between two excavations is influenced by the construction sequence of the two excavations. The zoned excavation can greatly reduce the diaphragm wall deformation. However, only the zoned excavation at the first excavation stage affects the deformation of the walls significantly. When the depth of the excavation increases, the zoned excavation has minor effect on the deformation of diaphragm walls.     

搅拌桩加固挤土效应及在地铁隧道保护中的应用

在减小地基变形进行的深层搅拌桩加固时,加固本身的挤土效应对隧道变形产生了影响。为此,模拟了搅拌桩（DCM）侧向挤土效应的4种荷载模式,结合实测资料并采用数值试验验证了侧向挤土荷载模式的合理性。进一步应用该模式,通过有限元模拟了搅拌桩的加固挤土效应,分析讨论了不同加固顺序对地面变形,隧道变形以及长期蠕变变形的影响,结论表明,搅拌桩加固对地面环境影响是不可忽略,隧道周围搅拌桩施工顺序对隧道变形影响较大,搅拌桩加固后长期蠕变效应相对加固过程的变形很小。     

重力式水泥搅拌桩挡墙在深基坑支护工程中的应用研究

本文从水泥搅拌桩与软土之间的物理化学反应过程以及水泥搅拌桩的力学特性两方面,分析了重力式水泥搅拌桩支护技术的作用机理,归纳分析了重力式水泥搅拌桩挡土墙的设计方法和施工技术特点。结合工程实例,对重力式水泥搅拌桩挡土墙的土压力计算、墙体稳定性验算等方面进行了探讨,对比分析了公式计算与电算结果,同时体会到深基坑支护工程除了优化设计外,采用信息化施工,对工程进行监测,及时反馈信息指导施工是工程成功的关键之一。实例表明无支撑重力式水泥搅拌桩挡墙作为7m以内基坑的围护结构具有显著的优越性,值得推广运用。     

深层水泥土搅拌桩在基坑支护中的应用

通过介绍水泥与土之间的一系列物理-化学反应,阐述了深层搅拌法加固地基土的原理[1]。结合室内试验及工程实例,介绍了深层水泥土搅拌桩重力式挡墙技术在基坑支护中的应用,对于基坑设计及施工有一定实际意义。     

Evaluation of buttress wall shapes to limit movements induced by deep excavation

Three-dimension finite element analyses of deep excavations with buttress walls were performed to evaluate the effect of buttress wall shapes on limiting movements induced by deep excavation. Results showed that a combination of the rectangular and the capital L-letter shapes (RL-shape) yielded the greatest performance in reducing wall deflections and ground surface settlements. The main deformation-control mechanism mainly came from the horizontal and vertical frictional resistances of buttress walls against adjacent soils which were pushed by wall deflections and the soil heave at the excavation bottom, respectively. Besides, the RL-shape buttress walls were successfully verified through a well-documented case history.     

Investigation of the integrated retaining system to limit deformations induced by deep excavation

A series of three-dimensional finite element analyses of deep excavations with the integrated system between buttress walls and diaphragm walls was conducted to investigate the effect of the buttress wall intervals, treatments, locations, height, and thickness on limiting deformations induced by deep excavation. The integrated retaining system was formed by maintaining buttress walls when soil was excavated. The wall deflection control mechanism of the integrated retaining system mainly came from the combined stiffness between the buttress wall and the diaphragm wall. In addition, the ground settlement control mechanism came from the combined stiffness between the buttress wall and the diaphragm wall, and the frictional resistance between the buttress wall and the surrounding soil. For achieving 50% reduction in the wall deflection and the ground surface settlement, the length and intervals of buttress walls that were applied to the integrated retaining system were at least 4 and 8 m, respectively. When the deflection at the diaphragm wall head was well restrained, for example, by the floor slab, the position of the buttress wall head could be located at a depth the diaphragm wall starts to bulge out. In such a case, the performance between the full height and limited height of buttress walls was quite close. Furthermore, a new well-documented excavation project was analyzed to verify the performance of the integrated retaining system. Results showed that the integrated retaining system worked excellently if the joints between buttress walls and diaphragm walls were constructed properly.     

Mitigation of levee failures using deep mixed columns and geosynthetics

Stability of levees is critical to the safety of human and structures, especially at high water levels. Levees may fail due to the existence of soft soil foundations or seepage of water through the levees or rapid drawdown. Deep mixing technology has been considered one of the good alternatives to solve foundation and seepage problems while geosynthetics can be used to stabilize slopes during rapid drawdown. Studies have shown that deep mixed columns and geosynthetics can increase the stability of highway embankments over soft soils. In those studies, however, no ponding water exists on either side of the embankment, which is not the case for levees. Experimental studies have shown that deep mixed columns under a combination of vertical and horizontal force could fail due to shear or tension/bending or rotation. A finite difference method, incorporated in the FLAC (Fast Lagrangian Analysis of Continua) Slope software, and a limit equilibrium method (specifically Bishop's method), incorporated in the ReSSA software, were adopted in this study to investigate the stability of the levee with ponding water or under rapid drawdown. In this study, deep mixed columns were installed in continuous wall patterns, which were modeled as 2D deep mixed walls. Geosynthetic layers were modeled using cable elements with grout properties between geosynthetic and soil in the numerical analysis. Mohr-Coulomb failure criteria were used for the levee, the soft soil, and the deep mixed walls. The stability of a levee at different stages (end of construction, average service condition, high water surge, and rapid drawdown from the service condition and the highest water level condition) was examined. The study clearly demonstrated that the deep mixed walls can enhance the stability of the levee by providing shear/moment resistance and hindering seepage through the levee and geosynthetics can enhance the riverside slope stability of the levee by providing tensile resistance to the soil.     

水泥土搅拌桩的施工质量问题和解决方法

介绍了水泥土搅拌桩在我国的应用情况和可行性、危机性,指出了当前搅拌桩施工质量上存在的搅拌不均和桩身不连续问题,分析了出现质量问题的3方面原因：规范检测方法严重滞后;成桩工艺不合理;施工管理混乱,并针对性的提出了3大对策和9项工艺改进措施。     

水泥土桩加固海洋土的渐进变形破坏试验与数值分析

应用一个高度非线性的三维各向异性粘弹塑性(EVP)模型及有限元法,对一水泥土桩(CDM)加固海洋土的复合地基进行数值模拟。海洋软粘土的渐进变形采用粘弹塑性模型来模拟,水泥土桩破坏前后的应力-应变关系采用一个双曲线模型来模拟,提出了一个简单且非常实用的数值分析方法来解决水泥土桩碎裂软化的数值模拟问题。将数值模拟结果与实测数据进行了比较,二者吻合较好。     

Study on Deformation Characteristics of Deep Foundation Excavation in Soft-Soil and the Response of Different Retaining Configurations

In recent years, many researchers have considered the mechanical characteristics of deep foundation excavation in soft-soil. The analysis of these deep excavations requires consideration of an uncertain, nonlinear, dynamic and complicated system, and involves consideration of soil strength, stability, deformation, fluid flow and interaction of soil and different retaining configurations. It is difficult to describe such a nonlinear system using traditional analysis. Therefore, in order to accurately describe the mechanical behavior of a representative deep excavation of the subway station, in this case, 3-Dimensional geotechnical numerical analysis method using FLAC3D software was applied. Using this tool, a study considering earth pressure, soil deformation and settlement was carried out. Furthermore, the response of different retaining configurations was deeply investigated. Triaxial cement mixing piles were considered as a way to optimize deformation of the deep excavation and reduce settlement of the ground surface and the railway embankment. The analysis indicated that the deeper the foundation excavation was, the larger the surface settlement and the smaller the earth pressure. The analysis also considered the mechanical effect of varying the wall thickness and the wall depth on the structure‘s deformation characteristics. The simulations indicated that a wall thickness of less than 1.4 m effectively reduced wall horizontal displacement, ground surface settlement and uneven settlement of railway embankment. While a variable wall embedded depth that was less than 52 m also changed the settlement of the excavation deformation and the ground surface. Therefore, the numerical results can agree with the practical project to imply that numerical method in the paper is applicable and reliable, which provides a new thought to research on deep excavation in soft-soil.     

Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions

The construction of sheet pile walls may involve either excavation of soils in front or backfilling of soils behind the wall. These construction procedures generate different loading conditions in the soil and therefore different wall behavior should also be expected. The conventional methods, which are based on limit equilibrium approach, commonly used in the design of anchored sheet pile walls do not consider the method of construction. However, continuum mechanics numerical methods, such as finite element method, make it possible to incorporate the construction method during the analyses and design of sheet pile walls. The effect of wall construction type for varying soil conditions and wall heights were investigated using finite element modeling and analysis. The influence of construction method on soil behavior, wall deformations, wall bending moments, and anchor forces were investigated. The study results indicate that walls constructed by backfill method yield significantly higher bending moments and wall deformations. This paper presents the results of the numerical parametric study performed and comparative analyses of the anchored sheet pile walls constructed by different construction methods.     

Modelling the behaviour of a retaining wall in residual soils for a cut and cover construction of a deep station in Metro do Porto

This paper presents a re-appreciation of the ground characterisation and the criteria to select the most representative geomechanical parameters to consider in a numerical model to predict the behaviour of a retaining wall of a deep excavation in highly weathered granite rock masses and residual soils. This study was focused in the construction of a deep station of Metro do Porto, which involved a cut and cover solution, with unusual proportions (in plant and in depth), built in the typical Oporto's granite weathered profiles, being the excavation conducted with retaining walls consisting of multi-anchored concrete piles. Specific sections were carefully instrumented, due to the presence of historic buildings in the vicinity. The definition of representative model parameters was based on precise laboratory tests over high quality soil samples, including oedometer and high-precision triaxial tests. Geotechnical and geological characterisation of all the area for the original design, was initially based on in situ tests, such as SPT and rock masses classification, and on the local experience on this type of ground. After this construction, the assumptions of parameterisation, using a constitutive modelling based on new laboratory tests over high quality block samples, allowed a reanalysis of the assumptions on the design phase. A back-analysis of monitored displacements and forces during the construction was made, assuming the designed structural solutions, which were in fact implemented in construction, but considering the new approaches on the definition of the geomechanical parameters for the prevailing weathered rock masses, necessary for the numerical simulation based on the commercial software Plaxis®, using the Mohr-Coulomb and “Hardening-Soil” models. Some specific changes of the constructive sequence during the excavation and activation of supporting system were attained, by looking at the information found during the construction. The results of this parametrical re-approach and analysis of the singularities of highly weathered granite and corresponding residual soils masses for modelling of retaining walls of large excavations are discussed.     

Reliability analysis of soil nail walls

In India, soil nail walls are being extensively used for supporting vertical excavations below ground level to accommodate construction of one-or two-storied basements. Generally, the depth of excavations for basement construction ranges from 10 m to 15 m. For such large depth of excavation, variability of in-situ soil properties has significant influence on the stability of the soil nail walls. In the present study, using reliability analysis, an attempt is made to study the influence of variability of in-situ soil properties on the stability of soil nail walls. For better understanding, a case of 10 m high soil nail wall constructed to support a vertical cut is considered for the study and its stability is evaluated for various failure modes. Additionally, the influence of correlation among soil parameters on soil nail wall stability is assessed. In-situ soil friction angle and correlation between in-situ soil cohesion and angle of friction are found to influence soil nail wall stability significantly. In general, reliability analysis provided a better insight into the assessment of stability of soil nail wall.