Deep Excavation In Soft Soils Research Articles

Estimation of deformation behaviour of deep excavation in under-consolidated soft ground

The under-consolidated (UC) state affects the deformation behaviour of deep excavation in soft soil and poses potential risk to the safety of adjacent facilities. However, deformation models for deep excavation in UC ground have not been well investigated. This paper presents a series of numerical analyses on the deformation characteristics of deep excavations in UC and normally consolidated (NC) ground, each retained by diaphragm walls with rigid struts and a bottom improvement layer. UC cases without excavation activities (simplified as consolidated cases) are included for comparison. The modelling results showed that, with the lateral constraint of the inner rigid support system, the UC ground resulted in only a slight increase in lateral wall deformation but a significant increase in ground settlement as compared with NC ground. The UC ground with lower initial average consolidation ratio, thicker surface fill, higher permeability and longer construction period produced greater wall deformation and ground settlement during excavation. An empirical method is proposed to estimate the settlement envelope for deep excavation in UC ground as the superposition of two parts: settlement induced by excavation activities and settlement induced by residual consolidation with consideration of average consolidation ratios before and after excavation.

Performances of a large-scale deep excavation with multi-support types and zoned excavation technique in Shanghai soft soil

This paper presents a comprehensive field investigation on a large-scale deep basement excavation in Shanghai soft soil propped by a multi-support system. Because of its large size, irregular shape, and different excavation depths, the excavation site was divided into Zone A and Zone B to optimize the construction process and improve the construction efficiency. The excavation was constructed using the “bottom-up” method following the principles of stratification and zone excavation. A notable innovation in this project is the implementation of three different support subsystems as a multi-support system to accommodate different deformation requirements in different areas. The excavation was densely instrumented to monitor the behaviors of retaining walls, columns, axial forces of struts, and surrounding ground throughout the whole construction process. The wall deformation and ground surface settlement of the three support subsystems are comprehensively compared to investigate the performances of the three support subsystems. The comparison of the corner-effect envelope summarized from nine reported cases indicates that the multi-support system can effectively alleviate the spatial corner effects of the excavation. The zoned construction technique in conjunction with the multi-support system presented in this case study provides an efficient and economic approach for large-scale deep excavation in soft soils.

Rebound Calculation for Deep Excavation in Soft Soil Based on Rebound-Recompression Method

The excavation-induced stress relief and inward movement of the retaining wall will result in soil rebound deformation at the bottom of the excavation, adversely affecting nearby existing tunnels and foundation piles. Various existing methods for calculating the excavation rebound rely on rebound parameters and void ratio obtained from laboratory tests, with considering the effects of sampling, specimen preparation and laboratory procedures on the rebound parameters. As a result, a novel method is proposed in this article for calculating excavation rebound based on rebound-recompression method (RRM). This method first modifies initial void ratio (e0) and laboratory recompression index (CLR) used in traditional methods (TM) for calculation, based on field rebound and recompression curve proposed by the RRM, to in situ void ratio (ev0) and field recompression index (CFR). Then, the final rebound at the bottom of the excavation is calculated using a layered summation method. In addition, through two engineering examples, the proposed method is compared with existing calculation methods and measurements, demonstrating that this method is easy of calculate, yields reliable results, and can accurately predict the final soil rebound at the bottom of the excavation.

3D Finite Element Analysis of Pile Behavior Inside the Deep Excavation in Soft Soil

3D Finite Element Analysis of Pile Behavior Inside the Deep Excavation in Soft Soil - written by Nghia Trong Le , Tuan Anh Nguyen published on 2021/01/06 download full article with reference data and citations

Collapse of a deep excavation and its reconstruction in soft soil of Nanjing, China

A collapse of deep excavation in soft soil happened in Nanjing, China. The collapse mechanisms were studied through in situ investigations, back-calculations and numerical simulations. It was found that the incorrect design (due to the incorrect in situ exploration) and poor construction of the retaining piles contributed to the final collapse. A reconstruction project using large-diameter cast-in place piles with a steel casing and a prestressed assembled steel strut (Pass) system was then introduced. Further, the advantages of the Pass system were summarised. The successful implementation of this reconstruction project can provide reference for relevant excavation projects with complex soil profiles and construction environments.

SDMT-Based Numerical Analyses of Deep Excavation in Soft Soil

AbstractThe paper explores the application of conventional (DMT) and seismic (SDMT) dilatometer tests to an important case of deep excavation design. The work presents finite-element analyses simul...

The choice of soil models in the design of deep excavation in soft soils of Viet Nam

The use of diaphragm wall to protect the depth excavation is quite common in Viet Nam. Prediction of diaphragm wall deformations is required to choose the method of construction, and also for control of process of erection of an underground construction. Currently, there are many programs to calculate the deformation of the diaphragm wall, including software Plaxis. This paper considers the choice of a computational model for soils in the Hanoi - Viet Nam region of Mohr Coulomb (MC) and Hardening Soil (HS) and comparison of the calculation results with the measured data. Such investigations were conducted first.

3D Finite Element Modelling of Sheet Pile Wall Excavation: A Case study in Bangkok

Sheet pile wall has been used extensively as a soil retaining structure during the excavation process in soft ground. Meanwhile, finite element method (FEM) has been widely used as a numerical tool to predict wall movements due to soil excavation. In FEM, many factors including soil parameters, structures’ parameters and construction stages simulation influence the analysis results. This paper presents a modelling of sheet pile wall at deep excavation using 3D FEM. The study focuses on the structures’ stiffness modelling and the stages of construction simulation. The hardening soil model and its parameters adopted from previous study was employed in the analysis. To validate the model, an excavation site located in the center of Bangkok was selected to model. PLAXIS 3D – a commercial software for solving finite element problem was employed in this study. In overall, the results of wall movements from 3D FEM agree well with the instrumented data confirming that the modelling could reflect the real behavior of sheet pile walls at deep excavation in soft soils in Bangkok

Prediction of behaviour of a deep excavation in soft soil: a case study

In this paper, the nature of wall deformation and surface settlement for deep excavation in soft soil was studied. Excavation was carried out in eastern metro city of India in the bank of Hooghly River. This excavation was the part of underground metro station construction. Numerical analysis was performed to predict the soil behaviour during excavation. Accordingly, the most suitable excavation method was finalised. Vertical and horizontal deformations were measured during construction on adjacent area to ensure the safety of nearby existing structures. Predicted deformation was relatively high when compared with measured one. Back calculation from measured deformation shows the conservative estimation of stiffness during investigation and anisotropic soil behaviour. This study indicates that the numerical analyses can be very effective tools to predict the behaviour of soil during excavation with installation of support systems. Thus, it helps for necessary planning to ensure the safety of nearby structures. However, accuracy of such analysis depends on the judicial estimation of soil parameters.

Analysis on Deep Excavation in Soft Soil Located on Sloped Bedrock

Deep excavations in soft-clay layer on sloped bedrock often leads to lateral displacement on retaining structures and uneven settlement due to unbalanced pressure generated from excavation. A construction project for which an excavation was complete in soft clay layer on sloped bedrock in Taipei City was adopted in the study. It is learned from the observation logs of the studied case that a significant difference exists in the lateral displacement of diaphragm wall and settlement between up and down-slope sides of sloped bedrock. Deep excavation is in fact profoundly complicated interaction between excavation strutting and soil. In general practice, the design of excavation is frequently simplified as a 2D strain behavior. However, the actual excavation on sloped bedrock is quite different from 1D or 2D simulation in a symmetric manner. Therefore, 2D finite element analysis program, PLAXIS, is introduced for the analysis on the behaviors of soil clay layer on sloped bedrock in excavation. The result is compared with onsite observation data, including displacement of retaining wall, settlement, axial loads of struts and others. The result of retaining wall displacement analysis is found consistent with the trend derived from onsite observation, which is possible for reference of similar engineering analyses and designs in the future.

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.

Mass behaviour of embedded improved soil raft in an excavation

In deep excavation in soft soils, a layer of soil below formation level is often improved to stabilise the excavation. The improvement, usually by deep cement mixing or jet-grout piling, will result in the construction of a raft of overlapping short columns (termed an ‘embedded improved soil raft'). Soil investigation carried out will usually provide information on vertical cores from such columns. But during excavation, the columns are loaded laterally by the inward-moving retaining wall: thus the mobilised mass properties in the lateral direction are of importance in the design. This paper examines the mechanisms of how mass properties are mobilised and their relation to the elemental properties, the variation of properties within a column, and how the columns are arranged. The analysis and the simulation of a reported field case history show that for soil-cement columns arranged just in contact with each other, the mobilised mass stiffness is less than 28% of the elemental material stiffness. The analysis also shows that the properties of the outer layer have a greater impact on the mass behaviour of the embedded improved soil raft than those of the inner layer, and this ought to be considered in any site investigations.

Cofferdam for BARTD Embarcadero Subway Station

Inward deformation of cofferdam walls during deep excavation in soft soils are inevitable but steps can be taken to minimize these movements. An excavation 1,160 ft (354 m) long by 55 ft (17 m) wide by 70 ft (21 m) deep in a busy downtown San Francisco street, flanked by major buildings, was performed for the BARTD Embarcadero station successfully in soil strata that contained varying depths of soft clay locally known as recent Bay mud. To provide an impervious rigid cofferdam wall of adequate strength and reasonable thickness a soldier pile and tremie concrete (SPTC) system of sheeting was selected. Successive excavation cuts below prescribed bracing levels were held to a practical minimum. Struts were preloaded to reduce compression deformations. Inclinometers recorded deflections of the walls while strain gages enabled determination of strut loads. The thicker the layer of soft clay the greater the inward movement of the walls notwithstanding the corresponding increases in wall sizes.