Excavation Case History Research Articles

Development of Plane Strain Ratio for Soil Nailing Wall in Clay

Deep excavation analysis, the numerical modelling is now often utilized. Deep excavation is, in fact, a three-dimensional issue. Two-dimensional analysis is still often used for practical purposes. For deformation analysis, plane strain ratio (PSR) is an option to acquire three-dimensional behaviour from two-dimensional analysis. For braced excavations, PSR is well established, but it is yet unknown how to construct retaining walls using soil nails. The aim of this paper is to propose the PSR chart for a soil nailing wall used as a retaining structure in clayey soil with an asymmetrical layout. The excavation case history is adopted for this study. Later, the length (L) and width (B) of the excavated area were varied as part of three-dimensional parametric research to create a soil nailing wall PSR chart.

One-Dimensional Reverse Consolidation Model for Basal Soil from Deep Excavation Based on the Continuous Drainage Boundary

Excavation-induced unloading effects and dewatering-induced groundwater seepage inevitably result in basal soil reverse consolidation in deep excavation. However, this reverse consolidation process is rarely considered because most previous analytical methods were developed based on total stress analyses. This study proposes a one-dimensional reverse consolidation model for basal soil of deep excavation. Based on the consolidation theory proposed by Terzaghi, governing equations of soil reverse consolidation caused by excavation and dewatering were separately established. The continuous drainage boundary was introduced to describe the construction processes. The reverse consolidation responses of basal soil were obtained by superimposing analytical solutions of the excess pore-water pressures that resulted from excavation and dewatering. The proposed model was verified by existing solutions and a well-documented excavation case history. Moreover, the reverse consolidation characteristics of basal soil were investigated by parametric analyses. Results indicate that the excavation-induced variations in pore-water pressure decreased with increasing excavation depth. The final pore-water pressure and effective stress were predominantly affected by excavation duration rather than interval distribution. In addition, a smaller coefficient of consolidation led to lower pore-water pressure and greater effective stress at a given excavation depth.

Analysis of observed performance of a deep excavation straddled by shallowly buried pressurized pipelines and underneath traversed by planned tunnels

The objective of this paper is to analyze the observed performance of a deep excavation facing severe restrictions from underground facilities or structures in Suzhou, China. The first source of the restrictions comes from two shallowly buried pressurized pipelines that straddle the excavation at an intersection angle of about 81°. The pipelines need to be well protected at the site during the construction to ensure the normal life of the local resident. The second source of the restrictions arises from the twin tunnels that are planned to be constructed beneath the excavation in the future for Suzhou Metro Line 14. The planned tunnels will be constructed using a tunnel boring machine, which necessitates the reservation of favorable conditions by the present excavation project. Because of these restrictions, the form of the excavation support system becomes complicated, and the performance of the excavation will inevitably differ from that of the common excavations. Based on a thorough analysis of the extensive field performance data collected by means of settlement points and inclinometers located within and adjacent to the excavation, the characteristics of the ground surface settlements, lateral ground movements, ground water table variations, vertical and lateral displacements atop diaphragm walls, vertical displacements atop lattice column, axial forces in concrete strut, and vertical and lateral pipeline deformations are captured. Despite the discontinuity of the peripheral retaining wall and the shortened wall height on the discontinuous sides, the ground and structural deformations are well controlled by the specially designed excavation support system. The pipelines are slightly uplifted by the suspension system, with the maximum vertical pipeline displacement being lower by an order of magnitude than the commonly observed maximum pipeline settlements in the literature. The existing empirical envelops are found to be site-specific in applicability, and are recommended to incorporate the field observations of this unusual deep braced excavation case history.

Observed Performance and FEM-Based Parametric Analysis of a Top-Down Deep Excavation in Soil-Rock Composite Stratum

This paper investigates the performance of a top-down deep excavation in soil-rock composite stratum. The behavior of the excavation bracing system, consisting of ground anchors and end-suspended piles, has not been well understood due to the lack of relevant research. Based on the observed data of a typical deep excavation case history for the May Fourth Square Station in Tsingtao, China, the characteristics of the horizontal and vertical pile displacements, ground surface settlements, building settlements, axial forces in ground anchors, earth pressure, and pore water pressure during excavation were analysed. Two-dimensional finite element simulations were carried out to further explore the deformation and internal force responses of end-suspended piles and to capture the effects of pile diameter, embedded depth, and rock-socketed depth on the horizontal displacement and bending moment distributions along the pile shaft. It was found that the pattern of the vertical pile displacements could be categorized into three types: rapid settlement, slow settlement, and rapid heave. The magnitudes of the ground and building responses can be well controlled within allowable limits by combining the top-down method with the adopted bracing system. Among the investigated parameters, pile diameter is dominant in affecting the horizontal pile displacement. The primary influence zone for pile bending moment varies, depending on the parameters. It is recommended that a combination of top-down method, ground anchors, and end-suspended piles be adopted for restraining excavation deformation and lowering construction costs of similar deep excavations in soil-rock composite stratum.

Comparative Study between 2D and 3D Analysis for Deep Excavation Case History

Comparative Study between 2D and 3D Analysis for Deep Excavation Case History

Comparative study of the effects of three data‐interpretation methodologies on the performance of geotechnical back analysis

SummaryBack analysis can provide engineers with important information for better decision‐making. Over the years, research on back analysis has focused mainly on optimisation techniques, while comparative studies of data‐interpretation methodologies have seldom been reported. This paper examines the use of three data‐interpretation methodologies on the performance of geotechnical back analysis. In general, there are two types of approaches for interpreting model predictions using field measurements, deterministic versus population‐based, both of which are considered in this study. The methodologies that are compared are (a) error‐domain model falsification (EDMF), (b) Bayesian model updating and (c) residual minimisation. Back analyses of an excavation case history in Singapore using the three methodologies indicate that each has strengths and limitations. Residual minimisation, though easy to implement, shows limited capabilities of interpreting measurement data with large uncertainty errors. EDMF provides robustness against incomplete information of the correlation structure. This is achieved at the expense of precision, as EDMF yields wider confidence intervals of the identified parameter values and predicted quantities compared with Bayesian model updating. In this regard, a modified EDMF implementation is proposed, which can improve upon the limitations of the traditional EDMF method, thus enhancing the quality of the identification outcomes.

Numerical study of the performance of jet grout piles for braced excavations in soft clay

Abstract Jet grout piles (JGP) is a ground improvement method that is constructed beneath the final excavation level to prevent basal heave instability, control wall deflections and reduce the strut loads for braced excavations in deep deposits of soft clay. This paper presents the effects of JGP on the wall deflections and strut forces for deep excavations. Firstly, finite element analysis calibrated by an excavation case history improved by JGP is carried out, followed by parametric studies that investigate the effects of various JGP geometric and mechanical properties on the excavation performances. The numerical analyses indicate that increasing the number of piles and the areal extent of reinforcement area, as well as decreasing the pile spacing, can significantly reduce the wall deflections and strut forces for excavation in weak clay. Additionally, use of a thicker layer of jet grout slab without the jet grout pile system is also studied. The feasibility of reducing one level of struts for sake of construction time and costs while adjusting the jet grout piles’ layout to maintain the stability is also discussed.

Adopting Numerical Models for Prediction of Ground Movements Induced by Deep Excavation

Control of ground surface settlement induced by deep excavation is of major concern in order to attain safety of adjacent structures and utilities against excessive or differential settlements. Accurate prediction of ground surface movements is an important design criterion in the analysis and design of excavation supporting systems. Many codes of practice are based on a design criterion that satisfies a factor of safety preventing collapse of the system and its surrounding soil. In this research, finite element modeling is adopted to numerically simulate the performance of deep excavation systems and the associated ground movements. The soil behavior was simulated using two types of models; the Mohr-Coulomb model (MC) and the Hardening Soil Model (HS). Field data from monitoring a real deep excavation case history of a retaining system was considered to check the validity of the proposed numerical modeling. A simpler equivalent section replacing the multi-layered soil profile was verified. Then, a sensitivity study has been conducted to study the influence of major parameters that affect ground movements induced by deep excavation. The results of the parametric study were accomplished to construct design charts and drive empirical equations by implementing a design parameter, called the "Stiffness Ratio (R)”, that represents the supporting system stiffness. From these suggested charts and equations, the percentage of maximum vertical ground movements to wall height can be estimated.

Bayesian model and parameter calibration for braced excavations in soft clays

This article presents a Bayesian approach for simultaneously calibrating models and soil parameters using field data on wall deflections and ground surface settlement from braced excavations. This approach is developed based on the Metropolis–Hastings algorithm, the Gibbs algorithm, and hybrid Markov chain Monte Carlo simulation. The Formosa excavation case history is adopted to demonstrate this approach. The predictions are made by using both calibrated models and soil parameters, and the predicted excavation-induced responses from the models are shown to have greater accuracy and less variability than the responses obtained when updating the soil parameters only. This Bayesian approach is effective given the availability of reasonably estimated prior distributions and field observation data with good quality.

An efficient inverse analysis procedure for braced excavations considering three-dimensional effects

In the literature, the plane-strain assumption usually forms the basis of inverse analyses of excavations, mainly due to the high computational cost of 3D analyses. In this paper, a procedure that quantifies three-dimensional effects of excavation is proposed. Such three-dimensional effects are represented as an uncertainty term that corrects plane-strain-based predictions, thereby offering an alternative approach to perform 3D inverse analysis without excessive computations. The results of an inverse analysis performed on an excavation case history show that (i) the procedure is able to quantify three-dimensional effects reasonably well and (ii) improved predictions of excavation field responses can be obtained.

A numerical study on the influence of anchorage failure for a deep excavation retained by anchored pile walls

This article aims to study the mechanical behavior of a deep excavation occurring in sandy soils retained by anchored pile walls under anchorage failure conditions. The primary causes and types of anchorage failures are summarized. Based on an excavation case history, a numerical investigation is conducted using a PLAXIS-based finite element method to reveal the crucial mechanical behaviors of the retaining structure and soil layers as a result of different types of anchorage failures. According to the simulation results, the responses to anchorage failures are analyzed, and the safety coefficient for the retaining structure is presented. The analysis for this numerical work indicates that dangerous failure positions appear at the top of the pile wall or in the region near the excavation bottom. Then, a three-dimensional numerical model is established to analyze the mechanical behaviors and the influence scopes of the retaining structure caused by individual anchor failures. Special emphasis is placed on the analysis of the bending moments and horizontal displacements in the wall, as well as on the prestress and internal forces in each of the row anchors, displacements, and plastic strains in the ground and stabilities of the deep excavation. Conclusions obtained from the numerical study can provide guidelines for good practices in the construction of anchored pile wall-retained excavations in sandy soil.

Multivariate Adaptive Regression Splines Approach to Estimate Lateral Wall Deflection Profiles Caused by Braced Excavations in Clays

Based on a database including a total of 30 case histories for braced excavation in stiff, medium and soft clays, a multivariate adaptive regression splines (MARS) approach for estimating wall deflection profile caused by deep braced excavations is presented in this study. For each soil type, ten case histories with information on subsurface soil conditions, geometry characteristics, excavation support system details, the maximum wall deflections and the wall deflection profile against embedded depth are provided. Seven input variables, including wall length, excavation depth, excavation length, system stiffness, average unit weight and undrained shear strength of the soil, and the depth below the ground surface, are adopted as inputs to the MARS deflection profile model. Comparison with three more excavation case histories indicates that the developed MARS model can give an accurate graphical representation of the wall deflection profile. It is capable of not only predicting the value of maximum wall deflection but also estimating the possible depth at which maximum lateral deformation occurs.

Design Analysis and Observed Performance of a Tieback Anchored Pile Wall in Sand

This paper aims to study the design process and service performance of a deep excavation supported by tieback anchored pile walls. The design procedure and design approaches for deep excavation in China are described. Based on the excavation case history for Shenyang, China, design results obtained using the elastic method and the finite element method (FEM) are compared and analyzed. Special emphasis is given to the analysis of horizontal wall deformations, internal forces in the wall, earth pressures on the wall, ground surface settlements, and stabilities of the excavation. The similarities and differences between the Chinese code (JGJ 120-2012) and the European code (EN 1997-1) for the design of geotechnical structures are presented based on a design example. Through the comparison, it is indicated that the Chinese code focuses on the design result, while the European code focuses on the design process. The crucial construction methods for reducing construction risk based on the excavation case history are described. The mechanical behaviors of the excavation retained by an anchored pile wall were investigated by analyzing observed field cases. The results provide good, practical guidelines for the design and construction of a tieback anchored pile wall retained excavation in sandy soil.

Application of an Enhanced Anisotropic Bounding Surface Model in Simulating Deep Excavations in Clays

AbstractFinite-element simulations are conducted to assess the capability of an anisotropic bounding surface model in simulating ground response due to deep excavations. The model assumes a nonassociative flow rule that allows for the simulation of not only strain-hardening, but also strain-softening response. It can be degenerated into anisotropic and isotropic forms of the bounding surface model employing associative flow. After integrating the model into a commercial finite-element code, two deep excavation case histories were simulated. The simulation results are compared with published field measurements and with those obtained using the anisotropic Sekiguchi-Ohta and MIT-E3 models. Satisfactory agreement between finite-element simulations and field measurements in terms of lateral wall deflections and ground surface settlements was realized. The difference in results between the nonassociative and associative versions of the anisotropic bounding surface model was found to be rather small given the f...

Finite-element analysis of a deep excavation case history

The design of deep excavations requires careful consideration of the influence of various soil/structure interaction mechanisms and detailed issues relating to the construction processes and the mechanics of the soil. Finite-element analysis provides a useful design tool for deep excavations, but care needs to be taken to ensure that an appropriate level of detail is included in the model. This paper describes a three-dimensional finite-element analysis of a deep excavation supported by a diaphragm wall, recently constructed in Shanghai. The principal purpose of the study is to investigate the level of detail that is required in the finite-element model to obtain results that provide a realistic representation of the wall and ground movements measured during the construction process. Studies are conducted on (a) the influence of the soil constitutive model on the quality of the results; (b) procedures to model the effect of post-cure shrinkage in the concrete floor slabs; (c) procedures to model the construction joints in the diaphragm wall; (d) the relative merits of using shell and solid elements to model the diaphragm wall; and (e) the sensitivity of the analysis to the assumed initial horizontal stresses in the soil.

Efficiency of excavations with buttress walls in reducing the deflection of the diaphragm wall

Installation of buttress walls against diaphragm walls has been used as an alternative measure for the protection of adjacent buildings during excavation, but their mechanism in reducing movements has not yet been fully understood. This study performs three-dimensional finite element analyses of two excavation case histories, one in clay with T-shape buttress walls and another in dominant sand with rectangular buttress walls, to establish analysis model. Then, a series of parametric study were performed by varying soil types, types and length of buttress walls based on the above-mentioned excavations. Results show that the mechanism of buttress walls in reducing wall deflections mainly came from the frictional resistance between the side surface of buttress wall and adjacent soil rather than from the combined bending stiffness from diaphragm and buttress walls. The buttress wall with a length <2.0 m had a poor effect in reducing the wall deflection because the soil adjacent to the buttress wall had almost the same amount of movement as the buttress wall, causing the frictional resistance little mobilized. Since the frictional resistance of buttress walls in a deep excavation has fully been mobilized prior to the final excavation depth, the efficiency of buttress walls in reducing the wall deflection in a deep excavation was much less than that in a shallow excavation. Rectangular shape of buttress walls was of a better effect than T-shape in the shallow excavation because frictional resistance between buttress walls and adjacent soil played a major role in reducing the wall deflection rather than bearing resistance of the flange. When the excavation went deeper, the difference in reducing the wall deflection between the R-shape and T-shape became small.

Finite-element analysis of a deep excavation case history

Finite-element analysis of a deep excavation case history

Updating Soil Parameters Using Spreadsheet Method for Predicting Wall Deflections in Braced Excavations

A back analysis procedure implemented in a spreadsheet environment that uses construction monitoring data for supported excavation systems to continuously recalibrate the soil parameters and thus enable updating of the lateral wall deflection predictions is presented. With this framework, the observed maximum wall deflections are used to update the soil parameters through a recently developed semi-empirical polynomial regression model. This paper demonstrates that the updated soil parameters recalibrated from the current stage of excavation can be used to predict the maximum wall deflection in the subsequent stages of excavation. The proposed framework is computationally efficient and can easily be implemented in a spreadsheet. The feasibility and effectiveness of the proposed methodology is illustrated through seven well-documented excavation case histories. The results of the case histories show that the updated soil parameters improve the accuracy of estimations of the wall deflections in braced excavations.

A comprehensive evaluation of strength and modulus parameters of a gravelly cobble deposit for deep excavation analysis

This study presents results of numerical back analysis for estimation of strength and modulus parameters of gravelly cobble deposit through various laboratory and field tests. A non-linear and stress dependent stiffness soil model implemented in the finite element software was used to simulate four conventional triaxial compressive (CTS) tests, two pure shear triaxial (PS) tests, one field plate load (PL) test, one field direct shear (FDS) test, and one natural slope stability (NSL) analysis. The internal friction angle ϕ and stress dependent stiffness modulus E50ref back analyzed from those various tests were summarized and evaluated considering the load conditions and stress states. Results show that the strength parameter ϕ tends to decrease linearly with an increase of the minor principal stress σ3′ when they were plotted in a semi-logarithmic graph. The stiffness moduli evaluated from the back analysis, cyclic triaxial test, and downhole test are compared and discussed. In addition, results of numerical analysis of the recent excavation case history in Taichung gravelly cobble deposit are presented to verify the summarized parameters, which are selected with respect to the in-situ stress state.

PROBABILITY-BASED SERVICEABILITY EVALUATION OF BUILDINGS ADJACENT TO AN EXCAVATION USING RANDOM FINITE ELEMENT METHOD

Excavation design is usually controlled by the serviceability-based analysis of buildings adjacent to the excavation site. In practice, it is not easy to implement the serviceability-based analysis due to the uncertainty of many factors affecting excavation behavior. It is desirable to incorporate the probability analysis into the procedure to deal with the effects of the uncertainty in an excavation problem. In this present study, the Monte Carlo simulation is equipped into the existing finite element method code, which is capable of representing the small-strain stress–strain characteristics of soils, to form the random finite element method (RFEM). Then, the RFEM is employed to perform the probability-based serviceability evaluation of buildings adjacent to an excavation on a quality excavation case history. A flow chart for RFEM-based evaluation of excavation-induced damage to buildings is established. In addition, a simplified method, called the KJHH method, is also adopted to conduct the probability-based analysis on the issue of building damage potential. Three scenarios were planned to compare the performance of the RFEM and the KJHH methods. The results reveal that the developed RFEM is validated to be a useful tool to consider the uncertainty of input parameters in the evaluation of the damage potential of buildings adjacent to excavation.