Braced Excavation Research Articles

Mechanical Behaviors of a Symmetrical Bolt Fasten Wedge Active Joint for Braced Excavations

Joint-failure of steel tube bracing structures usually causes an excavation accident. A symmetrical bolt fasten wedge (BFW) active joint has been innovatively developed in this paper, which overcomes the shortcomings of the traditional steel wedge (SW) active joint. Three full-scale BFW active joint specimens were manufactured for field operation tests. The test results show that the proposed active joint had a good construction feasibility and a wide applicability. After the field operation tests, axial compression experiments were implemented. Meanwhile, a series of numerical simulations were performed. The numerical results agreed with that of the experiments. Both showed that the working principle and mechanical principle of the BFW active joint was reasonable and ingenious; it had outstanding mechanical properties. Overall, the BFW active joint had the advantages of structure and mechanical properties, and it is suitable for popularization and application.

3D numerical analysis of passive pile groups adjacent to deep braced excavation in soft clay

3D numerical analysis of passive pile groups adjacent to deep braced excavation in soft clay

Numerical analyses of an experimental excavation supported by panels of lime-cement columns

The influence of ground improvement with panels of overlapping lime-cement columns on the behavior of a braced excavation loaded to failure has been investigated using 3D numerical analyses and the results are compared with an experimental full-scale failure test. The analyses reveal that stress-induced strength anisotropy of lime-cement improved clay needs to be considered when the stress path for the actual field conditions differs from that in conventional laboratory testing. In addition to strength parameters, the modulus of deformation that is consistent with the actual encountered stress path is also needed for reasonable predictions.

A new empirical approach to estimate temperature effects on strut loads in braced excavation

In deep excavation designs, strut loads play a key role to ensure excavation safety. During the construction, temperature fluctuation inevitably leads to a variation in strut loads. Therefore, how to quantitatively estimate the effects of temperature on strut loads is a matter of concern. In this study, the incremental changes in wall deflection due to temperature fluctuation were assumed to be piecewise linear. Based on the beam-on-elastic-foundation (BEF) model, an empirical approach accounting for the variation in temperature-induced strut loads at all levels was established. This model was further calibrated against a reported case study for a more precise predictive performance.

Estimation of failure probability in braced excavation using Bayesian networks with integrated model updating

A probabilistic model is proposed that uses observation data to estimate failure probabilities during excavations. The model integrates a Bayesian network and distanced-based Bayesian model updating. In the network, the movement of a retaining wall is selected as the indicator of failure, and the observed ground surface settlement is used to update the soil parameters. The responses of wall deflection and ground surface settlement are accurately predicted using finite element analysis. An artificial neural network is employed to construct the response surface relationship using the aforementioned input factors. The proposed model effectively estimates the uncertainty of influential factors. A case study of a braced excavation is presented to demonstrate the feasibility of the proposed approach. The update results facilitate accurate estimates according to the target value, from which the corresponding probabilities of failure are obtained. The proposed model enables failure probabilities to be determined with real-time result updating.

Bayesian updating of subsurface spatial variability for improved prediction of braced excavation response

This paper introduces an approach that utilizes field measurements to update the parameters characterizing spatial variability of soil properties and model bias, leading to refined predictions for subsequent construction stages. It incorporates random field simulations and a surrogate modeling technique into the Bayesian updating framework, while the spatial and stage-dependent correlations of model bias can also be considered. The approach is illustrated using two cases of multi-stage braced excavations, one being a hypothetical scenario and the other from a case study in Hong Kong. Making use of all the deflection measurements along an inclinometer, the principal components of the random field and model bias factors can be updated efficiently as the instrumentation data become available. These various sources of uncertainty do not only cause discrepancies between prior predictions and actual performance, but can also lead to response mechanisms that cannot be captured by deterministic approaches, such as distortion of the wall along the longitudinal direction of the excavation. The proposed approach addresses these issues in an efficient manner, producing prediction intervals that reasonably encapsulate the response uncertainty as shown in the two cases. The capability to continuously refine the response estimates and prediction intervals can help support the decision-making process as the construction progresses.

Reliability-based analysis of braced excavation in unsaturated soils considering conditional spatial variability

This study presents a practical approach for reliability analysis of braced excavation in spatially varied unsaturated soils. To take full advantage of the measured data, random fields are conditionally simulated and implemented in a finite element analysis. In this approach, the uncertainties involved in the soil-structure interaction are considered by performing the analysis for perfectly rough and perfectly smooth interfaces leading to upper and lower bounds on the full range of solutions. To assess the influence of the unsaturated condition on the braced retaining system (i.e., wall and struts), the probabilistic analysis of excavation is carried out with and without considering suction. The results demonstrate that applying the unsaturated state not only causes significant changes in the statistical parameters of wall responses; it also moves the location where the maximum values occurred.

System reliability assessment on deep braced excavation adjacent to an existing upper slope in mountainous terrain: a case study

Due to rapid urbanization, the land resources available for construction are becoming increasingly scarce in many built-up environments, especially for infrastructure development in mountainous terrain. For deep excavations developed for basements of high-rise buildings in cities with mountainous terrains, the excavation activities may have an influence on the stability or performance of existing nearby upper slopes. Based on a case study in Chongqing, this paper numerically investigates the effects of the excavation geometry, the retaining wall system stiffness and the distance between the excavation and slope on the performances including the global factor of safety and retaining wall deflection of the excavation-slope system. Subsequently, simplified ultimate and serviceability limit state response surface models have been developed and implemented into the First-Order Reliability Method to determine the probability that the ultimate or serviceability limit state is exceeded by performing probabilistic analysis on the global factor of safety through setting the threshold maximum wall deflection as an optimization constraint.

Displacement and force analyses of braced structure of deep excavation considering unsymmetrical surcharge effect

This paper describes the displacement and force behavior of a braced deep excavation under unsymmetrical surcharge effect. The horizontal displacements and bending moments in diaphragm walls, bending moments and axial forces in amplitude modulation (AM) piles, and lateral ground movements were analyzed employing the finite difference (FD) method. The FD method was validated by yielding wall displacements and bending moments matching closely the results of the beam on elastic foundation method and the field monitoring data. It was found that the surcharge effect was detrimental to the wall and pile responses and the excavation stability. The wall displacements reached maxima at depths slightly above the final excavation bottom due to the principal stress rotation resulting from the surcharge effect. The maximum wall bending moments developed at a depth which was independent of the final excavation depth. The depths at which the maximum axial pile forces developed increased with progressing the stage of excavation. The lateral ground movements were deteriorated by the unsymmetrical surcharge load, guiding the design and optimization of the excavation bracing system for minimizing the adverse effects on the adjacent structures.

Behaviors of Wall and Ground due to T-shaped Excavation

Extensive monitoring was performed on a T-shaped site with a length of 280 m, a width of 16 m, and a maximal depth of 20.4 m. The braced excavation was performed under retaining comprised of cast-in-situ bored piles and jet grouting piles. The field databased wall performance and its influences to ground were assessed by detailed comparisons with that in other excavations. It is showed that the wall deflections, ground movements and bracing forces all exhibited a typical camelback-shaped characteristic along the length direction, and a maximum 100% reduction of which was observed due to the jet grouting. The rebar near sections with varied excavation depths was in a tension state both on the excavation and the retained sides due to the two-dimensional unbalanced force, and the corresponding bracing force was small. The maximum wall deflection δhm decreased as the partitioning excavation moved horizontally, and it was less than 0.04% of the final horizontal excavation length Le. The normalized Fσmax approximately approached to be identical with the normalized Fbmax, while most of the occurring depths for Fσmax were greater than that for Fbmax.

Estimation of strut forces for braced excavation in granular soils from numerical analysis and case histories

Construction work of many projects in urban areas requires deep braced excavations, under which circumstances, appropriate design method to assure both the safety and economy of the retaining walls and supporting systems is therefore of paramount importance. For multi-propped retaining walls, Apparent Pressure Diagram (APD) method developed by Peck (1969) is commonly adopted to determine the forces acting on the struts. Unfortunately, most previous studies focused on the performance of braced excavations in soft to stiff clays, where limited publications involve braced excavations in dense sand and gravel with medium rigid walls. In this study, both 2D and 3D finite element analyses were carried out to examine the strut forces for braced excavations in granular soils. Subsequently, based on the numerical results as well as the field measurements from a number of reported case histories, empirical charts have been proposed to determine strut loads for deep excavations in granular soils with medium rigid walls.

Numerical investigation of pile responses caused by adjacent braced excavation in soft clays

ABSTRACT The pile responses to soil movements induced by adjacent braced excavation have attracted increasing attention from geotechnical design engineers. The lateral displacement of soil caused by deep braced excavation could produce significant effect on adjacent piles according to field measurement data. In this paper, the finite element (FE) analyses considering soil of the hardening small strain (HSS) constitutive model were carried out to establish the excavation-induced pile behaviours by varying the depth of excavation, pile diameter, pile length, pile distance away from the excavation, pile-head fixity, unsupported depth of excavation and axial loadings exerted on the pile head. A set of design charts is developed to estimate the behaviours of piles subjected to adjacent braced excavation in soft clay, targeting to provide design guidelines for the lateral wall deflection assessment of pile foundations and stability evaluation.

Back analysis of Budapest Rákóczi square metro4 station deep excavation

AbstractThe construction monitoring of Ráckóczi square metro station of Budapest Metro Line 4 offered a great opportunity to back analyse the braced deep excavation in the building period. Building movements, strut forces and wall displacement measurements have been compared to numerical results derived by PLAXIS using HSsmall soil model. Considering the influence of small strain stiffness of plastic soils, good fit of the measured and calculated movements could be observed and the strut forces could be calculated reasonable. Curve characteristic of surface settlements was explored, as well. Analysis were carried out to observe influence of the following inputs on the numerical results: soil material model; consolidation effects, boundary conditions. A detailed evaluation is given about the results of parametric analysis.

Basal stability analysis of braced excavations with embedded walls in undrained clay using the upper bound theorem

The embedded wall has beneficial effects on the basal stability of braced excavation in clay. Conventional basal stability analysis methods cannot evaluate the lateral resistance afforded by the wall reasonably. This paper presents a new failure mechanism to evaluate the basal stability of excavations with embedded walls in undrained clay based the upper bound theorem. The proposed mechanism consists of a rigid block and three shear zones, and the horizontal reinforcing effects of wall penetration blow the excavation base are considered. The embedded wall is regarded as an elastic beam and deforms consistently with the velocities of adjacent soil in the shear zone. The elastic strain energy stored in the wall is incorporated in the upper bound calculation to increase the stability of excavation. The proposed mechanism has also been extended to the basal stability of excavations in anisotropic and non-homogeneous clay. The applicability of the proposed mechanism was validated by comparison with the results of numerical limit analysis and FEM, as well as five field cases near or at failure. Two failure field cases in anisotropic clay were also studied using the proposed mechanism. The results showed that the proposed method is capable to yield reasonable estimation by using both isotropic and anisotropic shear strength.

Estimation of Ground Movement and Wall Deflection in Braced Excavation by Minimum Potential Energy Approach

Estimation of Ground Movement and Wall Deflection in Braced Excavation by Minimum Potential Energy Approach

Behavior of braced excavation in sand under a seismic condition: experimental and numerical studies

The behavior of braced excavation in dry sand under a seismic condition is investigated in this paper. A series of shake table tests on a reduced scale model of a retaining wall with one level of bracing were conducted to study the effect of different design parameters such as excavation depth, acceleration amplitude and wall stiffness. Numerical analyses using FLAC 2D were also performed considering one level of bracing. The strut forces, lateral displacements and bending moments in the wall at the end of earthquake motion were compared with experimental results. The study showed that in a post-seismic condition, when other factors were constant, lateral displacement, bending moment, strut forces and maximum ground surface displacement increased with excavation depth and the amplitude of base acceleration. The study also showed that as wall stiffness decreased, the lateral displacement of the wall and ground surface displacement increased, but the bending moment of the wall and strut forces decreased. The net earth pressure behind the walls was influenced by excavation depth and the peak acceleration amplitude, but did not change significantly with wall stiffness. Strut force was the least affected parameter when compared with others under a seismic condition.

NUMERICAL SIMULATION OF STAGED BRACED EXCAVATION IN SAND ~ O6 MRT STATION

A numerical model for long-term drained analysis of staged excavation is developed and validated with a case history in this paper. Kaohsiung O6 station in Taiwan is selected for this purpose. A complete soil-structure interaction is considered in the current analysis which involves structural elements for diaphragm wall and strutting with preloading. FISH scripts are used to simulate staged strutted excavation using the finite difference program FLAC. Within the framework of drained analysis for excavation in the sand, dewatering is performed for each stage of the excavation work. Field monitoring data from other published sources are used to compare with the results produced by the numerical model. The main purpose of this paper is to demonstrate the usefulness of the development in assisting design and monitoring construction of deep braced excavation. The developed model can be used to assist future design and construction projects.

A REVISIT TONICOLL HIGHWAY EXCAVATION IN SINGAPORE

This paper demonstrates the use of explicit finite difference program FLAC to numerically simulate staged braced excavation involving various stages of excavation and strutting. The short term undrained condition is considered in the analyses. A FISH script is developed, with automatic mesh generation capacity, to study the behavior of wall deformation and ground settlement during the stages of construction. The numerical model is developed using the case study of Nicoll Highway in Singapore. Both the geotechnical design and the causes of failure of the Nicoll Highway excavation are re-visited through extensive comparisons with existing published data. It is considered as a successful development with the main purpose in mind to develop a numerical model for the analysis of staged excavation in the construction of underground basements. The success of the development is to be used in assisting other design projects in the future.

2D and 3D numerical analysis on strut responses due to one-strut failure

In deep braced excavations, struts and walers play an essential role in the whole supporting system. For multi-level strut systems, accidental strut failure is possible. Once a single strut fails, it is possible for the loads carried from the previous failed strut to be transferred to the adjacent struts and therefore cause one or more struts to fail. Consequently, progressive collapse may occur and cause the whole excavation system to fail. One of the reasons for the Nicoll Highway Collapse was attributed to the failure of the struts and walers. Consequently, for the design of braced excavation systems in Singapore, one of the requirements by the building authorities is to perform one-strut failure analyses, in order to ensure that there is no progressive collapse when one strut was damaged due to a construction accident. Therefore, plane strain 2D and three-dimensional (3D) finite element analyses of one-strut failure of the braced excavation system were carried out in this study to investigate the effects of one-strut failure on the adjacent struts.

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.