Diaphragm Wall Panels Research Articles

Identification of defects in underground structures using machine learning aided distributed fiber optic sensing

Despite the extensive use of distributed fiber optic sensing (DFOS) in monitoring underground structures, its potential in detecting structural anomalies, such as cracks and cavities, is still not fully understood. To contribute to the identification of defects in underground structures, this study conducted a four-point bending test of a reinforced concrete (RC) beam and uniaxial loading tests of an RC specimen with local cavities. The experimental results revealed the disparity in DFOS strain spike profiles between these two structural anomalies. The effectiveness of DFOS in the quantification of crack opening displacement (COD) was also demonstrated, even in cases where perfect bonding was not achievable between the cable and structures. In addition, DFOS strain spikes observed in two diaphragm wall panels of a twin circular shaft were also reported. The most probable cause of those spikes was identified as the mechanical behavior associated with local concrete contamination. With the utilization of the strain profiles obtained from laboratory tests and field monitoring, three types of multi-classifiers, based on support vector machine (SVM), random forest (RF), and backpropagation neural network (BP), were employed to classify strain profiles, including crack-induced spikes, non-crack-induced spikes, and non-spike strain profiles. Among these classifiers, the SVM-based classifier exhibited superior performance in terms of accuracy and model robustness. This finding suggests that the SVM-based classifier holds promise as a potential solution for the automatic detection and classification of defects in underground structures during long-term monitoring.

Providing stability to a highway cut in a landslide area with the use of diaphragm wall panels

The route of the D1 highway in the Lietavská Lúčka - Dubná Skala section near Žilina is taken forward by a 17 m deep cut in a section about 300 m long. The first proposed solution of the highway route was based on the assumption that it is a stable area. However, during the construction of the highway cut an old landslide area with a depth of shear surfaces of up to 25 m was identified right in the area of the cut. Therefore, massive measures had to be designated to assure that the area above the highway and the access to the tunnel portal were not threatened by landslides. When addressing the security of the cut, an unusual solution was used - anchored diaphragm wall panels of underground concrete walling oriented towards the slope. Currently, this given solution is under construction and is controlled by inclinometers incorporated into selected diaphragm wall panels.

Bentonite and polymeric support fluids used for stabilization in excavations

Bentonite is a natural and finite mineral resource. Dilute suspensions of sodium montmorillonite clay in water represents bentonite slurries. Suspension and orientation of colloidal clay particles define rheological properties in bentonite slurry (BS). The BS has been used about seventy years to temporarily support the excavations. More recently, polymer support fluids (PSF) gained much popularity and are widely used compared to bentonite support fluids. The PSF are categorized into natural (pure) and synthetic polymers. Physico-chemical properties of PSF are different than BS irrespective of the quite similarity in the mode of action. Synthetic polymer fluids are molecularly engineered fluids that can be a popular alternative of conventional BS deployed as excavation support fluids in different foundation applications such as diaphragm wall panels and pile bores. The synthetically engineered fluids of polymers (water-soluble) are different from conventional BS. The PSF offer additional benefits because their use is cost effective, eco-friendly, and these polymers need smaller site footprint as well as easy preparation, mixing, handling, management and ultimately the final disposal. Nevertheless, synthetic polymers have advantage over bentonite, however, foundation engineers and scientists have also certain concerns about their use because of their performance related issues. For an efficient use of polymers, specific properties and in situ behavior of polymers as well as their sorption onto the soils must be recognized because the polymer concentration in the solution is decreased with time during their use. The present manuscript reviewed the relative performance of excavation support fluids and displayed an arranged marriage of physicochemical and rhelogical properties of natural and synthetic excavation support fluids used in the foundation industry. This information will be highly useful to scientific community for their future ventures and will lay a foundation to understand the mechanisms of stabilization in open and deep excavations.

Finite element investigation of ground response during diaphragm wall panel installation

The construction of diaphragm wall panels inevitably changes the initial stress condition and causes movements in the surrounding soil mass. The ground response is affected by a number of aspects including the ground condition and construction procedure, thus creating complexities and uncertainties to explain the field observation and predict the potential influence on adjacent assets. This paper analyzes observed data from field and model tests, and identifies important aspects which may affect the ground response during the diaphragm wall panel installation process. These aspects mainly include (i) construction details in the trench excavation, (ii) concreting characteristics, (iii) initial ground conditions, and (iv) the panel width. Finite element analyses are then conducted to understand their impact on the ground response in a systematic manner. The analysis procedure follows some typical test programmes reported in the literature, to reproduce the observed phenomenon and explain the mechanisms more consistently. This study demonstrates some key issues that should be considered carefully in practice to mitigate risks in the diaphragm wall installation process.

Catastrophic Failure of Shanghai Metro Line 4 in July 2003: Postaccident Rehabilitation

The destructive failure of Shanghai Metro line 4 across underneath the Huangpu River on July 1, 2003, led to massive ground subsidence and collapse of pre-existing structures in the proximity; additionally, the failure generated large amounts of human-made rigid obstructions in the strata, which made rehabilitation of the collapsed metro line be very technically challenging. By extensive comparisons in terms of technical difficulties, economic costs, and construction durations, a plan of reconstruction of the collapsed metro line at its original location rather than relocation was selected finally for the postfailure rehabilitation. The majority of the failed metro line was replaced by a 262-m-long×23.7-m-wide×38.5–41.2-m-deep cut-and-cover tunnel bottoming out in the confined aquifer, whose two ends were connected with the existing intact metro line via two 10-m-long tunnels mined within the aquifer by the ground-freezing method. Excavation of the cut-and-cover tunnel was retained by 1.2-m-thick and 65-m-high perimeter diaphragm wall panels braced by 9–10 levels of steel-reinforced concrete struts. To ensure the project safety and mitigate potential adverse influences of field works on the urban environments, extensive innovative construction plans, measures, and techniques were designed and then implemented into the reconstruction, e.g., a lightweight cofferdam and pile-supported steel platform for diaphragm walling through the existing metro line underneath the river; unique protective solutions and construction techniques for execution of vertical and horizontal ground-freezing programs for diaphragm walling and mining; special procedures of cleaning up the existing tunnels previously filled with water, sand, and cement; innovative joints and grouting measures adopted between the reconstructed tunnel and the pre-existing intact metro line, and so on. Furthermore, the initial designs were optimized and adjusted with the progress of field construction works. Lastly, the long-term field performance data demonstrated the success of this rehabilitation project, which affected the urban environments rarely. The construction plans, measures, and techniques outlined in this paper are practical, useful references for professionals worldwide to deal with similar underground projects in the future.

Centrifugal Test Replicated Numerical Model Updating for 3D Strutted Deep Excavation with the Response-Surface Method

Centrifugal tests provide an efficacious experimental process to predict the behavior of deep excavations, and numerical models are indispensable for demonstrating the test results and analyzing the engineering demand parameters. Uncertainty in material properties can cause simulations to differ from tests; therefore, updating the model becomes inevitable. This study presents a response-surface-based model updating technique for the nonlinear three-dimensional simulation of the centrifugal testing model of strutted deep excavation in sand. An overview of the fundamentals of the response-surface model is provided, including selecting uncertain parameters as input factors, creating a design order for training the model, building a second-order polynomial surface, and updating the input factors through targeted centrifugal results. The bending strains of diaphragm wall panels at multiple points along the depth are used to form the multiobjective function. Response-surface model predictions were well-matched with actual numerical responses, with less than a 0.5% difference. Parametric analyses could be conducted utilizing this updated strutted deep excavation model.

Effect of concreting pressure on ground response caused by installation of diaphragm wall panels

The ground response during diaphragm wall panel installation is strongly affected by the support slurry pressure, and by the subsequent lateral pressures during the tremie placement and set-up of wet concrete. The support slurry pressure can be reliably assumed as hydrostatic within the excavated trench, whereas the lateral concreting pressure is much more complex due to the stiffening effect of wet concrete. This paper reviews the measurements of wet concrete pressures from a series of instrumented diaphragm wall panels which show that lateral pressures in the concrete slurry increase as the tremie displaces bentonite slurry and reach a maximum condition about 1 h after placement, while conditions over the longer term closely approximate the hydrostatic pressure of the bentonite-filled trench. The maximum increase of pressure varies at different heights of the same trench, and also varies for two separate panels in the same project, reflecting the varying velocity of concreting. Model tests of concreting pressure on formwork also demonstrate that the lateral pressure reduces from hydrostatic wet concrete pressure a few hours after casting. A time-dependent lateral concreting pressure model is developed to represent these measurements more realistically. This proposed model is implemented into a finite-element program to investigate its effects on the ground response due to the concreting process. Computed results from a reference case can capture the progressive variations of ground movement, earth and pore water pressure in the soil adjacent to the trench during and shortly after the concreting process, which is consistent with the reported measurements.

Mindlin solution on ground deformation caused by the trench excavation during installation of concrete diaphragm wall panels

Concrete diaphragm walls (CDWs) are widely used as support of deep excavation in soft ground in urban areas. Ground deformation occurs during the excavation of trenches for the installation of the CDW panels due to ground stress release. This paper investigates the ground deformation during slurry trenching using Mindlin solution. The pressure of slurry used to protect the trench stability during excavation is simulated as a triangularly distributed load on the trench walls, the soil stress state is solved using Mindlin solution, and the horizontal and vertical ground displacement are obtained through integral transformation of ground strain. The rationality of the solution is verified through comparison between the analytical solution and field measurement. Sensitivity analyses are performed on soil elastic modulus, Poisson's ratio, trench excavation depth and panel length. Simplified formulation is proposed for the prediction of horizontal ground deformation. The impact of excavation stages and adjacent panels on the ground deformation is explored. The study finds that the Mindlin solution is able to provide an approximate solution on the ground deformation during slurry trenching, and the simplified formulation can be used to provide a fast estimate of the horizontal ground movement in engineering practice.

Application of Fiber Bragg Grating Sensor Technology to Leak Detection and Monitoring in Diaphragm Wall Joints: A Field Study.

Joints between diaphragm wall panels are weak spots in wall construction. It is essential that potential leak sites are detected prior to excavation. In this study, a novel leak detection and monitoring system is presented that is based on fiber Bragg grating (FBG) sensing technology. A field study was performed in a deep excavation supported by diaphragm walls (in Hohhot, China) to validate the feasibility and effectiveness of the proposed method. Two schemes were trialed; one using pipes made of stainless steel, and one used a pipeless method. The results of the field study are presented and discussed. They show that potential leak sites in the wall joints could be determined prior to excavation using the proposed detection method. Stainless steel is a good material to use to make the detection tube because it can protect the FBG sensors and heating belts from damage and is more sensitive to water leakage. The field study provides good evidence for the feasibility of the new detection system. It also provides valuable experience for the field application of the system and has generated useful data to use in follow-up work.

Observed performance of three adjacent 48 m depth concrete diaphragm wall panels in silty soils

The performance of three adjacent 48 m depth concrete diaphragm wall panels in silty soils is measured throughout the complete process of the wall installation including trench excavation, trench concreting, and concrete hardening. The analysis of the field measurement data shows that the ground movement induced by the wall installation is a function of construction activities. The maximum lateral movement occurs during the trench excavation. The trench concreting reduces the lateral movement. Field measurement at the end of wall installation may not catch the value of the maximum lateral movement. The ground settlement continues to increase during the complete process of panel installation, but with a decreasing rate. Roughly 90% of the settlement occurs during the trench excavation. The ground movement is also closely related to the properties of subsurface strata. The weak superficial fill materials sustain high disturbance, which consequently results in relatively large ground movement. With current engineering means and methods and good construction workmanship, the ground movement can be well controlled. In the current project, the maximum lateral ground movement was controlled within 2.0 cm and the maximum settlement was controlled within 0.05% of trench depth.

Field measurements of disturbance of ultra-deep diaphragm wall construction to weak stratum

This study investigates ground movement based on field instrumentation during the installation of deep concrete diaphragm wall panels for the Suzhou Metro Line 5 Project. The field measurements, including lateral ground movement, ground surface settlement, subsurface settlement, adjacent building settlement, soil and hydrostatic pressure, are conducted during the installation of three adjacent wall panels. The measurement shows that the construction disturbs the ground significantly at shallow depth and generates relatively large lateral movement. The maximum lateral ground movement occurs at the ground surface and the movement decreases along depth. The trench excavation releases ground stress and generates the major portion of the lateral ground movement. The trench concreting reduces the lateral ground movement by compensating the ground stress release. The concrete hardening releases the ground stress again but on a much smaller magnitude and the soil displacement tends to be stabilized. The installation of adjacent panels contributes to the ground movement which occurs along a line perpendicular to the centerline of the previously installed panel, but the contribution is relatively small. The settlement and inclination of surrounding buildings caused by the panel installation are relatively small and controllable.

Research Progress on Stability of Slurry Wall Trench of Underground Diaphragm Wall and Design Method of Slurry Unit Weight

This paper performs an extensive literature survey and example investigation on the stabilisation of slurry wall trenches during the construction of diaphragm wall panel trenches, and the failure modes of slurry wall trench instability, the stability theoretical analysis models and methods, the slurry formation and its protection mechanism, the influence of related factors on slurry wall trench stabilisation, and other related problems are summarized and analyzed emphatically. And then, based on the limit equilibrium analysis method, the mechanical models of the overall stability and local stability of the trench wall are established, respectively, and the design method of slurry unit weight is derived to ensure the stability of the trench wall. Furthermore, an example application shows that the established slurry unit weight design method is reliable. At last, this paper also proposes the focus and direction for follow‐up work, that is, to construct an accurate and effective theoretical analysis model of slurry wall trench instability considering the influence of multiple factors and the calculation method of the slurry cake and its mechanical or mathematical relationship with slurry quality.

Improved characterization of underground structure defects from two-stage Bayesian inversion using crosshole GPR data

Abstract Crosshole ground-penetrating radar (GPR) is a widely used measurement technique to help inspect the structural integrity of man-made underground structures. In a previous paper, we have introduced a Bayesian framework for inversion of crosshole GPR experiments to help back out defects in concrete underground structures. Here, we evaluate the practical usefulness of our inversion framework by application to waveform data from a real-world GPR survey of a diaphragm wall panel with two embedded structure defects. We also use this case study to further refine our methodology by introducing the elements of a two-stage inversion method to help delineate the exact location and shape of small structure defects. Herein, a low-resolution inversion composed of relatively few inversion coefficients (stage-1) is used to determine roughly the presence of structure defects, followed by a second inversion (stage-2) with much enhanced spatial resolution in those areas classified with anomalous or suspicious permittivity values. This two-stage inversion approach uses more wisely CPU-resources by focusing primarily on those areas of the concrete structure that have been classified as anomalies. We investigate the benefits of this two-stage inversion scheme using a synthetic and real-world case study involving waveform data of a diaphragm wall panel measured with crosshole GPR. Our results demonstrate that the proposed two-stage inversion method recovers successfully the location and shape of structure defects, at a computational cost that is considerably lower than the original inversion framework.

Investigation into performance of deep excavation in sand covered karst: A case report

Performing deep excavations in karst regions is likely lead to geo-hazards, e.g. sinkholes, water ingress, and ground surface settlement. This case report presents the performance of a 16-m deep excavation in sand covered karst in Metro Line no. 9, Guangzhou, China. The performances during deep excavation, including lateral wall deflection, vertical wall movement, ground surface settlement, and settlement of adjoining buildings, were monitored and evaluated. Based upon the interpretation of the field measurements, the following major findings were obtained: (i) the lateral wall deflection and surface settlement measurements were smaller those cited in other case studies with similar geological conditions, (ii) both the grouting of the karst cavern and the use of the socket diaphragm wall panel dominated the performance of the deep excavation; (iii) the type of the founding structure could significantly affect the settlement development of the adjoining buildings. This case report provides insight into the design and construction of deep, narrow excavations in sand covered karst in a metropolitan environment.

The influence of trenching diaphragm wall panels on deflection and bending moment of existing piles within piled foundation

Conducting deep excavations for underground construction inside the cities requires diaphragm wall due to space limitation. The ground surface is affected by the trenching process. The construction of the diaphragm walls near deep foundations may not be avoided specially in the crowded cities. This research focuses on the deflection and bending moment of piled foundations near slurry trench. Since the trench is considered as a three-dimensional problem, a three-dimensional numerical analysis was used in this research. It was conducted using a commercial analysis software known as FLAC 3D. The numerical analysis method was verified using two case histories, one in Hong Kong and another one in Giza, Egypt. The results from the numerical analysis were in quite a good contrast with the field data results. This indicates that the trenching process could be numerically modeled with the proposed method and provides good results. The verified numerical analysis method was used to conduct a numerical parametric sric study that discusses the effect of multiple panels on adjacent connected pile groups or piled raft foundation. The parametric study showed that the pile deflection and bending moment are affected by the panel construction stages, pile location within the foundation. The pile deflection and bending moment are affected by the stages of construction. The piles near the trench are affected by the trenching process, while the piles far from the trench are affected by the dragging force of the pile near the trench.

Diaphragm wall and piles set record in Africa: Foundation for Africa’s largest suspension bridge in Mozambique

As part of the master plan for the infrastructure of the road network in the southern Africa area, a suspension bridge is being built in Maputo, Mozambique. The bridge over the Maputo Bay has an open span of 680 m between pylons and is anchored on the north and south side of the Bay by massive anchor blocks that are built in deep shafts constructed by circular diaphragm walls. With an outer diameter of 50 m and an excavation depth of 15 m and 36 m respectively, these shafts are among the largest presently being built in the world for a suspension bridge. The diaphragm wall panels of the walls reach down to a depth of 56 m into the fine sandstone and siltstone.The pylons and the approach bridges north and south are founded on piles with diameters of 1.50 m to 2.20 m and reach a maximum depth of 110 m. Construction material has been carefully selected and the use of local material as well as approved imported material and products to guarantee a 16-hour retarding time needed for the concreting procedure of the foundation elements.

Shear bearing of cross-plate joints between diaphragm wall panels – II: numerical analysis and prediction formula

In this, the second part of two companion papers, three-dimensional finite-element modelling of the shear load tests is presented and the shear bearing behaviour of the cross-plate joints is predicted. The isotropic damage plasticity model is adopted to model the non-linear behaviour of concrete and the interface, and the quasi-static load test process is approximately simulated with an explicit technique employing an artificial timestep to decrease the kinetic energy. The numerical analysis method is verified using the experimental results presented in the first part of the two companion papers, and the shear behaviour and shear bearing capacity of the cross-plate joints are evaluated numerically. The numerical results and parametric study reveal the load-bearing mechanism and failure modes of the joints. By introducing a correction coefficient considering the effect of the hole ratio in the longitudinal plate and based on the analytical expression for the ultimate shear load of perfobond connectors, a new and modified prediction formula for the shear bearing capacity of cross-plate joints is proposed. The close correlation of the prediction formula with the numerical and experimental results demonstrates that the proposed formula is reliable for predicting the ultimate shear load of cross-plate joints.

Shear bearing of cross-plate joints between diaphragm wall panels – I: model tests and shear behaviour

This paper, the first part of two companion papers, describes a test programme involving seven specimens and experiments to investigate the effect of three main design parameters of longitudinal steel plates in cross-plate joints on the shear capacity. The load–displacement relationship, cracking of the concrete and strain of the longitudinal steel plate were experimentally measured. It was found that the failure modes of the specimens can be categorised into two types – concrete crushing and steel plate yielding. The failure mode is mainly affected by the relative stiffness of the cross-plate joints, while the ultimate shear load is primarily influenced by the number and area ratio of holes in the longitudinal plates. The paper shows that the available analytical expression for the shear capacity of rib shear connectors cannot be readily applied to cross-plate joints, and therefore further investigations should be conducted to develop a new shear capacity prediction formula for cross-plate joints. This experimental study on the performance of concrete diaphragm walls with cross-plate joints sheds light on the shear failure behaviour of diagram walls and provides some benchmark data for numerical analysis and development of empirical design formulae.

Covered Semi-Top-Down Excavation of Subway Station Surrounded by Closely Spaced Buildings in Downtown Shanghai: Building Response

AbstractBased on field data, this study investigates responses of six pre-existing buildings to an adjacent 24.8–25.2 m deep subway station excavated by the covered semi-top-down method. During excavation of 1 m wide and 50.2–50.5 m deep slurry trenches for diaphragm wall panels (i.e., diaphragm walling), buildings on both shallow and deep foundations developed noticeable settlements up to 15 mm. Subsequent excavation of the upper 15.9–17.7 m thick soils inside the pit only incurred limited building settlements less than 10 mm, which contrasted sharply with remarkable displacements of the diaphragm wall and ground nearby. As excavation continued to the final level, buildings on strip footing, stiffened raft foundation, strip footing atop long piles, and short piled raft foundation settled dramatically up to 40 mm, but those on long piled raft foundation still were hardly displaced. Like diaphragm wall and ground, all buildings developed significant postexcavation settlements up to 34.5 mm. Eventually, int...

Numerical modelling of a deep circular excavation at Abbey Mills in London

For several forthcoming tunnelling projects circular shafts will need to be constructed in London. Better understanding of their behaviour could result in more reliable movement predictions and efficient designs. A previous large-scale monitoring scheme of Thames Water's Abbey Mills shaft provided measurements of diaphragm wall deflections and bending moments. In this paper, the results of a parametric study in Flac2D are compared with the field data to understand the shaft's performance during construction. Very small wall movements are calculated and are confirmed by the inclinometer data. Previous researchers have proposed that the joints between the diaphragm wall panels reduce the circumferential wall stiffness. Out of several simulations with reduced hoop stiffness, the results obtained from the isotropic wall, that is, with strong joints, give closest agreement with the field measurements. In the initial design, the Chalk was allowed to yield below excavation level, but the field data indicate elastic behaviour. The elastic Chalk caused the wall to bend more sharply at the excavation level and hence bending moments exceeded the design predictions. Further simulations show that the wished-in-place assumption with a K0 of 1·0 in the Chalk was appropriate, and that, when the wall installation is modelled, the applied bentonite and wet concrete pressures during the construction process govern the performance of the shaft wall. These findings provide valuable information for the construction of future shafts in a similar geological setting.