Excavation Support System Research Articles

Cause investigation of large deformation of a deep excavation support system subjected to unsymmetrical surface loading

This paper presents a case study to investigate the large deformation of a deep excavation supporting system closed to a railway with high embankment. The in-situ monitoring is used to determine the effects of the unsymmetrical surface loading on the support system. Meanwhile, the optimization of its design schemes is discussed by the finite element method (FEM). The monitoring results present the support system rotates away from the railway side during the excavation process. The deflection of the diaphragm walls significant affects the railway settlement especially when the maximum movement of lateral wall exceeds 21 mm. The observed maximum lateral deflection exceeded 50.9 mm, which caused considerable settlement (106.0 mm) to the nearby railway. The results of the FEM indicate that soil strengthening inside the deep excavation is an optimized scheme. It can reduce 35.8% maximum wall deflection on bias loading side so that largely control the rotate deformation of the support system, and the scheme may also reduce about 70% maximum settlement of the railway bed.

Method for estimating wall deflection of narrow excavations in clay

The support system stiffness is crucial to designing the excavation support system by displacement control, but the current methods of estimating the system stiffness are inapplicable to narrow excavations. The field data from narrow excavations in the same site in Shanghai soft clay were analyzed and it was found that the deformations of these excavations were significantly affected by the width-depth ratio (B/H). 111 numerical simulations were carried out to study the contributions of wall bending stiffness (EI), average vertical support spacing (Sv) and width-depth ratio (B/H) to the support system stiffness. A parameter was proposed to quantify the support system stiffness for narrow excavations. It can consider the coupled effects of EI, Sv and B/H. An equation for the maximum wall deflection was presented based on the parameter and it was applicable to the cases with B/H less than 1.5. The proposed method was verified against the measured data from several narrow excavations in clay and the overall agreement was satisfactory.

Influence of Surcharge Loading, Retained Soil and Restrained Soil on Design of Diaphragm Wall

Many different types of embedded retaining wall are constructed due to the increasing demands. In Yangon, Myanmar is encountered deep excavation problem. Many buildings are damaged due to excavation of adjacent building. Therefore, embedded retaining wall as excavation support system is necessary to be sustainable buildings. There are important that influence of surcharge loading, retained soil and restrained soil on design of embedded retaining wall. In this paper, diaphragm Wall is emphasized and solved using soil structure interaction analysis. Behaviour of diaphragm Wall wall is based on various factors. Consider with natural and increasing of shear strength parameter of retained and restrained soils to get the level of the dredge line is stiff soil and various distances from wall to surcharge. Sites are located in urban setting, there are near building and separately from main structure. This project involves the construction of 5 m depth retaining wall. In case study (A) retained soils is soft, medium (low) clay, restrained soil is mostly cohesive soil. There are medium (low), stiff, medium, hard soil layers. In case studies (B to H) are increasing shear strength parameter of retained soil and restrained soil. All cases are considered with various distances from wall to surcharge. According to the soil conditions and distance from wall to surcharge, Wall depth, horizontal and vertical movement of ground and wall deflection are described. When retained and restrained soil reach stiff condition, then ground movement and wall deflection reach acceptable limit and wall depth become more safe and economical condition.

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.

The deformation of retaining piles and ground surface under various support systems during deep excavation

BackgroundSince the high-rise buildings and deep underground structures dramatically increases in urban area, the decent design and construction of deep excavations is essential before these structures are actually built up. The building area is very susceptible to the changes of their geo-environment, and even minor insecurity of a deep excavation may lead to a catastrophic failure of structures and deformation of the ground. Therefore, the adverse influence of deep excavation on the surrounding environment should be monitored and controlled stringently.ResultsBased on the real case of deep excavation in Xuzhou, China, a FDM numerical model, which was accomplished by FLAC-3D, was developed to evaluate the deformation of retaining piles and ground surface under various excavation support systems. The original pile-anchor support system was simulated as well as the proposed quincunx double-row piles support system. For the original support system, the deformations were discussed by comparing the numerical results with monitoring data. For the proposed support system, orthogonal tests were designed to evaluate the influence of multiple factors on the effectiveness of excavation support. The optimum solution for the proposed support system was obtained through orthogonal tests.ConclusionResults show that the pile space is the primary factor for the excavation-induced deformations, which can be reflected by the lateral displacement of retaining piles and the settlement around the excavation, while the row space has insignificant influence on the deformations. The conclusions of this paper can contribute to the design of similar projects and avoid excessive deformation of the ground during excavation in the future.

An Analysis of Excavation Support Safety Based on Experimental Studies

Abstract The article presents the results of inclinometric measurements and numerical analyses of soldier-pile wall displacements. The excavation under investigation was made in cohesive soils. The measurements were conducted at points located at the edge of the cantilever excavation support system. The displacements of the excavation support observed over the period of three years demonstrated the pattern of steady growth over the first two months, followed by a gradual levelling out to a final plateau. The numerical analyses were conducted based on 3D FEM models. The numerical analysis of the problem comprise calculations of the global structural safety factor depending on the displacement of the chosen points in the lagging and conducted by means of the φ/c reduction procedure. The adopted graphical method of safety estimation is very conservative in the sense that it recognizes stability loss quite early, when one could further load the medium or weaken it by further strength reduction. The values of the Msf factor are relatively high. This is caused by the fact that the structure was designed for excavation twice as deep. Nevertheless, the structure is treated as a temporary one.

Three-Dimensional Analyses of Excavation Support System for the Stata Center Basement on the MIT Campus

The basement of the Stata Center building on the MIT campus required 12.8 m deep excavations covering a large open-plan site and underlain by more than 25 m of lightly overconsolidated Boston Blue Clay. The excavations were supported by a floating, perimeter diaphragm and braced with a system of internal corner struts, rakers, and tieback anchors. The project involved a complex sequence of berms, access ramps, and phased construction of the concrete mat foundation. One of the key goals of the design was to limit ground movements in order to prevent damage to adjacent structures, including the Alumni Pool building, which was founded on shallow caissons and located less than 1.5 m from the south wall. Lateral wall movements and building settlements were closely monitored throughout construction, while photos from a network of webcams located around the open-plan site provide a detailed time history of the construction processes. This paper describes the development of a comprehensive three-dimensional (3D) finite-element (FE) model for the Stata Center basement excavation, which has been enabled by recent advances made available in a FE software package, including efficient multicore iterative solving capabilities, importing of geometric data from computer-aided design files, and use of embedded pile elements to represent tieback anchors. The analyses highlight the effects of the 3D excavation and support geometry on wall and ground movements. The base case results using a simple elasto-plastic Mohr–Coulomb soil model with undrained conditions in the clay are generally in very good agreement with measured performance. The effects of refined constitutive modeling and partial drainage within the clay have a secondary role in numerical predictions for this project.

Prediction and Performance of Deep Excavations for Courthouse Station, Boston

AbstractConstruction of the Silverline Courthouse Station in South Boston involved 18-m-deep excavations at a site underlain by more than 24 m of normally and lightly overconsolidated Boston blue clay (BBC). The excavations were supported by 27-m-deep floating diaphragm wall panels and five levels of preloaded cross-lot bracing. This paper compares the measured performance of the excavation support system with the Class A finite-element (FE) predictions prepared during the original design phase and with the results of Class C analyses using information obtained during construction. The numerical analyses used data from a special test program of laboratory and in situ tests at a nearby site. The analyses represent coupled consolidation within the soil mass and the anisotropic stress-strain-strength properties of BBC using the MIT-E3 soil model. The Class A analyses generally overestimate the lateral wall deflections and underestimate the measured strut loads, as preloading was not included in the original ...

Geomechanical characterization, 3-D optical monitoring and numerical modeling in Kirkgecit-1 tunnel, Turkey

The geomechanical characterization of a twin-tube tunnel during exploration and excavation stages is presented and used as a base (a) to interpret the monitored long-term displacements, (b) to numerically investigate if the displacement pattern at a given section could be predicted using an equivalent continuum model and (c) to assess adequacy of the NATM recommended excavation method and support systems. The pre-excavation rock mass classification by RMR and Q systems utilizing logs of two exploration boreholes lead to reasonably similar predictions of the as-built rock mass quality.The resultant displacement magnitudes and angles are interpreted with reference to the as-built geological cross-sections of the tunnel. All displacement patterns monitored at five posts in each section are consistently effective in providing advance warning for the presence of zones of contrasting conditions despite very low magnitudes of displacements. Numerical simulations predict no imminent danger of stress-induced instability and that the twin tubes could stand unsupported. A light support system as recommended by NATM (and similarly by RMR and Q systems) is necessary to prevent structural instability, loosening and overbreaks. The numerical analysis using the core-softening approach also exemplifies the sensitivity of tunnel convergence behavior to moving position of the excavation face, and consequently supports the combined use of the displacement vector magnitudes and angles to predict the presence and to appreciate the magnitude of changes in rock mass stiffness ahead of the excavation.

Physical and numerical tests of the excavation walls in jointed rock masses

This paper examines the magnitude and distribution of earth pressure on the support systems of open cuts in jointed rock masses. A physical model test was carried out using concrete blocks with man-made joints to represent a jointed rock mass. The model test was simulated numerically to provide a justifiable basis for extended numerical parametric studies. This study focused on the overall procedures of the physical model test, its numerical simulation, and extended numerical parametric studies. A comparison of the results from both the physical model test and numerical simulation confirmed that the applied numerical approach and methodology were suitable for further extended numerical parametric studies. The controlled parameters were the different rock types and joint characteristics including joint shear condition, joint spacing, and joint inclination angle. Results of the earth pressures from the numerical parametric tests in jointed rock masses were compared with Peck’s empirical earth pressure for soil ground. The comparison showed that the earth pressure in jointed rock masses can be very different from that in the soil ground. Accordingly, the effect of the rock types and joint characteristics needs to be considered when designing excavation support systems in jointed rock masses.

Engineering classification of soil masses for design of tunnel support based on observations in Tehran eastern main sewerage tunnel

The need to provide an index of subsurface soil behaviour which would include allowances for intact soil strength, soil texture and its matrix properties, discontinuity frequency, groundwater condition together with stress condition at the depths encountered has led this author to develop a soil mass classification system to isolate in a qualitative manner categories of soil which, under a particular set of engineering constraints, will behave in a similar way. The principle on which this classification system is based is that the selected parameters which are important in determining soil mass behaviour can be determined quickly and easily by a field geologist or engineer. For a particular soil stratum, a numerical assessment is made within designated limits of the quality of the soil as it relates to a particular parameter. These numbers are then summed to give an index number which can be related to the behaviour of soil mass culminating in the prediction of excavation procedures and support system density for a 3⋅5 m diameter hand excavated tunnel. The classification provided very good feed-back for the NATM tunnelling method, including stand-up time and support requirements.

Displacement Prediction for Soil Nailing Based on ANN

Soil nailing has become an important excavation support system for its good performance and cost-effectiveness. It is complicated to predict deformation of soil nailing during excavating. The Artificial Neural Network (ANN) is developed very quickly these years, which can be applied in diverse applications such as complex non-linear function mapping, pattern recognition, image processing and so on, and has been widely used in many fields, including geotechnical engineering. In this paper, the artificial neural network is applied for deformation prediction for soil nailing in deep excavation. The time series neural networks-based model for predicting deformation is presented and used in an engineering project. The results predicted by the model and those observed in the field are compared. It is shown that the artificial neural network-based method is effective in predicting the displacement of soil nailing during excavation.

Method for Estimating System Stiffness for Excavation Support Walls

Excessive excavation-induced movements are major concerns for most underground construction projects in urban areas. These movements can lead to significant damage in adjacent structures. When average to good workmanship is employed during the installation process of the excavation support systems, the consequent ground movements are most influenced by the support system stiffness. Therefore, choosing the most appropriate stiffness for an excavation support system is crucial to minimizing excavation-related damage to adjacent buildings and utilities. This paper presents a semiempirical design methodology that facilitates the selecting of the excavation support system stiffness in such a way that limits excavation-related ground movement. As part of the proposed design methodology, a new parameter was developed called the relative stiffness ratio. This new parameter relates the strength and stiffness of the soil with the stiffness of the excavation support system and was developed from a comprehensive parametric analysis that incorporated a fully three-dimensional finite-element analysis of a generalized excavation that realistically modeled the excavation geometry, excavation support system configuration, and excavation activities. The performance of the proposed methodology was evaluated using several excavation case histories reported worldwide. The results of the evaluation show that the new relative stiffness ratio performed well in predicting the support system bending stiffness and the actual excavation-induced lateral deformations of the case history support systems.

Cracking in Walls of a Building Adjacent to a Deep Excavation

A major concern for projects involving deep excavations in urban areas is the response of adjacent buildings and utilities to excavation-related ground movements. Unfortunately, a purely theoretical approach to estimating building response to excavation-related deformations is not possible due to the variability of the many factors that contribute to the response. Consequently, building response must be estimated and evaluated primarily based on empirical observations and various structural approximations. The goal of estimating and evaluating building response is to provide limiting criteria that will safeguard the structure against unacceptable damage. Thus, estimating the extent of the building response and consequently the severity of excavation-related building damage is critical to establishing rational limiting criteria for excavation support system designs. The most common measure of damage severity is the onset and growth of cracks in interior walls of adjacent structures. Although several proced...

Finite-element analysis of secant pile wall installation

A common practice in the USA for excavation support system design is to assume that the complete support system is ‘wished in place'. This implies that the construction of the in situ wall component and the installation of the supports do not cause any movements or changes in the in situ stress state. However, the installation can cause appreciable changes in the in situ soil stress conditions, which result in significant movements in the surrounding ground. This paper presents the results of efforts to evaluate the factors that influence movements associated with excavation support wall installation, using a series of three-dimensional finite-element models. These analyses demonstrated that soil responses and the processes that produce them are three-dimensional in nature. Thus it can be concluded that fully three-dimensional analyses are required to provide insight into these complex relationships.

Three-Dimensional Responses Observed in an Internally Braced Excavation in Soft Clay

Several three-dimensional effects were observed in the performance monitoring data collected during excavation for the Ford Engineering Design Center in Evanston, Illinois. The elevations of the soil around the excavation varied and the excavator removed the soil in a nonuniform excavation process, both of which contributed to the observed three-dimensional (3D) effects. This paper describes the excavation support system and subsurface conditions at the site, summarizes the construction procedures, and presents the lateral soil movements measured by inclinometers, ground-surface movements measured by an automated total station, tilt of components of an adjacent structure, and forces in internal braces. These responses are compared with expected responses from current design methods. The 3D nature of the excavation resulted in smaller movements on the side of the excavation, where the retained soil was lowest, an unexpected pattern of axial strut loads and very slight damage to an exterior wall that paralleled one of the excavation walls.

Reliability assessment of excavation systems considering both stability and serviceability performance

Excavation projects related to urban redevelopment and infrastructure improvement are often governed by serviceability-based design, rather than failure prevention criteria. Deformation tolerance specifications are often prescribed based on minimizing potential damage to adjacent structures. A risk-based approach to serviceability performance that systematically incorporates design parameter uncertainty will allow engineers to address soil uncertainty in performance-based design. This paper demonstrates the use of various kinds of reliability methods, such as response surface method (RSM), first-order reliability method (FORM), second-order reliability method (SORM), adaptive importance sampling (AIS), Monte Carlo simulation (MCS) and system reliability, to assess the risk of stability and/or serviceability failure of an entire excavation support system throughout the entire construction process. By considering multiple failure modes (including serviceability criteria) of an excavation, the component and system reliability indices for each excavation step are assessed during the entire excavation process. Sensitivity analyses are conducted for the system reliability calculations, which demonstrate that the adjacent structure damage potential limit state function is the dominant factor for determining excavation system reliability. An example is presented to show how the serviceability performance for braced excavation problems can be assessed based on the system reliability index.

Assessment of Local Excavation Support Systems: A Case Study of Nablus City, Palestine

Excavations of soil and rock are one of the most important elements in laying the subsurface structures. These excavations usually require excavation support systems that have fundamental influence on the safety, profitability, speed and quality of construction projects. Despite the great importance of the support systems, most designers and contractors know very little about their design and construction and they rely heavily on experience. The goal of this paper is to present a review of excavation support systems available worldwide and to survey the current state of practice in the local area (Nablus - Palestine), including available types, reasons for failure, and methods of design and construction of excavation support systems. This paper also suggested new techniques that may be adopted locally as an excavation support system. Conclusions of this study are presented and recommendations are suggested to identify what research and development should be carried out to improve the process of design and construction of excavation support systems.

Rock mass characterization for an underground excavation support system: the Sabzkuh water conveyance tunnel, Iran

There are various structural and geological stratigraphies in study area. The main lithology includes the shale, marl, limestone and dolomite that belong to the Cambrian to upper Cretaceous besides Quaternary deposits. To identify the engineering and geotechnical characteristics of the rock mass along the tunnel route, the results of laboratory and in situ tests, geophysical explorations (geoelectrical methods), field observations and borehole logging charts have been used. The well-known rock mass classification systems for tunnelling purposes (RMR, GSI and Q-system) have been used. Using the GSI classification system for rock mass, the modified Hock- Brown criterion parameters of the rock mass for typical section were determined. The RMR was used to determine the required support for the entire length of tunnel. Also the Q system was used to compare the required support from Q system with the RMR system. Finally rock-support interaction analysis was conducted for a typical cross section of the tunnel. Using the above empirical and analytical methods, the required supports were compared for a typical section.

Rock mass characterization for an underground excavation support system: the sabzkuh water conveyance tunnel, Iran

Rock mass characterization for an underground excavation support system: the sabzkuh water conveyance tunnel, Iran