Excavation In Clay Research Articles

A continuous upper bound limit analysis for the transverse stability of shallow shield tunnelling in undrained clays

For a cross sectional ‘convergence-oval deformation-sagging/uplifting’ composite volume loss model of shallow shield tunnels in undrained clays, the ground displacement field is calculated using complex variables, and the most critical tunnel stability number as well as its supremum are determined by using continuous upper bound limit analysis. Illustrative calculations reveal: (1) the method can be verified via finite element modelling and comparisons with other limit analysis solutions and centrifuge model test results; (2) with the increase of the tunnel sagging coefficient or oval-deformation coefficient, the ground surface settlement increases mainly in magnitude but minor in trough width; (3) the influence of the tunnel uplifting coefficient on ground displacement is similar in pattern but opposite in direction to that of the tunnel sagging coefficient; (4) the effect of the tunnel oval-deformation coefficient on the supremum of the tunnel stability number is more significant than that the tunnel sagging/uplifting coefficient; (5) the uniform convergence volume loss model can reflect the most critical pattern of tunnel instability and the most critical tunnel stability number with sufficient accuracy. The proposed method and the relevant observations can serve as references for limit support pressure calculations concerning shallow shield tunnel excavations in undrained clays.

Determination of Soil Stiffness Parameters for Clays as a Construction Subsoil for Warsaw Underground

Analysis of soil-structure interaction demands properly determined parameters for a particular calculation method. In the case of deformation modulus determination, it is essential to take actual construction activities into consideration. It means that these moduli should correspond to stress-strain range of a particular construction. The use of numerical methods, allowing for more realistic soil-structure interaction, compatibility of displacements, as well as to account for the stiffness of the structure and the soil, may provide more realistic prediction of displacements. The article presents the methods of stiffness parameters determination including all factors that should be taken into consideration in geotechnical design. The article characterizes clays of Poznań formation, a subsoil for many structures in almost 75% area of Poland. Investigations conducted for clays in Warsaw were used as a background with the focus on an underground Metro station. Direct measurements of the unloading of the bottom of the excavation in clays were used for validation. Finite Element Method (FEM) was used to verify stiffness parameters and their determination methods. The correctness of obtained results was verified by comparison with deformation measurements of station construction and displacement measurements in the adjacent area. As a conclusion, the article shows that the type of a construction and its character should be taken into consideration in geotechnical parameter determination. The analysis with stiffness parameters determined in the small-strain range of deformations allows to obtain displacement values approximate to real ones. In the case of an underground station, large area of soil interacts with a structure, and in this case, the deformations are small, which should be taken into account in a design analysis.

Non-invasive techniques to assess restrictive lung disease in workers exposed to free crystalline silica.

Objectives:To compare the reliability of spirometry and body plethysmography in detecting restrictive lung disease in clay excavation workers exposed to free crystalline silica (FCS). The exhaled breath condensate (EBC) biomarkers of oxidative stress were also assessed in order to evaluate early lung damage.Methods:The study involved 62 workers (58 males and 4 females) at a company that extracts and processes clay.Results:Body plethysmography (total lung capacity below the lower normal limit) and spirometry respectively indicated restrictive pattern prevalence rates of 22.6% and 1.6%. EBC 4-hydroxynonenale levels were not sufficiently sensitive to highlight a restrictive deficit, but did distinguish low and high rates of occupational exposure. There was no correlation between plethysmography values and the intensity or duration of exposure.Conclusions:Only one out of 14 cases of restrictive deficit diagnosed on the basis of body plethysmography values was also identified by means of spirometry. This finding supports the need to use body plethysmography in the health surveillance of clay workers exposed to FCS.

A Multivariate Adaptive Regression Splines model for determining horizontal wall deflection envelope for braced excavations in clays

The horizontal wall deflection of the braced excavations is influenced by many factors, including the excavation geometry, wall stiffness, soil parameters, struts spacing and so forth. In this study, three-dimensional (3D) finite element (FE) analyses were carried out to examine the corner restraining effects. A series of 3D FE analyses using the Hardening Soil (HS) model were carried out to investigate the effects of the strength of the clay, wall stiffness, excavation length, excavation depth and width, on the horizontal wall deflection envelope induced by braced-excavation in clays. Based on the results, a Multivariate Adaptive Regression Splines (MARS) model able to accurately learn the complicated implicit relationship between the maximum wall deflection envelope and these influential factors as well as the various interaction factors is developed. The developed MARS model is of good interpretability and enables the design engineer to estimate the shape of the wall deflection profiles. In addition, wall deflection profiles computed by this method compare favourably with a number of field and published records.

Parametric study of deep excavation in clays

Parametric study of deep excavation in clays

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.

Single Pile Settlement and Load Transfer Mechanism due to Excavation in Silty Clay

In densely built areas, development of underground transportation system often involves excavations for basement construction and cut-and-cover tunnels which are sometimes inevitable to be constructed adjacent to existing piled foundations. In order to gain new insights into single pile responses (i.e. settlement and load transfer mechanism) to an adjacent excavation in saturated silty clay, a three-dimensional coupled- consolidation numerical analysis is conducted in this study. An advanced hypoplasticity (clay) constitutive model with small-strain stiffness was adopted. A linear increase in pile settlement was observed due to excavation-induced stress release. This is because part of the pile is placed within the boundaries of a major influence zone due to excavation-induced ground movement. Based on a settlement criterion, apparent loss of pile‘s capacity is 14%. A maximum bending moment of about 350 kNm is induced in the pile with the maximum deflection of 28 mm. In addition, mobilisation of shear strength at the pile-soil interface was found to be a key factor governing pile-soil-excavation interaction. During excavation, a downward load-transfer mechanism in the piles can be identified.

Single Pile Settlement and Load Transfer Mechanism due to Excavation in Silty Clay

Single Pile Settlement and Load Transfer Mechanism due to Excavation in Silty Clay

MARS inverse analysis of soil and wall properties for braced excavations in clays

A major concern in deep excavation project in soft clay deposits is the potential for adjacent buildings to be damaged as a result of the associated excessive ground movements. In order to accurately determine the wall deflections using a numerical procedure such as the finite element method, it is critical to use the correct soil parameters such as the stiffness/strength properties. This can be carried out by performing an inverse analysis using the measured wall deflections. This paper firstly presents the results of extensive plane strain finite element analyses of braced diaphragm walls to examine the influence of various parameters such as the excavation geometry, soil properties and wall stiffness on the wall deflections. Based on these results, a multivariate adaptive regression splines (MARS) model was developed for inverse parameter identification of the soil relative stiffness ratio. A second MARS model was also developed for inverse parameter estimation of the wall system stiffness, to enable designers to determine the appropriate wall size during the preliminary design phase. Soil relative stiffness ratios and system stiffness values derived via these two different MARS models were found to compare favourably with a number of field and published records.

Behaviour of anchored pile wall excavations in clays

A database of 35 case histories of anchored pile walls, collected mainly from Chicago, Boston, Texas and Washington, DC, USA as well as Taiwan, has been evaluated to predict the performance of deep excavations in clays. Various factors (including excavation depth, undrained shear strength, wall bending stiffness, factor of safety and pile embedment) affecting the deformation characteristics of excavations supported by anchored pile walls are presented and compared with findings of previous researchers. The normalised maximum wall displacement δHmax/He falls in the range of 0·1–0·7% and the location of δHmax is at a depth of 0·4He to 1·0He. Normalised maximum ground surface settlement δVmax/He is in a range of 0·1–0·4%. The ratio of maximum surface settlement to maximum lateral displacement falls in the range of 0·7–1·4, with an average value of 1·0. Maximum wall displacements are dependent on undrained shear strength and factor of safety and decrease with an increase in these factors. Maximum wall displacements decrease by increasing wall stiffness and most of the cases are bounded by values of EI/He between 35 MNm and 120 MNm. The effect of system stiffness is more pronounced in soft clays whereas deformations in stiff clays are less dependent on system stiffness.

Cantilever Contiguous Pile Wall for Supporting Excavation in Clay

In urban areas, there are many situations where basement of new constructions or underground utilities are proposed to be constructed adjacent to old buildings. Of greatest concern are buildings with shallow foundations that do not extend below the zone of influence of the adjacent excavation. For deep excavation, the presence of a cantilever stage at the beginning of a construction sequence can often lead to excessive movements. Therefore over-excavation can also be a primary cause of excessive movements. In the present study, shallow excavation was considered. Three Dimensional Finite Element (3D FE) study was carried out in the present study. A cantilever contiguous pile wall was selected because it is common and relatively economic to be used in cohesive soil. Parametric study were performed considering excavation depth, pile embedded depth, and wall stiffness. Some design recommendations were given to provide a safe supporting system in clay.

Numerical Investigation of Pipeline Responses to Tunneling-Induced Ground Settlements in Clay

Tunnel excavation causes stress changes in the ground, leading to soil movements that may affect the safety and serviceability of existing pipelines. Although the tunnel-soil-pipe interaction has attracted increasing research attention, previous studies primarily focused on the responses of existing pipelines due to tunneling in sand. In this study, an extensive numerical parametric study with 540 numerical runs is conducted to investigate the effects on existing pipelines due to tunneling in clay. Soil responses to uplift and downward pipe-soil relative movements are explicitly simulated. Parameters considered in the tunnel-soil-pipe interaction include ground settlement profile, pipe dimension, material properties, pipe burial depth, and undrained shear strength. A dimensionless plot of relative pipe-soil stiffness and the ratio of maximum pipe curvature to ground curvature are developed to directly estimate pipeline curvature due to tunnel excavation in clays. When the relative pipe-soil stiffness is less than 1×10-4, the existing pipeline deforms with tunneling-induced ground settlement. The existing pipeline is relatively rigid when the pipe-soil stiffness is larger than 10.

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.

A new framework for probabilistic analysis of the performance of a supported excavation in clay considering spatial variability

This technical note presents a new framework for probabilistic analysis of the performance of a geotechnical system such as a diaphragm wall-supported excavation. The new framework consists of two components: a sampling algorithm and an integration method. With the new sampling algorithm, samples of uncertain variables are ‘uniformly’ generated in the possible domain of uncertain variables; the new integration method is then used to construct the cumulative distribution function of the system performance, based upon the integration weights of the generated samples of uncertain variables. To demonstrate this new framework, an illustrative example focusing on a diaphragm wall-supported excavation in clay is analysed; the system performance of the diaphragm wall-supported excavation is studied with Flac (a finite-difference computer code) and the spatial variability of soil properties, in the context of numerical modelling, is modelled with random field theory. Finally, a parametric analysis is conducted to examine the influence of the statistical characterisation of soil properties on the results of the probabilistic analysis. The innovation of the new framework is demonstrated with the results presented.

Assessment of strut forces for braced excavation in clays from numerical analysis and field measurements

One important consideration in the design of a braced excavation system is to ensure that the structural bracing system is designed both safely and economically. The forces acting on the struts are often determined using empirical methods such as the Apparent Pressure Diagram (APD) method developed by Peck (1969). Most of these empirical methods that were developed from either numerical analysis or field studies have been for excavations with flexible wall types such as sheetpile walls. There have been only limited studies on the excavation performance for stiffer wall systems such as diaphragm walls and bored piles. In this paper, both 2D and 3D finite element analyses were carried out to study the forces acting on the struts for braced excavations in clays, with focus on the performance for the stiffer wall systems. Subsequently, based on this numerical study as well as field measurements from a number of reported case histories, empirical charts have been proposed for determining strut loads for excavations in stiff wall systems.

Estimation of Design Parameters for Braced Excavation in Clays

In this study, the optimum values of the design parameters (position of struts, embedment depth of the wall, thickness of the wall and the strut stiffness) for braced excavation in clayey soil have been determined by using the numerical tool, FLAC. The design parameters are determined by studying their effects on the factors such as strut force, wall deflection, wall moment and displacement of the ground surface adjacent to the braced excavation which play significant role in the design of braced excavation. The results of the present numerical model are compared with the observed values obtained from a case study on braced excavation in a clayey soil. A close agreement between the result as obtained from the present numerical study and that measured in the field has been observed. On the basis of the parametric studies done on two different clayey soil profiles, it is found that the most effective design of excavation in a clayey soil can be done when the embedment depth of the wall, the thickness of the wall and the strut stiffness are kept within the range of (80–100)% of the depth of excavation (6–7)% of the depth of excavation and (5–25) × 105 kN/m/m, respectively. The top strut can be kept at a height of (2–3) m below the ground level without endangering the safety of the system.

Sustainable adobe bricks with construction wastes

The objective of this research was to manufacture sustainable adobe bricks from mixtures integrating clay excavation wastes, recycled binding aggregates from concrete having a particle size distribution of 0·96 mm and recycled mixed aggregates and binding wastes with a particle size distribution ranging from 0·64 mm to fines, wood cutting wastes and a liquid mixture of water and Opuntia ficus-indica (mucilage) extract as a natural additive. A sustainable manufacturing process was used in which the clay material typically used in the traditional manufacturing of adobe bricks was replaced by excavation wastes of originally unknown composition obtained during the construction of two buildings, in order to investigate whether various excavation wastes having appropriate clay contents can be used as raw materials for manufacturing sustainable adobe bricks. Before using these wastes, preliminary particle size distribution, plasticity and hardness tests were performed in order to determine the appropriate residue dosages for manufacturing adobe bricks. The results have shown that the use of excavation wastes different from the clay soil usually used ensures the quality requested by the Mexican standard for non-structural adobe bricks. Currently, further studies are being conducted to promote their use in building elements.

Basal heave stability of supported circular excavations in clay

Deep circular shafts are commonly used for example to construct access shafts for transit tunnels, pump stations for wastewater tunnels, and launch shafts for tunnel boring machines. Since the earth pressures acting on a circular shaft subjects the shaft support to ring compression, the reinforcement in the structural elements can be reduced and the need for internal support is eliminated, thereby speeding up excavation. The design of circular support systems for excavations in clays involves assessing the ground stresses and ground movements, the capacity of the structural elements, and the basal heave stability. This study focused on assessing the basal heave stability of diaphragm wall supported circular excavations in clays using the finite element method. The analyses have shown that the basal heave factor of safety is dependent on the undrained shear strength of the clay, the geometrical properties of the excavation system and the thickness of the soil stratum. Based on these results, a simplified method is proposed for assessing the basal heave factor of safety for axisymmetric supported excavations in clay.

Stability of unsupported conical excavations in non-homogeneous clays

New plasticity solutions for the undrained stability of unsupported conical excavations in homogeneous and non-homogeneous clays were solved by axisymmetric finite element limit analysis. Three parametric studies were performed on excavated height ratios, slope inclinations and dimensionless strength gradients. In all cases, the exact stability factors were accurately bracketed by computed bound solutions within 0.6%. An accurate closed-form equation of the stability factor was proposed from nonlinear regression analysis of lower bound solutions. New conical stability factors for soil cohesion, strength gradient, and coupling effect of these components were deduced to conveniently and accurately predict a safe solution in practice.

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...