Showing papers in "Geotechnique in 2008"

Micro- and macro-mechanical behaviour of DEM crushable materials

Abstract: This paper provides a micro-mechanical commentary on the macroscopic behaviour observed in DEM simulations of the compression of individual crushable grains, and of triaxial tests on assemblies of both crushable and uncrushable grains. A fragmentation ratio is defined to describe the bond breakage processes, and the significance of other micro-mechanical parameters such as sliding contacts ratio, average coordination number, deviator fabric, and internal energies per unit volume is discussed. Three different modes of grain damage were observed: asperity breakage, internal shear cracking, and internal tensile cracking leading to fast fracture. Energy balances both for the compression of a single grain, and for triaxial tests on assemblies of grains, showed that the loss of elastic energy due to bond breakage was a negligible fraction of the significantly enhanced dissipation encountered with crushable materials. This extra dissipation was associated with frictional sliding triggered by the creation of new degrees of freedom among the breaking fragments. Different modes of grain breakage were found to be representative of different regions of soil states of stress defined with respect to the virgin compression line. The secondary role of elastic grain deformability, increasing coordination number but reducing dilatancy, has also been demonstrated.

Quantifying and modelling fabric anisotropy of granular soils

Abstract: This paper describes an integrated study of the effects of fabric anisotropy on granular soil response, in which the microscopic measurements are properly linked with the macroscopic modelling. Using an image-analysis-based technique and an appropriate mathematical approach, the inherent fabrics of sand specimens prepared in the laboratory using different sample preparation methods were measured, quantified and compared at a microscale level. It was found that the specimen prepared by the dry deposition method had a more anisotropic microstructure than the specimen prepared using the moist tamping method, which is considered directly associated with the experimental observation that different sample preparation methods produce samples with distinctive responses under otherwise identical conditions. An existing platform model was then extended so that the combined effects of initial fabric and shear mode dependence were accounted for in a simple yet rational manner. To calibrate and verify the model, a ser...

Tunnelling and geotechnics: new horizons

Abstract: New developments in both the theory and the practice of tunnelling are covered in the lecture. The important relationship between tunnelling and geotechnics is highlighted, and recent advances in r...

Effect of embedment on the undrained capacity of shallow foundations under general loading

Abstract: Results of a finite element study addressing the effect of embedment on the undrained bearing capacity of shallow strip foundations under uniaxial and combined loading are reported. Uniaxial vertical (V), moment (M) and horizontal (H) capacity and combined VH, VM and general VMH capacity are assessed. Ultimate limit states and the kinematic mechanisms accompanying failure are presented. Ultimate limit states under combined loading are presented as failure envelopes expressed in terms of loads non-dimensionalised by the foundation geometry and soil undrained shear strength and loads normalised by the ultimate uniaxial limit load, in order to describe their size and shape respectively. The results presented in this paper show that the size and shape of failure envelopes defining the undrained capacity of shallow foundations under general loading are dependent on embedment ratio.

Anisotropic creep model for soft soils

Abstract: In this paper a new anisotropic model for time-dependent behaviour of soft soils is presented. The formulation is based on a previously developed isotropic creep model, assuming rotated Modified Cam Clay ellipses as contours of volumetric creep strain rates. A rotational hardening law is adopted to account for changes in anisotropy due to viscous strains. Although this will introduce some new soil parameters, they do not need calibration as they can be expressed as functions of basic soil parameters through simple analytical expressions. To start with, the one-dimensional response of the model is discussed, making it possible to explore how the model is capable of capturing key features of viscous soft soil behaviour. Subsequently, the three-dimensional generalisation of the model is presented, followed by comparison with experimental data, showing good agreement in both triaxial undrained compression and extension. In the authors' opinion, the simple formulation of the model makes it attractive for use in engineering practice.

Compaction behaviour of clay

Abstract: This paper presents an experimental study of the compaction behaviour of non-active clay. One-dimensional static compaction tests were carried out at high and medium water content with matric suction monitoring using Trento high-capacity tensiometers. At lower water contents, a transistor psychrometer was used to measure post-compaction suction. Samples were compacted on the dry side of optimum to cover a wide range of compaction water contents and vertical stresses. Three water content regions were identified in the compaction plane depending on whether post-compaction suction increased, decreased or remained constant as the degree of saturation was increased at constant water content. Hydraulic paths of specimens subjected to loading-unloading cycles at constant water content have clearly shown that post-compaction suction may increase as the degree of saturation increases. This non-intuitive behaviour was demonstrated to be associated with the coupling between mechanical and water retention behaviour. To this end, a coupled mechanical water retention model was formulated. Irreversible one-dimensional mechanical paths were modelled by a boundary surface in the space average skeleton vertical stress, modified suction and void ratio. Irreversible hydraulic 'wetting' paths were modelled by a boundary surface in the space suction, degree of saturation, and void ratio. This study was completed by investigating the pore size distribution of compacted samples through MIP tests.

The uplift resistance of pipes and plate anchors buried in sand.

Abstract: The design of buried anchors and pipelines requires assessment of the peak uplift resistance. This paper describes a limit equilibrium solution for the uplift resistance of pipes and plate anchors buried in sand. The geometry of this solution reflects observations from model tests. Peak angles of friction and dilation are found using established correlations that capture the influence of stress level and density. These angles govern the geometry of the failure mechanism and the mobilised resistance. The solution is validated using a database of 115 model tests on pipes and strip anchors assembled from the published literature. Good agreement with the overall database is shown, without optimisation of any input parameters. The method overpredicts the uplift resistance of smooth model pipes by ∼10%, highlighting the influence of pipe roughness. In contrast, it is shown that the solution for uplift resistance based on the limit theorems of plasticity is generally unconservative. The assumption of normality, ...

Gas flow through clay barriers

Abstract: A model for the simulation of gas flow in clayey rocks and clays is described. Gas migration is associated with the development of preferential paths along existing or pressure-dependent discontinu...

Analysis of the shaft resistance of non-displacement piles in sand

Abstract: The paper examines, using numerical modelling, the problem of the limit shaft resistance of non-displacement piles installed in sands. The modelling makes use of an advanced, two-surface-plasticity constitutive model. The constitutive model predicts the soil response in both the small- and the large-strain range, while taking into account the effects of the intermediate principal effective stress and of the inherent anisotropy of the sand. Finite element analyses of shearing along the pile shaft are performed in order to examine the development of limit unit shaft resistance and the changes in stress state around the shaft upon axial loading of the pile. Special focus is placed on the operative value of the lateral earth pressure coefficient when limit shaft resistance is reached. The analyses offer useful insights regarding the factors controlling the value of unit shaft resistance in sands. The simulations predict a significant build-up of horizontal effective stress for dense sands. Based on these simu...

Thermo-mechanical behaviour of a compacted swelling clay

Abstract: Compacted unsaturated swelling clay is often considered as a possible buffer material for deep nuclear waste disposal. An isotropic cell permitting simultaneous control of suction, temperature and ...

Determination of the anisotropic shear stiffness of an unsaturated decomposed soil

Abstract: The shear modulus at very small strains (0·001% or less), G0, is one of the most important parameters for predicting ground movements and dynamic responses of many engineering earth structures Wit

Spatial variation of void ratio and shear band thickness in sand using X-ray computed tomography

Abstract: A detailed study of spatial variation of void ratio and shear band thickness measurements is presented in this paper. A prismatic sand specimen that initially measured 57·4 mm wide × 120·5 mm long × 182·1 mm high was sheared under plane-strain (biaxial) loading conditions. The specimen was prepared at a relative density of 79% using F-75 Ottawa sand. X-ray computed tomography was used to scan the specimen before shearing and after the onset of the shear band. The specimen failed through a single shear band with an inclination angle of 65·6° measured from the direction of the minor principal stress. Computer algorithms were developed to calibrate CT images and quantify void ratio (e) variation within the specimen. CT data analysis revealed cross-sectional spatial variation in void ratio where density is higher in regions close to the edges of the specimen due to membrane effects and confining pressure with no significant variation in void ratio in the axial direction (top to bottom) before shearing. The sh...

Revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying clay

Abstract: The safe and economic use of mobile jack-up structures is still hindered by limited understanding of the installation of their large conical 'spudcan' footings in layered sand over clay sites. This paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally consolidated clay. Digital images were captured continuously by installing a half-spudcan against a transparent window, and analysed using a particle image velocimetry technique coupled with close-range photogrammetry correction. As the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case. The experimental evidence provides: (a) failure modes at different spudcan penetration depths; (b) the transitional failure mechanism at the event of peak bearing resistance; and (c) the changes in overall failure mechanism due to varying geometric and strength conditions of the layered soil. The results provide useful references for the development and validation of analytical or numerical solutions for this problem.

The ultimate undrained resistance of partially embedded pipelines

Abstract: On-bottom pipelines for transporting oil and gas in deep water undergo significant changes in temperature and pressure during operating cycles, which cause a tendency for lateral buckling. Prediction and control of this phenomenon are required for the safe design and operation of these pipelines. However, the soil response under combined vertical and lateral loading is a significant area of uncertainty, and current practice relies on empirical expressions for the estimation of lateral pipe–soil resistance. This paper reports the results of finite element (FE) analyses of shallowly embedded pipelines under vertical and horizontal load. These analyses have been compared with collapse loads calculated using the upper-bound theorem of plasticity, and are used to construct yield envelopes defining the limiting combinations of vertical and horizontal load. The FE limiting loads were found to compare well with upper-bound plasticity solutions, and the internal soil displacements calculated in the FE analyses mat...

Undrained monotonic and cyclic simple shear behaviour of carbonate sand

Abstract: The paper presents a study of the undrained behaviour of an uncemented carbonate sand (Quiou sand) under simple shear loading conditions. The experimental study was conducted through cyclic and monotonic undrained/constant volume simple shear tests carried out on reconstituted specimens prepared by using the sedimentation in water (WS) method. Tests were carried out on specimens reconstituted at two void ratios (i.e. loose and dense) and different effective consolidation stresses. Furthermore, to account for the effect of non-zero mean shear stress level, cyclic simple shear tests were performed under both symmetrical and non-symmetrical cyclic loading. Two types of failure modes have been observed in cyclic tests, that is ‘cyclic liquefaction’ or ‘cyclic mobility’, depending on whether or not they were conducted under shear stress reversal conditions. A unified framework seems to exist whereby undrained monotonic and cyclic response can be comparatively analysed. In particular, normalising the cyclic liq...

Variation of suction pressure during caisson installation in sand

Abstract: A series of model tests was conducted in a geotechnical centrifuge to investigate the variation of suction pressure during caisson installation in dense sand. The tests were carried out for caissons of different size and surcharge (or effective weight). Finite element (FE) simulations were also conducted to study and explain the experimental results. Data from two field installations and other model tests were investigated, and compared with the results from the centrifuge model tests. The suction pressure was observed to increase relatively linearly with embedment depth, following a distinct slope that is related to establishing a critical hydraulic gradient along the inner caisson wall. The total driving force (weight plus pressure differential) required for suction installation is significantly reduced compared with the force required to install the caisson by jacking. When a larger surcharge was used, the required suction pressure decreased for a given penetration depth, but the total force increased....

Shear strength model for overconsolidated clay-infilled idealised rock joints

Abstract: Saturated infilled joints can contribute to the instability of rock masses during undrained shearing. This paper reports an experimental investigation into the effect of the overconsolidation of infilled rough joints on undrained shear behaviour. A revised model is presented for predicting the shear strength of rough infilled joints on the basis of experimental tests carried out on idealised sawtoothed joints with natural silty clay as the infill material. Tests were conducted under consolidated undrained conditions in a high-pressure triaxial apparatus on joints having a dip angle of 60°. Pore pressure development in the infill materials was monitored. The results show that the effect of asperities on shear strength is significant up to a critical asperity height to infill thickness ratio (t/a), whereas the shear behaviour is controlled by the infill alone beyond this critical value. The proposed model for predicting the shear strength of rough infilled joints describes how the OCR influences the shear strength, pore water pressure development, and critical t/a ratio.

A comparison of the bearing capacity of flat and conical circular foundations on sand

Abstract: This paper describes a series of centrifuge model tests investigating the vertical bearing capacity of flat and conical circular foundations on sand, which are representative of the spudcan foundations of offshore jack-up units. It is found that at shallow embedment the bearing capacity factor, Nγ, mobilised by a conical footing is significantly lower—by a factor of up to 2—than that for a flat footing. Plasticity solutions indicate that this discrepancy cannot be attributed to the theoretical differences in Nγ that arise from the roughness and geometry of the cone: these theoretical differences are minor (<10%). Instead, it is proposed that in dense sand the pre-shearing induced by the conical shape of the footing leads to a form of progressive failure, so a lower operative friction angle is applicable to conical foundations than for flat footings. Since accurate solutions for Nγ exist, the key uncertainty in the prediction of bearing capacity is the appropriate friction angle, which is affected by this ...

Limit analysis with anisotropic fibre-reinforced soil

Abstract: Fibre reinforcement is a technology to improve the strength of soils used as fill in geotechnical structures. The properties of the fibre-reinforced sand are anisotropic because of the distribution of orientation of the fibres, which is characterised by a preferred bedding plane. The development of an anisotropic yield condition for fibre-reinforced sand is presented. Application of the kinematic approach of limit analysis to anisotropic frictional materials is demonstrated. An interesting implication of anisotropy is a lack of uniqueness in the internal friction angle along a shear surface with given orientation. The anisotropic internal friction angle was found to be a function of the major principal strain rate direction, and not just a function of the orientation of the shear surface. While the inclination of principal strain rate directions can be inferred from the kinematics of the mechanism for a given boundary value problem, the internal friction angle cannot be determined a priori as a unique fun...

Monotonic and cyclic behaviour of sand under torsional loading

Abstract: Comprehensive tests were performed to obtain the stress–strain and stiffness characteristics of a fine uniform sand in a hollow cylinder apparatus under monotonic and cyclic torsional loading. Unstable response with a sudden reduction in shear strength after peak was observed under undrained monotonic loading. After a quasi-steady state the specimens showed a phase transformation and a tendency to dilation. Drained torsional hollow cylinder tests showed an equivalent characteristic state marking the transition between contractant and dilatant behaviour and the same line at phase transformation as their undrained counterparts. The instability line, defined under monotonic loading, appears to form a boundary to the behaviour of sand under cyclic loading. Unstable behaviour is introduced when the effective stress path followed by the specimen during cyclic loading reaches the instability line at a shear strain of approximately 0·5%. The first loading cycle is at variance with the following cycles in terms of...

Pile and penetrometer end bearing resistance in two-layered soil profiles

Abstract: The paper examines the influence of soil strength and stiffness on the end resistance mobilised by a cone or displacement pile in a two-layered soil profile (i.e. strong soil overlying or underlying weak soil). The investigation involved a series of numerical analyses that idealise pile/cone penetration assuming the spherical cavity expansion analogue. It is shown how the end resistance mobilised by a penetrometer in a dense sand (i.e. strong soil) varies with the number of penetrometer diameters (D) from a weak soil (assumed here to be either a loose sand or soft clay), and depends on the ratio of the steady-state penetration resistances of the strong and weak layers. The penetrometer resistance in the weak soil is not affected by an adjacent strong layer at penetrations greater than 2D. A design procedure based on a numerical parametric study is proposed, and this is subsequently shown to compare well with centrifuge test data and with previous numerical research.

Insight into seismic earth and water pressures against caisson quay walls

Abstract: Motivated by the need to explain the large displacement and rotation that numerous caisson-type quay walls suffered in the port of Kobe during the devastating 1995 earthquake, a detailed numerical analysis is presented for the response of such a wall from Rokko Island. Utilising the Pastor–Zienkiewicz elastoplastic constitutive model, an effective stress dynamic analysis is performed using as input the accelerogram recorded 32 m below the ground surface in the nearby Port Island. The evolution during shaking of lateral displacements, plastic strains and pore water pressures sheds some light on the complex interplay of several simultaneously occurring phenomena: the development of oscillatory inertia forces on the wall, in phase or out of phase with the backfill soil and water pressures; the simple-shear seismic deformation of the soil and the ensuing initial development of positive excess pore water pressures in the backfill and the foundation soil; the extensional deformation developing in the ‘active we...

Geotechnics: the next 60 years

Abstract: The authors were set the challenge of thinking about the next 60 years, not only of Geotechnique but of geotechnical engineering in general They have attempted to do this by talking to colleagues, by dreaming a little, but mainly by trying to extrapolate the trends they see in society and in present-day geotechnical activity Although the principal emphasis of Geotechnique tends to be research, this is driven by the requirements of design and construction Hence, in looking to the future, even the future of research, likely trends in geotechnical construction activities are considered first, particularly noting the emerging importance of energy conservation and carbon dioxide reduction This leads to discussion of the characterisation of material behaviour and investigation of the ground, and the development of design processes The future of dissemination of geotechnical information is also discussed

Buoyancy of tunnels in soft soils

Abstract: INTRODUCTION The construction of bored tunnels in soft soils is one of the major achievements of civil engineering in the 20th century. The process minimises the disturbance in the environment, especially in built-up areas, but there may still be some disturbance of the stress field, and some deformation of the soil surrounding the tunnel. In particular, some subsidence above the tunnel may occur, because of the excavation process at the front of the tunnel, and because the dimensions of the tunnel-boring machine are necessarily somewhat larger than the tunnel constructed in its interior. The latter effect is often reduced by the injection of grout between the tunnel elements and the soil, but experience shows that there is usually some subsidence above the tunnel. The simplest problem for theoretical analysis arises when considering the ground loss problem: the deformations caused by a uniform reduction of the radius of a cylindrical cavity. For the surface subsidence due to ground loss, various methods of analysis have been suggested, ranging from analytic solutions assuming elastic or elasto-plastic behaviour to numerical solutions using advanced material modelling of soil behaviour, and empirical methods using some assumed subsidence curve (Peck, 1969). In most of the elastic methods (Sagaseta, 1987; Verruijt & Booker, 1996) the problem considered is an imposed deformation of a circular ring in a half-plane, ignoring other effects. It has been observed (e.g. Loganathan & Poulos, 1998) that the subsidence curve predicted by such elastic solutions is wider than the subsidence curve observed in engineering practice, with the observed subsidence curves being some 50% narrower than the curves predicted from elastic solutions for the ground loss problem. To improve the predictions, various improved methods of analysis have been suggested. Loganathan & Poulos (1998) considered the possible ovalisation of the tunnel and, preferably, the addition of a semi-empirical correction factor. Other possible causes for deviations from an elastic solution are differences between the excavation face pressure and the in situ stresses, plastic deformations of the soil (Osman et al., 2006), and consolidation or creep of the soil after the construction process of the soil. Perhaps the ultimate solution is to use a numerical method, for instance based upon the finite element method, in which elasto-plastic soil behaviour and creep can be incorporated, and even the effect of grouting (Van Jaarsveld et al., 1999; Brinkgreve et al., 2006). Buoyancy will automatically be included in such a model if it starts with an initial state of stress due to gravity of the soil, and then considers the construction of the tunnel as a removal of the soil, the installation of the tunnel, and consolidation or creep of the soil after the construction process, taking into account the effective weight of the various components. If desired, the grouting process can also be included. The effect to be considered in this paper is the difference in weight of the completed tunnel and the excavated soil, to be denoted as the buoyancy effect (Strack, 2002; Strack & Verruijt, 2002). Because the weight of the tunnel is usually less than the weight of the excavated soil, an upward force is exerted on the surrounding soil, and this will affect the deformation and stress field, including a reduced subsidence. The purpose of the present paper is to investigate the influence of the buoyancy effect on the subsidence curve at the surface, and in particular the width of this curve, using a simple homogeneous isotropic elastic model for the soil. Of course, other effects, such as plasticity, consolidation and creep, may also influence the shape of the subsidence curve, but this paper considers the ground loss and buoyancy effects only.

A new yield and failure criterion for geomaterials

Abstract: This paper proposes a new isotropic yield and failure criterion for geomaterials under general stress conditions. The new criterion is a function of the three stress invariants and the paper shows the well known criteria by Matsuoka & Nakai (1974) and Lade & Duncan (1975) are included in the new formulation. In order to include different features characterising soil and rock behaviour, the obtained function is successively modified to account for tensile strength and curvature in meridian planes. The influence of each parameter is discussed and finally the comparison between the new criterion and some experimental data on soils and rocks is shown. Finally, the domain of the new criterion is analysed in order to show if the yield function takes negative values also outside the shown elastic domain.

Experimental pile subjected to long duration thrusts owing to a moving slope

Abstract: An instrumented pipe pile has been installed in an unstable slope to measure long-duration P-Δy lateral reaction curves. After outlining the main features of the experiment, results are given over a period of 16 years. These comprise the soil movements, the pile displacements, the bending moments and the reaction pressures. A Menard pressuremeter method and a self-boring pressuremeter method are used to predict the P-Δy reaction curves, which are then used in a numerical model to derive the overall pile behaviour. The results are compared with measured pile displacements and bending moments and show that the pile displacements and bending moments calculated by the numerical model are markedly overestimated.