Showing papers in "Geotechnique in 2009"

Energy pile test at Lambeth College, London: geotechnical and thermodynamic aspects of pile response to heat cycles

Abstract: Very limited information is available regarding the impact of heating and cooling processes on the geotechnical performance of piled foundations incorporating pipe loops for ground-source heat-pump systems (so-called energy piles). A pile-loading test that incorporated temperature cycles while under an extended period of maintained loading was undertaken to investigate the behaviour of an energy pile installed in London Clay. Testing was carried out over a period of about seven weeks, with conventional loading tests carried out either side of an extended loading test with thermal cycles. Using an optical fibre sensor system, and other more conventional instrumentation, temperature and strain profiles were observed in the test pile, an adjacent bore-hole, two of the anchor piles, and the heat sink pile. Details of load and movement at the pile head, of ambient air temperature and of the input/output temperature of fluid within the heating system were also recorded. Thermodynamic behaviour observed during t...

UH model: three-dimensional unified hardening model for overconsolidated clays

Abstract: Based on the relationship between the current yield surface and the reference yield surface, a new model, called the three-dimensional unified hardening model for overconsolidated clays (the UH model), is proposed in this paper. A current yield surface is used to describe overconsolidated behaviour, and a reference yield surface to describe the yield characteristics corresponding to normally consolidated clays. The UH model can model many characteristics of overconsolidated clays well, including stress-strain relationships, shear dilatancy, strain-hardening and softening, and stress path dependence behaviour. The key feature of the model is the adoption of a unified hardening parameter that is independent of stress paths. Based on the SMP criterion and the corresponding transformed stress method, the proposed model can be applied conveniently to three-dimensional stress states. Compared with the Cam-clay model, the UH model requires only one additional clay parameter, the slope of the Hvorslev envelope. The validity of this new model is confirmed by data from triaxial drained and undrained compression and extension tests for clays with different overconsolidation ratios, true triaxial tests with different Lode's angles, and cyclic loading tests.

Energy from earth-coupled structures, foundations, tunnels and sewers

Abstract: Ground-embedded structures such as shallow foundations, bored piles, diaphragm walls, tunnel cut-and-cover walls, tunnel linings, anchors for NATM tunnelling and even sewer systems can be used as absorber elements for ground heat exchange. The technology is based on absorber pipes installed within concrete elements and a heat carrier fluid circulated through the system; heat is extracted from the ground or stored in the ground for heating and cooling of associated buildings. This promising technology leads to a reduction of fossil energy consumption at moderate installation and low operation costs. This paper describes some of the varied applications of the technology in Austria, ranging from the high-rise building Uniqa Tower in the city of Vienna to the Vienna metro extension U2 and, most recently, at the new 15 km long Lainzer tunnel at construction lots LT44 and LT32.

Three-Dimensional Stability of Slopes and Excavations

Abstract: Three-dimensional (3D) limit analysis of stability of slopes is presented. Such analyses are not common, because of the difficulties in constructing three-dimensional mechanisms of failure in frictional soils. A class of admissible rotational mechanisms is considered in this paper. The failure surface has the shape of a curvilinear cone (‘horn'), with upper and lower contours defined by log-spirals; all radial cross-sections of the surface are circular. In the special case of cohesive soils (undrained behaviour), the shape of the failure surface reduces to a torus. An alternative failure surface is generated when the axis of rotation intersects the circle that generates the surface. The 3D mechanism is further modified with a plane-strain central insert to ensure the transition to a plane-strain mechanism if no restraint is placed on the slope width. Also, the spherical failure surface considered in the literature is re-examined. The critical height of slopes with finite width is determined, and the resul...

A water retention model for deformable soils

Abstract: The paper presents an experimental study on water retention behaviour of two reconstituted soils, a silty clay (Speswhite kaolin) and a sandy clayey silt (Barcelona silt) and two compacted soils, a silty clay (Speswhite kaolin) and a silty sandy gravel (Adige river embankment). These soils were used with the aim of encompassing a wide range of soil types and soil fabrics. Specimens were subjected to drying and wetting ‘hydraulic' paths by removing or adding water under zero total stress and to ‘mechanical' wetting paths by compressing specimens at constant water content. To investigate a broad range of void ratios, the reconstituted sandy clayey silt was initially reconstituted from slurry by applying different consolidation vertical stresses, whereas the compacted Speswhite kaolin and Adige river embankment were compacted to a broad range of water contents and compaction energy. On the basis of the experimental data, a modification to the van Genuchten's model is proposed to account for the effect of voi...

Experimental investigation of thermo-hydro-mechanical behaviour of an unsaturated silt

Abstract: Experimental research is conducted on a compacted sample of silt using a triaxial equipment modified for testing unsaturated soils at elevated temperatures. An extensive array of isothermal and non-isothermal tests is conducted, including temperature-controlled soaking and desaturation tests, temperature- and suction-controlled isotropic consolidation tests, and suction-controlled thermal loading and unloading tests. Temperature values applied ranged from 25°C to 60°C, and the suction values varied from 0 to 300 kPa. Baseline data are presented on the influences of temperature and suction on critical-state parameters, yield surface, preconsolidation pressure, strength properties, and water retention characteristics of the soil. All data are carefully analysed and prevailing trends are highlighted.

A full-scale in situ heating test for high-level nuclear waste disposal: observations, analysis and interpretation

Abstract: The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level nuclear waste. In the experiment, heaters are emplaced in the axis of a tunnel excavated in granite to simulate the heat production of radioactive waste. The test is fully instrumented, and attention is focused on the thermo-hydro-mechanical (THM) behaviour of the near-field region constituted by the compacted bentonite barrier surrounding the heater and the immediately adjacent rock. Interpretation of the test is assisted by the performance of a coupled numerical analysis based on a formulation that incorporates the relevant THM phenomena. Initial and boundary conditions for the analysis as well as material parameters are determined from an extensive programme of field and laboratory experiments. The paper presents and discusses the thermal, hydraulic and mechanical observations in the bentonite barrier and in the host rock. Special attention is paid to the progress of hydration in the barrier, to the effects of heating and vapour transport, and to the development of swelling pressures in the barrier. After five years of heating, one of the heaters was switched off and the experiment was partially dismantled, allowing the final state of the barrier to be observed directly. The numerical analysis performed has proved able to represent the progress of the experiment very satisfactorily. In addition, predictions concerning the final state of the clay barrier are very close to the observations obtained during dismantling. The performance and analysis of the in situ test have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity.

THM-coupled finite element analysis of frozen soil: formulation and application

Abstract: A fully coupled thermo-hydro-mechanical (THM) finite element (FE) formulation is presented that considers freezing and thawing in water-saturated soils. The formulation considers each thermal, hydraulic and mechanical process, and their various interactions, through fundamental physical laws and models. By employing a combination of ice pressure, liquid pressure and total stress as state variables, a new mechanical model has been developed that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework. Important frozen soil features such as temperature and porosity dependence of shear strength are captured inherently by the model. Potential applications to geotechnics include analysis of frost heave, foundation stability or mass movements in cold regions. The model's performance is demonstrated with reference to the in situ pipeline frost heave tests conducted by Slusarchuk et al. Detailed consideration is given to FE mesh design, the influence of hydraulic parameters, an...

Modelling of cryogenic processes in permafrost and seasonally frozen soils

Abstract: This paper investigates the thermo-hydro-mechanical (THM) behaviour of soils subjected to seasonal temperature variations in both permafrost and seasonally frozen conditions. Numerical modelling of soil freezing and ice segregation processes is presented, and compared against small-scale physical modelling experiments. The coupled THM model presented, which is solved by way of a transient finite element approach, considers a number of processes, including conduction, convection, phase change, the movement of moisture due to cryogenic suctions, and the development of ice lenses. Two seasonal freezing scenarios are considered: (a) for soils with no permafrost, where freezing is from the surface downward (one-sided freezing); and (b) for soils underlain by permafrost, where large thermal gradients in the uppermost permafrost layer can cause active layer freezing in two directions, from the permafrost table upwards and from the ground surface downwards (two-sided freezing). In the case of one-sided freezing, ice lens formation occurs as the freezing front advances downwards from the surface, and is limited by water supply. However, during two-sided freezing, ice segregation takes place in a closed system, with ice lenses accumulating at the base of the active layer and near the ground surface, leaving an intervening ice-poor zone. Numerical modelling is able to represent the development of both the thermal field and ice segregation observed in the physical models. The significance of this contrasting ground ice distribution is considered in the context of thaw-related slow mass movement processes (solifluction).

Rate effects on penetrometer resistance in kaolin

Abstract: This paper describes the results from a centrifuge investigation into the rate dependence of T-bar and ball penetrometer resistance in kaolin. Four T-bar diameters and two ball diameters were installed in kaolin with overconsolidation ratios of 1, 2 and 5 at penetration velocities varying over five orders of magnitude. The penetration resistances are compared with the properties of kaolin as measured in element tests to assist in the development of a framework that describes this resistance over the full velocity range. Consistent trends emerge when the soil viscosity is assumed to affect the penetration resistance in both the partially drained and undrained conditions, enabling the dependence of the T-bar and ball resistances on diameter, velocity and soil characteristics to be quantified.

The strength of unstabilised rammed earth materials

Abstract: Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted between forms to build walls. Primarily a historic method of construction, it is now receiving considera...

The undrained capacity of skirted strip foundations under combined loading

Abstract: The response of skirted foundations to combined vertical, horizontal and moment loading is important for the design of offshore installations. The vertical skirts beneath the footing interact with the soil and increase the foundation capacity, compared with a surface footing. Previous researchers have assumed that the soil within the skirts remains rigid during undrained loading, but this assumption has not been investigated rigorously. A series of plane-strain finite element analyses has been conducted to investigate directly how the skirt geometry affects the undrained strip foundation capacity under combined horizontal–moment loading and the mechanisms occurring at failure. Conditions of both uniform and non-uniform undrained strength soil have been considered. The results show that deformation of the soil between external skirts can lead to significantly less foundation capacity than that of an equivalent solid embedded foundation. Hence the specific geometry of the foundation must be considered in de...

Resistance of full-flow penetrometers in rate-dependent and strain-softening clay

Abstract: Accurate determination of in situ shear strength using full-flow penetrometers (cylindrical T-bar and spherical ball) requires thorough investigation into soil characteristics such as strain rate dependency and strain softening, which have been shown to have a significant influence on the resistance factors relating penetration resistance to the soil shear strength measured in laboratory tests. In this paper, these effects have been quantified using a large deformation finite element technique, modelling both the strength enhancement owing to high strain rate and gradual degradation of soil during penetration disturbance. A parametric study confirmed that the separate factors quantifying effects of high strain rate and partial softening may be multiplied together to predict the combined effect, to derive a resistance factor for a given soil. Simple expressions are presented allowing the resistance factors for T-bar and ball penetrometers to be expressed as a function of rate and strain-softening parameter...

A clustered overlapping sphere algorithm to represent real particles in discrete element modelling

Abstract: An algorithm is presented for representing irregular particles as clusters of overlapping spheres, for use within discrete element method (DEM) simulations of particulates. The algorithm is sufficiently fast to be implemented on desktop computers. Although volume and moment of inertia comparisons of resulting clusters with prototypical geometric data show that in the region of 50 to 100 spheres may be needed to represent a particle, actual DEM simulations of an unstable pile of simulated particles show that only 10 or so spheres may be sufficient to capture the mechanics of the system.

Thermomechanical model for saturated clays

Abstract: This paper aims to simulate the thermomechanical behaviour of saturated clays in the triaxial stress space at states wetter than critical condition. A thermoelastic-plastic (TEP) mechanical model was developed under the framework of critical state soil mechanics and the modified Cam-Clay model. It consists of two parts. The first part involves an isotropic TEP model that is able to reproduce the thermally induced volume change of saturated clay specimens under normally consolidated and overconsolidated conditions. This part was introduced earlier and validated by the authors. The second part, the focus of this paper, involves extension of the isotropic TEP model into triaxial stress space. The model is simple enough to require merely a few constants more than those of the modified Cam-Clay model. The adequacy of the proposed model has been verified using the experimental results from three different types of clay reported in the literature.

Explaining thermal failure in saturated clays

Abstract: Failure conditions in soils at elevated temperatures appear to be strongly dependent on the history of the application of stress and temperature. Four cases of such history leading to various modes of failure are identified and interpreted in terms of thermal Cam-clay models. Particular attention is given to the influence of thermal variability on the coefficient of the critical state, M, or the angle of internal friction. A detailed analysis of the material history offers an explanation of an apparent confusion about whether the soil strength is decreased or increased by temperature. In a companion paper, numerical analysis of the development of axisymmetric thermal and stress fields around a cylindrical heat source suggests that thermal failure may arise in conditions that are far from any mechanically critical situation.

Post-liquefaction undrained monotonic behaviour of sands: experiments and DEM simulations

Abstract: This paper presents the results of a series of servo-controlled cyclic triaxial tests and numerical simulations using the three- dimensional discrete element method (DEM) on post-liquefaction undrained monotonic strength of granular materials. In a first test series,undrained monotonic tests were carried out after dissipating the excess pore water pressure developed during liquefaction. The influence of different parameters such as amplitude of axial strain,relative density and confining pressure prior to liquefaction on the post-liquefaction undrained response have been investigated.The results obtained highlight an insignificant influence of amplitude of axial strain, confining pressure and a significant influence of relative density on the post-liquefaction undrained monotonic stress-strain response.In the second series, undrained monotonic tests were carried out on similar triaxial samples without dissipating the excess pore water pressure developed during liquefaction. The results highlight that the amplitude of axial strain prior to liquefaction has a significant influence on the post-liquefaction undrained monotonic response.In addition,DEM simulations have been carried out on an assembly of spheres to simulate post-liquefaction behaviour.The simulations were very similar to the experiments with an objective to understand the behaviour of monotonic strength of liquefied samples from the grain scale. The numerical simulations using DEM have captured qualitatively all the features of the post-liquefaction undrained monotonic response in a manner similar to that of the experiments.In addition,a detailed study on the evolution of micromechanical parameters such as the average coordination number and induced anisotropic coefficients has been reported during the post-liquefaction undrained monotonic loading.

Particle breakage during cyclic triaxial loading of a carbonate sand

Abstract: Dynamic loading of embankment, foundation and pavement structures results in particle breakage of the constituent granular materials, when the stresses imposed on their particles exceed their strength. This paper presents the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of Dogs bay carbonate sand. It is observed that particle breakage is dependent on stress level, cyclic stress ratio, and creep and is directly related to volumetric strain. Drained cycling increases volumetric strain and therefore more breakage occurs when larger numbers of cycles are imposed. The increase in particle breakage from one cycle to the next indicates that while particles may not be loaded to their full capacity in a given cycle, they can be crushed in subsequent cycles without any variation in the amplitude of cyclic loading.

Numerical investigations into cycling of full-flow penetrometers in soft clay

Abstract: T-bar and ball full-flow penetrometers have important potential in evaluating the remoulded shear strength in situ, by means of cyclic penetration tests. This paper presents numerical investigation...

Experimental measurements of the global anisotropic elastic behaviour of dry Hostun sand during triaxial tests, and effect of sample preparation

Abstract: Experimental measurements have been carried out with a new, accurate, triaxial device (Triaxial StaDy) to determine the quasi-elastic properties of dry Hostun sand. The experimental investigations consist in successively applying very small axial cyclic static loadings (strain single-amplitude cycle, esa ≈ 10–5 m/m) and four types of wave, generated by piezoelectric sensors (compressive and shear waves in the axial and radial directions), at different levels of the stress–strain curve. The use of both static and dynamic results allows for an entire description of the anisotropic elastic compliance tensor at a given stress–strain state to be made. The evolutions of this tensor during triaxial loading and unloading are presented. The isotropic and deviatoric triaxial stress paths considered underline the effects of inherent and induced anisotropy respectively. The different methods used to set the sample involve clearly different kinds of anisotropic elastic behaviour, which are probably indicators of diffe...

A Continuum-Based Model for Analysis of Laterally Loaded Piles in Layered Soils

Abstract: An analysis is developed to calculate the response of laterally loaded piles in multilayered elastic media. The displacement fields in the analysis are taken to be the products of independent functions that vary in the vertical, radial and circumferential directions. The governing differential equations for the pile deflections in different soil layers are obtained using the principle of minimum potential energy. Solutions for pile deflection are obtained analytically, whereas those for soil displacements are obtained using the one-dimensional finite difference method. The input parameters needed for the analysis are the pile geometry, the soil profile, and the elastic constants of the soil and pile. The method produces results with accuracy comparable with that of a three-dimensional finite element analysis but requires much less computation time. The analysis can be extended to account for soil non-linearity.

Aquifer thermal energy storage: theoretical and operational analysis

Abstract: Aquifer thermal energy storage (ATES) systems provide a method of improving the performance of more commonly installed mono-direction groundwater heating and cooling systems. Rather than using the ...

A predictive framework for liquefaction instability

Abstract: A predictive framework for flow liquefaction instabilities in sands is presented. A general criterion for liquefaction, based purely on the laws of physics, is presented and adapted to the important case of radial loading (e.g. triaxial, simple shear) using the Cambridge p′–q plane. Three important contributions are made in the paper. First, the instability concept is unified, and it is shown that the liquefaction criterion coincides with other instability criteria proposed previously. Second, the mechanics triggering liquefaction instabilities are highlighted using a simple instability criterion—underscoring the role of the material state and the underlying constitutive response. Third, the proposed framework is compared with experimental data from samples of sand under undrained triaxial compression, and it is shown that the proposed criterion correctly predicts the onset of liquefaction instability as a function of the sand state. Contractive samples encounter the so-called Lade's instability line, whe...

Investigating the time-dependent behaviour of Boom clay under thermomechanical loading

Abstract: Boom clay, a stiff clay, has been selected as a potential host formation for the geological disposal of radioactive waste in Belgium. The underground research facility HADES has been constructed to enable various in situ experiments to be performed on Boom clay so as to study the feasibility of high-level radioactive waste disposal, and to provide reliable data on the performance of Boom clay as a host formation. Among the various laboratory studies performed on samples extracted from the HADES facility to investigate the thermo-hydro-mechanical behaviour of Boom clay, relatively few were devoted to the time-dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The tests started with constant-rate thermal and/or mechanical consolidation and end...

Use of energy piles in a residential building, and effects on ground temperature and heat pump efficiency

Abstract: Piled foundations for residential buildings have predominantly been used in areas where conventional trench footing foundations have not been possible. The reasons for this have included loose and unstable overburden and contaminated ground, which result in the use of concrete piles as an economically technical solution. More recent methods have enabled concrete pile foundations to become an economic option for ground conditions that would be otherwise considered for trench-fill. It is also realised that the heat load of modern residential buildings has reduced significantly in recent years, and will fall further in forthcoming years. It is now considered that the residential energy piles can not only be economically installed, but can also provide the necessary heat requirement, which is sustainable over the life of the building. A test plot has been produced with 21 concrete piles, 10 m deep, with a single U-tube pipe in each. A heat rejection facility was devised to simulate the heat load of a two-stor...

Ground energy systems: from analysis to geotechnical design

Abstract: Ground energy systems use the ground and groundwater beneath a site as a heat source or sink to reduce energy costs and improve the environmental performance of buildings. The design and performance of the ground element of these systems (boreholes and ground loops) are dominant factors in the capital and operating costs of the system, yet, at present, such systems are often specified with little geotechnical input. This paper reviews some of the existing design approaches from a geotechnical perspective, and identifies potential failure modes (short term, long term and regulatory related) for ground energy systems. Short-term failures may result from deficiencies in the capacity of the infrastructure forming the ground element and/or from poor connection between the infrastructure and the ground. Long-term failures may derive from misestimation of loads and/or ground parameters. Possible future directions in the design of ground energy systems are discussed, and the need for informed geotechnical input to ground energy system design is highlighted.

Assessing the impact of quartz content on the prediction of soil thermal conductivity

Abstract: Accurate predictions of soil thermal conductivity k are strongly influenced by the volumetric fraction of quartz, Θq, data for which are very scarce. This paper reveals a new approach to estimate Θq from measured k records. First, an equation that relates the normalised k of soil (Ke) to the degree of saturation Sr is fitted to experimental k data, and the k at full saturation is assessed; then Θq is calculated from a geometric mean model. This modelling approach was applied to the k data of 10 Chinese soils obtained by Lu et al., also containing k measurements at full dryness, and to soils investigated by Kersten with measured quartz content data. The fitted Θq data for Chinese soils are noticeably different from the sand mass fraction, commonly assumed in the past as an equivalent of quartz content, consequently leading to irrational k estimates. Acceptably good agreement was obtained between fitted and measured quartz content for Kersten's soils. Five Ke(Sr) functions were tested against the experiment...

Experimental study of uplift resistance of shallow skirted foundations in clay under transient and sustained concentric loading

Abstract: Wind, wave and current forces cause significant overturning loads to be transmitted to foundation systems of fixed-bottom offshore structures, while buoyancy forces, in conjunction with environmental loading, impart direct tensile loads to foundations of floating facilities. Shallow skirted foundations are a potentially attractive foundation solution when overturning or tension loading is significant, as suctions mobilised within the soil plug provide tensile resistance. While passive suctions are maintained, undrained reverse end bearing will govern failure providing enhanced uplift resistance compared with vented pull-out of the foundation unit (with or without the soil plug), governed by skirt/soil friction. Currently uncertainty exists over the limiting ratio of skirt depth to foundation diameter to mobilise undrained reverse end bearing and the duration over which passive suctions can be sustained. This paper reports results from beam centrifuge tests investigating the response of shallow skirted fou...

A method of estimating the form of fine particulates

Abstract: In the companion paper we noted that a significant body of literature exists to suggest that form can have an important influence on the behaviour of an assembly of particles during shear, and that at present there are no practical methods of measuring form. A simple definition of form was adopted, and a method of measuring form was proposed for coarse particulates, and applied to a range of materials. This paper considers the form of fine particulate materials. It reviews the evolution of the size and shape of clastic and crushed rock particles, and examines the applicability of methods that have been developed in other fields for determining the form of fine particles. A new method of determining form, based on turbidity measurement and laser diffraction particle sizing, is proposed and applied to seven different materials with widely varying form.