bluuebunny/crossref_metadata_2025 huggingface dataset:
Miniature sounding tests on soft saturated cohesive soils
Authors: Van Kessel T. , Fontijn H. L.
Published: October 2009

Knowledge of the yield strength of subaqueous sediment beds is essential to calculate their sensitivity to erosion and failure. Yield strength is a parameter of the utmost importance in dredging operations. This paper describes measurements of the yield strength profiles of soft cohesive sediment samples with a miniature sounding instrument capable of measuring bed strengths of only a few pascals. Strength profiles are calculated from the forces acting on a small pile slowly penetrating into a sediment sample. During some experiments, pore and total pressures were measured. The influence of the geometry of the penetration probes used and the penetration speed is discussed. The method is shown to be suitable for measuring bed strength profiles fairly accurately. Several sediment types with varying consolidation times were tested.
CPT-based liquefaction analysis, Part 1: Determination of limit state function
Authors: Juang C. H. , Chen C. J. , Tang W. H. , Rosowsky D. V.
Published: October 2009

This paper is the first of a set of two papers that address the issue of earthquake-induced liquefaction potential. In this paper, a CPT-based limit state function for assessing the cyclic liquefaction resistance of sandy soils is presented. The liquefaction resistance of a soil is generally expressed as cyclic resistance ratio (CRR), while the seismic load that causes liquefaction is expressed as cyclic stress ratio (CSR). By definition, CRR is equal to the maximum CSR that a soil can sustain without the occurrence of cyclic liquefaction. In the present study, a new method to establish a limit state function for evaluating cyclic liquefaction resistance is developed. This new method is based on an artificial neural network (ANN) modelling and analysis of 225 field liquefaction performance records. First, an ANN model is developed to predict the occurrence of liquefaction based on historic field performance records. Second, a search procedure is developed to locate data points on the limit state surface. Third, another ANN model is created to approximate the multi-variable limit state function. The established approximate function, an ANN model, can be used to determine the CRR of a soil using CPT data. The developed CPT-based limit state function forms the basis for the development of a risk-based method for assessing cyclic liquefaction potential.
Secondary compression of clays as a local dehydration process
Authors: Navarro V. , Alonso E. E.
Published: October 2009

Long-term volumetric deformations of clays are interpreted in terms of water transfer from clay aggregates to the bulk water. Chemical potentials for the two states of water are first derived and then integrated into balance equations in order to find expressions for the rate of secondary deformation. The resulting model requires the determination of three material parameters, which have a physical interpretation. A procedure to interpret conventional consolidation tests and to find model parameters is described. The model seems to capture well the observed features of secondary clay behaviour, and reproduces in a consistent manner the response of samples of different thickness.
Modelling consolidation accelerated by vertical drains in soils subject to creep
Authors: Nash D. F. T. , Ryde S. J.
Published: October 2009

The settlement of embankments and reclamations over soft soils is frequently accelerated by the use of vertical drains. The magnitude of long-term settlement is sometimes reduced by the use of surcharge, although there is often uncertainty about how long the surcharge should be maintained to minimise creep movement. The design of vertical drains is generally based on closed-form solutions of Terzaghi's consolidation equation, and rarely takes into account non-linear stiffness and creep of the soil. In this paper a one-dimensional finite difference consolidation analysis is outlined showing how vertical and radial drainage of a multi-layer soil profile in the zone of influence of a vertical drain may be modelled. The analysis allows inclusion of a zone of peripheral smear around the drain and drain resistance, permeabilities may be varied with void ratio, and creep is modelled both during and after primary consolidation. The application of the model is illustrated with back-analysis of field data from construction of an embankment with temporary surcharge over estuarine alluvium.
Plane strain instability of saturated elasto–plastic soils
Authors: Iai S. , Bardet J. P.
Published: October 2009

The two-phase instability of saturated elasto-plastic soils is investigated under plane strain conditions. This type of material instability is characterised by a non-homogeneous field of solid deformation with vortex-like patterns, which grows rapidly and is accompanied by a non-uniform flow of interstitial water exhibiting source–sink flow patterns. Three simple criteria are established to define the conditions for the onset of two-phase instability within elasto-plastic soils saturated with an incompressible fluid and subjected to plane-strain undrained loadings. The simplest criterion states that the onset of instability almost coincides with the peak deviator stress during undrained loading. This analysis shows that two-phase instability in elasto-plastic soils is different from strain localisation, strain softening and con-stitutive singularities. Two-phase instability is likely to emerge before shear bands in contractant elasto-plastic soils of the Mohr–Coulomb type. The study suggests that two-phase instability may disrupt numerical liquefaction analyses using elasto-plastic soil models, and thus prompts the need for further analytical and experimental investigations.
Kinematic pile bending during earthquakes: analysis and field measurements
Authors: Nikolaou S. , Mylonakis G. , Gazetas G. , Tazoh T.
Published: October 2009

The passage of seismic waves through the soil surrounding a pile imposes lateral displacements and curvatures on the pile, thereby generating ‘kinematic’ bending moments even in the absence of a superstructure. These moments are concentrated in the vicinity of interfaces of alternating soft and stiff soil layers and, in the case of restrained-head piles, at the pile head. The scope of this paper is threefold: (a) to critically review some existing design methods for kinematic pile loading; (b) to develop new analytical results for piles in homogeneous and layered soils; (c) to present a case study in which theoretical predictions are tested against field measurements. To this end, an approximate beam-on-dynamic-Winkler-foundation (BDWF) model is implemented, specifically developed for the seismic response of piles in layered soil. Both fixed- and free-head piles, and different boundary conditions at the pile toe, are considered. It is shown that the magnitude of kinematic moments depends mainly on the stiffness contrast between the soil layers, the pile–soil stiffness contrast, the excitation frequency, and the number of excitation cycles. A unique case history involving the instrumented pile foundation of a multistorey building in Japan is presented. Time histories of bending and axial strains recorded at six locations along two piles are successfully compared with results computed from simple formulae and methods presented in the paper.
Fuzzy partitioning algorithms applied to the interpretation of distinct element modelling results
Authors: Harrison J. P. , Ferrero A. M. , Cravero S.
Published: October 2009

The distinct element method (DEM) has emerged as one of the principal computational methods for problems involving the analysis of fractured rock masses. The DEM is a recursive, time-stepping technique, and complex models may take many hundreds of thousands of cycles to converge to equilibrium. Even with fast computers this can be time consuming, and—depending on the model under consideration—may be strictly unnecessary, as the instability of the model is clear. Here, we demonstrate how the fuzzy c means (FCM) clustering algorithm can be used to analyse the kinematic and dynamic behaviour of the material in a DEM model. Clustering based on kinematics allows blocks to be interpreted in terms of the form of their instability (i.e. falling, sliding or stable), and clustering based on dynamics (i.e. forces and moments) can be used to delineate the zone of unstable rock around an excavation. Determination of the size of the zone of unstable rock leads directly to a calculation of support pressure.
Sols renforcés par Boulonnage—Etude numérique et application au front de taille d'un tunnel profond
Authors: Dias D. , Kastner R. , Jassionnesse C.
Published: October 2009

Soils or soft rocks reinforced with linear inclusions constitute a composite material which is difficult to numerically model, due to large contrasts between the deformation module and differences of scale. The homogenisation of recurring environments means that the behaviour of macroscopic materials such as these can be considered under certain conditions. In this article, the technique is described and the anisotropic behaviour of the homogenised material and its positioning in an axial symmetry numerical code is outlined. The comparison with a three-dimensional analysis of homogeneous loadings allows the limits of this approach to be studied. Lastly, the comparison of the homogenisation technique with three-dimensional modelling is carried out on deep tunnels reinforced at the coalface.
Centrifuge model tests on embedded retaining walls supported by earth berms
Authors: Powrie W. , Daly M. P.
Published: October 2009

Earth berms are often used for the temporary support of embedded retaining walls, but there is a lack of information concerning the factors that influence their behaviour. In this paper, the results of a series of plane strain, 1:100 scale centrifuge model tests on berm-supported embedded retaining walls in overconsolidated clay are presented. It is shown that earth berms will probably be more effective in reducing soil and wall movements than an increase in the depth of wall embedment; that an increase in the depth of embedment of a wall of given stiffness supported by a berm of a given size will lead to an increase in wall bending moments, but only a small reduction in wall and soil movements; and that the presence of the berm in limiting wall movements and possibly preventing collapse becomes more significant as drainage occurs.
Three-dimensional modelling of NATM tunnelling in decomposed granite soil
Authors: Shin J. H. , Potts D. M. , Zdravkovic L.
Published: October 2009

Three-dimensional analyses of tunnel excavation and lining construction are rarely performed in practice, because it is difficult to reproduce the exact boundary conditions involved in this process, and because such analyses require considerable computing resources if realistic non-linear soil behaviour is to be reproduced. This paper presents a pseudo-three-dimensional analysis of tunnel construction by utilising the Fourier series aided finite element method (FSAFEM). The theory behind this approach, and its validation when applied to a tunnelling problem, are described. The method is then used to predict the behaviour of tunnels forming part of the Seoul Subway system in Korea. The results are compared with field measurements.
Cylindrical cavity expansion in dilatant cohesive-frictional materials
Authors: Mántaras F. M. , Schnaid F.
Published: October 2009

A procedure for analysing the expansion of a cylindrical cavity in cohesive-frictional materials is described, with the aim of providing a theoretical framework on which to base analyses of pressuremeter tests. The concept that plastic dilatancy is inhibited by the presence of soil bonding is investigated and used to describe the plastic components of the tangential and radial increments. This new solution is formulated within the framework of non-associated plasticity in which the Euler method is applied to solve simultaneously two differential equations leading to the continuous variation of strains, stresses and volume changes produced by cavity expansion. Application of the proposed approach is particularly relevant in hard soils and soft rocks.
On the plastic hardening of sand
Authors: McDowell G. R. , Nakata Y. , Hyodo M.
Published: October 2009

Recent research on the micromechanics of sands has suggested that plastic hardening is due to grain fracture. A theory based on a work equation suggests that, for a sand subjected to plastic deformation, suitable soil hardening parameters are the isotropic preconsolidation pressure relating to plastic reduction in voids ratio, the surface area per unit mass of solids, and the plastic work done. The theory also suggests that these parameters are linear functions of one another. An alternative theory based on fractal crushing also suggests that surface area is a suitable hardening parameter. This paper examines the behaviour of silica sand subjected to a range of high-pressure stress path tests in the triaxial apparatus, in order to try to establish suitable parameters for plastic hardening, and whether the recent theories can be supported. For this sand a simple state boundary surface can be established, and it can reasonably be assumed that strains inside the surface are elastic and isotropic. The size of the yield surface is indeed shown to be a linear function of the increase in sample surface area and of the plastic work per unit volume done on the sample, so that for stress paths to points on the state boundary surface the plastic work done per unit volume and increase in surface area are approximately independent of the path taken. However, although particle breakage is necessary for plastic hardening, most of the plastic work done is shown to be dissipated in friction.
Cyclic settlement behaviour of spudcan foundations
Authors: Ng T. G. , Lee F. H.
Published: October 2009

This paper examines the cyclic settlement behaviour of spudcan foundations in sand, using centrifuge model data. By means of a servo-electrohydraulic system, loading episodes comprising several thousand cycles of sinusoidal loading were applied to model spudcan foundations. The findings showed that, in dry sand, non-preloaded spudcans settle by penetration into the soil, resulting in a large increase in cyclic stiffness. In contrast, preloaded spudcans appear to settle by densification of the sand, leading to a smaller increase in cyclic stiffness. In saturated sand, where excess pore pressure can accumulate, the effects of preloading are less evident. A non-preloaded spudcan tends to generate a bulb of negative pore pressure around itself, conferring some apparent preloading on the near-field soil, and thereby resisting further penetration. In a preloaded model, the negative pore pressure bulb is smaller and surrounded by a zone of positive pore pressure. As the positive pore pressure propagates into the negative pore pressure bulb, the effective stress of the sand around the spudcan decreases, allowing further penetration. This is manifested as an apparent loss of preload. Thus non-preloaded and preloaded spudcans may settle by densification and penetration.
Ground response to dynamic compaction of dry sand
Authors: Gu Q. , Lee F.-H.
Published: October 2009

The mechanics of dynamic compaction are studied in this paper using two-dimensional finite element analyses with a large-strain dynamic formulation and a cap model for soil behaviour. Comparison with centrifuge model results shows that stress wave attenuation and improvement effects are realistically predicted. The analyses show that, in the initial blows, stress wave propagation induces transient elasto-plastic K0 compression due to lateral inertia. This preserves the plane wavefront and reduces the attenuation rate of the dynamic stresses with depth. With multiple blows, the effect changes to one of triaxial compression: this sets a limit on the degree of improvement that can be achieved in the near field. Deeper down, the wavefront adopts a bullet shape, and the attenuation rate rises: this sets a limit on the depth of improvement. Both these phenomena are consistent with the existence of a ‘threshold’ state that has been noted in previous literature on dynamic compaction. The results also show that the depth of improvement is dependent upon the momentum per blow, as well as on the energy per blow.
Methods for determining representative density–depth profiles using nuclear density gauges
Authors: Winter M. G. , Clarke B. G.
Published: October 2009

Most fills when compacted in layers will, to some degree, experience variations in density with depth. When measured using a nuclear density gauge in direct transmission mode, such variations in density through the layer will lead to unrepresentative density–depth profiles. This is because each measurement used to generate the profile is the mean density between the surface and the depth of measurement, and no correction for this is currently made. Uniform sands and silts, and pulverised fuel ash (PFA), often go through a process of over-stressing during compaction, leading to significantly lower densities in the upper part of a compacted layer. Under these circumstances the errors recorded in nuclear density gauge determinations of the density–depth profile may become significant. In extreme cases the usual guidance is to remove the upper part of the compacted layer prior to density testing. This paper describes numerical techniques to determine the mean density between successive measurement depths for the cases of both equal and unequal depth increments, thus giving a density–depth profile that is much closer to reality.
Modelling tailings dams and mine waste dumps failures
Authors: Pastor M. , Quecedo M. , FernÁndez Merodo J. A. , Herrores M. I. , GonzÁlez E. , Mira P.
Published: October 2009

This paper presents a depth-integrated model that can be used to simulate the flowslides, mudflows and debris flows that are caused by the failure of tailing dams, mining waste dumps and other similar structures. The sliding mass is first determined using a coupled elastoplastic finite element code and a suitable constitutive equation. Once failure has been triggered, propagation of the mobilised material is analysed considering flow properties averaged over depth. A simple law of pore pressure dissipation is postulated for cases in which the consolidation and propagation times are of the same order of magnitude.
Basin-scale compressibility of the northern Adriatic by the radioactive marker technique
Authors: Baú D. , Ferronato M. , Gambolati G. , Teatini P.
Published: October 2009

The paper presents a constitutive model for the one-dimensional vertical compressibility, mv, of the northern Adriatic sedimentary basin based on in-situ deformation measurements by the radioactive marker technique. Gas reservoir compaction and expansion occurring during the field production life and after its abandonment have been measured in three deep boreholes of the northern Adriatic by FSMT (formation subsidence monitoring tool) and CMI (compaction monitoring instrument) developed by Schlumberger and Western Atlas respectively. The compressibility values, as obtained by relating the shortening of the distance between several pairs of adjacent markers to the pore pressure decline experienced by the productive levels during the monitored time interval, are statistically processed to reduce the influence of operational and instrumental errors. The method of moving weighted averages is used over clustered depth intervals. The compressibility thus derived is related to the in-situ vertical effective stress to provide basin-scale constitutive relationships for the expected mv in both virgin loading and unloading–reloading conditions, together with the 68% and 95% confidence intervals characterising the law uncertainties. The results indicate that for virgin loading mv ranges between 7 × 10−4 MPa−1 and 5 × 10−5 MPa−1 over the effective vertical stress interval 10< p′ < 80 MPa—that is, for a depth between 900 m and 7000 m in undisturbed conditions—and decreases by two to three times in unloading–reloading conditions.
Modelling the variation of degree of saturation in a deformable unsaturated soil
Authors: Gallipoli D. , Wheeler S. J. , Karstunen M.
Published: October 2009

An improved relationship for the variation of degree of saturation in an unsaturated soil is presented, incorporating the influence of changes of void ratio. When combined with an elasto-plastic stress–strain model, this is able to represent irreversible changes of degree of saturation and changes of degree of saturation caused by shearing. Experimental data from tests on compacted Speswhite kaolin are used to demonstrate the success of the proposed new expression for degree of saturation. The experimental data involve a wide variety of stress paths, including wetting, isotropic loading and unloading under constant suction, constant suction shearing, and constant water content shearing. Improved representation of the variation of degree of saturation has important consequences for numerical modelling of coupled flow-deformation problems, where the expression used for the degree of saturation can influence significantly the suction generated within the soil and hence the predicted stress–strain behaviour.
Elastic-plastic modelling of unsaturated soil using results from a new triaxial test with controlled suction
Authors: Blatz J. A. , Graham J.
Published: October 2009

This paper examines the constitutive behaviour of a compacted highly plastic sand–clay mixture. Three series of tests evaluated components of a possible elastic-plastic constitutive model for unsaturated stress–strain behaviour. The tests examined the influence of suction on yield stress under external isotropic loading and the influence of suction on shear strength. The results examine the behaviour of highly plastic clay subjected to generalised (triaxial) stresses that include shear, isotropic and suction loading. The data represent an important new resource of information on a densely compacted, strongly expansive, sand–bentonite mixture that was loaded independently by external stresses and internal suctions. The suctions were also measured by sensors buried in the specimens. They have been interpreted in an elastic-plastic framework to produce values for some of the material properties needed for the model. The results and associated discussion provide new insights into the behaviour of unsaturated highly plastic clay materials.
Instabilities on moraine slopes induced by loss of suction: a case history
Authors: Springman S. M. , Jommi C. , Teysseire P.
Published: October 2009

In comparison with saturated conditions, negative pore water pressures arising from partial saturation increase the available shear strength on a potential slip surface. This additional contribution is lost progressively during infiltration of rainfall, leading to instabilities, sometimes before full saturation is reached. In such cases, reliable prediction of the safety factor may be achieved, by taking the suction history of the soil into account. A combination of field and laboratory tests was carried out recently to investigate the triggering effect of rainfall on shallow slips in alpine moraine slopes. The problem is complicated by the strong heterogeneity of these soils, with particle sizes varying from silts to boulders. The data highlight the influence of suction on the peak shear strength, and allow for the calibration of simple models, which take into account the dependence of shear strength on the saturation degree. An infinite slope stability analysis is performed. The results are compared with the field data from two test sites, where the development of a shallow planar slip in the steeper 42° slope was observed after two days of artificial rainfall. The less steep 31° slope remained stable.
The vadose zone soil-water balance and transpiration rates of vegetation
Authors: Blight G. E.
Published: October 2009

A conventional geotechnical site investigation usually provides only a snapshot of soil-water conditions at a site, particularly for the vadose zone. Although it is generally recognised that soil suction and water contents will vary from season to season and from year to year, designs are almost invariably based on what is discovered during a single short-term investigation. As a result, long-term problems may, and often do, arise, which were not foreseen as a result of the site investigation. An evaluation of the annual water balance for a soil system provides a means of examining and understanding the effects of seasonal and longer-term changes in soil-water conditions, and for predicting the effects of future changes of water conditions in the soil. The paper gives an illustration of the use of the water balance technique to investigate annual changes of soil-water conditions in situ. Evapotranspiration is a major component of any soil-water balance, and as it is probably the most difficult component to estimate, special attention is given to its characteristics and estimation, and particularly to the transpiration component of total evapotranspiration and to evapotranspiration from trees.
PSD measurement using the single particle optical sizing (SPOS) method
Authors: White D. J.
Published: October 2009

A new technique for measuring the particle size distribution (PSD) of a soil, known as single particle optical sizing (SPOS), has been evaluated. A series of tests was conducted to compare the results obtained from sieve analysis and the SPOS method, using both uniform and well-graded sands of varying particle size and angularity. It was found that the SPOS method oversizes particles compared with sieving by 20–30%, as predicted by theoretical analysis. Measurements of particle size gathered from sieving and SPOS methods are not equivalent. However, as neither of the definitions of particle size implicit in each method can be considered as ‘correct’, this exercise does not represent a ‘validation’ or otherwise of the SPOS method. The repeatability of the SPOS method was demonstrated by sizing multiple samples obtained by riffling from the same batch of sand. The accuracy of the SPOS method was demonstrated by the high correlation between the calculated and measured PSD of mixed samples. Finally, a series of tests showed that the system can easily resolve a small (<1%) introduction of fine material into a coarser sample. As the technique requires only a small sample volume, it is particularly suited to the assessment of changes in PSD during geotechnical model or element testing.
Proposal for a new plasticity chart
Authors: Polidori E.
Published: October 2009

This study investigates the consistency limits of the pure clay minerals kaolinite and montmorillonite, and their respective mixtures with fine silica sand in various proportions. Plotting the plasticity index data as a function of the liquid limit allows the zones where mixtures with the same clay contents fall to be defined. In particular, the line corresponding to 50% clay (designated as the 0·5C-line) makes it possible to distinguish the points that lie below the line, namely clay, from the points lying above the line in the silt zone. The clay zone includes inorganic soils with clay contents ≥50%, whereas the silt zone includes inorganic soils composed of silt and/or sand (2–425 μm) in percentages >50%. A new plasticity chart, which aims to classify soils (<425 μm) using the Atterberg limits, is defined herein. It differs from Casagrande's plasticity chart, especially in terms of its silt and clay zones, whose positions are reversed compared with Casagrande's chart. This can be explained by the fact that, contrary to what is commonly believed, in inorganic soils—liquid limits being equal—the plasticity index increases as the clay content decreases. Save only a few exceptions, the examples of inorganic soils plotted on the new plasticity chart lie above the 0·5C-line in the silt zone or below the line in the clay zone when their clay contents are less or greater than 50% respectively. On Casagrande's plasticity chart, the same soils lie above the A-line in the clay zone, regardless of their clay contents (9·7–100%). Only the kaolinite samples (relatively pure) lie below the A-line in the silt zone.
Estimation of active earth pressure against rigid retaining walls considering arching effects
Authors: Paik K. H. , Salgado R.
Published: October 2009

It is known that the distribution of active earth pressure against a translating rigid wall is not triangular but nonlinear, owing to arching effects in the backfill. In the present paper, a new formulation is proposed for calculating the active earth pressure on a rigid retaining wall undergoing horizontal translation. It takes into account the arching effects that occur in the backfill (or retained soil mass). In order to check the accuracy of the proposed formulation, the predictions from the equation are compared with both existing full-scale test results and values from existing equations. The comparisons between calculated and measured values show that the proposed equations satisfactorily predict both the earth pressure distribution and the lateral active earth force on the translating wall. In order to facilitate calculation of active earth pressures using the proposed equations, a modified active pressure coefficient and an effective height of application of the lateral active force are also provided as functions of the soil friction angle, ϕ, and the wall friction angle, δ.
Elastic response of circular footings embedded in a non-homogeneous half-space
Authors: Doherty J. P. , Deeks A. J.
Published: October 2009

This study uses a semi-analytical technique (the scaled boundary finite-element method) to evaluate the load–displacement response of rigid circular footings embedded in a non-homogeneous elastic half-space. Results are presented in terms of dimensionless elastic stiffness coefficients. The footings are subjected to vertical, horizontal, moment and torsional loads. The cross-coupling between the horizontal and moment load case is also evaluated. The variation of soil shear modulus with depth may correspond to normally consolidated sand or clay, and the footing geometries considered cover a range of footings commonly encountered in engineering practice.
Behaviour of calcareous soils in undrained cyclic simple shear
Authors: Mao X. , Fahey M.
Published: October 2009

The behaviour of three calcareous sediments—a muddy silt, a silt and a sand from offshore areas on the North-West Shelf of Australia—is studied through undrained simple shear tests. In monotonic tests, the muddy silt shows contractive behaviour and the silt and sand show dilative behaviour. In cyclic tests, a cyclic mobility failure mode is observed for the three soils. Responses in non-symmetrical cyclic tests were significantly different from those in symmetrical cyclic tests. A ‘cyclic phase transformation state’, which separates undrained cyclic stress paths into contractive and dilative phases, was identified: this was found to be different from the phase transformation line obtained from monotonic tests, and it is unique for a particular soil. The three soils show strain hardening behaviour in plots of stress ratio against shear strain, which can be characterised by a backbone curve and hysteresis loops.
Modelling of natural soft clay destruction processes using viscoplasticity theory
Authors: Rocchi G. , Fontana M. , Da Prat M.
Published: October 2009

A viscoplastic constitutive model is proposed that can reproduce both strain rate dependence of natural soft clay behaviour and destruction processes (DPs) induced by strain or stress increments. The model tracks changes in the yield locus from the initial position (INYL), which reflects the presence of metastable structures, to the intrinsic position (IRYL), which represents structurally stable conditions, where DPs are almost complete. The model is able to simulate in oedometric conditions Bjerrum's 1967 framework, and makes a clear distinction between the vertical effective yield stress, σ′vy, above which significant irrecoverable time-dependent strains are irreversibly activated, and the preconsolidation pressure, σ′vp, from Casagrande's construction, helping to avoid possible sources of error in consolidation settlement predictions. Notwithstanding some simplifications, the model fits well the experimental results of oedometric tests on a natural soft clay with distinct σ′vy and σ′vp for which viscoplastic modelling becomes essential. The model, which the authors have implemented in the finite element code TOCHNOG, is presented in sufficient detail for application to back-analysis and solution of boundary value consolidation problems under loading conditions.
Permeability of a bentonite–crushed granite rock mixture using different experimental techniques
Authors: Mata C. , Ledesma A.
Published: October 2009

The use of bentonite-based mixtures as a sealing material for deep geologic repositories is common to many different concepts for isolating nuclear fuel waste. In this context, saturated hydraulic conductivity becomes a fundamental parameter when analysing the long-term behaviour of these facilities. Despite that, measurement of hydraulic conductivity is still far from being routine for very low-permeability materials. A mixture of 30% sodium bentonite and 70% crushed granite rock is being considered as a sealing material in a future repository in Sweden. This paper describes the experimental programme carried out to determine the permeability of this mixture. Constant-head tests, pulse tests and oedometer tests have been used for that purpose, but because of the characteristics of the material—large grain size and very low permeability—special test procedures and equipments had to be developed. The paper also presents the analysis of the results, including a numerical simulation of the pulse test by using finite elements. A comparison of all results established an average hydraulic conductivity of 6·5 × 10–12 m/s for a dry specific weight of the mixture of 15·9 kN/m3. Darcy's law was also confirmed for this material over the range of gradients applied (4 to 100). Finally, the paper describes a methodology suitable for the measurement of hydraulic conductivity of low-permeability laboratory samples based on the pulse test concept.
Catastrophic failure of submerged slopes in normally consolidated sediments
Authors: Puzrin A. M. , Germanovich L. N. , Kim S.
Published: October 2009

The application of fracture mechanics methods to shear band growth in granular materials requires careful consideration of the elasto-plastic nature of the material behaviour. In this paper, the Palmer and Rice fracture mechanics approach is applied to the problem of catastrophic shear band propagation in a submerged slope built of normally consolidated sediments. This allows for catastrophic (unstable) and progressive (stable) types of soil failure to be properly distinguished. Many submerged landslides are triggered by earthquakes, via the shear strength degradation, due to the excess pore water pressure accumulation. The conventional analysis assumes that the excess water pressure zone develops simultaneously along the entire landslide length, often reaching several kilometres. When a mechanism of the catastrophic shear band propagation is considered, a much shorter excess water pressure zone (e.g. several hundred metres) can still cause a full-scale landslide (often generating a tsunami wave of considerable height). This mechanism also suggests possible measures for mitigation of the landslide hazard.
Crushing and plastic deformation of soils simulated using DEM
Authors: Cheng Y. P. , Bolton M. D. , Nakata Y.
Published: October 2009

Cheng and co-workers showed how to make numerical simulations of crushable soils by the discrete element method (DEM). Stress-path tests on triaxial elements comprising crushable agglomerates have now been simulated. The plastic behaviour of this numerically generated soil closely resembles that of real sand. Crushing in the aggregate begins at stresses less than one tenth of the characteristic strength of single grains. The yield surfaces of isotropic ‘lightly overconsolidated’ DEM simulations are also contours of breakage, and are elliptical on Cambridge-style (q, p′) plots and symmetrical about the p′ axis. The points of maximum deviator stress at yield lie along lines of stress ratio My = ±0·8, but the plastic strain increments at yield are non-associated, giving more contraction than normality would allow. Significantly, therefore, the stress ratio My was found not to coincide with critical states. All stress-path simulations yielding with q/p′ > My were found to satisfy the requirements of stress–dilatancy theory. In particular, their yielding was best described using a unique Mohr–Coulomb angle of internal friction ϕ, correlated with dilatancy rate. Points of zero dilation were found within this regime, providing a critical-state friction angle ϕcrit = 42° for these very ‘rough’ agglomerates. They also coincided with the location of a critical-state line on an e–log p′ plot. The peak angle (ϕpeak) developed in a variety of tests showed a unique correlation, reducing by progressive grain crushing as log σ′1 increased. As macroscopic stress levels approached the characteristic crushing strength of grains, it was impossible even to mobilise ϕ = ϕcrit, owing to large strains and high degress of breakage.
A comparison of the combined load behaviour of spudcan and caisson foundations on soft normally consolidated clay
Authors: Cassidy M. J. , Byrne B. W. , Randolph M. F.
Published: October 2009

Experimental data from loading tests of model circular footings on soft normally consolidated clay are presented. The experiments were carried out on a drum centrifuge at a radial acceleration level equivalent to 100 times Earth's gravity, ensuring conditions of stress similitude between the model and the prototype scale. The aim of the experiments was to compare the undrained response of different offshore foundations to the same loading conditions. Two different types of foundation were targeted for investigation: spudcans and suction caissons. The spudcan, typically an inverted shallow cone, is the traditional footing for mobile drilling units (also known as jack-up rigs). An alternative foundation concept that is being increasingly considered is that of foundations skirted about the perimeter and installed by suction. A loading arm that incorporated an internal hinge was used so that combinations of load appropriate to the foundations of jack-up units could be applied to the models. Although it was anticipated that, by using skirted foundations there would be increased moment capacity, this was not found to be the case (for the caisson skirt length and soil strength profile investigated). However, there was a stiffer response and additional horizontal capacity at the foundation level. Significantly the use of skirted foundations allowed for a greater combined loading capacity under tensile vertical loads. The results have been interpreted within the framework of strain-hardening plasticity theory, and comparisons with existing yield surfaces are detailed.
An improved direct shear apparatus for sand
Authors: Lings M. L. , Dietz M. S.
Published: October 2009

A conventional direct shear apparatus (DSA) has been modified to improve its articulation by moving the point of shear load application to the sample centre. Jewell's symmetrical arrangement has been adopted for a 100 mm square shearbox, resulting in increased dilation rate and reduced rotation. Testing using coarse Leighton Buzzard sand has enabled the performance of the modified DSA to be optimised by exploring different test configurations. Optimum results were obtained with a substantial initial gap between the shearbox frames of 5D50, the use of thin rubber edging strips to contain the sand, and the omission of grid plates at the sample boundaries. Upper-frame rotations persist during testing, but have almost no effect on measured parameters. A framework of relationships between parameters measured in a direct shear test ((ϕ′ds)p, ψp and (ϕ′ds)Id) and relevant plane strain parameters ((ϕ′ps)p and ϕ′crit) is set out, based on Rowe's flow rule and Davis's (1968) relation. Stroud's simple shear apparatus (SSA) data for coarse Leighton Buzzard sand have been examined and shown to fit Rowe's flow rule. The output from the modified DSA also fits these relationships well, and shows good internal consistency. A simple relationship enables modified DSA peak direct shear friction angles to be converted into plane strain friction angles. Large displacement direct shear friction angles for the modified DSA are related to critical state friction angles by tan(ϕ′ds)Id = sin ϕ′crit.
A numerical study of cone penetration in clay
Authors: Lu Q. , Randolph M. F. , Hu Y. , Bugarski I. C.
Published: October 2009

This paper presents results from large-displacement finite-element analysis of cone penetration into clay. The soil is idealised as a homogeneous elastic-perfectly plastic material obeying a Tresca yield criterion, and the analysis is carried out using an ‘arbitrary Lagrangian–Eulerian’ technique, with periodic remeshing and interpolation of all field values. This allows the cone to be advanced by several diameters, thus achieving steady-state conditions. A full parametric study has been undertaken, quantifying the influences of the rigidity index, in situ stress anisotropy and the cone roughness. A theoretical correlation for the cone factor, Nkt, is developed from this study, and compared with previous correlations developed using the strain path method. Characteristics of the stress distribution around the cone, the extent of the plastic zone and apparent incremental movements are discussed, allowing new insights into this problem.
A constitutive model for structured clays
Authors: Baudet B. , Stallebrass S.
Published: October 2009

A simple constitutive model for structured clays is presented. The model is based on an existing constitutive model for reconstituted clays. It uses a fixed relationship between change in sensitivity and plastic strain to represent destructuration so that all predicted effects of structure are included in the value of sensitivity. Only three added parameters are required, which can each be derived from a single isotropic compression test. All other parameters are the same as those used in the base model, which are derived from test data on reconstituted specimens. A value of ultimate sensitivity greater than unity allows the model to simulate clays with stable and/or metastable structure. Results from a parametric study show that the model predicts responses during compression and shearing typical of natural clays. The model was applied to test data from a natural soft clay using parameters derived where possible from isotropic compression tests. Results from the analyses prove that the simplified formulation of the model is sufficient to significantly improve predictions.
Two- and three-dimensional bearing capacity of foundations in clay
Authors: Salgado R. , Lyamin A. V. , Sloan S. W. , Yu H. S.
Published: October 2009

Bearing capacity calculations are an important part of the design of foundations. Most of the terms in the bearing capacity equation, as it is used today in practice, are empirical. Shape factors for square and rectangular footings could not be derived in the past because three-dimensional bearing capacity computations could not be performed with any degree of accuracy. Likewise, depth factors could not be determined because rigorous analyses of foundations embedded in the ground were not available. In this paper, the bearing capacities of strip, square, circular and rectangular foundations in clay are determined rigorously based on finite element limit analysis. The results of the analyses are used to propose rigorous, definitive values of the shape and depth factors for foundations in clays. These results are helpful in reducing the uncertainties related to the method of analysis in bearing capacity calculations, paving the way for more cost-effective foundation design.
On microstructural tests for poroelastic materials and corresponding Gassmann-type relations
Authors: Wilmanski K.
Published: October 2009

This paper considers a transition from the microstructure to a linear thermodynamic model of poroelastic media that yields Biot's model. It investigates a two–component poroelastic linear model in which a constitutive dependence on the porosity gradient is incorporated, and this is compared with the classical Biot's model without added mass effects. It analyses three microstructural tests (Gedankenexperiments)—jacketed undrained, jacketed drained, and unjacketed—and derive a generalisation of classical Gassmann relations between macroscopic material parameters and microstructural (true) compressibility moduli of the solid, and of the fluid. Dependence on the porosity is particularly exposed owing to its importance in acoustic applications of the model. In particular it is shown that Gassmann relations follow as one of two physically justified solutions of the full set of compatibility relations between microstructure and the macroscopic model. In this solution the coupling to the porosity gradient is absent. Simultaneously, the paper demonstrates the second solution, which lies near the Gassmann results but admits the coupling. In both models couplings are weak enough to admit, within the class of problems of acoustic wave analysis, an approximation by a ‘simple mixture’ model in which coupling of stresses is fully neglected.
Probabilistic assessment of stability of a cut slope in residual soil
Authors: El-Ramly H. , Morgenstern N. R. , Cruden D. M.
Published: October 2009

A probabilistic slope analysis methodology based on Monte Carlo simulation using Microsoft Excel and @Risk software is applied to investigate the failure of the Shek Kip Mei cut in Hong Kong. The study demonstrates the techniques used in quantifying uncertainties in shear strength of granitic soils based on a large database of triaxial tests. Probabilistic back-analyses of the failure are applied to estimate the probability distribution of the pore water pressure. Using the back-calculated pore pressure, the inclination of the Shek Kip Mei slope is redesigned to a flatter inclination, and the probability of unsatisfactory performance and reliability index are estimated.
Towards reliable and effective site investigations
Authors: Jaksa M. B. , Goldsworthy J. S. , Fenton G. A. , Kaggwa W. S. , Griffiths D. V. , Kuo Y. L. , Poulos H. G.
Published: October 2009

It is widely appreciated that, in civil engineering and building projects, the largest element of financial and technical risk usually lies in the ground. Almost exclusively, the scope of geotechnical investigations is governed not by what is needed to characterise the subsurface conditions appropriately but, rather, by how much the client and project manager are willing to spend. There is often little correlation between the variability of the ground and the scope of the investigation. This paper presents the results of a Monte Carlo simulation incorporating many 3D single-layer soil profiles with different statistical characteristics. A three-storey building founded on nine pad footings is used to assess the reliability of various site investigation scopes and test methods. The pad footings are designed on the basis of settlement, and are examined using 3D finite element analysis and Schmertmann's method. It is observed, as expected, that the likelihood of underdesigning or overdesigning a footing decreases as the scope of the investigation increases. The relationship between these likelihoods and the variability of the ground is presented.