Showing papers in "Geotechnique in 2001"

Piled raft foundations: design and applications

Abstract: In situations where a raft foundation alone does not satisfy the design requirements, it may be possible to enhance the performance of the raft by the addition of piles. The use of a limited number of piles, strategically located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful. Some of the characteristics of piled raft behaviour are described. The design process for a piled raft can be considered as a three-stage process. The first is a preliminary stage in which the effects of the number of piles on load capacity and settlement are assessed via an approximate analysis. The second is a more detailed examination to assess where piles are required and to obtain some indication of the piling requirements. The third is a detailed design phase in which a more refined analysis is employed to confirm the op...

Natural slopes and cuts: movement and failure mechanisms

Abstract: Movements and failure of cuts and natural slopes constitute an important geotechnical problem that involves a variety of geomaterials in a variety of geological and climatic contexts, and which has a major socio-economic impact in many countries. The paper reviews the state of knowledge in this domain, examining the basic aspects of soil mechanics that are relevant to the context of slopes, the importance of water on slope behaviour, and then the different stages of slope movements: pre-failure, failure, post-failure and reactivation. Finally it is shown how the geotechnical characterisation of slope movements can be used, in particular for assessing risk associated with such movements. Emphasis is put on for the brittleness of soils and its practical implications for the progressive failure developing at the pre-failure stage and on the characteristics of post-failure movements. The influence of other factors such as creep, fatigue, destructuration, partial saturation and infiltration is also considered.

Particle shape characterisation using Fourier descriptor analysis

Abstract: A novel technique for the objective assessment of particle shape is presented. The technique uses Fourier shape descriptors and image analysis of scanning electron microscope photographs of sand grains to provide an accurate quantification of particle morphology and texture. Three lower order Fourier descriptors, denoted ‘signature descriptors’, provide measures of elongation, triangularity and squareness, while an additional descriptor, denoted ‘asymmetry’, provides a measure of particle irregularity. Together, these four descriptors quantify the overall shape of soil particles (defined as ‘morphology’). A summary of higher-order descriptors provides textural information that is related to local roughness features (defined as ‘texture’). The results of studies on three silica sands (two standard, laboratory-use and one natural, unprocessed) and one carbonate sand are presented. Breakage of particles by crushing is shown to affect the morphological signature differently depending on the type of sand, thou...

Bearing capacity of spatially random soil: the undrained clay Prandtl problem revisited

Abstract: By merging elasto-plastic finite element analysis with random field theory, an investigation has been performed into the bearing capacity of undrained clays with spatially varying shear strength. The object of the investigation is to determine the extent to which variance and spatial correlation of the soil's undrained shear strength impact on the statistics of the bearing capacity. Throughout this study, bearing capacity results are expressed in terms of the bearing capacity factor, Nc, in relation to the mean undrained strength. For low coefficients of variation of shear strength, the expected value of the bearing capacity factor tends to the Prandtl solution of Nc = 5·14. For higher values of the coefficient of variation, however, the expected value of the bearing capacity factor falls quite steeply. The spatial correlation length is also shown to be an important parameter that cannot be ignored. The results of Monte Carlo simulations on this non-linear problem are presented in the form of histograms, ...

Displacement and strain paths during plane-strain model pile installation in sand

Abstract: The underlying mechanisms governing the behaviour of displacement piles in sand are not well understood, leading to unreliability in design methods. A series of plane-strain calibration chamber tes...

Development of stabilisation and solidification in lime-clay mixes

Abstract: Two refined clay minerals, English China Clay (predominantly kaolinite) and Wyoming Bentonite (predominantly sodium-montmorillonite), were used to assess the time-dependent effects of mineral structural chemistry on the lime–clay reaction. The two clays, representing the extremes of structural negative charge development, were treated with two lime contents. Changes in the physico-chemistry of the samples were monitored after three curing periods by performing batch leaching tests and monitoring changes in undrained shear strength and Atterberg limits. The effects of changing the batch test operating parameters and increasing the curing period were assessed by measuring changes in solution pH and conductivity, and by analysis of eight elements in solution. The solidification mechanisms due to lime addition were found to be different for the two minerals. Pozzolanic reactions involving English China Clay and Wyoming Bentonite were found to occur owing to the dissolution of aluminium and silicon from the re...

Stability of plate anchors in undrained clay

Abstract: Soil anchors are commonly used as foundation systems for structures requiring uplift resistance, such as transmission towers, or for structures requiring lateral resistance, such as sheet pile walls. To date, the design of these anchors has been largely based on empiricism. This paper applies numerical limit analysis to rigorously evaluate the stability of vertical and horizontal strip anchors in undrained clay. Rigorous bounds on the ultimate pull–out capacity are obtained by using two numerical procedures that are based on finite element formulations of the upper and lower bound theorems of limit analysis. These formulations follow standard procedure by assuming a rigid perfectly plastic clay model with a Tresca yield criterion, and generate large linear programming problems. By obtaining both upper and lower bound estimates of the pull–out capacity, the true pull–out resistance can be bracketed from above and below. Results are presented in the familiar form of break–out factors based on various soil s...

Theory and practice of the fall cone test

Abstract: The fall cone is considered as a more reliable method for determining the liquid limit than the Casagrande method, and is standardised in many countries as the preferred liquid limit test method. In this paper the theory and practice of the fall cone test are described. First the penetration mechanism of a fall cone into clay is analysed, introducing the concept of dynamic strength to the static results. Next the applicability of dynamic analysis to the fall cone test is examined with regard to Hansbo's cone factor, K, for various cone angles. The theoretical K value for the 60° cone with a semi-rough surface is found to agree better with experimental results than is the case for the 30° cone. It is proposed that the liquid limit be redefined internationally as the water content at which a 60°, 60g fall cone penetrates 10 mm. Finally the applicability of the fall cone test as a device to relate the strength of a clay with the index properties is examined.

Kinematic pile bending during earthquakes: analysis and field measurements

Abstract: 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 su...

Undrained cavity expansion in modified Cam clay I: Theoretical analysis

Abstract: This paper presents an analysis of the undrained expansion of a cavity in modified Cam clay. The problem is formulated in small strain in the elastic zone and large strain in the plastic zone. Exact closed-form solutions of the effective stresses around the expanded cavity are presented. Closed-form expressions for the excess pore water pressure and the total stresses have also been derived based on a simplified assumption regarding the shear stress variation around the cavity. The analytical solution shows clearly the influence of the overconsolidation ratio on the excess pore water pressure induced by cavity expansion. As both the excess pore water pressure and the cavity pressure are expressed explicitly in terms of the primitive modified Cam clay parameters, the solutions may provide a theoretical basis for the interpretation of in-situ tests such as the piezocone test and the pressuremeter test.

Factors affecting rainfall-induced flowslides in laboratory flume tests

Abstract: Using a small flume, a series of tests has been conducted to cause rainfall-induced flowslides. Based on monitoring of sliding distance and pore pressures, the process of pore-pressure generation in relation to sliding distance is examined. A set of test results on the temporal and longitudinal variation of sliding velocity is presented. By changing the initial dry density and sample thickness in tests on a silica sand, the effects of these two factors on the pore pressure build-up after failure and flowslide motion have been analysed. The test results showed that there was an optimal density for pore pressure build-up, at which both the moving velocity and run-out distance were greatest, and this optimal density was dependent on sample thickness. Additionally, a series of tests on mixtures of silica sand with different loess contents has been carried out to investigate the effects of fine-particle content on the flowslide motion, in which different modes of flowslide motion were observed; these probably ...

Combined loading of spudcan foundations on clay: numerical modelling

Abstract: The load–displacement response of a spudcan foundation on clay is described by means of an incremental plasticity model with three degrees of freedom (vertical, rotational, horizontal). The model is termed ‘Model B’ and employs a yield surface and flow rule that are derived from a programme of carefully controlled small-scale laboratory tests. Behaviour inside the yield surface is defined by a set of elastic stiffness factors for embedded conical footings, determined from three-dimensional finite element analysis. The hardening law, which defines the vertical bearing capacity as a function of plastic spudcan penetration, is based on a set of theoretical lower bound bearing capacity factors for embedded conical footings. Care is needed to ensure consistent treatment of partially and fully penetrated spudcans under combined loading. Full details of the elastic and elasto-plastic stiffness matrices needed to predict incremental load–displacement behaviour are provided, and the performance of the model is dem...

Quantitative characterisation of the geometry of discrete particles

Abstract: A new technique is described to quantify particle shape and angularity using an image analyser. The method relies on discretising the two-dimensional projection of the particle and comparing the projected outline with that of a standard geometric shape, namely a circle. Two new parameters for particle shape and angularity were formulated and their values were determined for various materials. The relationship between the new parameters and large-strain (steady-state) internal friction angle as well as pluviated void ratio was examined. Overall, the results indicate that as shape and angularity parameters increase, the drained friction angle and pluviated void ratio increase. In addition, shape and angularity were found to influence the measured maximum void ratio and, to a lesser extent, the minimum void ratio.

Analysis of wave-induced liquefaction of sand beds

Abstract: This paper describes finite element analyses of wave-induced liquefaction of sand beds. The authors were motivated by previous experimental results in which a series of centrifuge wave tank tests were performed on loosely packed fresh deposits of sand with viscous scaling introduced. The principal finding was that the resistance to liquefaction under progressive waves was considerably smaller than the resistance exhibited under standing waves. An important factor for this marked difference was suggested to be the rotation of the principal stress axes that occurs under progressive wave loading. The present study extends an existing cyclic-plasticity constitutive model to account for the effect of stress axis rotation of sands. This model was incorporated into a finite element analysis procedure, which was applied to soil responses to progressive wave and standing wave loading. The predicted results compare well with observed soil behaviour. The predicted effective stress paths and associated stress–strain ...

A model for rockfill compressibility

Abstract: The paper presents a macroscopic constitutive model for rockfill that includes the effect of water on compressibility and collapse phenomena. Breakage of rock particles and fracture propagation are basic underlying mechanisms controlled by the relative humidity of the air filling the rockfill voids. A conceptual deformation model based on these mechanisms is first proposed and discussed. The results of oedometer tests on a quartzitic slate rockfill, in which the air relative humidity was controlled, are then presented. A significant finding is that bringing the relative humidity within the specimen to its maximum (100% RH) leads to a collapse strain equal to that observed in flooded specimens. An elastoplastic constitutive model, consistent with the basic deformation framework, is developed. Its parameters have a clear physical meaning. Guidelines for parameter determination are given. Model performance is finally compared with the results of the experimental programme.

Stiffness of geomaterials at very small strains

Abstract: Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry–Perot laser interferometer. A displacement accuracy as high as ±0·027 μm was obtained, leading to an axial strain accuracy of the order of ±10−4%. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress–strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0·002–0·003%) for all three materials, despite the maximum stiffness (Emax) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0·01% strain being of the order of 80–95% of Emax. Effects of loading path were observe...

An investigation of the effect of soil state on the capacity of driven piles in sands

Abstract: A series of model pile tests was carried out in the centrifuge, pushing the pile into two sands of very different geological origins and particle strengths. This was accompanied by series of triaxial tests allowing the critical state line of each sand to be established so that the initial in situ state of each sample, prior to pile installation, could be quantified. The work demonstrated that the initial state, defined as the combination of density and stress level relative to the critical state line, is a controlling factor in determining pile capacity, and hence that design approaches based solely on relative density could not be expected to be widely applicable. It was also found that state had to be defined as a ratio of stresses rather than the more usual state parameter in terms of volume. Attempts to verify the method using existing field pile test data were hampered by a dearth of laboratory data accompanying the in situ tests.

Case study of lateral spreading forces on a piled foundation

Abstract: A case study is presented of the effect of lateral spreading on the piled foundations of the Landing Road Bridge, Whakatane, New Zealand, during the ML 6·3, 1987 Edgecumbe earthquake. Trenching at two piers on the floodplain of the left bank revealed slip surfaces in the 1·5 m thick non-liquefied crustal layer, consistent with passive failure as the buried piers and raked piles resisted its displacement towards the river channel. The passive force on the buried portion of the slab piers is estimated at 850–1000 kN per pier, compared with roughly 50 kN in drag forces between the underlying liquefied sand and the set of eight, 400 mm square raked piles per pier. The collapse load of the foundation system is estimated to be about 1200 kN. Thus the force imposed by the non-liquefied crust was close to the ultimate capacity of the foundation. Our main conclusion is that the chief threat to piled foundations from lateral spreading comes from loads imposed by the non-liquefied crust, not from the drag forces of ...

Analysis of progressive liquefaction as a moving-boundary problem

Abstract: The propagation of liquefied zones in sand beds under fluid-wave loading is theoretically analysed in the present study. The completely liquefied state of sand is modelled as an inviscid fluid of a particular density, and the underlying sub-liquefied soil is modelled as a poro-elastoplastic material obeying a simple law of cyclic plasticity. It is shown that the proposed theoretical model is capable of consistently predicting the prograssive nature of liquefaction as observed in centrifuge wave tank tests on loose deposits of fine-grained sand. The theoretical model can also predict the final depth of the liquefaction front under a given wave loading.

Consolidation equation valid for both darcian and non-darcian flow

Abstract: In an earlier paper, a comparison was made between the consolidation process experienced in practice and the theoretical consolidation process both determined according to the classical consolidation theory based upon the validity of Darcy's law and obtained on the assumption of an exponential correlation between hydraulic gradient and flow velocity (non-Darcian flow). The results obtained, based on back-analysis of field data, showed a better agreement between theory and practice if non-Darcian rather than Darcian flow was assumed. In this paper new equations are demonstrated covering the exponential as well as the linear correlation between hydraulic gradient and flow velocity, including the effect of smear and well resistance. A comparison is made between the results obtained by the solution based merely on Darcian flow and the new solution based on exponential flow. It is shown that the new, more general solution can replace the solution based on Darcian flow.

Upper-bound load estimates on square and rectangular footings

Abstract: Limit analysis of square and rectangular rough footings is presented in this paper. All mechanisms of failure considered in the analysis consist of four regions, each characterised by plane deformation. However, the geometry of the mechanisms is three-dimensional. Both continuous deformation and multi-block patterns are considered. A common feature in all mechanisms is truncation of the blocks with conical surfaces. Standard calculations of the work dissipation rate are complex because of the elaborate three-dimensional geometry. However, a theoretical development is shown indicating that the tedious calculations of work dissipation on curved velocity discontinuity surfaces and within the deforming regions of cohesive-frictional soils can be substituted with an integral over the surface of the mechanism. While this method cannot be used easily for soils with an arbitrary distribution of properties, its application is straightforward for homogeneous soils, and it can also be used for layered soils. Calcula...

Winkler modulus for axially loaded piles

Abstract: The problem of elastic soil–pile interaction and its modelling using the concept of a Winkler support are revisited. It is shown that dividing the vertical soil shear tractions and the corresponding settlements along the pile generates depth-dependent Winkler springs that accurately describe the pile–soil interaction, contrary to the widespread belief that the Winkler representation is always approximate. A simplified theoretical model is then derived for analysing the response of an end-bearing cylindrical pile in a homogeneous soil stratum. Explicit solutions are obtained for: (a) pile settlement; (b) depth-dependent Winkler modulus; (c) average (depth-independent) Winkler modulus to match pile head settlement. Both infinitely long piles and piles of finite length are examined. The approximate analytical solution compares favourably with finite-element and boundary-element solutions. A simple regression formula for the average Winkler modulus is developed.

The effect on anisotropy of rotating the principal stress axes during consolidation

Abstract: It is well known that Ko-consolidated natural sediments have anisotropic strength, yielding and stiffness characteristics, particularly at low overconsolidation ratios. For example, the undrained triaxial compression shear strengths developed after Ko-consolidation far exceed those available in extension. This paper describes recent hollow cylinder apparatus research into other types of stress-induced anisotropy, reporting experiments on dense silt samples that involved rotation of the principal stress axes during consolidation. The results show strikingly different anisotropies from those developed by Ko-consolidated samples. The effective stress paths chosen for the laboratory studies were guided by the conditions expected under multi-stage embankments built on soft ground, but the results have considerable significance in other practical geotechnical problems.

Radiographic and microscopic observation of shear bands in granular materials

Abstract: Shear localisation in granular materials has been studied radiographically and microscopically. Thin columns and rows (lines) of lead silicate granules of distinct colour were embedded in a large hollow cylindrical specimen of granular materials, and a series of X-ray photographs were taken to capture the local micromechanical response during shear deformation, in both drained and undrained experiments. In drained tests, shear localisation, produced by monotonic deformation of the specimen, was directly observed. In undrained tests, the local deformation of the granular mass during liquefaction, induced by cyclic shearing, was examined. It was directly observed that shear localisation does occur in liquefied specimens. After shear bands were formed in drained tests, the specimen was frozen in situ. Later, samples were cut from the frozen large specimen, and their ice was replaced by a polymer resin, using a novel technique. The portion of the sample containing a shear band and a sheared section of a colum...

Pre-failure strain softening and pre-failure instability of sand: a comparative study

Abstract: Strain softening in the form of decreasing shear resistance and instability, the inability to sustain given loads, can occur before the failure state is attained. This so-called pre-failure strain ...

A parametric study of the pull-out capacity of bucket foundations in soft clay

Abstract: This paper investigates the effect of several parameters on the pull-out capacity of bucket foundations in soft clay. These parameters are load inclination, skirt length, foundation diameter, soil adhesion and soil anisotropy. Initially the soil is assumed to be isotropic soft clay, and is represented using a form of the modified Cam clay model. Some recent laboratory experiments performed in a hollow cylinder apparatus, which were designed to investigate strength anisotropy of a particular silt soil, are then used to represent the behaviour of an anisotropic soft clay. The anisotropic behaviour is simulated using anisotropic soil model MIT-E3. Both models are then used in finite element analyses to predict the behaviour of bucket foundations on pull-out. All analyses were performed using the three-dimensional Fourier series aided finite element method (FSAFEM).