the development and evaluation of a constitutive model for the prediction of ground movements in overconsolidated clay the stress strain response of overconsolidated clay depends both on its current state and on the loading history followed to reach that state in particular the relative directions of the current and previous loading paths a constitu tive soil model is developed which predicts this behaviour by allowing elasto plastic deformations controlled by two nested kinematic hardening surfaces inside a conventional modified cam clay state boundary surface this relatively straightforward model requires only eight para meters each with a rational basis and which can be determined from a small number of well controlled stress path tests predictions of soil behaviour using this model are compared with data from triaxial stress path tests the close agreement confirms that the essential features of soil behaviour are predicted by the model the wider implications of the use of the model in geotechnical analysis are illustrated by compar ing predictions made using the model in conjunction with finite element analys

unexpected tunnelling volume losses in the westminster area london during construction of the running tunnels for the jubilee line extension london large variations in tunnelling volume loss were measured over about a 2 km distance between waterloo south of the thames and the area north of st james s park at the design stage a value of volume loss of 2 was adopted which was thought to be conservative in practice volume losses of greater than 3 were measured in westminster and in st james s park south of the lake north of the st james s park lake volume losses were less than 2 as expected reliable estimates of volume loss are required for the planning and approval of future tunnelling projects in london and so a detailed investigation was undertaken to establish the reasons for the observed differences two primary and interlinked causes for the large volume losses are identified first the method of tunnelling and construction control south of the lake up to 1 9 m of unsupported tunnel heading was often advanced ahead of the shield and as a result abo

ground movements due to shallow tunnels in soft ground i analytical solutions this paper presents simplified closed form analytical solutions that can be used to interpret and predict ground movements caused by shallow tunneling in soft ground conditions these solutions offer a more comprehensive framework for understanding the distribution of ground movements than widely used empirical functions analytical solutions for the displacement field within the ground mass are obtained for two basic modes of deformation corresponding to uniform convergence and ovalization at the wall of a circular tunnel cavity based on the assumption of linear elastic soil behavior deformation fields based on the superposition of fundamental singularity solutions are shown to differ only slightly from analyses that consider the physical dimensions of the tunnel cavity except in the case of very shallow tunnels this work demonstrates a simplified method to account for soil plasticity in the analyses and illustrate closed form solutions for a three dimensional 3d tunnel heading a companion paper describes applications of these analyses to interpret field measurements of ground response to tunneling

characteristics of the london clay from the terminal 5 site at heathrow airport the innovative engineering approach adopted for the new terminal 5 at heathrow airport called for an advanced investigation of the london clay strata including detailed in situ profiling and stress path laboratory testing on high quality rotary cored samples although the scope of the investigations exceeded that normally specified for conventional design questions relating to the structure and anisotropy of stiffness and strength of the clay remained that could not be answered further research was required and the deep excavations at the site provided the opportunity for a team from imperial college to take multiple block samples from three depths supplemented by two additional dedicated rotary cored boreholes intensive research was performed at imperial college on these samples as described in three companion papers by gasparre et al and nishimura et al this overview paper integrates the findings from the recent research with those from the commercial investigation and with earlier studies to e

predicting 2d ground movements around tunnels in undrained clay a new analytical method is introduced for calculating displacements due to tunnelling this is conceived within the framework of the bound theorems of plasticity but allowing for soil strain hardening the ground displacements due to tunnelling are idealised by a simple displacement mechanism of distributed shearing in the plane of the tunnel cross section the tunnel support pressure corresponding to a certain volume loss is calculated from energy balances of the work dissipated in distributed shear the potential energy loss of soil flowing into the tunnel and the work done by this soil against the tunnel support pressure the calculations are carried out in steps of small volume loss accompanying small reduction in support pressure after each of which the tunnel geometry is updated in this way each reduced tunnel support pressure is related to a complete ground displacement field a simplified closed form solution is also provided for the prediction of maximum surface ground settlement for the par

analysis of undrained soil deformation due to ground loss closed form solutions are presented for obtaining the strain field in an initially isotropic and homogeneous incompressible soil due to near surface ground loss this problem fits into a category of cases in which the imposed boundary conditions are only or mainly in terms of displacements strain controlled problems in these cases there is a possibility of eliminating the stresses and obtaining the strains by using only the incompressibility condition the presence of the top free surface is considered by means of a virtual image technique and some results for the elastic half space the results are simple especially for the movements of the soil surface the application to some typical problems such as soft ground tunnelling and pile driving or extraction shows that the calculated movements agree well with the experimental observations and compare favourably with commonly used numerical methods some hints are given for extending the method to other cases such as to compressible materials des sol

the influence of soil anisotropy and k0 on ground surface movements resulting from tunnel excavation finite element fe analysis is now often used in engineering practice to model tunnel induced ground surface settlement for initial stress regimes with a high coefficient of lateral earth pressure at rest k0 it has been shown by several studies that the transverse settlement trough predicted by two dimensional 2d fe analysis is too wide when compared with field data it has been suggested that 3d effects and or soil anisotropy could account for this discrepancy this paper presents a suite of both 2d and 3d fe analyses of tunnel construction in london clay both isotropic and anisotropic non linear elastic pre yield models are employed and it is shown that even for a high degree of soil anisotropy the transverse settlement trough remains too shallow by comparing longitudinal settlement profiles obtained from 3d analyses with field data it is demonstrated that the longitudinal trough extends too far in the longitudinal direction and that consequently it is difficult to establish steady state set

the influence of structure on the behaviour of london clay an intensive investigation is described into the london clay units from heathrow terminal 5 intrinsic properties and composition were established and relating the behaviour of intact and reconsti

spatial and temporal arctic ocean depositional regimes a key to the evolution of ice drift and current patterns sediment physical properties measured in cores from all the major ridges and plateaus in the central arctic ocean were studied in order to analyze the spatial and temporal consistency of sediment d

analysis of tunnel excavation in london clay incorporating soil structure recent studies on london clay have identified a number of different units in the geological profile and have highlighted the role of soil structure in mechanical behaviour in fact structure is the dominant factor determining the differences in the mechanical response of different units in the paper numerical analyses simulating the undrained excavation of a tunnel in st james s park are presented london clay behaviour is characterised by a kinematic hardening structured soil model incorporating structure and stiffness degradation the parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the london clay formation and the different degrees of soil structure the analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short term surface and subsurface displacements as well as pore pressures the paper concludes with a discussion of the results in the context of other analyses performed

seabed erosion on the lomonosov ridge central arctic ocean a tale of deep draft icebergs in the eurasia basin and the influence of atlantic water inflow on iceberg motion 1 the submarine lomonosov ridge which bisects the arctic ocean is covered by 450 m of hemipelagic sediments high resolution chirp sonar and side scan data have documented erosion of a section of the ridge here we present multichannel seismic and chirp sonar data which show removal of unconsolidated sediments along the eurasia basin side of the lomonosov ridge where the central ridge is shallower than 1000 m the incomplete erosion of the hemipelagic drape and probable topographic control of ice motion suggest that the deepest draft glacier ice which reached the central arctic ocean from the eurasia basin was in the form of armadas of large icebergs embedded in sea ice rather than a single continuous floating ice shelf massive rapid discharge of glacier ice was probably required to produce the deepest draft 800 m icebergs eastward and northward drift trajectories from a likely iceberg source area on the kara barents sea margin would reflect the relative strength of atlantic inflow through the fram strait however only icebergs with drafts exceeding bathymetric thresholds on the lomonosov ridge would leave an erosive imprint on the seabed advection of atlantic water may have been relatively strong at times of discharge from the saalian ice sheet icebergs exiting the arctic ocean caused massive erosion and redeposition of the hemipelagic sediments on top of yermak plateau

arctic perennial ice cover over the last 14 million years knowledge of the long term history of the perennial ice is an important issue that has eluded study because the cenozoic core material needed has been unavailable until the recent arctic coring expedition acex detrital fe oxide mineral grains analyzed by microprobe from the last 14 ma 164 m of the acex composite core on the lomonosov ridge were matched to circum arctic sources with the same mineral and 12 element composition these precise source determinations and estimates of drift rates were used to determine that these sand grains could not be rafted to the acex core site in less than a year thus the perennial ice cover has existed since 14 ma except for the unlikely rapid return to seasonal ice between the average sampling interval of about 0 17 ma both north america and russia contributed significant fe grains to the acex core during the last 14 ma

the measurement of soil stiffness in the triaxial apparatus this paper describes a simple technique for accurately measuring the mean local axial strains of triaxial samples over a central gauge length the technique makes use of an axial displacement gauge which is a development of one devised by burland symes 1982 which makes use of electrolytic levels the device can resolve to less than 1 m over a range of 15 mm is simple to mount on the specimen and is not damaged when the sample is taken to failure the results of undrained triaxial tests are presented for a wide spectrum of soil types ranging from sands through intact reconstituted and remoulded low plasticity till undisturbed london clay to intact unweathered chalk the test results show that conventional external measurements of displacement contain errors which are frequently so large that their use in the determination of soil stiffness at working levels of stress is invalid the errors mainly result from tilting of the sample bedding at the end platens and the effects of compliance in the app

the stiffness of natural london clay an investigation of natural london clay is reported involving advanced triaxial hollow cylinder apparatus hca and dynamic testing techniques significant anisotropy was revealed at all scales of deformation and the framework of cross anisotropic elasticity was found to apply broadly to the initial elastic behaviour the stiffness parameters obtained by independent techniques generally exhibited good agreement with the greatest deviation being seen in the poisson s ratios which fell far from the values usually assumed in conventional foundation analysis probing tests established the limits to the elastic domain over a range of depths showing that these scaled in proportion to the mean effective stress level as did those of a second kinematic surface that surrounded the elastic domain once engaged this second surface signified a new pattern of strain increment directions faster elastic plastic stiffness decay with strain and also a greater dependence of behaviour on recent stress history howev

simple approach to predicting ground displacements caused by tunnelling in undrained anisotropic elastic soil an attempt to develop a kinematically based approach to predicting ground settlements due to tunnelling is represented by using the more reliable data of the commonly observed settlement troughs rather than imposing the boundary conditions on the tunnel surface which have a high level of uncertainty the anisotropic elastic model used in this work is highly simplified and the resulting analytical solution is of an approximate nature nevertheless it seems to provide good agreement with both the shape and the magnitude of the observed settlement trough based on this solution a modification is proposed for the widely used empirical gaussian curve by correlating the trough width parameter k with the ratio between the two anisotropic shear moduli ghh gvh

major element geochemistry and geomorphic relationships in brazilian cerrado soils abstract a thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the cerrado region of central brazil a region with some of the oldest soils on earth the objectives of this paper are to summarize the major element composition of cerrado soils in the context of their morphology and geomorphology to describe impacts of biogeochemical cycling on the chemical properties of cerrado soils and to use elemental data to provide insights on the possible origin of soil parent materials on the south american erosion surface and on the serra geral basalts we sampled three major subregions selected to be representative of the main areas of the cerrado namely i eastern goias ii northwestern minas gerais and iii the tri ngulo mineiro area of western minas gerais five soils were sampled in each subregion each represented by three pedons sampled at 0 0 2 and 0 8 1 0 m depths geomorphologically the soils were on the south american velhas i and velhas ii erosion surfaces all sites were carefully selected to minimize potential anthropogenic contamination general chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength dispersive x ray fluorescence spectroscopy the soils ranged from loamy sands to clays with si 142 947 g kg 1 sio 2 al 14 439 g kg 1 al 2 o 3 fe 10 331 g kg 1 fe 2 o 3 and ti 3 101 g kg 1 tio 2 contents closely following the clay contents organic c averaged 17 g kg 1 for the surface and 7 g kg 1 for the subsurface horizons the na ca mg and k contents were very low tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition and were higher in soils containing hard plinthite nodules the th zr ratios suggest that the parent materials of the clayey soils on the south american erosion surface have a common origin in sediments from rocks in the central plateau of brazil that were transported eastwards to cover the underlying sandstone trace element signatures indicate that basalt derived soils occurring at different elevations in the tri ngulo mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events

numerical assessment of the effects of end restraints and a pre existing fissure on the interpretation of triaxial tests on stiff clays conventional laboratory triaxial tests apply axisymmetric boundary conditions to a cylindrical sample which has an axisymmetric geometry for a homogeneous sample this implies that the deformed sha

the use of kinematic hardening models for predicting tunnelling induced ground movements in london clay the use of a kinematic hardening soil model for predicting short and long term ground movements due to tunnelling in london clay is investigated the model is calibrated against oedometer and triaxial tests on intact samples from different units of the london clay the calibrated model is then used in finite element analysis to simulate the field response at st james s park during excavation of the jubilee line extension tunnels the finite element predictions compare well with the available field monitoring data the importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted the stiffness response of different regions of the finite element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short term predictions significantly the long term predictions confirm that the compression characteristics of the soil control the mag

effect of recent stress history on the stiffness of overconsolidated soil the stress strain behaviour of overconsolidated soil is non linear and the stiffiiess depends on the current state and stress history in addition to the overall stress history which is best described by the overconsolidation ratio the recent stress history either a sudden change in the direction of the stress path or a period of time at a constant stress state has a major effect on subsequent stiffiiess the effects of a change in the direction of the loading path on the stiffness of overconsolidated soil were examined in laboratory stress path tests on reconstituted samples of london clay further tests were carried out on reconstituted samples of other soils with a range of plasticities the test results showed that the behaviour of over consolidated soil is largely inelastic and highly nonlinear and that the recent history has a major effect on the subsequent stiffiiess particularly for relatively small changes of stress or strain for reconstituted samples of london clay the stiffiiess near the s

pore water pressure and total horizontal stress response to epbm tunnelling in london clay the ground response in terms of surface and subsurface displacements to twin bore crossrail tunnel construction beneath a research monitoring site in hyde park london using earth pressure balan