Showing papers in "Geotechnique in 1993"

Liquefaction and flow failure during earthquakes.

Abstract: Two aspects of seismically-induced liquefaction are discussed which are of vital engineering significance: the triggering condition and the consequences of liquefaction. The triggering condition is examined with respect to liquefaction analysis, note being taken of the onset condition which is governed by cyclic strength. Consequences of liquefaction are discussed with respect to post-seismic stability analysis, in which the residual strength plays a major role. Procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced, and the applicability of this method for estimating associated ground settlements is discussed. The evaluation of residual strength requires a better understanding of undrained sand behaviour. Results of extensive laboratory rests on Japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better. The results of laboratory tests on silty sands are examined in the same way. AH the results are...

Subsurface settlement profiles above tunnels in clays

Abstract: Designers assessing the possible effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop, and how these relate to surface settlement profiles. The effect on piled foundations or existing tunnels depends on the width of the subsurface settlement profile and on the magnitude of the settlement. This note uses available field measurements and centrifuge model test data to determine how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays. The results reveal a fairly consistent pattern of behaviour: the width of the subsurface settlement troughs at depth is significantly greater than would be predicted by assuming a constant trough width parameter K of 0.5. Smaller more realistic subsurface settlements and strains will be predicted if account is taken of the proportionally wider trough widths at depth.

Nor-sand: a simple critical state model for sand

Abstract: A generalized Cambridge-type constitutive model for sand is developed from the fundamental axioms of critical state theory. An infinity of normal consolidation loci for sand prevents the direct cou...

The mechanics of cemented carbonate sands

Abstract: The behaviour of artificially cemented carbonate sand was investigated in triaxial tests at confining pressures of up to 9 MPa. The results show that an important effect of cementing is a reduction in specific volume resulting from the increase in fines content This influences both the stress-strain behaviour and the peak strength at strains beyond those required to fracture the cement bonding. Comparisons between the behaviour of cemented and uncemented soils should, therefore, be carried out on samples with the same gradings. For cemented samples it is possible to identify a yield curve outside the state boundary surface of the uncemented soil. A framework for the behaviour has been defined which depends on the relative magnitudes of the confining pressure and cement bond strength. The behaviour of a natural cal-carenite agreed well with this framework which is also likely to be applicable to other cemented soils. Le comportement d'un sable carbonate artificiellement cimente a ete etudie a l'aide d'essa...

Limit load in translational failure mechanisms for associative and non-associative materials

Abstract: Translational failure mechanisms for estimation of the limit load in geotechnical boundary-value problems are considered. The equivalence of the energy balance approach and the equilibrium of force...

A new instrument for the measurement of soil moisture suction

Abstract: Current methods of determining soil moisture suction are outlined and their shortcomings identified. A new instrument is then described which will overcome a number of these problems by the use of a miniature pressure transducer, and minimum volume of water between the active face of the transducer and the porous stone. The instrument uses an Entran EPX-500 stainless steel miniature pore water pressure transducer, with a working range up to 3500 kPa, shrouded by a stainless steel sheath fitted with a porous stone. The mechanics of the instrument and its evaluation against known values of suction are outlined. Reliable measurements of suction were achieved up to 1500 kPa.

Seepage beneath water retaining structures founded on spatially random soil

Abstract: The effect of stochastic soil permeability on confined seepage beneath water retaining structures has been studied. Random field concepts for the generation of soil permeability properties with a fixed mean, standard deviation and spatial correlation structure have been combined with finite element methods to perform Monte Carlo simulations of the seepage problem. Analyses have been performed for a dam with two cut-off walls. The results of parametric studies to gauge the effect of the standard deviation and correlation structure of the permeability on the output statistics relating to seepage quantities, exit gradients and uplift pressures are presented. In all cases, comparison is made with results that would be achieved on a deterministic basis. Flow rates and other quantities are shown to be affected significantly by the standard deviation and the correlation structure of soil permeability. L'article etudie l'influence de la permeabilite d'un sol stochastique sur l'infiltration confinee existant sous ...

Large-scale triaxial testing of greywacke rockfill

Abstract: This Paper describes the results of a series of large-scale triaxial tests conducted on greywacke rockfill, used in dam construction in Southern Thailand. The tests are conducted at low to moderate confining stresses to relate their findings to the stability of rockfill dams. Considering the current test results in conjunction with previous laboratory data, revised failure criteria for rockfill are proposed in non-dimensional form. For both low and high confining stresses, lower and upper bounds of strength envelopes have been established, based on a wide array of granular materials. The influence of the confining stress on the shear strength of rockfill is studied in depth, and the implications of a non-linear envelope at low normal stress levels on the stability of rockfill dams are discussed. Although two parallel rockfill gradations for specimens compacted to similar porosities are considered, the exact role of particle size effect on shear strength is not examined in detail, as the difference in maxi...

Coupled fluid, electrical and chemical flows in soil

Abstract: Renewed study of coupled flows in soils has been stimulated by the potential uses of electrokinetics for remedying waste sites. Most suggested uses involve the movements of fluid, electricity and c...

Kinematic seismic response and bending of free-head piles in layered soil

Abstract: The paper studies the kinematic response of freehead piles. Such pile deformation has triggered structural damage in many strong earthquakes. In this Paper dimensionless parametric graphs for pile bending moments are presented which pertain to characteristic two-layer soil profiles. The results are derived by using an existing rigorous dynamic finite-element code, and by implementing a realistic beam-on-dynamic-Winkler-foundation formulation specifically developed for the kinematic response of piles in layered soil. The Winkler model is shown to reproduce quantitatively even detailed trends observed in the finite-element results; a simple analytical expression is thereby developed for estimating the Winkler stiffhess in terms of the local soil Young’s modulus and key dimensionless pile/ soil parameters. The study concludes that even relatively flexible piles may not exactly experience the wavy and abruptly changing ground deformation of the free field. The critical region of pile distress due to such kinematic loading is shown to he at or near the interface between alternating soft and stiff soil layers. The magnitude of the bending moment at such critical interface locations depends mainly on the stiffness contrast of the two layers through which the pile penetrates, the excitation frequency and the relative rigidity of the pile. A constraining cap may exert an important effect on such kinematic deformations.

Evaluation of a constitutive model for overconsolidated clays

Abstract: This Paper evaluates the performance of a generalized effective stress soil model for predicting the rate-independent behaviour of K" normally to moderately overconsolidated clays. The model formulation comprises three components: an elasto-plastic model for normally consolidated clay including anisotropic and strain softening behaviour, equations to describe the small strain non-linearity and hysteretic response in unloading and reloading, and bounding surface plasticity for irrecoverable, anisotropic and path-dependent behaviour of overconsolidated clays. Model complexity is controlled through the use of input parameters which can be obtained from a relatively small number of standard soil tests. The Paper illustrates the selection of model input parameters for a low plasticity, moderately sensitive clay and compares the selected values for different types of clay. Extensive comparisons with measured data from undrained shear tests performed in different modes of shearing and with overconsolidation rati...

Seismic bearing capacity of soils

Abstract: This paper derives the seismic bearing capacity factors for a C- phi soil within the framework of the Mohr-Coulomb theory. Horizontal and vertical accelerations, and the effects of: a) inertia forces of the soil both below and above the footing; and b) the applied loads are all taken into consideration. The method is used to derive the ultimate seismic bearing capacity for a square footing 2.5m wide founded 1.0m below a homogeneous soil with C (soil cohesion parameter) = 7 kPa, phi = 25 degrees and a unit weight of 18 kN per cubic metre. The results indicate that the horizontal accelerations as well as soil cohesion and vertical accelerations affect the bearing capacity coefficients considerably.

Hydraulic fracturing of soil during laboratory experiments Part 1. Methods and observations

Abstract: Hydraulic fracturing appears to have useful environmental and geotechnical applications, but the details of fracture morphology and propagation in soil are poorly known. To identify those details, ...

Displacement phase differences in a harmonically oscillating pile

Abstract: Analytical solutions are developed for harmonic wave propagation in an axially or laterally oscillating pile embedded in homogeneous soil and excited at the top. Pilesoil interaction is realistically represented through a dynamic Winkler model, the springs and dashpots of which are given values based on results of finite element analyses with the soil treated as a linear hysteretic continuum. Closed form expressions are derived for the phase velocities of the generated waves; these are compared with characteristic phase velocities in rods and beams subjected to compressionextension (axial) and flexural (lateral) vibrations. The role of radiation and material damping is elucidated; it is shown that the presence of such damping radically changes the nature of wave propagation, especially in lateral oscillations where an upward propagating (reflected) wave is generated even in a semi-infinite head-loaded pile. Solutions are then developed for the phase differences between pile displacements at various depths. For most piles such differences are not significant and waves emanate nearly simultaneously from the periphery of an oscillating pile. This conclusion is useful in analysing dynamic pile to pile interaction, the consequences of which are shown in this Paper.

On the behaviour of quay walls in earthquakes

Abstract: The failure of quay walls induced by earthquakes has been widely observed in the field. A study of the seismic response of anchored flexible quay walls based on the data of dynamic centrifuge model tests is reported, and the applicability and limitations of the widely used pseudo-static calculations are investigated. It is found that amplification of motion due to resonance may cause a greatly increased earthquake response over that predicted by a pseudo-static analysis. Resonant vibration may be a consequence of the deterioration of soil stiffness in the backfill, which is itself a result of cyclic loading and excess pore pressures induced by base shaking. This analysis is strongly supported by the data of centrifuge model tests. Principles for design are proposed, which by a model test are shown to be effective. La rupture des murs de quai provoquee par des seismes a ete largement observee in-situ. L'article presente une etude de la reponse sismique de murs de quai flexibles et ancres reposant sur les r...

The Static Compaction of Soils

Abstract: This paper describes a new test called the "static compaction test" for soils. This test is based on the variable peak stress-constant stroke compaction process. Although this test does not lend itself to a constant energy impact compaction, an attempt has been made to derive the energy input OMC (optimum moisture content) relationship indirectly. Drawbacks of standard compaction tests are outlined, and a case is made for a static compaction test in which the energy input per unit volume can be easily varied. The concept and objectives of the new static compaction test are outlined and the procedure described. Results are presented graphically and compared with results using the Proctor compaction test. A difference is found due to the lack of a consolidation phase in the Proctor test. The use of static compaction test results using the relationship between optimum moisture content and energy input is discussed.

Undrained response of loosely compacted sands during monotonic and cyclic compression tests

Abstract: Undrained monotonie compression tests were conducted on loose sand samples subjected to an initial isotropic stress state, and undrained cyclic compression tests with no shear stress reversal were performed on loose sand samples subjected to initial anisotropic stress, in order to establish the stress conditions needed for triggering contractive flow deformation and the corresponding steady-state characteristics. It was shown that the peak strength envelope was also the triggering strength envelope, which appears to be unique for samples prepared by the moist tamping method and independent of stress path. It was established that the failure envelope for a given sand is unique but that the steady-state strength was not unique and was dependent on void ratio and stress conditions at the onset of flow failure. The use of normalized stress space is very helpful in the analysis of undrained monotonie and cyclic compression tests and permits ready prediction of strain-softening and strain-hardening behaviour. A...

Behaviour of embedded model screw anchors in soft clays

Abstract: In recent years helical anchors have been used extensively in the construction of transmission tower foundations, pipelines and braced excavations. This Paper describes an experimental programme carried out to show the behaviour of multiple helical anchors at different embedment ratios in soft marine clays. The number of helical plates, their diameters and spacings are varied. Pullout tests were conducted by anchors installed in a soft marine clayey bed prepared in test tanks and at embedment ratios of 0-10. The results indicate that the anchor capacity increases with embedment ratio. The uplift capacity at any embedment ratio is explained in terms of the contribution of cohesive resistance between the top and bottom helical plates, the bearing resistance of the top helical plate alone and the shaft adhesion above. Based on embedment ratio and observations during testing, the anchors are classified as shallow, transit and deep anchors. Formulations are suggested for estimating and calculating the capaciti...

Stress state-permeability relationships for fine-grained soils

Abstract: This Technical Note investigates the possibility that stress state- permeability relationships might be developed for clays within the framework of generalisations already made in the analysis and prediction of soil behaviour. Consolidation and permeability tests carried out on red, brown, black cotton and bentonite soils, chosen for their varying liquid limits, are described. The results are presented and interpreted. For a given void ratio the experimental and predicted permeability coefficients showed a good correlation. The interchangeability of stress state and permeability is discussed.

Hydraulic fracturing of soil during laboratory experiments. part 2. propagation

Abstract: The propagation of hydraulic fractures in soil was studied in the laboratory by glycerine injected at a constant rate into rectangular samples of silty clay confined in a triaxial cell. During propagation, the trace of a hydraulic fracture appeared on the side of the cell as a nearly straight, narrow line that tapered from 0-05 mm to negligible aperture at the tip. Pressure of the glycerine was recorded as a function of time and is characterized by a period of nearly linear increase, a break in slope followed by a period of increasing pressure but decreasing slope, a maximum, and then a period of decreasing pressure. Tests terminated early in that sequence of events indicated that the break in slope marks the onset of hydraulic fracturing. The pressure at the onset of fracturing was predicted from a determination of the fracture toughness of the silty clay. The forms of the pressure records were markedly affected by the water content of the soil: increasing water content decreased the pressure required to...

Laboratory testing and parameters controlling the shear strength of filled rock joints

Abstract: The shear strength of iofilled rock joints has been studied for a long time but a complete understanding of the mechanisms and of the parameters controlling the process has never been reached. This Paper reviews test results in the literature, and compares them with results obtained by the Authors. The main parameters controlling the shear strength of infilled joints are discussed and a framework for understanding the failure mechanisms involved in the shearing of such discontinuities is given. La resistance au cisaillement des joints rocheux remplis a ete etudiee depuis tres longtemps mais le mecanisme et les parametres controlant le processus n'ont jamais ete totalement compris. Cet article passe en revue les resultats des essais publies et les compare avec les resultats obtenus par les auteurs. Les parametres principaux controlant la resistance au cisaillement des joints remplis sont discutes et un cadre pour comprendre les mecanismes de rupture associes au cisaillement de telles discontinuites est donne.

The design of diaphragm-type boundary total stress cells

Abstract: The compliance of boundary total stress cells is normally the primary criterion governing their design. It is widely believed that they will not under-read the pressures applied to them by soil if the ratio of the diameter of the cell diaphragm to the displacement at its centre is not smaller than some threshold value (typically 2000). This Paper reports the results of experimental and finite element studies which show that this approach is unsound, and that boundary total stress cells are more properly designed by taking into account the relative stiffness of the diaphragm with respect to the soil. Care must be taken in the design of calibration experiments, and soil stiffness must be measured locally and at small strains. There is then good agreement between observed cell action factors and predictions made using finite element analyses. Charts are presented which may be used for the design of field and laboratory instrumentation. La conformite des cellules de contrainte totale limites est normalement l...

Hydraulic fracturing of soil during laboratory experiments Part 3. Theoretical analysis

Abstract: A theoretical analysis is developed to explain observations from laboratory experiments where hydraulic fractures were created in silty clay soil. The analysis is based on a critical stress intensi...

The failure of Carsington Dam

Abstract: The upstream slope of Carsington Dam failed just before its completion in 1984. The slip started on the right abutment and spread across the valley to a length of nearly 500m. Extensive investigations were conducted, and further information was obtained when the dam was reconstructed. The initial slip sheared through the core, which contained shear surfaces due to rutting, and along a layer of Yellow Clay which contained solifluction shears. Both materials were brittle, with low residual strengths. The factor of safety based on measured peak strengths was about 1·4; with reductions to allow for shears it was 1·2. It had been reduced to 1·0 by progressive failure. Finite element (FE) analyses in which strain-softening was reproduced confirmed this. It was concluded that the slide spread by lateral load transfer, through sections of the dam with a safety factor greater than 1. In the vailey centre, the slide passed through the base of the mudstone fill. A combination of FE and limit equilibrium analysis ind...

Homogenization analysis of diffusion and adsorption macrotransport in porous media: macrotransport in the absence of advection

Abstract: A model for pollutant transport in a heterogeneous medium, including diffusion accompanied by adsorption, has been developed. The macroscopic equivalent diffusion tensor and the macroscopic governing equations for the average concentration field can be deduced by homogenization. The necessary condition for this theory to be applied is the existence of a small parameter E defined as the ratio of the scale of the pores and the scale of a sample, which can be distinguished in the heterogeneous medium. Transport processes with respect to different dominating phenomena, related to E, have been examined. Using the analysis performed the condition has been specified when the homogenization is not possible, i.e. when it is impossible to find the rigorous macroscopic equivalent description. Un modele de transport de polluants, prenant en compte diffusion et adsorption, a ete developpe. Le tenseur de diffusion macroscopique equivalant ainsi que les equations d'etat macroscopiques du champ de concentration moyen peu...

The effects of pressuremeter geometry on the results of tests in clay

Abstract: The effects of the geometry of a self-boring pressuremeter on the results that would be obtained from tests in clay have been studied by means of finite element analysis. This technique has enabled a parametric study to be carried out without the complicating factors of test variability that are inherent in field testing. The effects of length-diameter ratio L/D and depth of penetration-diameter ratio H/D have been examined. In the study of the effects of L/D on the apparent strength, this was found to depend also on 6/.v"; a chart of correction factors for strengths derived from the pressuremeter test is presented. The L/D ratio has a much smaller effect on the measurement of elastic properties. The depth of the pressuremeter below the ground surface in a homogeneous clay deposit was found to have little effect on the measurement of the elastic strength properties, even for very shallow tests. L'influence de la geometrie d'un pressiometre autoforeur sur les resultats obtenus lors d'essais realises dans d...

Interaction behaviour of steel grid reinforcements in a clayey sand

Abstract: The interaction behaviour between steel grid reinforcements and a clayey sand has been studied in laboratory and field pullout tests. The clayey sand is potentially useful as cheap, low quality, locally available and cohesive-frictional backfill in the construction of mechanically stabilized earth walls and embankments, especially in coastal areas. The laboratory tests were conducted under undrained conditions at three compaction moisture conditions. The field tests were conducted on dummy reinforcements embedded at different elevations in a full-scale test embankment resting on soft clay foundation. The laboratory tests revealed that the moisture content of the compacted soil, compaction stress, applied normal stress level, diameter and spacing to diameter ratios of the transverse members of the steel grid, all affect the soil-reinforcement interaction, and thereby, also the magnitudes of the pullout resistances. Interferences between the bearing transverse members of the grid were found to be less signi...

Analysis of the arching action in granular mass

Abstract: When the lining of a tunnel is more flexible than the surrounding soil, the pressure applied to the lining is in general far smaller than the value estimated from the weight of the overburden. Pressure applied to an underground conduit or a trapdoor is also smaller than the weight of overburden. Similarly, earth pressure applied to a retaining wall or to the timbering of an open cut, and particularly to the lower part of the wall or the cut, is sometimes smaller than the values estimated from the theory of earth pressure. Some studies of this arching action to the soil assume that the soil remains in an elastic state; others assume a plastic soil model with a Tresca yield criterion. In this Paper the stress distribution in the soil is analysed assuming that the soil is in a state of plastic equilibrium and that Mohr-Coulomb's failure hypothesis is satisfied. Results are compared with those obtained by measuring the inner air pressure of models made of vinyl tubes or bags buried in dry sand or gravel. Si l...