Search

Assessing foundation response to cyclic loading is vital when designing transport infrastructure, such as road pavements and rail tracks, as well as offshore, port, and tall tower structures. While detailed guidance is available on characterizing many soil types' cyclic behavior, relatively few studies have been reported on stiff, geologically aged, plastic clays. This paper addresses this gap in knowledge by reporting cyclic loading experiments on three natural stiff UK clays that were deposited and buried between the Jurassic Age and Eocene Epoch before geological unloading to their currently heavily over-consolidated states. High-quality samples taken at relatively shallow depths were reconsolidated to nominally in situ K0 stresses in triaxial and hollow cylinder apparatus before imposing cyclic loading. The completely stable, metastable, or unstable outcomes invoked by different levels of undrained cyclic loading are interpreted within a kinematic yielding framework that is compatible with monotonic control experiments' outcomes. The cyclic limits marking the onset of significant changes in permanent strain accumulation, pore pressure development, and stress–strain hysteresis demonstrate that the weathered Gault clay offers the lowest cyclic resistance. The experiments show that energy considerations provide a promising way of evaluating undrained pore pressure generation and stiffness degradation. They also provide a basis for developing cyclic constitutive models and analysis procedures for cyclic foundation design in stiff, high-OCR, plastic clay strata. [ABSTRACT FROM AUTHOR]

This note presents the application of the mobilisable strength design (MSD) method to the monitoring results of the multi-propped excavation in the south area of the British Library Euston, constructed in a highly overconsolidated stiff clay deposit. The MSD method is an energy-based approach (a nonlinear finite-element method for a single-degree-of-freedom soil-wall system) introduced to develop a simplified design methodology that satisfies both ultimate and serviceability limit states. Wall displacement predictions based on the MSD method are compared with considerable field monitoring data. The sensitivity of the method to reasonable variations in input parameters is considered. A spreadsheet and python code demonstrating the MSD analysis from this paper are provided in the online supplement alongside details of the mathematical formulation. [ABSTRACT FROM AUTHOR]

This paper presents a unified critical state parameter model for evaluating sand and overconsolidated clay, which is named as CASM-S. The model is formulated in terms of the modification of the clay and sand model, i.e., CASM, by incorporating the subloading surface theory with an overconsolidation state parameter R. Although the standard CASM proves to be useful in modelling overconsolidated clay and sand, the model (1) requires the rate of overconsolidated ratio to be constant which is in contrast to the observed behaviour of soils and (2) only allows elastic deformation to develop at the overconsolidated state that may underestimate the deformation corresponding to the peak stress or be unable to predict the actual stress path. By introducing a new material parameter u to those material properties of the standard CASM, the rate relationship of stress–strain for the CASM-S model is developed. The standard CASM can be either recovered or approximated as a special case of the overconsolidation state assumed in the CASM-S. The numerical integration of the newly proposed model is performed by the modified forward Euler method. Validation with undrained and drained overconsolidated clay and sand have demonstrated the capability of the CASM-S model to well predict the observed soil behaviours. [ABSTRACT FROM AUTHOR]

Plate anchors are an attractive technology for mooring floating facilities; as relative to piles, suction caissons, and drag anchors, they provide a much higher capacity relative to their mass. Plate anchors may experience an extreme loading event that will cause geotechnical failure, although they will still retain a residual capacity. The displacement associated with bringing the anchor to failure will induce excess pore pressures that initially reduce soil strength but will dissipate over time, leading to regains in soil strength and hence anchor capacity. This paper considers the time scales and magnitude of this anchor capacity regain through a series of model scale experiments conducted in a geotechnical centrifuge. The experiments involved vertical loading of pre-embedded horizontally orientated circular anchors in normally consolidated kaolin clay. The results show that anchor capacity regain is a function of consolidation time and the level of resistance maintained on the anchor, with the longest consolidation time and highest maintained resistance leading to a capacity regain of approximately 60%. These capacity increases are described here using a simple hyperbolic function, which provides a basis for estimating the time needed for the residual anchor capacity to regain sufficient capacity following a movement event. [ABSTRACT FROM AUTHOR]

The p–y method as a simplified analysis tool has been widely used to analyze the behavior of laterally loaded piles. This paper develops a novel cyclic p–y elastoplastic model within the framework of the single-surface bounding surface theory. The model can capture the soil stiffness degradation during cyclic loading by incorporating the cumulative plastic displacement to an interpolation function of the elastoplastic resistance coefficient. The model is relatively simple with only four parameters that can be determined from standard soil properties and stress–strain responses measured in direct simple shear tests. The performance of developed model is validated by predicting the cyclic lateral response of piles installed in soft clay during field and centrifuge tests published in the literature. The model can reliably simulate monotonic and cyclic responses of piles under different lateral loading patterns, and capture main characteristics of the pile-head load–displacement curve, such as nonlinearity, hysteresis, displacement accumulation, and stiffness degradation. It can also predict the evolution of the lateral deflection and sectional bending moment along the pile during cyclic loading. [ABSTRACT FROM AUTHOR]

This paper presents the results of a series of cyclic nonsymmetrical thermally loaded model-scale tests conducted on floating pile groups, with and without a rigid cap in saturated clay. The energy pile in each group was subjected to 10 two-way (heating–cooling) thermal cycles with an amplitude of 15 °C to investigate the potential effects of a rigid cap on the thermomechanical behavior of energy pile groups. It was found that the rigid cap restrained the expansion/contraction of the energy pile in response to temperature change and reduced its irreversible settlement, resulting in a lower differential settlement between the energy and nonenergy piles. Consequently, a lower tilting was observed in the pile group with a rigid cap. In addition, the thermally induced axial load was significantly increased (especially in cooling mode) due to the restriction of a rigid cap. The thermomechanical load transfer profile of the energy pile was also affected by it, resulting in the downward transmission of shaft resistance. [ABSTRACT FROM AUTHOR]

The main objective of this paper is investigation of clayey soil setup effects on axial "resistance" and "stiffness" of small-scale piles with special attention on "overconsolidation ratio" (OCR) and "installation method". A consolidation chamber is developed and an instrumented model pile is either driven or jacked-in through saturated consolidated Kaolinite clay at different OCRs of 1.2 (normally consolidated, NC) and 4.8 (over consolidated, OC). The pile was static load tested at different time intervals of 1 h through 90 days. Higher pore pressures are induced during pile installation in NC compared to OC clay. The frictional resistance of OC soil is higher than NC at end of drive as well as end of setup in both driven and jacked-in piles. The frictional resistance of jacked-in piles is greater than driven piles at end of drive, but due to higher rate of setup, the long-term frictional resistance of driven piles is shown to be greater, attributed to more disturbance of the soil during installation of driven piles. Attempts are made to differentiate between the contributions of dissipation of excess pore pressure and non-pore pressure components for each consolidation condition and pile installation method. A relation is proposed to quantify the pile stiffness variations over time due to soil setup. [ABSTRACT FROM AUTHOR]

The suffusion of particles classified as coarse-grained soils in the Unified Soil Classification System has been relatively well investigated in previous studies, whereas suffusion of clay particles has not been thoroughly explored. For clay particles, the surface interaction energy between clay and sand can be a critical factor in conjunction with hydraulic gradient. Therefore, this paper presented a series of designed soil-column experiments for assessing the reduction of ionic strength on suffusion of clay particles in the sand–clay mixture. The three most typical clay particles (kaolinite, illite, and montmorillonite) were selected and the ionic strength was decreased during the injection. The results indicate that the sequence of decreasing ionic strength induced substantial suffusion of clay particles, mainly attributed to the decreased attraction between sand grains and clay particles. In addition, among three types of clay particles, the high swelling potential of montmorillonite particles led to the most substantial increase in relative hydraulic conductivity caused by suffusion. [ABSTRACT FROM AUTHOR]

Wind turbines are typically designed based on fatigue and serviceability limit states, but still require an accurate assessment of bearing capacity. Overconsolidated clay deposits in Canada often have a thin layer of crust with a relatively high undrained shear strength developed from weathering, desiccation, and geochemical processes. However, existing design methods only assess the bearing capacity using effective area and inclination factor without consideration of surficial crusts. This paper studies the undrained VHMT (vertical, horizontal, moment, and torsional) failure envelope of circular foundations founded on a surficial crust underlain by a uniform soil with a zero-tension interface condition using finite element analysis. An analytical expression for the VHMT failure envelope is derived. [ABSTRACT FROM AUTHOR]

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil's undrained strength under long-term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in kaolin and reconstituted Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil. [ABSTRACT FROM AUTHOR]

Suction caissons supporting offshore wind turbines are exposed to great horizontal loading above the soil surface, which may lead to overturning failure of the caisson. This paper presents a modified three-dimensional failure mechanism to analyze the anti-overturning bearing capacity of suction caissons. The modified failure mechanism is composed of a meniscus-conical wedge with a meniscus shape at the soil surface and a scooped shape. The analytical solution to the anti-overturning bearing capacity of the suction caisson is deduced in terms of the limit equilibrium method, followed by a parametric study of the wedge depth ratio (c) to optimize the critical failure mechanism that satisfies both the force and moment equilibriums. Thus, the methodologies are relatively easy to implement in traditional spreadsheets, and the analyses tend to perform very quickly. The effects of gap formation at the rear side of the caisson, loading eccentricity, and adhesion factor at the caisson–soil interface on the anti-overturning bearing capacity are also investigated. Compared with the finite element limit analysis results, experimental data, and existing theoretical solutions, it is proved that the presented limit equilibrium analysis can satisfactorily predict the anti-overturning bearing capacity of suction caissons with low aspect ratios for offshore wind turbines in uniform and linearly increasing strength clays. [ABSTRACT FROM AUTHOR]

In this paper, the behaviour of an idealized excavation carried out in a clay sensitive to destructuration is studied through a series of finite element analyses, employing an advanced bounding surface model developed for structured clays. Several cases are examined to investigate the influence of factors including the velocity of destructuration, damage to the soil–wall contact produced by the wall construction, and width-to-height ratio of the excavation. The case of a soil deposit insensitive to microstructural damage is also studied for comparison. Results of the numerical analyses show that the progressive dissipation of excess pore-water pressures generated during the excavation stage can damage the clay microstructure severely enough to trigger an instability phenomenon. If the clay structure deteriorates rapidly, the instability is concurrent with the dissipation of excess pore-water pressures. However, for a clay less sensitive to microstructural damage, the instability can occur towards the end of the consolidation process, or even be preceded by a deceptively stable time interval, during which small redistributions of pore-water pressure can trigger an important destructuration and collapse of the excavation. In a final part of the paper, results of the numerical analyses are used to provide indications about the most appropriate quantities to monitor to provide an effective early warning of the instability phenomenon. [ABSTRACT FROM AUTHOR]

This paper examines the possible use of a bio-cementation process to reduce excessive bulging that occurs during the loading of granular columns in soft clays. A 12-phase percolation bio-chemical treatment technique was used to bio-cement part of the columns targeting the columns upper section, where bulging usually occurs. Upon application of unit cell surface loads up to 350 kPa, placement of a bio-cemented granular column substantially reduced the vertical strains by 43% to 48% compared with an un-cemented granular column and 56% to 60% compared with unreinforced kaolin clay for just a low replacement ratio of 11%. Bulging was reduced by 62% to 75% following bio-cementation and mostly occurred where bio-cementation was less evident. Scanning electron microscopy and energy dispersive spectroscopy validated the existence of calcium carbonate. This study presents a relatively new alternative to reducing bulging in granular columns using a promising approach. [ABSTRACT FROM AUTHOR]

A new experimental series on the highly plastic (plasticity index, IP = 34%) Lower Rhine clay (LRC) is presented. The study comprises tests on normally as well as overconsolidated samples under monotonic and cyclic loading. The loading velocity has been varied to evaluate the strain rate dependency of the LRC behaviour, testifying the well-known reduction of undrained shear strength with decreasing displacement rate. Isotropic consolidation followed by a cyclic loading with constant deviatoric stress amplitude leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the tests. The inherent anisotropy has been additionally evaluated using samples cut out in either the vertical or the horizontal direction. Furthermore, the behaviour of LRC is compared with the behaviour of low plastic kaolin silt (IP = 12.2%). A new visco-hypoplastic-type constitutive model with a historiotropic yield surface has been used to simulate some of the experiments with cyclic loading. Even the eight-shaped stress loops at cyclic mobility are reproduced well with this model. The data of this paper can be also used by other researchers for the examination, calibration, improvement, or development of constitutive models dedicated to fine-grained soils under monotonic and cyclic loading. [ABSTRACT FROM AUTHOR]

This paper presents the result of five pressuremeter tests conducted in deep three clay shale formations at a thermal operation site. Pressuremeter loading was imposed parallel to the bedding plane under an approximately undrained condition. Test data are analyzed using the axisymmetric cavity expansion theory. In parallel with triaxial test results, the analyses reveal several constitutive characteristics of the Westgate clay shale, in particular, the limited nonlinearity and the stress−strain-path dependency of shear modulus. A procedure is proposed to correct the data including multi-azimuth radial displacement measurements in the caliper plane and the anisotropic response of the borehole is studied. Two aspects of azimuthal anisotropy are assessed — (i) in-plane anisotropy of borehole stiffness and (ii) the anisotropy in expansion after borehole plastic yielding. The latter provides the implication of anisotropic in situ horizontal stresses, demonstrated by both experimental and numerical evidence. The influence of the radial and azimuthal variations of the elastic stiffness around borehole on the interpreted orientation of in situ horizontal stresses is discussed. An agreement is shown between the interpreted azimuth of major or minor in situ horizontal stress and the reported crustal stress orientation for the test least affected by such stiffness variation. [ABSTRACT FROM AUTHOR]

Methods of computation for soil heave based on an 'equivalent' net normal stress path approach often have built-in assumptions regarding the relationship between in situ matric suction, overburden stress, and swell pressure. The term 'matric suction equivalent' has been previously defined as the difference between in situ state of stress projected on the net normal stress plane (represented by swell pressure) and overburden stress. Correlations between matric suction, overburden stress, and swell pressure for matric suction values in excess of the air-entry value have not been well studied and are explored here for expansive soils. Filter paper (in-contact) and constant volume swell pressure tests were performed on undisturbed tube samples of expansive soils taken from Arizona, Colorado, and Texas. In general, the ratio of the matric suction equivalent to the matric suction decreases with increasing soil suction. An empirical relationship was established in which the matric suction equivalent was found to be proportional to the logarithm of the matric suction for in situ matric suctions greater than a certain value, termed as the 'intercept matric suction'. A correlation between the intercept matric suction and the soil's plasticity index (PI) was found for the clays of this study. The developed correlations represent a method for obtaining approximate estimates of in situ matric suction for expansive soils from PI and swell pressure data. Implications of the study findings for the use of an equivalent net normal stress approach for estimation of soil heave are explored. [ABSTRACT FROM AUTHOR]

Sinkholes in clay soils can be considered as the collapse of a soil layer previously bridging a void. Here, flexural deformation in the clay drives the formation of tensile cracks from the lowest surface of the layer and the consequent soil collapse is from crack propagation. Considering a simplified model of the sinkhole geometry, this paper aims to describe the tensile and fracture behaviour of clay soils with different plasticity indices. Speswhite kaolin, London, and Durham clays were tested using direct tensile and bending tests. Moderate- and high-plasticity clays showed a nonlinear fracture response with increasing moisture content, while low-plasticity clays demonstrated a linear response. Bending tests confirmed the importance of the moisture content while the plasticity index confirmed the difference in ductile or fragile collapse for fracture propagation. To assess the results, elasto-plastic fracture mechanics (EPFM) theory was applied to clays with appropriate modifications. The analysis demonstrated that EPFM theory provides a good baseline for predicting tensile fracture behaviour in clay soils, which can be extended in future research. [ABSTRACT FROM AUTHOR]

The location near the touchdown zone of a steel catenary riser at the seabed is a primary "hot spot" for fatigue assessment, with seabed stiffness having a major influence on the predicted fatigue life. This paper presents the results of laboratory model tests in the lateral direction with the motivation to appropriately capture the fundamental mechanism of soil interaction with the pipeline or riser in the lateral direction. The objectives of this study are to evaluate (i) the fundamental mechanism of soil interaction with the pipeline or riser in the lateral direction subjected to monotonic and cyclic loading, (ii) the evolution of lateral resistance with different (small to large) displacement amplitudes, (iii) the degradation of lateral resistance while increasing the number of cycles, and (iv) the recovery of the soil strength with time. The primary findings from the tests are that (i) the lateral resistance on the riser–pipeline drops sharply after trench formation, (ii) the lateral resistance across the trench approaches zero and reaches a steady state at a large number of cycles, (iii) the shape of trenches depends on the lateral displacement amplitude and the initial penetration depth, and (iv) some regain in strength occurs after a period for consolidation. [ABSTRACT FROM AUTHOR]

Traditional soil stabilization by chemical additives such as cement and lime is a well-established technique for ground improvement of problematic soils. However, with the advantage of lower carbon emission and energy consumption, fly-ash-based geopolymer has recently become an attractive alternative to traditional stabilizers. Nevertheless, the literature lacks systemic approaches that assist engineers to apply this promising binder for soil stabilization, including the proper dosages required for an effective treatment. This paper introduces a systematic approach to assess the applicability of fly-ash-based geopolymer for stabilization of clay soils, through a comprehensive experimental program where engineered and natural clays were examined and evaluated, including soil compaction, plasticity, compressive strength, durability, pH level, and impact of pulverization. The results revealed several factors that influence the level of enhancement of geopolymer-treated clays, including the soil mineralogy, plasticity–activity properties, geopolymer concentration, curing time, and pulverization. [ABSTRACT FROM AUTHOR]

This paper describes a program of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile, and type of footing (i.e., mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid, and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons groups, respectively, at which a turning point can be identified in the load–displacement curve. This was found to be attributed to the adhesion on the caisson–soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissons group exhibits much greater normalized uplift resistance than the mono-caisson group before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large-amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism. [ABSTRACT FROM AUTHOR]

Piles and prefabricated vertical drains (PVDs) are two well-established inclusions used by geotechnical practitioners when dealing with soft compressible foundations. Induced movements in highly compressible soil can adversely influence the pile response by inducing additional movements and stresses in the piles. Especially, undesirable soil–pile interaction often leads to the development of excess pore-water pressure during pile installation and negative skin friction caused by the settlement of compressible soil surrounding the piles. Additional drainage by PVDs prior to the installation of a pile could reduce excess pore-water pressure, lateral soil movement, and negative skin friction on the pile. In this paper, full-scale field testing on two trial embankments built on soft soil is reported and the relative behaviour of these two embankments is compared and discussed. Soft soil underneath both embankments was consolidated before one pile was installed at the centre of each embankment. The pore-water pressure, lateral soil movement, surface settlement, and associated strain at the pile shaft were recorded. The pile capacity was tested immediately and 3 h after pile installation. The monitoring and testing results indicated that preconsolidation with PVDs before piling can effectively reduce the excess pore-water pressure, lateral soil movement, and downdrag on the pile. [ABSTRACT FROM AUTHOR]

After unloading, the deformation of soils cannot be stable immediately, but continues to expand over time even under constant pressure. In this paper, the expansive deformation over time when effective stress is kept constant is defined as the negative creep, while the compressive creep is described as the positive creep. The division between positive creep and negative creep is named the stable normal compression line (SNCL), on which the stress–strain behaviour of the soil is time-independent. Based on the concept of the SNCL and test results, a new formula for creep is proposed. This formula is simple in form and has less parameters, and both negative creep and positive creep can be well predicted. By incorporating this formula into the current yield function of the unified hardening model, a new time-dependent current yield function is built. Combining the yield function, a flow rule, and transformed stress method, a new three-dimensional time-dependent constitutive model considering both positive and negative creep for clays is derived and presented. The new model is then validated by test results, including multistage loading oedometer tests, triaxial undrained creep tests, and triaxial undrained compression tests at the constant strain rates. [ABSTRACT FROM AUTHOR]

This paper presents an efficient, practical, and automated strategy for deriving modified Cam clay parameters from undrained self-boring pressuremeter (SBPM) data. A mixed approach involving a parametric sweep and numerical optimization is used, with a focus on parameter groups that dominate the modified Cam clay response in undrained cavity expansion. The proposed technique is illustrated using data from SBPM tests carried out in soft estuarine clay. The resulting parameters are used to back-analyse large-scale foundation load tests and are shown to provide an excellent match to the measured foundation response. [ABSTRACT FROM AUTHOR]

The constant rate of strain (CRS) oedometer test, using an imposed vertical strain rate --> ε ˙ I --> equal to 10 times the end-of-primary (EOP) vertical strain rate --> ε ˙ p --> , requiring a test duration of about 2 days produces reliable information on both the e versus log --> σ v ′ --> relation and e versus logkv relation of soft clays and fibrous peats. An empirical correction for the strain rate effect on preconsolidation pressure leads to the EOP e versus log --> σ v ′ --> relation and EOP --> σ p ′ -->. The imposed vertical strain rate --> ε ˙ I --> produces excess pore-water pressures at the impervious bottom of the specimen, corresponding to --> u b ′ / σ v --> values in the range of 3%–15% and allows, use of the Darcy flow equation, a reliable calculation of the coefficient of permeability. Compressibility and permeability data are from CRS and incremental loading (IL) oedometer tests on specimen quality designation (SQD) A samples of seven soft clays and two fibrous peats are presented in this paper. [ABSTRACT FROM AUTHOR]

A stiffened deep mixed (SDM) column can significantly increase the bearing capacity, reduce settlement, and enhance the slope stability of soft clays as compared with a conventional deep mixed (DM) column. This technique involves inserting plain concrete, reinforced concrete or a steel pile into the center of the DM column after the DM column is constructed. In this paper, a series of centrifugal modeling tests were conducted to investigate the performance of an SDM column-supported embankment over soft clay. A model embankment supported only by DM columns was constructed for comparison. Two ideal numerical models of column-reinforced soil under equal stress and equal strain conditions were established to explore the role the column played in accelerating soil consolidation. A parametric study was conducted to investigate the influence factors of the length of the core pile, column spacing, thickness of the underlying soil, modulus and thickness of the cushion, and modulus of the slab on the load transfer of the system, and some recommendations were proposed for its application. The load-transfer mechanism of an SDM column-supported embankment system with a slab was established based on the development of the volumetric strains and the principal stresses in the numerical models. [ABSTRACT FROM AUTHOR]

This paper summarizes 239 static load tests to evaluate the performance of four static design methods for axial resistance of driven piles in clay. The methods are ISO 19901-4:2016, SHANSEP, ICP-05, and NGI-05. The database is categorized into four groups depending on the load type (compression or uplift) and pile tip condition (open or closed end). The model uncertainty in resistance prediction is quantified as a ratio between measured and calculated resistance, which is called a model factor. The measured resistance is interpreted as a load producing a settlement level of 10% pile diameter. Database studies show that the four methods present a similar accuracy, where the mean and coefficient of variation (COV) of the model factor are around 1 and 0.3, respectively. The COV values are smaller than those for driven piles in sand available in literature. The model statistics determined from the database are applicable to a simplified or full probabilistic form of reliability-based design (RBD) of driven piles in clay. As an illustration, the resistance factors in load and resistance factor design (LRFD, a simplified form of RBD) are calibrated by Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

The determination of reliable geotechnical parameters from laboratory testing is highly dependent on sample quality. Over the past decades, undisturbed sampling of soft sensitive clays has been performed using various apparatuses and procedures. This paper outlines details of the design and performance of a new Laval-type tube sampler employed for the investigation of five soft clay sites located in Finland. The investigation was conducted using the new tube sampler and two different piston samplers. The sample quality was evaluated based on the recompression volume during reconsolidation to the in situ effective stress in constant-rate-of-strain oedometer tests. Test results show that tube samples are generally characterized by higher quality, especially in low-plasticity clays. In particular, the quality of piston samples is highly affected by the apparatus condition and sampling operations. Furthermore, the influence of storage time on tube samples was investigated. To guarantee a proper confinement, and thus reduce swelling, a pressurized system was applied to the tube samples obtained in two soft clay sites. Results demonstrate that the sample quality is not significantly affected by storage time as long as the soil is properly stored in the tube. [ABSTRACT FROM AUTHOR]

Clays exhibit creep in compression and shear. In one-dimensional compression, creep is commonly known as 'secondary compression' even though it is also a significant component of deformations resulting from shear straining. It reflects viscous behaviour in clays and therefore depends on load duration, stress level, the ratio of shear stress to compression stress, strain rate, and temperature. Research described in the paper partitions strains into elastic (recoverable) and plastic (nonrecoverable) components. The plastic component includes viscous strains defined by a creep rate coefficient ψ that varies with plasticity index and temperature ( T), but not with stress level or overconsolidation ratio (OCR). Earlier elastic-viscoplastic (EVP) models have been modified so that ψ = ψ( T) in a new elastic-thermoviscoplastic (ETVP) model. The paper provides a sensitivity analysis of simulated results from undrained (CIŪ) triaxial compression tests for normally consolidated and lightly overconsolidated clays. Axial strain rates range from 0.15%/day to 15%/day, and temperatures from 28 to 100 °C. [ABSTRACT FROM AUTHOR]

The mineral structure of soft clays is extremely fragile. Sampling operations and laboratory handling cause unavoidable disturbance. Undisturbed sampling is a theoretical concept because the stress release imposes disturbance. Consolidation tests on disturbed samples provide one-dimensional (1D)-compression curves very different from field response, which leads to inaccurate settlement estimates. Among the approaches for curve reconstruction, Schmertmann's method is probably the most commonly adopted in engineering practice. However, it presents difficulties in determining preconsolidation stress. The paper primarily addresses Schmertmann's and Nagaraj et al.'s propositions and discusses their advantages and shortcomings. A new approach for reconstructing the 1D-compression curve is proposed with the main objective being its independence of the interpretation of the experimental results. The validity was examined by comparing the reconstructed curves with those obtained by other methods. Experimental results on high- and low-quality specimens have been analyzed as well. The results revealed that the reconstructed curves for the cases analyzed are almost unique and independent of specimen quality. The proposed method allows both graphical and analytical implementation and the reconstructed curves are unaffected by experimental curve interpretation. [ABSTRACT FROM AUTHOR]

Estimation of the bearing capacity of shallow foundations on layered soil profiles, such as a sand layer of finite thickness over clay, is mainly based on empirical models resulting from the interpretation of experimental test results. While it is generally accepted that such models may be applicable to soil properties and footing geometries outside the range tested experimentally, they offer limited insights on how the assumed failure mechanism affects their range of application. In particular, the contribution of the sand layer to the overall capacity is accounted for via simple considerations, which are valid only for a specific range of problem parameters. This paper addresses the estimation of the undrained bearing capacity of a rigid strip footing resting on the surface of a sand layer of finite thickness overlying clay, using finite element limit analysis (FELA). The rigorous upper and lower bound theorems of plasticity are employed to bracket the true bearing capacity of the footing, and identify the geometry of possible failure mechanisms. Insights gained from FELA simulations are used to develop a new simple bearing capacity model, which captures the variation in shear resistance from the sand layer with the dimensionless undrained strength of the clay layer. The proposed model provides results that are in close agreement with published experimental studies, and allows treating simple problems, such as the design of working platforms, without having to resort to numerical simulations. [ABSTRACT FROM AUTHOR]

As a type of shallow foundation, a mudmat serves as the seabed support structure for subsea wells, pipeline manifolds, and pipeline terminations. The shallow foundations are usually designed with perforations to facilitate installation and removal, but the influence of these perforations has not been fully understood. This paper presents a method to analyze the bearing capacities of both two-dimensional (2D) and three-dimensional (3D) perforated shallow foundations using finite element analysis. The soil was idealized as a Tresca material, with the undrained strength increasing linearly with depth. The outcome indicates that perforations have nonnegligible effects on the bearing capacity of shallow foundations. The bearing capacity decreases with increasing perforation ratio, R, and the degree of reduction increases with the increase of the dimensionless ratio kB/Suo, where k is the shear strength gradient, B is the width of the foundation, and Suo is the shear strength at the mudline. For 2D shallow foundations, there exists a critical perforation ratio, Rc; when the perforation ratio is lower than the critical perforation ratio, the perforated foundation does not lose its bearing capacity. For 3D shallow foundations, the bearing capacity decreases directly with the increase of perforation ratio, R. [ABSTRACT FROM AUTHOR]

The main codes of practice governing jack-up preloading presently do not consider the possibility of beneficial effects of soil resistance generated by leg chord and leg brace members as they move downwards through disturbed soil that has been squeezed past the spudcan to form part of the backfill. This paper argues that these effects can be significant for the special case of deep penetrations in soft clay. An approximate method of estimating the effects is proposed and discussed. Results are found to be broadly consistent with recent centrifuge model tests and numerical analyses. Further work is recommended to explore these potentially important and certainly complex effects. [ABSTRACT FROM AUTHOR]

This paper presented a new approach for ground improvement of deep marine clay in which the conventional booster tube in the current air booster vacuum preloading technology was replaced by a booster prefabricated vertical drain (PVD). In comparison to the ordinary PVD, the booster PVD could provide inflow channels for the compressed air when the booster pump was in operation. To examine the performance of this new air booster vacuum preloading technology, in situ field tests were conducted at Oufei sluice project in Wenzhou, China, where the thickness of the soft soil layers (i.e., marine clay) was more than 20 m. An extensive monitoring system was implemented to measure the vacuum pressure, pore-water pressure, settlement, and lateral displacement at this reclamation site. With the collected field monitoring data, a comprehensive data analysis was carried out to evaluate the extent of ground improvement. The study results depicted that this new air booster vacuum preloading technology was more effective for the ground improvement of the deep marine clay layers, in comparison to the conventional vacuum preloading technology. [ABSTRACT FROM AUTHOR]

Excess pore-water pressures induced by pile driving could have a detrimental effect on a project, especially on the construction sequence and ground settlement. Measures that can effectively accelerate dissipation of pore pressures are therefore valuable. One way to reduce the pore pressure is the use of permeable piles. This paper presents a series of model-scale laboratory tests conducted on piles with drainage holes around the pile circumference. It is found that permeable piles could accelerate soil consolidation significantly, and the beneficial effects are more apparent for group pile tests. An approximate influencing zone of permeable piles was derived in this study based on the experimental observations. [ABSTRACT FROM AUTHOR]

This paper presents an analytical solution for drained expansion in both spherical and cylindrical cavities with a unified state parameter model for clay and sand (CASM). The solution developed here provides the stress and strain fields during the expansion of a cavity from an initial to an arbitrary final radius. Small strains are assumed for the elastic region and large strains are applied to soil in the plastic region by using logarithmic strain definitions. Since its development, the unified CASM model has been demonstrated by many researchers to be able to capture the overall soil behaviour for both clay and sand under both drained and undrained loading conditions. In this study, the CASM model is used to model soil behaviour whilst a drained cavity expansion solution is developed with the aid of an auxiliary variable. This is an extension of the undrained solution presented by the authors in 2017. The parametric study investigates the effects of various model constants including the stress-state coefficient and the spacing ratio on soil stress paths and cavity expansion curves. Both London clay and Ticino sand are modelled under various initial stress conditions and initial state parameters. The newly developed analytical solution highlights the potential applications in geotechnical practice (e.g., for the interpretation of cone penetration test data) and also provides useful benchmarks for numerical simulations of cavity expansion problems in critical state soils. [ABSTRACT FROM AUTHOR]

This paper presents results of a series of hollow cylinder tests carried out to investigate the undrained behaviour of saturated railway track foundation materials during cyclic loading involving principal stress rotation. Four sand-clay mixes representative of real railway track foundation materials were investigated. It was found that moderate additions of clay (up to ∼14% by weight) increased the cyclic shear stress threshold at which significant excess pore pressures started to accumulate. After the cyclic shear stress threshold had been exceeded, the rate of pore pressure increase with the logarithm of axial strain was greatest for the material having a clay content of 11%. Excess pore pressure generation reduced with increasing intergranular and global void ratio, with the global void ratio being perhaps the more useful indicator because of the reduced amount of scatter and higher correlation of the idealized relationship. [ABSTRACT FROM AUTHOR]

A test fill at Onsøy, close to the town of Fredrikstad in Norway, was built on a very soft marine clay with in situ vane strength as low as 11 kPa and a plasticity index varying from 34 to 50. The dimensions at the bottom of the long test fill were 20 m × 60 m with a height of 2.3 m. The fill, as placed in the first stage, had a computed factor of safety against foundation failure of 1.35 based on in situ vane strength. The fill was allowed to sit for about 3 years (from 1972 to 1975) before it was, in a second stage, brought to failure in about 4.3 days by raising the height. This second stage is the subject of this paper. Back-calculation of the failure by undrained stability analysis based on in situ vane tests performed shortly before the start of the second stage, based on undrained triaxial and constant volume direct simple shear tests on specimens from tube samples taken shortly before the start of the second stage and reconsolidated to the in situ effective stresses, gave factors of safety varying from 0.88 to 1.45 depending on type of analysis. Settlement values computed at the centreline of the fill for the second stage based on the results from undrained compression triaxial tests agreed reasonably well with the field measurements. [ABSTRACT FROM AUTHOR]

In this paper, time-dependent clogging is studied considering a unit cell consisting of a single stone column interacting with the surrounding soft clay. Clogging is assessed quantitatively and the corresponding void space of the column is determined using computed tomography. It is observed that the extent of clogging is substantial in the upper part of the column, but diminishes rapidly with depth. The soil properties in the clogged zone are determined indirectly through additional tests of clay-aggregates mixtures with various clay fractions. An equal strain consolidation model based on the principle of unit cell analysis is developed to capture both the initial and time-dependent clogging. The model accounts for a reduction in permeability and an increase in compressibility of the column. This current model, as expected, offers identical results to some previous studies if clogging is ignored, while the comparison with other selected models demonstrates the influence that clogging of the stone column can have on the consolidation of the surrounding soil. Furthermore, load-settlement predictions from the proposed 'equal strain' model are also compared with the consolidation response of a previously developed 'free strain' model. [ABSTRACT FROM AUTHOR]

In this paper, the physics of scour hole in a mixed sand-clay bed downstream of an apron is studied experimentally. Seven combinations of sand-clay mixtures including clay contents, Cc, ranging from 0 to 0.4 were used. The results show that Cc = 0.4 can reduce the maximum scour depth, εm, up to about 80% for all the densimetric Froude numbers in the range of the present study. An empirical equation has been proposed for calculation of εm in sand-clay mixtures with the mean error of 0.12. The removal mechanism of sediments from the bed was different based on the Cc. For low clay contents, i.e., Cc ≤ 15%, individual particles were detached from the bed. At higher Cc, clusters of particles were separated and moved downstream with the flow. A new equation has been proposed to predict the dimensionless scour hole profile in mixed sand-clay sediments. Dimensionless graphs have been presented for incorporating the effect of tailwater depth and sediment grain size on the main characteristics lengths in sand-clay mixtures. [ABSTRACT FROM AUTHOR]

Conventional seismic assessments of river embankments have focused on liquefaction damage of sandy ground. However, following the 2011 earthquake centered off the Pacific Coast of Tohoku, extensive damage of river embankments built on a clayey foundation has garnered greater attention. This paper presents seismic response analyses of river embankments constructed on soft and sensitive estuarine clay, as well as analyses of countermeasures implemented with sheet piles with a succession of penetration depth and placement. River embankments have kept stable if the clayey ground was assumed to be a nonsensitive condition. However, if a strong shake impacts actual sensitive clay, there is a risk of slippage generated from the clayey layer. If the penetration depth of piles is shallow in the clayey layer, the clay is strongly disturbed by the tip of the pile, generating extensive damage; if the penetration depth is sufficiently deep in the supporting layer, significant deformation control may be obtained at the reinforced side, although there is a risk of promoting deformation at the opposite side. These results indicate the importance of the specific inspection of pile penetration depth; otherwise, not only is it not possible to obtain a sufficient countermeasure effect, but also an adverse effect may be generated. [ABSTRACT FROM AUTHOR]

This paper describes a design framework for inclined tensile loading capacity (holding capacity) of dynamically installed anchors in soft clay. Centrifuge model test and numerical results indicate that, while ultimate inclined holding capacity increases with a loading angle smaller than 45° (to the vertical at anchor padeye), anchor failure is controlled predominantly by the ultimate vertical holding capacity, which can be predicted using the American Petroleum Institute method put forth in 2007 with interface friction ratio of 0.25-0.34 and 0.60-0.82 for short- and long-term conditions, respectively. For a loading angle larger than about 65°, anchor failure is controlled largely by the ultimate horizontal holding capacity. It is proposed herein that the ultimate lateral holding capacity can be estimated by generalizing Broms' method put forth in 1964 for lateral anchor capacity to variable cross section; thereby allowing anchor geometry to be considered in a rational manner. Good agreement can be obtained between the estimated ultimate horizontal holding capacity and that computed using the finite element method by using a lateral resistance of 10.5 su, where su is the undrained shear strength. For a loading angle between 45° and 65°, there is significant interaction between vertical and horizontal failure modes. This allows a normalized horizontal-vertical interaction curve to be defined by fitting data from this and previous studies. [ABSTRACT FROM AUTHOR]

The presence of a thin soft clay layer inside a bed of sand may significantly reduce the bearing capacity of the sand layer, imposing a risk of punch-through failure. In this paper, finite element (FE) simulations are reported using a hardening soil (HS) model for sand. The FE model has been verified against centrifuge tests involving loose and dense sand layers overlying clay soil. The effects of sand stiffness, foundation roughness, sand friction angle, undrained clay strength, clay strength nonhomogeneity, and sand and clay layer geometries on the foundation peak capacities have been studied. Punch-through failure is initiated with an inclined sand plug being sheared and pushed into the underlying soft clay. During punch-through, the clay layer fails due to significant radial squeezing. Existing analytical models do not capture the combined failure mechanism of sand shearing and clay radial squeezing. A new analytical model is developed to estimate the peak punch-through capacity of a spudcan in sand with an interbedded clay layer, showing improved performance over the current industry guidelines. [ABSTRACT FROM AUTHOR]

This paper constructs a 10-dimensional multivariate probability distribution covering 10 clay parameters. The parameters are the liquid limit, plasticity index (PI), liquidity index, effective vertical stress, undrained shear strength, sensitivity, and three piezocone test parameters. A CLAY/10/7490 database is compiled in a companion paper for this purpose. The database consists of 7490 data points from 251 studies. The number of data points associated with each study varies from 1 to 419 with an average 30 data points per study. The clay properties cover a wide range of overconsolidation ratios (but mostly 1~10), a wide range of sensitivity (St) (sites with St = 1~tens or hundreds are fairly typical), and a wide range of PI (but mostly 8~100). The constructed multivariate probability distribution can be used as a prior distribution to derive the joint distribution of design parameters based on limited but site-specific field data. Note that the entire joint distribution of the 10 clay parameters is derived, not marginal distributions or simply means and coefficients of variation. These multiple design parameters can be updated from multiple field measurements, which is more useful than updating one design parameter using one field measurement that is typical in current practice. This paper also demonstrates that it is practical to build multivariate probability models by combining available bivariate models, which are prevalent in the geotechnical engineering correlation literature. The proposed approach circumvents the need to collect multivariate data, which are rarely found in typical site investigation programs. [ABSTRACT FROM AUTHOR]

The push-in resistance test (PIRT) is an in situ means of verifying the strength of stabilized soil columns. During the test, a winged penetrometer penetrates the stabilized column at a constant rate and the column strength is estimated from the recorded probing resistance using a bearing capacity factor ( N). While N values between 8 and 20 (although typically between 8 and 15) are quoted in the literature (based almost exclusively on empirical and Scandinavian experience), there have been few field tests and no laboratory investigations aimed at investigating the value of N and the factors upon which it depends, thereby limiting international confidence in the method. This paper presents the findings of a unique laboratory-scale PIRT series conducted at the National University of Ireland, Galway (NUI Galway). The development of appropriate column construction and testing methods is discussed. The results of 11 PIRT tests on pre-drilled stabilized columns with unconfined compression strengths (UCS) from 150 to over 800 kPa are reported, as well as those of a complementary cone-only series of tests to assess additional frictional forces acting on the penetrometer. Appropriate corrections to the data for temperature and time consistency between the probing forces and UCS values are discussed. Test results indicate that the strength of the column has a mild influence on the N value. Further investigation of this finding is recommended at both laboratory and field scales. [ABSTRACT FROM AUTHOR]

Predicting the final embedment depth of a dynamically installed anchor is a key prerequisite for reliable calculation of anchor capacity. This paper investigates the embedment characteristics of dynamically installed anchors in normally consolidated and overconsolidated clay through a series of centrifuge tests involving a model anchor instrumented with a microelectric mechanical system (MEMS) accelerometer, enabling the full motion response of the anchor to be established. The data are used to assess the performance of an anchor embedment model based on strain-rate-dependent shearing resistance and fluid mechanics drag resistance. Predictions of a database of over 100 anchor installations - formed from this study and the literature - result in calculated anchor embedment depths that are within ±15% of the measurements. An interesting aspect, consistent across the entire database, relates to the strain rate dependence on frictional resistance relative to bearing resistance. The predictions reveal that strain rate dependency may indeed be higher for frictional resistance, although only if a soil strength lower than the fully remoulded strength is considered as the reference strength, which suggests that water may be entrained along a boundary layer at the anchor-soil interface during installation. [ABSTRACT FROM AUTHOR]

Jack-up rigs are commonly employed to perform offshore oil and gas drilling. During the installation of the spudcan foundation, the large volume of soil displaced may induce severe stresses on the piles supporting the adjacent platform. The increasing numbers of jack-up rigs being deployed at close proximity to piled platforms accentuates the need for research into quantifying the effect of spudcan penetration on adjacent piles. The main difficulty with the numerical simulation of spudcan-pile interaction lies in the need to model continuous spudcan penetration in a three-dimensional (3-D) modeling space. In this paper, the three-dimensional Eulerian numerical technique is adopted to perform a coupled interaction analysis whereby the process of continuous spudcan penetration is simulated and the corresponding pile responses are obtained. The salient features of the proposed numerical approach with particular emphasis on the incorporation of a pile in the numerical model are first described. The validation of the numerical model against centrifuge experimental data under different geometric configurations and soil profiles are then presented. The final part of this paper demonstrates the usefulness of the proposed numerical approach through a series of parametric studies. This numerical approach can be readily applied to perform site-specific assessment to mitigate the risks associated with spudcan-pile interaction. [ABSTRACT FROM AUTHOR]

This paper summarizes a detailed geotechnical and geochemical characterization program for the Champlain clay deposit in Lachenaie, Quebec. Presently, the clay pore water is saline. The main objective of the testing program was to study the leaching of the clay saline pore water since its deposition. Four issues are discussed in this paper. First, the deposit is shown to comprise two distinct clay layers with contrasting geotechnical properties. The liquid limit and grain-size distribution are the properties that best distinguish the two materials. The upper layer is more plastic while the lower one is relatively silty. The occurrence of these two layers is shown to be in agreement with some recent findings in Quaternary geology. Second, the relatively high pre-consolidation pressures of the clay in the Lachenaie area are shown to be coherent with an erosion scenario and the elevation of a neighbouring bluff previously interpreted as an erosion terrace. Third, the relatively constant hydraulic conductivity values obtained from permeability tests throughout the deposit are shown to be consistent with the variations of liquid limit and void ratio in the two clay sublayers. Fourth, the occurrence of sensitive clay pockets (sensitivity greater than 50) in this otherwise saline clay body is elucidated. [ABSTRACT FROM AUTHOR]

Offshore pile foundations are always subjected to cyclic lateral loads, which can result in the remolding and softening of the surrounding seabed soil. Cyclic T-bar penetrometer testing provides a rapid and effective method for assessing the remolded shear strength. It is widely believed that the soil strength degrades with the accumulation of plastic strain, but the strain cannot be measured. Numerical analysis described in this paper shows that the accumulated plastic displacement of the T-bar in a cyclic range of two diameters is approximately equal to its accumulated displacement. By using T-bar test data, a cyclic degradation model based on the accumulated (plastic) displacement is developed to describe the soil strength degradation at a given depth. Furthermore, an improved p-y curve model based on the cyclic degradation model is proposed to estimate the lateral response of pile under cyclic loads. The improved p-y curve model was embedded into the OpenSees program to investigate the cyclic lateral responses of soil elements and the pile. A case study was conducted to verify the improved p-y curve model by comparing it with published centrifuge experiment data. Results indicate that the improved p-y curve model based on T-bar test data is highly precise and practicable. [ABSTRACT FROM AUTHOR]

Although engineers often make use of pile geometry to improve the axial load capacity of piles, geometrical effects on floating piles in consolidating clay are still not fully understood. This paper reports two centrifuge model tests to investigate the responses of a Y-shaped pile and a circular pile subjected to an induced dragload and applied axial loads. Three-dimensional numerical back-analyses were performed considering the elastoplastic slip. The Y-shaped and circular piles developed similar downdrag, but the dragload induced on the Y-shaped pile was larger than that induced on the circular pile. As the axial load increased, the neutral plane shifted upward along a nonlinear path, of which the gradient of the Y-shaped pile was gentler. The ultimate bearing capacity of the Y-shaped pile was 1.73 times that of the circular pile. The dragload on the Y-shaped and circular piles was eventually eliminated at approximately 0.56 and 0.83 times the corresponding ultimate pile capacities, respectively. Three flanges of the Y-shaped pile "hung-up" adjacent soil that settled together with the pile shaft. The lateral extent of vertically nonuniform trapped soil decreased with increasing axial load. Even though the Y-shaped and circular piles encountered a similar serviceability limit state, Y-shaped pile had advantages in bearing capacity. [ABSTRACT FROM AUTHOR]

Challenges in obtaining high quality samples of sensitive low-plasticity clay in an effective manner have been overcome by the development of the mini-block sampler. The starting point for the development of the new sampler was the Sherbrooke block sampler that was first introduced in Canada in 1979. Although the Sherbrooke block sampler can produce high quality samples, its use, particularly in industrial projects, has been limited due to the perceived high costs, practical difficulties, and the time required. This paper outlines details of the development and design of the mini-block sampler together with developed techniques for protection and transportation of the samples and preparation of specimens for laboratory testing. The sampler has been used successfully at five Norwegian clay sites. For two of the sites comparative Sherbrooke samples were available and it is shown that the quality of the mini-block samples is very high and is at least as good as that of the Sherbrooke samples. The work also demonstrates that rigging, preparation, and sampling with the mini-block sampler is fast, practical and is not labour intensive. Furthermore the resulting samples are easy to transport and handle, but still provide sufficient material for extensive laboratory testing. [ABSTRACT FROM AUTHOR]