Bothkennar Clay Research Articles

An evaluation of non-linear undrained behaviour in the moderate strain range for fine-grained soils

To select appropriate stress-strain parameters for serviceability limit state calculations, an understanding is needed of the likely variation of stress-strain behaviour within the model displacement mechanism. One approach that may be utilised to investigate variations in stress-strain behaviour is by employing a simple non-linear model (with a small number of physically significant parameters) that simulates experimental measurements of soil stress-strain with reasonable precision. By testing the sensitivity of the model parameters to changes in physical properties that can be expected to be related to them the reliability of different models can be established. Recently, empirical analysis of the published triaxial test database RFG/TXCU-278 identified a significant positive correlation between 50 and OCR for four test modes (CIUC, CIUE, CKUC and CKUE). In this paper, a new experimental dataset from a programme of reconstituted soil tests on Kaolin and Bothkennar Clay is used to investigate the validity of a simple non-linear model.

Hydraulic conductivity of fine-grained soils subjected to freeze-thaw cycles

This paper presents laboratory data from tests on four fine-grained soils: reconstituted Kaolinite, destructured Bothkennar clay, reconstituted Bothkennar clay, and reconstituted Gault clay. The soil samples were conditioned in an oedometer cell while being subjected to varying numbers of freezing and thawing cycles. The influence of freeze-thaw cycles on key soil parameters, including the hydraulic conductivity, Atterberg limits, compression and swelling index was studied. The experimental results were then compared with the analysis of a previously published database of hydraulic conductivity measurements on fine-grained soils called FG/KSAT-1358. The paper demonstrates that while multiple cycles of freezing and thawing affect some of the studied soil parameters, such as the Atterberg limits and the compression characteristics, the effects on the hydraulic conductivity transformation model parameters, linking the water content ratio to hydraulic conductivity are less apparent. The results are useful for geotechnical and pavement engineers when making assessments of freeze-thaw effects on subgrade materials in cold regions.

Evaluation of stress history and undrained shear strength of three marine clays using semi-empirical methods based on Piezocone Test

The paper presents a comparative study between semi-empirical methods for the estimation of pre-consolidation pressure and undrained shear strength from Piezocone (CPTu) data. The first method, proposed by Massad, was developed from observing the variation of these parameters with depth; the second method, proposed by Mayne, was developed from simplifications and relationships between the Spherical Cavity Expansion Theory (SCET) and the Critical State Theory; the third method was proposed by Mayne, which considers the variations due to soil type from the CPT Index to estimate the pre-consolidation pressure. The methods were validated based on their applications to the marine clay from Santos Coastal Plain, Brazil, Bothkennar clay from Scotland, and Torp Clay from Sweden. It is intended to verify if the results are consistent with each other, with the stress history of these soils and with the available test results. The application of the Massad’s method led to results close to the available reference values. The results of the Mayne’s method based on SCET showed great variability in behavior comparing to the test data depending on the case study. By the Mayne’s method based on CPT Index values, the calculated pre-consolidation pressures were slightly higher than the values of the available test data. The variations in the results highlighted the importance of validating estimates based on semi-empirical methods through specific tests and the knowledge of geological history contributes to predicting the behavior of clays, since they showed good agreement with the available data from oedometer tests.

Analysis of Cyclic Response of Bucket Foundations Based on Simplified Kinematic Hardening Model

The response of bucket foundations for offshore wind turbines subjected to cyclic loading in saturated clay is explored through three‐dimensional finite element numerical analyses. In the analyses, nonlinear cyclic hysteretic behavior of clay under undrained condition is modeled through a simple kinematic hardening constitutive model embedded in ABAQUS. The finite element model is validated against published in situ tests of bucket foundations under quasistatic cyclic loading in Bothkennar clay. The computed results agreed generally with those from in situ tests. The behavior of bucket foundations with different aspect ratios under displacement‐controlled cyclic loading mode is investigated. Then, the evolution of foundation displacement with increasing number of cycles is studied subjected to wind and wave combined loading. The results show that, for the cycles of low‐amplitude rotation, dimensionless moment‐rotation curve is approximately elastic; however, the curve engenders obvious hysteresis loop, whose shape is influenced by soil‐sidewall interface condition, during high‐amplitude cycles. Under thousands of loading cycles, for bucket foundations of low aspect ratio, the oscillatory displacement component is smaller; however, the residual component will accumulate gradually until the serviceability rotation is exceeded. For foundations of high aspect ratio, the oscillatory component is relatively larger, but the accumulation rate of residual displacement decreases gradually.

Modeling Pile Setup in Natural Clay Deposit Considering Soil Anisotropy, Structure, and Creep Effects: Case Study

AbstractThis paper reports the results of a numerical investigation of the behavior of a natural soft clay deposit under the installation of a case study pile. The case study problem included installation of an instrumented close-ended displacement pile in a soft marine clay deposit, known as Bothkennar clay, in Scotland. The site has been used for a number of years as a geotechnical test bed site, and the clay has been comprehensively characterized with both in situ tests and laboratory experiments. The soft soil behavior, both after pile installation and subsequent consolidation, was reproduced by using an advanced critical-state-based constitutive model that accounts for the anisotropy of soil fabric and destructuration effects during plastic straining. Furthermore, a time-dependent extension of the model was used to study soil creep and the significance of its consideration in the overall pile-installation effects. The simulation results were compared against field measurements; furthermore, for compa...

Numerical study on the response of two footings at Bothkennar research site

This paper presents a numerical study of the performance of two instrumented surface footings at the Bothkennar Clay research site in the UK. Footing A was loaded to failure over 4 days, reaching a net bearing capacity of qr = 138 kPa; footing B was loaded to 89 kPa, at an identical loading rate, and left to consolidate under maintained load for about 11 years. The preloaded footing was then loaded to failure over 3 days, reaching qr = 204 kPa. The increase in bearing capacity was significantly larger than that expected due to consolidation effects alone, and it is anticipated that the occurrence of creep and other ageing processes may have played a major role in the observed response. The complete loading history of the two footings is simulated by means of coupled axi-symmetric finite-element analyses in which the foundation soil is described using an elastic–viscoplastic model that mimics isotach viscosity. The ground profile and the model parameters are derived based on the extensive laboratory and field test data available in the literature. The numerical analyses are able to describe accurately the footings behaviour during first loading, the development of delayed settlement under maintained load and the increase in bearing capacity due to preloading. The paper emphasises various issues regarding the application of elastic–viscoplastic models to model boundary value problems in conditions close to failure.

Influence of stone column installation on settlement reduction

The paper presents numerical simulations investigating the settlement reduction caused by stone columns in a natural soft clay. The focus is on the influence of the soft soil alteration caused by column installation. A uniform mesh of end-bearing columns under a distributed load was considered. Therefore, the columns were modelled using the “unit cell” concept, i.e. only one column and the corresponding surrounding soil in axial symmetry. The properties of the soft clay correspond to Bothkennar clay, which is modelled using S-CLAY1 and S-CLAY1S, which are Cam clay type models that account for anisotropy and destructuration. The Modified Cam clay model is also used for comparison. Column installation was modelled independently to avoid mesh distortions, and soft soil alteration was directly considered in the initial input values. The results show that the changes in the stress field, such as the increase of radial stresses and mean stresses and the loss of overconsolidation, are beneficial for high loads and closely spaced columns but, on the contrary, may be negative for low loads, widely spaced columns and overconsolidated soils. Moreover, whilst the rotation of the soil fabric reduces the settlement, in contrast the soil destructuration during column installation reduces the improvement.

Numerical simulations of stone column installation

This paper describes the results of numerical simulations investigating the installation effects of stone columns in a natural soft clay. The geometry of the problem is simplified to axial symmetry, considering the installation of one column only. Stone column installation is modelled as an undrained expansion of a cylindrical cavity. The excess pore pressures generated in this process are subsequently assumed to dissipate towards the permeable column. The process is simulated using a finite element code that allows for large displacements. The properties of the soft clay correspond to Bothkennar clay, modelled using S-CLAY1 and S-CLAY1S, which are Cam clay–type models that account for anisotropy and destructuration. Stone column installation alters the surrounding soil. The expansion of the cavity generates excess pore pressures, increases the horizontal stresses of the soil, and most importantly modifies the soil structure. The numerical simulations performed allow quantitative assessment of the post-installation value of the lateral earth pressure coefficient and the changes in soil structure caused by column installation. These effects and their influence on stone column design are discussed.

Effects of long-term pre-loading on the performance of a footing on clay

An instrumented footing test in Bothkennar clay is described, which examines the effects of a long period of drained pre-loading on the footing's undrained bearing capacity. It is seen that pre-loading induced a significant increase in the subsequent foundation stiffness due to consolidation but also due to creep and ageing processes that led to an outward expansion of the clay's large-strain yield surface. Pre-loading also resulted in a downward shift of the bearing capacity mechanism and an ultimate bearing capacity that was substantially greater than that expected due to consolidation effects alone. Evolving anisotropy and creep are identified as being important contributory factors to the enhanced undrained capacity after pre-loading.

Effects of long-term pre-loading on the performance of a footing on clay

An instrumented footing test in Bothkennar clay is described, which examines the effects of a long period of drained pre-loading on the footing's undrained bearing capacity. It is seen that pre-loading induced a significant increase in the subsequent foundation stiffness due to consolidation but also due to creep and ageing processes that led to an outward expansion of the clay's large-strain yield surface. Pre-loading also resulted in a downward shift of the bearing capacity mechanism and an ultimate bearing capacity that was substantially greater than that expected due to consolidation effects alone. Evolving anisotropy and creep are identified as being important contributory factors to the enhanced undrained capacity after pre-loading.

Continuous hyperplastic models for overconsolidated clays

This paper presents four constitutive models for prediction of the behaviour of overconsolidated clays under triaxial loading conditions. Their formulations are based on a new continuous hyperplastic framework. Continuous hyperplasticity allows the derivation of plasticity models which simulate smooth stress-strain behaviour, have continuous memory, and are guaranteed to obey the laws of thermodynamics. All four models are capable of simulating the triaxial stress-strain behaviour of overconsolidated clays in a wide strain range. The ability of the models to predict the stress path dependency of the clay behaviour is evaluated against experimental data obtained on undisturbed Laval samples of Bothkennar clay.

Self-boring pressuremeter permeability measurements in Bothkennar clay

Self-boring pressuremeter permeability measurements in Bothkennar clay

Self-boring pressuremeter permeability measurements in Bothkennar clay

Keywords: clays ; earth pressure ; in situ testing ; permeability ; pore pressures ; site investigation ... Show All

Stiffness of geomaterials at very small strains

Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry–Pérot laser interferometer. A displacement accuracy as high as ±0·027 μm was obtained, leading to an axial strain accuracy of the order of ±10−4%. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress–strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0·002–0·003%) for all three materials, despite the maximum stiffness (Emax) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0·01% strain being of the order of 80–95% of Emax. Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using field geophysical techniques, despite assumptions of isotropy, and the effects of sampling. The implications of these observations for practice are discussed.

Stiffness of geomaterials at very small strains

Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry–Pérot laser interferometer. A displacement accuracy as high as ±0·027 μm was obtained, leading to an axial strain accuracy of the order of ±10−4%. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress–strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0·002–0·003%) for all three materials, despite the maximum stiffness (Emax) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0·01% strain being of the order of 80–95% of Emax. Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using field geophysical techniques, despite assumptions of isotropy, and the effects of sampling. The implications of these observations for practice are discussed.

Kinematic hardening model for overconsolidated clays

Abstract The proposed model puts a particular emphasis on accurate simulation of stress-strain behaviour of overconsolidated clays both within the small strain region of high stiffness and during the large-scale yielding. To achieve this, the model utilizes existing formulations which proved to be successful in description of clay behaviour in these regions, and makes an attempt to bridge between them. The model utilizes the principles of the multiple surface kinematic hardening plasticity and is constituted by three elliptical yield surfaces in triaxial stress space. Two of the surfaces are rotated ellipses of the same shape, representing the boundaries of the linear elastic and small strain regions, while the third one is the rotating elliptical MIT-E3 large-scale yield surface with a non-associated flow rule. All three surfaces are subject to both kinematic and isotropic hardening with corresponding hardening rules. The model requires 17 parameters, out of them eight are small strain and nine are large strain parameters. Performance of the model in prediction of the stress path dependency of clay behaviour was evaluated against experimental data obtained on undisturbed Laval samples of Bothkennar clay.

Role of organic material in the plasticity of Bothkennar clay

Keywords: chemical properties ; clays ; organic ; soils ; plasticity ; sedimentation ; soil classification ... Show All

In situ determination of G hh at Bothkennar using a novel seismic method

Abstract The paper describes a novel seismic method for determining the variation of velocity with depth of horizontally propagating, horizontally polarized shear waves ( S h,h ). The technique exploits the curved paths of seismic waves in deposits with increasing velocity properties with depth. The new method has been applied at the soft clay test site at Bothkennar, and the results of this survey are compared with S h,h and S v,h wave velocity profiles obtained using conventional crosshole survey techniques at the site. The seismic data are further compared with the results of recently reported bender element tests on unconfined samples from the bedded and mottled Bothkennar clay. It is shown that, at the site investigated, the new method presents results comparable to those from the established field survey techniques. Neither the seismic nor the bender element data indicate significant stiffness anisotropy at the site.

Non-linear model of small-strain behaviour of soils

The paper describes a general approach to the development of a constitutive model for predict-ing the generalized small-strain behaviour of soils. The model incorporates non-linear stress— strain behaviour, stress-path dependence and a consistent definition of the small-strain region (SSR). Based on this general approach, a simple model is proposed which incorporates crossanisotropic elasticity within the linear elastic region (LER) and elliptical boundaries for the LER and SSR. The model leads to unique normalized stress—strain behaviour of undisturbed Bothkennar clay for the complete range of stress probe directions in triaxial stress space. The comparison confirms the uniqueness of the normalized volumetric and deviatoric stress— strain behaviour. The model can easily be incor-porated into numerical procedures for solving two- and three-dimensional boundary value problems. ĺarticle décrit une conception générale de la mise au point ´un modéle constitutif pour prédlre le comportement général des sols soumis à de petites contraintes. Le modèle regroupe la réaction aux tensions/déformations non liné aires, la variabilité en function du parcours de déformation et une définition cohérente du domaine des petites contraintes. A partir de cette conception générale, les auteurs proposent un modéle simple qui incorpore ĺléasticité transanisotrope dans le domain é1astique linéaire et les limites elliptiques du domaine élastique lin&aci\ute;aire et du domaine des petites contraintes. Le modèle produit un comportement spécifique de ĺargile Bothkennar non remaniée face à des tensions/déformations normalisées dans toute la gamme de directions de la sonde de tension, dans ĺespace de contrainte triaxiale. La comparaison confirme la spécificité de la réacdon à des tensions/déformations hydrostatiques et dé viatrices normalisées. Il est facile ´incorporer le modéle dans des formules numériques pour résoudre les problèmes de valeurs limites à deux ou trois dimensions.

Discussion: The yielding of Bothkennar clay

Discussion: The yielding of Bothkennar clay