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Silty alluvial soils are highly compressible. Deep soil mixing is being increasingly used for remediating such ground conditions, which produces artificially cemented soil columns that gain strength with curing. This study investigated the one-dimensional compression and shear stiffness degradation behaviour of an alluvium in its reconstituted, undisturbed and artificially cemented states. The binder used to stabilise the soil was a low-carbon alkali-activated blast-furnace slag. Oedometer and triaxial data indicated that the nature of the soil structure evolved from being chiefly meta-stable when undisturbed to dominantly stable in its cemented state after 28 days of curing. A new framework has been presented to quantify continuous changes in structure within the alluvium during one-dimensional compression in its undisturbed and cemented states, with respect to its intrinsic properties. This better captured structure degradation during earlier stages of compression compared with previous frameworks and provided insights into defining the limits of meta-stable and stable components of structure within materials of higher strength. A new formulation has been developed for predicting shear stiffness degradation of samples with strain under triaxial conditions. This showed efficiency and good performance in modelling experimental data and was successfully used to quantify initial structure and degradation of structure within the undisturbed and cemented alluvium.

This study aims to understand the causes of anomalous pore fluid pressures within sedimentary sequences of an active tectonic basin through well log analysis, pressure data evaluation and thermo-hydro-geomechanical modelling. The study focuses on the East Coast Basin (ECB), New Zealand, an active convergent margin, where anomalously high pore pressures have been encountered in deep-water systems at burial depths as shallow as 200 m. A regional investigation including analysis of the Cretaceous to Holocene tectono-stratigraphy and diagenetic histories of the ECB, was combined with seismic and well log interpretation to understand the structural and sedimentation history of the ECB, and thus the main factors that were likely to contribute to overpressure generation/dissipation and porosity loss. 1D hydro-geomechanical models were then built to undertake a parametric study to investigate the effect on porosity and pore pressure evolution of different sedimentation and erosion rates, hiatus periods, different erosion thicknesses, and tectonic compression. The parametric models show that high overpressures can be preserved during rapid erosion events due to the relatively small timeframe for pore pressure dissipation, depending on sediment permeability. Furthermore, only recent erosion events are relevant to the present-day overpressure. In addition, high levels of tectonic compression (12.5 %) applied in recent events can produce both high pore pressure values and significant porosity reduction if the sedimentary column was undercompacted prior to the tectonic compression. Learnings from the parametric studies were used as a starting point to understand controls on the pore pressure and porosity in the Opoutama-1 well, located on the onshore area of the ECB. Results from the Opoutama-1 well show that the high pore pressure registered at shallow depths (< 1 km) in this well is significantly driven by tectonic compression as a result of high subduction rates (presently 48 mm/yr). Disequilibrium compaction also contributed to overpressure generation due to high sedimentation rates (up to ~3000 m/Ma). However, much of the disequilibrium compaction-related overpressure was dissipated during uplift, hiatus, and erosion. Where overpressure is preserved, it is related to thick (up to 1 km) mudstone packages deposited during the Cretaceous to Holocene, thin low permeable layers of limestones deposited during the Miocene to Pliocene and marl intervals with high content of smectite.

Offshore wind turbines (OWTs) must withstand harsh environmental loads over their 20- to 30-year design life. Fragility analysis investigates the probability of damage over a range of hazard intensities and is integral to a performance-based engineering approach. The focus of this paper is on monopiles, which are widely used to support OWTs in water depths up to around 40m. The paper presents a fragility analysis of monopiles in extreme storms in terms of geotechnical performance, measured by permanent rotation of the foundation. Geotechnical fragility has so far not been comprehensively addressed due to the challenge of predicting soil behaviour under cyclic loading and estimating the probability of extreme responses. On the latter, the paper develops efficient Karhunen-Loeve representations of wind and wave loading that can be combined with inexpensive probabilistic methods to compute fragility. The framework was demonstrated using a representative scenario of a 5MW OWT installed in clay. Non-linear foundation response was captured by a dynamic 3D finite element model. Fragility curves were generated using subset simulation for storms with return periods (RPs) from 1 to 100 years. Fragility during extreme storms (with 50- and 100-year RPs) was significantly higher than storms with RPs of 10 years or less.

The construction industry in Abu Dhabi is thriving and its coastline has some of the most ambitious structures in the world. Whilst the subsurface evaporitic and calcareous soft rocks of this region are of great geological interest, they are relatively poorly understood from a geotechnical engineering perspective, forcing foundation designs to be overly conservative. Understanding the stiffness of the underlying geology at small strains is of great importance for the accurate estimation of ground movements around excavations and foundations, and yet routine post-SI laboratory testing programmes tend to focus on basic rock mechanics tests such as UCS tests. These procedures are generally unsuitable for use with calcareous rocks due to their friable and moisture sensitive nature, and rarely obtain parameters representative of actual in situ behaviour. The calcareous mudstone investigated in this paper has mechanical and structural characteristics falling between those of a soil and those typical of a rock and, as such, requires a geotechnical testing approach that combines methods from both soil and rock mechanics disciplines. The mineralogical, micro-structural and mechanical characteristics of this lithology have been examined via a suite of testing techniques, including XRPD, SEM, advanced triaxial with bender elements, along with industry standard procedures. Shearing, tensile and consolidation behaviours have been explored. Examination of the micro- and macro-scale features of this material shows it to be highly structured, with strength and stiffness being controlled by inter-granular bonding of Dolomite grains, as well as by mean effective stress state and rate of strain. The presence of fibrous Palygorskite acts to reduce the degree of bonding, causing specimens rich in this clay mineral to have a more ductile mechanical behaviour.

Alkali-activated alumino-silicate-based industrial waste products have recently proven to be beneficial as low-carbon alternatives to ordinary Portland cement binders for strengthening soft soils. This paper examines the small-strain stiffness behaviour of a UK silty alluvial soil in its natural state and artificially cemented using sodium hydroxide-activated ground-granulated blast-furnace slag (GGBS). Undrained triaxial testing with bender element measurements revealed that the initial small strain shear stiffness (Gmax), shear strength and hydraulic conductivity of the alluvium were all significantly enhanced after 28 d curing. Microstructural and mineralogical analyses were carried out on stabilised soil to understand the mechanisms better through which the enhanced engineering performances were achieved. Through hydration and pozzolanic reactions, a significant proportion of the clay minerals within the original soil had been converted into new cementitious hydrates. These were observed to infill pore spaces, coating soil and GGBS particle surfaces and increased interparticle bonding throughout the matrix of the material. The outcomes from this study have the potential to contribute towards improving current practices for modelling cemented soils and ultimately making geotechnical designs involving deep soil mixing less conservative.

Observations show that many soils in linear geotechnical infrastructure including embankments and cuttings undergo seasonal volume changes, and different studies confirm that this is due to cycles in climatic and hydrological conditions. These cycles can give rise to progressive failure of the soil mass, which in turn may lead to deterioration of performance and ultimately slope failure. It is expected that the magnitude of the seasonal cycles of pore pressure will be increased by more extreme and more frequent events of wet and dry periods predicted by future climate scenarios. In this paper, numerical modelling has been undertaken to simulate a continuous time series pore water pressure within a representative cutting in London Clay. The approach uses synthetic control and future climate scenarios from a weather generator to investigate the potential impacts of climate change on cutting stability. Surface pore water pressures are obtained by a hydrological model, which are then applied to a coupled fluid-mechanical model. These models are able to capture the significant soil–vegetation–atmospheric interaction processes allowing the induced unsaturated hydro-mechanical response to be investigated. The chosen hydraulic conductivity variables in the model are shown to affect the total magnitude of pore pressure fluctuation and hence the rate of progressive failure. The results demonstrate for the first time that higher total magnitude of annual variation in pore pressures caused by future climate scenarios can have a significant effect on deformations in cuttings. This in turn leads to increased rates of deterioration and reduces time to failure.

The self-boring pressuremeter (SBP) test was designed to measure in situ engineering properties of the ground with a relatively small amount of disturbance. The properties that may be inferred from the test depend on the mechanical model used for its interpretation and on the significance given to other previously available information. In this paper, numerical modelling using the advanced kinematic hardening structure model (KHSM) for natural soils has been performed to investigate the influence of the initial structure and the degradation of structure on the SBP cavity pore pressures and expansion curves within London Clay. The validation of the KHSM against well-known analytical solutions and the calibration procedure used to identify the material parameters are presented. The numerical analyses reveal that the simulations of the SBP tests using the KHSM model provide a very close match of the expansion curves to the experimental data, but underestimate the pore pressures at the initial stage of the SBP expansion test. A parametric study has been carried out to determine the effects induced by the parameters of the destructuration model along with the disturbance experienced during the SBP installation, which is difficult to estimate in situ. Two disturbance scenarios were considered where the initial structure was assumed to vary linearly across an area close to the wall of the cavity. These simulations indicate that accounting for installation disturbance leads to a substantial improvement in pore pressure predictions for the SBP. The first author would like to acknowledge funding provided by the Engineering and Physical Sciences Research Council (EPSRC) GR/S84897/01. The authors are grateful to Cambridge Insitu Ltd for making available the pressuremeter data on London Clay.

Nonlinear time domain numerical approaches together with elasto-plastic effective stress soil constitutive models are nowadays available to geotechnical researchers and practitioners interested in geotechnical earthquake engineering. The use of such advanced two- and three-dimensional schemes allows the analysis and design of complex geotechnical structures within a performance-based framework, considering the build-up of excess pore water pressure during the earthquake, dynamic interaction between the soil deposit and the above surface buildings and infrastructures and effects of multi-directional seismic loading. Within this context, the paper focuses on the dynamic finite element (FE) method and presents a review of the key ingredients governing its predictive capabilities. These include i) the description of the fully coupled solid–fluid interaction formulation and time integration, ii) the calibration of Rayleigh damping and the soil constitutive model parameters, iii) prescription of the boundary conditions for the generated mesh and iv) input motion selection/scaling strategies. For each of the above points, the paper summarises the current knowledge and best practice, with the aim of providing protocols for a confident application of nonlinear FE schemes to evaluate the performance of critical geotechnical infrastructures. Useful hints to promote familiarity of advanced nonlinear soil dynamic analysis among geotechnical practitioners and to indicate areas for further improvement are also highlighted.

Alkaline cements have been extensively tested for soil stabilisation in the last decade. However, only a few studies have focused on the assessment of such performance by establishing the constitutive behaviour of the cement. In this paper, we focus on the mechanical behaviour, using triaxial testing of a clay with high water content stabilised with an alkali-activated binder and the subsequent prediction of the experimental stress–strain response using a kinematic hardening constitutive model initially developed for natural clays. Monotonic consolidated undrained triaxial tests were conducted on reconstituted and stabilised clay specimens cured for 28 days to evaluate the effects of cementation on the overall shear behaviour. Alkali-activated binder was synthetised from blast furnace slag and sodium hydroxide. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were performed to study the microstructure, whereas leachate analyses were performed after 28 and 90 days of curing to investigate the contamination potential. The main product formed was calcium aluminosilicate hydrate (C-A-S-H) with a low CaO/SiO2 ratio, and no risk of soil contamination was found. The compressibility and undrained shear strength in the pre-yield state was found to be independent of the initial mean effective stress (p’0), unlike what was observed in the post-yield state, where the shear strength seemed to be affected by p’0. The model provided reliable predictions of the experimental results and captured the main features of the artificially cemented clay for the tested p’0 range. Such studies are fundamental to establish adequate confidence in such alternative binders—an essential aspect if their use is to become widespread in the near future., This work was funded by the Project “MINECO- New Eco-innovative Materials for Mining Infra” with reference ERA-MIN/0002/2018 and by a Ph.D. scholarship with reference SFRH/BD/132692/2017 financed by the Portuguese Foundation for Science and Technology (FCT) and the European Social Fund (FSE).

This paper investigates the nano to core-scale geomechanical properties of a maturity series of organic-rich, calcareous shales buried to 100-180°C, with a focus on: (a) the mechanical properties of organic matter; (b) the elastic response and anisotropy of the shale composite at micro and core scale; and (c) the creep response. Atomic force microscopy was used to target kerogen at nanoscale resolution, and it was found that the elastic stiffness increased with thermal maturity from 5.8 GPa in an immature sample to 11.3 GPa in a mature sample. Nanoindentation testing of the shale matrix showed that diagenesis is a key factor in determining the bulk elasticity, with increasingly intense carbonate cementation at higher thermal maturities contributing to a stiffer response. A multiscale model was formulated to upscale the elastic properties from nanoscale solid clay minerals to a microcracked composite at core scale, with good predictions of the micro and core-scale stiffness in comparison to indentation and triaxial results. A negative correlation was found between the creep modulus and clay/kerogen content, with greater creep displacement observed in nanoindentation tests in the immature clay- and kerogen-rich sample compared to samples of higher thermal maturity.

Soft rocks are of great geological and geotechnical interest, due to their mechanical and structural characteristics falling between those of a soil and those of a rock. This presents several challenges for constitutive and numerical modelling. Here we propose a calibration strategy of an advanced kinematic hardening model with structure degradation against experimental data on soft rocks, accounting for their features at the micro- and macro-scales. The experimental results show a highly structured material, with strength and stiffness being controlled by inter-granular bonding as well as by mean effective stress. The clay content of soft rock samples seems to be a dominant factor: the specimens show lower stiffness and strength when the content is higher. Accordingly, the samples have been classified into two different types (A and B). The constitutive model reproduces well both the mechanically strong and stiff behaviour of Type A as well as the more ductile mechanical response of Type B specimens observed during drained triaxial compression tests. With increasing confining pressures, the transition from brittle and dilatant to more ductile and contractant behaviour observed for both types of rocks is well captured by the adopted constitutive model.

The paper focuses on the numerical prediction of the stress-strain behaviour under triaxial compression of a sandy clay soil improved with an environmentally friendly binder at the short-term (28 days) and long-term (90 days) using a kinematic hardening constitutive model. The binder was synthesised by ground granulated blast furnace slag (GGBS), an industrial by-product (IBP) of the steel industry, and sodium hydroxide (NaOH). The model, which is being used to reproduce artificially cemented soil behaviour for the first time, was able to successfully capture the smooth elastic-plastic transition response in the unimproved soil, while a ductile response observed in the improved soil prior to a fragile post-peak and residual states was also well predicted by the model.

Quantification of risk to seal integrity in CCS, or gas extraction from hydraulic fracturing, is directly affected by the accessibility of organic pores within organic rich mudrocks. Knowledge of the host organic matter's mechanical properties, which are influenced by depositional environment and thermal maturity, are required to reduce operational risk. In this study we address the effect of both depositional environment and maturity on organic matter Young's modulus by means of Atomic Force Microscopy Quantitative ImagingTM, which is a nondestructive technique capable of nanomechanical measurements. Shales from varying marine depositional environments covering kerogen Types II (Barnett), IIS (Tarfaya), and II/III (Eagle Ford/ Bowland) are analyzed to capture variance in organic matter. The findings show organic matter has a Young's modulus ranging between 0.1 and 24 GPa. These marine shales have a bimodal distribution of Young's modulus to some degree, with peaks at between 3–10 and 19–24 GPa. These shales exhibit a trend with maturity, whereby Young's modulus values of

This paper details development of a numerical modelling approach that has been employed to forecast the long-term performance of a cut slope formed in high plasticity clay. It links hydrological and mechanical behaviour in a coupled saturated and unsaturated model. This is used to investigate the influence of combined dissipation of excavation-generated excess pore water pressures and seasonal weather-driven near-surface cyclic pore water pressures. Deterioration of slope performance is defined in terms of both slope deformations (i.e. service) and factor of safety against shear failure (i.e. safety). Uniquely, the modelling approach has been validated using 16 years of measured pore water pressure data from multiple locations in a London Clay cut slope. Slope deterioration was shown to be a function of both construction-induced pore water pressure dissipation and seasonal weather-driven pore water pressure cycles. These lead to both transient and permanent changes in factor of safety due to effective stress variation and mobilisation of post-peak strength reduction over time, respectively, ultimately causing shallow first-time progressive failure. It is demonstrated that this long-term (90 year) deterioration in slope performance is governed by the hydrological processes in the weathered near surface soil zone that forms following slope excavation.

This paper presents a model that describes the gradual yielding of unsaturated cemented soils subjected to isotropic loading. The model relies on the definition of a “cementation bonding function” which accounts for the progressive breakage of inter-granular cementation caused by loading. The combination of this cementing bonding function with the unsaturated model of Gallipoli and Bruno (2017) leads to the formulation of a “cemented unified normal compression line” (CUNCL), which describes the virgin behaviour of both cemented and uncemented soils under saturated and unsaturated conditions. Gradual yielding is described by assuming that, as the soil state moves towards the CUNCL, the slope of the loading curve tends towards the slope of the CUNCL. The model describes the hysteretic variation of void ratio for both cemented and uncemented soils under saturated and unsaturated conditions by using only seven parameters, i.e. five parameters for the uncemented behaviour plus two extra parameters accounting for the effect of cementation. The model has been calibrated and validated against the experimental data of Arroyo et al. (2013) demonstrating a good performance to describe the uncemented and cemented behaviour of soils under saturated and unsaturated conditions.

Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. In fact, structure is the dominant factor determining the differences in the mechanical response of different units. In the paper, numerical analyses simulating the undrained excavation of a tunnel in St James's Park are presented. London Clay behaviour is characterised by a kinematic-hardening structured soil model incorporating structure and stiffness degradation. The parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the London Clay formation and the different degrees of soil structure. The analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short-term surface and subsurface displacements, as well as pore pressures. The paper concludes with a discussion of the results in the context of other analyses performed previously, and puts forward some considerations concerning design issues.

Deep dry soil mixing is a popular ground improvement technique used to strengthen soft compressible soils, with Portland cement being the most popular binder. However, its continued use is becoming less sustainable given the high CO2 emissions associated with its manufacture. Alkali-activated cements are considered to be viable low carbon alternative binders, which use industrial waste products such as blast furnace slag. This study focusses on the stabilisation of a potentially liquefiable soft alluvial soil using a dry granulated binder comprising sodium hydroxide-activated blast furnace slag (GGBS-NaOH). This binder has previously been demonstrated by the authors to have potential as a replacement for Portland cement due to its excellent engineering performance, positive contributions towards the circular economy, reducing energy usage and CO2 emissions in the construction sector. A detailed comparison in mechanical behaviour is presented between the soil in its reconstituted, undisturbed and cemented states after 28 days curing through the use of advanced monotonic triaxial testing techniques, including small strain measurements. Mechanical behaviour was specifically analysed regarding peak deviatoric strength, pore pressure response, stress – volumetric dilatancy, shear stiffness degradation over small to large strain ranges, critical state and failure surfaces. Using 7.5% GGBS-NaOH increased the stiffness and shear strength of the soil significantly, whereby the shear strains at which initial shear stiffness degrades is three times higher than the untreated undisturbed soil. As a result, larger amounts of dilation was observed during shearing of the material and resulted in an upward shift of the soil’s original critical state line due to the creation of an artificially cemented soil matrix through the precipitation of C-(N)-A-S-H gels.

This paper presents a constitutive modelling approach for the viscoplastic-damage behaviour of geomaterials. This approach is based on the hyperelasticity framework, where the entire constitutive behaviour is derived from only two scalar potentials: a free-energy potential and a dissipation function. The novelty of the new proposed model, in addition to being thermodynamically consistent, is that it requires only a few parameters that can be derived from conventional laboratory testing. The model has been specifically tested for its ability to reproduce a series of triaxial compression tests on core rock samples. The comparison between the viscoplastic-damage model predictions and experimental results shows that the model is remarkably successful in capturing the stress–strain response both at peak stress and in the region of material softening and the time to reach failure.

The paper presents novel results from the advanced numerical modelling of the effect of cycling pore water pressures on landslide processes. It combines the stability analysis of a prototype natural slope through a hydro-mechanical nonlinear finite element approach with the calibration of a kinematic hardening model against representative laboratory data, to draw conclusions of significance to both researchers and designers. The analyses have been carried out for two permanent hydraulic steady-state conditions representing the average pore water pressure regime at the end of the winter and summer season, thus replicating in a simplified way the seasonal fluctuations of the piezometric levels resulting from transient seepage processes generated by the slope-atmosphere interaction. The work shows the ability of the constitutive law, seldom used in these kind of analyses, to predict a progressive accumulation of plastic deformations during the cyclic fluctuation of pore water pressures associated to climate. More importantly, the output of the advanced modelling are useful to support the phenomenological interpretation of the landslide processes associated to natural hazards and to provide guidance for the sustainable management of marginally stable slopes affected by a constant evolution of permanent displacements.

The effect of floating objects has so far been little considered for hazard risk assessment and structure design, despite being an important factor causing structural damage in flood-prone and coastal areas. In this work, a novel two-way method is proposed to fully couple a shock-capturing hydrodynamic model with a discrete element model (DEM) for simulation of complex debris-enriched flow hydrodynamics. After being validated against an idealized analytical test, the new coupled model is used to reproduce flume experiments of floating debris driven by dam-break waves. The numerical results agree satisfactorily with the experimental measurements, demonstrating the model’s capability and efficiency in simulating complex fluid-debris interactions induced by violent shallow flows.

There is a growing interest in the geomechanical behaviour of low performing soils strengthened with alkali-activated materials, which have been promoted as low-carbon-footprint binders. This paper focuses on the performance of a sandy clay stabilised with NaOH-activated blast furnace slag after short (28 days) and long (90 days) curing periods. Triaxial compression experiments were conducted at a range of mean effective stresses (41 to 600 kPa) and overconsolidation ratios (1 to 12.2). The experimental data was used to calibrate a kinematic hardening constitutive model and the ability of the model to capture the behaviour of artificially stabilised sandy clay was investigated. Triaxial results carried out on the stabilised soil at both curing periods showed a behaviour resembling that observed for cement-mixed clays. The model, which had never been tested in artificially stabilised soils, successfully predicted the smooth elastoplastic transition observed on the non-stabilised soil specimens and the peak/residual shear strains and strain-softening behaviour after peak strengths in its stabilised state after 28 and 90 curing days.

Down-hole array observations are extremely useful to investigate site amplification effects and to validate numerical modelling techniques for site response. In this paper the ground response of the Lotung experiment site (Taiwan), measured along a down-hole accelerometer array during a weak and a strong motion event, is simulated using different numerical techniques of increasing level of complexity: 1) a simple equivalent-linear visco-elastic procedure, 2) a total stress time-domain scheme using a pressure-dependent hyperbolic model and 3) a fully-coupled approach implementing an advanced elasto-plastic soil model. The numerical models are calibrated against resonant column data and in-situ cross-hole measurements. The two horizontal components of the input motion are applied separately at bedrock level. The results of the simple and advanced numerical simulations are compared with the down-hole motions recorded in-situ during the investigated seismic events in terms of acceleration time histories and response spectra. The comparison between in-situ measurements and predicted results highlights the well-known limitations of the frequency-domain technique. It also shows some improved predictive capabilities of the total stress time-domain scheme and demonstrates the excellent performance of the fully-coupled advanced non-linear approach.

We present coupled, critical state, geomechanical-fluid flow simulations of the evolution of a fold-and-thrust belt in NW Borneo. Our modelling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold-and-thrust belt. The present-day structure predicted by the model contains the key first order structural features observed in the field in terms of thrust fault and anticline architectures. Stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb. Between the compressive zones, near to the axial plane of the anticline, the modelled stress field represents an extensional regime. The predicted overpressure distribution is strongly influenced by tectonic compaction, with the maximum values located in the two laterally compressive regions. We compared the results at three notional well locations with their corresponding uniaxial strain models: the 2D thrust model predicted porosities which are as much as 7.5% lower at 2.5 km depth and overpressures which are up to 6.4 MPa higher at 3 km depth. These results show that one-dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings. Finally, we performed a drained simulation during which pore pressures were kept hydrostatic. The predicted geological structures differ substantially from those of the coupled simulation, with no thrust faulting. These results demonstrate that pore pressure is a key control on structural development.

There has been a huge surge in the construction of marine facilities (e.g., wind turbines) in Europe, despite the many unknowns regarding their long-term performance. This paper presents a new framework for design strategy based on performance measures for cyclic horizontally loaded monopile foundations located in saturated and dry dense sand, by considering pile deformations and pore pressure accumulation effects. A three-dimensional finite element model was developed to investigate the behavior of large-diameter piles. The model accounts for nonlinear dynamic interactions in offshore platforms under harsh combined moment and horizontal environmental loads, with emphasis on the cyclic mobility of the surrounding cohesionless subsoil and associated shear. The maximum moment applied in the cyclic analyses is varied from 18% to 47% of the ultimate resistance. The considered data reflect behavior at the expected load amplitudes and cycle numbers during the service life of operation. For low numbers of load cycles (

Skirted foundations are used in offshore applications to resist the large horizontal and moment loads that are characteristic of the ocean environment. The combination of vertical–horizontal-moment (VHM) loading results in complicated stress conditions in the seabed and design is often based on VHM failure envelopes. These have generally been constructed by numerical analysis using a deterministic characterisation of soil properties and disregard the natural spatial variability of marine sediments. In this study, spatial variability is taken into account by coupling a random field model with finite element analysis. The paper presents a probabilistic analysis of the ultimate capacity of skirted foundations in spatially variable undrained clay. The increase of strength with depth typical of a marine clay is included in the modelling framework. Probabilistic failure envelopes are constructed to analyse the effect of spatial variability when skirted foundations are subjected to different combinations of VHM loading. The results show that the probability of failure increases under high vertical loads and at peak moment capacity in the HM plane, suggesting that care should be taken in design at these areas of the failure envelope. The methodology demonstrates a straightforward and effective way of quantifying uncertainty in the ultimate limit state design of offshore geotechnical structures and the results presented provide specific guidance for the design of skirted foundations.

Determination of the mechanical response of shales through experimental procedures is a practical challenge due to their heterogeneity and the practical difficulties of retrieving good-quality core samples. Here, we investigate the possibility of using multiscale homogenisation techniques to predict the macroscopic mechanical response of shales based on quantitative mineralogical descriptions. We use the novel PeakForce Quantitative Nanomechanical Mapping technique to generate high-resolution mechanical images of shales, allowing the response of porous clay, organic matter, and mineral inclusions to be measured at the nanoscale. These observations support some of the assumptions previously made in the use of homogenisation methods to estimate the elastic properties of shale and also earlier estimates of the mechanical properties of organic matter. We evaluate the applicability of homogenisation techniques against measured elastic responses of organic-rich shales, partly from published data and also from new indentation tests carried out in this work. Comparison of experimental values of the elastic constants of shale samples with those predicted by homogenisation methods showed that almost all predictions were within the standard deviation of experimental data. This suggests that the homogenisation approach is a useful way of estimating the elastic and mechanical properties of shales in situations where conventional rock mechanics test data cannot be measured.

The Material Point Method (MPM), which can be thought of as a mesh-free technique, has been shown to be very efficient in avoiding the mesh distortion problem in large deformation analyses. However, for the widely used explicit dynamic MPM formulation, the time step must inevitably be very small in order to guarantee convergence, especially in the case of quasi-static problems. In this paper, an incremental updated Lagrangian quasi-static MPM formulation is developed, which requires less computation effort by using much larger time steps. Issues pertaining to the implementation of the present MPM formulation are discussed. Strain softening, which may potentially lead to localisation phenomena, is also considered in the constitutive model. Scale effects and mesh size dependency in the solution are accounted for by applying a spatial averaging approach to the strains using a weighting function defined by an internal length scale characterising the non-local deformation. A progressive failure of a slope is simulated in order to demonstrate the efficiency and good performance of the proposed formulation.

Summary The effective exploitation of unconventional shale hydrocarbon resources requires the robust geomechanical characterisation of the reservoir. However, there is still a paucity of mechanical data on well-characterised shale, partly due to the technical and economic difficulties in obtaining high quality core for laboratory testing. Here, we present the results of a study which combines PeakForce Quantitative Nanomechanical Mapping and nanoidentation testing on three shales from the Posidonia of NW Germany, covering a range of thermal maturities. PF-QNM is used to determine the mechanical properties of individual organic, clay mineral and other phases, including their anisotropy. Estimates of bulk elastic and viscoelastic parameters are determined through use of high load instrumented indentation testing. These data form the building blocks for the estimation of the geomechanical properties of shales at larger scales, as a function of mineralogy and organic matter type and content.

The paper presents novel results from advanced numerical simulations of the transverse behaviour of shallow circular tunnels in natural clays accounting for soil structure degradation induced by earthquake loading. It combines the calibration of a kinematic hardening model against real laboratory data with outputs from a parametric study with different degradation rates of soil structure to demonstrate the good performance of the model and draw conclusions of significance to both researchers and designers. The sensitivity analysis indicates an increase in the maximum and minimum values of the lining forces attained during the earthquake motions and in the lining hoop force and bending moment increments in response to the seismic events when higher rates of destructuration are accounted for. Hence, the paper highlights for the first time the importance of the initial structure and its degradation in controlling the magnitude of the tunnel lining forces and, consequently, the overall seismic tunnel design.

The performance of monopiles in cohesive soils is of great interest for future offshore wind farm developments, particularly under the cyclic loads experienced in the ocean environment. Clay behaviour during undrained cyclic loading is complex and involves the accumulation of plastic strains, generation of excess pore-water pressures and degradation of initial stiffness. In this paper, the cyclic performance of a laterally-loaded monopile in spatially variable clay is investigated for the first time. A kinematic hardening constitutive model is used in a 3D finite element analysis to capture the hysteretic stress-strain behaviour of the clay. The monopile is installed in overconsolidated London Clay, which is present at several offshore wind farms in the Thames Estuary. The finite element model is coupled with random field representations of initial stiffness and clay structure. The statistical characterisation of the random fields was undertaken considering parameter ranges observed in laboratory tests. Under one-way cyclic loading, the monopile showed ratcheting behaviour, where pile rotation accumulates with increasing numbers of load cycles. The cyclic secant stiffness also increased due to the generation of negative excess pore-pressures in the clay. This behaviour occurred in both homogeneous and spatially variable clay. The monopile was also subjected to an extreme dynamic event and the soil response around the monopile showed increasing variability in stress-strain response and generation of excess pore-water pressure over time as plastic strain accumulated. However, the overall behaviour of the foundation was governed by a spatial average of the mobilised clay. The range in monopile response demonstrates how the natural spatial variability of clay can have a strong influence on monopile performance.

Suction caissons are increasingly used in offshore energy production to moor floating facilities in deep water. The holding capacity of a suction caisson is dependent on the angle of the mooring line and is often described in terms of a vertical-horizontal (VH) load interaction diagram, or failure envelope. These envelopes have commonly been defined by numerical methods using deterministic soil parameters, ignoring the natural spatial variability of seabed sediments. In this paper, spatial variability is modelled using a random field and coupled with finite element analysis to obtain a probabilistic characterisation of holding capacity. The increase of strength with depth that is characteristic of a marine clay is taken into account. A non-parametric approach using kernel density estimation is presented for constructing probabilistic VH failure envelopes that allow an appropriate envelope, associated with an acceptable level of risk, to be selected for design. A study of the autocorrelation distance, a quantity often difficult to obtain accurately in practice, has shown that the vertical autocorrelation distance has a much greater influence on the variability of holding capacity than the horizontal and should be carefully chosen in offshore applications.

As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.

Soft alluvial soils present unfavourable conditions for engineering developments due to their poor bearing capacities and high potential for experiencing shrinkage and swelling. This paper focusses on deep dry soil mixing (DDSM), which introduces cementitious binders to soft soils via a rotating auger drill, thereby producing soil-cement columns. Ordinary Portland cement (CEM-I) is globally used across the construction industry and is the most commonly used binder for DDSM applications due to its high strength performance. However, CEM-I production is one of the world׳s most energy intensive and expensive industrial processes, contributing 5–7% of the world׳s total CO2. There is now significant pressure on the cement and construction industries to greatly reduce their CO2 emissions by developing “greener” alternatives to CEM-I, which are both more environmentally and financially sustainable in the long-term. Alkali activated industrial by-products (IBP׳s) such as ground granulated blast furnace slag (GGBS), known as geopolymers have been identified as potential alternatives. These are advantageous due to negating the need to transfer IBP׳s to landfill, their abundance, negligible or zero production costs. Geopolymers are capable of reducing greenhouse gas emissions by up to 64%. Calcium-bearing slags have also been found to possess potential for carbon capture and storage (CCS). Comparisons with the strength and durability of untreated and stabilised soils have been made in this study. Results indicate that stabilising an alluvial soil with sodium hydroxide (NaOH) activated GGBS produced significant strength and durability improvements surpassing CEM-I. The addition of NaOH allowed pozzolanic reactions to occur, leading to improved mechanical properties with time, with a particularly marked improvement in strength.

The stability of geotechnical structures under repeated loading depends to a large extent on the induced cyclic shearing stresses. The design of these structures usually requires engineers to employ advanced soil models in their analyses. While a number of such models do exist, their validation against cyclic laboratory tests is still very limited. In particular, the influence of the initial structure of the clay and its subsequent degradation under cyclic loading appears to be insufficiently investigated from both experimental and constitutive modelling standpoint. The work outlined in this paper adds a new contribution to the theoretical understanding of cyclic response of clayey materials, presenting the extensive validation of an advanced kinematic hardening model against laboratory data on a number of natural and compacted clays found in literature. In order to analyse in detail the evolution of shear and hysteretic soil behaviour over a wide strain range, further modelling accounting for the effects of overconsolidation ratio and structure degradation is undertaken. The modelling results of shear stiffness degradation, hysteretic dissipation and pore pressure accumulation are presented and compared with experimental data. The results show that the enhanced kinematic hardening model gives very satisfactory predictions of clay response during cyclic loading.

The thermal properties of soils are of great importance in many thermo-active ground structures such as energy piles and borehole heat exchangers. In this paper the effect of the porosity and degree of saturation on the thermal conductivity of a sandy soil that has not been previously thermally tested is investigated using steady state experimental tests. The steady state apparatus used in these tests was designed to provide high performance in controlling all boundary conditions. Twenty thermal conductivity experimental tests have been carried out at different porosity and saturation values. The performance of selected prediction methods have been validated against the experimental results. The validation shows that none of the selected models can be used effectively in predicting the thermal conductivity of Tripoli sand at all porosity and saturation values. However, some can provide good agreement at dry or nearly dry condition while others perform well at high saturations. The performance of most of the selected models also increases as the soil approaches a two phase state where conduction plays the dominant role in controlling heat transfer. An empirical equation of thermal conductivity expressed as a function of water content and porosity has been developed based on the experimental results obtained.

Homogenisation techniques have been successfully used to estimate the mechanical response of synthetic composite materials, due to their ability to relate the macroscopic mechanical response to the material microstructure. The adoption of these mean-field techniques in geo-composites such as shales is attractive, partly because of the practical difficulties associated with the experimental characterisation of these highly heterogeneous materials. In this paper, numerical modelling has been undertaken to investigate the applicability of homogenisation methods in predicting the macroscopic, elastic response of clayey rocks. The rocks are considered as two-level composites consisting of a porous clay matrix at the first level and a matrix-inclusion morphology at the second level. The simulated microstructures ranged from a simple system of one inclusion/void embedded in a matrix to complex, random microstructures. The effectiveness and limitations of the different homogenisation schemes were demonstrated through a comparative evaluation of the macroscopic elastic response, illustrating the appropriate schemes for upscaling the microstructure of shales. Based on the numerical simulations and existing experimental observations, a randomly distributed pore system for the micro-structure of porous clay matrix has been proposed which can be used for the subsequent development and validation of shale constitutive models. Finally, the homogenisation techniques were used to predict the experimental measurements of elastic response of shale core samples. The developed methodology is proved to be a valuable tool for verifying the accuracy and performance of the homogenisation techniques.

The use of industrial by-products in the context of soil stabilisation has recently been explored as a mean to substitute the massive use of ordinary Portland cement (OPC), whose production has severe environmental impacts. Therefore, the purpose of the present investigation was to assess the geomechanical behaviour of a soft soil stabilised with a sustainable alkaline binder incorporating ground granulated blastfurnaces slags (GGBS), a residue from the iron industry, activated with NaOH at short (28 days) – and long (90 days) term. The mechanical characterisation included the performance of oedometer and triaxial testing, where the impact of both, initial mean effective stress (p’0) and overconsolidation ratio (OCR) effects, was analysed on the stress-strain and volumetric strain curves, stiffness degradation and, triaxial failure surfaces of the specimens. Also, SEM-EDS, XRD and leachate analyses were also carried out at both curing ages. The mechanical results showed a significant increase of the shear strength and stiffness, and a stress-strain behaviour typical of artificially cemented soils with OPC. The detection of Si, Ca and Al suggests the formation of C-A-S-H, which explain the more pronounced development of strength and stiffness up to 28 days. No risk of soil contamination was detected due to the addition of activated GGBS to the soil.

Understanding the mechanical response of coal to CO2 injection is necessary to determine the suitability of a seam for carbon capture and underground storage (CCUS). The bulk elastic properties of a coal or shale, which determine its mechanical response, are controlled by the elastic properties of its individual components, i.e. macerals and minerals. The elastic properties of minerals are relatively well understood, and attempts have been made previously to acquire maceral elastic properties (Young's modulus) by means of nanoindentation. However, due to the resolution of a nanoindent and small size of macerals, the response is likely to be from a combination of macerals composition and minerals. Here atomic force microscopy is used for the first time to give a unique understanding of the local Youngs modulus of individual macerals, with a precision of 10 nm in both immature and mature coals/shale. Alginite, cutinite, inertinite and sporinite macerals are analysed from a samples of cannel coal (rich in cutinite), paper coal (enriched in sporinite), Northumberland coal (higher rank coal, rich in vitrinite and inertinite) and alginite rich New Albany Shale. Initial findings on the New Albany Shale indicate that kerogen isolation is not a suitable preparation technique for atomic force microscopy and as such, no alginite maceral moduli are accurately reported. Therefore results of the coal derived macerals (cutinite, inertinite and sporinite) are included in this study. The results at this length scale indicate that the mean and modal Young's modulus values in all coal macerals is less than 10 GPa. This range is similar to Young's modulus values acquired by nanoindentation within previous studies. A major difference is that the modal modulus values obtained here are significantly lower than the modal values obtained within previous studies. Thermally immature liptinite macerals (cutinite/sporinite) have a lower modal modulus (1.35–2.97GPa) than the inertinites (1.44–3.42 GPa) from the same coal. The modulus response is also non-normally distributed and most likely conforms to a gamma distribution with shape parameter between 1.5 and 2.5. The modal Young's modulus of all macerals increases with maturity, but not at the same rate, whereby the liptinite macerals become stiffer than the inertinites by the dry gas window (1.56 % Ro in Northumberland Coal). Modelling of volumetric strain under CO2 injection indicates an inversely proportionate relationship to Young's modulus, which suggests that differential swelling is more likely to occur in immature coals. It is therefore preferable to target mature coals for CCUS, as the reaction of macerals at higher maturities is more predictable across an entire coal seam.

The shear wave velocity profile and dynamic soil properties are known to be affected by aleatory uncertainty. This paper aims to investigate the effect of a statistical variation in the initial stiffness profile, stiffness degradation and damping curves on ground response predictions by conducting stochastic analysis. The Large Scale Seismic Test site in Lotung, Taiwan, is back-analysed with a fully-coupled finite element procedure using an advanced kinematic hardening soil model. Two ground motions recorded at the site, one strong and one weak, are applied at bedrock level. The results reveal that the site response prediction is sensitive to the seismic intensity of the input motion. When the level of induced shear strain is higher, i.e. in the case of the stronger motion, the spatial variability of the stiffness degradation and damping curves has a pronounced effect on the predicted site response. In contrast, when the weaker motion is considered the prediction is particularly sensitive to the statistical variation in the initial stiffness profile. This is mainly due to the stiffness degradation at very small strains shown by the laboratory data on LSST soils, which is captured in the paper by assuming an appropriate elastic domain in the constitutive model calibration.

Robust geomechanical characterisation of shale reservoirs is necessary for safe and economic resource exploitation but there is still a lack of mechanical data on well-characterised shale, partly due to the difficulties of obtaining high quality core samples for laboratory testing. The composition of shale also presents challenges when attempting to constrain the mechanical response. Multi-scale homogenisation techniques have recently been used to predict the macroscopic behaviour of shales based on quantitative mineralogical descriptions. However, there is a considerable amount of uncertainty associated with some key inputs into these homogenisation schemes. In particular, the organic matter of shale encompasses a range of scales, from nanometre to micrometre-size material, and its mechanical properties are not well understood. Here, PeakForce Quantitative Nanomechanical Mapping (PF-QNM), a recently developed form of atomic force microscopy (AFM), is combined with nanoindentation testing to characterise the mechanical response of the organic matter and clay phases of Posidonia shale from north-west Germany. The nanoscale testing revealed a clear peak in the histograms of the reduced elastic modulus, which can be attributed to kerogen in the shale matrix. Upscaling of the mechanical properties through homogenisation showed a reasonable prediction when compared with experimental data, including capturing the inherent anisotropy of the shale response. The influence of factors such as the volume fraction of silt inclusions and the applicability of different homogenisation formulations warrant further investigation.