Numerical analysis of suction embedded plate anchors in structured clay

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.Peer ReviewedPostprint (author's final draft

Numerical evaluation of mean-field homogenisation methods for predicting shale elastic response

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 geocomposites 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

A new low carbon cementitious binder for stabilising weak ground conditions through deep soil mixing

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

A simple model for tertiary creep in geomaterials

This paper presents a constitutive modelling approach to 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, it requires only a few parameters which can be derived from conventional laboratory testing. The model has been specically 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 show the model is remarkably successful in capturing the stress-strain response at both peak stress and in the region of material softening and the time to reach failure

Thermal conductivity of a sandy soil

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.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.applthermaleng.2016.06.01

A new low carbon cementitious binder for stabilising weak ground conditions through deep soil mixing

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

Numerical analysis of suction embedded plate anchors in structured clay

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

Analysis of tunnel excavation in London Clay incorporating soil structure

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

Stress and pore pressure histories in complex tectonic settings predicted with coupled geomechanical-fluid flow models

Most of the methods currently used for pore pressure prediction in sedimentary basins assume one dimensional compaction based on relationships between vertical effective stress and porosity. These methods may be inaccurate in complex tectonic regimes where stress tensors are variable. Modelling approaches for compaction adopted within the geotechnical field account for both the full three dimensional stress tensor and the stress history. In this paper a coupled geomechanical-fluid flow model is used, along with an advanced version of the Cam-Clay constitutive model, to investigate stress,pore pressure and porosity in a Gulf of Mexico style mini-basin bounded by salt subjected to lateral deformation. The modelled structure consists of two depocentres separated by a salt diapir. 20% of horizontal shortening synchronous to basin sedimentation is imposed. An additional model accounting solely for the overpressure generated due to 1D disequilibrium compaction is also defined. The predicted deformation regime in the two depocentres of the mini-basin is one of tectonic lateral compression, in which the horizontal effective stress is higher than the vertical effective stress. In contrast, sediments above the central salt diapir show lateral extension and tectonic vertical compaction due to the rise of the diapir. Compared to the 1D model, the horizontal shortening in the mini-basin increases the predicted present-day overpressure by 50%, from 20 MPa to 30 MPa. The porosities predicted by the mini-basin models are used to perform 1D, porosity-based pore pressure predictions. The 1D method underestimated overpressure by up to 6 MPa at 3400 m depth (26% of the total overpressure) in the well located at the basin depocentre and up to 3 MPa at 1900 m depth (34% of the total overpressure) in the well located above the salt diapir. The results show how 2D/3D methods are required to accurately predict overpressure in regions in which tectonic stresses are important

Small to large strain mechanical behaviour of an alluvium stabilised with low carbon secondary minerals

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