The vertical capacity of grillage foundations

Grillage foundations may provide an economical alternative to offshore ‘mudmat' foundations for seabed infrastructure, owing to their improved hydrodynamic characteristics, which are important during installation. Grillage foundations consist of a mesh of vertical grilles that penetrate the seabed during loading. Offshore loadings on these types of foundation are likely to consist of vertical (mostly dead weight) loading and horizontal ‘in-service' loads. However, to date there is no accepted method of design, as foundation capacity may differ significantly from that of conventional solid shallow foundations. This paper presents an analytical method designed to calculate the variation of vertical bearing capacity with grille penetration in sand. The results show that grillages are able to achieve the same capacity as solid foundations of the same breadth, but this requires significant penetration of the grillage. Consequently, design choices are likely to depend on the amount of settlement the structure can tolerate. Simplified analytical equations have been presented to allow calculation of the load–settlement response, and to calculate how much settlement is required to mobilise the flat-plate capacity of a solid mudmat of the same overall breadth. The methodology has been validated by comparing results with those from model tests. </jats:p

Screw pile design optimisation under tension in sand

Many applications in offshore engineering, such as floating or jacket-founded wind turbines or wave energy converters, require a significant uplift capacity of their foundations to be kept in place. Straight-shafted or suction piles in sands have a limited uplift capacity as they resist by friction only. In contrast, screw piles or screw anchors are a promising solution which provides a similar capacity to plate anchors and does not generate disturbance for marine mammals (e.g. from pile driving operations). The optimisation of the screw pile design does not rely only on the geotechnical assessment of the uplift capacity based on soil strength, but also on operational (installation requirements) and structural (helix bending, core section stress, limiting steel plate thick-ness) constraints. This paper develops a methodology for the design optimisation of screw piles under pure ten-sion in sand, incorporating all of these constraints, based on simplified analytical or semi-analytical approaches. The results show that the uplift capacity provided by an optimised screw pile is able to meet the needs of the offshore industry, across a range of soil densities and different applications (jacket foundation pile or tension leg platform anchor), providing that adequate installation plant could be dev

Centrifuge modelling of the influence of slope height on the seismic performance of rooted slopes

This paper presents an investigation into the influence of slope height on the role of vegetation to improve seismic slope stability. Dynamic centrifuge modelling was used to test six slope models with identical soil properties and model slope geometry within different centrifugal acceleration fields (10g and 30g, respectively) representing 1:10 and 1:30 scale slopes, that is, slopes of different height at prototype scale. A three-dimensional (3D) root cluster analogue representing a tap-root system, with root area ratio, root distribution and root length representative of a 1:10 and 1:30 scale tree root cluster (of rooting depth 1·5 m at prototype scale) was modelled using 3D printing techniques. A sequence of earthquake ground motions was applied to each model. The influences of filtering out low-frequency components of the earthquake motion, such as was necessitated at the lowest scaling factor owing to the practical limitations of the earthquake simulator, on dynamic amplification of motions within the slopes and the seismically induced slip, were first revealed. Subsequently, the effects of slope height on acceleration and deformation response of vegetated slopes were illustrated. It was found that the beneficial effects of roots on improving the seismic performance varied with the height of the slope. As an individual engineering technique for slope stabilisation, root reinforcement will not be such an effective solution for taller slopes, and complementary hard engineering methods (e.g. piles, retaining walls) will be necessary. For slopes of smaller heights (e.g. low-height embankments along transport infrastructure), however, vegetation appears to represent a highly effective method of reducing seismic slip. </jats:p

Sources and Transport Pathways of Fecal Bacteria and Pathogens to Aquifers in Rural Bangladesh

During the 1980’s millions of households in Bangladesh switched from drinking surface water to private groundwater wells to reduce their exposure to fecal microorganisms. Sadly, this switch to shallow groundwater resulted in the largest example of drinking water poisoning in history, with approximately 100 million people exposed to high concentrations of naturally occurring Arsenic in the groundwater. Spatial distribution of Arsenic in the shallow aquifers tends to be patchy, so the most economical mitigation option has been lateral switching from high Arsenic wells to nearby low Arsenic wells. The recently developed Arsenic flushing conceptual model, which explains the spatial distribution of Arsenic throughout the shallow aquifers in Bangladesh, suggests however, that low Arsenic zones are recharged via coarse-grained, rapid flow pathways and therefore represent a higher risk for waterborne pathogens. The objectives of this dissertation are to evaluate new methods for sampling and detection of waterborne pathogens, while also identifying sources of fecal contamination and transport pathway(s) to private wells emplaced within the shallow aquifers. It was demonstrated that private wells are broadly contaminated with E. coli, with prevalence ranging from 30 to 70%. The fact that E. coli was detected more frequently in private wells than sealed monitoring wells (p\u3c0.05) suggests that well construction and/or daily pumping contribute to fecal contamination of the private wells. Using DNA-based molecular fecal source tracking, contamination was demonstrated to originate from human fecal waste. Unsanitary latrines, which spill effluent onto the open ground, were demonstrated to cause elevated levels of fecal bacteria in ponds, found in every village. These ponds were demonstrated to have an influence on concentrations of fecal bacteria to at least distances of 12m into the adjacent aquifer. In a culture where latrines, private wells and ponds are frequently clustered closely together, these findings suggest that improvements in the management of human fecal waste changes in placement and construction of private wells could substantially reduce exposure of people to fecal pathogens. Fecal contamination was found to be pervasive in low Arsenic, unconfined, shallow aquifers, and therefore gains from well switching to avoid Arsenic need to be balanced with the risk of consuming waterborne pathogens

Evaluation of continuum modelling approaches for reinforced concrete in geotechnical applications

Modelling the structural response of reinforced concrete (RC) elements in geotechnical applications has been implemented using various numerical approaches with different levels of confidence; ranging from simple linear elastic approximations to non-linear section behaviour using embedded beams with moment-curvature (M-κ) relationships within dummy elements. However, the non-linear structural response of continuum RC approaches has not been widely employed in the geotechnical analysis of soil-structure interaction problems. This paper evaluates and compares different combinations of modelling approaches for the concrete and reinforcement, as implemented within the FE code PLAXIS 2D, to simulate the structural response of RC beams using the continuum approach for the concrete with discretely modelled reinforcement. The Concrete Model ‘CM’ and an equivalent Mohr-Coulomb ‘MC’ approach are compared for the concrete alongside the use of either embedded plates (with interfaces) or embedded beam rows to efficiently simulate the reinforcement. These approaches are validated against well-documented experimental data of singly and doubly reinforced concrete beams obtained from the literature. The results can be utilised to improve structural precision in Finite Element models in various soil-structure interaction problems (e.g., piles, shallow foundations, retaining walls, tunnel linings) within an integrated geotechnical environment