Natural polymers (biopolymers) are discussed as environmentally friendly and sustainable grouting chemicals. This paper presents guidelines for selecting potentially useful biopolymers for strengthening cohesionless soil. Agar and six modified starches were identified for further study over a range of concentrations (1–4% agar and 0.5–1% starch). Experimental results demonstrated the compatibility of agar and starch. Depending on the biopolymer concentration, the unconfined compressive strength of the sand treated with agar and starch biopolymers ranged from 158 to 487 kPa. Triaxial compression tests over a range of confining pressures also indicated that the biopolymers effectively increased the cohesion intercept and stiffness of the treated sand.

Microbial-induced carbonate precipitation using urea hydrolysis is a relatively new improvement technique for granular soils. An important factor in achieving uniform calcite deposition (and hence consistent improvements in geomechanical properties) throughout the treated soil mass is the protocol adopted for injecting the reagents of ureolytic bacteria, urea and calcium. This paper reports a laboratory study investigating a technique to treat two loose medium quartz sands using different injection strategies. Staged injection including retention periods, and with a pressure head applied during injection of the bacterial cell solution, proved most effective. Sand specimens were treated using different concentrations of bacterial cell and urea–calcium chloride solutions and for a single injection cycle. Measured strength and stiffness values from unconfined compression tests ranged from 50 to 240 kPa and from 6 to 56 MPa, respectively. Permeability coefficient values were reduced by up to approximately one order of magnitude. Hence, a single injection cycle adhering to the proposed treatment method did not significantly affect the drainage capacity of the sand media. Greater improvements in stiffness and strength were achieved for lower bacterial cell and higher cementation solution concentrations, with a higher molarity of urea (non-equimolar solutions) proving even more effective. These findings were confirmed by scanning electron microscope observations.

Microplastics (MPs) are emerging persistent contaminants in the terrestrial subsurface, and evidence has emerged for significant effects of MPs on the biological and ecosystem functions of soils. Main MP sources include land spreading of sewage sludge and biowaste composts, plastic mulching film used in horticultural fields, waste water irrigation and leachate from the landfills, among others. This updated state-of-the-art review paper describes recent experimental and numerical research and developments in understanding the accumulation and fate and effects of MPs in the soil environment (focusing on their storage, degradation, transportation, leaching to groundwater etc.), followed by mitigation and bioremediation measures, including MP-eating soil bacteria and fungi and the best management practices for reducing MP pollution of soil. Other areas covered are the combined effects of MPs and various other environmental contaminants (heavy metals, organic pollutants and antibiotics) in soil ecosystems and the standardisation of methods for detection, quantification and characterisation of MPs in soils, which is critical for MP research. The paper concludes by identifying knowledge gaps and presents recommendations on prioritised research needs.

This paper reviews the percussion-cup liquid limit, thread-rolling plastic limit (PL) and various fall-cone and other approaches employed for consistency limit determinations on fine-grained soil, highlighting their use and misuse for soil classification purposes and in existing correlations. As the PL does not correspond to a unique value of remoulded undrained shear strength, there is no scientific reason why PL measurements obtained using the thread-rolling and shear-strength-based fall-cone or extrusion methods should coincide. Various correlations are established relating liquid limit values deduced using the percussion-cup and fall-cone approaches. The significance of differences in the strain-rate dependency on the mobilised fall-cone shear strength is reviewed. The paper concludes with recommendations on the standardisation of international codes and the wider use of the fall-cone approach for soft to medium-stiff clays in establishing the strength variability with changing water content and further index parameters.

Offshore wind turbines (OWTs) are generally supported by large-diameter monopiles, with the combination of axial forces, lateral forces, bending moments, and torsional moments generated by the OWT structure and various environmental factors resisted by earth pressures mobilized in the soil foundation. The lateral loading on the monopile foundation is essentially cyclic in nature and typically of low amplitude. This state-of-the-art review paper presents details on the geometric design, nominal size, and structural and environmental loading for existing and planned OWT structures supported by monopile foundations. Pertinent ocean-environment loading conditions, including methods of calculation using site-specific data, are described along with wave particle kinematics, focusing on correlations between the loading frequency and natural vibration frequency of the OWT structure. Existing methods for modeling soil under cyclic loading are reviewed, focusing in particular on strain accumulation models that consider pile–soil interaction under cyclic lateral loading. Inherent limitations/shortcomings of these models for the analysis and design of existing and planned OWT monopile foundations are discussed. A design example of an OWT support structure having a monopile foundation system is presented. Target areas for further research by the wind-energy sector, which would facilitate the development of improved analyses/design methods for offshore monopiles, are identified.