Acta Geotechnica最新文献

In our recent paper, we reported the development and application of the aerosol injection technique to accelerate the consolidation in soft soils. Aerosol injection technique is a process that involves injecting air under high pressure into soft soil to create a network of fractures, which gives rise to enhanced permeability. In this paper, we present an experimental study on the permeability of soft soil specimens with pre-made fractures in a triaxial device. Some emphasis is given to the variation of permeability with time. Some major influence factors are considered, e.g. the fracture roughness, confining pressure, pre-consolidation pressure, soil structure and mineral composition. The decrease in permeability with time is mainly due to the self-healing mechanism in soft soils, which is dictated by the soil deformation and migration of fines driven by the confining pressure and hydraulic gradient.

This paper presents an analytical methodology that provides the vibration characteristics of monopile–wheel composite foundation embedded in homogeneous saturated soil, when the top of the foundation is subjected to a harmonic horizontal load. In the proposed frame, the horizontal resistances along the pipe pile due to the vibrations of the outer and inner elastic soil and compressible seawater are considered by using the Biot porous medium theory, plane strain model and radiation wave theory. The closed-form expression of the frictional force caused by the wheel vibration is calculated through the three-dimensional continuum mechanics theory. Based on the Euler beam model, the dynamic governing equations of different pile segments in the composite foundation are simulated as a one-dimensional linear elastic rod. Analytical solutions of dynamic impedances of composite pile in the frequency domain can be derived by virtue of the boundary and continuity conditions. Following the validation of the proposed methodology, the sensitivity of the dynamic response and natural vibration frequency of this innovative foundation to the main geometrical problem parameters is studied. The results show that increasing the wheel radius, wheel thickness and embedded length can improve the dynamic stiffness of the composite foundation, and increase the natural frequency of foundation–soil system simultaneously. Finally, the differences of the dynamic responses between the composite foundation and single pile with the same fabricating cost are discussed in detail, and the corresponding analysis proves the superiority of the composite pile under offshore loading conditions. Meanwhile, the contribution of the friction wheel to the dynamic behavior of composite foundation under different parameters is also investigated.

The design of column-supported embankments requires a comprehensive evaluation of overall stability and failure mechanisms. Previous research has investigated stress characteristics and failure modes of deep mixing (DM) columns under embankment loads. Depending on column configurations, those near the embankment toe experience flexural potential bending or tilting failure, while those closer to the centre are prone to shear failure. As column spacing decreases, a shift from bending to tilting failure occurs. Inadequate column bending capacity increases the risk of tilting-induced embankment collapse. This study initially showcases the efficacy of geosynthetics in mitigating tilting failure in columns with smaller spacings. The interaction between geosynthetics and diverse configurations on the failure mechanisms of DM columns is meticulously examined. A crucial shift in the failure mechanism from tilting to bending is facilitated by the application of conventional geosynthetics with tensile stiffness, particularly in scenarios with restricted spacing. Geosynthetics effectively mitigate lateral soil displacement, enhance column bending capacity, and intricately redistribute lateral pressures exerted on the columns. Ultimate load, shear strain, and stress are analysed both with and without geosynthetics. Lastly, the influence of geosynthetics on soil reaction distribution and internal forces within columns is deliberated.

The response of partially saturated granular cargoes during maritime transportation has resulted in the capsize and sinking of 27 bulk carriers at sea and the loss over 90 seafarers’ lives in the last decade. The partially saturated granular material response to energy imparted during cargo loading, ship engine vibrations and vessel rolling motions from sea states causes a change in state of the granular cargo, which can lead to vessel instability and ultimately capsize. However, the mechanisms driving the response of partially saturated granular cargos within a bulk carrier hold are not well understood. This paper presents results from an experimental study of rolling table centrifuge model tests on a partially saturated silica sand to contribute to improved understanding of the response of granular cargoes during maritime transport. Observed settlement, pore pressure, moisture content and density changes during and/or following a sequence of large amplitude rolling motions are presented. The results indicate that for the conditions considered, a progressive upwards migration of pore water during rolling led to creation of a free surface of water above the granular sample that was left in a denser, lower moisture content sample compared to its initial state. Sloshing of free water on top of even a competent cargo during rolling motions of the ship can contribute to loss of ship stability and ultimately capsize.

The freezing strain characteristics of unsaturated soil are quite different from those of saturated soil. Besides the commonly observed frost heave, unsaturated soil may also experience frost shrinkage, causing uneven surface deformation and deterioration of soil properties. Matric suction and water–ice phase change are key factors affecting the freezing strain characteristics of soil. In this study, the freezing strain characteristics and underlying mechanisms of lean clay samples with different initial matric suctions at varying temperatures were investigated. A mathematical relationship between soil temperature and matric suction was established, enabling the estimation of variation trends in matric suction. The internal mechanisms of different freezing strain characteristics were analyzed based on the mesoscopic structures of frozen samples with low and high initial matric suctions. The results showed that samples with low initial matric suction and frozen at low negative temperatures are prone to frost heave. Frost heave occurs when the volume expansion of the soil caused by water–ice phase change exceeds the volume reduction due to increased matric suction; otherwise, frost shrinkage occurs. The morphology of pore ice in samples with different initial matric suctions varies, reflecting the degree of water–ice phase change in the soil. Using the Pearson correlation coefficient method, an empirical model for axial freezing strain applicable to unsaturated frozen lean clay was established, and the model’s validity was confirmed with experimental data.

Geotechnical problem analyses often involve three-dimensional stress conditions. In this paper, a series of drained true triaxial tests were conducted under stress-controlled conditions to investigate the effect of the intermediate principal stress on the mechanical behavior of calcareous sands. All stress paths, characterized by different intermediate principal stress coefficients ((b)-values) and terminal stress ratios ((eta)), were maintained on the same deviatoric plane. The experimental results indicate that the deformation behavior of calcareous sands in three-dimensional stress is highly influenced by the (b)-values, particularly in the intermediate and minor principal directions, leading to notable anisotropy. The deviatoric stress–strain curves show that the deviatoric strain effectively represents the three-dimensional strain behavior of calcareous sands. As the (b)-value increases from 0 to 1, a simultaneous decrease in the stiffness and volumetric contraction behaviors of calcareous sands has been observed.

As a hydrocarbon reservoir rock, shale is generally composed of highly compacted clay particles with submicrometer sizes and includes nanometric porosity and different hard particles, like quartz, pyrite, etc. One of the key reasons for the formation of a complex fracture network via hydraulic fracturing is the multiscale heterogeneity of shale, especially heterogeneity on the microscale. This paper conducted on an experimental investigation of shale and explored the intrinsic relationship between the microstructure, the related mechanical properties at the micrometer level and the anisotropic failure mechanism. Small-scale specimens with micrometer dimensions in the form of cantilever beams with rectangular cross-section were fabricated by means of a focused ion beam (FIB) and tested via bending with a nanoindenter. The load–deflection curves of these bending beams were monitored up to failure, and the tensile strength of the shale composite was directly derived from the load–deflection curves at 474.5 MPa (parallel to the bedding plane) and 168.9 MPa (vertical to the bedding plane). The results show that the strength anisotropy of shale at the micrometer scale is driven by the clay particles and other minerals, and the bonds of these particles. The modulus anisotropy of the shale composite at the microscale is dominated by the orientation of clay particles. Moreover, the shale composite embedded with pyrite exhibited strong softening characteristics.

The fabric anisotropy in granular soils is a very important character in soil mechanics that may directly affect many geotechnical engineering properties. The principal objective of this study is to develop an efficient approach for assessing the degree of fabric anisotropy as a function of grading, particles shape and weighting specifications. By assuming cross-anisotropy, the anisotropic shear stiffness values of 1042 implemented tests on 200 various sandy and gravelly soil specimens from 43 different soil types were collected from the literature. Those were combined with their corresponding void ratios, stress conditions, grading parameters, particles shape and weighting attributes to generate a global database of anisotropic shear moduli in terms of testing conditions. The magnitudes of fabric anisotropy ratio were obtained using a well-known empirical equation, and they were plotted against the relevant soil grading and particles information to examine the dependency level of this ratio to the particularities. A series of multiple regression analyses were carried out to develop a global correlation for evaluating fabric anisotropy ratio in granular soils from grading, particles shape and weighting characteristic. The results showed that reliable quantities of fabric anisotropy ratio can be estimated using the surface appearance soil specifications. The findings may serve as an appropriate technique to yield good approximations for fabric and shear stiffness anisotropies using soil grading and particle properties.

In tropical islands, calcareous sand with poor engineering properties usually needs to be treated before it can be used as building materials. Considering the scarcity of freshwater in these areas, this study proposes seawater-based enzyme induced carbonate precipitation (EICP) technology to enhance the properties of calcareous sand. It is to induce calcium carbonate crystals to bond calcareous sand particles together using the seawater-based crude soybean enzyme and cementation solution (i.e., urea and calcium chloride). In this study, the crude soybean urease extraction test was firstly carried out using seawater and it was also investigated what components of seawater had a greater effect on the soybean urease extraction. Afterwards, the solution test was conducted to explore the ability of the extracted urease in inducing calcium carbonate through analyzing the variation of concentration of calcium ions and pH of the solution. Finally, the biocementation effect of EICP treated calcareous sand using the seawater extracted urease solution was evaluated by the unconfined compressive strength (q_uc) and microscopic analysis. Test results show that the turbidity of the seawater-extracted soybean urease solution can be reduced by 66.7% compared to that of deionised water extracted urease, with only a slight reduction in urease activity. Among all the components of seawater, NaCl, MgCl₂, CaCl₂, NaHCO₃ and KBr can significantly reduce the turbidity of soybean urease solution. The lower turbidity can effectively avoid bioclogging and contribute to the homogeneity of the EICP-treated calcareous sands, and thus improve the biomineralization efficiency and strength enhancement. Seawater-based EICP treatment will be a great promising technology in freshwater-scarce tropical islands, because it can directly use seawater for biomineralization treatment of calcareous sand, and meanwhile effectively avoid local clogging of biocementation.