Acta Geotechnica

For deterministic scenarios, adaptive finite element limit analysis has been successfully employed to achieve tight bounds on the ultimate load of a geotechnical structure in a much more efficient manner than a dense uniform mesh. However, no probabilistic studies have so far considered finite element limit analysis with adaptive remeshing. Therefore, this research explores the benefits of combining adaptive mesh refinement with finite element limit analysis for probabilistic applications. The outcomes indicate that in order to achieve tight bounds on probabilistic results (such as the probability of failure), the ultimate load in each individual simulation (e.g. factor of safety or bearing capacity) has to be estimated with a very high level of accuracy and this can be achieved more economically using adaptive mesh refinement. The benefits, assessed here for undrained conditions, are expected to be much more pronounced in the case of frictional soils and complex geometries.

The piezocone penetration test (CPTU) is commonly used as a fast and economical tool to identify soil profile and to estimate relevant material properties in soils ranging from fine to coarse-grained. Moreover, in the case of fine-grained soils (clays and silts), the consolidation coefficient and the permeability can be estimated through the dissipation test. Undrained conditions are commonly assumed for the interpretation of CPTU in fine-grained soils, but in soils such as silts, penetration may occur in partially drained conditions. This aspect is often neglected in data interpretation thus leading to an inaccurate estimate of soil properties. This paper investigates numerically the effect of partial drainage during penetration on the measured tip resistance and the subsequent pore pressure dissipation response contributing to a more accurate interpretation of field data. A realistic simulation of the cone penetration is achieved with the two-phase Material Point Method, modelling the soil response with the modified Cam-Clay model. The approach takes into account large soil deformations induced by the advancing cone, soil–water, and soil–structure interactions, as well as nonlinear soil behavior.

Các kỹ thuật siêu âm chủ động và thụ động đã được sử dụng để nghiên cứu sự tiến hóa ba chiều của hư hỏng trong các đĩa sa thạch dưới thử nghiệm Brazil. Một đĩa tiêu chuẩn, có đường kính 50 mm và độ dày 25 mm, đã được trang bị một mảng cảm biến 3D gồm 12 cảm biến Nano30 và bị nén đường kính dưới tải trọng tuyến tính một cách tĩnh. Tốc độ sóng P dọc theo 64 đường đi trong mẫu đĩa đã được đo trước và trong suốt thí nghiệm, được phát hiện là có tính dị hướng mạnh mẽ và sau đó được sử dụng để xây dựng một mô hình tốc độ đồng nhất theo thời gian. Mười hai kênh dữ liệu dạng sóng đầy đủ đã được ghi lại liên tục bởi hệ thống theo dõi phát xạ âm thanh (AE) cho đến khi hỏng hóc hoàn toàn, từ đó các sự kiện AE riêng biệt đã được kích hoạt. Dựa vào mô hình tốc độ đã xây dựng, tổng cộng 1775 sự kiện AE đã được xác định thành công bằng thuật toán tìm kiếm lưới sụp đổ. Hai phương pháp phân loại khác nhau đã được áp dụng cho các tensor mô men đã được phân tách, tiết lộ một hỗn hợp các vi vết nứt kéo và cắt tập trung tại điểm khởi phát nứt. Hướng của trục P và T cũng được tìm thấy là một chỉ số không hoàn hảo nhưng có thể chấp nhận về trạng thái căng thẳng của mẫu đĩa. Hơn nữa, một thử nghiệm Brazil tương tự đã được thực hiện với một mảng cảm biến 2D để so sánh. Phân tích mảng cho thấy rằng mảng cảm biến 3D thường hoạt động tốt hơn trong không gian ba chiều nhờ vào khả năng bao phủ không gian đầy đủ, trong khi mảng cảm biến 2D có độ chính xác định vị cao hơn trên mặt phẳng nơi cảm biến đặt. Cả hai trường hợp đều nhận thấy có vị trí khởi phát nứt tương tự, khoảng 10 mm từ trung tâm đĩa, điều này cho thấy rằng độ bền kéo thu được có thể đánh giá thấp giá trị thực.

The slurry infiltration process contributes significantly to the stability of tunnelling face during the application of slurry pressure balanced tunnel boring machine (SPB TBM) in construction. A coupled computational fluid dynamics and discrete element method (CFD-DEM) model is developed to simulate the formation and long-term impermeable stability of filter cake considering different infiltration conditions, e.g., the cohesiveness of bentonite slurry and the permeability of sandy ground. The morphology of formed filter cakes and the pressure drops are analysed macroscopically. Then, the long-term impermeable stability of filter cakes under increasing slurry pressure is evaluated based on the evolution of the pressure gradient magnitude along the infiltration direction; additionally, the contact and force fabrics of filter cakes are examined to investigate their failure process from the micro-mechanical aspect. The numerical results indicate that the increase of size ratio of sand to slurry particle and the slurry cohesion restrain and enhance, respectively, the formation of filter cakes and their long-term permeable and mechanical stability subjected to dynamic hydraulic loads. This work contributes to the further development of the phenomenological model and the micromechanical constitutive model of slurry infiltration involving multifield and multiphase media.

The paper applies the Structure from Motion photogrammetry technique to measure three-dimensional shrinkage deformations of a cylindrical specimen during drying. A clay–binder mixture was statically compacted, and its shrinkage behaviour was observed for four weeks with a newly established photogrammetric system. The complete surface information including XYZ coordinates and RGB values was then reconstructed and formed into dense point cloud data, with which the volume change and shrinkage deformations are computed. In volume change determination, the reconstructed dense point cloud achieves a fine reproduction of the specimen surface, leading to satisfactory accuracy. The computed volume deviation was within 495.3 mm3 (0.24%) and 4170 mm3 (1.83%) for the dummy cylinder and clay specimen, respectively. Both values are significantly smaller than the volume deviations computed with a classical approach that involves artificially creating markers on the specimen surface. In the computation of shrinkage deformations, the paper proposes a novel method that detects the fast point feature histogram of the point clouds and then matches the detected descriptors, leading to an improvement in three-dimensional deformation determination. The compacted sample shrinks, and the computed axial deformations increase quasi-linearly with height. However, due to the noticeable tilt that occurred during drying, the calculated radial deformations are scattered at the same height and appear misleading. Circumferential deformations and strains are therefore adopted to represent the true material behaviour.

This paper investigates the swelling mechanism of Callovo-Oxfordian (COx) claystone grain in its mixtures with bentonite which are potential candidate for engineered barriers in the French high-level radioactive waste disposal. The swelling pressures of MX80 bentonite/COx claystone mixtures with various bentonite fractions and dry densities were first experimentally determined by carrying out infiltration tests under constant-volume conditions. According to the experimental results, the claystone void ratio after hydration in the mixtures was estimated by taking into account the bentonite/claystone grain interaction. Then, the volume change of claystone grain under different pressures applied by swollen bentonite was determined. The volumetric strain of claystone grain was found to decrease linearly with the increase in pressure. This allowed the indirect determination of the swelling pressure of claystone grain. The determined value was found to be comparable to that estimated from the relationship between the void ratio and the swelling pressure in the direction perpendicular to the stratification of intact COx claystone, confirming the identified swelling mechanism of bentonite/claystone mixtures.

Soft clay generally cannot be directly used as subgrade material due to its poor engineering characteristics. The application of rice husk ash (RHA) for the solidification of problematic soil contributes to the recycling of waste and reduces carbon emissions. A series of mechanical, chemical, and microscopic tests were conducted to assess the early effectiveness of RHA in stabilizing soft clay. The pH and conductivity Ec also were measured to evaluate the effect of active ions. The results show that the use of RHA alone can only improve the compressive strength qu of soft clay to a certain extent. However, it cannot improve the soaked strength of the solidified soil. Carbide slag (CS) and metakaolin (MK) as activators were used to modify RHA, which significantly improved the compressive strength, shear properties, and soaked strength of soft clay. During initial curing, the compressive strength qu and cohesion c showed a marked correlation with pH and Ec. This is closely related to the dissolution and consumption behaviors of more abundant active ions contributed by the active oxides in the modified RHA. Finally, combined with XRD, SEM, EDS, and MIP tests indicated that the mechanical properties and microstructure improvement of soft clay can be attributed to the early physical reinforcement and meshing effect due to the porous granular structure of RHA. Subsequently generated CaCO3 crystals and C–S–H/C–A–S–H cementitious hydrates played a crucial role through carbonization and pozzolanic reactions supported by more abundant active ions.

This paper presents a comprehensive investigation on the microstructural evolutions of expansive clay during a drying–wetting cycle, including pore size distribution (PSD) via mercury intrusion porosimetry and water distribution via nuclear magnetic resonance (NMR). The soil water characteristic curves at different soil densities and soil shrinkage curve are also obtained, and a threshold suction can be identified to distinguish the adsorptive and capillary regimes of pore water. Combined with the water distribution obtained by the NMR technique, the evolutions of the adsorptive water and capillary water during drying–wetting cycle were addressed. The measured PSD curves of the expansive soils at different suctions showed two distinct peaks, corresponding to micropores and macropores, respectively. Both variations of macropores and micropores are irreversible during the wetting–drying cycle, which partly explain the adsorptive water content decreasing when the suction is small.