Post-grouting pile technology has gained extensive application in collapsible loess regions through the injection of slurry to compress and consolidate the soil at the pile base, thereby forming an enlarged base that enhances the foundation's bearing capacity and reduces settlement. Despite the prevalent unsaturated state of loess in most scenarios, the conventional design methodologies for piles in collapsible loess predominantly rely on saturated soil mechanics principles. The infiltration of water can significantly deteriorate the mechanical properties of loess due to the reduction in matric suction and the occurrence of collapsible deformation, leading to a substantial degradation in the bearing behavior of piles. To explore the variations in load transfer mechanisms of post-grouting piles in collapsible loess under conditions of intense precipitation, a comprehensive large-scale model test was conducted. The findings revealed that the post-grouting technique effectively mitigates the adverse effects of negative pile shaft friction in saturated zones on the pile's bearing behavior. Furthermore, the failure criteria for piles may shift from the shear failure of the base soil to excessive pile settlement. By incorporating principles of unsaturated soil mechanics, modified load transfer curves were developed to describe the mobilization of both pile shaft friction and base resistance. These curves facilitate the extension of the traditional load transfer method to post-grouting piles in collapsible soils under extreme weather conditions. The proposed revised load transfer method is characterized by its simplicity, requiring only a few soil indices and mechanical properties, making it highly applicable in engineering practice.

Calculation and prediction of the uplift capacity of squeezed branch piles (SBP) are still immature. This study develops a method to predict the load-displacement relationship and ultimate capacity of SBP under pullup load by using a hyperbolic model to describe the nonlinear load transfer between pile-soil and plate-soil. The uplift bearing behaviors of SBP are analyzed through six sets of indoor model tests in homogeneous soils. The results, along with field tests of single-plate piles in layered soils and the indoor tests, confirm the high accuracy of the theoretical prediction method. The effects of three factors, including the pile side soil damage ratio (Rf), the horizontal earth pressure coefficient (k) and the damage angles of the soil under plate (psi), on the prediction results are analyzed. The results show that these factors significantly affect the second half of the loaddisplacement curve of SBP. Furthermore, as the Rf rises, the anticipated ultimate uplift capacity of SBP decreases linearly; as the k rises, it increases linearly; and as the psi rises, it increases nonlinearly.

Multiangle helical piles are used to support multidirectional loads. The load transfer behavior of inclined piles may differ from that of vertical piles. Vertical compressive and oblique uplift load field tests were conducted on a multiangle helical pile group and two single helical piles embedded in silty clay. The load-bearing capacities, group effects, load transfer behavior, earth pressure, and excess pore water pressure were investigated. The results show that the vertical compressive and oblique uplift capacities of the 10 degrees-inclined single helical pile were improved by 12% and 95% compared to those of the vertical single helical pile, respectively. The inclined installation of helical piles significantly optimized the load transfer mechanism of the piles under oblique loads. The group efficiency of the multiangle helical pile group was approximately 102%, attributed to the increased pile spacing resulting from the inclined installation. During loading, the helices and pile toe together contribute more than 50% of the bearing capacities of helical piles. The earth pressure and excess pore water pressure around the grouped helical pile, particularly near the bottom helix, exhibited less variation than those around the single pile, suggesting a smaller disturbance in the surrounding soil.

Pile-supported embankments are one of the most commonly used techniques for ground improvement in soft soil areas. Existing studies have mainly focused on embankments supported by end-bearing piles under static loading, with limited research on floating pile-supported embankments under cyclic traffic loading. In this study, model tests for unreinforced floating, unreinforced end-bearing, geosynthetic reinforced floating, and geosynthetic reinforced end-bearing pile-supported embankments were conducted. Cyclic traffic loading was simulated using a three-stage semi-sinusoidal cyclic loading. Comparative analyses and discussions are performed under floating and end-bearing conditions to investigate the influence of floating piles on the soil arching evolution and membrane effect under cyclic loading. The results indicate that floating piles result in earlier stabilization of surface settlement. There is less arching and membrane effect induced by floating piles, and the arching does not continue to degrade under cyclic loading. Less membrane effect in floating pile-supported embankments results in less geosynthetic and pile strain. The degree of membrane effect in floating pile-supported embankment largely depends on the pile-end condition.

A new type of pressure-cast-in-situ pile with a spray-expanded frustum (PPSF) developed in recent years, which is constructed by spiral multifunction auger drill. Due to the expanded body of PPSF, the mechanical properties of pile-soil interface are greatly improved. The traditional settlement calculation method of tapered pile is not applicable to PPSF due to the assumption of cylindrical cavity expansion theory, and the relevant calculation methods of squeezing branch pile cannot explain the strengthening effect of the surface normal resistance on the tangential resistance at the lower frustum of the expanded body. To get a simplified approach for load-settlement prediction of PPSF, considering the extrusion effect of the expanded body, a load transfer model is proposed to simulate the relationship between end resistance and corresponding settlement of the expanded body. The load-settlement responses from two field tests are compared to illustrate the reliability of the present method. Furthermore, the effects of embedment depth, diameter and frustum angles of expanded body on the bearing capacity of PPSF are studied. The research results show that the settlement calculation method of PPSF is reasonable and reliable (with a maximum error of 12.6%). With the distance from expanded body to pile end increases from 0.5 m to 6.5 m, the bearing capacity decreases from 4993 to 4770 kN, with reductions ranging between 1.1% and 4.5%. The bearing capacity increases by approximately 11.4% for every 100 mm increase in the diameter of the expanded body.

This paper presents a discrete element method (DEM) investigation into the load transfer mechanisms and failure surfaces of geosynthetics reinforced soil (GRS) bridge abutments. A local strain-dependent reinforcement contact model is developed to accurately simulate the nonlinear tensile behavior of reinforcement. The study analyzes both the macroscopic deformation response and the microscopic fabric evolution of backfill soil under bridge load. The findings reveal that as the bridge load increases, the micro-bearing structure of the soil within the potential failure surface evolves through progressive loss of effective contacts, particle rotation, and fabric reorganization. These micromechanical phenomena underlie the development of shear bands and the global failure mechanism of GRS abutments. Furthermore, a parametric analysis is conducted to evaluate the effects of reinforcement stiffness, reinforcement vertical spacing, and backfill soil friction angle on failure surfaces of GRS abutments. The results demonstrate that higher reinforcement stiffness constrains failure surface development, while wider reinforcement spacing and lower soil friction angles lead to deeper and more pronounced failure surfaces. Overall, the study highlights the critical role of reinforcement-soil interactions and micromechanical processes in determining the deformation and failure surfaces of GRS bridge abutments.

An analytical framework to analyze the progressive failure behavior of axially loaded single pile embedded in unsaturated soils is presented by means of the load transfer method (LTM) coupling with shear displacement method (SDM). In this study, the proposed DSC-based interface constitutive model and modified small-strain stiffness model undertake the role of load-transfer mechanism for the pile shaft and pile end, respectively, and the shear displacement method is adopted to take the soil deformation surrounding the pile shaft in consideration. This study adopts a stress loading approach in the solution process, differing from traditional strain loading methods, which involves assuming a segment of displacement at the pile end. By successively applying loading increments, the entire load-displacement relationship of the pile is accurately determined, effectively reproducing the authentic stress process of pile foundation, and analyzing the gradual failure processes of friction-dominated piles and end-bearing friction piles under varying suction conditions in unsaturated soil. Customized model piles with smooth, rough and ribbed surfaces and a stress-controlled servo system were developed to conduct static load tests on pile foundations in unsaturated sand-clay mixture and grey clay. Model parameters were calibrated through suction-controlled unsaturated ring shear tests. Finally, the validity of the solutions proposed in this study was verified by comparing the results of static load tests on smooth, rough and ribbed model piles. Subsequently, the effects of suction, interface dilation, and environmental factors on the load-displacement response were analyzed. The research findings of this study can provide a theoretical basis for the design of pile foundations with displacement control in unsaturated soil.

Solidified soil prefabricated pile (PPSS) is a new type of pile formed by extruding solidified soil with hydraulic equipment. The PPSS includes two parts: precast pile and core pile, which can be used to strengthen soft foundation. To study the deformation characteristics of PPSS under vertical load, the nonlinear mechanical behaviour of the double-contact interface of PPSS is analyzed by using the bond slip model and hyperbolic model. A settlement calculation method is proposed considering the displacement coordination of the doublecontact interfaces, e.g., interface between precast pile and surrounding soil, and interface between core pile and precast pile. The bearing characteristics of the double-contact interfaces are studied by using the numerical results. Based on the numerical results, the effects of elastic modulus ratio, diameter ratio, length and initial cohesion on the deformation characteristics of PPSS are analyzed.

To further study the load transfer mechanism of roof-multi-pillar-floor system during cascading pillar failure (CPF), numerical simulation and theoretical analysis were carried out to study the three CPF modes according to the previous experimental study on treble-pillar specimens, e.g. successive failure mode (SFM), domino failure mode (DFM) and compound failure mode (CFM). Based on the finite element code rock failure process analysis (RFPA2D), numerical models of treble-pillar specimen with different mechanical properties were established to reproduce and verify the experimental results of the three CPF modes. Numerical results show that the elastic rebound of roof-floor system induced by pillar instability causes dynamic disturbance to adjacent pillars, resulting in sudden load increases and sudden jump displacement of adjacent pillars. The phenomena of load transfer in the roof-multi-pillar-floor system, as well as the induced accelerated damage behavior in adjacent pillars, were discovered and studied. In addition, based on the catastrophe theory and the proposed mechanical model of treble-pillar specimen -disc spring group system, a potential function that characterizes the evolution characteristics of roof -multi-pillar-floor system was established. The analytical expressions of sudden jump and energy release of treble-pillar specimen-disc spring group system of the three CPF modes were derived according to the potential function. The numerical and theoretical results show good agreement with the experimental results. This study further reveals the physical essence of load transfer during CPF of roof -multi-pillar-floor system, which provides references for mine design, construction and disaster prevention. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

This paper presents a numerical study to investigate the load transfer mechanism of a geosynthetic encased stone column (GESC) under embankment loading. The soils were modeled with a nonlinear elasto-plastic constitutive model incorporating a hyperbolic stress-strain relationship and the Mohr-Coulomb failure criterion. The geosynthetic encasement was modeled using a linearly elastic embedded liner element. Two interfaces were used to simulate the interaction between the geosynthetic encasement and the soils on either side. The validation of the numerical model was conducted using test data from vertical loading tests of the individual GESC installed in loose sand, including applied vertical stress-settlement curves and the circumferential strains profiles. Then, the influences of different design parameters on the load transfer mechanism of the GESC unit cell were investigated through a parametric study. Results indicate that the development of stress concentration ratio depends on the mobilization of tensile strains. The circumferential strains are significantly larger than the longitudinal strains, indicating that the circumferential tensile effect is dominant under embankment loading. The load transfer effect was gradually enhanced with increasing tensile strains. Increasing the geosynthetic encasement stiffness can be considered as an alternative to increasing the column infill friction angle in improving the load transfer effect.