The reliability of high-rise buildings’ rigid-pile composite foundations primarily relies on the load test results of single-pile composite foundations. Field load tests were conducted for a high-rise building in the loess area of China, the results conformed to the design requirements. Nevertheless, the buildings experienced significant settlement after construction, which was quite different from the test results. Investigating bearing disparities between the single-pile and group-pile composite foundations in high-rise structures was necessary. Firstly, conducting an indoor interface shear test to propose a pile–soil hyperbolic contact model and integrate it into the numerical model. Subsequently, a finite element model was employed that accounts for pile–soil interactions in order to analyze discrepancies in bearing. The results show that the settlement difference rises by 56.4% as the load escalates by 100 kPa from the initial level and increases by 28.9% as the load ascends by 100 kPa to the final level. The settlement difference changes with an increasing load in a hardened curve pattern. Furthermore, differences in pile bearing characteristics and pile–soil interaction were analyzed. Negative friction occurs within 1/4 of the pile length within group-pile composite foundations, while in single-pile composite foundations, the range contracts to 1/10.

Water migration changes the water distribution of loess and significantly affects the engineering properties of loess. However, because of the characteristics of loess such as water sensitivity and special structure, it is still difficult to understand the water migration trend of loess. The objectives of this study are to investigate the water diffusion coefficient and soil‐water characteristic curve (SWCC) of loess, focusing on the effect of temperature and density, and establish a mixed migration equation for gaseous and liquid water to develop a new calculation method of loess water migration. The results show that the density of loess with larger liquid water content has a more significant effect on the diffusion coefficient than that of less liquid water content. Furthermore, the density significantly affects the matric suction of unsaturated loess, while the temperature change at normal temperature has slight effect. Based on the data obtained in this study, the soil water permeability coefficient was obtained. Moreover, considering the characteristics of mixed migration of gas and liquid water in unsaturated loess, an equation was derived for the gas‐liquid water transfer in unsaturated loess. The calculation results are consistent with the experimental results of water migration experiment. Based on the calculation results of water migration, the trend of water migration and proportion of gaseous water during migration were analyzed.

This paper investigates the skin friction transfer characteristics of the rock‐socketed section of a rock‐socketed pile resting on thick sediment by conducting in situ core‐drilling tests and static loading tests. Test results show that when using the impact hole‐forming method in weakly cemented soil, a layer of sediment is deposited at the pile bottom. Due to the existence of sediment, when the load reaches a certain value, sudden and large subsidence is observed. This indicates that the end resistance does not contribute to the bearing capacity. Thus, it is not appropriate to consider both end resistance and side resistance in the existing design method of a rock‐socketed pile. The bearing capacity of a single rock‐socketed pile should be determined according to the side resistance of the soil layer and rock‐socketed section only. Numerical analysis is performed to determine the deformation and load‐carrying capacity of the pile and the distribution of friction on the sides of the rock‐socketed segment. Under a given applied load, small settlement is observed when socketed thickness and rock strength are relatively large. The distribution of side friction of the socketed segment along the vertical direction shows a double‐peak saddle shape. When the socketed thickness and rock strength are relatively smaller, the lower peak is higher than the upper peak, and conversely, when the socketed thickness and rock strength are relatively larger, the lower peak is smaller than the upper peak. For a given applied load on the pile top, smaller socketed thickness results in larger settlement and side friction. Due to the thick layer of sediment, the axial force of the rock‐socketed segment of the pile gradually decreases along the vertical direction from the applied load on the pile top to zero at the bottom. According to the mechanical properties at different shear stages, a function is derived for the complete constitutive model for a pile‐rock interface. Analytical solutions for the friction of a single pile are obtained under the conditions of failure and elasticity deformation of the surrounding rock. Its load transfer equation is derived as well. Accordingly, an equation is proposed for calculating the bearing capacity of rock‐socketed piles resting on sediment at the bottom.

Collapsible loess is generally characterized by a sudden and substantial decrease in volume that occurs when is applied under constant stress. To evaluate the loess collapse potential, the self‐weight collapse and collapse coefficients have been defined by the code for building construction in collapsible loess regions. However, the method in the code does not account for the vertical stress variation. The loess collapse process commonly occurs with stress variation in practice. This paper documents a low‐cost, quantitative evaluation scheme using regression analysis to evaluate the loess collapse potential by varying the unloading levels. The results show that the factors that prominently account for loess collapse deformation are the initial pressure, unloading ratio, and collapse completed‐ratio. At a constant collapse‐completed ratio, the remnant collapse coefficient significantly decreases with the decreasing unloading ratio; at a constant unloading ratio, the remnant collapse coefficient increases with a decreasing collapse‐completed ratio. Decreasing unloading and collapse‐completed ratios decreased the loess collapse potential with an initial pressure that exceeds the threshold value. Finally, an unloading collapse deformation calculation of loess was prepared to analyze practical project problems of loess based on the unloading collapse test.

A series of shear interface experiments on a type of loess and rough concrete interface under conditions of different initial water contents (16%, 21%, and 26%), dry densities (1.30 g/cm³, 1.52 g/cm³, 1.70 g/cm³) and normal stresses (50 kPa, 100 kPa, 200 kPa) were conducted to further understand shear deformation and strength characteristics of a loess and rough concrete interface combined with loess deformation monitoring method of gypsum powder line method. A discrete element method (DEM) model was then established, calibrated against the experimentally obtained shear stress–displacement curves, and run to investigate the shear deformation, contact force chain and fabric evolution processes at the microscopic level. The results show the following: (1) The shear deformation and strength behaviors of the loess and rough concrete interface were significantly impacted by the initial moisture content, dry density and normal stress. (2) The shear deformation of the loess increased with the increase in initial moisture content, and decreased with dry density and normal stress. (3) The shear strength of the loess and rough concrete interface increased with the increase in dry density and normal stress, and decreased with the increase in initial moisture content. (4) The evolution of the shear deformation, contact force chain and fabric of the loess-concrete rough interface were explored and analyzed from a microstructural perspective. This study contributes insights critical to construction of the pile-loess systems in Chinese Loess Plateau regions.

In the loess area, the bearing capacity of rigid pile composite foundation is mainly determined based on the load test of single pile composite foundation. However, it has been observed that there is a significant difference in settlement between on-site loading tests in settlement between on-site loading tests and the actual settlement of the building. In order to identify the reasons for the difference in settlement between single-pile composite foundation and pile group composite foundation, and to lay a foundation for revising the results of loading tests for singlepile composite foundation, this article studies the bearing differences between single pile composite foundation and group pile composite foundation under high-rise building loads. Based on an example of a first-class high-rise building, it was found that although the settlement results of the loading test were smaller, the high-rise building experienced excessive settlement. Therefore, it is necessary to further establish a finite element model considering the pile-soil interaction for single pile composite foundation and pile group composite foundation, and compare and analyze the differences in pile axial force and side friction. The difference in bearing capacity is due to the higher load borne by the soil between piles in the pile group composite foundation. The soil around piles generates downward movement relative to the surface of the pile. This will lead to a decrease in the lateral friction resistance of the pile, thereby weakening its bearing capacity and increasing its vertical settlement. Resulting in the difference in settlement between the loading test and the actual post-construction settlement.