The physical and mechanical properties of red clay have a significant influence on the bearing characteristics of pile foundations. Therefore, this paper conducted model tests to study the influence of different red clay water content (16~−20%) and different compaction degrees (88~−92%) on the bearing characteristics of pile foundations. The results show that the load–displacement curves of the test pile generally exhibit a steep type. The ultimate bearing capacity of the test pile increases gradually with the compaction degree from 86% to 92%. However, the ratio of load borne by the side of the pile increases from 54.8% to 67.8%, while the ratio of load borne by the end decreases from 45.2% to 32.2%. The ratio of load borne by the pile end to that borne by the pile side also decreases gradually from 0.825 to 0.476. The ultimate bearing capacity of the test pile decreases gradually with the moisture content from 17% to 26%, showing a decreasing trend. However, the ratio of load borne by the pile side decreases from 69.5% to 51.9%, while the ratio of load borne by the end increases from 30.5% to 48.1%. The ratio of load borne by the pile end to that borne by the pile side also increases gradually from 0.439 to 0.927. This indicates that increasing the compaction degree or reducing the moisture content has a more significant effect on increasing the side resistance. In this paper, the ultimate bearing capacity of test piles are calculated by the Mohr–Coulomb criterion (M-CC) and Duncan–Chang hyperbolic model (D-CHM), respectively. The results show that M-CC and D-CHM methods can accurately calculate the bearing capacity of the pile foundation in red clay. The conclusion of this study can provide a reference for the design and construction of pile foundations in red clay areas.

In order to study the strength characteristics of organic-matter-contaminated red soil and the improvement effects of different modifiers, the red soil in the Yulin area was taken as the research object, and triaxial compression tests were carried out to study the effects of different mass fractions (0%, 2%, 4%, 6%, 8%) of organic matter (sodium humate) on the strength characteristics of red soil. Unconfined compressive strength (UCS) tests and scanning electron microscopy (SEM) tests were carried out to study the improvement effects of different amounts of lignin, fly ash, and xanthan gum on organic-matter-contaminated red soil (organic matter content of 8%). The results of the tests showed that the cohesion and internal friction angle of red soil both tended to decrease with the increase in organic matter content. When the organic matter content increased from 0% to 8%, the cohesion of the red soil decreased from 60.98 kPa to 40.07 kPa, a decrease of 34.29%; and the internal friction angle decreased from 17.42° to 7.28°, a decrease of 58.21%. The stress–strain relationship curves of organic-matter-contaminated red soil all show a hardening type. Under different confining pressures, as the organic matter content increased, the shear strength of the red soil decreased continuously. The unconfined compressive strength of organic-matter-contaminated red soil increased with the increase in lignin content, and increased first and then decreased with the increase in fly ash content and xanthan gum content. Through comparative analysis, it was found that the fly ash with a content of 15% had the best improvement effect. The lignin-amended red soil enhanced the connection of soil particles through reinforcement, reduced pores, and improved soil strength. Fly ash improved the acidification reaction, and the hydrates filled the pores and enhanced the soil strength. Xanthan gum improved the red soil by absorbing water and promoting microbial growth, further enhancing the bonding force between soil particles. This study can provide a reference for engineering construction and red soil improvement in red soil areas.

Rock-socketed piles are a common type of end-bearing pile, but when there is deep sediment or holes at the pile bottom, the load is primarily supported by side resistance. In this study, based on such conditions and considering the influence of pile shaft roughness, model tests were conducted to investigate the bearing characteristics of rock-socketed friction piles. The results show that the failure mode of rock-socketed friction piles is the formation of a penetrating cylinder in the rock layer, with the cylinder diameter directly approximating the pile diameter. The load–displacement curves of the test piles are steeply variable. After reaching the ultimate bearing capacity, the residual bearing capacity of rough test pile is approximately 60% of the ultimate bearing capacity, while that of smooth test pile is 72.4%. The maximum side resistance of the test pile is located within a depth range of 25 mm below the soil–rock interface, and the upper load of 41.0% to 48.9% on the test piles was born by the pile side resistance within this depth range. As the roughness factor (RF) increases gradually from 0.0 to 0.3, the ultimate bearing capacity of the test pile shows nearly linear growth, the ultimate displacement increases sharply first and then decreases slowly, and both the axial force attenuation and the percentage of side resistance within the depth range of 25 mm below the soil–rock interface gradually increase slightly. In this paper, two existing methods are employed to calculate the ultimate bearing capacity of friction piles under the conditions of this study. Based on a comparison of the results, the applicable conditions for each method are proposed. The findings of this study can serve as a reference for the design of rock-socketed piles in similar geological formations.