To study the settlement and deformation characteristics of a long‐short pile composite foundation and its influencing factors, the indoor centrifuge model test and numerical simulations were performed under the actual working conditions to treat the saline soft soil foundation in the Soltan salt lake area of Iran. Firstly, the settlement of a long‐short pile composite foundation and the law of pile‐soil stress ratio development with time were studied using a centrifuge model test. In addition, FLAC3D was used to simulate the test conditions, and it has been observed that the numerical and experimental results were in good agreement. Based on the numerical model, the effects of the replacement rate of long and short piles, cushion modulus, and pile length on the settlement of the composite foundation and the stress ratio of long and short piles were analyzed. The results show that the long‐short pile composite foundation can effectively control settlement. Compared to the untreated foundation, the total settlement of a long‐short pile composite foundation was reduced by 67%, and the postconstruction settlement was reduced by 47.7%. Compared to the final settlement of the composite foundation obtained using the centrifuge model test, the error range of the numerical simulation results was between 2.4% and 8%. The influence of pile spacing on the settlement and stress characteristics of a long‐short pile composite foundation is greater than that attributable to the arrangement of the piles. Under this geological condition, it is reasonable to choose a pile spacing of 4d∼5d. When the long pile reaches the bearing stratum, the length of the short pile should not exceed 0.6 times the length of the long pile. The setting cushion not only reduces uneven settlement of the composite foundation but also improves the joint bearing capacity of the pile and the soil between the piles.

In order to study the long-term bearing capacity of concrete pile composite foundation in the Salt Lake area, based on the Tehran Isfahan high-speed railway project in Iran, the full (semi) immersion drying test and rapid freeze-thaw test was carried out, and the specimens were scanned by electron microscope. Numerical calculations were used to study the effects of different pile strengths and design parameters on the long-term bearing capacity of the composite foundation. The main conclusions were as follows: The concrete specimens in the adsorption zone deteriorated earlier and faster. In the rapid freeze-thaw tests, the strength attenuation of high-strength (C40, C50) specimens was smaller than that of low-strength specimens (C20). Within 20 years after construction, the additional settlement of low-strength (C20) piles was 12.21 mm, while high-strength concrete was less affected by deterioration. With pile spacing ranging from 1.8 m to 4.5 m, the maximum increase in additional settlement under the C20 condition was about 20 mm. The pile-soil stress ratio under the three conditions increased by 2.42, 6.59, and 8.63. As the pile length and diameter increased, the peak stress of the pile body moved towards the pile end, and the changes in the pile-soil stress ratio under the three conditions were similar.

To determine whether coarse‐grained saline soil meets the deformation requirements of the DY high‐speed railway subgrade, a study was conducted by a combination of field‐monitoring and laboratory tests. First, several temperature sensors were buried vertically in the ground of a typical section, and the temperature at different depths was monitored for nearly one year and analysed dynamically. It was determined that a depth of 4.8 m can be set as the constant‐temperature layer. Then, based on the field‐monitoring results, laboratory tests were carried out on a large‐scale subgrade model under freeze‐thaw and precipitation conditions. The change of temperature, moisture content, and soil deformation of the subgrade under long‐term freeze‐thaw and precipitation conditions were obtained. The results show that the temperature changes periodically with a V shape during the entire cycle. Twenty centimetres below the top surface is the sensitive depth of the sample, and salinity has little effect on temperature change. In the process of cycles, the average moisture content of soils with higher salinity is about 0.5% lower than that of soils with lower salinity. After nine freeze‐thaw cycles, the sample finally shows dissolved settlement deformation. Precipitation mainly affects the deformation of the sample; however, the influence on salt‐expansion and frost‐heave deformation is less significant. Finally, by predicting the deformation of coarse saline soil, it is proven that the soil can meet the deformation requirements of high‐speed railway foundations.