Deformation characteristics of long-short pile composite foundation for high-speed railway in salt lake region

Based on the Tehran-Isfahan high-speed railway project in Iran, centrifuge model tests were conducted for full-length pile (concrete pile) composite foundation, full-short pile (gravel pile) composite foundation, and long-short pile (concrete pile-gravel pile) composite foundation, their settlement processes were simulated in embankment construction and following two years, and the effects of pile length ratio, pile spacing, short pile layout and filling height on the settlement characteristics were studied. Research results show that under the same geological condition, the post-construction settlements of the soils between piles for full-length, long-short, and full-short pile composite foundations are 28.16, 36.17, and 53.95 mm and reduce by over 70%, 60%, and 40%. Therefore, the post-construction settlements for full-length and long-short pile composite foundations meet the regulatory requirement of no more than 50 mm, and the post-construction settlement of long-short pile composite foundation is between the settlements of full-length and full-short pile composite foundations. For long-short pile composite foundation, increasing pile length ratio by 0.1 may reduce settlement by 7%-12% for the pile spacing of 3 times pile diameter and around 8% for the pile spacing of 4 times pile diameter, indicating that increasing pile length ratio can significantly improves settlement control, and the control is more effective as pile spacing increases. When the pile spacing increases from 3 times pile diameter to 5 times pile diameter, composite foundation settlement increases from 36.56 mm to 55.71 mm for a pile length ratio of 0.5, and from 28.38 mm to 45.93 mm for a pile length ratio of 0.7, indicating that settlement control is less effective at larger pile spacing. To meet the regulatory settlement requirement, the pile length ratio should not be less than 0.5 when the pile spacing is 5 times pile diameter. For a pile length ratio of 0.7, composite foundations settlements with one long-one short, one long-two short, and one long-three short pile arrangements are 28.37, 38.06, and 43.69 mm, indicating that placing more short piles between adjacent long piles can balance settlement control and economic efficiency. As the number of short piles increases, the stress on the long piles increases, but the stress distribution trend remains unchanged. Under different filling heights, in long-short pile composite foundation, gravel piles primarily move downward, long piles penetrate cushion layer, short piles penetrate soft soil, and localized potential slip surfaces form due to shear deformation and lateral extrusion of gravel piles.