Huiyun Chen

The load transfer mechanism of piles in karst cavity areas was investigated through field tests, and an orthogonal test was carried out to establish a calculation method for negative skin friction induced by backfilling. The results indicate that the negative skin friction of piles is strongly influenced by the type of cavity. When cavities were completely filled with limestone breccia mixed with silty clay and the applied load reached 3628 kN, the unit side friction ranged from 15 to 22 kPa. In contrast, when cavities remained unfilled, soil settlement occurred around the pile after backfilling, leading to the development of negative skin friction. For cavities with heights of 3–12 m, it is recommended that the bearing capacity of piles be calculated by considering negative skin friction at depths of 0H, 0.106H, 0.214H, and 0.271H (where H denotes the cavity height). Based on 21 orthogonal tests, the sensitivity ranking of factors affecting negative skin friction was determined as follows: cavity height > elastic modulus of backfill > pile diameter > cavity span > pile length > cavity position. The calculated values of negative skin friction were further validated against engineering data, showing a variation trend consistent with the test results, with a relative error of only 7.4%.

Caves affected the load transfer mechanism of bridge pile foundation, and then the safety of the bridge was threatened. This study was to investigate the effect of karst cave under bridge pile foundations on the vertical bearing characteristics of bridge pile foundations by static load test, finite element analysis and mechanical model. The settlement of the pile was measured by displacement meter, and the axial force were measured by stress gauges in the test. The load-settlement, the axial force, the unit skin friction and the ratios of side and tip resistances were compared with the result of the simulation. Then sixteen conditions were selected in finite element analysis, one of them was a conventional pile not on cave. The others were about five kinds of height, five kinds of span and six kinds roof’s thickness of the cave. The simply supported and fixed wide beam were established to calculate the allowance roof thickness. The results reveal that when the cave span is greater than 9 m × 9 m or the roof thickness is less than 2 D (pile diameter), the stress and deformation of piles are significantly affected.

A new method was used to study the performance of pile across cave. This paper investigated the vertical bearing characteristics of piles cross caves using centrifugal model tests and a theoretical model of sensitivity. Twelve pile scenarios were selected, the first was a conventional pile, 24 cm long and 2.5 cm in diameter, with no karst cave as a control. In the other eleven scenarios the piles passed through karst caves of four different heights, of four different spans, and three different numbers of caves. The results reveal that increasing the height, span, and number of caves all are negative for vertical ultimate bearing capacity of piles. The axial force and unit shaft resistance of piles are great different. According to the ratios of the tip and shaft resistance, caves change the type of piles. The sensitivity of vertical ultimate bearing capacity to these factors from high to low is height, number, and span of caves. Importantly, the bearing characteristics of piles decrease faster once the height of the prototype karst cave is higher than 9 m, but decreases slowly when the cave’s span is greater than 9 m × 9 m.