Abstract: In order to understand the load transfer mechanism of highway bridge pile foundation passing through karst cave, the field test of bridge pile foundation treated by backfilling method is carried out. Combined with the numerical simulation method, the vertical bearing characteristics and load transfer mechanism of pile foundation passing through karst caves with different heights are studied, and the variation law of the maximum value of the negative skin friction caused by backfilling material in different cave heights and its proportion of distribution range are discussed. The results show that the negative skin friction in the karst area is affected by the type of karst cave, i.e. when the settlement of the soil on the side of the pile of the filled karst cave is smaller, the pile side can provide less positive friction for the pile foundation; when the settlement of the soil of the pile side of the unfilled karst cave is larger, the pile side can produce negative friction. After the karst cave is treated by the backfilling method, the vertical ultimate bearing capacity of the bridge pile foundation passing through the karst cave decreases with the increase of the cave height, e.g. when the cave height increases from 3 m to 12 m, the distribution range of the negative skin friction corresponding to the vertical ultimate bearing capacity of pile foundation increases from 0% to 27.14%. It is suggested that in the actual design, when the backfilling method is used to deal with the karst cave, the influence of the negative skin friction caused by the pile foundation passing through the karst cave on the vertical bearing characteristics of the pile foundation should be considered. When the height of karst cave is 3–12 m, the bearing capacity of the pile foundation should be calculated according to negative skin friction in the ranges of 0, 0.106H, 0.214H and 0.271H (H is the cave height) below the top surface of karst cave, so as to ensure the bearing safety of bridge pile foundation during the consolidation and settlement of backfill materials after cave treatment.

Key words: bridge engineering, karst cave, load transfer mechanism of pile foundation, field test, negative friction