Abstract: There exist many technical issues of thickly-filled open-cut tunnel with highly-filled and deeply-excavated foundation, such as high load-bearing capacity of structure, complex foundation treatment, and uneven settlement deformation. A case study is conducted on the Guangzhou-Zhanjiang high-speed railway, and the structural load bearing characteristics of open-cut tunnel and pile arrangement plans are analyzed, proposing a rational structural design plan. The load bearing capacity calculation methods of the pile in various specifications are compared, and the evolution pattern of negative frictional resistance around the pile under the action of soil properties and stacking pressure is examined, clarifying the calculation principles load bearing capacity of pile. Additionally, the calculation methods of the foundation settlement in composite strata are summarized, and feasibility assessment is conducted, proposing settlement control measures in transition section between filling and excavation. The research results reveal the following: (1) Inclined wall, flat bottom, and large arch foot structure with large-diameter and large span piles should be adopted for the open-cut tunnel in the filled section, whereas straight wall and inverted arch structure should be adopted for that in the excavated section. (2) The allowable stress method is more conservative than the probabilistic limit state method in calculating pile foundation load. The negative friction resistance around the pile is dissipated, thus a single-time backfill thickness should be controlled and the unfavorable strata around piles should be conditioned. (3) The foundation settlement of the open-cut tunnel in the filled section can be calculated as the rigid piles and using the pile compression method. The plain concrete piles can be used to effectively connect drilling piles and cement fly-ash gravel piles, and the thickness of the pile top cushion can be adjusted to achieve a uniform transition of foundation settlement.

Key words: highly-filled and deeply-excavated foundation, thickly-filled soil, open-cut tunnel of high-speed railway, negative frictional resistance, transition section between filling and excavation, uneven settlement