Abstract: In order to solve the cavity problem of surrounding rock at tunnel bottom, the cavity evolution of surrounding rock at tunnel bottom under different soil surrounding rock conditions and structural styles are analyzed based on the laboratory experiments and numerical simulations, and the cavity types of tunnel bottom surrounding rock under different soil surrounding rock conditions are given. Furthermore, the stress characteristics of tunnel bottom structure under different caving types are cleared. The results show that: (1) Under the condition of the same tunnel bottom surrounding rock type, the cavity phenomenon is easier caused by floor structure than inverted arch; under the condition of same tunnel bottom structure, the cohesive soil can cause cavity phenomenon most easily, followed by pebble soil and sandy soil. (2) The type of tunnel bottom surrounding rock cavity is mainly related to the type of surrounding rock, and the cavity types of surrounding rocks of pebble soil, cohesive soil and sandy soil are local cavity in a small area, global cavity in a whole area and large scale cavity in the central area of tunnel bottom, respectively. (3) Under the condition of the same cavity type, the stress characteristics of the floor structure vary more dramatically than the inverted arch structure, the stress concentration is more obvious, and the tunnel bottom structure is more prone to damage. (4) With the increase of the cavity range, the variation of bending moment and the decrease of axial force of the tunnel bottom structure all show an increasing trend, and the distribution of the axial force gradually shows an uneven pattern of "large in the middle and small at both ends".

Key words: heavy-haul railway, tunnel, surrounding rock cavity, model test, tunnel bottom, structural stress