Abstract: Gravel was widely applied as the cushion materials for most foundations of the immersed tunnels, while the pebble as an alternative material has not been used till now. The differences in physical characteristics such as surface smoothness, alignment, contact methods and natural porosity between the gravel and pebble will affect its mechanical performance. In this paper, the mechanical deformation characteristics of pebble and gravel cushion were comparatively studied through physical modelling tests and numerical simulations. The results show that: 1) The compression curves of the two material cushions both show a two-phase anti-bending trend. The compression of pebble cushions is higher than that of the gravel under the same loading condition, whereas the overall secant modulus of pebble is approximately 30% lower than that of the gravel. 2) When the thickness of the cushion increases from 0.8 m to 1 m, a 13% increase in the secant modulus for the pebble cushion is observed, while 2.2% for the gravel. The mechanical deformation properties of the pebble cushion are more prone to be influenced by the cushion thickness than the gravel cushion. 3) When the preload increases from 52.5 kPa to 84 kPa, the secant modulus of the pebble cushion increases by 23.5%, while the modulus of the gravel cushion increases by merely 7.6%. A larger preload load leads to an earlier inflection point for the pebble cushion so as to show more stable mechanical properties. The increase in the preload load can more significantly improve the overall mechanical deformation performance for the pebble cushion than for the gravel cushion. 4) As the ditch width increases, the secant moduli of both cushions decrease. The pebble cushion is less sensitive to the changes in ditch size than the gravel cushion in the full load range. 5) The performance of the overall mechanical properties of the gravel cushion are better than the pebble cushion, but the cushion materials have limited effects on the structural settlement and stress. The pebble material can be used as an alternative cushion material for the immersed tunnels after a full analysis of the sensitivity to construction deviation. 6) Investigations on the optimal pebble grading will be the priority in the follow-up study.

Key words: immersed tube tunnel, pebble cushion, gravel cushion, dynamics, deformation