Gravel cushion is often used as the foundation of immerse tunnel; but the experience of using gravel cushion as the force transmission between pile and immersed tube tunnel structure has been lacking. Physical model tests have been carried out to reveal the deformation characteristics of gravel cushion and to examine the effects of ditch dimension

horizontal displacement of structure

sediment

ditch diverge

top incline and so on

upon the deformation characteristics of gravel cushion. The experimental results show that 1) small particle size gradation is not suitable for the foundation gravel cushion

and in selecting gravel gradation the compression modulus

porosity

particle size and other factors must be taken into account; 2) the existence of the groove can reduce compression modulus of gravel cushion greatly

and the main reason for this is that the joint effect of compaction and groove causes lateral extrusion of gravels; 3) at piled foundation sections

the ultimate bearing capacity of gravel cushion is about 480 kPa. As the force transmission structure between pile and immersed tube tunnel structure

the force mechanism of the gravel cushion is complicated

and several factors may come into play in influencing the behaviors of the gravel cushion. Hence

some measures need to be adopted to control the stress of the pile top within the ultimate bearing capacity of gravel cushion