As a connecting part between surrounding rock and the segment

the backfill layer is used to stabilise the lining

transfer the load

and absorb the deformation. During the construction of shielded TBM tunnel

the backfill layer had two main states: loosened before grouting and consolidated after grouting. The effect of the backfill layer and characteristics of the segment stress depend on the state of characteristics of segment stress. The similar model test was used to analyze the mechanism of backfill at different states

and the following conclusions were obtained. When the backfill is loosened before grouting

due to the radial compression and circumferential movement

the load value of the surrounding rock to the segment is reduced

and the distribution is more uniform. At this time

the backfill is used as the "compressible layer" to reduce pressure and to make the pressure uniform. The backfill can significantly reduce the force and deformation of the segment. When the backfill becomes consolidation after grouting

the backfill layer can bear a small amount of load and deformation

but the bearing capacity is limited. At this time

it becomes the "transfer layer" between the surrounding rock and the segment. The extrusion of the shield rock TBM tunnel construction may use the backfill compressibility before grouting

and reduce the segment force and deformation. Shallow and subway tunnels should be grouting as soon as possible

so the lining and the surrounding rock can form a stable force system. The increase of elastic modulus of the backfill layer can improve the support stiffness of the backfill to the segment system

but the effect is not obvious.