In order to research the stability of shield tunnel face during shield tunnelling

soil is regarded as pure clay

and obeying elastic-perfectly plastic Mohr-Coulomb yield criterion.According to three-dimensional Hill mechanism

which is used in the bearing capacity of rectangular foundation

the failure mode is established when shield tunnel face is in the active or passive limit equilibrium state.Based on the plastic upper-bound limit analysis

the three-dimensional upperbound solution of limit support pressure is obtained during shield tunnelling.Combined with an example

comparison between the three-dimensional upper-bound solution and plane strain upperbound solution is made for limit support pressure in shield tunnel face.The relationship between limit support pressure and soil cohesion is discussed

and the influence of the ratio between thickness of the overburden hand the diameter Dfor tunnel on the limit support pressure is studied.The results show:the three-dimensional active limit support pressure is smaller than the plane strain solution

the three-dimensional passive limit support pressure is larger

and the difference increases gradually with the soil cohesion increasing.The active and passive limit support pressure increases with the ratio between hand Dincreasing

and the larger the ratio is

the faster the change rate of limit support pressure with soil cohesion will be.