Bearing behaviors of steel foot pipes for tunnels in soft foundation

 Numerical simulations are conducted with FEM to investigate the bearing behaviors of steel foot pipes under vertical concentrated load.When a foot steel pipe bears the vertical load,mechanical effects mainly occur at the lateral zone,and the maximum stress occurs at the lateral end of the pipes.With the increase of the vertical load,the maximum stress point turns into the plastic state firstly,and the pipes are pulled out.The influencing factors for the bearing capacity of the steel foot pipes are studied,including the strength of rock and the parameters of the pipes.Some important results different from engineering experience are as follows: 1) for the steel foot pipes that can not penetrate the soft foundation,the reasonable dip angle is very small for the low axial bearing capacity;2) the steel pipes with diameter of 42 mm are unsuitable for the foot steel pipes because of the low horizontal bearing capacity;3) under given geological conditions and pipe parameters,the reasonable length of the steel pipes is certain,and extra length of pipes can’t lead to larger capacity;4) the surrounding rock near the support structure should be especially reinforced by drilling grouting holes along the whole steel pipes.Based on those results,the design and construction methods are presented.