Analysis of dynamic stress path in inhomogenous subgrade under moving aircraft load

Considering the cut and fill alternating characteristic of airport subgrade in mountainous area, especially in the runway direction, a semi-analytic finite element method was proposed to analyze the dynamic response of inhomogenous subgrade under moving aircraft load. This method can decrease the computational cost effectively by using the Fourier Transform in the transverse direction of the runway to reduce the dimension of the 3D problem and account for the traveling wave effect of moving load. The dynamic responses of a soil element in homogenous and inhomogeneous subgrade subjected to single-wheel aircraft load were analyzed comparatively. The results show that the dynamic stresses generated in soil element are mainly vertical normal stress and in-plane shear stress, forming stress paths with the characteristic of complex principal cyclic stress axis rotation. Due to the traveling wave effect, the dynamic responses of soil are not antisymmetric when the moving load approaches and leaves the observation point. In the homogeneous subgrade, the dynamic stress path in the plane of the two shear stress components is egg-shaped with a tilting angle. As the moving load velocity grows, the long axis of the egg-shaped stress path rotates counterclockwise. In the inhomogeneous subgrade, the dynamic stress is concentrated near the junction of cut and fill areas, and the peak value of the vertical dynamic stress is 1.35 times that in the homogeneous subgrade. Moreover, the stress path near the junction tends to be more prolate and spindle-shaped.