Impedances of Soil Layer with Boundary Zone

When calculating dynamic response of embedded foundations or piles, impedances (stiffness and damping) of the adjacent soil are needed. To account approximately for soil nonlinearity under high strain, contact deficiencies and disturbances caused by foundation installation, a cylindrical annulus of softer soil around the foundation is used. The soil properties in this weakened boundary zone differ from those of the outer zone but both regions are assumed to be homogeneous. The mass of the cylindrical annulus can be either neglected or accounted for. If this mass is considered, a seemingly more accurate approach, wave reflections from the interface between the two media can occur resulting in marked undulations of the impedances with regard to frequency and, possibly, in negative stiffness. It is the purpose of the paper to show that such undulations are unrealistic and that the impedances featuring them can be less suitable for practical application than those obtained neglecting the mass of the inner zone.