Stability of flexible conduits embedded in soil

Abstract

After final installation, underground flexible conduits are subjected mainly to ring compression. Therefore, it is important to check the stability of the conduit walls, which may govern the load carrying capacity of such composite soil–steel structures.The approaches developed by different investigators to deal with the problem of buckling are discussed and examined critically in the paper. This leads to the recommendation of the following formula for calculating the elastic buckling stress fb:[Formula: see text]in which E = modulus of elasticity of the conduit material; R = radius of the conduit; r = radius of gyration of the conduit walls; ψ = reduction factor for buckling stress of conduits under shallow cover h, where[Formula: see text]and[Formula: see text]in which E′ = modulus of soil reaction; I = moment of inertia per unit length of cross section of conduit wall; B = 1.22 for the sides and bottom in which the conduit walls displace towards the soil and B = 1.22[1 + 2(EI/E′R3)1/4] for the upper portion of the conduit wall where the radial displacements are towards the inside of the conduit.Inelastic buckling is considered to start when fb = half the yield stress fy. A parabolic equation is assumed for the transition zone between full yielding (kR/r = 0) and the elastic zone at fy/2:[Formula: see text]