Soil-structure interaction analysis of buried rigid pipes under surface load

Abstract

The effect of the surface live load on buried rigid pipe has been investigated based on soil-pipe interaction analysis. Two-dimensional and three-dimensional finite element analysis was performed to investigate the interaction between the pipe and soil under surface load. For the finite element aJ.1alysis, a mesh-sensitivity analysis was carried out to eliminate errors associated with the density of mesh as well as extent of the boundary taken into consideration. The analyses were performed using linear elastic and elastoplastic material models. A parametric study was conducted to understand the effects of pipe and soil modulus on the pipe responses (i.e. thrusts, moments on pipe wall) calculated from two dimensional and three dimensional analysis. The study indicates that 20 plane strain idealization is valid for pipe with low modulus while a three dimensional analysis will be required for concretc pipes or pipes with stiffer materials. Finite element results were compared with different design standards for shallow burial condition. It is observed that ASCE and AASHTO design standards provide 30 to 50 percent unconservative estimate with respect to finite element results due to the effccts oflive load in region near to crown. The study revealed that thc pattern of stress distributions, due to concentrated surfacc load, in longitudinal and transverse direction of the pipe at crown level is almost similar. The maximum stress was directly under the load, which decayed gradually away from the load. However, a tensile stress develops at the pipe-soil interface at some distance from the load in longitudinal direction when the pipe is buried at a depth less than 0.5 times the diameter. Stress distributions in longitudinal direction of the pipe showed that stresses deerease and become insignificant at a short distance (1.5 m) away from the pipe. Thus, stresses reaching to the pipe from any wheel load may not be affected by the stresses from the other wheel of the same axle if the wheels are located at a distance farther than 1.5 m. Pipe with greater stiffness appeared to attraet greater surfaee load due to arching. Three-dimensional finite element analysis appeared successful in predicting the observed response of full-scale test pipes under surface live load. The test, undertaken by Ontario Concrete Pipe Association in Canada, was modeled effectively using the' FE method. The three-dimensional analysis was then used to develop influence lines for different stresses for various positions of the concentrated surface load with respect to the pipe axis. The influence lines for stresses on the pipe and the surrounding soil are found useful to calculate the ground stresses developing due to the live load.