Seismic Design and Analysis of Buried Culverts and Structures

This paper presents a method to analyze and evaluate buried culverts and cut-and-cover tunnels for seismic loading in addition to standard static loading from dead and live loads. Using CANDE-2007, a plane-strain finite-element program, the soil-structure problem is characterized by a cross-sectional slice through the structure and surrounding soil envelope. First, the static-design loads are applied with a series of incremental load steps. Next, the seismic loading condition is simulated by specifying quasistatic displacements at the peripheral boundaries of the soil envelope to produce a shear-racking distortion equivalent to the maximum free-field seismic shear strain from the design earthquake. The proposed method is based on two recently completed NCHRP projects and is presented here in detail. An initial linear-elastic study investigates two basic culvert shapes, circular and rectangular, wherein moment and thrust distributions from CANDE-2007 are shown to compare favorably with closed-form solutions. A second study investigates a nonlinear reinforced concrete box culvert installation under the combination of static and seismic loadings. Plots of moment, thrust, and shear distributions show how and where the seismic loading alters the response of static loading including the effect on safety factors for steel yielding, concrete crushing, and shear failure. It is concluded that the proposed seismic method is rational, easy to apply, and fulfills an engineering need heretofore unfulfilled. The procedure applies to any culvert shape, size, and material, and the safety of the culvert installation may be assessed either by working stress or load resistance factor design.