Earthquake design procedures for rectangular underground structures

Abstract

Severe damage to six out of a total of 21 subway stations in the Kobe area during the 1995 Hyogoken-nanbu earthquake indicated a need for more attention to be given to the earthquake design of rectangular underground structures. The research report presents work undertaken to extend the present knowledge of the dynamic interaction of box-section structures with the surrounding soil, and a design method for predicting the earthquake loads on underground structures such as basement walls, tanks, subways, utility boxes, highway underpasses, and culverts.

Rectangular underground structures experience transverse racking deformations due to earthquake-induced shear strains in the surrounding soil. Deformation of the cross-section is usually more critical in design than the axial or curvature deformations induced along the axis of the structure. A pseudo-dynamic finite element procedure was used to generate the first mode earthquake-induced shear deformations in a range of soil profiles and to analyse the interaction of typical box structures embedded in the soil profile. A large number of analyses were undertaken to study the influence of the depth of cover, soil stiffness profile, relative stiffness of soil and structure, and the geometry of the box section. The accuracy of the simplified analysis procedure was verified against published results from more detailed time-history dynamic analyses.

A design procedure developed from the analysis work involves calculating a relative stiffness using the soil shear modulus and the box section dimensions. Charts are presented to enable the racking deformation induced in the structure by interaction with the soil, and the internal forces for typical single and multi-barrel structures, to be rapidly assessed from the free-field shear strain in the soil and the soil-structure stiffness ratio. For design, the earthquake free-field shear strain can be estimated by considering the response of a number of standard soil profiles analysed in the report. More sophisticated numerical analyses may be required for complex major underground structures but the method presented will assist preliminary design and analysis verification work.

Application of the simplified design method to several box culverts and pedestrian subway structures showed that the internal forces from earthquake induced racking deformations can be large in comparison to actions produced by gravity and water pressure loads. It was concluded that earthquake-induced deformations need to be considered in the design of most underground rectangular structures.