CKI-24: SEISMIC RESPONSE OF UNDERGROUND STRUCTURES IN SOFT SOILS
• J. Enrique Luco
As part of a research project on the seismic response of underground structures embedded in soft soils we have completed the following tasks:
(i) Derivation of correct 'radiation' conditions to determine the reflected wave field in viscoelastic media. This is a critical step in the calculation of the deterministic free-field ground motion.
(ii) Development and validation of a method to calculate the two- and three-dimensional response of unlined tunnels (cylindrical cavities) embedded in layered viscoelastic media and subjected to obliquely incident waves.
(iii) Development and validation of two independent methods to obtain the two- and three-dimensional response of cylindrical pipelines or lined tunnels buried in layered viscoelastic media and subjected to obliquely incident waves. The first method is based on an analytic Donnell model for the shell and applies to tunnels or
pipelines of circular cross-section. The second method is based on a finite element model for the shell and applies to tunnels of arbitrary cross-section.
(iv) Development of a hybrid formulation and of basic subprograms to calculate the response of a flexible axisymmetric tank and a lined vertical shaft subjected to obliquely incident seismic waves.
The research effort was concentrated on the development and validation of methods to calculate the seismic response of underground structures subjected to spatially varying ground motions. For the purposes of the study the soils were represented as horizontally layered viscoelastic half-spaces.
Two classes of structures were considered. The flrst class included buried cylindrical pipelines or tunnels which can be considered to be infinitely long. In this case, excitations in the form of incident waves impinging in a direction normal to the axis of the pipeline or tunnel lead to two-dimensional plane-strain problems for P, SV and Rayleigh waves and to two-dimensional anti-plane shear problems for SH-waves. In the more general case of excitations at an oblique angle with respect to the axis of the pipeline or tunnel, the response is fully three-dimensional.
The second class of structures considered in the project includes flexible axisymmetric shells with a vertical axis of symmetry. The structures are buried in a layered viscoelastic medium representing the soil. The shells represent flexible underground tanks and lined vertical shafts. The shells are subjected to spatially varying ground motions in the form of non-vertically incident waves or spatially random ground motions.
2. Spatial Variation of the Free-Field Ground Motion
3. Response of Pipelines and Tunnels to Obliquely Incident Waves
4. Response of Underground Tanks and Vertical Shafts to Spatially Varying Ground Motions
6. List of Publications Resulting from Project
July 15, 1992 - January 14, 1993