Seismic design and analysis of underground structures

Abstract

Underground facilities are an integral part of the infrastructure of modern society and are used for a wide range of applications, including subways and railways, highways, material storage, and sewage and water transport. Underground facilities built in areas subject to earthquake activity must withstand both seismic and static loading. Historically, underground facilities have experienced a lower rate of damage than surface structures. Nevertheless, some underground structures have experienced significant damage in recent large earthquakes, including the 1995 Kobe, Japan earthquake, the 1999 Chi-Chi, Taiwan earthquake and the 1999 Kocaeli, Turkey earthquake. This report presents a summary of the current state of seismic analysis and design for underground structures. This report describes approaches used by engineers in quantifying the seismic effect on an underground structure. Deterministic and probabilistic seismic hazard analysis approaches are reviewed. The development of appropriate ground motion parameters, including peak accelerations and velocities, target response spectra, and ground motion time histories, is briefly described. In general, seismic design loads for underground structures are characterized in terms of the deformations and strains imposed on the structure by the surrounding ground, often due to the interaction between the two. In contrast, surface structures are designed for the inertial forces caused by ground accelerations. The simplest approach is to ignore the interaction of the underground structure with the surrounding ground. The free-field ground deformations due to a seismic event are estimated, and the underground structure is designed to accommodate these deformations. This approach is satisfactory when low levels of shaking are anticipated or the underground facility is in a stiff medium such as rock. Other approaches that account for the interaction between the structural supports and the surrounding ground are then described. In the pseudo-static analysis approach, the ground deformations are imposed as a static load and the soil-structure interaction does not include dynamic or wave propagation effects. In the dynamic analysis approach, a dynamic soil structure interaction is conducted using numerical analysis tools such as finite element or finite difference methods. The report discusses special design issues, including the design of tunnel segment joints and joints between tunnels and portal structures. Examples of seismic design used for underground structures are included in an appendix at the end of the report.

Introduction

Underground structures have features that make their seismic behavior distinct from most surface structures, most notably (1) their complete enclosure in soil or rock, and (2) their significant length (i.e. tunnels). The design of underground facilities to withstand seismic loading thus, has aspects that are very different from the seismic design of surface structures.

This report focuses on relatively large underground facilities commonly used in urban areas. This includes large-diameter tunnels, cut-and-cover structures and portal structures (Fig. 1). This report does not discuss pipelines or sewer lines, nor does it specifically discuss issues related to deep chambers such as hydropower plants, nuclear waste repositories, mine chambers, and protective structures, though many of the design methods and analyses described are applicable to the design of these deep chambers.

Large-diameter tunnels are linear underground structures in which the length is much larger than the cross-sectional dimension. These structures can be grouped into three broad categories, each having distinct design features and construction methods: (1) bored or mined tunnels; (2) cut-and-cover tunnels; and (3) immersed tube tunnels (Power et al., 1996). These tunnels are commonly used for metro structures, highway tunnels, and large water and sewage transportation ducts.

Bored or mined tunnels are unique because they are constructed without significantly affecting the soil or rock above the excavation. Tunnels excavated using tunnel-boring machines (TBMs) are usually circular; other tunnels maybe rectangular or horseshoe in shape. Situations where boring or mining may be preferable to cut-and-cover excavation include (1) significant excavation depths, and (2) the existence of overlying structures.

Cut-and-cover structures are those in which an open excavation is made, the structure is constructed, and fill is placed over the finished structure. This method is typically used for tunnels with rectangular cross-sections and only for relatively shallow tunnels (<15 m of overburden). Examples of these structures include subway stations, portal structures and highway tunnels.

Immersed tube tunnels are sometimes employed to traverse a body of water. This method involves constructing sections of the structure in a dry dock, then moving these sections, sinking them into position and ballasting or anchoring the tubes in place.

This report is a synthesis of the current state of knowledge in the area of seismic design and analysis for underground structures. The report updates the work prepared by St. John and Zahrah (1987), which appeared in Tunneling Underground Space Technol. The report focuses on methods of analysis of underground structures subjected to seismic motion due to earthquake activity, and provides examples of performance and damage to underground structures during recent major earthquakes. The report describes the overall philosophy used in the design of underground structures, and introduces basic concepts of seismic hazard analysis and methods used in developing design earthquake motion parameters.

The report describes how ground deformations are estimated and how they are transmitted to an underground structure, presenting methods used in the computation of strains, forces and moment in the structure. The report provides examples of the application of these methods for underground structures in Los Angeles, Boston, and the San Francisco Bay Area.

This report does not cover issues related to static design, although static design provisions for underground structures often provide sufficient seismic resistance under low levels of ground shaking. The report does not discuss structural design details and reinforcement requirements in concrete or steel linings for underground structures. The report briefly describes issues related to seismic design associated with ground failure such as liquefaction, slope stability and fault crossings, but does not provide a thorough treatment of these subjects. The reader is encouraged to review other literature on these topics to ensure that relevant design issues are adequately addressed.