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105 CHAPTER 9 Buried Structures This chapter provides results of analyses and sensitivity above-ground structures. Seismic performance records for studies conducted for buried structures. These studies dealt culverts and pipelines have been very favorable, particu- with the TGD and not PGD. The primary objectives of the larly when compared to reported damages to other highway/ TGD work were to: transportation structures such as bridges. The main reason for the good performance of buried struc- Identify methodologies for evaluating the ovaling response tures has been that buried structures are constrained by the of circular conduits, as well as the racking response of rec- surrounding ground. It is unlikely that they could move to any tangular conduits, and significant extent independent of the surrounding ground or Conduct parametric studies and parametric evaluations be subjected to vibration amplification/resonance. Compared for the methods being proposed. to surface structures, which are generally unsupported above their foundations, buried structures can be considered to dis- Results of analyses conducted to address these objectives are play significantly greater degrees of redundancy, thanks to the summarized in the following sections. These analyses focused support from the ground. The good performance also may be on deriving a rational procedure for seismic evaluation of partly associated with the design procedures used to construct buried culverts and pipelines that consider the following sub- the embankment and backfill specifications for the culverts jects: (1) general properties and characteristics of culverts and and pipes. Typical specifications require close control on pipes, (2) potential failure modes for buried culverts and pipes backfill placement to assure acceptable performance of the subject to seismic loading, (3) procedures used in current de- culvert or pipe under gravity loads and to avoid settlement of sign practice to evaluate seismic response of buried structures, fill located above the pipe or culvert, and these strict require- (4) derivation of detailed rational procedures for seismic eval- ments for static design lead to good seismic performance. uation of both rigid and flexible culverts and pipes subject to It is important that the ground surrounding the buried TGD, taking into consideration soil-structure interaction, and structure remains stable. If the ground is not stable and large (5) providing recommendations on a general methodology PGD occur (for example, resulting from liquefaction, settle- for seismic evaluation under the effects of PGD. These results ment, uplift, lateral spread, or slope instability/landslide), consider both flexible and rigid culverts, burial depths that then significant damage to the culvert or pipe structures can range from 0.5 to 5 diameters, various cross-sectional geome- be expected. Although TGD due to shaking also can damage tries (for example, circular and rectangular) and wall stiff- buried structures, compared to the effects of PGD, the damage nesses, and different properties of the surrounding soil. is typically of a more limited extent. 9.1 Seismic Performance of Culverts 9.2 Culvert/Pipe Characteristics and Pipelines Culvert/pipe products are available over a large range in Damage to buried culverts and pipelines during earth- terms of material properties, geometric wall sections, sizes, quakes has been observed and documented by previous in- and shapes. Pipe sizes as small as 1 foot and as large as culverts vestigators (NCEER, 1996; Davis and Bardet, 1999 and 2000; with spans of 40 feet and larger are used in highway applica- O'Rourke, 1999; Youd and Beckman, 2003). In general, buried tions. They can be composed of concrete, steel, aluminum, structures have performed better in past earthquakes than plastic, and other materials. Detailed information about their