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133 are conventionally used to investigate the seismic stability of studies will be required to advance design methods for buried engineered slopes and natural cut slopes. structures: The primary topics that require further study area are as follows: Methods suggested in Chapter 9 need to be tested on a range of pipe configurations, ground conditions, and earthquake The appropriate liquefaction strength to use when assessing shaking levels to confirm that the recommended approaches the stability of slopes comprised of or resting on liquefiable for TGD design are practical. Experimental studies involv- materials needs to be established. A number of issues about ing TGD also are needed to confirm the validity of the nu- the liquefaction strength remain difficult to quantify, and merical methods being suggested. these difficulties lead to uncertainty in design. Issues in- Further guidance needs to be developed for modeling clude simple methods of estimating the liquefied strength pipeline behavior in conditions where PGD occurs. These at locations involving sloping ground (that is, where a static developments include appropriate spring constants to shearing stress is imposed) and appropriate liquefied use in modeling soil-pipe interaction for moving ground strength values for cohesionless soil where limited defor- conditions. mations occur. Included within this topic is the potential The seismic effects of transient racking/ovaling deforma- for ratcheting movements and how to adequately represent tions on culverts and pipe structures need to be incorpo- this mechanism. rated into the updated CANDE analysis. It is anticipated Stability of rock slopes requires further evaluation. This topic that an option will be required in the CANDE program to was not addressed during this Project because of the com- allow ground displacement profile as a loading input to the plexity of the problem. Although a transparent approach CANDE analysis. does not seem possible, some additional guidance on factors to consider when conducting a site-specific seismic evalua- 10.5 Need for Confirming Methods tion would assist designers when they have to deal with rock slope stability. One clear conclusion from this Project was that various methods are available to the designer to use for the seismic design of retaining walls, slopes and embankments, and buried 10.4 Buried Structures structures. These methods range from simple equations to The buried structures portion of the Project provided design advanced numerical methods. The focus of this Project has equations for rigid and flexible culverts and pipelines subjected been to develop simple methods of analysis suitable for use in to TGD. Guidance also was provided on design considerations AASHTO LRFD Bridge Design Specifications. By focusing on for PGD such as might occur during liquefaction-induced simple methods, a number of simplifying assumptions and lateral spreading or seismic-induced embankment failures. Sec- approaches had to be taken. Whereas checks and then exam- tion 12 of the current AASHTO LRFD Bridge Design Specifica- ple problems were completed to test these proposed methods, tions does not cover seismic response of culverts and pipelines, additional test cases will be required to identify areas where the and therefore the developments summarized in this report simplifications are not appropriate, are too conservative, or address a current deficiency in the AASHTO Specifications. lack conservatism. For example, test cases involving advanced The treatment of buried structures in this Project was numerical methods or experimental centrifuge testing could be relatively limited in terms of levels of effort, and additional used to confirm the simplified methods.