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15 A number of observations relative to the overall goals of this cedures have been based on post-earthquake evalua- Project can be made from the results of the literature review. tions of damage to water and sewer pipelines. The Further discussion is provided in Chapter 3. procedures consider both the TGD and PGD. Most examples of damage are associated with PGD. Pressures Retaining Walls on the walls of buried structures are typically estimated M-O equations are used almost exclusively to estimate using conventional earth pressure equations, including seismic active and passive earth pressure. Little atten- the M-O equations for seismic loading. tion seems to be given to the assumptions inherent to the Experimental studies have been conducted with cen- use of the M-O equations. The seismic coefficient used trifuges and shake tables to estimate the forces on cul- in the M-O equation is assumed to be some percent of verts and pipes that result from seismically induced the free field ground acceleration--typically from 50 to PGD. Only limited attention has been given to experi- 70 percent--and the soils behind the retaining structure mental studies involving the effects of TGD on pipelines are assumed to be uniform. and culverts. There is widespread acceptance, particularly in Europe, Observations from past earthquakes suggest that per- of displacement-based methods of design, although it is formance of culverts and pipe structures located beneath recognized that displacements are sensitive to the nature highway embankments has generally been good. This of earthquake time histories. good performance is most likely associated with the Only limited experimental data exist to validate the forces design procedures used to construct the embankment estimated for the design of retaining walls. These data are and backfill specifications for the culverts and pipes. from shake tables and centrifuge tests. In most cases they Typical specifications require strict control on backfill represent highly idealized conditions relative to normal placement to assure acceptable performance of the conditions encountered during the design of retaining culvert or pipe under gravity loads and to avoid settle- walls for transportation projects. ment of fill located above the pipeline or culvert, and The overall performance of walls during seismic events these strict requirements for static design lead to good has generally been very good, particularly for MSE walls. seismic performance. This good performance can be attributed in some cases The most common instances of culvert or pipe structure to inherent conservatism in the design methods cur- damage during past earthquakes is where lateral flow or rently being used for static loads. Slopes and Embankments spreading associated with liquefaction has occurred. In Except in special cases the seismic stability analysis for these situations the culvert or pipe has moved with the slopes and embankments is carried out with commer- moving ground. cially available limit-equilibrium computer codes. These codes have become very user friendly and are able to 2.3 DOT, Vendor, and handle a variety of boundary conditions and internal Consultant Contacts and external forces. Limited numbers of laboratory and field experiments Contacts were made with staff on the Project Team, staff have been conducted to calibrate methods used to esti- in geotechnical groups of DOTs, vendors, and other con- mate seismic stability or displacements. These experi- sultants to determine the availability of design guidelines to ments have used centrifuges to replicate very idealized handle seismic design of retaining walls, slopes and embank- conditions existing in the field. Usually the numerical ments, and buried structures. During these contacts, an method is found to give reasonable performance esti- effort also was made to determine the normal approach fol- mates, most likely because of the well-known boundary lowed when performing seismic design and analyses of conditions and soil properties. retaining walls, slopes and embankments, and buried struc- Slope and embankment performance during earthquakes tures. This was viewed as a key step in the data collection has varied. Most often slopes designed for seismic load- and review process, as the procedures used by this group of ing have performed well. The exception has been where practitioners represent the current state-of-the practice and liquefaction has occurred. The most dramatic evidence of should form the starting point for the development of any seismically induced slope instability has occurred for new methodology. oversteepened slopes, where the static stability of the Some of the key design guides and references identified slope was marginal before the earthquake. from these contacts are summarized here: Buried Structures A number of procedures have been suggested for the Caltrans: Contacts with California Department of Trans- design of culverts and pipelines. Most often these pro- portation (Caltrans) personnel focused on the design

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16 requirements for retaining walls and the approach used to site-adjusted PGA with a target factor of safety of 1.1. evaluate seismic slope stability. Caltrans personnel con- Newmark-type analyses were allowed where an esti- firmed that the retaining wall design requirements are doc- mate of deformations was needed. umented in the Caltrans Bridge Design Specifications dated Seismic earth pressures on walls were determined using August 2003. Specifications include a 14-page Part-A on the M-O equations. WSDOT staff specifically pointed General Requirements and Materials and 106-page Part-B out the difficulties that they have had in dealing with high on Service Load Design Method, Allowable Stress Design. acceleration values and steep back slopes when using the Some of the key design requirements for retaining walls M-O equations. include the following: ODOT and ADOT&PF: Both the Oregon Department of A minimum factor of safety of 1.3 for static loads on Transportation (ODOT) and the Alaska Department of overall global stability. Transportation and Public Facilities (ADOT & PF) have A minimum factor of safety of 1.0 for design of retain- recently worked on developing guidelines for addressing ing walls for seismic loads. the effects of liquefaction on embankment stability. Some Seismic forces applied to the mass of the slope based on of this information is useful for addressing the response a horizontal seismic acceleration coefficient (kh) equal of slopes in liquefiable soils. to one third of the site-adjusted PGA, the expected peak Vendors: Design methods used by several vendors of MSE acceleration produced by the maximum credible earth- walls (for example, Keystone, Hilfiker, and Mesa) were quake. Generally, the vertical seismic coefficient (kv) is reviewed. Generally, these vendors followed methods considered to equal zero. recommended by FHWA. Both the inertial force within Caltrans specifications go on to indicate that if the factor of the reinforced zone and the dynamic earth pressure from safety for the slope is less than 1.0 using one-third of the M-O earth pressure calculations were used in external sta- site-adjusted PGA, procedures for estimating earthquake- bility evaluations. Guidelines also were provided for eval- induced deformations, such as the Newmark Method, may uation of internal stability in the approach used by some be used provided the retaining wall and any supported vendors. structure can tolerate the resulting deformations. Consultants: Contacts also were made with geotechnical WSDOT: Initial contacts with WSDOT's geotechnical engineers and structural designers to determine what staff focused on WSDOT's involvement in develop- they perceived as the important issues for seismic design ing technical support for load and resistance factors used of retaining walls, slopes and embankments, and buried in geotechnical design. While this work was not specifi- structures. Below is a list of some of the issues identified cally directed at seismic loading, both the methodology from this limited survey: and the ongoing work through the AASHTO T-3 group There was consensus that there needs to be clarification appeared to be particularly relevant to Phase 2 of this on the responsibility between geotechnical engineers Project. WSDOT efforts included evaluation of load and and structural engineers in the overall design process. resistance factors through Monte Carlo simulations. The view was that a lack of communication occurs Subsequent discussions took place with WSDOT on seis- between the two parties resulting in much confusion at mic design methods for retaining walls in general and times. MSE walls in particular. One key concern on the part The design practice varied tremendously from state to of WSDOT was how to incorporate load and resistant state and from project to project on many fundamental factors in the seismic design process. This concern was requirements, including whether retaining walls need particularly critical in the use of the M-O procedure for to be designed for the seismic load case at all. A com- determining seismic earth pressures. WSDOT found mon practice was to design retaining walls for static that if no resistance factors were applied to the dynamic loading only with its inherent factor of safety, and case, as suggested in NCHRP 12-49 Project report and many designers believed that retaining walls have per- other similar documents, it was possible that the seis- formed well in past earthquakes and traditional static mic earth pressure will be lower than the static earth design practice and its inherent conservatism were pressure determined using load and resistance factors in adequate. the AASHTO LRFD Bridge Design Specifications. WSDOT A major objective in future effort should be to devote also provided a preliminary copy of their draft seis- some effort to clarifying basic steps involved in design- mic design requirements for retaining walls, slopes, and ing retaining walls. embankments. Pseudo-static methods are typically used to evaluate sta- For pseudo-static analyses, WSDOT proposed using a bility of slopes and embankments during seismic load- horizontal seismic coefficient equal to 0.5 times the ing. There seems to be a divergence of opinion on the