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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 24
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Suggested Citation:"Chapter 3 - Problems and Knowledge Gaps." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
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18 The goal of Task 2 of the NCHRP 12-70 Project was to identify, illustrate, and document problems and knowledge gaps in current seismic analysis and design of retaining wall, slopes and embankments, and buried structures. This objective was based on the Task 1 data collection and review, as well as the Project Team’s experience gained from conducting seismic design studies for retaining walls, slopes and embankments, and buried structures in seismically active areas. The discussion of knowledge gaps and problems is organized in four subsections. The first three summarize knowledge gaps and problems for retaining walls, slopes and embankments, and buried struc- tures, respectively. The final section provides key conclusions about knowledge gaps and problems. As with the previous chapter, the primary focus of this effort was to identify prac- tical problems and knowledge gaps commonly encountered by design engineers when conducting seismic design studies. 3.1 Retaining Walls The discussion of problems and knowledge gaps for retain- ing walls focused on three primary types of retaining walls: gravity and semi-gravity walls, MSE walls, and soil nail walls. Various other categories of walls exist, such as nongravity can- tilever walls and anchored walls. The discussions for gravity and semi-gravity walls are generally relevant to these other walls as well, though additional complexity is introduced from the constraints on deformation resulting from the structural system and the need to meet structural capacity requirements. 3.1.1 Gravity and Semi-Gravity Walls Current AASHTO Specifications use the well-established M-O equations developed in the 1920s for determining pseudo-static seismic active earth pressures behind conven- tional gravity or semi-gravity retaining walls (that is, cast- in-place gravity walls or cast-in-place concrete cantilever or counterfort walls), where the maximum inertial forces acting on the wall and backfill soil are computed from the peak ground acceleration coefficient at ground level. This approach is still widely used in general geotechnical practice since being suggested as a standard method by Seed and Whitman (1970). A number of problems and related knowledge gaps with the above approach have been identified, as discussed in the fol- lowing subsections. 3.1.1.1 Use of M-O Approach for Seismic Earth Pressures The following problems are encountered when using the M-O equations for the determination of seismic earth pressures: • How to use the M-O equations for a backfill that is pre- dominantly clayey, for a soil involving a combination of shear strength derived from both c (cohesion of the soil) and φ (friction angle of the soil), or where backfill conditions are not homogenous. • How to use the M-O equations for sloping ground behind the wall where an unrealistically large seismic active earth pressure coefficient can result. • How to use the M-O equations when high values of the selected seismic coefficient cause the M-O equation to degenerate into an infinite earth pressure. These concerns reflect the limitations of the M-O equations as discussed in the Commentary within the NCHRP Project 12-49 Guidelines (NCHRP Report 472, 2002). As noted in the commentary, these limitations in the M-O approach are the result of basic assumptions used in the derivation of the M-O methodology. For the case of seismic active earth pressures, the M-O equation is based on the Coulomb failure wedge assumption and a cohesionless backfill. For high accelera- tions or for steep backslopes, the equation leads to excessively high pressures that asymptote to infinity at critical accelera- C H A P T E R 3 Problems and Knowledge Gaps

19 tion levels or backslope angles. For the latter conditions, no real solutions to the equation exist implying equilibrium is not possible. A horizontal backfill with a friction angle for sand of 40 degrees, a wall friction angle of 20 degrees, and a peak acceleration coefficient of 0.4 has a failure surface angle of 20 degrees to the horizontal. It will lead to very large seis- mic earth pressures due to the size of the failure wedge. For a peak acceleration coefficient of 0.84, the active pressure becomes infinite, implying a horizontal failure surface. Since many areas along the West Coast and Alaska involve peak ground accelerations in excess of 0.3g and it is common to have a backslope above the retaining wall, it is not uncommon for the designers to compute what appear to be unrealistically high earth pressures. In practical situations cohesionless soil is unlikely to be pres- ent for a great distance behind a wall and encompass the entire critical failure wedge under seismic conditions. In some cases, free-draining cohesionless soil may only be placed in the static active wedge (say at a 60 degrees angle) with the remainder of the soil being cohesive embankment fill (c − φ soil), natural soil, or even rock. Under these circumstances, the maximum earthquake-induced active pressure could be determined using trial wedges as shown in Figure 3-1, with the strength on the fail- ure planes determined from the strength parameters for the soils through which the failure plane passes. This approach (in effect the Culmann method identified for use with non- cohesionless backfill in the 2007 AASHTO LRFD Bridge Design Specifications for static wall design) will provide more realis- tic estimates of seismic active pressure. The above problem becomes further unrealistic in the case of a sloping backfill, where earthquake active pressures become rapidly infinite for small seismic coefficients and relatively shallow slope angles, as illustrated in Figure 3-2. As discussed in Chapter 4, these problems with the M-O active earth pressure equation appear to be avoidable through the use of commercially available computer programs based on the method slices—the same as conventionally used for slope stability analyses. This approach can be used to com- pute earthquake active earth pressures for generalized and nonhomogeneous soil conditions behind a retaining wall. The determination of seismic passive earth pressures using the M-O equation for passive earth pressure also suffers limi- tations. In many cases the soil is not a homogeneous cohesion- less soil. However, more importantly, the use of the Coulomb failure wedge is not necessarily conservative, potentially result- ing in an underestimation of passive pressures. For some cases (for example, where the wall height is shallow), a sufficient approach for the computation of seismic passive earth pres- sures can be the use of the static passive earth pressure equa- tions, as discussed in the NCHRP 12-49 guidelines (NCHRP Report 472, 2002). However, this approach fails to consider the earthquake inertial effects of the soil within the passive pres- sure zone. A preferred approach involves use of a log spiral method that incorporates seismic effects, as described by Figure 3-1. Trial wedge method for determining critical earthquake induced active forces.

Shamsabadi et al. (2007). The passive case is important for establishing the resisting force at the toe of semi-gravity walls or for the face of a sheet pile wall or a cantilever wall comprised of tangent or secant piles. 3.1.1.2 Wall Sliding Assumption The concept of allowing walls to slide during earthquake loading and displacement-based design (that is, assuming a Newmark sliding block analysis to compute displacements when accelerations exceed the horizontal limit equilibrium yield acceleration) was introduced by Richards and Elms (1979). Based on this concept, Elms and Martin (1979) sug- gested that a design acceleration coefficient of 0.5A in M-O analyses would be adequate for a limit equilibrium pseudo- static design, provided allowance be made for a horizontal wall displacement of 10A inches. The coefficient “A” used in this method was the peak ground acceleration (in gravitational units, g) at the base of the sliding soil wedge behind the retain- ing wall. This concept was adopted by AASHTO in 1992, and is reflected in current AASHTO LRFD Bridge Design Specifica- tions. However, the concept is not well understood in the design community, as designers often use values of 33 to 70 percent of the peak ground acceleration for pseudo-static design without a full understanding of the rationale for the reduction. Observations of the performance of conventional semi- gravity cantilever retaining walls in past earthquakes, and in particular during the Hyogoken-Nambu (Kobe) earthquake in 1995, have found significant tilting or rotation of walls in addition to horizontal deformations, reflecting cyclic bearing capacity failures of wall foundations during earthquake load- ing. To represent permanent wall deformation from mixed sliding and rotational modes of deformation using Newmark block failure assumptions, it is necessary to formulate more complex coupled equations of motions as described, for exam- ple, by Siddharthen et al. (1992) and Peng (1998). A coupled deformation approach also has been documented in the MCEER report Seismic Retrofitting Manual for Highway Struc- tures: Part 2—Retaining Walls, Slopes, Tunnels, Culverts, and Roadways (MCEER, 2006). Peng (1998) indicates that such an analytical approach (including P-Δ effects) appears to provide a reasonable simulation of observed rotational and sliding wall deformations in the Kobe earthquake. From the standpoint of performance criteria for the seismic design of new conventional retaining walls, the preferred design approach is to limit tilting or a rotational failure mode by ensuring adequate factors of safety against foundation bear- ing capacity failures and to place the design focus on perfor- mance criteria that ensures acceptable sliding displacements. For weaker foundation materials, this rotational failure require- ment may result in the use of pile or pier foundations, where lat- eral seismic loads would of necessity be larger than those for a sliding wall. For retrofit design, the potential for wall rotation may have to be studied, but retrofit design is not within the scope of the proposed AASHTO specifications for this Project. 3.1.1.3 Rigid Block Sliding Assumption Much of the recent literature on the seismic analysis of con- ventional retaining walls, including the European codes of practice, focuses on the use of Newmark sliding block analysis methods. The basic assumption with this approach is the soil in the failure wedge behind the retaining wall responds as a rigid mass. Intuitively, for short walls, the concept of a backfill failure zone deforming as a rigid block would seem reasonable. However, for very high walls, the dynamic response of the soil in the failure zone could lead to nonuniform accelerations with height and negate the rigid block assumption. Wall flexibility also could influence the nature of soil-wall interaction. A number of finite element or finite difference numerical response analyses have been published in recent years, model- ing the dynamic earthquake response of cantilever walls. Unfor- tunately, many of these analyses are based on walls founded on soil layers leading to wall rotation. In addition, numerical diffi- culties in modeling interface elements between structural and soil elements, along with problems modeling boundary condi- tions, tend to cloud the results. Many of the analyses use only one wall height, usually relatively high—greater than 30 feet, for example. 20 Figure 3-2. Effect of backfill slope on the seismic active earth pressure coefficient using M-O equations.

Many conventional gravity retaining wall designs involve heights between 5 and 30 feet for economic reasons, with MSE walls being favored for greater wall heights. For this range of heights, and considering the frequency range of likely input ground accelerations, the rigid block assumption is probably adequate; however, as discussed in the next chapter additional studies were required to confirm this expectation. 3.1.1.4 Earthquake Time Histories for Wall Displacement Analyses The existing AASHTO Specifications use an empirical equa- tion based on peak ground acceleration to compute wall dis- placements for a given wall yield acceleration. This equation was derived from studies of a limited number of earthquake accelerograms. However, recent studies including publica- tions related to the seismic response of retaining walls have clearly indicated the sensitivity of displacement computations (based on Newmark sliding block analyses) to the frequency and duration characteristics of earthquake acceleration records. Studies by Martin and Qiu (1994) showed sensitivity to both peak accelerations and peak ground velocity. Whereas site-specific design time histories could be developed for projects, the approach identified in Chapter 4 involved developing new design charts reflecting differences between WUS and CEUS time histories. To develop these charts, it was necessary to have separate sets of time histories representative of WUS and CEUS characteristic earthquakes. As will be discussed in the next chapter, a database of these records was available for use on this Project for developing the proposed charts. 3.1.2 MSE Retaining Walls MSE walls generally have performed well in past earthquakes, based on case histories reported in the Northridge, Kobe, and Nisqually earthquakes. Minor damage patterns included ten- sion cracks on soil behind reinforced zones and cracking of con- crete facing panels. In some cases significant wall displacements were observed. For example, roughly 12 and 6 inches of lateral displacements at the top and bottom of a 20-foot-high wall in Kobe were noted, where ground accelerations were 0.7g. Such minor damage did not affect the integrity or stability of wall, and the wall continued to function. Based on the above evidence, it could be argued that cur- rent seismic design methods for MSE walls are adequate. However, the lack of monitoring data and the lack of case his- tories for wall heights greater than 30 to 50 feet, together with the limitations and uncertainties of current design method- ologies, suggest that improvements in design approaches are still needed. As an extreme example of this need, an MSE wall with a height of over 100 feet was designed and constructed for the third runway extension at the Seattle–Tacoma Inter- national Airport. The firm-ground PGA value for this site will vary from approximately 0.3g to 0.6g for return periods rang- ing from 500 to 2,500 years. The combination of large PGA and very high wall height poses questions as to the appropri- ate seismic coefficient to use for design. Whereas model studies using centrifuge or large shaking tables, together with numerical analyses using finite element of finite difference programs, are providing insight on the complex physical behavior of MSE walls under seismic load- ing, current practical design approaches described in the lit- erature rely on pseudo-static, limit-equilibrium analyses, such as those used for conventional gravity walls. Data from such models or numerical studies often are used to calibrate pseudo-static approaches, which have been developed over the past 20 years. Based on the literature survey carried out for Task 1 of this Project, the following general observations summarize the data gaps and uncertainties related to aspects of published design approaches using limit equilibrium analyses of MSE walls. • Limit equilibrium approaches to the seismic design of MSE walls entail consideration of the following two stability modes: – Internal or local stability, which considers the potential for rupture or pullout of tensile reinforcement; and – External or global stability, which considers the over- turning or sliding stability of the reinforced fill, assumed a coherent mass. • Existing design guidelines or procedures use different assumptions in addressing internal stability. Current AASHTO guidelines assume inertial forces act only on the static active pressure zone, leading to additional tensile forces in reinforcement strips. A horizontal acceleration coefficient kh = (1.45-A) A is used to determine inertial loading, where A is the peak ground acceleration coefficient. This empirical equation reflects potential amplification of low ground accelerations in the reinforced zone. A maxi- mum acceleration of 0.45g is assumed reflecting a potential sliding failure mode at this acceleration level. Choukeir et al. (1997) describe a procedure where kh is a function of the natural frequency of the reinforced soil mass and the domi- nant earthquake input frequency. To improve design guide- lines, a better understanding of the influence of reinforced fill height and stiffness and the frequency characteristics of input motions on design acceleration levels is needed. It is also clear that the geometry of the earthquake-induced active pressure zone will be influenced by the level of accel- eration. The Bathhurst and Cai (1995) analysis method adopted in the 2006 MCEER report Seismic Retrofit Guide- lines for Highway Structures (MCEER, 2006) assumes a seis- mic active pressure zone defined by the M-O Coulomb 21

failure surface and is used in conjunction with maximum ground accelerations. Other analytical approaches search for a critical active pressure zone defined by a bi-linear failure surface. • External stability is addressed in most guidelines by assum- ing the M-O method for determining the earthquake- induced active earth pressures in the fill behind the rein- forced soil mass. To evaluate the potential for sliding, the AASHTO LRFD Bridge Design Specifications assume only 50 percent of the earthquake active pressure acts in con- junction with the reinforced soil mass inertial load on the assumption that the two components would not be in phase, which is questionable and requires further evaluation. In addition, the limitations and problems with the use of the M-O equations for external stability assessments are simi- lar to those previously described for conventional semi- gravity retaining walls, and along with performance criteria based on allowable wall displacements, can be addressed in a similar manner to approaches described for semi-gravity walls. As discussed in the next chapter, studies related to wall height/stiffness and ground motion dependent seismic coefficients for design, along with improved approaches for evaluation of internal and external seismic stability, are clearly needed. 3.1.3 Soil Nail Walls Soil nail walls act in a similar manner to MSE walls, but are typically a ground reinforcement technique used for cut slopes as opposed to fill slopes in the case of MSE walls. As described in an FHWA Geotechnical Engineering Circular No. 7 Soil Nail Walls (FHWA, 2003), soil nail walls have per- formed remarkably well during strong earthquakes, with no sign of distress or permanent deflection. Choukeir et al. (1997) note a seismic design methodology similar to that previously described for MSE walls. Caltrans have developed a computer program SNAIL for the design of soil nail walls based on a limit equilibrium approach using a two-wedge or bilinear failure surface for both inter- nal and external stability considerations, including the spec- ification of horizontal and vertical seismic coefficients. The computer program GOLDNAIL also is widely used in prac- tice during the design of soil nails. This software also can be used to evaluate the performance of anchored walls by replacing the nail with a tendon having a specified strength and pullout capacity As the design issues for MSE and soil nail walls are gener- ally similar, analysis methods for development were also somewhat similar, with potential applications of the SNAIL and GOLDNAIL programs also requiring review. 3.2 Slopes and Embankments The dominant theme in the literature on the topic of eval- uating the seismic stability or performance of slopes and embankments was the use of either pseudo-static or the New- mark sliding block methods of analysis. Whereas dynamic response analyses (particularly of large earth structures such as dams) using computer programs such as FLAC were finding increasing use, for routine seismic design of slopes and embankments related to highways, the pseudo-static method has found wide acceptance, while the use of New- mark sliding block deformation method was gaining favor, particularly where pseudo-static methods resulted in low factors of safety. Often results of the deformation analysis indicated that the amount of deformation for a slope or embankment was tolerable, say less than 1 to 2 feet, even when the factor of safety from the pseudo-static analysis is less than 1.0. 3.2.1 Seismic Considerations for Soil Slopes A number of considerations relative to the seismic analysis of slopes and embankments are summarized below. • As described in both the MCEER (2006) Seismic Retro- fitting Manual for Highway Structures and the SCEC (2002) Guidelines for Analyzing and Mitigating Landslide Hazards in California, recommended practice for the analysis of seismic slope or embankment performance is a displacement-based analysis using a Newmark sliding block approach. This approach also was adopted by the NCHRP 12-49 Project for evaluating liquefaction-induced lateral spread displacement at bridge approach fills or slopes. • Newmark displacements provide an index of probable seis- mic slope performance. As a general guideline, a Newmark displacement of less than 4 inches often is considered to rep- resent a “stable” slope, whereas more than 12 inches is con- sidered unstable from a serviceability standpoint. Several design charts correlating Newmark displacement with the ratio of yield acceleration (defined as the acceleration required to bring the factor of safety 1.0) to the peak acceler- ation exist. The approach identified in Chapter 4 involved review of the existing data for the purpose of developing improved design charts applicable to nationwide seismic hazard conditions—with different charts produced for WUS versus CEUS sites. • As previously discussed for retaining wall design, studies described in the literature suggest that displacement-based analyses are very sensitive to the frequency and amplitude characteristics of earthquake acceleration time histories and to earthquake duration, together with the earthquake response characteristics of higher walls, slopes, or embank- 22

ments. Whereas design charts or simplified expressions are available to provide design guidance, improvements were needed to better reflect the above variables and to provide a basis for nationwide application and to use as a screening tool to establish “no seismic analysis” criteria based on appropriate serviceability criteria. Caltrans guidelines, for example, use a “no analysis” screening criteria based on pseudo-static factors of safety greater than 1.1 when a seis- mic coefficient of 1⁄3 of the maximum ground acceleration was used. • For slopes and embankments of limited height, say less than about 30 to 40 feet, the assumption of a rigid sliding block and the use of ground acceleration parameters to define inertial lateral forces was thought to be a reasonable approx- imation. For higher slopes and embankments, however, where the dynamic response of the sliding mass may influ- ence displacement magnitudes, modifications to computed Newmark displacements were required, depending on the comparative natural period characteristics of the earth- quake ground motion and the slope. Such modifications are included, for example, in the design methods documented in the SCEC (2002) recommended procedures. An approach for analytical development is described in Chapter 4 to address this issue. 3.2.2 Seismic Considerations for Rock Slopes Rock slopes are encountered in many situations—both urban and mountainous terrain. Some considerations related to these types of slopes are summarized below. • In regularly bedded or foliated rock, cut by joints, there are many possibilities for block movement along weak planes. Where there are multiple sets of discontinuous planes intersecting at oblique angles, three failure modes must be examined: plane sliding, wedge sliding, and toppling. A plane slide can form where a block of rock rests on an inclined plane that dips downward and intersects the face of the slope. A wedge slide can occur where two planes of weakness intersect to define a tetrahedral block. Toppling failure can develop from overturning of certain types of rock, such as slates and schists, that have bedding planes inclined steeply into the hillside. • In practical solutions, the plane failure is examined using a two-dimensional limit equilibrium approach treating the seismic inertial load as a constant horizontal acceleration acting on the potential failure block. For the wedge failure, three-dimensional limit equilibrium wedge analyses using stereographic projection of joints and open free surface orientations are used for gravity loading. While the con- sideration of seismic loads in terms of pseudo-static accel- eration can readily be implemented for the plane failure which can be carried out with most two-dimensional slope stability programs, a wedge failure under seismic excitation is not widely analyzed. Analyses for the toppling failure, which generally involves moment equilibrium, rarely are used in practice due to the complexity of the problem and lack of adequate rock properties for carrying out meaning- ful solutions. • Often the seismic performance of the rock slope is expressed in terms of a pseudo-static factor of safety. The challenge faced by the practicing engineer involves assigning appro- priate shear strength parameters on the weakness plane where sliding is anticipated. Some engineers may be reluc- tant to assign cohesion to the joint surface due to lack of ‘stickiness’ as found in a clayey soil. In fact, this assumed cohesive strength is defined by the intercept on the shear strength axis, of a tangent of a curvilinear Mohr envelope. This curvature is the result of the interlocking of aspirates on the matching surface of the joints. Furthermore, labo- ratory direct shear tests are generally conducted on small rock specimens, and thus dilation due to waviness (undu- latory nature) of the joint that has a wave length longer than the specimen size is not captured in the test. These conditions would increase the gross shear strength proper- ties of the joint planes when a large failure surface is consid- ered. When a large block failure is considered, the potential failure plane is likely to go through the existing discontinu- ities and to shear the intact rock that bridges the joint planes. In this case, the shear strength parameters assigned to the potential failure plane in a limit equilibrium analysis should include some portion of the intact rock strength. These increases in shear strength play a crucial role in the stability of rock slope. • The seismic design of the rock slope can be further improved by a deformation analysis involving a Newmark sliding block analysis on the failure plane. The Newmark sliding analysis for a plane failure is relatively simple to perform; however, for the wedge failure, it requires modification to deal with sliding on two planes under three-directional loading. The resultant vector of the inertial body forces act- ing onto each joint plane due to the three-component acceleration is checked against the yield acceleration of the joint. Sliding can take place on either plane or along the interception of the two planes depending on the direction of the loads at any given instance of time. This type of analysis provides a rational basis for deformation analysis of the wedge failure. Although these seismic performance considerations can be identified, it was also apparent that a transparent approach for evaluating the seismic response of rock slopes could not be developed into a guideline consistent with the simplified approaches needed for these AASHTO LRFD Bridge Design 23

Specifications, at least within the scope of this Project. Rather, the seismic design of rock slopes would be more accurately treated on a case-by-case basis. For rock slope stability evaluations, geologists and geo- technical engineers will be required to define the potential mechanisms of failure, the strength parameters representing the failure mechanisms, and the seismic loads. With this information an assessment of available computer software is required to investigate seismic stability. In some cases where two-dimensional conditions are predominant, conventional stability software similar to programs used for soil slopes could be used. Otherwise, more complete or specialized pro- grams, involving two- and three-dimensional wedge-failure surfaces would be needed. 3.3 Buried Structures Almost all highway culverts and buried pipes have been designed and built without regard to seismic effects. Cur- rently, there are no seismic provisions in AASHTO LRFD Bridge Design Specifications for culverts and buried structures, except for a general requirement stating that “earthquake loads should be considered only where buried structures cross active faults.” Unless there is a global slope stability problem within the embankment through which the culvert of pipeline passes, it is unlikely that existing highway culverts or buried structures (other than tunnels) have been designed and built with the consideration of fault displacements. While this approach may be acceptable for drainage culverts and most pipelines, it may not be an acceptable approach for a well-used pedestrian tunnel. In recent years, a great deal of attention has been given to the study of seismic performance of underground structures to improve the understanding of factors influencing the seis- mic behavior of underground structures. Design and analysis procedures also have been proposed by some researchers and design engineers, but they are generally developed either for pipelines (for example, gas and water) or tunnels (that is, transportation or water) systems. These procedures have not been directly applied to culvert installations. The potential problems and knowledge gaps associated with the current seismic design and evaluation procedures for buried structures were considered. • Culverts and buried pipes have performed much better than other highway structural components (for example, bridges and foundations). The “no-analysis required” cri- terion proposed for the bridge structures may not be appli- cable to the culvert structures. A separate and less stringent screening criterion, taking into account both the ground shaking intensity and the project geological site conditions, was needed. • Current design and analysis methodologies for pipeline and tunnel systems were developed typically for long, linear structures. For most highway applications, the culvert or pipe, however, is typically with limited length. The effect of the short length of the culvert or pipe on seismic response, as well as on the analysis procedure, had to be evaluated. • Current design and analysis methodologies for pipeline and tunnel systems were developed typically for level-ground conditions. Culverts and pipes, however, are typically con- structed within a built-up embankment. There was a lack of data of how to determine the appropriate TGD parameters for culverts and pipes embedded in embankments, especially in high embankments. • The effect of soil overburden thickness (or embedment depth) and the effect of vertical components of the ground shaking on culvert or pipe performance was not well under- stood. Further studies in these aspects were required. • When subjected to the TGD effect, the response of a buried linear structure can be described in terms of three principal types of deformations: (1) axial deformations, (2) curvature deformations, and (3) ovaling (for circular cross section) or racking (for rectangular cross section) deformations. The first two types, axial and curvature deformations, are induced by components of seismic waves that propagate along the culvert/pipe axis. The ovaling/racking deforma- tions are induced along the transverse cross section when seismic waves propagate perpendicularly to the culvert/ pipe axis. Previous observations have suggested that smaller diameter pipes (or small diameter highway culverts) are more resistant to ovaling deformations than the tunnel structures (and large diameter/size culverts). On the other hand, tunnels and large-size highway culverts have per- formed better than small diameter pipes under the effects of axial/curvature deformations. A further understanding of the factors resulting in this different performance between large and small buried structures was important. Once identified, these factors were considered in the design and analysis procedures. • Simplified ovaling and racking analysis procedures devel- oped for tunnel structures (for example, mined circular tunnels and box type cut-and-cover tunnels) can be applied to large-span circular and rectangular culverts, respectively. Simplified procedures for noncircular and nonrectangular sections (for example, ellipse, arch, arch top 3-sided, etc.) were nonexistent. Numerical analysis was required in this case and specific procedures related to performing this type of analysis were needed. • Various approaches for analysis or design of pipeline sys- tems (for gas and water) have been proposed, particularly under the effect of PGD, including fault displacements, lat- eral spread, and slope deformations (slump). Significant disparity exists among these approaches. There are also 24

different performance requirements and loading criteria being used or proposed from different studies. A consistent methodology and design criteria compatible with the other components of the highway facilities have yet to be devel- oped for the culvert structures. 3.4 Conclusions Knowledge gaps and problems identified in the literature review, through discussions with various individuals at DOTs and those conducting research in the area, and through the completion of Task 2 have not identified any additional or new knowledge gaps or problems; the ones cited above are relatively well-known and documented. It appeared that in most cases, existing simplified methodologies with appropri- ate improvements and documentation could be used to address these knowledge gaps and problems. While many problems could be handled by existing sim- plified methodologies, the complexity of some topics, such as the seismic design of geosynthetic MSE walls, was seen as more complex than originally anticipated. This complexity resulted in part from the changing approach to the static design of this wall type. It also appeared that the seismic design of other wall types, such as soil nail walls, still lacked the rigor needed to be considered state-of-the-practice. As noted in the discussion of earthquake design basis, current practice with some of these wall types involved sufficient con- servatism in the ground motion specification, as well as the inherent conservatism in static design, that these shortcom- ings were not a serious design issue. In fact, overall current design methods have worked surprisingly well. On the basis of the work carried out for this task, the pri- mary development needs were identified as follows: • Retaining walls – Numerical procedure that avoided deficiencies in the M-O procedure at high acceleration levels and steep back- slopes and that handled mixed soil (c-φ) conditions. The recommendation was to use either a wedge-equilibrium equation or a limit-equilibrium stability program to determine the forces needed for stability. – Charts for estimating wall displacement for representa- tive areas of the United States (for example, CEUS ver- sus WUS). – Guidance on the selection of the seismic coefficient for limit-equilibrium and displacement-based design and its variation with wall height. • Slopes and embankments – Procedures for determining the appropriate seismic coefficient and its variation with slope height. – Charts for estimating displacement for representative areas of the United States (for example, CEUS versus WUS). (These charts are the same as those used for esti- mating the displacement of conventional rigid gravity walls.) – Procedures for introducing the effects of liquefaction. – Procedures for treating rock slopes. • Buried structures – Simple-to-use design charts for medium-to-large-size culverts and pipes under the effect of transverse seismic racking deformations, taking into account soil-structure interaction effect. – Guidance on how to select transient ground defor- mation (or strain) parameters for design and analysis purposes. – Development of a consistent and rational procedure for buried structures subject to various forms of PGD, including lateral spread, embankment slope movements or flow, and faulting. An overall need for the three areas was a screening procedure that would provide guidance to the designer as to when a seis- mic analysis could be neglected, because the reserve capacity for static design was sufficient to meet seismic demands during the design seismic event. Further, guidance was needed on the selection of appropriate ground motions to use for seismic design and the determination of appropriate soil strengths to use in the capacity estimate. 25

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 611: Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments explores analytical and design methods for the seismic design of retaining walls, buried structures, slopes, and embankments. The Final Report is organized into two volumes. NCHRP Report 611 is Volume 1 of this study. Volume 2, which is only available online, presents the proposed specifications, commentaries, and example problems for the retaining walls, slopes and embankments, and buried structures.

The appendices to NCHRP Report 611 are available online and include the following:

A. Working Plan

B. Design Margin—Seismic Loading of Retaining Walls

C. Response Spectra Developed from the USGS Website

D. PGV Equation—Background Paper

E. Earthquake Records Used in Scattering Analyses

F. Generalized Limit Equilibrium Design Method

G. Nonlinear Wall Backfill Response Analyses

H. Segrestin and Bastick Paper

I. MSE Wall Example for AASHTO ASD and LRFD Specifications

J. Slope Stability Example Problem

K. Nongravity Cantilever Walls

View information about the TRB Webinar on Report 611: Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments: Wednesday, February 17, 2010

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