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18 CHAPTER 3 Problems and Knowledge Gaps The goal of Task 2 of the NCHRP 12-70 Project was to on the wall and backfill soil are computed from the peak identify, illustrate, and document problems and knowledge ground acceleration coefficient at ground level. This approach gaps in current seismic analysis and design of retaining wall, is still widely used in general geotechnical practice since being slopes and embankments, and buried structures. This objective suggested as a standard method by Seed and Whitman (1970). was based on the Task 1 data collection and review, as well as A number of problems and related knowledge gaps with the the Project Team's experience gained from conducting seismic above approach have been identified, as discussed in the fol- design studies for retaining walls, slopes and embankments, and lowing subsections. buried structures in seismically active areas. The discussion of knowledge gaps and problems is organized in four subsections. 188.8.131.52 Use of M-O Approach The first three summarize knowledge gaps and problems for for Seismic Earth Pressures retaining walls, slopes and embankments, and buried struc- tures, respectively. The final section provides key conclusions The following problems are encountered when using about knowledge gaps and problems. As with the previous the M-O equations for the determination of seismic earth chapter, the primary focus of this effort was to identify prac- pressures: tical problems and knowledge gaps commonly encountered by design engineers when conducting seismic design studies. · 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 3.1 Retaining Walls (friction angle of the soil), or where backfill conditions are The discussion of problems and knowledge gaps for retain- not homogenous. ing walls focused on three primary types of retaining walls: · How to use the M-O equations for sloping ground behind gravity and semi-gravity walls, MSE walls, and soil nail walls. the wall where an unrealistically large seismic active earth Various other categories of walls exist, such as nongravity can- pressure coefficient can result. tilever walls and anchored walls. The discussions for gravity · How to use the M-O equations when high values of the and semi-gravity walls are generally relevant to these other selected seismic coefficient cause the M-O equation to walls as well, though additional complexity is introduced from degenerate into an infinite earth pressure. the constraints on deformation resulting from the structural system and the need to meet structural capacity requirements. 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 3.1.1 Gravity and Semi-Gravity Walls commentary, these limitations in the M-O approach are the Current AASHTO Specifications use the well-established result of basic assumptions used in the derivation of the M-O M-O equations developed in the 1920s for determining methodology. For the case of seismic active earth pressures, pseudo-static seismic active earth pressures behind conven- the M-O equation is based on the Coulomb failure wedge tional gravity or semi-gravity retaining walls (that is, cast- assumption and a cohesionless backfill. For high accelera- in-place gravity walls or cast-in-place concrete cantilever or tions or for steep backslopes, the equation leads to excessively counterfort walls), where the maximum inertial forces acting high pressures that asymptote to infinity at critical accelera-
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19 tion levels or backslope angles. For the latter conditions, no Specifications for static wall design) will provide more realis- real solutions to the equation exist implying equilibrium is tic estimates of seismic active pressure. The above problem not possible. A horizontal backfill with a friction angle for becomes further unrealistic in the case of a sloping backfill, sand of 40 degrees, a wall friction angle of 20 degrees, and a where earthquake active pressures become rapidly infinite for peak acceleration coefficient of 0.4 has a failure surface angle small seismic coefficients and relatively shallow slope angles, of 20 degrees to the horizontal. It will lead to very large seis- as illustrated in Figure 3-2. mic earth pressures due to the size of the failure wedge. For As discussed in Chapter 4, these problems with the M-O a peak acceleration coefficient of 0.84, the active pressure active earth pressure equation appear to be avoidable through becomes infinite, implying a horizontal failure surface. Since the use of commercially available computer programs based many areas along the West Coast and Alaska involve peak on the method slices--the same as conventionally used for ground accelerations in excess of 0.3g and it is common to slope stability analyses. This approach can be used to com- have a backslope above the retaining wall, it is not uncommon pute earthquake active earth pressures for generalized and for the designers to compute what appear to be unrealistically nonhomogeneous soil conditions behind a retaining wall. high earth pressures. The determination of seismic passive earth pressures using In practical situations cohesionless soil is unlikely to be pres- the M-O equation for passive earth pressure also suffers limi- ent for a great distance behind a wall and encompass the entire tations. In many cases the soil is not a homogeneous cohesion- critical failure wedge under seismic conditions. In some cases, less soil. However, more importantly, the use of the Coulomb free-draining cohesionless soil may only be placed in the static failure wedge is not necessarily conservative, potentially result- active wedge (say at a 60 degrees angle) with the remainder of ing in an underestimation of passive pressures. For some cases the soil being cohesive embankment fill (c - soil), natural (for example, where the wall height is shallow), a sufficient soil, or even rock. Under these circumstances, the maximum approach for the computation of seismic passive earth pres- earthquake-induced active pressure could be determined using sures can be the use of the static passive earth pressure equa- trial wedges as shown in Figure 3-1, with the strength on the fail- tions, as discussed in the NCHRP 12-49 guidelines (NCHRP ure planes determined from the strength parameters for the Report 472, 2002). However, this approach fails to consider the soils through which the failure plane passes. This approach earthquake inertial effects of the soil within the passive pres- (in effect the Culmann method identified for use with non- sure zone. A preferred approach involves use of a log spiral cohesionless backfill in the 2007 AASHTO LRFD Bridge Design method that incorporates seismic effects, as described by Figure 3-1. Trial wedge method for determining critical earthquake induced active forces.
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20 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- 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- Figure 3-2. Effect of backfill slope on the seismic mance criteria that ensures acceptable sliding displacements. active earth pressure coefficient using M-O equations. 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 Shamsabadi et al. (2007). The passive case is important for sliding wall. For retrofit design, the potential for wall rotation establishing the resisting force at the toe of semi-gravity walls may have to be studied, but retrofit design is not within the or for the face of a sheet pile wall or a cantilever wall comprised scope of the proposed AASHTO specifications for this Project. of tangent or secant piles. 184.108.40.206 Rigid Block Sliding Assumption 220.127.116.11 Wall Sliding Assumption Much of the recent literature on the seismic analysis of con- The concept of allowing walls to slide during earthquake ventional retaining walls, including the European codes of loading and displacement-based design (that is, assuming a practice, focuses on the use of Newmark sliding block analysis Newmark sliding block analysis to compute displacements methods. The basic assumption with this approach is the soil when accelerations exceed the horizontal limit equilibrium in the failure wedge behind the retaining wall responds as a yield acceleration) was introduced by Richards and Elms rigid mass. Intuitively, for short walls, the concept of a backfill (1979). Based on this concept, Elms and Martin (1979) sug- failure zone deforming as a rigid block would seem reasonable. gested that a design acceleration coefficient of 0.5A in M-O However, for very high walls, the dynamic response of the soil analyses would be adequate for a limit equilibrium pseudo- in the failure zone could lead to nonuniform accelerations with static design, provided allowance be made for a horizontal wall height and negate the rigid block assumption. Wall flexibility displacement of 10A inches. The coefficient "A" used in this also could influence the nature of soil-wall interaction. method was the peak ground acceleration (in gravitational A number of finite element or finite difference numerical units, g) at the base of the sliding soil wedge behind the retain- response analyses have been published in recent years, model- ing wall. This concept was adopted by AASHTO in 1992, and is ing the dynamic earthquake response of cantilever walls. Unfor- reflected in current AASHTO LRFD Bridge Design Specifica- tunately, many of these analyses are based on walls founded on tions. However, the concept is not well understood in the design soil layers leading to wall rotation. In addition, numerical diffi- community, as designers often use values of 33 to 70 percent of culties in modeling interface elements between structural and the peak ground acceleration for pseudo-static design without soil elements, along with problems modeling boundary condi- a full understanding of the rationale for the reduction. tions, tend to cloud the results. Many of the analyses use only Observations of the performance of conventional semi- one wall height, usually relatively high--greater than 30 feet, for gravity cantilever retaining walls in past earthquakes, and in example.