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