Factores que intervienen en el rendimiento académico

Y.8.1 General C.Y.8.1

If liquefiable soils are predicted to occur within or below the slope or embankment, the potential for slope instability during liquefaction shall be evaluated using either a limit equilibrium or displacement-based approach:

• Limit-Equilibrium Analysis: For a limit equilibrium analysis the C/D ratio shall be determined using the strength of the liquefied soil in the slope stability analysis. If the C/D ratio for the liquefied case is greater than 1.0 (i.e., FS > 1.), the site is considered stable during the design seismic event. If the C/D ratio is less than 1.0 (i.e., FS < 1.0), the seismic-induced displacements shall be estimated using a displacement-based approach.

• Displacement-Based Analysis: For the displacement-based approach, permanent displacements shall be estimated using one of the following methods: (1) the simplified Newmark equations (Article Y.7.3), (2) the time history displacement method (Article Y.7.4), or (3) numerical modeling with a 2- dimensional computer code. For the displacement analysis, reductions in soil strength due to pore-water pressure buildup associated with liquefaction shall be accounted for in the displacement

The limit equilibrium approach for assessing the performance of slopes during liquefaction is the same as for non-liquefiable slope, except that the soil strength for static loading is replaced by the residual strength of the soil As noted in Article C.Y.4.1.2, considerable uncertainty exists regarding the appropriate strength to use for liquefied soils that are not undergoing large deformation or flow in displacement-based evaluations. One option is to conduct laboratory cyclic tests to estimate soil response under the anticipated seismic stress path. Another alternative, which is thought to be conservative, is to use the residual strength of liquefied derived from back analysis of flow failures.

The seismic coefficient used in this analysis is the same as for the nonliquefiable case. No reductions or modifications in seismic coefficient are made to account for the modifications of ground motions from liquefaction. This assumption is usually conservative. Nonlinear effective stress analyses and field studies (e.g., Youd and Carter, 2005) usually show that the peak ground acceleration above the liquefied zone will be decreased; however, the amount of reduction depends on the characteristics of the site and the seismic design event. For conservatism the recommendation is to use the peak ground surface acceleration with

determination.

Predicted demand to capacity ratios or deformation shall be reviewed with the Owner, and a decision shall then be made on whether ground improvement methods are required to limit flows or lateral spreading movements.

adjustments for wave scattering and permanent movement (i.e., 50% reduction if 1 to 2 inches of displacement are acceptable) but without an reduction for liquefaction.

Three approaches are currently being used or proposed for evaluating slope displacements where liquefaction is involved:

• Youd Empirical Method: The simplest are the empirical relationships, such as suggested by Youd et al. (2002), for estimating displacement during lateral spreading. These relationships are based on empirical correlations between observed lateral displacement, earthquake parameters, and soil conditions. This approach is typically applied near rivers or other locations where slopes are gentle and a free face might exist. Generally results from these methods are considered more suitable for early screening of potential displacement issues and involve too much uncertainty for site-specific design.

• Two-Step Method: The second approach involves used of the simplified Newmark equations in a two-step analysis. This approach is based on a Newmark sliding block approach. A pseudo-residual strength is assigned to the liquefied layer using empirical relationships (Seed and Harder, 1990; Olson and Stark, 2002; or Idriss and Boulanger, 2007) for flow failures. The first step involves determination of stability after the end of shaking using the residual strength of layers that have liquefied. Under this condition a seismic coefficient is not applied. If the ratio of capacity-to- demand is less than 1.0, a flow failure is predicted. In this case very large deformations are predicted to occur. An approximate estimate of the magnitude can be made using the Youd et al. (2002) empirical method. If the capacity-to- demand ratio is greater than 1.0, the stability analysis is repeated using the residual strength of the soil and also

imposing the seismic coefficient. The yield acceleration is determined, and deformations estimated in accordance with procedures recommended in Article Y.7.3.

• Numerical Modeling Method: The third method involves the use of numerical modeling methods. Various computer programs, such as FLAC and PLAXIS, are commonly used to investigate the seismic stability problem where liquefiable soils have been identified. These methods seem to be used extensively by designers – often without having a particularly good understanding or appreciation for the uncertainties of the model. One of the significant criticisms of this approach is that thin layers that lead to ground displacement during liquefaction are not correctly modeled.

Various approaches for dealing with liquefaction-related slope instability will continue to be identified as future research studies are conducted. Unfortunately, there is no current consensus within the profession on the best approach for dealing with liquefaction- related slope stability – each has its pros and cons. The current difficulty in developing a consensus results from uncertainties in two areas: (1) the capacity of the soil in its liquefied state, particularly where there are static shearing stresses (i.e., sloping ground effects) and dilation effects during cyclic loading, and (2) the ground motions to use after the seismic wave travels through the liquefied soil. While numerical methods, such as DESRA (1978), are available to address the latter issue, these methods are limited in availability to most designers.

For many sites the two-step Newmark method identified above can be used. This approach represents a relatively simple method that allows “order of magnitude” displacements to be estimated. While this approach is relatively simple to apply, it is often criticized as it does not address the

complexity of the triggering relationship for liquefaction on sloping ground, and it does not properly account for the overall complexity of the problem, particularly the appropriate for liquefied soils undergoing limited deformation. Results of centrifuge research programs also indicate that the methodology may not replicate important mechanisms that occur during seismic loading. Many of these issues are being studied in research being conducted by the Pacific Earthquake Engineering (PEER) Center. Until a consensus is reached on a better simplified method of analysis, the two-step method will be sufficient.