Antecedentes del estudio

For competent soils that do not undergo strength degradation under seismic loading, static strength parameters shall be used for seismic design.

• For cohesive soils total stress strength

parameters based on undrained tests shall be used during the seismic analysis.

• For clean cohesionless soils, the

effective stress friction angle of the soil shall be used.

For saturated, sensitive cohesive soils or saturated cohesionless soils, the potential for earthquake-induced strength degradation shall be considered.

The static design of retaining walls in

Section 11 of the AASHTO LRFD

Bridge Design Specifications is based on

the use of effective stress strength parameters. For conservatism the effective cohesion intercept is normally neglected and only the effective friction angle is used. The use of effective stress strength parameters is appropriate for design of retaining walls for long-term, gravity loads. However, for transient seismic loading, total stress parameters are more appropriate for cohesive soils.

Selection of Strength Parameters

Seismic stability analyses for retaining walls require the determination of strength parameters (c and φ) for either or both compacted fill and natural soils. In the case of fill wall construction,

specifications for wall construction usually require backfill materials to be cohesionless and free draining materials (i.e., amount of soil passing the No. 200 sieve less than 5 to 10%). For these soils cohesion (c) is assumed to be zero, and the effective (drained) friction angle (φ’) should be used to characterize the soil strength parameters. This strength can be obtained by conducting effective stress, or drained, laboratory strength tests or through the use of empirical correlations to field measurements, such as the Standard Penetration Test (SPT) blowcount or the cone penetration test (CPT) end resistance.

For wall construction involving cuts in natural ground, a high likelihood of encountering soils with cohesive content exists. The undrained (total stress) strength parameters should be used to characterize these soils for seismic loading analyses. The undrained strength can be determined on the basis of total stress strength parameters by in situ testing (e.g., vane shear tests), or through empirical correlations to results of CPT soundings.

In some geographic areas the availability and cost of clean granular backfill soil is becoming a significant construction issue, and backfill soils with a cohesion component due to fines content are increasingly being used. Gravity walls which involve the use of these “dirty” granular backfill soils may also require determination of total stress strength parameters for evaluation of wall design requirements.

Additional information regarding the characterization of soil strength by field and laboratory testing methods is provided in Section 10 of the current

AASHTO LRFD Bridge Design

Contributions from Soil Capillarity – Cohesionless Soils

In many situations it may be appropriate to include the effects of apparent cohesion from soil capillarity in the assignment of strength properties for the seismic loading analyses. This contribution will occur in many relatively clean sands or silts above the water table. The magnitude of apparent cohesion is difficult to establish without conducting special field and laboratory tests, as discussed, for example, by Fredlund and Rahardjo (1993). For this reason the following conservative guidelines are suggested, in the absence of specific testing that demonstrates higher values of apparent cohesion from capillarity.

Percent Passing No. 200 Sieve (%) Maximum Allowed Apparent Cohesion from Capillarity (psf) 5 - 15 50 15 - 25 100 25 - 50 200

Note that for backfill materials characterized by large particle sizes (e.g., gravels, quarry spalls, or larger particles) the effects of apparent cohesion from capillarity stresses should be ignored. Silts and sands permanently located below the water table, or where fluctuations in water table occur, also should not include apparent cohesion from capillarity. In these locations either capillarity will not develop or cannot be reliably included in the analysis.

Influence of Slope Geometry on Soil Parameter Evaluation

The presence of cohesive soils often leads to steep cut face during

construction – with angles of 1H:1V or steeper. For these conditions the use of simple Coulomb theory for seismic active pressure computations as shown in Figure X.5-1 becomes problematic, as critical failure surfaces leading to maximum seismic active pressures that develop. For this situation, total stress

strength parameters (c and φ) for the

natural ground need to be established. If the fill immediately behind the wall is a clean granular backfill (with little or not cohesion), then the seismic design must consider strengths from both drained (clean granular backfill) and undrained (natural soil) when evaluating wall stability.

Liquefiable Soils

For soils that are susceptible to liquefaction, the most common method of determining the strength of liquefied soil (often referred to as the residual strength) involves use of Standard Penetration Test (SPT) correlations to liquefied strength or similar Cone Penetrometer Test (CPT) correlations. These correlations are documented by Seed and Harder (1990), Olson and Stark (2002), and Idriss and Boulanger (2007). In view of the various factors that affect the strength of liquefied soil, it is important to establish potential variations in the liquefied strength, and then use this variation during the retaining wall analysis.

Figure C.X.5-1 Typical Cross-Section of Semi-Gravity Retaining Wall next to Steep Cut Slope

X.6 LIMIT STATES AND RESISTANCE