Environmental Engineering Reference
In-Depth Information
and thus to obtain better estimates of permanent deformations under seismic loading. The
effective stress codes may be divided into three main categories: fully coupled, semi-
coupled and uncoupled.
12.6.4.2.1 Fully coupled codes
In the fully coupled codes, the soil is treated as a two-phase medium, consisting of soil and
water phases. Two types of pore pressures are considered, transient and residual. The
transient pore pressures are related to recoverable (elastic) deformations and the residual
pore pressures are related to non-recoverable (plastic) deformations. A major challenge in
fully coupled codes is to predict residual pore pressures. The residual pore pressures,
unlike the transient pore pressures, are persistent and cumulative and thus exert a major
influence on the strength and stiffness of the soil skeleton. The transient pore pressures are
cyclic in nature and their net effect within one loading cycle is often equal to zero. An
accurate prediction of residual pore pressures requires an accurate prediction of plastic
volumetric deformations. In the fully coupled codes this is often achieved by utilizing
elasto-plastic models based on kinematic hardening theory of plasticity (utilizing multi-
yield surfaces) or boundary surface theory with a hardening law. These models are very
complex and put a heavy demand on computing time.
Generally speaking, fully coupled prediction of pore pressures under cyclic loading is
very complex and difficult. To date, many of the numerical codes developed in this area
are not fully validated and still are at their developmental stages. The validation studies
performed on a number of these codes suggest that the quality of response predictions are
strongly path dependent. When the loading paths are similar to the stress paths used in
calibrating the models, the predictions are good. As the loading path deviates from the
calibration path, the predictions become less reliable. Apart from the numerical difficul-
ties, part of this unreliability is also due to the poor or less than satisfactory characteriza-
tion of the soil properties required in the models. For instance, because of sampling
problems, it is often very difficult to accurately determine volume change characteristics
of loose sands as required by these models. In general, the accuracy of pore pressure pre-
dictions in fully coupled modes is highly dependent upon the quality of the input data.
Typical of the fully coupled codes are DNAFLOW (Prevost, 1981), DYNARD (Moriwaki
et al., 1988), SWANDYNE (Zienkiewicz, 1991) and SUMDES (Li et al., 1992).
12.6.4.2.2 Semi-coupled codes
Compared to the fully coupled codes, the semi-coupled codes are more robust and less
susceptible to numerical difficulties. However they are theoretically less rigorous. In these
codes empirical relationships, such as those proposed by Martin et al. (1975) and Seed
(1983a), are used to relate cyclic shear strains/stresses to pore pressures. The empirical
nature of the pore pressure generation in these codes generally puts less restriction on the
type of plasticity models used in the codes. The semi-coupled codes are in general less
complex and computationally demanding. Also, the parameters they require are often
routinely obtained in the laboratory or in the field. There is extensive experience in using
semi-coupled codes in practice.
Typical of the semi coupled codes are DESRA-2 (Lee and Finn, 1978), DSAGE (Roth,
1985), TARA-3 (Finn et al., 1986) and FLAC (Cundall, 1993).
12.6.4.2.3 Uncoupled codes
In the uncoupled analysis the pore pressures are estimated separately, using either a program
of laboratory testing or an empirical relationship such as the one proposed by Seed (1983a).
Then they are incorporated into an elasto-plastic non-linear code to obtain permanent
deformations. The uncoupled analysis is widely used in practice and is generally believed to
provide indicative estimates of the post liquefaction behaviour of earth dams. The perma-
nent deformations obtained using this approach are often on the conservative side, as the
analysis does not allow for dissipation with time of the estimated pore pressures.
