Soil Behavior: Dynamic Soil-Structure Interactions - Dynamic Behavior of Concrete and Seismic Engineering - page 141

model are necessarily equal to zero. Whenever a more accurate representation of the
soil is necessary, use of non-linear models is inevitable.
4.2.3. High strain domain non-linear models where JtJ v
In this strain domain, important changes in micro-structure occur (rearrangement
of grains), which generates significant volume and shear irreversible strains (Figures
4.3 and 4.9). These microstructure changes involve volume variations inside
unsaturated materials, and an increase of interstitial pressure within saturated
materials. This interstitial pressure increase can lead to canceling the actual stresses
and therefore the soil strength: it is a liquefaction phenomenon. Apart from this loss
of strength, the increase in interstitial pressure brings about a decrease in the initial
stiffness, which is a function of the state of the real stresses withstood by the soil
(Figure 4.10). Both factors combined - the stiffness loss and the loss of strength -
make saturated soils subjected to amplitude strains in which J is higher than J v
highly degradable.
As this type of behavior generates significant irreversible strains, it can only be
studied using non-linear models. Accumulated experience shows that strain
hardening elasto-plastic models are the most appropriate for describing this behavior
([PRE 78], [PRE 87]); the models have to be written using real stresses as these
govern soil behavior. In saturated soils, coupling of the fluid phase with the solid
phase can be described within the frame of porous medium mechanics due to Biot's
relationship.
Measuring the values that are used to formulate the behavior laws must be
carried out in laboratory conditions, on intact samples, under strictly controlled
environment conditions and over a range of stress paths that are all the more
numerous because the behavior law includes a greater number of parameters. In any
case, these tests are difficult to implement, because taking geo-material samples is
always delicate and can induce sample modifications, especially deep sampling. The
nature of the soil to be sampled (clay, marl) plays a part in the extent of the
modification. The properties most susceptible to modifications are soil stiffness and
the capacity of saturated sands to withstand liquefaction. At present, the most
versatile tool for obtaining this data is the triaxial device, which makes it possible to
reach important strain amplitudes with different stress paths.