Study of Passive Isolation of Deep Foundations in Sandy ... - Ejge.com

Vol. 16 [2011], Bund. Q 1302
Hm
Bf
Lm
Figure 2: Problem definition of passive isolation by open trench
GEOMETRIC MODEL
In order to reduce the computation time and in accordance with the axisymmetric ½ of the actual model is
built in two-dimensional model. To prevent any wave reflection from the base of model, the depth of model is
not less than 30m (Jesmani 2008).
FINITE ELEMENT MODEL STRATEGIES
The properties which affect the wave propagation in low strain are Stiffness, Damping, Poisson, Ratio and
Density. Stiffness and damping are more important and effective than other properties because the predicted
strains are lower than 10 -3 ; therefore, linear elastic is applied to simulate the soil behavior and the above
parameters are involved (Figure 3) ( Ishihara. K, 1996).
In two-dimensional model the computation is done under the assumption of plain strain condition. To
simulate the behavior of soil in places which we have stress concentration and we need exact strains, two
dimensional PLANE82, In places which the strains and stresses gradient don’t play an important role in results,
two-dimensional PLANE42, and for simulating the behavior of concrete foundation, two-dimensional plane82
have been employed.
To evaluate the behavior of soil and the foundation such as sliding or any probable separation at the soil
structure interface, two-dimensional surface-to-surface contact elements (TARGE169, CONTA171) have been
employed. Because of the rigidity of the foundation compared with the underlying soil, the soil surface and pile
are taken as contact surface and target surface. Normal contact stiffness and maximum contact friction
coefficient are presumed to be equal to 1 and 0.6 respectively. PLANE42 is defined by four nodes having two
degrees of freedom at each node and PLANE82 is the developed form of PLANE42 with eight
nodes(figure4)and all of them have plasticity, creep, swelling, stress stiffening, large deflection and large strain
capabilities (ANSYS Manual).