As the population increases, more infrastructure and housing facilities needed. To satisfy the need especially in highly dense cities, construction should be performed safely in anywhere, regardless steep-sloped areas, soft-grounds and near the bank of the river or sea-shore. There is no limit to build any structure for urbanization if it is necessary. Reinforcement is an important method for such kind of safe constructability. There are several methods for reinforcement that can be applied to soil itself or can be used with concrete structures. Reinforcement materials such as geosynthetics can supply effective soil improvement when necessary to avoid the failure mechanism. If the resistance of the deformation which is called as “stiffness” of soil is less, reinforced structure (reinforcement+soil) would be the solution.
Usually the soil itself doesn’t have enough resistance to keep its form under pressure. Geosynthetics help the soil to hold itself as a massive body especially at the embankments and the steep slopes. The soil gain resistance after reinforced by geosynthetics. This type of applications forms a composite body, which have better stiffness. Mechanical response of the composite material is far stiffer than the soil structure itself, that’s the reason that they are widely used all around the world. This solution is achieved after many experimental procedure and monitoring. From this process empirical results are obtained. These results are used to form an applicable equation to estimate the equivalent elastic modules of geosynthetic reinforced soil. This mechanism is applied to the field to have safe and cost-effective solutions at the risky areas.
The interaction between the soil and the reinforcement material (geosynthetics) is the starting point for the parametric study. The mechanical interaction between the soil and the geosynthetics governs the behavior of the whole structure. The stiffness and the failure threshold give an idea for parametric study of stress-strain relationship that will guide the constitutive equation of the soil to use in stability calculations and analysis. Experimentally, the stiffness of the composite body can be
calculated from the deformation of the model structure. The followings are important points for application;
A. The way how to use the reinforcement material,
B. The density of the reinforcement (amount per square),
C. In using geosynthetics, how much stiffness can be expected,
D. How can the stiffness that comes from soil and geosynthetics be increased, (in which condition or composite system, the stiffness can be maximum).
E. How can the embankment be built in the field successfully as designed.
The compound material reinforced by geosynthetics can never be accepted as linear elastic body. It is rather nonlinear accompanied by irreversible deformation.
The final deformation of each structure can be used to calculate the elastic coefficient equivalent to Young’s Modulus of the structure. Experimental studies on this type of reinforced soil structures indicate that the reinforcement contributes an increase of the equivalent Young’s modules but there seems to be a limiting value of Young’s modules.
In order to explain the above-mentioned experimental results, the following general expression can be used;
E: The global Young’s modules
Eg: Young’s moduli of geosynthetics
Es: Young’s moduli of soil
α: Density of reinforcement (reinforcement amount per unit area)
If effects of Eg and Es can be combined into a parameter β, then the equation is written as
However, it should be noted here that, above equation is
an empirical equation that can only be applied to some special
cases. In this case, the equation is suitable for sandy soils and
geosynthetics. The sandy soil is modeled by non-linear elastic
model using hyperbolic stress-strain relations (Duncan-Chang
1970). The mechanical behavior of soil is highly non-linear,
and they exhibit stress dependency according to their stiffness.
And since geosynthetics can be regarded as the linearly elastic
material based on the uni-extension test result. Then the
geosynthetics can be modeled by linearly elastic bar element
under two-dimensional space. On the other hand, an elasto/
visco-plastic constitutive model proposed by Sekiguchi and Ohta
(1977) is employed to model the mechanical behavior of clay
materials. The necessary soil parameters (stress and strain data)
can be determined from the triaxial CU test [1-5].
The displacement of the soil structure that leads to
deformation and finally failure, vary according to the density
and placement method of the reinforcement material. This
phenomenon can be predicted by modeling the structure by
using suitable empirical equation of soil. It is beyond doubt
that stiffness of the structure can be improved considerably by
using more reinforcement materials, however the application
procedure is also very important. Geosynthetics have brought
successful development in soil mechanics. Recently various
techniques for the improvement of soil strength are designed
and applied successfully by the help of geosynthetics. For this
reason, they have been extensively used and proved to be very
effective by many researchers.