Reinforced soil is a set of embankments, in which the reinforcing elements in the soil are in the form of rebars. Reinforced soil is generally resistant to pressure and shear, but is weak to tension. In reinforced soil, the presence of reinforcing elements in the direction of tensile strain improves soil behavior (similar to reinforced concrete). Reinforced soil walls are in the group of flexible walls.
The main parts of the reinforced soil
Embankment
Grain soils are commonly used.
Reinforcing elements
In most cases, narrow and wide belts are used at regular intervals.
Shell
The pieces of shell are attached to one end of the reinforcing elements and cover the outside of the wall. The ratio of width to height in reinforced soil walls is usually large and therefore, unlike conventional retaining walls, the phenomenon of stress concentration is not seen. So, they are suitable for substrates with low bearing capacity.
In reinforced soil, the shell elements and consequently the embankment are braced using reinforcing elements that are placed inside the soil and attached to the shell elements. In this method, the embankment applies lateral pressure on the shell components and its interaction with the belts braced the soil.
Implementation method and conventional applications
Embankment in reinforced soil structures is applied in layers and is compacted like conventional embankments. The reinforcing elements are located in the space between these layers. Shell elements are used in two forms, metallic with a semi-elliptical cross section and prefabricated concrete with cruciform-shaped.
Comparison of reinforced soil with other common retaining wall systems
The following results can be obtained from comparing reinforced soil with other common retaining wall systems:
- It is possible to implement reinforced soil quickly and easily, especially with the use of prefabricated facilities.
- Costs are reduced compared to other conventional retaining wall systems
- Its adaptability is high and it is possible to implement it in different types of slopes and with different soil conditions.
- The presence of shell elements allows the designer to create the most harmony between the structure and the surrounding environment.
- Relatively high flexibility and ductility of reinforced soil allows for high subsidence.
Use of reinforced soil
The main problem of reinforced soil is the corrosion of reinforcing elements in the soil. Therefore, in designing and implementing reinforced soil, special attention should be paid to the durability and reliability of the reinforcing elements in the soil. Generally, the experiences obtained from the implementation of different samples of reinforced soil indicate its appropriate performance for the following.
- Reinforced soil walls in mountain roads which installed on weak beds or unstable slopes.
- Reinforced soil walls for freeways where fast execution and low cost are very important.
- Reinforced soil walls around railway lines where high vibration resistance is considered.
- Reinforced soil walls as coastal walls because of their good resistance to waves and erosion.
Specifications of components of reinforced soil
- Embankment
- Reinforcing elements
- Shell elements
Embankment
To increase the efficiency of reinforced soil, special attention should be paid to the characteristics of the embankment. The important features in this case are as follows.
- Long-term and short-term embankment sustainability
- Mechanical properties of soil (adhesion and internal friction)
- Chemical properties (durability and stability issues, corrosion of reinforcing elements)
In the case of granular and cohesive soils, the following should be considered when selecting embankments for reinforced soils. It is more appropriate to select compacted granular soils that increase in volume when shear is applied. In well-drained granular soils, the effective vertical stress after the application of each embankment layer is rapidly transferred between the embankment and the reinforcing elements and the shear strength is reduced by vertical loading without phase delay. In the normal load range of reinforced soils, these soils exhibit elastic behavior, so that the phenomenon of residual deformation (after implementation) does not occur.
On the other hand, in fine-grained soils, which are usually not well drained, the effective stress is not transferred quickly and as a result, the safety factor of the run time is greatly reduced. Also, due to the presence of elastoplastic or plastic behavior in these soils, there is a possibility of deformation of the residue (after implementation). On the other hand, in these soils, reinforcing elements with high stress are prone to creep and corrosion is more seen in them. Based on this, the use of fine-grained soils is not suitable for reinforced soil embankments.
Reinforcing elements
These elements are key factors in reinforced soil to transfer force from the stimulus area to resistant area. These elements must have good continuity and friction with embankment materials, their durability and reliability must be good, and they must have high ductility during rupture. Their degree of yielding under tensile stresses should be low. Accordingly, materials that have been successfully used in the reinforcement of various engineering structures include galvanized steel, aluminum-magnesium alloy, stainless steel, and polymeric materials.
Non-metallic reinforcing elements are usually made of polymers. The use of geosynthetics (such as geotextiles, geogrids, and geocomposites) is also common. These elements are usually weaker than similar metal elements and do not corrode, but are attacked by other factors and the creep phenomenon is usually very important for these materials.
Shell elements
These elements are in fact a cover for the reinforced soil and their main function is to prevent soil from falling between the reinforcing elements. These components are used to prevent surface erosion and create a suitable appearance. Reinforced soils because of its high flexibility are often applied on soft soils where a lot of subsidence occurs. Accordingly, the shell element must also have the necessary flexibility. The most common types of shell elements that are commonly used include the following.
Metal or steel parts
The shape of these elements is semi-elliptical cross-section, which are very flexible and durable. To install these elements, they are screwed together and the reinforcing elements are placed in the distance between them. The use of this type of shell element is very suitable for areas that have transportation and access problems because of their high lightness.
Concrete parts
The shape of these elements is usually cruciform-shaped and they are prepared using prefabricated concrete. Their joints are installed in such a way that they can withstand significant deformation without cracking in the concrete. Concrete parts allow the implementation of various facade coatings.
Durability and reliability
The behavior and performance of reinforced soil in the long-term is a function of the behavior of its reinforcing elements over time. Based on the mechanical properties required for reinforcing elements, steel is one of the best choices, but the phenomenon of corrosion in steel reduces the behavior of reinforced soil.
Behavior of reinforced soil
To understand the behavior of reinforced soil structures, first the behavior of laboratory samples of reinforced soil materials and then the friction between the soil and the reinforcing elements and finally the behavior of reinforced soil structures are investigated.
Behavior of samples of reinforced soil materials
A laboratory sample of reinforced soil is tested as a type of material. The results of a series of triaxial experiments on sand samples reinforced with horizontal aluminum discs show the effect of sand density, distances and tensile strength of reinforcing elements on the behavior of the sample. In these experiments, two failure modes can be observed.
- Failure due to rupture of reinforcing elements
- Failure due to slipping between soils and reinforcing elements
Behavior and mechanism of reinforced soil structure
The experiments show that the tensile force changes along the reinforcing elements and reaches a maximum value at this length. The geometric location of the maximum stress location in the reinforcing elements for different layers defines the maximum tensile force line. This line separates the two areas of stimulus and resistance.
Stimulus area
In this area, the soil tends to separate from the structure and the friction factor along the reinforcing element inhibits it.
Resistant area
In this area, the shear stress prevents the sliding of the reinforcing elements. The boundary line between these two areas (maximum tensile line) is the probable level of rupture in the structure. The position of this line is a function of several parameters such as the geometry of the structure, the forces applied and the dynamic effects. In addition, the rigidity of the reinforcing elements also affects its shape. This rupture level is different from the Mohr-Coulomb failure wedge. This line is in the upper part of the vertical wall, and this is due to the presence of relatively rigid reinforcing elements in the soil, which changes the stress distribution and strain in the soil.
Reinforced soil designing method
Designing of reinforced soil structures includes control of external or overall stability and internal stability.
The external or overall stability of the structure
Common failure modes in this case are as follows.
- Slipping
- Reversal
- Insufficient load carrying capacity of the bed (distortion)
- Deep slipping
Seismic behavior of reinforced soil
Based on shaking table experiments, observation of real samples after earthquakes, and numerical analysis of finite elements, it has been determined that reinforced soil structures because of their good flexibility generally withstand severe earthquakes without damage. The distribution of reinforcing elements in the mass of the structure causes the distribution and dissipation of seismic energy, thus reducing the probability of creating a concentrated force and consequently failure.
The effect of earthquake on the overall stability of reinforced soil structures can be estimated based on the assumptions of other flexible gravity retaining systems. In the case of internal stability, an earthquake leads to an increase in dynamic forces in the reinforcing elements. The distribution of forces in the dynamic state is different from its distribution in the static state.
