The idea of reinforced earth technique was proposed by Casagrande and was first applied by Henry Vidal in France in 1966. Reinforced earth is a composite material in which strength of soil is increased by placing flexible reinforcement in granular non adhesive soil. The method of construction using reinforced earth has proved to be a better solution for increasing cost and poor quality of conventional material volume. This is a rapid, predictable and easy construction process which has lead to great economic use of this technology.
A cohesive material of great strength and stability is formed by the combination of granular soil and reinforcement. Through friction, the mechanical properties of soil were improved by reinforcement.
The reinforced earth structure is a single coherent gravity mass that can be engineered for specific load requirements. Reinforced earth is used in both mountain and urban highway projects for earth retaining structures and bridge abutments. In seawalls, dams, bulk storage facilities, civil and military projects and through an increasing range of applications this technology has been proven and accepted in standard practice.
1.1.1 Why we go for these kinds of techniques?
The main reason behind this is that comparedto other reinforcing techniques, it is very economical and easy to work with.Another important reason is that this can be incorporated inregions where the space required for the construction of retaining wall is limited.For a normal retaining wall structure we have to provide slope as the height increases which in turn requires more space.But nowadays going for these kinds of techniques is not practical mainly in cities and urban areas.
1.2 MATERIALS USED IN THIS TECHNIQUE
For a retaining wall construction using reinforced earth technique, main components used are:
1.Soil or backfill
3.Facing element or panel fascia.
1.2.1Soil: The soil used as backfill material must conform to some physical, chemical and electrochemical characteristics. All materials with less than 15% particles smaller than 80mm is automatically accepted on condition that it does not include particles larger than 250 mm. Similarly, other properties like resistivity, PH, chlorides, sulphatesetc are checked.
1.2.2Reinforcing Elements: The material used for reinforcing should be strong, stable, durable, low cost and easily available. It should have a good coefficient of friction. Steel, fibres, rubber, plastics etc are commonly used as reinforcing element.
Figure 1: A typical cross-section of a reinforced earth retaining wall
1.3 METHOD OF CONSTRUCTION
First, the geotechnical investigation of site is carried out. Bearing capacity, angle of internal friction, density, location of water table etc are found out. After these operations, the different elements, reinforcement and panels are designed. The construction of reinforced earth structure is very simple and quick. As panels are installed the structure is gradually backfilled in 37.5cm thick layers; the level of compaction is larger or equal to 95% of normal proctor density. The strips are laid in horizontal layers with vertical spacing of 75cm and bolted with facing plate.
At first, the foundation soil is laid at a thickness of 60- 70cm.Above that, footings are provided at both sides of the road for fixing the panels. These panels contain anchors for the fixation of the reinforced element. Normal soil is then laid at a thickness of about 37.5 cm and compacted tightly using dozers. Reinforced elements are held tightly and then laid horizontally over the compacted soil and finally clamped.The next layer of panels is then fixed and the process is repeated till it reaches the required height.As mentioned above, the vertical spacing between the strips is 75cm.
1.4 DIFFERENT TYPES OF REINFORCING MATERIALS
A variety of materials can be used as reinforcing materials. Some of them are:
· Mild steel and galvanized steel
· High alloy steel
· Glass fiber reinforcing elements
Of these, geo-synthetics are rapidly emerging as the most important family of materials used in wide variety of applications. Geo-synthetics consist of geo-textiles, geo-membranes geo-grids, geo-nets and geo-composites and are used to improve soil behaviour. They are made from polymeric materials.
These form the largest marketable material among geo-synthetics. According to ASTM, a geo-textile is “any permeable textile material used with foundation soil, rock, earth or any other geotechnical engineering related material as an integral part of a man-made project, structure or system”. These are porous flexible fabrics available as both woven & non-woven varieties. They perform four major functions – Separation, Reinforcement, Filtration and Drainage.
These represent the second largest group of geo-synthetics and their sales are equal to that of geo-textiles. They are impermeable or impervious sheet of plastics or rubber material used primarily as lining for liquid or solid storage/flow facilities. Their primary function is to act as an impermeable barrier to liquids/vapours. Most of the geo-membranes fall into the category of polymer resins 9PVC etc which are thermoplastic materials.
Geo-grid is defined as a synthetic planner structure formed by a regular network of tensile elements with aperture of sufficient size to allow interlocking with surrounding soil. The main function of geo-grid is reinforcement. The polymer material used in the manufacture of geo-grids is high-density polyethylene, high tenacity polyester/poly propylene.
They are combination of:
· Geo-textile and geo-grid
· Geo-grid and geo-membrane
· Geo-textile and geo-grid with geo-membrane.
· Anyone of the above geo-synthetic materials with another material such as steel cables/sheets, fire glass, plastics etc.
· Anyone of above synthetics with non-synthetic materials such as coir, jute, bamboo etc.
Cellular structure with the confined soil made of grocells can serve as a strong and stable mattress. Earth embankments have been built on such mattresses placed on weak/soft soils.
1.5 CASE STUDY
1.5.1 Flyover at Calicut
Location: Arayadathupalam(Flyover connecting Mavoor Road and Medical College Road)
The PWD department had gone for reinforced earth retaining wall and precast panel fascia over here. The main reason behind it is that the width of the road available at the sitewas very less and moreover it was the busiest road in the town. Thus, if the slope of the retaining wall was increased, then the width available for the road would increase. With the introduction of fascia panel, the earth is reinforced with geo-grids.
Figure 2: Casting of panel fascia
Figure 3: Erecting of Panel Fascia
Figure 4: Laying of geogrids
1.6 ADVANTAGES OF REINFORCED EARTH RETAINING WALLS
· Main advantage is that they are economical compared to other construction methods.
· Panels are used inorder to control the erosion of soil as it covers soil strata.
· They are easy to work with.
· In regions of lesser area,these kinds of retaining wall constructions are effective.
· They add beauty to the structure.
The world wide acceptance and utilization of reinforced earth makes it one of the most significant civil engineering developments in last three decades. This method has the advantage of economy, adaptability, reliability, flexibility, excellent appearance etc.
The technique has very wide applications in construction of retaining walls, embankments and sloped structures, foundation beds, sub grades and insitu strengthening of soil mass.
From small walls for commercial developments to bulkheads at industrial terminals, reinforced earth has been selected for its outstanding performance characteristics and combined cost savings that have ranged from 20-50% below alternate solutions.
All pre-manufactured components are delivered to job site and expert advice is provided. All these make construction very much easier and faster.
1. National Workshop in “Reinforced Soil Retaining Wall”, Mumbai, November 1998, Indian Institution of Bridge Engineers.
2. Garg K.G and Sridevi Jade, “Finite element analysis of reinforced earth wall”, Indian Geotechnical Journal.
3. Krishna Raj P, “Soil Reinforcement”, Indian Geotechnical Journal, January 1982.
4. Uppal K.B, “Reinforced Earth and advanced Construction Technology ,Civil Engineering Construction review, Volume 10,September 1997.