N2 Road Upgrade (Lynnfield to Asburton)
Reinforced Earth® Retaining Walls on the Lynnfield to Ashburton N2 Road Contract
Introduction
Reinforced Earth South Africa (RESA) was awarded a contract for the design and supply of materials for construction of retaining walls on the project for the upgrading of the N3 between km 30.6 and km 31.7 (Lynnfield to Asburton).
The face area of the Reinforced Earth® structures is 9950 m2; their height varies from 4m to 10m with a 30 degree surcharge and their total length is approximately 2 km.
Construction innovation technology
Reinforced Earth® is a composite material comprising frictional backfill, reinforcements and facing elements and was invented by French architect and engineer, Henri Vidal, in 1964. This invention was one of the most important civil engineering innovations of the twentieth century. RESA introduced Reinforced Earth® to South Africa in 1975. Ongoing innovation has enabled the material to offer state of the art solutions tailored specifically to meet the requirements of each structure.
The constituent materials for the construction of the Lynnfield Ashburton Reinforced walls comprised:
Backfill: coarse backfill (G7) material from a nearby quarry.
Reinforcements: 50mm x 4mm high adherence, medium tensile steel, hot dip galvanized to ISO 1461: 2009 specifications.
Facing: TerraTrel™ manufactured from 12mm, 10mm and 8mm hard drawn wire, welded into a grid with 100mm x 100mm apertures. The grid is shaped, and hot dip galvanized to ISO 1461: 2009 specifications.
Corporate Social Investment
The construction of Reinforced Earth® with TerraTrel™ facing is labour intensive and allowed the Contractor, Rumdel Cape, to sub-contract the construction of the Reinforced Earth structures to Small, Medium and Micro Enterprises in line with the project’s Contract Participation Goals (SMME/CPG).
Design Innovation
The TerraTrel™ facing allows for internal settlement of the facing elements during construction without bulging of the elements between layers of reinforcing strips.
A recent innovation, a flexible spacer named SpringTrel™, keeps the initial vertical spacing between the elements constant, until squeezed by the weight of the ongoing backfill placed above it during the construction process.
Environmental Impact Consideration
TerraTrel™ construction for permanent works requires rock with diameter greater than 100mm to be packed behind the facing. This presents a pleasing gabion-type appearance. Vegetation takes easily to the facing and could, relatively easily, be encouraged to become a “green” wall.
The appearance of the structure is dependent on the construction. In this case the MMSE/CPG sub-contractors did an excellent job within the required tolerances.
Health & Safety
Since work takes place from the back of the facing, while personnel stand on top of the compacted backfill layers, and because the advancing facing provides a barrier during construction, there is reduced risk associated with working at height.
Quantifiable Time, Cost and Quality
The pacing time for construction is the placing and compaction of the earthworks. When the earthworks are complete, so is the structure. This allows a significant saving in time compared to reinforced concrete retaining walls.
The cost is influenced by the rate of construction, the availability of the backfill and the height of the structures. The Reinforced Earth® solution succeeded in replacing an alternative solution, which was accepted by the contractor, further illustrating it’s cost effectiveness.
Quality: All materials were manufactured in South Africa to strict codes of practice. The construction was according to the specifications.
Risk Management
The excavation for the walls had to be made into the existing N3 embankment slope while the N3 remained operational during the works. In addition, poor foundation conditions were expected.
Reinforced Earth (Pty) Ltd was not only responsible for the internal stability of the walls, but also for the global stability. To ensure the global stability, RESA appointed geotechnical engineers Melis & Du Plessis to assist in the works.
To monitor the long-term degradation of the buried reinforcing strips, test strips were installed. These strips can be extracted at any stage during the life of the structures. Tests to determine the amount of corrosion as well as tensile strength will be carried out on the test strips by an accredited laboratory.
The TerraTrel™ facing cannot easily be vandalized. To our knowledge, no TerraTrel™ structures have been vandalized on any of the hundreds of RE projects throughout 16 African countries.
Motivation Facts about the Project
Reinforced Earth® Retaining Walls on the Lynnfield to Ashburton N2 Road Contract were:
- Designed according to Codes of Practice and constructed as per the specifications.
- Undertaken for safe construction by MMSE/CPG contractors.
- A factor in ensuring safe excavation of the existing embankment of the N3.
- Selected because the wall facing does not bulge and is aesthetically pleasing.
- Selected because the wall facing will not be vandalized.
- Selected because the walls are durable and can be monitored to track the durability of the reinforcing strips over the structures’ service life.
- Designed using the TerraTrel™ solution, which allowed the contractor to optimise his program – due to the speed of installation – when compared to the initial plan.
Project details:
Client: SANRAL
Main Contractor: Rumdel Cape
Consulting Engineers: NME (Nathoo Mbenyane Engineers) & Leporogo Consulting Engineers