By Kaytech – When a major roadway upgrade was approved for the city of Ballito, KZN Province, South Africa, mechanically stabilized earth (MSE) walls were essential to the project’s success. The scope of works entailed the expansion of an undivided two-lane road into a three-lane divided road. MSE walls were needed to bring the extra lanes to level, due to the site’s undulating topography.
Ballito is located approximately 40km northeast of Durban, which has the second most populous metropolitan area in the country.
The MSE walls system selected for the project was the Tensar TW1 System, which was introduced into the South African market by Kaytech. The system has been extensively applied in Europe and other world regions. The Ballito project offered an opportunity to install not just one of the first such systems in South Africa but one of the largest local MSE applications.
MSE WALLS – BENEFITS
The lane widening work had to be constructed within the road reserve to eliminate encroachment into existing developments. To reach this objective, two near-vertical, MSE walls of 11m and 5m, covering a total length of over 400m, and 2000 m2, were proposed.
The intricate design, led by project engineers SMEC South Africa, needed to ensure that the system complied with internal and external stability and project technical requirements. These included, for example, concerns for sliding and the bearing/tilt and overturning of the MSE block. They took into account preventing failures from block compression, bulging, poor connection strength, geogrid pullout, rupture, etc. Through all of this careful analysis, and in utilizing the MSE system, the engineers realized a number of cost and construction benefits.
One benefit was that lower quality fill could be safely used. As with many projects, the most readily available and least expensive fill sources would require reinforcement to properly serve a role in the engineered design. The geogrids in the MSE walls in Ballito provided the appropriate reinforcement strength and significant coverage and soil adherence. Local soils were, thus, permitted. Furthermore, the polymeric reinforcement in the system was resistant to chemical and biological degradation in these soils.
Another benefit that greatly helped the project was that adjacent landowners were satisfied. Not only was less land disturbed but the aesthetics of the split-face blocks for the MSE walls could be selected to complement the local architecture.
SOUTH AFRICA – MORE INFRASTRUCTURE OPTIONS
The MSE system, developed by Tensar International of the UK, comprises the specially designed TW1 block, combined with high-density polyethelene (HDPE) geogrid panels (Tensar uniaxial geogrids). These geogrid panels are attached by special connectors to the wall blocks and extended horizontally to secure and reinforce the fill. The process converts the whole structure into a monolithic mass.
The positive connection to the cladding or split-block face is an important attribute of the system and allows it to be used on near-vertical walls exceeding 7m.
For Ballito, that height was important. The other systems available in the project zone were not able to be constructed above 7m. The introduction of the TW1 approach allowed the final wall inclination to be 86°. It also helped extend the region’s options for MSE walls.
Internationally, tiered wall heights of 60m have been achieved with this same system. Single-tier heights have even been realized at 22m at a project in Fujairah, UAE.
The international engineering record and the works in Ballito are providing a strong base on which to build South Africa’s infrastructure.
GEOTECHNICAL INVESTIGATION AND DESIGN
As this was one of the first of these walls in the country, the design of the wall was a close collaborative effort between Kaytech, Tensar, and SMEC South Africa. SMEC undertook the final design checks to ensure overall stability of the system and compliance with project specifications and local codes. These included integration of the system with the new roadway and Jersey barriers along the top of the wall, as well as cognizance of the overall geotechnical conditions.
The geotechnical investigation of the site revealed the site to be underlain by mudrock of the Karoo Supergroup, overlain by Tertiary to Recent sediments. At the location of the MSE walls the site was underlain by thick coastal dune Berea deposits, and bedrock was present at depths exceeding 30m.
The design of the MSE walls was based on South African National Standard SANS207:2006 (Design and construction of reinforced soils and fills), which provides guidance applicable to the design of reinforced walls.
The internal and external analyses and modeling were aided by proprietary design software from the geogrid manufacturer. The overall stability of the wall was checked using geotechnical Finite Element software, Phase 28.
OPTIMIZING MSE WALLS
A key consideration in the design was to optimize the use of lower quality fill material, whilst simultaneously minimizing the amount of lateral support required in cutting back and benching into the existing roadway; i.e. the back excavation slope. Limited space was available for the 11m high wall, which restricted the length of the strips to 7m. At the same time it would be beneficial to the project if Berea sands could be utilized. However, by using the lower quality fill, strip lengths would need to be increased, which implied either increased cut or the use of a near vertical back excavation slope requiring the use of shotcrete and ground anchors or nails.
After a number of iterations, the final design for the 11m high wall comprised the use of 7m long strips, a granular (COLTO G6) backfill for most of the height and 1m thick granular soil-raft foundation. No lateral support was thus required and conventional benching into the existing fill was utilised. For the upper 3m of the 11m wall and for the 5m high wall, Berea sand was used throughout.
Again, the geogrids selected for site use, and which ensure optimal, long-term performance, must take into account the soil properties of the reinforced, retained, and foundation materials. These soil properties contribute to determining the tensile strength, stiffness requirements and spacing of the geogrid. The geogrid will only be able to withstand the tensile forces once attached to the facing and once normal stress is applied to its length.
The ultimate tensile strength of the geogrid is factored, giving rise to the calculated Long Term Design Strength (LTD) which is provided and discussed in detail in the manufacturer’s design guidelines.
CONSTRUCTION OF MSE WALLS
The TW1 system offered some labor benefits, in that the need for cranes and other heavy lifting equipment was eliminated. The local manufacturing and availability of the TW1 block by a Tensar-licensed provider, Remacon, was also a strong advantage for the site work.
Though the site labor was not deeply familiar with the system, that inexperience was not a challenge. Proper technical oversight was available for the contractor; and the strength system itself and the high-quality design further ensured safety. Typical actions of concern, such as base block setting (to achieve the 86° face inclination), stormwater management, and proper compaction were all executed effectively.
The Kaytech and Tensar teams provided technical assistance to the contractor and consultant’s supervising team with regards to installation, testing standards and quality control and assurance.
PROJECT PARTNERS
Contractor: Afriscan Construction
Supplier: Kaytech Engineered Fabrics and Tensar International
Design and Supervising Engineer: SMEC South Africa
Construction Value: R45 million (Walls and fill: R 8,5 million )
For more information on Kaytech products and systems, visit www.kaytech.co.za.
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