Construction World October 2015

ROADS AND BRIDGES

ALTERNATIVE to TRADITIONAL retaining wall options

Introduced into the South African market by Kaytech, the Tensar TW1 System was developed as an alternative to traditional retaining wall options and has been used extensively in Europe and elsewhere internationally.

optimise the use of lower quality fill material, whilst simultaneously minimising 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 11 m high wall, which restricted the length of the strips to 7 m. At the same time it would be beneficial to the project if Berea sands could be utilised. 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 11 m high wall comprised the use of 7 m long strips, a granular (COLTO G6) backfill for most of the height and 1 m thick granular soil-raft foundation. No lateral support was thus required and conventional benching into the existing fill was utilised. For the upper 3 m of the 11 m wall and for the 5 m high wall, Berea sand was used throughout. Construction Some of the further benefits of the Tensar TW1 System are that it is labour-intensive and eliminates the need for cranes and other heavy lifting equipment. Additionally, the In utilising the new system a number of challenges were experienced during construc- tion. These included: the setting of the base block, which is key to achieving the final face inclination of 86°; compaction criteria; stormwater control; and the use of labour inexperienced in building these walls. However, these problems were quickly resolved through close collaboration between the contractor, consultant and supplier. The Kaytech and Tensar teams were able to provide technical assistance to the contractor and consultant’s supervising team with regards to installation, testing standards and quality control and assurance. Conclusion The project has showcased the level of knowledge and experience required to design and construct a Tensar TW1 Mechanically Stabilised Earth Wall. This has been a major achievement for Kaytech, considering this is the first wall of this size to be constructed in South Africa. The system provides a number of benefits over other block and other Mechan- ically Stabilised Earth systems, including: the effective connection between block and geogrid; a near-vertical face inclination; locally manufactured blocks; aesthetic appeal; labour intensive construction; and eliminating the use of heavy lifting equipment. TW1 block is manufactured locally by Remacon, a Tensar licensee, for that s pecific block manufacture.

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that the system complied with internal and external stability and project technical requirements. A benefit of the system was that lower quality fill, which was more readily available and less expensive, could be utilised as the grids provide greater coverage and soil adherence than other systems on the market and the product is also not prone to degrada- tion or chemical attack by natural soils. A further benefit to the system was that adjacent landowners were satisfied that the appearance of the split-face blocks would provide a high aesthetic appearance which would complement the local architecture. Tensar TW1 System The system, developed by Tensar Interna- tional of the UK, comprises the specially designed TW1 block, combined with high density polyethelene (HDPE) grid mats – known as Tensar uniaxial geogrids – that are attached by a special connector into the blocks and extend horizontally to secure and reinforce the fill, thereby turning 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 7 m; which is the present maximum height attainable with other retaining systems available locally. Internationally, maximum tiered wall heights of 60 m have been achieved with the TW1 system, with a maximum single tier height of 22 m in Fujairah, UAE. 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 New Jersey barriers along the top of the wall, as well as cognisance of the overall geotechnical conditions. Design optimisation A key consideration in the design was to

The system was recently used on a major SANRAL road upgrade contract in Ballito, one of the first and largest local applications of the

system to date. Project engineers, SMEC South Africa, were involved in the detailed design of the system and realised a number of cost benefits on the project by using the Tensar TW1 System. The scope of works entailed the widening of a two-lane, single carriageway to a three- lane, dual carriageway in Ballito, about 40 km north east of Durban. Due to the site’s undu- lating topography, earth retaining structures had to be built to bring the extra lanes to level. The lane widening had to be constructed within the road reserve to eliminate encroach- ment into existing developments. To reach this objective, two near-vertical, Mechanically Stabilised Earth (MSE) walls of 11 m and 5 m, covering a total length of over 400 m, and 2 000 m 2 , were proposed. MSE walls, broadly, consist of fill material with horizontal layers of reinforcing elements which may take the form of sheets, grids, strips or meshes. The reinforcing elements, which are either metallic or polymeric, are able to sustain tensile loads and the effects of deformation or soil strains developed in the fill, part of which is transferred to the clad face through some form of positive connection. Following a competitive tender process, the tender for construction of the project was won by Afriscan Construction, which included the use of the Tensar TW1 system. The project consulting engineers were satisfied that the system would meet the technical require- ments and were subsequently closely involved in the detailed design of the system. The intricate design needed to ensure

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Tensar Polyethylene RE560 uniaxial geogrid.

CONSTRUCTION WORLD OCTOBER 2015

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