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INTRODUCTION

 

Ballito Drive is situated about 40km north east of Durban. The scope of works entailed the widening of a two lane, single carriageway to a three lane, dual carriageway. Due to the site’s undulating 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 encroachment into existing developments.  In order to reach this objective two, near-vertical, Mechanically Stabilised Earth (MSE) walls of 11m and 5m, covering a total length of over 400m, and 2000 m2 were proposed.

MSE walls, in broad terms, 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 capable of sustaining 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.

A number of propriety MSE products are currently in the market and, following a competitive tender process, the winning tender for construction of the project, that by Afriscan Construction, included the use of the Tensar TW1 system. The project consulting engineers, were satisfied that the system would meet the technical requirements and were subsequently closely involved in the detailed design of the system. The intricate design needed to ensure 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 degradation 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 International 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 7m; which is the present maximum height attainable with other retaining systems available locally. Internationally maximum tiered wall heights of 60m have been achieved with the TW1 system, with a maximum single tier height of 22m 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.

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: “The design and construction of reinforced soils and fills”, which provides guidance applicable to the design of reinforced walls. A reinforced soil structure must be checked for external stability and internal stability. External stability considers sliding, bearing/tilt and overturning of the MSE block. Internal stability considers not only the essential checks of failure against pullout of the geogrid and failure against rupture but also a number of ancillary checks, including compressive failure of the blocks, block rotation and bulging and connection failures.

The type of geosynthetic reinforcement selected 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.

Critical sections were analysed for internal and external stability. These were  modelled and checked with the proprietary design software packages from Tensar International. The overall stability of the wall was checked using geotechnical Finite Element software, Phase 2. Modelling material properties for the membrane and interface elements in a finite element model can be problematic, especially considering that the geogrid’s pullout resistance is derived from friction generated on the soil-reinforcement boundary, which in this case are not continuous and have perforations. This is further complicated by the fact that the in-soil stiffness of a geogrid is stiffer compared to the in-air stiffness under which the geogrid is tested. An approximation thus needed to be made and the reinforcing layers were simulated using liner elements which had capacity to resist tensile forces whilst the interface between the soil and liner was modelled with zero thickness interface elements, as per the recommendations of Potts (2005).

 

 

DESIGN OPTIMISATION

A key consideration in the design was to 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 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 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 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.

 

CONSTRUCTION

Some of the further benefits of the Tensar TW1 System is that it is labour-intensive and also eliminates the need for cranes and other heavy lifting equipment. The TW1 block is also manufactured locally by Remacon, a Tensar licensee, for that specific block manufacture. 

In utilising a new system a number of challenges were experienced during construction. This 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 not experienced in building these walls . However, these 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

This project has showcased the level of knowledge and experience required to design and construct a Tensar TW1 Mechanically Stabilised Earth Wall and this has been a major achievement for Kaytech, as this is the first wall of this size to be constructed in South Africa. This system provides a number of benefits over other block and other Mechanically Stabilised Earth systems, including the effective connection between block and geogrid, a near-vertical face inclination, locally manufactured blocks, aesthetic appeal and labour intensive construction, eliminating the use of heavy lifting equipment.

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Northern Portal with rail-bound hydraulic drill rig and loader system

The tunnel was constructed using drill-and-blast techniques. The tunnel sub-contractor Bomar Projects, used a rail-bound Montebert H181 jumbo for the drilling operations. This hydraulic drill rig has two booms and can drill to a depth of 3,7m. The mucking operations will be conducted using a rail-bound Atlas Copco LM71 loader in conjunction with a Hopper loading system.

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