CASE STUDY

Retaining Walls | The Grove

THE GROVE MALL:
TERRACED RETAINING WALL

Fusing principles of civil engineering, geosynthetics, architecture and landscaping, to construct a retaining wall that is economical, structurally sound and aesthetical - achieving optimal outcomes all-round.

Background

The Grove Mall on the corner of Lynnwood and Simon Vermooten Roads in Pretoria East had to commence with trading while a six-metre high earthen embankment which was created during construction still abutted the southern and western boundaries.

EIS was commissioned by Resilient Real Estate Investment Trust (REIT) to design and construct a solution to retain the soil embankment from encroaching on the parking areas of the mall as well as the pavement next to these roads, whilst finishing off the construction of the centre in an aesthetic manner.

Challenges

There was the parking area of the shopping mall that was already operational on the bottom side and the erf boundary top side of the embankment, plus it was close to existing Telkom lines. This translated into limited access and highly restricted space for construction of a retaining wall. As the mall was open for business-as-usual, the project furthermore had to be completed as quickly as possible. Another major challenge at this particular site, is that it has extensive groundwater, which requires creative solutions to get the soil dry enough for compaction.

Highlights from approach and methodology

As opposed to the gabions that were originally earmarked for the project, EIS proposed that a terraced retaining wall is constructed instead, for numerous reasons. Firstly, a terraced retaining wall is typically built by staggering its different parts on each other, so it almost resembles a series of steps, which is a much more aesthetic option. Secondly, a terraced retaining wall is generally more functional for such a sloped area that could potentially present a host of civil engineering works problems. Finally, based on formal cost estimates prepared by EIS for both options, the terraced retaining walls were much more economical for this particular purpose, than gabions would have been.

Due to ultra-limited space and the lack of an access road, the EIS team had to ‘cut out’ areas of the earthen embankment and formed soil stockpiles to create their own building elevations to work on. This had to be done in short phases or ‘pieces’, as and when they progressed with each next step of constructing the retaining structure.

Considering that each piece of the retaining wall presented different challenges, the senior engineering executives remained hands-on involved for the duration of the entire project, to conceptualise solutions for each of these new challenges, as and when they arose. Once such challenge, was that the area had significantly more twists and turns than usual, which made it much more complex to build.

For this reason, EIS opted to utilise the Aveng InfraBlok® 350 retaining block system (Sahara Sand colour) to construct a close-faced retaining wall with an 80-degree face angle. This system is renowned for its unique flexibility which made it possible to seamlessly achieve this angle and create convex and concave curves and complex ‘steps’ which varies anywhere between two to six and a half metres in height and length.

Other advantages are that the closed-faced construction method allows for maximum wall mass and effective backfill compaction which adds to the stability of the retaining wall. Plus the blocks can be filled in the offset configuration which enables landscaping and ensures a wall that is not only aesthetic, but basically also maintenance-free.

EIS further reinforced the soil with a range of geosynthetics, blending polymers with soil, rock and other geotechnical materials, which is proven to enhance the bearing capacity, promote aggregate interlock to ensure distribution of load forces and increase the stability and lifespan of the structure substantially. A general misperception that exists, is that the soil is retained by the precast blocks that the wall is constructed with. In reality, its true efficiency and durability, is actually determined by its reinforcement and the interlocking methods used to connect the soil with the various components of the retaining wall.

To counteract the issue of the excessive groundwater, EIS created multiple subsoil drains on the first terrace, using no fines concrete (NFC), which is a superior mix of coarse aggregate and cement. This was a critical step, as adequately compacted soil is much denser (stronger) than normal soil and has a reduced rate of water infiltration. It also has structural benefits as it contributes to the stability of the structure.

Outcomes

EIS provided The Grove Mall with a turnkey retaining wall solution that was not only economic and effective, but that also has an exceptional visual appeal. This has been reported to contribute to a notable increase of feet at the mall, since the completion of the project.

The project was completed within eight months, effectually safeguarding the parking area and surrounding pavements, with little to no disruption to the trading of the mall and no impact on the traffic on the main roads alongside the structure.

In addition, because of the fact that EIS proposed a terraced retaining wall instead of the gabions that were originally specified, the project was completed for around 15% cheaper than what the gabions would have cost.

Project scope

Main contractor: EIS

Face area: 2200 square metres

Value: R6.5 million

Duration: 8 months

Year completed: 2016