CASE STUDY

Retaining Walls / Valley Hyper Centre



VALLEY HYPER CENTRE: MULTIPLE CLOSED-FACED RETAINING WALLS


Ideating and implementing innovative solutions to counteract construction space constraints and civil engineering works problems to create multiple close-faced retaining walls that unlock maximum floor space, at minimum cost.


Background

Valley Hyper Centre is a new ±16 700 square metre shopping centre located in Nelspruit (Mbombela), Mpumalanga. As the earthworks for construction of the centre commenced, EIS, in conjunction with Jones & Wagener, was commissioned by Endecon Ubuntu Engineering Consultants to design and build three retaining walls along the perimeter of the development, to prevent environmental soil from encroaching on the development and retain optimal usable floor space.


Challenges

One of the major challenges at this site was that it had excessive groundwater, to such a degree that up to 18,000 litres of water were drained daily during the earthworks phase, with no signs of submission.

Another key issue was that there was extremely limited space to work in whilst building the retaining walls, due to an existing road along the top side of the retaining wall and the building of the new shopping centre in front of the walls.



Highlights from approach and methodology


  • To counteract the high groundwater issue, EIS engineered innovative blanket drain outlets to direct the water into existing stormwater systems - and prevent it from affecting the retained soil or impacting the structural integrity of the retaining walls. The outlets were inserted over the retaining wall foundation and below ground level - to ensure that it is also out of sight. The blanket drains were constructed with 19 mm stone, bidim® A2 and an open structured geotextile to accommodate the high fines content in the decompose granites. Bidim® A2 is a non-woven needle-punched geotextile that serves the dual purpose of filtration for subsoil drainage layer, as well as separation for stabilising and reinforcing the soil.


  • To meet the client’s requirement for maximum usable floor space, whilst addressing the space constraints for building the three retaining walls, EIS opted for the design and construction of three open-faced retaining walls at an 80-degree angle. The length of these three walls were 340, 135 and 222 metres respectively, varying between 2.1 and 8.5 metres in height, and 450 square metres to 1500 square metres in face area.


  • Factors that had to be taken into consideration during construction, include the fact that the distance between the building and the walls were only 600mm at some extremities - and in some areas, the available tieback was limited to less than 25% of the wall height. Plus there was an additional surcharge load from the adjacent road.


  • To construct the open-faced retaining walls, EIS opted for Bosun’s cutting-edge Robust Block system which has proven to have better weight distribution and height tolerances when compared to standard retaining wall blocks. Whilst consistency in height is a common issue with many other retaining wall blocks, the Bosun Robust Block fully complies with SANS 508 which stipulates dimensional differences of no more than ±3mm.


  • The Bosun Robust Block system also has a purposefully designed nib which provides excellent shear resistance against horizontal forces exerted by retained soil. In addition, it has a V-shaped stiffener which enhances load distribution, especially in such instances where there are additional surcharge loads from vehicles or buildings. This combination of nibs and stiffeners furthermore made a significant contribution to the structural soundness of the retaining walls, despite the minimal tieback.


  • Some other advantages include that the system can be stacked with only 5.1 blocks per square metre and has no highs and lows, due to its novel manufacturing method, which in turn ensures greater accuracy in installation. Then there is the ease of handling and packing of these blocks, a factor that is often overlooked, but in reality, it has a substantial impact on the project bottom line - saving both time and money.


  • Outcomes



  • The inventive blanket drain systems that were installed over the retaining wall foundations ensures ongoing retention and management of the excessive ground water in the area, and contributes to the stability of the structures constructed.


  • The centre received superior retaining walls that are both functional and aesthetic, whilst obtaining maximum floor space for operations, at minimum cost (EIS were able to construct these structures at R1 million less than quoted by other contractors).


  • Project scope



  • Main contractor: Belo and Kies Construction

  • Face area: 2,534 square metres (for a total of three walls)

  • Value: R5.2 million

  • Duration: 11 months

  • Year completed: 2020