The new main distribution warehouse for Pepkor is located in the Keystone Park Light Industrial, Warehousing and Logistics Precinct in Hammersdale KZN. This immense 110 000 m 2 warehouse was constructed as the main, fully automated distribution warehouse for the Pepkor Group. This project was the winner in the ‘Specialist Contractors or Suppliers’ category in the 2023 Best Projects awards.
During construction, the structural engineers noticed an issue with differential settlement of the earth platform. Due to the strict tolerances for the automation of the warehouse, the settlement would cause the robotics operating on the warehouse floor to lose track of position and render a large part of the warehouse unusable.
The size of the warehouse meant a large cut-to-fill exercise was undertaken initially to create the platform (+-500 m x 350 m) for the warehouse. Three fifths of the warehouse footprint was situated on sandstone or shallow fills following the in-situ rock profiles. The remaining two fifths of the warehouse was situated on an outer shell of choked rockfill and a wedge of fill that in some instances was constructed up to a depth of 25 m. This was the area that suffered from large settlements, and SOIL IMPROVEMENT was required.
The client went to tender to acquire a turnkey geotechnical solution to stabilise the platform from further settlement. GEOCIV Group submitted an offer that was not only suitable as a solution but was vastly superior regarding cost, programme, and effectiveness than competitors. Another “Innovative Geotechnical Solution,” says Managing Director Greg Whittaker.
The design for the structure was based on large conventional pad footings as the foundation. The foundations and majority of the superstructure had already been constructed at the time of tender.
Based on further geotechnical studies, it was concluded that the fill consisted of coarse granular material, mainly obtained from the sandstone in the cut area on site. As a result, the bulk fill has numerous sandstone rock fragments up to 400 mm in diameter, making conventional piling problematic. The expected settlement for a well-constructed fill platform on incompressible founding can be in the order of 0,25% of the depth of the fill under its own weight alone. GEOCIV calculated that this would result in settlements in the order of 40 mm to 65 mm in the deepest fill areas. In addition, the floor slab carries a surcharge load of 75 kPa, and the columns a structural load of 150 kPa. Further calculations indicated additional settlements under the floor slab of between 50 mm and 75 mm, and under the columns of 60 mm to 90 mm with the assumption that the fills were constructed correctly as specified.
The tender required a solution to reduce the variable settlement in the fill zone to guarantee a maximum settlement of no more than 25 mm over 20 years. However, most of these solutions are not viable due to the nature of the structure, the depth of the fill, the nature of the fill, and the limited access within the partially completed warehouse. Rigid inclusions – a thin slab was designed to transfer the load to the soil, and a piled solution would therefore not work, as the slab could not span between fixed pile positions. GEOCIV therefore proposed the installation of rigid inclusions into the incompressible rock below the fill and in-situ soil.
This would require the construction of a 1 m thick soil transfer ‘mattress’ to transfer the load to the rigid inclusions, evenly spreading the support across the surface bed. Where rigid inclusions overlapped the column bases, reinforced underpinning rigid inclusions would have to be installed through the column bases.
Because of the nature of the fill, the contractor reverted to the installation of rigid inclusions by means of percussion drilling techniques. The nature of the fill and the grouting process would result in a very high friction value on the rigid inclusions, resulting in the full weight of the soil column being transferred to each inclusion. Furthermore, the rigid inclusions would be socketed into an incompressible stratum, which allowed higher loads at each position.
The final design required installation of a total of 700 No rigid inclusions installed, and a total of 750 No underpinning piles extending up to 8 m past the footprint of the warehouse externally, to meet the settlement criteria of the fill. To complete the system an engineered soil raft was required to transfer the load evenly across the total area of the treatment zone.
This system was designed in collaboration with Fisheagle Geotechnique and Eco Elementum Engineering. The design was reviewed by Soilmechanics GmbH in Germany as an independent consultant and finally by Jones and Wagener on behalf of the client. It was ultimately approved in all instances.
After the design process and review was completed, the execution of works was like any construction project, but GEOCIV Group was under an immense time constraint. The client, Pepkor, was at this stage unable to occupy the warehouse as its main distribution centre.
The construction programme was based on completion of the 7 710 No inclusions within a 7-month duration; total metres drilled accumulating to 125 000 metres of percussion drilling and grouting through the variable fill stratum.
This, including the soil raft and final floor slab was to be completed in record time to mitigate large financial losses for the client. All critical path milestones were met without fault, even with large quantities of redrilled and re-grouted positions due to localised collapse within the fill.
Record keeping to this scale required a team of site engineers, skilled management, and staff, including the management of an independent third-party testing service to confirm the correct installation and reporting.
Rigorous integrity testing of grouted inclusions as well as static load testing were conducted to prove the system with great results. The difficult execution of this project and soil conditions was further complicated by the limited access nature of the existing structure.
In certain areas the contractor experienced headroom constraints, the lowest of which did not exceed 4,5 m, by using our fleet of specialist limited access drilling rigs and custom engineered drilling equipment and grout pumps made the execution of the proposed design possible.
As with most modern construction projects, it is any contractor’s duty to get involved in giving back to the community. With the specialist nature of our company’s field, it is not always possible to employ large quantities of general labour from localised community wards.
To complete the team for the monitoring and quality control to this scale, the contractor required a team of site engineers. The company took it upon themselves to employ five graduate engineers from the KZN region, who under supervision and design training by Fisheagle Geotechnique, kept accurate records of each borehole and multiple tests completed during the project.
“Coming across a project like this is certainly a once in a lifetime opportunity,” says Jean Breedt, Operations Director – “not only to test the limits of an ‘innovative design’, but mostly the successful execution of the works with specialist geotechnical rigs and plant, and something that has not previously been done.” It also allowed GEOCIV Group to showcase what a team of professionals in their field can achieve if put to the test, to help resolve a very sensitive and urgent problem.
A total of 125 000 metres were drilled and 7 710 No Rigid Inclusions drilled and successfully grouted
and tested. The Pepkor Distribution Centre in Hammersdale is currently operating to its full extent and is certainly a masterpiece of engineering prowess and determination, not only with regards to its scope of works, but the majesty of its sheer scale and complexity as a mega structure completed in South Africa.
