Construction World December 2023

SPECIALIST CONTRACTORS OR SUPPLIERS

PEPKOR DISTRIBUTION CENTRE – KEYSTONE PARK – SOIL IMPROVEMENT WORKS

T he new main distribution warehouse for Pepkor was 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. 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 our 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. We 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

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