Anchor construction challenge solved for Rosebank

Posted On Tuesday, 27 April 2010 02:00 Published by Commercial Property News
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An ingenious construction method has overcome a challenge posed by the Gautrain tunneling project and excavation for the R850 million expansion of The Zone @ Rosebank in Johannesburg by Old Mutual Investment Group Property Investments

Brent Wiltshire Old Mutual Investment Group Property The Zone phase 2 is schedule for an opening in April (the 160-room Holiday Inn) and a September retail opening, says Brent Wiltshire, development executive of Old Mutual Investment Group Property Investments.  Apart from the Holiday Inn, the project entails five underground parking levels, two levels of above-ground retail, and commercial office space.

Wiltshire says the challenge faced by the engineering and construction team of AKi Consulting and WBHO arose because the project site abuts directly on the Gautrain railway reserve that runs down Oxford Road.

“The complex nature of the Gautrain tunneling operation inside their reserve made it impossible for The Zone’s lateral support piles to be constructed using the conventional method of  ‘ground anchoring’. Gautrain’s concerns were that The Zone’s anchors were to occupy the same space below ground as their anchors did. A clash between the anchors under the two sites could have resulted in a potential catastrophic failure of the anchoring systems.”

AKi had to devise a method of achieving a 15m deep lateral support excavation without the use of ground anchors. The only solution to this problem, said Peter Beretta of Aki Consulting, was top down construction.

The lateral support piles were designed to cantilever to parking level P2 (6metres). After the bulk earthworks operation reached that level, an earth wedge of 8 000m3 of unexcavated material was left in position to maintain lateral support.

From level P2 construction could now proceed conventionally “upwards”. Structural piles were sunk from level P2 down past P5 to a founding level below P5 and construction proceeded conventionally. The remainder of the basement structure outside of the area taken up by the 8 000m3 earth wedge could also continue conventionally.

Only once all of the concrete structure had reached ground floor level across the entire basement area did AKI deem it safe to commence with the top down construction

The concrete slabs on P2, P3 and P4 needed to act in the same way as the lateral support anchors. The earth surface on P2 was leveled off and compacted. A power-floated concrete layer was cast on top of this earth platform and a bond breaker of masonite boarding was nailed down on top of the blinding layer. This masonite layer performed the function of conventional shuttering for the P2 suspended slab which was reinforced and cast conventionally. After 14 days strength had been achieved on the slab the earth below the slab could be excavated down to level P3.

As excavation progressed the lateral support piles had to be shotcreted over to achieve continuous support. Once the excavation had reached level P3, the structural piles that were now exposed needed to be “converted” into columns by casting a concrete jacket around them utilizing a complex drill and epoxy system of reinforcing and a high slump concrete mix, “funnelled” into position. The whole process was then repeated over again through levels P3 and P4 until the surface bed on level P5 could be cast.

A further complication here was that 50% of the “top down” slabs were not flat but in the form of spiral vehicle ramps between the levels. This led to an interesting shaping of the ground to form the masonite shutter face.

Material handling of the 8 000m3 excavated earth was a major challenge throughout this process, said Peter Browne, contracts director  of WBHO. The limited space available at street level meant there was no storage area for excavated material. The timing of the mini excavators, loading bobcats, transporting dumpers and tipper trucks to haul the earth away took careful planning and co-ordination.

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