AECOM showcases its geotechnical expertise at Exxaro head office

14 November 2019
Challenging dolomitic ground conditions at the site of the new Lakeside head office in Centurion, Pretoria for resources company Exxaro resulted in integrated infrastructure company AECOM collaborating closely with AMA Architects to optimise the building design.
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application/msword iconAECOM showcases its geotechnical expertise at Exxaro head office_approved40.5 KBDownload
PreviewAECOM oversaw the structural, civil, geotechnical, and quantity surveying aspects of the project.2.47 MBDownload
PreviewKim Timm (Pr Eng), Executive – Structures, Buildings and Places at AECOM.2.15 MBDownload
PreviewThe Exxaro head office at Lakeside in Centurion, developed by Growthpoint Properties.1.95 MBDownload
PreviewThe winning AECOM team (Martin Smith, JP Kitshoff, Kim Timm, Michael Sykes and Xonlani Mandidi)4.93 MBDownload

AECOM was responsible for the structural, civil, geotechnical, and quantity surveying aspects of the project. The earthworks contract was completed by Stefanutti Stocks Geotechnical, while the raft and first suspended floors were cast by main contractor WBHO.

“In terms of the architecture, we had to reshape the building design to account for the ground conditions. We ended up with about 16 different variations of the initial format of the building shape compared to the underlying ground conditions. The challenge was to mitigate the risk in the most cost-effective manner possible,” AECOM Executive – Structures, Buildings and Places Kim Timm (Pr Eng) explains.

AECOM’s involvement with the project resulted in it being awarded the AfriSam Innovation Award for Sustainable Construction at the Construction World Best Projects Awards 2019 at a gala banquet at the Rand Club in Johannesburg on 6 November. It also received a Highly Commended Award in the Consulting Engineering Category.

Sites underlain by dolomite rock are prone to sinkhole formation. The Lakeside site was specified as a D4 site with Inherent Hazard Class 8 Dolomite. The site had four known cavity locations that could potentially produce 15-m-wide sinkholes. Hence an expert reviewer was required by the Council for Geoscience to verify AECOM’s geotechnical solution.

The original plan was for ultra-deep piling, as dolomite conditions tend to vary a lot. As an example, there was one place in the basement where a dolomite outcrop was at ground level. A mere 3 m away, the dolomite level dropped 48 m below surface. Hence the footprint of the underlying geography was so varied it was actually incredibly difficult to predict the dolomite pipe.

AECOM’s final design saw the site excavated to the anticipated founding level, ranging from 7 m to 1 m to below Natural Ground Level (NGL). At 5 m below the founding level, the bedrock was blasted, excavated, and blended with an on-site chert residuum mix, and recompacted. Simultaneously, high-lying Weathered Altered Dolomite (WAD) was removed and replaced.

The entire footprint of the site was then dynamically compacted. A 14.5 t pounder was dropped about 4 600 times on-site, then followed by the smoothing or ironing phase. This created an even soil mattress above the highly-variable rock profile, and below the structural foundation.

It also assisted in pre-collapsing any high-lying potential cavities and weak spots. About 17 700 m3 of material was removed from site during this process. A further 23 000 m3 was excavated, blended, and recompacted back into place.

“Instead, of piling, we adopted a combined soil raft and concrete approach,” Timm notes. The structural foundation system was a 2.25 m reinforced concrete raft, designed to span a 15 m sinkhole. The raft was designed on a mattress of variable spring stiffness, adjusted for the depth to bedrock, and the anticipated depth of the WAD and the enhanced soil mattress zone.

The raft footprint was about 82 m x 68 m, requiring 13 200 m3 of concrete and 1 500 t of reinforcement steel. The raft construction was divided into nine main continuous concrete pours of about 1 400 m3 each, with a smaller tenth pour thereafter. Three readymix batch plants supplied up to 18 trucks on continuous rotation.

AECOM also designed a system to monitor and track the resultant ground, raft and structure movement. This was achieved by logging location tags on the rafts at each column location, surveyed bi-weekly. In addition, three-rod extensometers were installed at three locations on the raft.

Each extensometer location was close to a previously-identified high-risk area. The three rods depths were founded according to profiles based on the anticipated locations of the movement and the mobilisation. In the short term, this provided a high-precision spot settlement value. This, in turn, is linked to the Lakeside BMS system, serving as an early-warning signal for developing sinkholes at depth.

While high-lying dolomite is a good substrate to build on, the problem areas were the deep rifts running through the middle of the site. “We looked at options such as towers or even bridging structures. Since the dolomite was such a defining feature of the site, the client wanted to bring it through in the entire shape and look of the project,” Timm notes. Hence even the bollards outside were designed to resemble the dolomite pinnacles themselves.

One of South Africa’s largest black-empowered coal and heavy mineral companies, Exxaro moved into its new head office at the beginning of April 2019. The project has targeted a Green Star SA Office v1.1 Design and As-Built rating from the Green Building Council of South Africa.

Now in its 17th year, the Construction World Best Projects Awards showcase excellence in the South African building, civil engineering, supply and project management sectors. It recognises projects across the entire construction industry: from civil and building projects to professional services to specialist suppliers and contracts.


Notes to the editor

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