AECOM showcases its geotechnical expertise at Exxaro head office
 | AECOM showcases its geotechnical expertise at Exxaro head office_approved | 40.5 KB | Download |
 Preview | AECOM oversaw the structural, civil, geotechnical, and quantity surveying aspects of the project. | 2.47 MB | Download |
 Preview | Kim Timm (Pr Eng), Executive – Structures, Buildings and Places at AECOM. | 2.15 MB | Download |
 Preview | The Exxaro head office at Lakeside in Centurion, developed by Growthpoint Properties. | 1.95 MB | Download |
 Preview | The winning AECOM team (Martin Smith, JP Kitshoff, Kim Timm, Michael Sykes and Xonlani Mandidi) | 4.93 MB | Download |
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.
Ends
Notes to the editor
To download hi-res images for this release, please visit http://media.ngage.co.za and click on the AECOM link to view the company’s press office.
About AECOM
AECOM is built to deliver a better world. We design, build, finance and operate critical infrastructure assets for governments, businesses and organisations. As a fully integrated firm, we connect knowledge and experience across our global network of experts to help clients solve their most complex challenges. From high-performance buildings and infrastructure, to resilient communities and environments, to stable and secure nations, our work is transformative, differentiated and vital. A Fortune 500 firm, AECOM had revenue of approximately $20.2 billion during fiscal year 2018. See how we deliver what others can only imagine at aecom.com and @AECOM.
AECOM Contact
Lara Lombard
Communications & Marketing Officer, EMIA, Africa
Phone: (012) 421 3832
Cell: 060 573 4240
Email: lara [dot] lombard [at] aecom [dot] com
Web: https://www.aecom.com/za/
Media Contact
Renay Tandy
NGAGE Public Relations
Phone: (011) 867-7763
Fax: 086 512 3352
Cell: 082 562 5088
Email: renay [at] ngage [dot] co [dot] za
Web: www.ngage.co.za
Browse the NGAGE Media Zone for more client press releases and photographs at http://media.ngage.co.za