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 400mm 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 40mm to 65mm in the deepest fill areas. In addition, the floor slab carries a surcharge load of 75kPa, and the columns a structural load of 150kPa. Further calculations indicated additional settlements under the floor slab of between 50mm and 75mm, and under the columns of 60mm to 90mm with the assumption that the fills were constructed as 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 25mm over 20 Years.

There were a number of potential solutions available for excessive fill settlement, such as:

• Dynamic Compaction
• Conventional Rigid Inclusions
• Jet Grouting
• Compaction Grouting
• Compensation Grouting
• Piling

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. We 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 1,0m thick soil transfer ‘mattress’ to transfer the load to the rigid inclusions, evenly spreading the support across the surface bed. Where rigid inclusions overlaped the column bases, reinforced underpinning rigid inclusions would have to be installed through the column bases.

Because of the nature of the fill, we 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. Correct spacing of each position would mitigate excessive loads per position as per the below diagram:

The final design required installation of a total of 700No rigid inclusions installed, and a total of 750No underpinning piles extending up to 8m 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 and 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.
Our construction programme was based on completion of the 7710 No inclusions within a 7-month duration; total metres drilled accumulating to 125 000 meters 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.
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Execution:
To make sure we were able to keep up with required daily productions, we deployed a crew of 55 staff, including an on-site workshop, multiple backup drilling tools, rigs and grout pumps permanently on standby for the duration of the project. NO delay was tolerated.
Design and consultation:
With the rigorous international design reviews undertaken before commencement of the works, this gave the client peace of mind in our offer as proposed for the SOIL IMPROVEMENT solution, with the renowned Jones and Wagener Geotechnical consultant Peter Day on watch brief on behalf of the client, as an additional safety net and expert advice on the project.

Monitoring:
A final hurdle that needed to be overcome regarding the design was the possibility of lateral movement of the fill during or after the Soil Improvement system was in place. The rigid inclusions were not designed for lateral loads that may result through lateral movement of the fill. To monitor this, various inclinometers were installed over the treatment area to further monitor movement. To date no settlement nor lateral movement has been noted.
Insurance:
In consultation with our insurance brokers, the client was professionally advised on the initiation of a Site-Specific Insurance Policy (SSIP), over and above our in-place insurance policies of professional indemnity and product guarantees to further assist in case of error in the execution of works and design.
In conclusion, all factors of risk were identified by the project team and a structured but “failsafe strategy” was implemented not only as an early warning system, but a preventative measure.

The difficult execution of this project and soil conditions was further complicated by the “limited access” nature of the existing structure.
In certain areas we experienced headroom constraints, the lowest of which did not exceed 4.5m. 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 and to our knowledge has never previously been done to such a scale and in the manner proposed by the design.

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. Nevertheless, we strived to employ up to 55% of our staff compliment out of the local community. To do this, we were required to facilitate specialist accredited OEM training for the operation of our equipment that they would be able to use in other sectors of construction.
To complete our team for the monitoring and quality control to this scale, we required a team of site engineers. We took it upon ourselves to employ 5 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.

With a total of nearly 9000m3 of 40Mpa grout required to complete the installation, we employed the expert services of Metier Mixed Concrete to assist in the design and supply of a mix that would still abide to the strict design criteria of our solution. Keeping in mind the site is situated near a large farming community and the risk of contaminating local water sources, our grout mix was designed to ensure the lowest possible carbon footprint for the execution of works. The mix was based on a reduced cement content, echo plasticisers and increased fly ash content that would over time still reach the specified 40Mpa criteria and stay workable for placement, without the risk of bleeding and segregation that may cause spillage and runoff outside the warehouse fill embankments.
During drilling works, we opted to use the AMC HV Foaming agent, which is a biodegradable, highly concentrated foam and has been formulated for use in mineral and water well drilling applications to ensure no contamination to underground water sources of the local farming community.

Due to most of the scope being completed within an enclosed structure, a large Health and Safety risk identified was the management of dust, which was caused using high-pressure percussion drilling techniques. Our equipment was fitted with specialist dust suppression systems using water and an echo friendly drilling foam to suppress hazardous dust and flying objects caused by drilling works.
A further temporary dust screen was installed to cordon off the treatment area. This was a massive screen installed from the top of the 25m high rafters of the warehouse roof to platform level, separating the Southern portion of the warehouse completely from all drilling activities and dust contamination.
A rigorous health and safety specification was drafted by the client’s Practitioner, C & M Safety Consultants and our in-house safety officer specifically based on the design proposal and construction techniques required and monitored by our permanent safety team appointed for the project.