Namakwa Sands Expansion Project

Tenova-Bateman approached George Stott early in 2012 to assist with the design, manufacture, and installation of a hereto underutilised steel foundation technology on a new 40 TPH Dry Magnetic Separation Plant at the existing Unattritioned Mags Material (UMM) installation of Tronox’s Namakwa Sands heavy minerals mine near Brand-se-Baai, South Africa.

George Stott is a multi-disciplinary steel manufacturing group based in Johannesburg and introduced screw piles to the South Africa market in 2004 under the brand name - GeoPile. The company has since tackled various foundation projects in the telecommunications, civils, and mining industries with great success.

Although screw piles are routinely used as a foundation system in the UK and USA, this project was one of the first large and complicated screw pile foundation installations undertaken in South Africa.

The whole plant consists of six separate plant areas connected by conveyor systems. Each of the areas consists of equipment, enclosed buildings, or a combination of both. This provided a challenge in terms of aligning equipment and building footprints, and the positioning of the actual screw piles. A foundation grillage was developed which could effectively connect the equipment and buildings with the screw piles. Tolerances on equipment and building connections to the grillage were extremely tight, and dictated that the screw piles be positioned exactly.

Why Helical Screw Piles?

Key attributes of screw piles are that the piles can be loaded to full capacity directly after installation, that no major civil work need to be undertaken and that they are completely removable should the site be decommissioned. Projects which are time and environmentally critical or where access to concrete is limited are ideal candidates for screw pile foundations.

The UMM plant’s expected lifetime is a mere three years, after which the plant will be decommissioned. Having had experience in erecting steel structures with no environmental damage and in extremely short construction times, George Stott’s steel foundation solution appealed to Tenova-Bateman’s engineering and project teams. A further benefit of a steel foundation system over the conventional concrete foundations is the ability to easily remove and recycle the complete foundation once the plant has fulfilled its purpose.
 
Having used the screw pile system no major civil works had to be undertaken on site and no permanent concrete had to be used other than for floor screeding. This is due to the fact that screw piles can be erected on uneven surfaces with the foundation grillage providing the necessary levelness for structure mounting.

Construction

A screw pile’s physical construction resembles that of a self-tapping screw. As with a self-tapping screw the pile consists of central shaft which has helix-shaped bearing plates attached at specific distances from each other and the expected ground level. When installed the screw pile forms a virtual cylindrical shape between helixes which offers shearing resistance whilst the top and bottom most helixes’ exert bearing resistance to tension and compressive forces.

Depending on the loads, soil type and required installation lifetime the central shaft is either manufactured from a solid round bar or thick wall tube. Each helix has a specific pitch and diameter and careful consideration is given to their design before a project can be successfully completed.

The supported structures can either be connected to the screw piles directly or alternatively a simulated foundation can be constructed from structural steel sections on which the structure can be mounted. The simulated foundation effectively distributes all of static and dynamic loads of the aboveground structure into the individual screw piles.

This project required 167 individual piles along with 32 tons of grillage work. The screw pile construction was designed to withstand a 10 ton/100KN tension load and a 15 ton/150KN compressive load.

The screw piles where manufactured from 100mm Nominal Bore Shed 80 tube with an overall length of 3m. Two helixes of 250mm diameter each with a pitch of 100mm where attached to the central shaft by Grade 8.8 HDG bolts. The helixes and central shaft where constructed in such a manner that the helixes could be re-positioned along the 3m shaft should the soil conditions vary considerably from the anticipated conditions.

The grillage was constructed from various size beams between 152x152mm to 203x203mm. The use of 50mm slots made the positioning and bolting of the piles to the grillage members possible, providing a small tolerance for screw pile placing. Absolute squareness and levelness was critical to ensure that the equipment and building baseplates aligned 100% to the 22mm holes provided on the grillage members.

Installation

These 3m screw piles were installed by fixing the 25mm driving plate head of the screw pile to an 80Kn.m torque motor. The torque motor was attached to a 22 ton PC200 Komatzo tracked excavator. The effectiveness of each pile was tested by means of a 30 ton hydraulic test rig. A load of 150kn was applied which is 50Kn over the 100Kn which the design specified. Each of the screw piles on this installation had zero creep due to the nature of the soil in which installation was done.

It is interesting to note that there is a direct correlation between the torque required to screw the anchor into the soil and its ultimate loading capacity in tension. Screw piles have been proven to perform 1.5 to 2.5 times better with compressive loads than in tension. To accommodate dynamic loading on the piles a grillage is used to distribute the loads evenly across all piles with some acting in tension and others in compression.

Considering that each pile had to be accurately positioned to within 50mm on the x-and y-axis and that with virtually no z-tolerance was allowed, the 35 minutes it took to install a pile was quite extraordinary.

All 167 piles and their related grillages were installed in 34 working days which was shortened from the initial 75 working days planned. A further benefit was that since the pile could be loaded immediately construction of the equipment and building could start as soon as one area’s piling and foundation grillages were completed, effectively shortening the whole construction of the plant considerably.

 

screwpiles used tronnox plant


Summary

In short we believe this project shows the adaptability of steel in almost any aspect of construction. In this project’s case the use of steel as a foundation system resulted in drastically reduced installation times. Also to be noted is the ease of removal after the usefulness of the project and the fact that it is 100% recyclable.

Considering that South Africa’s civil engineering community is fairly traditional and generally not open to new ideas, the fact that such a large undertaking was successfully completed in such a short time is truly a remarkable example of what can be done by the innovative use of steel screw piles.