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De Put Abstraction Works | Ductile Iron Piled Foundations

New water abstraction works at De Put, Senekal, Free State – on the Zandspruit River, required the use of piled foundations for support.

Geopile Africa was recently contracted to install piled foundations at the De Put site in the Free State. It was deemed necessary for the installation of piles to improve the structural integrity due to the working platform level being completely saturated and unstable to at least 6m below grade base working level.  To add to that, the site was also underneath an existing earth dam wall.

Furthermore, there was little in the way of geotechnical information available at the time of tender. All that was known, was that there was bedrock, and it could be expected before 15m.

During the tender stage of the project, it was determined that ductile Iron Piled foundations proved to be the most economical piling solution.  This decision was made due to site- and time constraints the client was facing. One of the main reasons they chose this method was that the location of the site would make it impossible for a CFA rig to access. The team made the decision to use a standard 30 Ton excavator fitted with a powerful hydraulic demolition hammer (2800kN/blow – up to 400 blows / minute) that would perform the task with ease. This eliminated the need for a high-cost piling platform due to the lightweight nature of the installation equipment.

There was also no need for further detailed geotechnical investigation as bedrock was expected to be within 15m from working platform level from previous investigations. This is the benefit when using ductile iron piles as they were designed to be driven to refusal.  

The Engineer, Zutari, required a 600kN working load per pile.  Geopile achieved this by supplying the 170/9.0mm ductile iron pile (170mm diameter, with 9.0mm wall thickness) rated for a safe working load of between 750 and 1050kN, depending on the design life of the structure and the surrounding geotechnical conditions (amount of salt or acid – to allow for suitable wall thickness loss).

Ductile iron piles come in 5m or 6m lengths and are joined together by means of the geometry of the head (socket) and toe (spigot).  The pile joint is limited to 150mm with a slight ridge to counter any minutely smaller toes and stopover driving. The joining pile sections are usually complete after 130mm of penetration, due to the joining spigot diameter matching the leading socket.  Once the entire pile length is hammered, the pile sections are permanently fused by “cold welding”. This means that the joining happens by adding pressure without the need to use fusion or heat at the interface. 

The wall thickness at the joint is 4 times thicker than the rest of the pile – this transfers all compression and bending resistance between the sections. A pile shoe was not required on these piles due to the relatively low working load.  Should a site require a pile shoe, it can easily be fitted inside the toe of the pile keeping the entire cross-section free of debris and filled with concrete. Would the need for a pile shoe arise, the pile capacity would have increase from750kN to around 900-950kN (with 30-40mPa concrete).

Bedrock level was established to be just 9m below platform level, with all 12 piles reaching practical refusal with 1m in depth of each other.  All piles were driven to refusal with only 8 hours of working time on site. Once the pile has reached refusal, it is cut at the final pile trim level and the remaining pile off-cut section is used as the starter length for the next pile locations. The pile is then filled with concrete and fitted with a centralizing rebar and load plate.

This was another successful project completed by Geopile Africa which left the client satisfied with how efficiently the scope of work was carried out.