Piling Hammers in Reconstruction Of The Port Of Durban - continued
Reconstruction began by demolishing the existing cope beam and excavating behind the quay wall to reduce the pressure acting on the existing wall. This ensured that the wall did not fail due to imposed loads during the construction phase. As a precaution all crane-based activities were situated over 10 m from the quay wall.
To install the foundation piles the specification advised the use of impact hammers instead of vibratory hammers, as the latter are known to have the potential to induce vibration into the surrounding ground due to their high frequency. This vibration could liquefy the soil in a localised area around the toe of the existing wall, which would reduce the shear properties of the sub soil, most likely causing the toe of the existing sheet pile wall to rotate, inducing a serious failure.
To carry out the piling operation, the contractors used two BSP hydraulic impact hammers. First a CG240, with a 16t dropweight, suspended from a Kobelco 180 t crawler crane and mounted on a 40 m x 15 m floating barge, drove the 27 t king piles. Then a suspended CX85 with a 7t dropweight was used to drive the sheet piles and form the combination wall.
Prior to installing the piles, BSP carried out an important modification to the CG240 hammer.
This involved having legs fitted rather than a pile sleeve which reduced the overall weight and also enabled the hammer to drive the king piles while avoiding any clash with the pile template which had to be kept in place until driving was complete.
Geotechnical investigations allowed the SSA JV to size the correct hammers for the project. During pile driving the soil conditions along the quay wall were no problem for the BSP hammers.
Pile-driving durations in excess of two hours previously encountered on Berth 12 were substantially reduced through the use of these more powerful and reliable impact hammers.
Steel piles used for the combination wall included double I section HZ 1180M piles (king piles) to form the structural elements, and AZ 18-700 sheet piles acting as filler sections amounted to some 10,550 tonnes. A seven-bay pile guide frame was used to ensure the piles were kept within their vertical and horizontal alignment tolerances without twisting.
Because of logistical difficulties and space restrictions the 27 m long king piles were located on a designated area within the harbour complex where they were profiled with a curved cutting edge to aid installation. The 402 anchor piles 33m long underwent additional preparations for the HP grout system.
Design consultant Railway and Civil Engineering Consultants (RCE), the design consultant for project manager Transnet Capital Projects, proposed that the Müller Verpress Pile (MV-Pile or HP) system was employed for anchoring the combination wall, as an alternative to the traditional dead-man anchor solution. This is the first time the system has been used on a construction project in South Africa.
The system makes use of H-section bearing piles supplied with thickened flanges. The piles are usually driven at 45 degrees to the combination wall through the centre of the king pile to which it is later pinned. While the pile is being driven to level, grout is pumped out near the toe of the pile at 10 bar. The grout acts as lubricant during driving, and once set, it bonds to the steel pile and the surrounding substrate, increasing the frictional resistance of the anchor as well as protecting it from the adverse effects of corrosion.
The success of HP piling was measured through the results of the test piles, which passed the stringent testing and specification requirements set forth in the contract and was an important aspect of the project that the Stefanutti Stocks Axsys JV prides itself on accomplishing successfully.
We thank Eswee Kruger, site planner on the Maydon Wharf project, for his photographs, and Tim Milner, consultant Bid Manager for Stefanutti Stocks Marine for the use of his photographs and the text in this article.