Contracting - Case Study

White Horse Bridge, Wembley Stadium Station, London
Main contractor Edmund Nuttall Ltd, were awarded a challenging contract to construct a new bridge and associated infrastructure, to service the new national stadium. An additional challenge was to cause minimal disruption to the railway line, local businesses and residents during construction. Dawson Construction Plant were awarded seven different subcontracts to carry out a wide variety of sheet pile installation and extraction works. For some applications novel equipment was developed and new methods of working had to be implemented with safety paramount.

Much of the work was carried out at night or with weekend railway possessions. Pictured here is a 50-hour weekend possession to install nearly 400No x 9.5m long x L604 piles to form a safety barrier between the works area and the railway line. Ground conditions comprised stiff London clay with cohesions in excess of 150kN/m2. Three ABI telescopic leader rigs were used to pre-auger and then to drive the sheets using 92.5t variable moment resonance free vibrators. Two 80t mobile cranes and two excavators were also used to move equipment, build access ramps and provide piling platforms. Pre-augering was carried out at night and vibratory driving was carried out during the day to limit disturbance.

The work was carried out successfully and safely, nearly 12-hours ahead of schedule. This allowed the main contractor to complete the possession and return the line to Chiltern railways well ahead of schedule.

During 2005 Dawson Construction Plant Ltd were awarded a series of contracts by main contractor Edmund Nuttall Ltd to carry out silent and vibration-less pile installation and extraction. Shown below is a two cylinder push-pull mounted on a Liebherr 255 rig. 140No x 8m long x L605 sheets were installed to form a temporary cofferdam for the construction of a new reinforced concrete storm water tank. The sheets were fully driven without pre-augering.

Shown below is a contract to install a line of 64No x 13m long x PU32 box piles. Two sheets are welded together back to back to form a box. This painted line of box piles formed a permanent retaining wall that was also required to carry significant vertical load from the bridge. Steel was chosen due to the speed of construction and the quality of the surface finish. This was the first time that such box piles were to be installed using silent piling techniques. A four-cylinder push-pull on a Liebherr 255 rig was used and the piles were successfully driven during a weekend railway possession. In addition to the box piles the same equipment was also used to drive 30No x 18.5m long PU32 sheet piles.

Shown below is a contract to extract the temporary sheets for the storm water tank, which are shown being driven above. Once the tank was constructed it was not possible to get access for a piling rig, to extract the piles and the presence of the IBIS hotel required the extraction to be carried out silently.

Dawson Construction Plant devised a unique way to extract these piles. A two-cylinder push-pull was suspended from a 60t crawler crane, to provide sufficient working radius so that the new concrete tank was not affected by the machine surcharge loading. This proved to be a very effective means of extracting the piles. By pulling on one pile, reacting off another pile and by utilising the crane line-pull it was possible to efficiently and quickly extract all the piles.