Introduction
Groundforce supplied its Super Mega Brace hydraulic frame and 500 tonne capacity MP500 hydraulic props to main contractor BAM Civil to support a deep cofferdam during construction of the cooling water system on the Dublin Waste to Energy project in Poolbeg, Dublin. The design and build contract, which client Covanta and their representatives, PM Group, awarded to BAM, involved the construction of a reinforced concrete pumping station to supply cooling water to the new Waste to Energy plant.
Challenges
BAM had to install a temporary cofferdam, 35m long, 12m wide and 12m deep within which to construct the pumping station. The cofferdam was designed using interlocking AZ sheet piles to provide a fully interlocked sealed structure so that the construction could be carried out in the dry. The considerable depth of the excavation meant that the cofferdam was subjected to very high lateral loads, requiring a robust and secure support system to retain the ground. Located in Dublin Port, the cofferdam was also surrounded by water on three sides which also contributed to the high loads
Solution
BAM employed the services of AGL Consulting to design the cofferdam and Groundforce to design and supply the temporary bracing and propping. BAM and AGL together with Groundforce agreed on a layout and designed a support system based around the Super Mega Brace frame. This component comprised a length-adjustable hydraulic unit connected to a series of modular fixed length extension pieces. Combined with cross struts for added support, it formed a continuous heavy-duty waling beam. The depth of the excavation required the installation of props at two levels, 5m apart vertically. The loadings on this scheme were some of the highest that the Groundforce design team had encountered, and the design pushed the frames close to the limit with the top level waler at 313.5kN/m ULS and the lower level at 728.6kN/m ULS.
The Super Mega Brace was installed at the lower level, with cross-bracing provided by 500 tonne and 250 tonne capacity MP500 and MP250 hybrid struts.
The top frame employed Groundforce’s Mega Brace, supported with HSK150 struts. “The ability of the system to be rapidly installed significantly reduced the client’s programme of works” commented Groundforce General Manger Joseph Lenihan. “The system also eliminated the requirement for onsite welding which saved time and increased safety.
The cofferdam remained in place for 4 months whilst the permanent structure was being completed.
The Equipment
The Super Mega Brace is the strongest proprietary hydraulic brace in the industry. It is a high load capacity modular hydraulic bracing system designed to support square, rectangular or multi-sided excavations ranging from 4.7m to 24.2m in plan. It comprises a length-adjustable hydraulic unit connected to a series of modular fixed length extension pieces and a rough adjuster unit via a simple pinned splice connector, which when combined with cross struts, form a continuous heavy-duty waling beam.
The MP500 strut is the flagship of the Groundforce range and was developed to meet the growing demand for large struts as a cost-effective solution for all types of major excavation
It is the market’s largest proprietary hydraulic strut and consists of a hydraulic unit with 500mm adjustment, connected to tubular 1220mm Ø extensions. The hydraulic unit also incorporates a mechanical lock off system, meaning that the prop benefits from the speed of hydraulic installation coupled with the peace of mind of a failsafe back-up. Axial loadings in excess of 5000kN can be accommodated over a 35m span, without the need for any intermediate support.
What the client said
We were very satisfied with the level of service provided by Groundforce. From the initial concept stage, through planning and finally during installation of the system, Groundforce was always available for advice and comment. “We have also received excellent customer follow up service. We would have no hesitation is using Groundforce solutions for cofferdam and deep excavations in the future.