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In 2013, Balfour Beatty Ground Engineering (BBGE) was awarded the contract to construct a new roof on the Olympic Stadium in Stratford. BBGE provided a combined Mini & Driven Precast piled foundation solution to support a new roof almost twice the size and weight of its predecessor.
The main change to the stadium was its new cantilevered roof, which at 45,000m2, is twice the weight of its predecessor and the largest of its type in the world. It was calculated that the new roof would result in additional loads of up to 2,400kN imposed on the pile caps beneath the iconic V-columns. This meant that the existing foundation solution would have to be significantly strengthened before the stadium’s official reopening in 2016.
BBGE was employed to develop an alternative foundation solution which originally specified the use of 450mm diameter CFA piles to support the V-columns and new seating & lighting columns. BBGE worked to provide a more sustainable and cost-effective solution, proposing an alternative of 323mm bottom-driven steel cased mini piles in restricted areas and driven precast piles in areas where height restrictions did not apply. In total, 350No. mini piles were installed to strengthen the existing pile caps. These worked in tandem with the original 450mm diameter driven cast insitu piles, each of which was 12m in length. 161No. 270mm2 precast piles were driven to a depth of 12m providing working loads of up to 400kN to support the external stair and lighting towers.
Up to 20No. 323mm diameter bottom driven steel cased minipiles were installed at each existing V column foundation base then tied in by larger concrete pile caps. Piles extend through up to 10m of made ground and found in the Terrace Gravels below. They have a challenging settlement & lateral deflection criteria of 5mm. 12 V-columns experienced particularly large increases in loadings; up to a maximum of 2,400kN (80% increase) in Compression & 1,440kN (280% increase) in shear. To deal with the substantially increased shear loads minipiles were reinforced with circular hollow steel sections (CHS) in the top 1.8m of the pile and heavy reinforcement cages which extend full depth. Eight of the piles from each group were designed to withstand the additional compressive forces, while the rest resist the lateral and shear loads.
The bottom-driven minipiling system is a perfect solution when it comes to keeping noise and vibration levels to a minimum because it uses an internal drop hammer. Noise and vibration levels at the Olympic Stadium were monitored by BBGE’s Specialist Testing & Analysis Division. A Section 61 order was in place throughout the works which required noise levels to be monitored at 4 different locations to ensure compliance. Vibration monitoring was carried out on 6 existing pile caps to ensure no damage or disruption occurred during the piling works.
Our in-house UKAS accredited Testing & Analysis Division also carried out lateral load tests in order to confirm the large shear capacity of the piles was achieved. Dust monitoring was also carried out at a local school to ensure that levels were within an acceptable range. All of the tests successfully verified BBGE’s calculations.
The site was originally comprised of housing and industrial areas with heavily contaminated soils. In the initial construction of the stadium some of this soil was remediated, to depths of up to 3m. It was specified that penetrating below this layer and removing subsequent arisings should be avoided as this would require further remediation. Therefore the use of a displacement system, such as bottom-driven steel tube mini piling was ideal as it produces no spoil for removal. Below the fill and soft soils the site is underlain by Terrace Gravels into which the bottom-driven mini piles are founded.
By following RUFUS (Re-use of Foundations in Urban Sites) guidelines we were able to reuse the existing piles and upgrade the capacity of the foundations.
Due to the congestion of reinforcement within the pile, with CHS and heavy cages required, BBGE opted to use a batched grout mix rather than concrete. This facilitated an easier flow between the various reinforcing elements to ensure a pile of sound integrity was constructed. We worked closely with our concrete suppliers to develop a suitable mix, and various trials were undertaken before the final product was selected.
In some instances BBGE was required to install piles internally, working approximately 60 metres within the building. For the installation of these piles we used an electric powered rig and forklift fitted with rubber tracks as well as weld fume extractors to ensure compliance with health and safety regulations governing welding operations. These works proved challenging due to the extremely limited headroom necessitating the use of a short rig mast and limiting casing sections to 1m. The lack of spoil generation is a significant advantage when working internally because the additional plant and labour in the works area required to control and dispose of the material is eliminated.
"We are very pleased with the outcome of the BBGE piling team’s work. They installed all the piles in safe and timely manner and brought forward an alternative driven mini-piling solution to save time and money, which also brought environmental benefits. They worked well with us as the contractor, and collaboratively with all the design and construction team members.”Colin Naish, Executive Director Queen Elizabeth Olympic Park (LLDC)
“The BBGE team did a first class job on this project, from bringing forward the alternative driven mini piling solution in the first place to installing the piles in a safe and timely manner. This was a job where the design solution had to be right and they got it spot on. The guys on site were a pleasure to work with and were eager to collaborate and assist the overall project team.”Stuart Fraser, Project Director BBCSUK
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