What lies beneath
It is in the nature of piling and foundation work to have more obstacles below ground than above, but with a disused underground shopping centre directly beneath the development site, 1, Aldgate Union provided some unique obstacles for leading geotechnical specialist and Bachy Soletanche.
Indeed, logistical questions needed to be answered even before Bachy Soletanche had mobilised equipment to Aldgate Union, as the underground construction in the area meant a weight restriction of 38 tonnes covered all access routes to the site; a significant obstacle when the rig required for the work weighs over 75 tonnes. The Bachy Soletanche team overcame the problem by engaging a drastic weight-loss programme for the hefty rig, dismantling the ballast weights from the main body of the rig and transporting them to the site individually – thus remaining under the 38 tonne weight restriction at all times.
However, the difficulties didn’t end with gaining access to the site, as once the piling rig had arrived, the tight development area at Aldgate Union provided more complications. As a high-rise development, the piling work involved the construction of vast Large-Diameter Auger (LDA) rotary-bored piles, and the 20m x 25m site situated between two existing office buildings proved tight confines in which to manoeuvre 112 tonnes of rig and crane. Tom Stewart, Bachy Soletanche’s site supervisor for the project commented:
“Access to Aldgate Union was extremely difficult, but this was only the start of the logistical considerations. The small size of the site itself made things very problematic once work had commenced, but we managed to overcome everything with forward thinking and strategic planning. In the end, everything went very smoothly.”
Even with the scale of the site in direct contrast with the scale of the work, the initial piling programme at Aldgate Union was completed on-schedule before Christmas. In total, this first piling project involved the construction of three LDA piles up to 36m deep, with diameters ranging from 2.1m to 2.4m.
With these initial piles in place, the designs for the further piling work was still being finalised. It had become clear that the problems with weight experienced in mobilisation to the site were simply the tip of the iceberg, as the further piling work required for the development would need the heavy LDA rig to work directly above the ceiling of the underground shopping centre, which wasn’t strong enough to withstand the weight. This time, removing weight from the rig wasn’t an option as it needed to be fully operational – so the Bachy Soletanche team ventured underground.
Barry Osborn, Contract Manager at Bachy Soletanche explained:
“As an organisation we are very experienced in dealing with unique and complex situations – so we knew there would be a solution to the specific problems at Aldgate Union. In order to complete the required piling work it was clear that we needed to find a way of supporting the ceiling of the shopping centre or it may have risked collapse when we began augering with the LDA rig directly above.”
“Together with Mace, we decided to use props to support the ceiling of the underground complex, but first we needed to strengthen the floor to ensure it could withstand the pressure placed on it by the props. To do this, we undertook a controlled programme of mini-piling in the floor of the shopping centre, before erecting the props that supported the ceiling.” He added.
Putting the plan into action, Bachy Soletanche sent a mini-piling rig underground and into the disused shopping centre. Once the mini-piling was complete, a forest of carefully positioned props was erected and the support for the ceiling of the underground complex was in place. The Bachy Soletanche team was then able to move back above ground level and begin the second LDA piling programme.
This stage of piling work was much larger than the initial piling programme conducted before Christmas; involving the construction of six 2.1m diameter piles with under-reams up to 5.5m in diameter, and three larger 2.4m diameter piles that required the rig to be situated directly above the supported area of the underground shopping centre. Each gargantuan pile contained over 140m3 of concrete and the Bachy Soletanche engineers needed to work through the night on a number of occasions to give the piles enough time to set.
Barry Osborn commented:
“The work at Aldgate Union is a prime example of how good planning, experience and careful execution can combine to overcome even the most difficult and unique circumstances.”