The challenge
Being built by BAM North West for an international development group, specialist sub-contractor Careys was responsible for creating the sub-structure, which required the excavation of approximately 150,000 cubic metres of spoil. The work has been made even more complex by the fact the site in Manchester was affected by significant ground contamination and riddled with hidden voids from its previous use as a gas works.
Having already worked with BAM on a number of schools and other projects in the region, we were favoured for the project by the main contractor for early engagement in the feasibility and design process. We were subsequently awarded a number of orders; covering the supply of trench shields, cofferdams and the sheet-piling to help retain the perimeter of the four- to ten-metre-deep dig. Many of the retention solutions supplied involved our engineering team in the design and customisation: including for one corner to the excavation, which was supported by two 42-metre long Super Bracing Struts.
Then, to permit the heavy excavators and other machines to track safely across the unstable ground, we also supplied a pair of 20-metre long BeamBridges. Although normally used to span rivers, sinkholes and other gaps, the flexibility and versatility of the modular bridging equipment makes it ideal for a range of applications.
Our solution
Working collaboratively with Careys, we supplied the temporary works equipment for Careys to deploy, while our own expert team was responsible for installing the highly sensitive vibration monitoring, which provided a constant assessment of the live piling operations.
Typically, our vibration monitoring equipment is installed on existing buildings and other above ground structures. In this instance, however, it was a section of the elevated existing access road linking to a nearby major destination which required monitoring, due to the proximity and potential for disturbance caused by the piling operations in the site’s basement area.
Our monitoring team attached five sensors to the concrete footing at the base of the batter (slope) stretching down from the roadway; at a depth some two metres below the current ground level. To protect the sensors, a substantial timber frame was built around them and the area backfilled for the 14-week duration of the exercise.
Each of the sensors was hard-wired back to a data acquisition system, powered by a UPS (Uninterruptable Power Supply) and solar panel with a modem to transmit data to the Cloud: thereby giving the client access via the sophisticated Insite system. This allows agreed trigger levels to be set and an alert automatically sent to the client should the limits be exceeded.
Dave Hanlon, the Engineering Manager for BAM, commented: “We’ve established a very good working relationship with Mabey Hire over several years; there has never been anything we’ve wanted that they haven’t been able to supply us. On this project, we originally engaged with them because of the breadth of its offering and I also knew one of the key personnel from a previous role.
“The new structure is a very challenging project, with a short programme and where the basement structure takes up almost the entire footprint of the site. This has required the sinking of a neighbouring piled retaining wall around a large part of the perimeter, and the construction of perpendicular diaphragm walls to help retain it. In fact, we were piling to within 150mm of existing structures, hence the need for us to carry out vibration monitoring. In actual fact, Mabey Hire’s sensors only picked up occasional, very short spikes in vibration. Elsewhere on the project, Mabey Hire supplied a wide range of groundwork support systems, including two very long Super Bracing Struts, which speeded up the construction of the diaphragm walls. Progress has been good, and we are on schedule to complete in December next year.”
