The props are being used to support the sides of a two-storey excavation situated in anincredibly tight site at 5 – 7 St Helen’s Place in the City of London.
Working for principal contractor Brookfield Multiplex, specialist contractor O’Keefe Construction (Greenwich) Limited has completely demolished the existing building, retaining the elegant 1919 Portland stone façade, in order to construct a new building which will home the Worshipful Company of Leathersellers, one of the City of London’s Livery Companies.
Part of the design requires the enlargement of the existing basement which O’Keefe is extending down another 6m to create a two-storey basement.
Poor ground conditions and the close proximity of neighbouring buildings have been the biggest challenge to the construction team. The excavation occupies the entire site, which measures approximately 44m x 25mand is hemmed in on every side by existing structures.
Most delicate of all is the 800 year old Grade I-listed Church of St Helens Bishopsgatewhich adjoins most of the southern boundary of the site.
After the installation of a concrete secant piled wall, a reinforced concrete ring-beam was cast on top with special concrete bearing blocks incorporated into it. These blocks provide perpendicular restraints for the Groundforce MP250 props.
“We required perpendicular connections as this reduces the acting shear forces; thus minimising the number of anchors required to fix the props – making for a simpler and quicker installation” explains Ajay Nagah, Senior Engineer (Major Projects) for Groundforce.
The temporary propping solution had to ensure that the potential for ground movement was absolutely minimal in order to avoid damage to the church and the retained façade which was supported on steelwork cantilevered over the edge of the excavation.
The traditional method – installing bespoke steelwork within the excavation – would have been very slow and labour-intensive; it would also have been difficult to assemble within the narrow confines of the site.
Far quicker was to install modular hydraulic props which could be lifted in by the site tower crane and quickly extended between end bearings and pre-loaded to provide stability.
The system chosen was Groundforce’s MP250 – one of the largest standard prop types available in the UK and each capable of supporting a load of up to 250 tonnes.
Most of these 22 props utilised Groundforce’s 1220mm diameter “super” extension tubes; only those providing additional corner bracing were fitted with the usual 610mm diameter tubes.
“This was all because we had to maximise the stiffness of the temporary supports” says Ajay. “The actual design load was typically around 50% of the capacity of the props for the upper level and 70% of the capacity for the lower level” he adds.
The 1220mm super extension tubes are usually used where the props have to span wide excavations without intermediate supports. Although these tubes allow clear spans of up to 50m, nowhere here was the span more than 25m and most were substantially less than this.
The props were installed at two levels, the lower level bearing on a steel waling beam fixed around the perimeter of the secant piled wall. The total depth of the excavation was 9.2m.
To further ensure maximum rigidity in the system, each of the props was mechanically ‘locked-off’ with steel rods once the correct extension had been set hydraulically. In most applications, the hydraulic rams themselves can be relied upon, but not here, says Ajay:
“The hydraulic system offers less stiffness than that of a fully steel prop, and this would allow a tiny bit of movement” he explains.
Lee Horsley, Operations Director for O’Keefe, says that this is the first time his company has used the Groundforce system."We opted to use the flying shores from Groundforce as it proved to be the most appropriate method of installing the props in sections to accommodate the complex excavation sequence"
Throughout the excavation process, the loads acting on the props were recorded in real time using Groundforce’s own monitoring system. Strain gauges in the prop’s end connection pin transmit signals to an onsite GPS/SMS transmitter which sends readings to a remote server which is accessed by a secure website by Groundforce, the client and other authorised parties.
The system not only records actual loads and temperature, but can also be configured to notify named recipients if readings approach or exceed pre-set levels.
As it turned out, this function was not required at St Helen’s Place. The propping system, installed during the summer, was removed in late November after completion of the basement slab and permanent supports.
Principal contractor Brookfield Multiplex is now busy constructing the new building behind the retained façade.