DAWSON WAM were appointed by GRAHAM-BAM Healthcare Partnership to provide all the piling requirements to enable the construction of both the new Generic Ward Block and the new Acute Services Block as part of the redevelopment of the Ulster Hospital. The piling works for both projects were completed while working adjacent to operational theatres and laboratories with the emphasis on keeping noise, vibration and dust to a minimum. On occasions piles were installed as close as 100mm to the existing live hospital building.
DAWSON WAM worked alongside their geotechnical engineers Byrne Looby Partners to provide the design and all engineering supervision for the installation of the following:
- 680 linear meters of 630mm diameter contiguous piled retaining walls
- 200 linear meters of 630mm diameter hard/hard secant piled retaining walls
- 1100 number 630mm diameter load bearing piles
- 50 number 630mm diameter tension piles
- 40 number 450mm diameter tension piles
- 2 number preliminary static load tests
- 11 number working static load tests
- 1 number working tension test
- 100% integrity tests
- Installation and monitoring of 16 number inclinometers
- Temporary propping to temporarily support piled walls retaining up to 10m in height
A large proportion of the contiguous and secant piled walls were designed in the short term to cantilever without the requirement for any temporary propping. Long term stability was achieved through the use of the basement and ground floor slabs (and on a few isolated occasions’ intermediate floor slabs) as permanent props. The acceptable designed deflection limits at the head of the piles was 30-40mm. In practice no greater than 19mm was recorded. The inclinometers were monitored weekly during the phased excavation of the basement by DATUM Ireland. A key feature of the solution was to allow the bulk excavation to proceed prior to the capping beam being formed. This was a significant programme and cost saving advantage provided to the main contractor particulary because of the requirement to face the wall with no fines concrete and install a proprietary tanking system. Pouring this wall was made easy by having full access to the top of the formwork for placing and compacting the concrete without the presence of the capping beam.
The piled retaining walls were installed at two different levels with the bottom basement in areas 11 meters below ground level. Part of the building included a double height basement where the piled wall was required to span the full 11 meters in the temporary condition. DAWSON WAM alongside our geotechnical engineers developed a 630mm diameter hard/hard secant wall that could transfer temporary prop forces with one level of temporary props to the lower level secant wall which would then resist the temporary prop forces until restraint was provided by the permanent structure at basement, intermediate and ground floor slab levels. Our one level temporary propping solution enhanced the constructability of the substructure for the follow on trades.
On completion of the 6.5m deep bulk excavations DAWSON WAM mobilised the piling rig into the bottom of the basements and from here the 1100nr load bearing piles and 90nr tension piles were installed. Piles were designed to take loads from 140t to 335t and varied in length from 8 - 22m. These piles were installed into weathered becoming strong sandstone with rock sockets varying from 3m to 11m. The drilling was made possible due to the use DAWSON WAM’s Cased CFA and Double Auger Drive systems. Both projects were completed within the 12 week and 18 week respective piling programme to the satisfaction of the Client and Graham/BAM Healthcare Partnership.