|Location||Aberdeen City Centre|
|Requirement||Basement Retention & Bearing Piles|
|Techniques||Cased , Drilled , Rotary Bored|
|Main Contractor||Bancon Construction Limited|
Following the completion of a successful trialling exercise, TechnikGS(TGS) was awarded the contract for the design and installation of a permanent contiguous pile retaining wall. The works were carried out at the site of a former garage and petrol station on Union Grove in Aberdeen’s City Centre.
The relatively restricted site was bounded by the gable walls of two existing three storey masonry buildings; Union Grove to the north and a relatively fragile masonry perimeter wall to the south.
The works included the provision of up to 4.0 metres of retention with vertical bearing capacity of up to 100kN/m run on the pile capping beam.
- Maximisation of the basement footprint meant working at up to 750mm from the centre line of the wall to existing vertical faces of the gable of existing buildings and the perimeter wall.
- Granite bedrock with sloping rockhead and of varying strengths over a very short distance in plan.
- Stringent criteria for the permissible deflection at the pile head.
- Sensitivity of adjacent structures to vibration which required vibration monitoring to be carried out constantly throughout the pile installation works.
- Restricted site requiring careful planning with regard to material storage, concrete deliveries and spoil management.
TGS was able to provide a solution comprising 300mm diameter and 450mm diameter piles installed using minipiling rigs with specially adapted, multi-‐hinged safety cages. This allowed the safe installation of contiguous piles at a distance of as little as 750mm from the existing walls (centre of pile to vertical face).
In order to minimise vibration levels, the majority of piles were cased and augured through the granular overburden material, containing granite boulders, to depths of between 2 and 5 metres onto the granite rockhead. The pile bores were then progressed in granite bedrock using drilling techniques to the required depth, using 300mm diameter and 450mm diameter down the hole hammers with air flush. Drill bit maintenance and replacement were critical in ensuring the pile bores were constructed efficiently and with minimum disruption in the form of vibrations.
During the drilling works, additional precautions measures were taken to suppress the dust and contain the air borne arisings.
All piles were reinforced using full length cages comprising either H16s or H25s before the high slump, C28/35 and C32/40 concrete was placed using a tremme. Cube test results for the concrete confirmed the design strength was reached well before the 28 days.
On the excavation of the basement, the pile installation tolerances observed appeared to be particularly good especially in light of the very difficult drilling conditions.