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Session 3: Ground Movements Due to Deep Excavations |
Integral Geotechnique Limited, Ove Arup & Partners, Bristol, 1 Woodford Close, Nailsea, Bristol BS19 2UH, UK
Ove Arup & Partners, 4 Unity Street, Bristol BS1 5HH, UK
The redevelopment of Brunei House in Bristol required that the 1820 facade be retained. The facade was badly cracked having been constructed on only a shallow masonry strip footing in thick historical fill and up to 5 m of soft Keuper clays and-sands. The groundwater table was at approximately 3 m depth. The Keuper Sandstone, at typically 7 to 10 m depth had weathered to a loose sand and the site investigation had encountered sub-artesian water pressures in this stratum.
The new building required piled foundations, which had to be installed alongside the facade. The inevitable vibration during driven piling meant that bored piles were the preferred solution. The integrity of continuous flight auger piling had not been adequately proven in these ground conditions, and such piles were also rejected.
It was appreciated that with conventional bored piling, ‘boiling sand’ would prove to be a problem in the saturated Keuper Sandstone, which had weathered to a sand. The paper describes how the volume of ground loss was reduced to an acceptable amount by maintaining satisfactory hydraulic head. Twin telescopic casings were also experimented with in an attempt to seal the pipe casings into the underlying Carboniferous sandstone bedrock, thus reducing the ground loss.
The facade was supported with a large concrete counter balance, and a scaffolding grillage was designed to allow the facade to articulate during the piling. Details are given of the thorough programme of monitoring which was undertaken.
The ground loss into the pile bores was successfully controlled and the facade moved in the manner predicted.
