Extensive ground investigation within existing building
The original Drapers' Gardens building stood at 100 metres (328 ft) tall, had 30 storeys and was completed in 1967; it was designed by Richard Seifert, who also designed Centrepoint and Kings Reach Tower.
The new building, designed by Foggo Associates, comprises 16 storeys. The Siefert tower was built on a central podium supported on a 3 m thick concrete piled raft. As far as possible, it was hoped that the new building could re-use this foundation.
GEA was instructed by Foggo in 2003 to carry out an extensive investigation to assist in the design of the new building. All of the ground investigation work needed to be carried out within the existing building. A range of exploratory techniques was adopted to obtain the optimum amount of geotechnical information, including opendrive sampling, cable percussion drilling using a dismantlable compressed air cable percussion rig, hand dug trial pitting and concrete coring. In addition to the ground investigation, GEA organised a range of geophysical and materials investigations to assess the capacity of the raft and the nature of the existing retaining walls.
The existing basement was to be lowered in part and careful consideration needed to be given to accurate assessment of ground water levels. Two tributaries of the former Walbrook stream also crossed the site and profiling of Alluvium levels and the effect on local ground water flow were of particular concern.
We were able to provide clear advice on the level of ground water and the implications for the new basement of ground water inflows and were able to confirm the reported design of the existing piles. Analyses were carried out of likely heave movements resulting from demolition, which indicated that the proposed approach at that time of re-using the raft should be an appropriate solution.
Exemplar & Canary Wharf Developments
Restricted access ground investigation
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