In this paper we describe an investigation and assessment process relating to excavation by blasting for a marine foundation, and how blast disturbance can be verified and rationalised through site trials and post blast investigation. The 210m high Central Tower of the Queensferry Crossing is founded on Beamer Rock, a tidal island comprised entirely of Dolerite, in the River Forth estuary. The Foundation design required that the Dolerite was excavated to almost 9 metres below high tide level, over a plan area of 25m × 35m. The preferred excavation method was blasting, but there was insufficient published or empirical data to alleviate concerns that the disturbance due to blasting could be constrained and would not damage the founding rock. An innovative and commercially viable trial blasting assessment exercise was developed, including inspection and topographic survey of the blast-pit, bedrock coring and down-hole geophysical logging. Following final excavation and cofferdam construction, geological inspection of the dry formation enabled comparison of the predictions of the assessment with actual rock condition. This exercise provided an excellent opportunity to make a detailed assessment of the impact of blasting on a constrained foundation excavation and formation.
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