Incheon Bridge Design visualisation (image courtesy
of Samsung JV)Mott MacDonald has been assisting
Yooshin Engineering Corporation in supervising the design of a
fixed link between the international business district of New
Songdo City and Incheon (Seoul) International Airport on Yongjing
Island.
Development of this US$1.2 billion project includes constructing an
800m main span cable stay bridge with back spans of 340m and
approach spans ranging from 50 to 145m. The total length of the
crossing is 12.3km. The main piers are supported by pile groups
consisting of 3.0m diameter piles. These are typically 50m long and
socketed in decomposed granite. As part of the pile design four
pile tests were undertaken by use of Osterberg Cells located at the
basis of the pile.
Our role has involved reviewing the Osterberg Cell tests and
offering advice on the rock socket design basis. In addition, we
assessed the performance of the pile groups and have supported
management of the risk factors associated with the highly variable
ground conditions and foundation redundancy.
Notably, the static bi-directional load tests mobilised a capacity
of up to 279MN, which broke the previous pile test load world
record.
Innovation
Mott MacDonald developed a finite difference numerical model to
replicate the axial loading of the test piles. The model was used
to calibrate the material properties against the O-cell test data
during ‘bottom up loading’. The calibrated model was then used to
assess the load distribution of the rock socket and the
contribution of the shaft and end bearing when ‘loaded from the
top’. The model illustrated that pile performance was vulnerable to
pile base cleanliness and the amount of penetration into competent
rock.
From these analyses design recommendations were given to minimise
the risk of excessive settlement, which considered the structural
redundancy of the foundation and the guidance given by AASHTO. Once
the individual behaviour of the piles had been established the
performance of the groups was assessed so to develop foundation
springs for the global assessment of the bridges
’
performance.
Information technology and innovative approach
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