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Panda casts eye over Millennium Bridge

SITE INVESTIGTION contractor Soil Solution used its Panda dynamic cone penetrometer to check the unusual foundation design for the Millennium Bridge in central London.

Foundation contractor Bachy Soletanche installed 12, 2.1m diameter cast insitu piles to 28m for the bridge's northern abutment.

Foundations were designed to resist horizontal loading from the bridge by lateral displacement of the soil without causing damage to the 200year-old river wall only 14m away.

This was achieved by surrounding each pile with a 75mm annulus filled with a weak cement bentonite slurry to 11m depth, which allowed distortion of the piles and limited loading on the wall (Ground Engineering January 2000).

Target characteristic strength of the cement bentonite slurry was a maximum undrained shear strength of 10kPa, strong enough to support the soil while limiting load on the ground as the piles deformed.A strength-depth profile was needed to ensure this was achieved and because of the restricted access and the narrow nature of the annulus Bachy chose the Panda penetrometer.

The lightweight penetrometer electronically measures cone resistance using variable energy to a maximum depth of 10m.

2(35mm diameter) cone was used to obtain a depth-strength profile of the annulus in cone resistance in MPa.Undrained shear strength was calculated using the correlation between cone resistance and S u(GE September 1999).

Results showed a background cone resistance of between 0.1 and 0.2MPa, suggesting an undrained shear strength of between 5 and 10kPa, which falls within the required limits. Some values of cone resistance over 0.5MPa were recorded, attributed to the cone rubbing along the pile wall or the insitu formation.

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