The Interchange included on and off ramps on both sides of divided US 63 and the Overpass involved a three bent, two span steel girder bridge with MSE abutments having heights of approximately 20 feet with 7 foot surcharges at end bents and associated 30 to 40 foot thick approach fills.  Control of approach fill settlement was critical since the approach fills would not only support pavements and bike paths but also cast in place cantilever retaining walls with decorative steel fencing. 

Subsurface exploration identified low strength loessial and post-glacial soil deposits over glacial drift and Pennsylvanian shales, claystones and limestones.  The end and intermediate bents were supported on deep foundations consisting of H piles driven to practical refusal in the underlying Pennsylvanian deposits.  However, consolidated undrained triaxial compression tests performed in Allstate’s Columbia laboratory and global stability analyses of the proposed MSE Walls using our ReSSA slope stability software indicated the site soils provided low factors of safety with regard to overall global stability.  Further analyses indicated removal of the site soils and replacement with a compacted granular shear key would increase safety factors to acceptable levels.  Settlement analyses also indicated the thick approach fills would undergo excessive post construction settlement that could adversely impact the performance of the supported pavements, cast in place walls and decorative fencing.  Anticipated settlements were mitigated by preloading the approach fills with a temporary earth surcharge to accelerate these settlements prior to construction of the cast in place walls, fencing and pavements.