Abstract:
This case history is an important milestone in the timeline of Helical Piles as well as Class One Railroads in North America. The Terminal Railroad association is owned and operated by five Class One Railroads who have railroad operations in St Louis. The Terminal RR controls traffic across the Mississippi River using two rail bridges dating back to the turn of the last century, the MacArthur is one of the two bridges. Installation of high capacity helical piles for a replacement span of the MacArthur Bridge is significant in that the authors believe this is the first use of high capacity helicals traversed by Class One Railroads as well as the first use of helicals for a bridge crossing the Mississippi River.
Helical piles were chosen over other deep foundation solutions for several reasons. Traditional H piles are generally selected for rail bridges, however due to headroom constraints could not easily be installed for this bridge. There was also a concern of vibration from driven piling operations causing damage to the existing bridge, which is still in service. Drilled shaft construction also had headroom limitations as well as concern of drilling near the existing foundations, causing settlement of the bridge. As this is an urban site there is also the concern of damaging existing utilities, some of which date back to when the bridge was constructed. Micropiles were also considered for the bridge, the helical piles were a more economical alternative to the micropiles.
The helical piles are installed within the restricted headroom limits of the site. The helical piles were designed to take the heavy lateral train braking forces by battering some of the piles on 2:12 batter. To fully analyze the AREMA load cases, a finite element model was constructed using SAP2000 software. This model provided clarity on the load path through the caps and into the group of piles below. The individual pile section consists of 9.625” x 0.420 wall pipe Gr 80, filled with grout. Piles were installed through a thick layer of moderately to highly plastic saturated clay and taken down to the limestone bedrock. A single 32” helix was used to advance the pile to bedrock as well as assist in transferring the loads from pile to the foundation soils and bedrock. Cyclic compression and tension load tests were conducted on test piles to assess pile capacity with the test piles loaded to 715-kip compression and 160-kip tension. The compressive behavior of the pile was generally elastic with small non elastic deformation of the foundation soils. The tensile capacity of pile was reduced during the tension testing program due to the absence of a stiff gravel layer that appeared in preliminary borings, but was not present at all locations.
The presentation will discuss the preliminary as well as final design of the piles, as well as detailed aspects of construction. The authors believe that this project exhibits several important “firsts” in the foundation industry which should be shared with the organization.