Introduction:
Improvements to the upper quad area around the former Rasche Hall on the Virginia Tech campus involved construction of an at-grade bridge spanning over an existing access tunnel along an exterior walkway. Factored compressive loads for the bridge footings were 17.2 kips/foot.
Geotechnical Conditions:
Soil conditions consisted of soft to stiff residual clay and silt extending to a depth of about 53 feet below grade where weathered rock was encountered.
Project Challenge:
Provide foundation support in the soft residual conditions while limiting access and vibrations to avoid damage to the existing tunnel and surrounding areas.
Advantages
- Cost savings over traditional micropiles
- Mobile equipment to achieve limited-access installations
- Rapid DIP installation
Design and Construction Solution:
A micropile solution was specified for foundation support of the heavy bridge loads because of the deep, soft soil conditions combined with limited (and light-duty) access requirements to prevent damage to the tunnel and surrounding areas. The contract documents delegated the micropile design to the specialty contractor. GeoStructures, Inc. approached the project team with a design/build driven Ductile Iron Pile approach as an alternative to the micropile requirement. The solution met both the foundation support requirements and the limited access considerations while also maintaining low vibration levels to protect the nearby structures. Further, the Ductile Iron Pile solution provided cost-savings over the micropile approach.
A total of 10 Ductile Iron Piles were installed along the bridge footings on either side of the tunnel. The Series 118/7.5 piles (118 mm diameter with 7.5 mm wall thickness) were designed as end-bearing elements and matched the specified capacity of the micropiles. The capacity of the DIPs was achieved by terminating into the weathered rock at depths on the order of 50 to 60 feet where the set criteria was achieved.
The versatile installation using the conventional medium-sized excavator allowed the GeoConstructors’ crew to install on both sides of the tunnel with access from only one side – limiting the potential impacts of maneuvering equipment around or over the tunnel. The 10 Ductile Iron Piles were installed in less than 2 working days, making short work of the pile scope of work for the job.
Project Team Members
DIP Design/Build Partner: GeoStructures, Inc./GeoConstructors, Inc.
Geotechnical Engineer: Froehling & Robertson, Inc.
General Contractor: Barton Malow
Structural Engineer: Anderson & Associates, Inc.
Architect: Clark-Nexsen Architecture & Engineering
