I-70/I-670 Interchange

I-70/I-670 Interchange

Double-T panels constructed in front of temporary shoring.
Reinforced Earth Product in Field
Columbus, OH
Ohio Dept. Of Transportation
Kokosing Construction Company Inc.
Norwalk Concrete Industries

Travelers going east or west through Columbus, Ohio, have three route options:  south of downtown along I-70, through downtown on I-670, or bypass the city using circumferential I-270.  All three routes are intersected at different points by I-71, which runs north-south across the city and straight through the heart of Columbus.  The downtown maze where I-70, I-71 and I-670 intersect has long been characterized by high traffic volumes, geometric deficiencies, closely spaced ramps requiring intricate weaving patterns, and a high percentage of both commercial vehicles and large, limited-maneuverability trucks.   By the end of the 1990s the heart of the problem, the I-71/I-670 interchange, had the highest crash rate in Ohio and was in critical need of reconstruction and widening to improve safety and traffic flow.  In 2001 the Ohio DOT (ODOT) began studying ways to improve the downtown interstate network in general and this interchange in particular. 

A law passed in 2010 authorized ODOT to use the best value design-build method (contractor selection based on technical qualifications, project schedule and cost), allowing interchange reconstruction to begin two and a half years earlier than planned and saving $40 million compared to traditional project delivery methods.  The project comprised one mile of I-71, one and a half miles of I-670 eastbound, 18 ramps, 18 new bridges, 2 new flyover bridges, 2 reconstructed bridges, 26 retaining walls, and 2 new local urban avenues.  Planners took a comprehensive approach to improve traffic operations, restore safe travel, and support the social and economic vitality of the area. 

An advisory committee was created and public involvement was maintained throughout the project using venues such as community meetings, surveys, newsletters and a project website.  This community outreach clearly identified protecting the Shiloh Baptist Church as a very high priority.  Completed in 1923 and designated an historic site, the church plays a central role in the Columbus black community.  The interchange widening would place the new two-lane ramp from I-71 north to I-670 east and west much closer to the church, eliminating the existing slope down to the highway.  The only way to adequately protect the church, along with several nearby residential and child care structures, was with a retaining wall constructed in the narrow space between those structures and the road.  Beginning at the portal of a short tunnel and running north along the ramp, much of the wall would rise 25-29 ft. above top of footing.

Design and construction of this retaining wall was the responsibility of the design-build general contractor, Kokosing Construction Company Inc., teamed with design consultant CH2M Hill and CH2M's foundation/geotechnical subconsultant, E. L. Robinson Engineering.  The first plan was a traditional ODOT-design cast-in-place cantilevered wall – on a spread footing thanks to the excellent bearing available on the granular material at this location – but Kokosing wanted a faster, easier and lower-cost way to build the wall.  Based on prior positive experiences building Reinforced Earth® mechanically stabilized earth (MSE) walls supplied by The Reinforced Earth Company (RECo), the contractor turned to RECo for assistance.  The problem with an MSE approach, however, was the extremely narrow space behind the wall face and the 2:1 slope rising 5-10 ft. from the top of the wall.  In an MSE design, the soil reinforcements behind the wall are about 70% as long as the loaded height of the wall.  Therefore, including the effect of the 2:1 slope, many of these reinforcements needed to be 23 ft. long, far longer than the 13 ft. available in front of the excavated and temporarily supported backslope.

Next Kokosing considered a back-to-back Reinforced Earth wall, often used for ramps that must be retained on both sides, but the space was too narrow to develop adequate sliding resistance and global stability given the forces exerted by the high backslope.  So when RECo suggested an alternative concept that could satisfy most structural requirements, meet the geometric constraints of the site, and also speed up construction, the contractor was interested.  In general, the proposal was to construct a precast post-tensioned double- or triple-tee retaining wall, using a modification of the original ODOT-design cast-in-place footing.  RECo had a form available for casting the very tall panels required for the project and on-staff designers with previous experience with post-tensioning design and the details of this wall system.

Panels were cast of 5000 psi concrete and ranged from 6.5 ft. to 29.75 ft. tall.  As shown in Table 1, heights up to 27 ft. were accomplished using double-T panels with 2 or 4 post-tensioning (P-T) bars (Figure 1), while panels taller than 27 ft. required triple-Ts and 6 P-T bars.  The cast-in-place footing design, performed by others, was modified by Reinforced Earth Company engineers to add a shear lug in front of the panels and the required P-T rod anchorages (located just above the bottom layer of footing rebar).  P-T bars were precisely positioned and cast into the footing by Kokosing, allowing a perfect fit when the panels were set down over the bars.  Panels rested on 1.5 in. thick structural shims, allowing precise levelling; the space under the panels was then dry-packed with 5000 psi non-shrink grout (Figure 2) prior to P-T bar stressing (Figure 3).

Slope stability during construction required excavation support behind taller portions of the retaining wall.  The contractor's choice was a soldier pile and lagging wall that would be left in place, with the soldier piles encased in 2.5 ft. diameter by 22 ft. deep or 4 ft. diameter by 25 ft. deep drilled shafts.  The geotechnical engineer took advantage of this configuration to offset the limited space for the heel of the wall (maximum 12.5 ft.).  A 5 ft. long piece of HP 10x42 steel pile was welded to the face of each soldier pile and embedded in the footing concrete of the permanent wall, acting as a "deadman" to anchor the wall in place.

Panel fabrication was performed by Norwalk Concrete Industries and used a series of form liners to produce a varying but repeating pattern.  Norwalk set up the complete form, including the T forms, pouring the 6.5 in. thick panel portion first.  The Ts were poured about 30 minutes later, reducing the chance that T concrete would displace facing concrete or flow out at the bottom of the form. With only one form and 125 panels, this process occurred once per day and required more than 6 months to complete.  Setting up the form liner was a major task, so the precaster had to cast all panels using a particular liner before moving on to the next liner type.  The only exception occurred at the tunnel end of the wall, where the first 20 panels – 200 ft. of wall – had to be erected on a fast-track schedule, during the winter, to protect and allow access to the child care center and residences directly above.  Winter and spring weather was generally terrible, pushing the balance of wall erection to the late spring and summer months.

Design-build projects inspire creativity by all parties, and the I-71/I-670 Interchange was no exception.  Where an MSE wall would not work, The Reinforced Earth Company customized and supplied a different precast wall system.  Where typical foundation design would not work, a temporary wall was repurposed as a deadman.  ODOT evaluated and approved new ways to solve problems and Kokosing Construction made it all happen.  And the community benefitted through sensitive design and better and safer traffic flow along I-71 and I-670 in downtown Columbus.