By Don Flemming, P.E.

Aerial view of the unwoven I-35E/I-694 Corridor. Courtesy Brett Fechheimer

Carrying 140,000 vehicles per day, the onerous intersection at I-35E and I-694, which sits about 10 miles northeast of downtown Minneapolis, has been a major roadway bottleneck for years. The intersection of the two major interstate highways consisted of two Y-shaped interchanges separated by 1.2 miles of shared or “common” roadway. The overall alignment of I-35E is north-south, while I-694 runs east-west through the interchange and common area.

At this area, I-35E joined with I-694 from the south at the west interchange, proceeded through an east-to-west common section before splitting off northward at the east interchange. This created a bottleneck that led to heavy congestion and severe safety hazards: Higher than normal crash rates resulted where six lanes of traffic (three from I-35E and three from I-694) converged into a common section.

Travelers on I-35E who entered the common section were required to “weave” — cross over lanes of shared traffic — to exit the common section and continue through the interchange on the signed route. Conversely, travelers on I-694 who entered the common section had to weave across shared lanes with I-35E to continue on I-694. The Minnesota Department of Transportation (Mn/DOT) determined that it was necessary to redesign the interchanges and common section into a safer, less-congested stretch of roadway and eliminate the weaving. The objective, in short, was to “unweave the weave.”

A complex design
The four-year, $140-million project, which was completed in October 2008, was exceptionally complex. It consisted of 3.76 miles of roadway construction, 23  , 4,000 linear feet of retaining walls, 27,000 square feet of noise walls, nine permanent bridges, two temporary bridges, and 54 temporary bypasses that maintained two lanes of traffic in each direction on both highways throughout construction. The roadway bypasses were designed to accommodate every conceivable traffic movement. The reconstruction required final roadway alignments and profiles that were vastly different from the existing roadway. In some areas, grade crossings were reversed (a roadway that previously was an underpass became an overpass or vice versa). This required complicated bypasses to accommodate dramatic changes in profile elevation.

URS was retained by Mn/DOT to provide final design for the roadway, including:

• retaining walls, noise walls, drainage, utilities, traffic control, lighting and traffic signals;
• an aesthetic design guide for the entire I-694 corridor;
• preliminary and final design for two temporary bridges and one permanent bridge; and
• preliminary design for eight bridges. Mn/DOT performed final design for eight of the bridges.

Meeting the challenges
Many obstacles were encountered and overcome during the course of completing the project. One of the most difficult challenges arose at the south end near the I-35E/Little Canada Road intersection, where I-35E was widened and the ramp from Little Canada Road to northbound I-35E was relocated. Grade differences and severely limited right-of-way (ROW) required construction of a retaining wall near the ROW line. Typical construction methods, such as ground sloping for the wall foundation or use of temporary sheet piling, were not practical because of the many private residences located adjacent to the ROW limits.

Construction of an innovative soldier pile retaining wall showing the permanent tie-backs being installed and grouted into the soil behind the previously placed H-piles. Right-of-way constraints required that this wall to be built from the top down with no excavation occurring behind the wall. Photos courtesy URS Construction of the soldier pile wall along northbound I-35E. The reinforcement shown was placed vertically for the concrete facing. Innovative soil nail wall designed and constructed to provide room for the construction of a critical by-pass roadway between the pier and abutment of an existing bridge.

To circumvent the problem, URS developed an innovative, top-down soldier pile wall construction method for the area. This involved placing H-piling into drilled holes in the ground and constructing a wall with tieback grouted anchors and wood lagging to support the in-place ground behind the wall. The area in front of the wall could then be incrementally excavated to the desired elevation as more tiebacks and lagging were placed. A concrete facing was attached to the piling to give the appearance of a conventional wall. This technique allowed the wall to be constructed from the top down, and required access to the front of the wall only, with no impact to the nearby residences.

Imaginative construction techniques also were involved in the design of a temporary bridge carrying southbound I-35E over eastbound I-694. The final design in this area required I-694 eastbound traffic to go over traffic on I-35E. Complex construction bypasses were needed to maintain two lanes of traffic on both I-35E and I-694 during construction.

Aesthetic texture of concrete railing and ornamental railing on the Edgerton Street Bridge located in the center of the I-35E/I-694 common area. Extensive aesthetic design guidelines were developed for the I-694 corridor from the Mississippi River to I-494 for use on this project and for future projects in the corridor. Courtesy URS

To meet the challenge, portions of the newly constructed bridge were used as an abutment for the temporary bridge. This bypass was built over the new I-694 permanent pavement, the wingwall of the new bridge, and over temporary bypasses for eastbound I-694. H-piling was attached to the footing and front face of the wingwall to support the end of the temporary bridge. Two 26-foot-high pile bents, the tallest ever used in Minnesota, were constructed to serve as the temporary bridge piers and to support another temporary bridge over bypasses for eastbound I-694.

Creative bypass use, design, and construction
At other locations in the project, bypasses and bridges were utilized or constructed in innovative ways to facilitate the project schedule. The new Edgerton Street Bridge over I-694 was used as a bypass for the northbound I-35E to westbound I-694 movement, greatly reducing the construction time for the new 1,900-foot-long flyover bridge for I-35E northbound to I-694 westbound traffic. To provide sufficient bypass capacity under the in-place Little Canada Road Bridge, a bypass was designed with northbound I-35E traffic lanes placed on both sides of the center bridge pier.

Innovative signing and temporary traffic barrier placement provided a safe and efficient use of the added lane, which significantly improved bypass capacity. Bypasses were needed under in-place bridges where the only area available for the bypass was closer to the bridge abutment than allowed by standard design specifications. Temporary soil nail walls were designed to allow cutting into the slope to provide space for the bypass. In some areas, temporary reinforced soil slopes were used in lieu of sheet piling to facilitate bypass construction.

Utility and surface water drainage issues
Significant utility problems also had to be resolved. Close coordination with several utility companies was required to relocate a number of utilities. A major coordination effort was required to successfully relocate a 345-kv Xcel energy transmission line and tower to provide adequate clearance for the high-voltage line. A 54-inch sanitary sewer in relatively soft soils also had to be protected because of the addition of approximately 20 feet of fill over the sewer line. A new 54-inch fiberglass sewer pipe was placed in a steel casing to protect the sewer. Settlement issues made it necessary to design an adjustment rack to support the sewer in the casing. As settlement of the fill occurred, the sewer support was adjusted to compensate for the settlement.

A northwesterly view of the unwoven I-35E/I-694 Corridor. Courtesy URS

The overall surface water drainage for the project required extensive design and analysis to satisfy the complex construction staging conditions. The plans included more than 64,000 feet of storm sewer, multiple center-line culverts (including 72-inch and triple 60-inch culverts), and 23 stormwater basins. Thirteen of the basins were designed for wet water quality treatment, and the remaining basins were designed as dry rate control. All were designed in accordance with the requirements of the Minnesota Pollution Control Agency and Ramsey Washington Metro Watershed District. An extensive effort was put into developing plans to address staged temporary drainage, erosion and sediment control, and turf establishment for each of the 11 stages of the project.

The project team assessed the impacts of the new Municipal Separate Storm Sewer Systems permit requirements for the project. Water quality modeling of the 990-acre project area was completed utilizing the P8 Urban Catchment Model. P8 models were created for the years 1988, the present, and future 2020 land use conditions to compare the changes in flow volume, total suspended solids, and total phosphorous. The modeling showed that extensive infiltration measures would be needed to meet requirements for flow volume and total phosphorous loading levels discharging from the project area. The modeling was then used to determine the size and cost of the infiltration measures required to meet the flow and phosphorous loading requirements.

Unweaving the Weave
The new common section shared by I-35E and I-694 now consists of 12 lanes — three in each direction for both roadways. Travelers along I-35E are no longer required to merge and cross over I-694 traffic. Traffic on I-35E joins the common section on the inside lanes and continues through the interchange without any lane changes. Travelers continuing through on I-694 enter the common section on the outside lanes and continue through without lane changes. Ample distance has been provided for travelers to shift lanes to go from one interstate to the other. Extra-wide shoulders were added to allow for additional lanes as needed with a future increase in traffic.

The redesigned interchange offers a safer, less congested stretch of roadway while unweaving the weave between the two major interchanges.

Don Flemming, P.E., is a vice president at URS Corporation in Minneapolis. He can be reached at 612-370-0700.