This project is one of the larger road extension projects ever undertaken by the Mat-Su Borough. The project extends Trunk Road from the Parks Highway south to Nelson Road. The project initially started as a reconnaissance study of two intersections and progressed into the design of a new collector connection. Lounsbury’s work included an RES, traffic study, Design Study Report (DSR), and PS&E. Ultimately we recommended a new road connection, a 6-leg roundabout at the Parks Highway interchange and two new bridges (one over ARRC ROW and one to replace the existing Wasilla Creek crossing on Nelson Road). We evaluated alternatives, coordinated with DOT&PF and ARRC, designed the road, roundabout, and secured permits.
We worked with the Mat-Su Borough, DOT&PF, ARRC, and others to establish the preferred alignment. Our team successfully overcame two major design challenges. The first was the development of the roundabout itself. The proposed roundabout was constrained by topography and ROW. Our team’s design ultimately ensured traffic demands were being served, intersection skew angles and grades were met, and turning movements from the six approaches were accommodated while minimizing the earthwork and ROW impacts. The balancing of these factors resulted in the largest diameter roundabout built to date in Alaska.
The second challenge was the ARRC crossing. Lounsbury worked with ARRC to develop an alignment with a maximum skew angle of 30 degrees and maximum ROW impact of 25%. A significant fill embankment over poor soils was needed to achieve the vertical clearance requirements. Similar to the roundabout, the balancing of bridge ROW, skew, and clearance constraints resulted in one of the longest single-span concrete girder bridges in the state.
