One of the last stretches of I-70 in Colorado had to pass through Glenwood Canyon where steep canyon walls slope to the edge of the Colorado River. There was little room to build a highway without ruining the landscape. To solve this problem, engineers designed tiered roadways that were built on a backfill supported by concrete retaining walls. The walls are precast twin tees, set on end and post-tensioned. Post-tensioning bars at the top of the twin tee stems are threaded into anchors embedded in the footing. After grout placed under the tees reached a strength of 4500 psi, the bars were stressed and ducts were grouted.