New Zealand's Bridges: Port Hills Overbridges across Route 73 in Christchurch (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
March 2011 (-43.573 Degrees, 172.695 Degrees) Port Hills Overbridges
I took this photo of the Port Hills Overbridges while standing between the bridge decks.
At-grade road crossings require a stop sign, a traffic light, or a traffic circle. However, if one of the roads is an expressway you need to separate them with a bridge (or maybe a tunnel). If they are both expressways, you usually need an interchange.
Typically, to carry vehicles onto a bridge you can use mounds of earth (embankments), retaining walls (like we saw on the Colombo Street Overbridge), or the bridge can start at grade and climb steeply with short and tall piers.
However, Port Hills Road is descending from hills, so instead of an embankment, Route 73 is in a cut section below the bridge. Note the crib wall holding back the toe of the slope. As I've said before, I've seldom seem earthquake damage to this type of earth retaining structure. Also note the steel keys keeping the girders from moving transversely. At Caltrans, our philosophy is to let the bridge move, and design for the displacement.