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Driving You Crazy: Why is there such a deep dip on northbound I-225 at the Iliff bridge?

It is especially noticeable in the left lane
Northbound I-225 dip
Posted at 5:06 AM, Feb 13, 2024
and last updated 2024-02-13 11:58:04-05

Kelvin from Denver writes, “What's driving you crazy? When driving on I-225 northbound going over Iliff Ave, the transition between the bridge and far side has a very abrupt "dip," which is especially noticeable in the left lane.”

I am all too familiar with that bump and when you hit it, depending on your speed, it can feel like you are going to be launched into the Carrabba’s parking lot. The cause of that dip is created when the bridge was constructed.

When a bridge is built, it is set on pilings that are set deep in the ground. The roadway that approaches the bridge isn’t laid down on such a sturdy base, so there is a tendency for the soil right next to the bridge structure to settle. It is the settling that causes that dip to form just before or just after the bridge structure. One such spot is along northbound Interstate 25 at University Boulevard. Vehicles in the right lane take a substantial dip down due to the soils settling there more than the soils in the left lanes.

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Sometimes the problem can stem from when the structure and approaches were constructed. As we all know in Colorado, temperatures fluctuate greatly from day to day and from night to day. Concrete cures slightly differently depending on the temperature, so if the bridge was constructed over several seasons, the soils could have expanded and shifted leading to the creation of the dip.

Another compounding factor is the bridge beams are set with a slight rise and fall. This is to dissipate the tension caused by the weight of the material and traffic rolling over it. That design is not just stronger than a level flat design but helps with runoff and height clearance for tall trucks that run beneath it. When a dip forms before the bridge structure, that slight angle up for drivers can compound the feeling of rising up from that dip.

The way to prevent this problem is to make sure the fill material is packed to the point where the dip does not form. One way to fix this problem after the dip forms is to remove the road surface, add more material, pack it down well and then resurface the roadway. All of that takes time and, more of an issue for CDOT, money.

CDOT engineers told me they are currently designing a bridge preventative maintenance project for the entire Interstate 225 corridor. That project will replace many of the bridge joints in this area, which can themselves be bumpy. CDOT is also hoping to address some of the other safety issues that require surface treatment, like that major dip at 225 - pending additional funding. That project is expected to begin in the summer of 2024.

Why is there such a deep dip on northbound I-225 at the Iliff bridge?

Denver7 Traffic Expert Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook,Twitter or Instagram or listen to his Driving You Crazy podcast on any podcast app including iTunes, iHeartRadio, Spotify and Podbean.