Why do some depressed highways have sloped banks instead of retaining walls?

[kernals12]

The Michigan DOT is planning to add a lane in each direction to I-94 in Detroit. They won't need to do much eminent domain because the freeway already has a wide ROW, being abbutted by sloped banks. To fit in an extra lane, they just need to replace the slopes with vertical retaining walls. My question is why they used that design when they first built the highway instead of just having retaining walls from the start? Is it just a way of preserving space for future expansions or is it a way of reducing costs?

[tolbs17]

Because they are less expensive. Walls are expensive, and to minimize right of way impacts, they do that.

[SEWIGuy]

Pretty obviously the initial cost plus maintenance.

[GaryV]

Are there still restrictions for hazardous materials on freeways with retaining walls?  I'm not sure if it's the walls themselves that are the cause for restrictions, or other facets of the design.

[Rothman]

Are there still restrictions for hazardous materials on freeways with retaining walls?  I'm not sure if it's the walls themselves that are the cause for restrictions, or other facets of the design.

I can't imagine this is a blanket rule given the proliferation of retaining walls in the system.

[Dirt Roads]

Are there still restrictions for hazardous materials on freeways with retaining walls?  I'm not sure if it's the walls themselves that are the cause for restrictions, or other facets of the design.

I can't imagine this is a blanket rule given the proliferation of retaining walls in the system.

You are correct that there is not a blanket rule.  Every jurisdiction has its own Emergency Response Plan that details out these kind of issues.  The primary issues related to Hazmat restrictions for tunnels and retained cut trenches was the inability of local firefighters to properly address these hazards (particularly TIH/PIH = toxic and poison inhalation hazards).  Over the years, these restrictions have been gradually removed as local and regional emergency response capabilities have been enhanced by projects funded by government-sponsored grants such as the FEMA Hazard Mitigation Assistance Grant program.

[wanderer2575]

^^  There used to be signs on the I-696 and M-10 freeways stating that trucks hauling hazardous materials had to exit the freeway before the retaining wall sections.  Those signs are long gone.

Also, while not full retaining wall height, several Detroit-area freeways have seen recent construction of Jersey-type concrete barriers along the outside shoulders to lengthen acceleration/deceleration lanes and prevent slopes from washing onto the freeways.  For example:  https://goo.gl/maps/e2qy7VFucfYqp8WJA

[Henry]

In Chicago, it's a mix of both. Like on the Dan Ryan, you will see retaining walls at the interchanges and slopes along the mainline, although this is not explicitly so because the reverse is also true.

[Rick Powell]

The Michigan DOT is planning to add a lane in each direction to I-94 in Detroit. They won't need to do much eminent domain because the freeway already has a wide ROW, being abbutted by sloped banks. To fit in an extra lane, they just need to replace the slopes with vertical retaining walls. My question is why they used that design when they first built the highway instead of just having retaining walls from the start? Is it just a way of preserving space for future expansions or is it a way of reducing costs?

It could be a lot of things. Today's urban land tends to be expensive, contaminated or both, and there is usually a trade-off in the cost and environmental remediation of acquiring more land vs. the cost of staying within existing right-of-way and building retaining walls. Furthermore, if there are buildings, frontage roads, etc. at the top of the cut, it becomes even more complicated. If a slope needs to be steepened, it may be un-mowable. Finally, there is a movement towards preserving or maintaining open space in urban freeways, and a retaining wall can enable the creation of flat, usable land outside the freeway rather than a fenced-in slope that no one can use. 

[wanderer2575]

One other point regarding the I-94 rebuild in Detroit:  Unless the plan has been modified, it includes construction of continuous service drives both eastbound and westbound along the entire project length.  Retaining walls will help make land available for these service drives in areas where they currently do not exist.

[Road Hog]

Another consideration is that retaining walls can help to absorb and mitigate traffic noise through residential areas.

[hotdogPi]

Another consideration is that retaining walls can help to absorb and mitigate traffic noise through residential areas.

No, they don't. They just redirect it to places farther out.

[jeffandnicole]

Another consideration is that retaining walls can help to absorb and mitigate traffic noise through residential areas.

No, they don't. They just redirect it to places farther out.

We live about about 4/10ths of a mile off the highway, which has sound barriers, and can certainly hear the traffic, especially in winter when the trees are bare. We moved in after the barriers were installed so we can't compare to what it was like before. But, this sound is merely background noise and doesn't impact us. We also have planes that take off from the airport 3 miles away and occasionally fly over our house. That is much louder, and causes us to cease conversations until they pass by.

[Terry Shea]

The Michigan DOT is planning to add a lane in each direction to I-94 in Detroit. They won't need to do much eminent domain because the freeway already has a wide ROW, being abbutted by sloped banks. To fit in an extra lane, they just need to replace the slopes with vertical retaining walls. My question is why they used that design when they first built the highway instead of just having retaining walls from the start? Is it just a way of preserving space for future expansions or is it a way of reducing costs?

Have you seen what happens when these recessed freeways with retaining walls have to endure a major rainfall?  Maybe the major automakers think they can rekindle sales by putting a good portion of the current fleet of cars under water.

[Big John]

^^ The road should be engineered to provide the stormwater drainage to handle heavy rainfalls.

[GCrites]

Today's drainage engineering standards are vastly different than seen in the near, let alone more distant, past. They've changed far more than more visible standards such as radii, slope and breakdown lane width.