Roadway Elevation vs. Existing Grade 1

[gjeppesen]

I live in South Florida where we fill 99% of the time (low lying areas, high water table). Minimum roadway elevations are based on design storms. For those of you that work elsewhere, where water table and flood elevations aren't an issue, how do you determine roadway elevations vs. existing grade? Do you match road crown and existing grade? Do you put it one foot above existing grade?

 

[Drumchaser]

Water is always a primary concern. Routinely we lower the PGL (in most areas) on a total rehabilitation of say a C & G section, providing mucho run off for adjacent businesses etc.

Rural section - cuts and fills are based primarily on sight distance issues etc., bringing the facility up to current standards.

hope this helps

 

[PELS]

Ideally we try to balance cuts and fills to minimize grading costs but you still have to comply with the local agency design standards including dealing with numerous criteria such as drainage, sight distance, available right of way, design speeds, trees, wetlands, endangered spiecies, subsurface soils, traffic volume, existing utilities, etc.

You perform preliminary Hor & Vert alignment studies that meet design criteria and evaluate the impacts, while endeavoring to minimize construction costs. There is probably no single criteria that dictates a relationship of road elevations versus existing grade. You just have to satisfy all these other criteria. It's a process that can vary based on location, terrain and the agencies involved.

 

[gjeppesen]

OK. What if you were dealing with flat terrain, a straight road, and a water table that is low enough that it's not a concern?

 

[civilperson]

Try for a balance between the cut section, (ditches or gutterlines), vs. the fill section.

 

[PELS]

I didn't think such a road project existed anywhere but I think you are asking a theoretical question anyway.

You should have some known criteria already such as: Traffic Index, Drainage Patterns, Road Section (AC+AB) and Geotechnical Data for shrinkage/swell. Basically you will be laying the road section on top of the natural ground. I believe your question is, “Does the crown of the road belong above, below or at grade?”

If you place the crown at, or below grade, you will have to deal with surface drainage over the roadway and you will displace a quantity of earth material equivalent to or greater than the road section volume. This means the contractor will have to export the material displaced or find a convenient nearby location where the material can be stockpiled without adverse impacts.

If you place the crown and road section above grade, the road would function as a “dam” to block the natural sheet flow of drainage but there will be no export. Drainage will have to be picked up in a road side ditch or underground drain system.

Again, you have to do some preliminary alternative studies of horizontal and vertical alignment to determine the best fit for the natural terrain that will yield the least amount of earthwork and drainage costs to your client, while satisfying standard design practices for public safety. Whoever ends up maintaining this roadway should have some input into the design in order to identify an optimum design that will minimize operation and maintenance costs.

Bottom line is there are many variables that can affect the ultimate acceptable alignment so you have to identify and prioritize these variables as your design criteria and design accordingly.

 

[Ron]

gjeppesen....the centerline of the roadway should be at least a foot above the 100 year flood elevation for the area

 

[Apakrat]

"Amen" Ron (Structural), when considering roads using drainage ditches sloped to suitable natural runoff route.

Sometimes, local situations require excavation of an open pit evaporation area(s) to collect runoff from the drainage ditches.

They tell me that we got a few pretty good fishing hole at these pits in and near Amarillo.





At 74th year working on IR-One PhD from UHK - - -

 

[cvg]

"centerline of the roadway should be at least a foot above the 100 year flood elevation for the area"

regarding 100-year floodplains, if there is a mapped floodplain and you construct your roadway above natural ground - you will need to make sure the roadway construction does not cause the floodplain elevation to rise or to divert the flow to another flowpath. That may cause increased flooding on adjacent properties which will be attributed to your design and construction of the road. Therefore, make sure you either construct a suitable culvert, bridge, channel or other feature to handle the 100-year runoff and maintain the existing floodplain elevation (or lower it).

 

[civilperson]

Buildings not roadways should be above the 100 year floodway elevation when the road is a part of the drainage way as in Louisiana and Nebraska. The 10 or 25 year return storm is used to design a dry lane at the center of a crowned road but the road section, gutter/curb and ROW contribute to the flow areas for larger storm events.

 

[BigH]

Are you in "warm" country or frost country? For me, I would always have my road about 1 m above the existing grade - assuming flat land. Of course, you would modify if in cuts - but still, I'd have the road crown high enough and the road high enough to have good depth of subbase and base - for frost/thaw conditions (in the spring).

 

[chicopee]

One foot above the 100 year flood elevation in Florida will soon not be high enough. Climate change will see to it.

 

[sixaparrat]

In addition to culverts etc.,you also need to ensure a pathway for water through the embankment at subgrade level (granular blanket) to balance any sideways pressure upon the structure

 

[BAWANY]

The design will be based on the City's requirement. Please read the local City's manual first.

The other things to consider for a roadway design is pre/existing conditions. Try to avoid changes. The way the water was entering before leave it in its original form. This is achieved by plotting the cross section before and after. If left and right properties were dumping water into the road before than they are supposed to dump water after too.

Start with longitudinal profile and check the x-sections