Side slope of Roads

[ebsico]

I m not an expert in the field of executing and designing road
but I m working on an electrical substation project
Where I should submit the design of the access road
And I have this small issue
The area next to the site is super flat so i need to backfill couple of layer of subgrade and sub-base approx. 1m height before installing the pavement
My question is what should be the side slope of the road 1/1 1/2 1/3 ….
Note that the road only leads to the substation and my backfill is a cohesive clay soil
Thank you

 

[GeoEnvGuy]

Your geotech report should have recommendations for pavement design. If it does not get it altered to include those recommendations.

You indicate that the site is flat but you are intending to backfill 1 m with cohesive clay soil before installing pavement. That is not standard road base material. Your geotech report should have this recommendation.

If you have 1 m of fill is a grading/drainage plan plan and are culverts necessary?

As for the geometry of the road that depends on juriisdiction in Canada there is a unified geometric design guide that specifies the geometry. Also is your access a driveway or a road? A road would require effort put into sightlines, grades and turning radius.

 

[ACtrafficengr]

If you're referring to the embankment slope next to the road, it's normally 1:2 or flatter. You should also have 0.6 m or more between the roadway edge and the top of bank, just to help prevent people from sliding off.

The driving surface grade should be about 2% if paved, and about 4% if gravel.

Depending on the traffic level, a layer of separation geotextile on top of the clay embankment may be a worthwhile investment.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[BridgeSmith]

For highways in the US, anything steeper than 1:3 requires a guardrail, since going off the road at more than 30 mph onto a steeper slope is considered hazardous. 1:2 is usually the max, because anything steeper is difficult to vegetate (get plants to take root) and requires select backfill (something with a higher internal friction angle than we normally see in native soils).

Edit: I just noticed you said clay backfill. In that case, due to creep considerations, probably best to use the geotextile (or a biaxial geogrid), or keep it to 1:3.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Waxwing]

I agree with ACtrafficengr: embankment side slope of 2H:1V or flatter.

No idea why the need for a 3 ft high embankment when everything is "super flat"...?--The whole original post doesn't make sense.

 

[SlideRuleEra]

No idea why the need for a 3 ft high embankment when everything is "super flat"...?

When a large area is "super flat", there is no meaningful drainage. During and after heavy rain, water just, more or less, sits... quickly turning a "low" road into "mush". If a drainage solution is thought to be ditches, there is nowhere for the ditches in a large "super flat" area to drain. The ditches just fill up with water which also just sits.

This is a common situation in the area where I work.

http://www.SlideRuleEra.net

 

[ACtrafficengr]

anything steeper than 1:3 requires a guardrail

Since this is probably a site driveway, a guiderail is probably not needed.

Research has found that roadside clear zones and traffic barriers are not generally cost-effective on roads with low traffic volumes, particularly for roads with design volumes of 400 vehicles per day or less.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[BridgeSmith]

Since this is probably a site driveway, a guiderail is probably not needed.

Agreed. The OP said "access road", so I mentioned it, but qualified that it was for high-speed highways.

Research has found that roadside clear zones and traffic barriers are not generally cost-effective on roads with low traffic volumes, particularly for roads with design volumes of 400 vehicles per day or less.

Obviously, AASHTO are the experts. As you know (but others may not), there's more to clear recovery zones than the slopes, and traffic barriers can sometimes be more harmful than helpful. I wasn't suggesting the use of a guardrail; I was just pointing out what AASHTO considers a slope that is 'recoverable', i.e. one that a vehicle can go out onto at higher speeds and still get back on the road. At low speeds, slopes greater than 1:3 can be recoverable, depending on the material.

Rod Smith, P.E., The artist formerly known as HotRod10