Designing subdivision intersections 2

[johnhan76]

What is the best way to grade subdivision intersections when there is a storm box at each radius. Typically engineers here dont account for the differing cross slopes of each road. They just let the contractor figure it out. I dont like this.

 

[RWF7437]

The easiest way I know is to draw the curb radius profile around each radius with a ver exaggerated vertical scale. Include the slopes into and out of the radius. Then draw a smoothe curve joining the two slopes and scale the elevation at the begin radius, 1/4, 1/2, 3/4 and end radius points. Crude, old fashioned, not AUTOCAD's idea of design but it works (has for 45 years for me).

good luck

 

[francesca]

We use Terramodel to do this, which works great for generating our proposed contours, but the contractor doesn't build to our "road points" layer!

I suggest placing one or two spot elevations around your corners. This is what we do for cul-de-sacs and when we have an intersection that may be difficult for the contractor to figure out.

 

[groundcontrol2005]

pick one of the roads for your control,and grade the other based off the control road

 

[johnhan76]

I am assuming that I need to make the "thru" road the control road and grade the other road "stop road, the road with the stop sign) to it. How do I consider the cross slope of the control road? When I am drawing the center line profile of the "stop" road do I draw it to the center line of the thru road or do I draw it to the edge of pavement of the thru road and then draw a line 2% up to the centerline of the thru road? Or is it different each time?

Thanks

 

[francesca]

Draw the CL of the stop road to the CL of the control road, but don't forget your crown. Your stop road profile should begin at -3% (or whatever your crown slope is) for the width of the control road (e.g. 14' for a 28' back of curb to back of curb). This would be the second option you stated. Make certain that your CL elevations match on both roads.

Now the trick is to ask yourself where you want the water to go, and to grade the corners accordingly. Be certain not to leave the possibility for low-points in the corners if there are no catch basins there. (The danger of unintentional low points is when the stop-road comes in at a downhill slope to the intersection and the crown of the stop-road leaves a pocket at the intersection.) Contractors are usually good at adjusting grades in field to eliminate low points in corners and make sure that the road drains correctly. If you're concerned, you can specify spot-elevations on the corners to essentially remove the crown of the stop-road at the intersection.

 

[johnhan76]

That clears things up a lot. If there are no catch basins (ie I want the water from the thru-road to cross the intersection it seems as if I need to eliminate the crown as you said. How and when should I make the transition? Should the transition be made in 10',20' or 50'? Thanks to everyone that has contributed to this.

 

[francesca]

We rarely account for this on our plans as the contractors are really experienced at this and usually adjust in the field.

On rare occasions, we specify a kind of concrete flume down the intersection, but that's only if we have a lot of water and/or if both roads slope away from the intersection and we want to control which route the water picks.

I would just put a spot elevation on the corner to help the contractor out, rather than specify removing the crown as it may lead to confusion rather than help things.

To make certain that your road drains properly at the intersection, you'd make the spot elevation on each corner equal to the through road (CL elevation - crown) at that station (CL intersection station - stop-road half-width). That way, your intersection will continue to slope at the same slope as the through-road. Your stop-road will effectively be superelevated at the intersection.

 

[johnhan76]

Lets say I have catch basins at both corners - I dont want the water to cross the intedersection. Now do I match the centerline of the stop-road with the centerline of the thru-road instead of the edge of pavement? I would in effect maintain the crown and trap the water so that it would trap the water

 

[francesca]

I wouldn't do it that way, because an experienced contractor probably would remove the crown to match the minor road to the major road's crown anyway.

You need to specify a Top of Grate (TOG) elevation on your catch basins, and your catch basins should go in before the subgrade is graded to your road elevations. The contractor will then grade the road so that you get drainage to your catch basin. Be careful with your TOG elevations: we usually specify for them to be equal on both sides of the road.

Depending on the slope of the major road, the difference in elevation across a 24' wide road can potentially be very minor (0.25 ft for 1%). If your major road is sloping at 5%, however, the difference in elevation would be 1.20 ft. Specifying equal TOG elevations in this case could lead to a bumpy ride across the intersection for those on the major road.

I would specify the TOG elevations as for superelevating the minor road and analyze the catch basin capacities as interceptors, not sumps. Your local storm water manual should provide equations for doing this, based on a K-factor for the grate, which should be provided by the manufacturer. If not, I can probably point you to some downloads. (The K-factor is based on geometry of the grate.)

We use an Excel spreadsheet to analyze our interceptor-catch basin capacities with a K-factor interpolated from a nomograph provided in a Neenah Inlet Grate Capacities for Gutter Flow and Ponded Water publication (dated 1987!)

 

[johnhan76]

Thank you very much for your help. We use curb inlets without grates and place them in the center of the radius. One more question: Assum that the the minor road is sloping down to the intersection. The cross slope of the major road is sloping in the other direction. Does this make for a bumpy ride when crossing the major road when driving on the minor road? Perhaps a vertical curve somewhere on the minor road is necessary?

 

[francesca]

Your intersection should have the major and minor roads intesecting at around 90 degrees, so the slope of the major road is immaterial to vehicles going straight on the minor road. Many subdivision roads are not sign-controled, but even still, at a speed of around 20-30 mph (the design speed for subdivision roads), the 3% crown (or in your case, I think 2%) is really not a problem. When last did you cross an intersection and think "woah!"?

If your inlets are in the center of the radius, make sure to specify a TOG elevation that will allow both roads to drain to the inlet.

Whether or not your inlet has a grate, you will still need to analyze it as an interceptor. I'm sure your local storm water manual will be able to tell you what calculations they require.

Best of luck.

 

[oudst]

Francesca,

Have you ben working with the design part of Terramodel and then with the inputfiles?
Do you have experience in MX software?
I am asking because as an MX designer i got hand of a 30 day trial for Terramodel and i am trying to find out the pro's and con's.

 

[francesca]

oudst,

I work in residential subdivision design and use Terramodel to perform things like contour drawing off DTM, road points generation and sewer design. Our transportation department used to use it, before TDOT decreed that Microstation be used for all their projects. The transportation guys use much more of Terramodel's functionality than we do in residential.

I don't edit input files and I don't know what MX software is.

We use Terramodel version 9.6, which is a long way from the most recent version. I can't say that I have many complaints about Terramodel, and even if I could highlight one, the chance is it's been fixed in later versions.

For subdivision design, I would consider Terramodel superior to AutoCAD, but I've hardly used AutoCAD in my career, and that was both long ago (2000) and not for residential subdivision design. AutoCAD has plenty of bolt-on software and can probably match Terramodel's functionality. I've never used Microstation in anger, so I can't comment on its value, but apparently it's the draughting tool of choice for TDOT.

Francesca