DCG Question - Geological Geometry

Leaving Cert Student

Does anyone know how to do cuttings and embankments for roads that are rising/falling? I can't seem to be able to make sense of differentiating between the two... and when there is a carpark or some obstacle in the middle of the road then what?

Just realizing how difficult it is to study for this subject!

maughantourig

Should look like this:

Suppose the contours of the map are 5m apart, the fill/embankment is 1 in 1 and the cut is 1 in 1.

Let's refer to the point where the road starts to rise as your 'starting point'.

At the starting point, draw a line perpendicular to the road, through the road. Where this line strikes the sides of the road (points 1 and 2), draw 2 semicircles radius 5mm on both sides of the road with centre point on the sides of the road. These are your cut cones, (but look like semicircles on map).

If the road is, we'll say, 1 in 15 rising, then 75m (5*15=75) up the road from your starting point, draw a line perpendicular to the road. At this line the road will have risen 5m.

Where this line crosses the sides of the road (points 3 and 4), draw 2 semicircles radius 5mm like you did previously. These are your fill/embankment cones.

From points 1 and 2, draw a tangent to the fill/embankment cones. All of your fill/embankment lines will be parallel to these ones.

From points 3 and 4, draw a tangent to the cut cones. All of your cut lines will be parallel to these lines.

(radius of cut semi circles)=(difference between consecutive contours)*(ratio of cut (1in1, 1in2, etc))
In this example: 5=1*5

(radius of fill semi circles)=(difference between consecutive contours)*(ratio of fill/embankment)
In this example: 5=1*5

The 75m was calculated by multiplying 15 ('1 in 15 rising') by 5 (contours were 5m apart)

If the road was falling, you would reverse the positions of the cut and fill cones.

You place your cut cones at the low end of the road and the fill cones at the high end.

maughantourig

If the road was straight, you would line up your sheet with the road and work away.

To deal with obstacles, (eg carpark), you turn the sheet to work on each individual side of the carpark.

Keep going until your final embankment/cut curves intersect.

kingcobra

Once you get the grasp of Geologic Geometry it's a really really nice question. I've only been doing it a month and now it will almost certainly be the best question in my exam paper, but that probably shows how I'm complete crap at everything else :pac:

Also my teacher said that obstacles like a car park etc. won't come up because they reduced the time per drawing from the old exam format so it'll take up too much time. It hasn't come up in the new curriculum yet, ie the last 3/4 years.

I couldn't put it much better than the poster above and that'll be a great reference for study too so thanks :P Just don't forget the position of the cones; I always think it as opposites: cuttings are on the high side of the road so cut cones are on the opposite side, the low side. Whereas embankments are on the low side of the road so you put the fill cones on the high side of the road.

koraDCG

you will find a sample question for a fill on a rising road here
http://www.dcgsolutions.org/Demo2/demo2.html

its the second geological geometry questions (first is a level road)

Leaving Cert Student

Cheers lads, mighty help... still one thing bugging me though... say i want to get a cut or fill for a rising road which is not straight? I know how to do it as long as it is flat but the rise changes things especially for a curved or turning road?

kingcobra

To be honest I don't know how to that; I take you mean that you're trying to find the cut/fill for a road that's curving and rising/falling simultaneously? I never saw that question in the exam papers and I certainly wasn't taught that anyway.

Leaving Cert Student

kingcobra wrote: »

To be honest I don't know how to that; I take you mean that you're trying to find the cut/fill for a road that's curving and rising/falling simultaneously? I never saw that question in the exam papers and I certainly wasn't taught that anyway.

Yeah it is unlikely something so cruel would come up but there was a rising road question on a mock a few years ago with a carpark on the rising surface. the marking scheme used a technique i certainly have never been taught

maughantourig

They surely wont ask that, but it is possible to draw. The best way I can think of managing this is as follows:

If the road was level, you would draw in your height lines with a compass.

If the road was we'll say, 1 in 15 falling over a 90 degree semi-circular curve in the road, you would have to divide up the curve:

Using your 60/30 set square, divide the curve in 30 degree sections.

At each point, draw a tangent.

If the contour lines were 5m apart, you would mark a point 75mm (15*5) along each of these tangents.

Place your cut/embankment cones at the point of contact and at the 75mm point.

Now you have divided up your circular problem into a series of linear ones. Work out each as normal. I imagine you should get a series of final cut/fill curves that overlap to leave you with the finished product :cool: