take the edge off Roads, Drains

One of the most common reconstruction jobs being done for Road Authorities these days is adding bus turning lanes to existing intersections.

The goal is to minimise the work on the existing roads by grading the new string to preserve and extend the existing road crossfall where possible. The challenge is to control the drainage around the new kerb - its a perfect example of reference planes in action.

This is a follow on from a previous post on the topic “Widen at Grade.. the concept”

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The Components

We can do the whole job with just 3 Horizontal Alignments and a surface model.

The Subassembly

We are going to use the “OverlayWidenMatchSlope1″ Subassembly as it is custom built to do exactly what we need.

The help file explains it nicely (see the illustration on the right).
Insertion point: Saw Cut String. This is where the reconstruction will begin so it is where we calculate our grades from
Overlay depth: 0 - we are projecting the existing surface so we don’t need to worry about overlay.
Sample point: We want the plane to go to the left so we sample to the right (+1m)
Surface: this is where we calculate the existing grade

There is a confusing side to the graphics when the subassembly is inserted to the drawing: - the insertion point appears to be at the sample point.
This is only a schematic representation - don’t let is confuse you, the insrtion point is always as per the diagram above.

This part is crucial to the whole gig - there will be a post specifically on it soon.

The Procedure

• Sample the Existing Roads Surface for each alignment
• Form corridors on the 2 saw cut alignments using the OverlayWidenMatchSlope1 assembly.
• Form surfaces from these corridors.
• Add these surfaces onto the profile of the Bus Lane String

Next Step? (coming soon..)

The next step is to design a profile based on the projected planes and to use that for the final corridor. There are also some checks we can put in place to ensure we are within max/min grades etc - watch this space!