Tolerances for gravity sewer horizontal bore under highway

[DScintille]

We are advising our state DOT on a project where they must bore under an existing state highway for a gravity sewer. The pipe must be at 1% minimum grade. The state DOT specs state the minimum tolerance should be 0.5 feet per 100 feet. We'd like to tighten up the spec a bit to keep the mom and pop contractors from bidding, but still not inflate the total cost of the project.

I have two main questions:

1. What is a reasonable achievable tolerance to hold the contractor?
2. What is a reasonable interval to take survey measurements to check tolerance?

Thanks for any help you can provide.

Drew

 

[TDAA]

First, if you can, go to at least a 2% grade, this will help with the tolerance issues. See if the civil can give a little extra at the crossing

Second, you say there is a minimum of 1% on the pipe, with a deviation of 0.5 feet in 100 feet. This means you are now limited to grades between 1.0 and 1.5% if the design is at the minimum slope. That seems pretty tight to me as is, depending on the length you are going.

In checking your minimum grade, I would go from the change in grade to change in grade. If you average, you could actually have flat or reverse grade areas. Also check at a specified interval (based on how far you are going – 2 lane highway, 4 lane divided with a couple turn lanes?)

Also, what method will be used to “bore”? Directional boring may lead to poor results at low slopes (based on exp), while horizontal drilling or pipe jacking may be better, but would require pits. A bit more information would be good.

 

[cvg]

normally a casing pipe would be jacked and the sewer would go inside the casing. Assuming the casing is sufficiently oversized, the sewer pipe can be installed with the proper slope by using skids / spacers on the pipe installed to give the desired slope.

 

[nackra]

might consider:
larger casing (allow for construction "tolerances)
verify elevations of installed casing
adjust skid (spacer) dimensions to maintain slope

good luck