Pipeline Road Crossing using API 1102

[TexasPE]

When doing the dead load and live load calculations on a pipeline road crossing a client wants to go 15 deep and bore a 12" nominal pipe under a Txdot FM road. An old 2001 guideline I have from ASCE/FEMA study has a table that shows there is no live load influence on AASHTO H-20 truck at below 8 ft for a vehicle road crossing. Yet my version 7 of API makes no mention of not having to calculate any stresses at burial depths below this depth. Of course the charts dont have lines for h below 10 ft and it states not to extrapolate

 

[JGard1985]

I would look to existing guidance on this one. By the time you solve the load coefficient using Newmark & Hall integration of the Bousssineq Equation, you will find out that the live load contribution at depths greater than 8' is negligable (See my discussion below). All books/industry standards talk to eachother and negate live load after 8' of cover for trucks.

It is very common to negate live loads (of trucks) at deeper depths. For more Info Read "Design of Buried Pipes By Moser", free PDF available online. Look at Figure 2.19, shows how much the Vertical soil pressure from H-20 Live Load drops off at deeper depths. You can calculate the load based on the coefficients and methods in table 2.6 in same chapter of book. Just be consistent with H-20 (vs. HS-20) and AASTHTO guidance (ASD or LRFD) which has different shaped soil prisms




Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[BigInch]

No effect below 8ft or so. I suppose you do know that you will need TX State Dept of Highways approval no matter how deep you make the crossing.

The only question I might ask is why is your client wants to place it 15ft below. Certainly not necessary for stress purposes.

Reaction to change doesn't stop it

 

[LittleInch]

texasPE,

I'm a bit confused about the question. The headline refers to API 1102, which is a pretty standard cook book design to check the stresses in a pipeline crossing a road or railway.

You just need to follow the calculation flowchart.

"Of course the charts dont have lines for h below 10 ft ".. Which charts? Ref no in API 1102 or a different one?

I think the issue is that as shown below the impact of depth on surface loads is a small percent below 2.5m/ 8 ft, but not zero. However some charts may just ignore it, but API 1102 doesn't so just include it, even if it is a small number. I'm struggling to see what your issue is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[auzie5]

How are you planning on crossing this road in your design (e.g. auger bored, jacked, horizontal directional drill (HDD))? How is your pipeline contractor planning on crossing this road during construction? Be careful that the crossing method stipulated in the design and submitted for approval is actually how your pipeline contractor intends to cross this road.

Rather than using a crossing technique covered by API 1102, for convenience many pipeline construction companies prefer to complete crossings using horizontal direction drills. This introduces a greater risk of settlement of the structure being crossed (since the drill path is not reinforced by the carrier pipe during the pilot hole, reaming, and cleaning passes) if the drill path is not located sufficiently below the structure being crossed. Some clients are insisting on deeper crossings to provide some protection in cases where the crossing is designed to be an auger bore (at a relatively shallow depth) but is actually installed using a HDD crossing method.

For these cases (i.e. where a horizontal directional drill is the crossing method), API 1102 alone may not be sufficient to demonstrate the design is adequate. Installation and operating stresses that are introduced due to the HDD crossing method may need to be considered in addition to any live surface loading covered by API 1102 (e.g. reference PRCI PR-277-144507-E01).