Pipe Stresses due to tunnelling activities
Pipe Stresses due to tunnelling activities
(OP)
First post here folks. Struggling a little bit and need to pick some brains.
Have been tasked with demonstrating that any settlement caused by our pipe tunnel crossings is not having a detrimental affect on existing services.
We were given a copy of some calculations by P.B. Attewell to demonstrate the forces exerted due to the bending curvature. But for settlement amounting to 1% of the existing pipe diameter, we get stresses amounting to 10% of the permissible strength. We think we should be getting figures much less than this.
Are there any other methods that can be used to work this out or does anyone have any ideas how best to approach this?
Many thanks.
James
Have been tasked with demonstrating that any settlement caused by our pipe tunnel crossings is not having a detrimental affect on existing services.
We were given a copy of some calculations by P.B. Attewell to demonstrate the forces exerted due to the bending curvature. But for settlement amounting to 1% of the existing pipe diameter, we get stresses amounting to 10% of the permissible strength. We think we should be getting figures much less than this.
Are there any other methods that can be used to work this out or does anyone have any ideas how best to approach this?
Many thanks.
James





RE: Pipe Stresses due to tunnelling activities
http://www.acipco.com/aswp/pdfs/ASWP2-Design.pdf
RE: Pipe Stresses due to tunnelling activities
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
Thanks unclesyd for the doc. We've used similar calcs for calculating the loading on our pipe, but we want to try to factor in the permanent axial stresses that will be put upon the pipe due to say 4mm deflection over a trough width of 12m.
Our example at the minute churns out figures of nearly 40N/mm2. When I calculate this as a percentage of the Ultimate tensile Stress of 420N/mm2 I get the 10% figure. Should I even be using the UTS figure?
It may be the case that we already have the answer here, but it just seems way too high for what we're doing. Can you think of anything else that we might be doing wrong?
Thanks again
RE: Pipe Stresses due to tunnelling activities
marston theory pipe axial forces
Question:
Did you do your calculations with the pipe flexible or rigid?
RE: Pipe Stresses due to tunnelling activities
The calulations we've been using are for a rigid joint pipeline and I've attached a copy so you can see what we've been working with.
We've downloaded some software and got similar results so are ready to admit defeat.
We're getting around 40N/mm2 for our stress when a 380mm pipe deflects 6mm over 12m. At the minute we're using the Ultimate tensile Stress (420N/mm2) to determine how this relates to the pipe's operating capacity.
Is this even the right way to go about it? Are there factors that should be accounted for relating to additional stiffness if the pipe is operating at high pressure? Should we even be accumulating these stresses with the stresses that the pressurised pipe is already subjected to, or is the affect only a fraction of the stresses already being exerted?
RE: Pipe Stresses due to tunnelling activities
MAOPs have a base calculated using either the "permitted stress", as in ASME B31.3, or "allowable stress" as in B31.4 & 8. You need to check the design codes of those pipelines in order to know what the limits are. The codes will also give you the load combinations, but they don't consider construction and operating loads simultaneously, which is what you may be looking at here, if pipeline operating pressure is not reduced when the construction activities are taking place. In that case you must use the full operating pressure of combined construction loads and then calculate the maximum combined stress for the most critical point in the span. A typical limit for that would be less than or equal to 0.72 * YIELD_STRESS, not ultimate stress.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
I did start to look at summating all the stresses that the pipe might be subjected to (i.e. ground loading, traffic loading and operating stresses) and then added these with the bending stresses, but the actual stresses involved with these other three components have little / no effect on the outcome of the calculation. They amount to a total of about 0.5N/mm2 whereas the bending forces are 42N/mm2 and the total amounts to about 15% of the yield stress of the pipe which is will below the minimum level of 50%. It's just that these sets of figures seem to be worlds apart.
Most of our pipework is installed below ground and trenchless techniques are a common way of traversing either a service, road, rail or river crossing here. We do however have to submit proposals to the owner and protective measures (such as ground monitoring) are put in place at the request of the owner as insurance measures. The proposals contain all the settlement calculations and pipe loading calculations normally, but this is the first time we've been asked for anything like this.
If we were to lay a pipe in a trench, it would have more than 6mm deflection along it's length in places and the stresses would be permanent due to the undulations in the ground so we know that the pipework can handle deflections of this nature. It is just a case of producing corroborating calculations. The client is hapy with what we have produced, but we are unsure of whether or not the values are correct so we would like to verify these for our own piece of mind before we start submitting incorrect calculations.
Obviously these the results of these calculations are theoretical values using 'worst-case' figures, so in reality would the bending stresses be significantly lower in operation?
RE: Pipe Stresses due to tunnelling activities
http://ww
RE: Pipe Stresses due to tunnelling activities
http:/
RE: Pipe Stresses due to tunnelling activities
The problem we found was that because this is an unusual request as far as the submition of standard calulations is concerned, there isn't really much support material. We've produced the calculations in a spreadsheet and tested the worked example and we are getting the same results.
When looking for other info, we did find research papers, not things put in best practice, and may just be documents produced by under graduates or post graduates. It's becoming increasingly frustrating as I've got to the stage now where I'm not even sure if we're not just creating work for ourselves.
RE: Pipe Stresses due to tunnelling activities
ht
Anecdotal:
The is major project in my area of relocating the Waste Water Treatment Plant that involves many miles of force mains. I watched a 20" D1 pipe being installed near my house. The pipe is about 6 foot deep in sandy soil with occasional outcropping of hardpan. The trench is excavated and the pipe dropped in on a bottom that looks like a catapillar as it moves. I watch 8 joints being laid and 3 of these had to adjusted to where they would make up.
RE: Pipe Stresses due to tunnelling activities
Don't understand, "It's just that these sets of figures seem to be worlds apart." Apart from what?
I also don't think you can look at it as "we know it will work" so it must be OK to do it kind of question either. Most of the time, and in fact, only under rare conditions; perhaps only during hydrotest, a pipeline is nowhere near full pressure for its entire length, so safety factors can cover a lot of installation errors, as far as the average piece of pipe is concerned. As long as you've got no big bending stresses in the first kilometer or so, or at bottoms of valleys, there are very few places where pressure hoop stress are likely to be anywhere near design levels. Well over half of a flat pipeline might not be at even be at 50% of hoop stress allowable. (Another reason why they seem to always blow under the middle of a river.)
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
By worlds apart I'm just referring to the fact that the figures I'd calculated for hoop stresses from the loadings and the operating pressure were on the whole negligible when compared to the bending stresses. So much so that the bending stress amounts to 15% of the yield stress, regardless of whether I decide to include these additional stresses or not.
The other two engineers that have looked at this with me were the ones that were originally dubious as to the validity of the results due to the relatively high figures encountered.
I appreciate the safety factor consideration and how the areas where levels of static head are greatest may require heavy wall pipe to extend hydrostatic test sections. And how 10m equates to approx 1bar of pressure. I have just taken a standard operating pressure of 75bar at the minute rather than the 135bar test pressure that may be used for a high pressure gas main, and as the one I'm dealing with currently is only 125mm and not one of the 1220mm ones.
Sorry if I appear not to be grasping something quite obvious here. This is the first time since I finished my degree I've done any engineering so to say I'm rusty is an understatement.
RE: Pipe Stresses due to tunnelling activities
Your hoop stress at max pressure should generally be much more significant, normally limited at 40%, 50%, 60% or 72% of yield stress (not ultimate), the % depending on B31.8 class location factor of the pipe segment. And ring bending might be 15% or so, with axial bending near zero for good bed contact. That must be a very very thick pipe wall at a very low operating pressure, if hoop stress is negliable. Normally a waste of material for a typical pipeline design.
Surface load effects on ring bending drop fast with clear cover. You can assume that surface point loads distribute laterally up to the width of the trench (more or less in a 45 deg cone) as depth increases. With a 2 m clear you could easily drive most heavy trucks over a line at full pressure, that perhaps without pavement of any kind, if the backfill is good.
And directional drilling isn't done in gravel or sand where cave in and bridging of overloads isn't possible, so there should be little effect of subsidence on a line above when the soil is right for HDD. Directional drilling depths are usually planned to be well away from any foreign structures to avoid any subsidence at all, as that's the real advantage, right?
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pipe Stresses due to tunnelling activities
http://cedb.asce.org/cgi/WWWdisplay.cgi?0529081
I used this url as some of the others were quite long.
http://
RE: Pipe Stresses due to tunnelling activities
The crossing was on a long sweeping curve where a straight shot would have been easy.
RE: Pipe Stresses due to tunnelling activities
You've both helped me a lot and definitely helped me to think about it a bit more logically, and stimulating memories of long forgotten lectures. Many thanks to you both.
As for the 90deg bends unclesyd, I never mentioned anything about them and our crossings are all via SBU or TBM machines. The bends we use are may be required to return the pipe to the normal buried level before and after the crossing.
Thanks again gents.