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settled pipeline

settled pipeline

settled pipeline

(OP)
Hello everybody.

I have another site problem.  An underground drain (beneath a little road) has deflected.  This was revealed during a CCTV inspection.  The pipe is 250mm dia 15m long and 3m below ground level.  The centre of the pipe appears to have settled by 150mm between manholes.

There are many ideas floating around our team from testing it/ partially filling it to reduce the dip/ post tensioning it to straighten it/ digging it out/ boring another pipe......

Is there any tried and tested way of repair which could be done without disturbing the road?

nb

RE: settled pipeline

nb,
The first thing you should investigate is why the pipe deflected.  Is it a result of soil during road construction which has setteled, etc...  or has a joint opened which is washing away the soil thru the pipe structure

RE: settled pipeline

The cause of pipe deflection and the surrounding soil/bedding characteristics must be determined before a good recommendation can be given.

I believe Compaction Grouting may be a possible solution for your situation. The grout injection below the pipe bedding could possibly partially realign and and provide support for the pipe.

I tend to prefer Compaction Grouting for underpinning because the lifting can occur quite a few feet below the structure, distributing the uplift forces and using the existing soils as a 'cushion' The grouting can also be accomplished without a lot of disturbance around the structure, oftentimes allowing continued use during the repair.

RE: settled pipeline

(OP)
Thanks for the replies.

The excavation beneath this pipe may not have been compacted properly after excavation.  The ground being sand with a high water table.

Would pressure grouting be able to raise the pipe and the 3m of soil above it or would we need to flush some sand away from the top of the pipe?

nb

RE: settled pipeline

The Compaction Grouting could raise the pipe, the overlying backfill and the ground surface. The trick is to grout 1-1/2 to 3 meters BELOW the pipe base. The pressure of the grout and 'pressurized' soil must not be too high in the immediate vicinity of the settled pipe, or pipe collapse could occur. The grout pipes should be spaced 1 to 1-1/2 meters apart and be grouted in progression, in several stages or steps. I would suspect the contractor would want to alternate the grout pipes on each side of the settled pipe.

Some minor raising or heave of the road will probably occur. The amount depends on the condition and density of the native and the backfill soils. This procedure is normally used to stabilize a settling pipe and partially raising the pipe is a secondary result. The task is not impossible but, you would be stretching the 'comfort zone' of most engineers and many of the Specialty Contractors.

You must also be advised that some grouting jobs, after a lot of effort and expense, do not accomplish the goal. I have generally had good to excellant success with compaction grouting, but only when the Specialty Contractor has been experienced and willing. This is one of those techniques in which the contractor is as much an 'artist' as a technical type. Those that have the 'touch' do the best work.

A final note, the best contractors want a detailed Geotechnical, Subsurface Report. Beware the ones who are ready to go at a minutes notice.

RE: settled pipeline

(OP)
Thanks,

I shall investigate if any specialists subbies are available.

nb

RE: settled pipeline

I am not aware of any way to straighten your pipe without destroying it.  sounds like there may be poor soils or bedding under it.  If you could sonmehow straighten it, you have not solved the issue of poor soils or bedding, and pipe would settle again.  If you cannot diturb the road, you may need to push a casing pipe under the road with your new carrier pipe inise the casing pipe.

RE: settled pipeline

Assuming you're satisfied that the deflection process is compete and you've established that there are no leaking joints as a result of the deflection, it seems to me that you haven't asked and answered a fundamental, commonly missed question:  Does it matter?

A 6" sag in a 12" pipe?  How does that affect your capacity?  Any?  Considering that intentionally sagged lines have been known to work quite happily in sanitary systems for many years, it does seem that answering this question should be your first step.

Bill Holt

RE: settled pipeline

(OP)
All good points. Actually we have considered the last one Bill (because it's the cheapest) but I was interested in other ideas.

I fear our Client will not accept any defective work regardless of what i can prove by calculation or otherwise.

In such circumstances an extended guarentee may assist - but this becomes a contractual issue.

An interesting problem better to get it right first.

nb

RE: settled pipeline

Ah yes, the good old "get it right" the first time approach.  I've heard of that.  Seen it a few times too.

I imagine, however, that in this situation any repair acceptable to the owners that leaves out the lawyers will be less costly than any solution that includes them.

Bill Holt

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