Best Way To Cut-In Backflow on Underground?

2/25/2020

What is the best way to cut in to an existing underground line that may have a thrust block at the end?

We are doing a project that has scope to retrofit an 8" backflow into an existing approximately 200' of 8" underground. There is an existing detector check valve in the line which will be removed. Because of the age of the building and underground (about 30 years old) it is assumed the piping has a thrust block at the end. But with cutting into the line and adding a couple 90 degree elbows to get the backflow above ground there is fear that the line may slip back to the water main because the thrust block at the end of the 200' run is no longer holding the entire line in place.

We are planning to add a thrust block to the new 90s, but the issue is that there is only a shut off valve that is in the straight line of the 200' of pipe. If we shut that valve, there will still be pressure on the front side of the valve and when we cut into the pipe, the pipe may slip.

Any suggestions on securing the pipe on the supply side of the new backflow or limiting the pressure prior to the concrete being poured on the elbows for the backflow?

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7 Comments

Darrell

2/25/2020 10:14:26 am

My understanding of thrust blocks is limited, but I believe they are only applying force when water is flowing. The thrust block is sized to apply a reaction force to the flowing water. If you have a shut off valve, where no water is flowing during the BFP addition, then the pipe is seeing no force.

RyanO

2/25/2020 10:15:30 am

I had a similar situation a few years ago where we just poured concrete between the two ells below grade. Everything else was assembled with Grip Ring retainer glands. It has never moved.

There are also a few manufacturers that make assemblies specifically for this. Look at Watts(Ames) website under backflow preventer accessories. They're called Valve Setters.

2/25/2020 10:18:10 am

I may have misread the problem. Are you concerned about the valve blowing off after you cut in downstream? Only way to mitigate this is to retain the valve with a large "deadman" upstream, or isolate the supply to the valve.

Peter

2/25/2020 01:34:03 pm

The concern is the valve or pipe before the valve pulling out of the tee (or 90) after the pipe is cut out, prior to setting the new MJ 90 ell that will be the supply side of the new backflow preventer. Once the new MJ 90 is set, a new thrust block can and will be poured but it is the time period between cutting into the pipe before a temporary thrust block (with wedges or some other anchoring) can be performed.

I had thought about using an EBAA 1100SDB in a dead man like you had suggested just upstream. We've cut in a number of backflows, but the valve had always been upstream of another 90 or tee. This one is about 4' in front of where we need to cut in. We are planning to dig back to the valve and we've already spoke to the water municipality who has ownership of the valve about the potential issue. They haven't offered too much advice.

C.T. Carbonari link

2/25/2020 04:37:16 pm

I would use a restraining collar and rod back to a deadman poured just downstream of the valve. (A collar from a Bell & Spigot restraint set should work just fine and is probably what I would grab if I was on the jobsite). If I understand your conditions correctly then rodding the pipe back to a deadman should prevent any longitudinal slippage throughout the run of pipe back to the Ell (or Tee).

You propose an interesting Risk/Assessment question. I would think the probability of any joints slipping loose is very minimal (under typical line pressures, with no thrust, and the weight of the ground acting as a retaining force). With that said, the results of being wrong could be catastrophic.

A healthy fear of pressure is a good trait to have in this business and I would always err towards the side of caution and safety.

2/25/2020 08:02:48 pm

I am probably being over cautious. The existing pipe underground is steel and the connections visible are welded flanges. There may be no thrust block at all and the pipe may be welded back to the tee. However given the set of circumstances on a 30+ year old install I feel it better to be over cautious. There is an existing concrete valve pit we can use as a dead man and rod back to. This should at the very least provide enough of a restraint until the thrust blocks on the new underground elbows are cured.

2/26/2020 09:08:40 am

Black Steel? Not Ductile Iron? That would give me even more concern of existing piping integrity due to increased likelyhood of corrosion.

Like you, I am overly cautious as well. I've seen deadend MJ Valves pop off. Granted, it was after ~3.5 hours of hydro @ 200 psi and we were running the jumping jack in the trench all morning during backfill (I'm convinced we were trapping air at the end of the lateral). Not one of my Boss's wiser ideas.

Best Way To Cut-In Backflow on Underground?

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