pipe trench backfill testing of compacted layers 1

[Good_Guy]

The specification for pipe laying (DICL) states
The frequency of testing shall be one test at the spring line of the pipe for each 50 linear metres of pipe laid or part thereof.

The material shall be placed in the excavation up to the level of the pipe barrel and shall be tamped and rammed in layers not exceeding 150 mm thick before compaction, to provide a dense well-compacted bed free from soft spots throughout the length of the pipeline

Now, to mitigate time and costs, the contractor proposes to lay 300 mm layers(150+150) and test the 2 layers of 150 mm at once

can 2 or more layers be tested in single test at different depths?

Please advise if this will be OK and so what test methods must be adopted to get a minimum 98 % Std Dry density

Thank you

 

[oldestguy]

Looks to me he only does one test in each 50 meters and then only at the spring line. Who ever wrote the spec needs to revise it due to confusion if he is to do more.
The spec doesn't call for more testing "can 2 or more layers be tested in single test at different depths?"

Also definition of the test method to check against is not a standard that I know of. You might call that "standard Proctor", but he can argue that.

 

[Good_Guy]

TThanks for your quick response
Just keeping the spring line of the pipe out of discussion, can one get multiple layers of compacted soil tested in one single test

Say I have 2 layers of 150 mm compacted soil. Can the test be done on top of the 2nd layer(300 mm). Will the test pass the bottom first layer?

 

[EireChch]

Good Guy - When you do a sand cone replacement test, you remove material to approximately 150mm (cant remember exactly)and replace it with sand and determine the dry density. You are essentially checking the density of the layer to 150mm only.

The contractor may be proposing to place two layers and do 2 sand cone replacement tests, one after the other. i.e. at the top of the second layer do a test and then remove another area of roughly 450 x 450mm x 150mm deep to get the tray placed on top of the first layer and then do another test.

In summary, a sand replacement test will only test to 150mm depth (the depth of the material replaced with sand).

Alternatively, they could be proposing to do a nuclear gauge test which would test the density of the 2 layers at once as it has a deeper testing influence. Note, this needs calibrating with the material onsite.

Plate bearing test is another alternative, however not too common for testing trench backfill. Likely have no access for an excavator for a reaction. That can test to approximately 1.5-2B of the plate. So a 300mm dia plate would get you down to 450-600mm depth.

 

[Good_Guy]

oldestguy & EireChch

Thank you for your valuable inputs. In this case , the tester has advised that he will be using a nuclear gauge meeting to all calibration and testing requirements

all suggestions provide a good knowledge

 

[Mad Mike]

With nuclear gauge testing, you would still want 2 x results for the 2No layers, as opposed to testing all 300mm as a single layer.

Have your Contractor test the upper 150mm layer; confirm the result, and then carefully excavate down another 150mm to test the second layer- so you will obtain clear results for each of the 150mm layers.

You should expect a slightly higher density in your lower layer when testing this way with 150mm layers...

All the best,
Mike

 

[EireChch]

If they are doing Nukes then it is a very quick test. There is no reason why they cant test all the layers immediately after each layer is compacted. Test takes about 2 minutes.

If it were me, I would test the first 150mm layer and then the next layer at a sufficient amount of locations to give you confidence that the contractor is doing good work and meeting the spec. Once you get confidence in the contractor then you could push it to testing a single 300mm layer. That assumes that your spec allows you some freedom for engineering judgement etc.

If your nailed to each 150mm layer by the spec then go and do it in each layer

 

[Good_Guy]

EireChch / Mad Mike and all

All of you have given me super tips

I am now more knowledgeable
thank you

 

[mikeofBelAir]

A few more thoughts...
For whom are you working? The Owner? the Excavator? The Government? If you are working for the Owner--it is his interests that you are protecting. Don't try to be the "nice guy" and go easy on the Contractor because you only incur liability for yourself with no benefit. Protect Your company's interests first and then your Client.

 

[oldestguy]

More"two cents" from OG. Compaction and required testing the soil just below the spring line is touchy, since one can actually lift the pipe with too much compaction alongside it. Again its a crummy spec and can be difficult to use, practically.

 

[geomane]

You might also need to run a speedy moisture test with the nuclear gage testing. If you are testing in a trench, the trench walls will influence the moisture readings taken from the nuke gage.

 

[fattdad]

What we know:

the specifications require 150mm lifts.
the specifications require each lift be tested.
it's 300mm to the spring-line.

What we don't know:

What's the pipe's use - is it a low-level outfall from an embankment dam?
What's the anticipated failure mode - is the concern related to piping along the pipe run, is it concern on lateral support, 'cause there's going to be 100 ft of rock-fill over the top?

Is there some reason that the specifications require 6-in thick lifts?

I just don't know what to say!

Yes, you can obtain dry density measurements on previously-placed lifts. You may find the compaction moisture content invalid; however. You can learn dry density and you can see if it relates to the specification. Folks have already mentioned the trench problems with nuclear methods. That's mostly an effect to the water content - so bring a Speedy or a frying pan and Coleman.

If the entire domain of fill is shown to be at the proper degree of relative compaction, who cares about lift thickness? I would not care in the least! But, the specifications REQUIRE 150mm lifts - so that should be built into the contractor's unit fees. Is there some reason that the contractor now wants relief? Not your place to provide that relief; unless you happen to own the contract.

If the pipe is carrying flowing water (i.e., as a pipe through an embankment), I'd think the entire design off. I'd put the upper 2/3rds in a concrete cradle and the lower 1/3 in a gravel blanket. Seen too many failures owing to piping along the concrete-to-haunch interface.

Have fun!

f-d



ípapß gordo ainÆt no madre flaca!