Problem:

Specification says that the contractor shall carry out 6 passes of a 15 ton vibrating smooth drum compactor.

What is a pass?  
   Is it a round trip?
   Is it a single pass-over?

Most specs are "mute" on this.  Wanting to see what kind of variation in responses - and by country.


 

BigH, you asked this question 2 years ago, I knew it sounded familiar.
My vote- a single pass-over.
Do compactors always retrace their paths?
Alaska, USA.  

From north Florida and south Georgia, a single pass-over, not round trip.
I don't think I've ever had this issue come up before.
If I was a contractor who read that statement, I would do 3 trips out and back.  I don't think I'd take it kindly if an engineer told me I had to do more.

Sorry CarlB - Senior Moment - but hoping to get more responses - especially in Asian and African countries . . .

Australia...six passes means the roller touches the surface six times, not necessarily three round trips.

Square trips??

I'm going with "B."

f-d

¡papá gordo ain't no madre flaca!

Single pass.  US - mid atlantic.

A round trip is always two passes ... and that's an awful spec regardless of how one interprets the passes.

As many know, I am now in a SE Asian country. In the performance of a trial fill to which the contractor must carry out up to 12 passes, the contractor was ready to do it as 12 round trips ( which would be, for the record, 24 passes in my book) . . . and this isn't the first Asian country I've seen this "idea" in . . . Suggestion if one is involved in writing specs or soils reports for overseas projects (perhaps Asian countries) it might be a good idea to clearly define the meaning . . . I convinced the contractor that he only need to do 12 "single" passes . . .

I'm still interesting in those practicing in SE Asia or in the sub-continent to please give their understanding . . .

It's a good question to resolve, because with a vibratory roller, one pass too many most likely will REDUCE the density of the material being compacted. If any material takes 12 passes with a vibratory roller, no matter how you define a "pass", either it is the wrong roller for the job, the moisture content of the material is not optimal (far from it), or the lift is too thick. Any spec that requires either an odd or even number of passes with a vibratory roller has at least a 50% chance of being more or less than optimal by one, one-way pass.

Six passes of a pretty large vib roller? Wow, that's a lot of good, good, good vibrations. What RD are you trying for and what lift thickness? That's not your filter, right? (Peck told us that for filters, 70% RD should be the max. He was concerned about particle breakdown -> loss of vertical permeability, and potential for brittle behavior and cracks.)

No, @Dave, it is not the filter . . . it is a clayey zone outside the core.

Oh. I had assumed it was filter, drain, or granular shell. I don't often see smooth vib rollers on clay in the US, mostly padfoot rollers on smaller projects, and lots of 825s on bigger ones (stock, or with aftermarket drums having small circular classic sheepsfoot feet).

Have you ever done an experiment to see how much the vibration helps the density of clay? For a fairly small fill, the contractor brought a small radio-control vib padfoot. The vibration made a very small difference in the density, but that was the difference between barely making the density spec and just missing it.

DRG

Doing both smooth drum and padfoot on the trial embankment of the material - there is a difference between USA and SE Asia . . . . and what the contractor can or is willing to put on the job . . .

BigH... using a smooth vibratory roller for clay? or did I misunderstand you? We typically use a sheepsfoot...

Dik

@dik - we are doing both . . . the equipment choice for the trial embankment is left to the contractor and then it is judged . . . I've seen a lot of smooth drum rollers used on clayey material (CI/MI) here in Asia.

OK... I assume you can get the compaction... not been my experience with drum rollers... we have a very high plastic clay in these environs... compaction not spec'd by Proctor?

Dik

Well, that's why we are doing the trials - the material at present is not overly plastic; compaction to be 95%. I'm more "worried" about ensuring no torturous water paths through the material. Thanks, dik and others, for your interest.

My experience is the same as Dik, no success with smooth drums on clay. If the smooth drum roller leaves traverse checking on the surface, that's typical of clay and a sure sign the roller isn't doing the job.

If your spec is by percent of proctor, why be concerned about the number of passes?

This is a big dam site, Hoaokapohaku. At present, we are looking at an embankment trial that will help us evaluate the most appropriate means and methods (remember, the contractor had a choice of equipment - so he has chosen to try a smooth drum roller in addition to a padfoot). As for the number of passes, it is important to the contractor (and us) to know how many passes he needs to make to reach the compaction needed. Considering perhaps more than 280 layers, the time element is very important - if it takes 8 passes to reach specification limit rather than 6, it may mean several weeks or longer of additional compaction over the course of the dam construction. On a highway project where the embankment is only a couple of metres high, it is not as critical.

Thanks BigH...that helps me understand. If the project were my responsibility, this would definitely be one of those "spare no reasonable expense to get the optimal compaction equipment" kind of projects, which is probably the 825's that dgillete mentioned earlier. But like you have said, you're in SE Asia and you must work with what you have.

Big H - Suppose you could post some photos of test pits in the fill, for my enlightenment/entertainment? Would love to see how the lift contacts compare with the two different rollers. What would be really cool, though probably not possible, is to see how they compare after another 10-20 m of fill have been placed over them. Would they be consolidated to form one tight, coherent impervious mass, or would there still be discontinuities (potential locations for initiation of piping)?

I assume you don't have handy access to testing equipment, but regardless of the specifications I'd do a quick nuclear density growth curve with the material at hand and see what the field conditions are during compaction. As many mentioned, too many passes could put you on the backside of the growth curve proctor test.

As much as the Contractor would like to just windup and go on a project, they need to adjust for actual conitions. Can I get density in 3 passes, maybe if the moisture % is just so. It is usually the Contractor's "means and methodology" to balance if they want to cover the cost of additional moisture (a water wagon) or drying (disking and waiting) versus adding additional passes with the roller to achieve target density.

All that being said, the 6 pass specification is more of a performance specification and opens the designer and owner to liability for field conditions they can;t really control, some of which are present by the Contractor's choosing (ie. moisture, equipment - too heavy or too light of a roller, ...)