Meaning of Passes

[BigH]

Simple question - will be interesting to see how this plays out engineer-wise and country-wise:

If the specification says that X passes of a compactor are required, does that mean that X is a single pass over a spot or a back and forth (double) pass?

 

[ishvaaag]

In Spain as per the language just every pass, back or forth. But I don't know if this agrees with slang in the trade.

 

[hokie66]

When the compactor goes across a spot, that is one pass. When it comes back across, that is two passes. US/Oz

 

[SirAl]

I consider a single pass as there and back. Canada, eh.

 

[Ron]

One pass is one pass...one time over the same spot.

 

[msucog]

each time over equals one pass.

Ryan Coggins, P.E., S.I.

http://www.linkedin.com/in/ryancoggins

 

[RHTPE]

One pass = one pass. Back & forth = 2 passes. New England.

Always assume to the conservative view without solid documentation to the contrary.

Ralph
Structures Consulting
Northeast USA

 

[fattdad]

I'm with the one time over is one pass.

Let me be the first to add, I hate specs like this (hopefully it's not your spec, BigH).

f-d

¡papá gordo ain’t no madre flaca!

 

[TDAA]

Agree with fd - bad specifying. The result should be speced, not the method. If you are trying to help a contractor dial it in, you should say what you mean. Where I deviate from fd is that I have usually used there and back, since that is what others in my company did. I was always very specific about this though.

 

[msquared48]

I almost passed out when I read this string.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RHTPE]

Be nice Mike.

Given the number of ancient specs I've seen, as well as the some of unintended consequences of modifications to the MasterSpecs, the question really isn't surprising.


Ralph
Structures Consulting
Northeast USA

 

[fattdad]

Let me be the first to add, I hate specs like this (hopefully it's not your spec, BigH).

. . . that is unless you're specifying construction of a test pad.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Its not really a spec but I hear all the time someone say "We've done 3 passes." - when pressed, it turns out it is three double passes or six, in my view. I agree, one time over is one pass. Canada. Doule passes = one pass I've seen in India, Indonesia.

 

[SirAl]

Interesting input to a simple question. I agree that it is not or at least should not be a spec. It is a terminology thing though, and I run in to the same question occasionally. When proof rolling I like to see a loaded vehicle pass the prepared area in at least 2 directions. Same thing when monitoring compaction of trench backfill ... if we are doing 'control strip densities' then we test the density of the material after the packer has gone down the trench and back again. (We started to monitor the placement of critical fill outside large CMP or multiplate culverts using control strips to establish the peak density vs. comparison to a laboratory proctor). Of course there are applications where a single - 1 directional pass is the only way a pass can be described. This would definitely apply to doing 'control strips' on a highway project where base gravel is compacted with a wobbly train or procession of multi-units passing a premarked density test location

 

[dgillette]

TDAA writes "Agree with fd - bad specifying. The result should be speced, not the method."

I don't think this is universally true. With relatively clean granular materials (generally speaking, RD material rather than Proctor), one can often get perfectly adequate compaction with a procedure spec, e.g., 12" lifts, each compacted by 4 passes of a single-drum 10-ton vibratory roller. We often do dam filters that way, and it's no problem to get 70% RD. It's a whole lot easier than sand cones or even nukes in a material with coarse gravel. Sometimes a test fill helps to nail down the required number of passes for whatever roller the contractor brings out. It might only be two.

This only applies to granular material, for which the exact moisture content is not too critical. For Proctor material, no way!

DRG

 

[TDAA]

So, in your filter, are you going to go with the number of passes, or for the 70% RD that you indicate. I guess if you know it is hitting 70% you must have checked, at least at some point.

If you are using past contractors ability to achieve a requirement with their equipment, and using with another contractor and different equipment, I still say bad specifying.Are you specifying the same amplitude, frequency and speed for the roller too? Is the material exactly the same as far as angularity, etc.?

I have seen to many instances of poorly functioning equipment to know that one contractor's POS is not the same as the next guys, nor does one operator run the equipment at the same settings as the next.

At a minimum, I would want to run a test section, with reasonable amounts of testing to show that the contractor is achieving what you want. What you want is not to run the hours up on the equipment, but to achieve the desired degree of compaction, right?

 

[dgillette]

There is a lot of collective experience behind procedure specs like that, and if the quantities are small, it sometimes makes more sense to put a little overkill in the compaction than to slow the process with a lot of testing. On any material with cobbles or even large gravel, the density measurements are a major undertaking, sometimes requiring a 6-foot ring with water replacement. It's mostly just record tests for those, although your field people will still curse you for requiring them.

Four passes of the 10-ton vib roller on a 12" lift is probably going to be overkill in any clean material. For large volumes, the contractor may want the test fill in order to be allowed to cut back on the passes. We have a standard 10-ton compactor spec that gives beats per minute, centrifugal force, and such, but it really isn't very difficult to hit 70% with any decent-sized equipment on clean fill. I don't think there is a whole lot that can go wrong with using a vib roller if the eccentrics are spinning at the correct speed (and the original factory eccentrics are still there).

You may be able to get 70 with 4 passes of a D8 on a 12" lift.

On a side note, we try not to overcompact filters because particles break down and create layers of low permeability at the top of each lift. [Note - I have no experience with fills where the minimum was more than 70.]

 

[Ron]

As for the specification part....the purpose of the passes needs to be stated. For instance is the specified number of passes to done for uniformity, for relative density or for "proofrolling"? The number of passes is obviously up to the Geotech (with of course, one pass being one time over spot)

Go Dirt.

 

[TDAA]

dgillette, I completely understand where you are coming from. I am just playing the devil's advocate.

All I was pointing out is that while you state one thing, you are looking for another, and you may not always get that by prescribing the method. As you stated, "if the eccentrics are spinning at the correct speed (and the original factory eccentrics are still there).) This was the same thing I was pointing out. Add to that some operator that does not like the way the vibration feels on his manhood, and who knows what you get.

 

[dgillette]

Never operated one myself so I'm not familiar with the comfort issue. I am tempted to make a rude and totally inappropriate comment about who should run them, but I will refrain in the interest of keeping this forum sort-of professional.

On a recent project with a small amount of dam embankment being replaced, they brought out a Wacker radio-controlled two-drum vib pad-foot roller that I'd never seen before. I didn't think it would work very well for the clayey core material, but with lots of passes, it got 98% of std. according to sand cones (lifts 8" LOOSE). They ran the vibrator, but I'm not sure if that really helped much. It was small enough that they could move it into small spaces by crane.