Aggregate Compaction? 3

[jyf]

Hi,

I hope someone can help me out of this. I have an industry site here. Originally? it was spec. as 100mm crushed gravel and 200mm pit-run with 98% SPD compaction. Then the contractor told me if I can reduce 98% SPD to 95% SPD, the compaction cost can drop 40%. So, any thought of that or any resource that I can take a look?

Thanks a lot!

JYF

 

[muuddfun]

You should find out why it was speced to 98 percent in the first place. Only when it is very critical is it usually speced that high. One reason could be it looks like you have a standard proctor spec, and not the modified proctor? Find out what the compaction energy required is. Usually though we only specify a compaction requirement for 3 inch minus portion (the matrix material), (ie. 75mm and less). For larger sizes then you want to specify a method spec, or a target density, or be prepared to take ring tests or something like that.

 

[jyf]

Hi Muuddfun,

Thank you for your reply.

I couldn’t find why they spec. 98% originally and yes, that is a standard proctor density. The crushed gravel and pit-run defines as below

Metric (mm) US Percent Passing
20.000 3/4" 100
16.000 5/8” 84-94
10.000 3/8” 63-86
5.000 #4 40-67
1.250 #16 20-43
0.630 #30 14-34
0.315 #50 9-26
0.160 #100 5-18
0.080 #200 2-10

Metric (mm) US Percent Passing
80.000 3” 100
50.000 2” 44-100
25.000 1” 38-100
16.000 5/8” 32-85
5.000 #4 20-65
0.315 #50 6-30
0.080 #200 2-10

I think that fits your 3” minus portion then. But I don’t know how much deferent between 98% or 95%. The majority of traffic loads for the site will be imposed by lightly loaded trucks. But during the construction, 80 tons mobile crane will come in to do the installation.

 

[msucog]

this sounds a lot like base course material. 98% std should be no problem what so ever if i'm seeing that correct. we typically test for 98% modified on base course. dot specifies 100% modified here.

if this will carry construction traffic (light trucks, heavily loaded concrete trucks, or anything else), i'd advise against 95% standard Proctor being your target. you might put to the contractor like this: "you can put it in at 95% standard with the cost savings to the owner and you guarantee the area for x-years". if you tell them it's ok to put it in less than well compacted (i consider 95% standard for base course material to be less than well compacted) and it tears up during construction, someone (the contractor) make "ask" for a sizeable change order to fix it all if they're not on the hook for the end product. that's my experience anyway.

 

[cvg]

agree with msucog and also question the proposed 40% savings claimed by the contractor. I doubt that one extra pass by the roller will increase the cost by 40%

 

[muuddfun]

jyf
I agree with Msucog, your original post had 200 mm (8inch), that is why I was questioning doing 98 percent on cobles. For road work you can get 98 percent on base without that much difficulty.

 

[jyf]

Thanks for all.

cvg,

So, to increase the compaction from 95% spd to 98% spd usually just need one more pass by the roller right? I will ask them about that. I think I need ask some nother contrators for the cost as well.

muuddfun,

sorry for the misunderstanding?that 200mm is the thickness of the pit-run layer, not the size.

 

[msucog]

you'd just about get to 95% of the standard proctor max dry density by spreading it (some exaggeration there...but not much). go with a minimum 95% of the modified proctor max dry density. getting 98% or 100% modified is what i recommend around here...some folks do stick with 95% modified. the moisture needs to be correct and the subgrade conditions need to be well compacted otherwise they likely won't get to 100% modified proctor.

and i also agree, there's no way they'll cut the costs 40% by simply making a few less passes. the only way i could see it is maybe they leave a bunch of crap in the subgrade whereby it reduces the amount of undercutting/replacement needed to obtain a stable subgrade.

 

[PhxCivilPE]

You should be able to get 95% with little to no effort. If it's going to be under a paved section, I'd make it 100% compaction, which again should be pretty easy to get with conventional equipment, ie roller with a water truck to get to optimum moisture content.

 

[dirtsqueezer]

Just a little add-on here, this is all accurate. In my experience it is easy to get 98% using the standard proctor pespecially if your thickness is only 6 inches. Shouldn't be a problem. For the future, thicker lifts would require more compaction, and in my experience take a lot more effort to compact. The very thin lift thickness is probably the reason he's trying to get you to ease your compaction requirements, because it's a lot more effort for him to fill in such lifts. Just reassure him that with the 6" lifts compaction is easy to get, and he shouldn't have to worry about any extra cost he's got in his mind. Very good comments here.

 

[HawaiianMarine]

The 3% difference is not likely to save the contractor any money or have a significan impact on the quality of the finished product. It sounds to me like the existing soil conditions must be somewhat favorable as your section is somewhat thin. Vigilance for soft spots in the grade due to excess moisture is probably more important than 98% vs. 95%.