Minimum Lift Thickness

[howardoark]

Does anyone have a reference for minimum lift thicknesses for vibratory compaction of road base (3/4 inch minus)? I have a contractor having trouble making compaction with a decent-sized vibratory compactor on virgin aggregate. I think his lift size (4 inches) is too thin and he's just bouncing the rock around. Contractors using too-thin lifts doesn't appear to be a big problem around the country.

Thanks

Jeff

 

[fattdad]

Minimum lift thickness is three times the largest aggregate size. I don't typically look at it that way however. I normally recommend a lift thickness that corresponds to the largest partical size size that I'd accept in the fill specification.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[howardoark]

Thanks - it turns out the nuke gauge has a problem (sigh) sand cone showed 100% maximum dry density.

 

[howardoark]

So I sent a second gauge to the site and it produced the same reading as the first gauge. Operating the gauge in back scatter mode matched the sand cone data pretty well. So, something is messing up the probe readings at depth.

All I can think is the gravel is somehow being rotated as the probe goes down and that's causing the probe to measure more air than it should be.

Any one have any ideas?

Thanks

 

[cvg]

check the following which recommends using backscatter mode:

thread261-215268

 

[iandig]

In the UK we have a lot of guidance for using 'method' compaction, which relates the class of material [determined by grading] to compaction plant with optimum depth of layer, number of passes etc... There is also quite a lot of really useful info from people such as Bomag. Also Parsons who worked at the TRL published all his reasearch a couple of years back comparing plant performance.
If you can let me know the mass per metre width, I will look it up and see what it says. I take it from your description that the material could be described as a well-graded 3/4" down crushed rock, this would be comparable to a Class 6F1 capping/Class 6N structural fill/Type 1 sub-base.

 

[genomty]

Is the bottom layer of the same material, perhaps the source at the tip of the rod is reaching the underlying material with a dry density lower than your fill material, try testing to 3 or 2 inches. Backscatter requieres corrections for lift thickness and you should know underlying lift density. Is sand cone device calibrated for dimensions similar to those produced in field?, different depths and dimensions may produce different sand density which mean higher or lower compaction, is the Maximum Density properly obtained? Is the stone composed by any material that contains high concentrations of hydrogen (gypsum, coal) or is being lime stabilized, cement treated?. Does the fill material match the materials density for which gage operates adequately (70 to 170 psf)? how about moisture readings against oven dried samples? is testing being conducted within a trench or nearby (near by minus 2 feet) a vertical structure?, As you see there are several factors that could being intefering in the gage readings that are not considered or produce the same effect in sandcone test. however both nukes are producing consistent results, obiously there are something affecting the gage operation that you haved noticed.

Always is a nuke fault, I mean this devices have good, bad and worst issues,but it seems that always something should be wrong with the apparatus function instead the guy operating him, it is curious, isn't it?

Technology does't try harder, people does!

 

[howardoark]

Thanks for your interest.

The base rock is the nicest stuff you ever saw. Virgin crushed blue rock (specific gravity or around 2.8) with this gradation:

¾? (19mm) 90-100
No. 4 (4.75 mm) 35-60
No. 30 (0.6 mm 10-30
No. 200 (0.075mm) 2-9

Maximum dry density 146 pcf (2.36 g/cc), 7% optimum water content with a flat curve. I had it done twice and the soils lab says they've done Proctors on the same material in the past and got the same results.

Compaction with a Caterpillar CS54 (Vibratory Roller) 23120 lb (10,500 kg) 84 in (2130 mm) 130 hp

The section includes 18 inches (.46 m) of rock compacted in 8 inch (0.2 m) lifts over a Tensar BX1100 geogrid. The subgrade has the lowest possible resistance value (4), but the site had to be filled, so there’s 4 feet (1.2 m) of compacted fill (native soil with the r-value of 4) underneath the geogrid. If you’re curious, the traffic index was larger than the Santa Monica Freeway's. If you’re now curious why the section is “only” 18 inches thick it’s because the client didn’t want to pay for 12 inches of asphalt.

Since we switched to only backscatter mode we’ve been getting 95 to 99% compaction which matches the sand cone results.

There’s no reason this material wouldn’t have compacted, so I believe the backscatter results. All I can think that was happening was that the probe was rotating some of the larger rocks opening voids in the probe hole which resulted in false low densities.

A fully-loaded water truck rolling over the compacted rock doesn’t leave any tire (tyre) tracks.

 

[iandig]

Thanks for the extra info on the roller. Looking through the SHW, and comparing the grading limits, the material would correspond to the following classes as per the UK standards:
Class 1A General Granular fill, compaction Method 2
Class 6F1 Selected Granular Capping Material [fine grading], compaction method 6
Class 6N Selected granular backfill to structures, compaction requirements of 955 of MDD [vib hammer].
Looking at Table 6/4 of SHW, then the reccomended compactions for 'method specification' for the roller type you are using would be:
Method 2: compacted layer thickness of 250mm with minimum 9 passes
Method 6: for depth of 110mm a min 3 passes; for depth of 150mm a min of 7 passes; for 250mm layer a min of 12 passes.
From refernce to Method 6, it suggests that there should not be a problem with the original depth of layer.
As mentioned in the previous responses, the impact of the moisture content can not be ignored which in turn can mean a material is unable to be placed to the specified density. The above methods all rely on the fill being within an acceptable moisture content range of the OMC, which in turn is typically OMC -2% to OMC +1%. Finally, and again as has been discussed in many other threads, there has always been some historical doubt over what depth the NDG reads the moisture, although it has been pointed out to me several times that that only relates to the older gauges when they were in their early stages of development.