dynamic cone penetrometer 1

[rowe]

Hi to All,

What is a reasonable number of DCP tests to verify compaction of fill soil compacted for a residential slab foundation (approx. 1800 sq ft of slab)? Thanks for your input.

 

[oldestguy]

Maybe two. Instead of elaborate cones I used to only go around the footing areas and poke (my weight) with a 30 inch long 1/2" rod (a "T" handle on top). Goes quick. Questionable areas and then be checked further. This assuming only the weight of the slab. Footing areas get separate checking. Old timers would just jam their heel down and use that.

Edit: In general this probe for the average person should not exceed 10" penetration for slabs and 6" for residential footings. One problem. In uniform sand it goes easily, but uniform sand has very little potential for settlement any how.

 

[GeoEnvGuy]

I have also seen and used the manual "nuclear" probe but usually use it on native deposits. The hard thing about fill is how much garbage is mixed in and if it was compacted in the first place. I would not have confidence in previously placed fill without a report stating compaction was exerted over the entire area with density testing completed. Fill is usually not compacted in areas outside previous foundations.

In terms of your slab size it should only require a one day investigation and I would not recommend dcp tests. SPT or test pits I believe are more suitable.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[EireChch]

OG - when in worked in NZ it was common to inspect a footing with a gum spear (similar to your T bar). This was in compacted clay, in a matter of minutes you could identify "softer" areas and focus your testing there.

I would disagree with GeoEnvGuy - SPT or test pits are a bit impractical imo. I think a DCP is a great little tool for testing density of compacted fill for light structures, when used correctly. To clarify, Sand with minor amounts of gravel. For cohesive soils you should be undertaking a shear vane test and aiming for a Su of 50kPa (depending on your spec).

In NZ a scala penetrometer (type of DCP, 10kg weight lifted by 500mm or so and dropped)is very common. See link

https://www.youtube.com/watch?v=FxK4d3eJucI

The NZ building code specified 2 blows per 100mm = an ultimate bearing capacity of 200kPa and 5 blows per 100mm = an ultimate bearing capacity of 300kPa.

 

[Mad Mike]

Shallow DCP test probes are all that is needed- given how quick it is to do the probes to a 4' depth, I would do about 6 of them in grid fashion across the proposed slab (can be done in an hour or two).

Vane shear etc. are not required since standard DCP data can be related to shear strength empirically. A couple of "pocket penetrometer" tests could give you an unconfined compressive strength to convert to shear strength.

No expensive testing necessary, unless your DCP results show very poor soil conditions.

All the best,
Mike

 

[EireChch]

Mad Mike - if you have a vane at hand why would you use an empirical relationship for something you can measure directly

Also interested to know what blow counts you would consider acceptable? Based on engineer judgment or a reference. The reason i ask is that NZ was the only country i have seen that actually had a prescriptive value of blow counts.

 

[Mad Mike]

EireChch - absolutely; could measure with a hand vane at the surface- I prefer the DCP probe to get down 4' or so, even deeper if necessary and especially for small residential sites where the civil work is often not up to scratch.

The DCP blow counts must be related empirically to SPT'N' before judgment can be made- it requires
care since you get DCPs with 45deg cones and 60deg cones, in addition to different drop hammer weights etc. In my area, we often see engineers specify that the subgrade must be "at least medium dense" or similar vague prescriptive, but the SPT'N' value could also be related to an in-situ CBR strength and a suitable safety factor applied.

All DCP check testing I've done has been judgment based...in sensitive applications I would never recommend it in isolation.

All the best,
Mike

 

[maxim22]

I might use DCP for about 1m of coarse soil without frequent nugget particles of more than 20 mm which skew results. Then compare them to what I know, not correlate to justify.

UK uses plate load test but that needs heavy plant for jacking under.

Vane tests/PP are for clay material strength not compaction validation. UK would not use compacted clay under a resi slab unless some movement was expected.

DCP is a correlation to a standard methodology pavement design based on stress/strain relationship in crushed rocks. Use with caution.

My heel is 100kPa and the whole boot 50kPa for shear bearing capacity not long term settlement.

 

[oldestguy]

I agree with all added comments except one. A lot of expensive fancy tests will drive you out of the business due to competition. With experience in a given area and knowing more of the earthwork practices, the requirements for testing and the cost of testing fill under floors should be low.

 

[Ron]

In granular soils, DCP is fine. I never use less than 3 tests for anything....my reasoning....one test is insufficient....two tests can create conflict if results are different.....three tests at least give you a referee to the two tests.

 

[darthsoilsguy2]

i always wanted to see 5. corners and center like a dice, with blow counts and visual description at 0', 1', 2', and 4'. no particular reason for that many tests except to avoid conflicts with a client who has an opinion on such things. the tech spends most of their time driving to/from the site and each test doesn't take that long. just looking for consistency.

 

[mikeofBelAir]

I have always used the method that OldestGuy described. Do any of you other old timers remember seeing the actual "rule of thumb"? Some text books gave unconfined strength numbers based on how far you could penetrate soil using your thumb..

 

[geojosh84]

For a slab subgrade I'd recommend compaction testing and proof rolling. I'd focus a few shallow DCPs in the turndowns with random probing.

If it's a clean sand, I'd run at least 5-6 increments at each DCP test depth.

 

[BigH]

With respect to maxim22, Professor Wesley - New Zealand - uses vane tests (i.e., undrained shear strength) as a measure of compaction especially for residual soils - Su values of 150 kPa are generally considered to be "well compacted". His criteria is: (1) Average of 10 tests greater than 150 kPa; (2) no single test less than 120 kPa; and, (3) air voids not greater than 8%. He has also found that the more compactive effort one puts into compacted residual clayey soils, the undrained shear strength ends up decreasing. I have attached a graph from the book. Of course, local experience may vary and it may need to be "proven" for non-residual soils. See Geotechnical Engineer in Residual Soils by Dr. Laurence D. Wesley, Wiley 2010.

I am in the process of trying to get some students at a university here in Malaysia to do a study (their final year paper) on the correlation between % relative compaction and undrained shear strength. Having just finished a job placing 4.5 M m3 of compacted residual soils, the use of a undrained shear strength criteria would have saved much time, unnecessary laboratory testing (the testing couldn't keep up with placing 30k m3 of compacted material a day, and also many headaches.

It would be interesting if others, who deal more with alluvial or lacustrine deposits could provide information on this from their experience.

 

[BigH]

Forgot the picture - and editing reply didn't seem to work . . .

 

[BUGGAR]

Perhaps a bit late, but regarding the thumb test, you used to have to "precalibrate" the test machine (triax test?) in order to pick the right spring to use in the apparatus for that soil. This was best done with the thumb on the sample from the split spoon. My boss made this test on the sample in the field and was usually right on. I lifted the sampler weight and hated "firm bearing soil".

 

[maxim22]

Shear strength vs compaction in clay. This is interesting to me at the moment.

Cant beat a big field test and I haven't seen the book. I suspect using HV or PP you are measuring original to remoulded shear properties.They are small size shears and would be higher as water content is lower. Clays may need a light enough compaction not to completely remould. But there is clay, and there is different clay. I may be missing something here?

Sand replacement density has scale error problem if the clods and lift are bigger than the test and you test in the middle of a clod = original density no air voids. Move test between clods and on excavation you can see the voids where clods haven't closed up together. Landfill base experience in the 1990s.

In the next 3 months we should be involved with a 150,000m3 alluvial clay and peat soil stabilisation and compaction. The specialist contractor will tell us what lift and effort they will use plus the binder ratios. Not sure what validation we will use.

 

[BigH]

Maxim22 - then you are in a good position to go the extra "kilometer/mile" . . . Not sure I would "work it" with peat - but if you could do some shear strengths and the compaction - it would add. Taking a bunch of shear vane tests and the like would be fast - could get some drive samples to do a UU test. Also, could do some UU tests on the laboratory compaction specimens. In our case trying to place so much fill per day we would be a layer or two higher than the lab value obtained. One reason we tried using the "average" of the last 10 available MDD values (same borrow area) and then adding 1 std deviation to get a "target" value to check the field work against. If the field compaction exceeded the requirement, then there would only be a 10% chance or so that the actual MDD value would be bigger. Seemed to work okay - but I'd rather, if one could, use the undrained shear strength as the "primary" criteria with back-up of lab tests say every 2 or 3 layers after you have developed a comfort level.

How are you going to stabilize the peat? How thick? Do you really want to use the peat as compacted material? If not thick, you might just take it out; else, you might consider rolling surcharge to displace it and then use compacted clay in its place. Just some thoughts.

 

[EireChch]

"Surely you cant be serious about peat stabilization' ,

"I am serious and dont call me Shirley"

^^A bad joke from a good movie.

Joking aside, it is not possible (at least economically possible) to place and compact peat....alluvial clay yes but peat??

 

[maxim22]

Ground investigation still ongoing, I will report back. Do we need a new thread?