compaction testing by feel

[KaBone]

Sorry in advance if this isn't the appropriate activity area for this thread. I'm doing some utility removal beneath a concrete slab on grade. It's a small area 5'x5'x~4'D. I want to backfill in 8" lifts and achieve a 95% MDD compaction. The replacement pad will be ~5" thick with #4 bar 12" oc each direction placed mid-depth. The pad will be dowelled into the existing with two #4 bar on each side embedded 8" with epoxy. The area gets some vehicular traffic. What kinds of manual field tests are done to see if the soil is compacted enough? I have used a boot technique. I kick or grind my boot on the sub grade and if I can make an impression with moderate effort it needs to be compacted more. I've only done this as a pretest to the actual ASTM testing and so far have had good results. This time there will be no ASTM testing. Please don't ask why; right, wrong or different, that's just the way it's going to be. Looking for feedback if anyone else does something similar or different also with good results.

 

[oldestguy]

Hey, that boot test is not so bad. I'll bet you have done that and then found by the ASTM tests that you were right. Just make sure you don't use too thick a lift each time and give plenty of passes. The firm I worked for always did a supplementary test along with sand cone, nuc and other methods. That consists of a 1/2" diameter rod about 30 inches long with a cross handle welded on top. With experience you ca pretty well tell how well the soil is compacted by shoving with your weight. For example, a few inch and stop with full weight is damn good. An 18" penetration with not much effort is not good. About8 8" penetration would begin to be questionable. However, for single grained, uniform sand, which has a very small range between loose and compacted, the test is no good. That easy penetration can be likened to shoving it into a bucket full of marbles.

First try it on a known suitable soil area to get some idea of how it works. Several firms that I know of use this along with regular testing to quickly assess a large area before testing with better methods. I've done it many times with no other "testing" to approve a footing excavation as OK.. Call it the "T" test. It is more impressive than using an umbrella.

 

[Ron]

Try to push a #4 or #5 rebar into the soil by holding the rebar extended at arms length. This keeps your body weight out of the issue and allows a qualitative assessment of soil density. Any penetration more than about 1 inch needs more compaction.

 

[BigH]

oldestguy - the push test you describe - I bet that my weight would push it further! I think we have all done the "boot" test before - when boots were boots and not sneaker-type . . Similar to the thumb test for determining (roughly) the consistency of a clay.

 

[oldestguy]

BigH: Funny thing, with several guys out doing field density testing, we never figured in the weight. Somehow, the jobs got done and no contractor complained. Currently in this area a number of testing firms use the dynamic cone penetrometer, but at least 2 still have those "T" bars. In sand country the wear on the tips changes them to more of a point than as original, but they still are used. For the subject job the ideas are above.

 

[GeoPaveTraffic]

The boot or T handle test should work just fine for what you are doing.

One comment on your concrete repair. I would suggest using a smooth dowel not deformed rebar. You need stiffeness for the load transfer and a smooth bar is considerably stiffer than deformed bars.

Mike Lambert

 

[KaBone]

Big mahalos for the very helpful replies. I always get nervous posting to boards like this with questions like that as I usually get ripped to shreds. Again, thanks for staying on point.

 

[M0nkeyb0y]

I know this is late, but...

However you do it, physically manipulating the soil is a legitimate way of gaining a "feeling" for its properties. I use a T-handle soil sampler to probe and sample the soil or fill below the surface. The resistance to pushing the sampler in gives me an idea of its consistency/relative density, and I get a soil sample that I can examine further. It lets me quickly "look" another 18" to 24" below the surface.

For your specific situation, the relatively small volume of the excavation, and the lack of testing, makes me think that that you should consider backfilling with a material that doesn't need much compaction effort to develop good engineering properties with respect to settlement, shear strength, and bearing capacity. For example, you could use a well-graded sand, will-graded crushed limestone, or flowable fill. You may also need to line the excavation with a geotextile to separate a fine-grained subgrade from a courser-grained fill. This approach may increase cost, but it should reduce risk. Clients should realize that reducing costs by eliminating testing and engineering involvement doesn't necessarily save money in the long run, but it does increase risk to the owner, contractor, and professionals involved.

If you ever have a bad feeling about a situation, sample and test at your own cost to cover yourself and others involved. You'll sleep better.

 

[fattdad]

don't like the boot test for compaction control. You can gain false security measuring dry strength, which will quickly reduce to mud when the winter comes.

I have no problem using the #5 rebar, or t-handle bar for natural soils when I have boring data to relate the field conditions.

f-d

¡papá gordo ain’t no madre flaca!