Backfill layers - bottom the worse (DCP)?

[BauTomTom]

Hi Guys

it is me again with some backfill problems. When I backfill a column base and compack every layer (250mm) must the bottom layer not be the strongest? the best compacted? It gets all the compactions from above and the weight of all the ground above as well.

Have a look on the attached grafik. Strange or?

What gives me actually the CBR % value?

BauTomTom

 

[ishvaaag]

In roads it is preferred the best compaction atop because stiffness there, being closer to the applied point loads, is thought to be more meaningful to sound behaviour than deeper in the soil, where such loads have already spread. It provides akin functionality to that of a slab bridging some weak ground or a void. If you make the upper layers weaker rutting is a problem far more apparent to everyone than some slight variation of settlement along big distances. Nor one or the other should be thought tolerable, and both are cause of costly repairs, so a proper statement of the specs to counteract the foreseeable bad aspects of behaviour must be in place. When there is and it is not respected you are already in some early stage of repairs' mode.

 

[Ron]

Just because a material lower in the strata does not mean that it is either stronger or better compacted than layers above it. If you look at a standard penetration test boring log, you will readily see that soil deposition and strength have little relationship. While it is more plausible in clayey or silty/clayey soils (considering the consolidation from overburden) it is less so in elastic materials such as sands.

As ishvaaag noted, upper layers are often chosen for stiffness and bridging capability as much as for comparative compaction and stability.

Also, keep in mind that a DCP does not measure stability nor compaction as isolated parameters...it only measures a resistance to penetration, which while related to compaction, is not in itself a measure of compaction....it is more closely related to a measure of stability (such as the CBR).

Variable correlation CBR values with depth from a DCP do not surprise me.

 

[BauTomTom]

Hi

ok it makes sensse but still I wouldnt aspect such an zigzag graph my a compacted ground in each layer

and how can I visualise the % of the CBR?
Somehow have a problem with it

TomTom

 

[ishvaaag]

In reality it is not as surprising that in layers compacted one after another such kind of irregularity appear. Think that when you are compacting at one point you are displacing soil elsewhere. Just as when you are threading on mud. Hence for a system of compacted layers some kind of jagged result may be expected for a general case.

 

[BigH]

Might I draw your attention to the work of D'Appolonia back in the 1970s? - Read about his work in Terzaghi Peck and Mesri - Article 44.2 (see figure 44.1).

 

[BauTomTom]

TO BIGH

and where can I get this article?

TomTom

 

[BigH]

In the book by Terzaghi Peck and Mesri (3rd Edition, 1995). I'll try to find the original article tomorrow - it was in ASCE Soil Mechanics Journal, I think.