Compaction of fills on a slope 1

[geotechguy1]

I've run into a few sites where my fellow geotechnical engineers have written specifications requiring uniform horizontal lifts. I've had some back and forth with different senior engineers but I'm still searching for a good reason for this specification and why they are so stringent on enforcing it.

Suppose you have a 6:1 slope, and you place and compact a uniform lift on the slope, with the same 6:1 slope. Why is that different than placing a uniform lift of the same thickness on horizontal ground? Usually these particular geotechs insist on placing overbuilt horizontal lifts on the 6:1 slope and then trimming them to grade at the end (imagine an overbuilt staircase of lifts if you would).

So, here's the million dollar question: Do the soil particles on the 6:1 slope know that they have been placed on a slope, and that they therefore are required to perform worse? Or do they only know that they've been compacted to whatever arbitrary percentage of SPMDD or MPMDD that particular consultancy has pulled out of their collective behinds?

As an aside, I always like asking them how they know compacting it to that percentage of SPMDD will achieve the shear strength and compressibility parameters they are looking for. No one has ever answered. I had one company that had assumed 1900 kg/m&^3 in a slope/w model, but all the fill was placed and compacted to 98% SPMDD, with a resultant of around ~1750 kg/m^3. Upon questioning the response was 'we know better than you'. Anyway, now I've gone off on a tangent.

 

[SlideRuleEra]

Suppose you have a 6:1 slope, and you place and compact a uniform lift on the slope, with the same 6:1 slope. Why is that different than placing a uniform lift of the same thickness on horizontal ground?

IMHO, a one word cause... Gravity.

Any real-world compactive equipment either uses or is affected by gravity.

On a horizontal surface all compactive energy is used for it's intended purpose - soil compaction.

On a sloped surface some of the compactive energy is directed parallel to the sloped surface. Remaining energy that is perpendicular to sloped surface is reduced a little, but that is usually minor compared to the disruptive "sideways" component.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[geotechguy1]

So the issue is that it's harder to achieve the targetted compaction?

Supposing that you actually achieve the SPMDD target, there should be no issue then?

I've also heard the argument that the requirement is based on nuclear densometers not working on a slope, based on an older version of the ASTM standard which implies this is the case.

 

[Ron]

Agree with SRE; however, when you achieve compaction, you're done. It's less efficient for compaction purposes, but if you achieve the result, no issue other than extra time.