I have always used Live load. Especially given the soccer field nature it will be moist soil so heavier than a drier soil. I will often use an inflated unit weight of 125-130pcf and a lower load factor (1.3 ish) instead of a dryer weight and a higher load factor (110pcf*1.6) but it ends up with a similar ultimate load.
@tngolfer: I would consider it as a deal load (superimposed) for designing for strength, including proportioning of the foundations. However, for stability checks, uplift calculations etc. I would not count on it, i.e. assume it does not exist.
I would say statistically, it's a dead load because it's known,ever-present and more predictable than a live load, thus justifying a lower load factor. I see no reason to not count it as being there for other combinations of load as DST148 has suggested, but would consider it's lowest/minimum weight in those calculations.
Canada it's an H load with a load factor = 1.5, which is the same as the live load factor. The code distinguishes soil, superimposed earth, trees, etc when considered as dead loads the code requires the load factor be 1.5. When the depth of the soil starts exceeding 1.2m in depth the load factor can be reduced to 1.25 as the depth increases. Guess they feel if there is only a couple of feet of soil it could be removed easily hence the 1.5 load factor?
In ASCE 7-10 (and probably all the rest of the editions), Chapter 3, Dead Load is defined as weight of "...all materials of construction... including but not limited to walls, floors, roofs, ..."
By that definition, it's not a Dead Load.
When you take the live load reductions allow per the IBC will you apply them to the weight of the soil? I would not, so I would have to say that it is a dead load.
Now this is a Special-Purpose roof (2009 IBC section 1607.11.2.2) but per the 2009 IBC section 1607.11.3 Landscaped roofs. "...the uniform design live load in the landscape area shall be 20 psf. The weight of the landscaping materials shall be considered as dead load and shall be computed on the basis of saturation of the soil."
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
Like others have mentioned I would consider it a dead load. Seems to me that it is a "material of construction". It's essentially, just unique flooring. How is soil any different than a concrete topping slab, tile, etc. Seems to me that I would design it as a dead load and use the saturated weight of the soil in the design. This is what we have done in the past for rooftop gardens.
AASHTO defines it as "...dead load of earth fill" for design of tunnels, culverts, and other rigid underground concrete structures. I don't see why it would be any different under the IBC. Also, the earth fill is not transient, so makes sense that it would be a sustained dead load. Just my 2 cents...
I consider it dead load, particularly when only a percentage of only the dead load can reliably be used to resist uplift.
For concrete detention vaults, the dead load of the soil above is used, in addition to the dead load of the vault, to prevent it from becoming bouyant.
In addition to treating it a dead load with the saturated unit weight, I would suggest that you investigate ponding/flooding potential and verify any vehicle loading if not already done. I once helped on a project with a heavily landscaped roof with HS-20 loading.
I agree that it is a dead load becasue it is permanently acting on the structure but I think that it is a highly variable dead load and should be assinged a load factor of 1.5. The unit weight of the soil is related to its compaction which cannot be assured.
The National Building Code of Canada specifically defines soil and trees above a structure as a dead load except when it's acting as the load restraining structure (retaining walls, etc) or acts as a counteracting load in certain other ways.
The occupancy above the soil would be a live load, though.
