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Concrete cover

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I am looking for a little additional background/past experience as 
it relates to minimum concrete covers recommended by ACI (318-89) in 
Section 7.7.1.  Part (a) gives a minimum cover for concrete cast 
against earth and permanently exposed to earth of 3".  Part (b) gives 
a minimum cover of 1-1/2" to #5 bar, W31 or D31 wire, and smaller for 
concrete exposed to earth or weather.  The commentary notes that "the 
condition "concrete surfaces exposed to the weather" refers to direct 
exposure to moisture changes and not just to temperature changes.  
Slab or thin shell soffits are not usually concidered directly 
"exposed" unless subject to alternate wetting and drying, including 
that due to condensation conditions or direct leakage from exposed to 
surface, run off, or similar effects."

My question relates to slabs on ground and to shotcreted walls for 
pools.  These types of members are typically 4"-6" thick and cast 
directly against the earth and the reinforcing is typically at 
mid-depth (or supposed to be).  Does anyone have any additional 
references as to why WWF in a 4" SOG is not required to be 1" from 
the surface?  (unfortunately, I haven't been able to review the ACI 
SOG committee provisions yet).

Thanks in advance,

Michael Bramhall