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Coefficient of Friction between Boulders

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To All:
Longtime lurker, first-time caller and I need your help.  I have an existing 6-ft. high, 4-ft. thick retaining wall consisting of 12" to 48" diameter granite boulders held together with concrete.  The retained material is decomposed granite and some planter soil.  About 20 feet beyond the planter is a BIG MOUNTAIN.  No permit was issued for construction of the boulder wall.  No footing that I can see.  It is 3 feet away from a proposed 5-ft. deep gunite pool.  I produced calcs for the pool wall reinf., footing, etc.  and the plan checker is okay with my design on that. 
 
He asked for stability calcs on the boulder wall.  I gave him an overturning calc showing the soil pressure was not exceeded and a sliding friction calc.  Now he is asking for a calc for frictional resistance between the boulders to prove the wall won't come apart in an earthquake.  
 
Anybody heard of this?
 
BTW, I'm in Southern California, more than 10KM from the nearest fault.
Thanks in advance.
 
Denise Poeltler, PE