(IAAC) Concrete mounting

Perhaps a little off-topic, but I think there would be enough expertise 
on the list to help me.
I'm planning to permanently mount my scope on a concrete pillar to 
eliminate the need to align the mount every time I use it.  However I am 
not sure about how deep the pillar would need to go to give reasonable 
stability.  The soil I would build it in is very clayey, and subject to 
gleying and saturation in the Winter and cracking in the Summer.  Whilst 
I would like to create the 'ultimately stable' pillar, I am limited by 
the fact that I probably won't be staying at my current residence for 
particularly long, and a four foot deep concrete-filled hole in the lawn 
probably wouldn't be a good selling point for the house.  The scope and 
mount and any accessories would never weigh any more than about 50 kg.  
What would the minimum sort of depth be?
Thanks in advance,
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