@223nut.
I assume your on the mainland. I would not recommend putting shock loads onto chains. Use wire rope with steel core a d proper choker bell and talurit. A double purchase opening block shackled ahead of the choker stop may halve the load.
The safest option is to use a swing boom crane of sufficient rating.
Failing this and depending on what is on top of the 15m cliff , you might consuder building an 'A Frame', the base of each leg of the A frame able to securely pivot back and forth in sockets well anchored. The crossbar of the A frame can support a snatch block. Your winch rope runs through the snatch block a d down to your log. The A frame is lowerwd so the block is suspended out over the cliff. As the log is winched up to the top the A frame can pivot on its feet to near vertical thus bringing the log over the top of the bank and laying stable in a position whence it can be unhooked and then rehooked to an extraction vehicle up on the flat. This setup requiresong strong logs or beams to construct the A frame and sold footings. You would need one winch additional to raise and lower the A frame boom as required.
Be aware of BUSHCODE Regs. Incorrect loadings on this sort of gear wire ropes chokers, deadmen and other anchors and tiebacks et al can be huge. You are talking some very hea y loads especially when trying to break out or drag over an obstruction.
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