A friend of mine decided to do something with her very worn out socks. An easy way to fix socks with big holes is to just knit a sole and then sew if over the holes.
Of course the best way to avoid big holes is using yarn that is suitable for socks. These socks were made of pure organic wool so it is not a big surprise that they wore out fast since wool is not really durable in socks.
When I knit warm socks like these I usually use yarn that is designed for socks and I use an extra thread in the heel and the toe to make them even more durable.

Here is a picture of one of the socks with the knitted sole sewed on: