I have found it in one of those small "travel grammar" books with phrases.
Actually it was related to the numbers+counters, not sure if it works with other words as well. I am sure there are many other exceptions. That's what my book says:
BEFORE H
The kana ち and く become a P and transform the following H into a P so to obtain a PP sound.
じゅう changes the H into a P and doubles it, at the same time the sound become short じゅ.
さん doesn't have any change in itself, but changes the following H into a B.
BEFORE S
The kana ち becomes a S to obtain a SS sound.
じゅう it double the following S so that the sound becomes SS, at the same time the syllable becomes short じゅ.
BEFORE K
The kana ち and く become K.
じゅう double the K and its sound becomes short じゅ.
As I said it is only mentioned when it speaks about numbers+counters, to know if it is a general rule you need to ask to someone who know Japanese better than me (not hard to find one
).