Hope you have Resistance.
You should have a Show IDs/Hide IDs button in the mission editor. Click on it till ids are shown. Then find the piece of wall that you want to breach, and get the object number.
Then make a trigger, with radio alpha, and type object *object's number* setDammage 1.
That will knock over walls but buildings just crumple, don't fall over. :'( Can't use it on them.
If you want, you could create a script that would detect when you placed a satchel, then do the above for you. It would look something like this:
_wall = _this select 0
_bomber = _this select 1
_time = _this select 2
_wpn = _this select 3
_ammo = _this select 4
? _wpn != "put" || ? _ammo != "PipeBomb" : exit
hint format["You have %1 seconds before your breaching charge detonates.",_time]
~_time
_bomber action ["TOUCHOFF"]
_wall setDammage 1
exit
then just add this event handler to anyone who's going to use it (in their init line)
this addEventHandler ["fired",{[object *wall id*,this,*detonation time*,_this select 1,_this select 4] exec "*scriptsname*.sqs"}]
example:
this addEventHandler ["fired",{[object 12456,this,25,_this select 1,_this select 4] exec "breachWall.sqs"}]