Is civil divorce OK for Catholics, are there mitigating circumstances? This is not a pastoral advice question, I'm happily married.  I just really don't know the answer to this question to help a friend, so I guess it's pastoral pastoral advice, but I definitely don't want a 'try to work it out' answer, just the facts please.

Sacramental marriage cannot be undone by civil divorce, that much is clear from scripture and Church teaching. However, what does civil divorce mean for the sacramental marriage?

If it is OK, what factors should a person take into account when forming their consciences to up and leave (for instance there could be a case where the laws in a particular region require someone to serve divorce papers in order leave their spouse with their kids and not be accused of kidnapping them) and if it is not OK then what recourse does one have to protect themselves and their children against their spouse, that does not constitute a grave sin?
As usual the Catholic Encyclopedia is helpful here.


  The Catholic doctrine on divorce may be summed up in the following
  propositions:



In Christian marriage, which implies the restoration, by Christ
Himself, of marriage to its original indissolubility, there can never
be an absolute divorce, at least after the marriage has been
consummated; 
Non-Christian marriage can be dissolved by absolute
divorce under certain circumstances in favour of the Faith; 
Separation from
bed and board (divortium imperfectum) is allowed for various causes,
especially in the case of adultery or lapse into infidelity or heresy
on the part of husband or wife.


I would take that last part to indicate that if the marriage is unlivable in (for example if there is violence or other abuse), then civil separation is a viable option. In most jurisdictions civil separation does not need to be followed by a divorce - I have known people who lived in a state of separation for decades. The only practical difference is that a separated person cannot remarry. As I'm sure you are aware, sacramental marriages can be annuled.