Personally I think road planings and similar should be banned on farms which have cloven-footed animals. The mud reforms on top of them, the foot sinks through and onto the planings, which are small and sharp enough to get in between the cleats... Serious welfare issue, imo.
I am going to be contentious about the other thing too : Pigs are not happy outdoors in winter if there is no way of giving them routes to and from their food, toilet and ark where they are not belly-deep in wet, cold mud. They are much,
much happier indoors with plenty of straw and or hay to play in and lie in, and a well-drained clean corner in which to toilet.
In your case, Julia, if you have an earth-floored pen that you can bear to let them use, (with plenty of straw to rootle in to try to reduce the amount of floor they dig into!) that would suit your girl best. Otherwise I would say she would be better on a well-strawed cement surface than left to struggle in the mud. You can always let them out for a bit of play and exercise when the ground is dry enough - but if the pen is somewhere that there are things for them to see, and you go by from time to time, I think you will be amazed at how happy they are