Legal Question in Real Estate Law in California

This seems to be more of an engineering problem than a lawyer problem. From a legal point of view, I would say that you undoubtedly have some kind of easement for the pipe under the driveway, probably "of record" but if not, I'd suppose if this situation has been in place five years or more, there is a prescriptive easement. It might be worth while to research the titles and other records to both properties to determine whether there is an express grant or reservation of easement somewhere, and if so, what if anything it says about maintenance.

As a general rule, the holder of an easement has both a right and a duty to maintain the facilities on the easement that are there for the holder's benefit.

Although you probably have the right, as the easement holder, to perform whatever repairs are necessary to get the benefit of the easement, you also have a duty to make those repairs in a responsible way and with minimum disruption to the property upon which the easement lies. You could be liable for damages if you do, or allow a contractor to do, the work negligently.

I would suggest telling the neighbor that you're going to hire an engineering consultant to advise you on the best combination of economical construction and high-quality results, and then have a contractor do the work as specified by the consultant. Most well-established, licensed contractors will know engineering consultants in their field.

You might also benefit from a discussion with your homeowners' insurance company; they have a stake in getting the work done in a way that avoids lawsuits, and they might be able to give you some practical tips.

I would advise keeping the neighbor informed and advised in advance of your intentions, but also don't be bullied into making unnecessary concessions or delay. Also, avoid shortcuts because you are probably liable for anything that goes wrong, perhaps even if you have acted prudently and followed good professional advice.