It depends how you look at it. I did the first year of a maths degree, passed, but found it boring and switched to Mechanical Engineering. I know it's not civil, but it will have many similarities.
In my opinion, Maths is a slightly tougher degree conceptually, and less people will be capable of passing it, no matter how hard they work, than any engineering degree. The reason it doesn't tend to have lower pass rates is just becuase I think smarter people tend to do maths. More people on my course came into engineering thinking it would be way less mathematically rigorous than it is, or wanted the high paying jobs at the end of it, and have found it to be beyond them.
On the flip side, if you are very intelligent you can do fine on a maths degree with minimal work. Very intelligent people still occasionally fail engineering degrees as they fall too far behind with the workload. And of course, maths is not the only degree all of this applies to in comparison to an email discipline. It's just the only one I can really comment on, and can be used to highlight these pointed well.