If by “grounded container of cables” you mean that you have conduit (pipe) running the entire distance… then you’re probably OK. The conduit should provide protection for the UTP cable. Plus, if there is a failure down the road, you can try again with minimal problems (ie. you don’t have to dig a trench again). So if that’s the case, if you want to try UTP cable then go ahead.
However, if the conduit isn’t sealed (or if I misunderstood and you don’t have conduit), you still may have issues with water ingress, soil acidity, and thermal flexing. So I’d still recommend proper cable. 8 conductor direct-burial sprinkler wire isn’t a whole lot thicker than UTP. Certainly it is less than 1 cm diameter. You don’t need 8 individual single-conductor direct-burial cables!