We’ve had some conversations with the team at CfAT about your questions, and have some answers 🙂
1. The bore depths. The current bore depths of 25m in some locations are actually ineffective and far too shallow. Some boreholes on the Cape are up to 90m, so the request for a 100m is to allow for the upper limit of this range.
2. Use of PVC Piping as the borecasing. This is simply due to the durability of PVC comparative to steel, which is what has been used historically. This video shows how a PVC borecasing is used in the context of a bore drill. https://www.youtube.com/watch?v=cKweHAwj9-A
Let us know if you’ve got further questions.