Joint with Tapered Hole

The wedge space between pipe wall and hole enhances compressive forces driving the grout deeper into the joint without resistance
The effect greatly improves the joint when a corrugated or ribbed OD pipe is used.
When Butyl sealant is used as a water stop the resulting wedge can stop the inward movement allowing the outer gap to be filled with non-shrink grout

Joint with Non-Tapered Hole