I installed corrugated pipe to take downspout water away from the house.
One downspout goes into a Y. The corrugated pipe is in ten foot sections.
I put this Y together while the ditch was empty, then filled it in. The
connections are not watertight. The fill was somewhat loose and the first
rain pulled the connection to the downspout down. In digging it out, I
found the dirt around the Y was very wet.

Testing before filling would not prove anything. I already know it leaks.
It is not watertight.

Are corrugated pipe connections supposed to be watertight? I would think
the dirt would provide the seal. While ther may be some leakage, in time it
would pack. I tried various kinds of tape. Nothing holds on the plastic
pipe. I tried silicone caulk, but I'm not very confident about it. There
is just plain too much wiggle in these parts.

John Stewart