The outside plumbing is pretty much finished now, so it's almost time to
install the liner. I was trying to fill the shaft around the gate valve
today, but the dirt was slumping badly into the pond. I did expect some of
this, but I thought I could overcome it by using mud to fill the shaft. But
it was not terribly successful. How about putting the liner in first and
filling the shaft with dirt as I add the water? The shaft is about 1x1x5 and
cut into the pond wall.

Any better ideas to reinforce the dirt or make it firmer?