Replacing fence posts set in wall

[DIYspanner]

Perhaps away to cement in is get the post ready to drop in, then fill the two channels with thickish mixed cement so when the posts are installed, the cement sags down and fills the gap. As the cement settles, you can pour some soggy cement down the channels to back fill.

 

[Tigercubrider]

If the posts are dry, they will swell when wet

 

[DIYspanner]

If the posts are dry, they will swell when wet

That's why fencing contractors use a dry cement mix when setting fence posts.

The one post I had to cement is last year, I used a soggy cement/sharp sand mix to get into all the gaps but to this day, its snug and secure.

 

[DIYspanner]

This is the idea for cement channels I had in mind. Apologies, I'm not the best at freehand drawing.

 

[Halitosis]

Is it an option to drive long/slim wedges down to steady/secure the posts?