As ever there was an example of "be careful what you wish for; you
might get rather a lot of it".
I needed to learn about buildings and here is an ever-present reality I am now much more familiar with...
Were getting seemingly through a hole through the house wall (former pipe run maybe?) and getting up the pipe riser conduit to a space between the joists the width of the upstairs hallway. They could get no further given the beams. Hence no-one had ever known.
Two remedial steps
Might have been getting to the wall here:
Cleaned and scraped-out hole so gave clean bond to what I did as remedy.
Made "trough" from cut-out top of plastic milk carton. Packed behind with plastic bag waste for support to keep it in place and shape. So cement mixture doesn't leak out and keeps cement away from copper pipes (cement corrodes copper).
Troweled-in cement and pushed down hole from level to above hole in "trough".
Prior effort which did not work. Trying to push wire-wool + broken glass + cement down hole. "Jammed". Hence went for the "normal" 4-and-1 builder's sand and cement option for fluidity to get deep plug down hole.
Did finish with pushing some of the wire-wool + broken glass + cement into the hole.
Got 4:1 sand/cement, 3 litres of the sand and the cement - so a few kg - to go into the hole.
At time of writing (Monday 25 September 2023 10:30) awaiting arrival of copper gauze to seal the riser.
(R. Smith, 25Sep2023, 26Sep2023 (cem. hole done))