The message
from Andy Hall contains these words:

Walls that go across a room part way across under the floor and
provide extra
support for the joists.

A friend had a house in Harrow with a suspiciously bouncy livingroom
floor. I had to go under it one day to replace a CH pump. Instead of
sleeper walls it just had rough piles of bricks vaguely mortared
together.

Many years earlier a new iron gas pipe had been run in, someone had
knocked over the pier in the middle of the floor 'cos it was in the way
of an easy pipe run.

Talking to other people in the road those that had been under the
livingroom floor had mostly found the same thing.

--
Skipweasel
Pay no attention to that man behind the curtain.