Mostly due to the ground not matching what the geo charts said, and proximity to trees.
The engineer really should have been aware of this in their design.
Also this is why it pays to have appoint local authority building control officer when you do a basement as in some boroughs they want to inspect every pin before its poured. It beggars belief that they didn't get the LA/their approved inspector to come out and inspect the first one at least to pick up this issue (haven't watch the programme).
I thought on the program they said it was due to the foundations not having enough cement content so not water proof rather than not thick enough, hence the solution was an internal tanking.
The engineer really should have been aware of this in their design.
Also this is why it pays to have appoint local authority building control officer when you do a basement as in some boroughs they want to inspect every pin before its poured. It beggars belief that they didn't get the LA/their approved inspector to come out and inspect the first one at least to pick up this issue (haven't watch the programme).