Toddbrook Reservoir dam

Cavitation is the culprit. When fast moving water passes over any imperfection in a surface it creates a roiling vacuum, sort of a Bernoulli effect which lifts, and erodes, the underlying surface. Roots are powerful; they lift slabs, creating imperfections, and they create paths for water infiltration. Then, water flowing under the slab is more or less invisible and of course it undermines the slab. Both sides of the slab suffer when managers fail to control vegetation or repair/fill cracks.

I wouldn't give much credibility to the engineer inspectors as they're too likely to be members of the same club, so to speak, with personal and professional reasons to understate the faults of colleagues. For the coming repairs, it's other peoples' money they'll be spending and no one wants to spoil their chance to participate in the jobs to come.