Soil Mechanics
The parts of soil mechanics that get used every week and are still got wrong: reading a grading curve, plotting on the plasticity chart, and the Mohr circle. These articles work through the arithmetic in full, because in each case the mistake is not conceptual — it is a factor of two, a logarithmic axis read as a linear one, or a threshold applied from memory.
2 articles
What a Mohr circle actually tells you
The Mohr circle turns a stress state into geometry. The angles on it are doubled, the envelope touches it once, and both facts are derived here from scratch.
Soil classification from a grading curve
Reading D10, D30 and D60 off a grading curve is a logarithmic interpolation, not a linear one. A worked USCS classification with every number recomputed.