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.
11 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.
Strength reduction: what the factor of safety reduces
A factor of safety of 1.990 means the slope stands on c of 2.5129 kPa and phi of 17.43 degrees — worked on a slope whose closed form checks the method.
Monopile p-y curves: what PISA was needed to fix
At 35 degrees the API p-y transition sits at 16.96 diameters whatever the diameter. On an 8 m monopile that is 135.67 m, deeper than the pile itself.
Dilatancy angle: what the choice of psi actually costs
Associated flow is convenient and wrong for soil. On one bearing capacity problem it inflates the collapse load from 200.8 to 301.4 kPa — a third more.
The K0 procedure: initial stresses before anything moves
A model starting from the wrong horizontal stress is wrong before the first load. Jaky, elasticity and overconsolidation give 0.429, 0.470 and 0.980 here.
Plane strain vs axisymmetric: the depth the stress reaches
A strip and a circle of the same width put very different loads on the ground below. The 10% contour sits at 1.85B under one and 6.34B under the other.
SPT: N60, (N1)60, and five routes to a friction angle
The same 18 blows become N60 of 11.5 or 20.4 depending on the rig. Five published correlations then spread the friction angle over eight degrees.
CPT interpretation: from qc and fs to Ic and su
A cone reading is not a soil type. Between them sit an area correction, two normalisations and a factor nobody measures — worked through on one sounding depth.
Effective stress: a worked profile, uplift and settlement
Total stress is arithmetic; pore pressure is a boundary condition you have to go and find. One profile, three questions, and the same subtraction behind each.
Atterberg limits: the flow curve and the plasticity chart
The liquid limit is a point read off a fitted flow curve, not a single reading. A full classification of a fine-grained soil, with every number recomputed.
Terms defined
- Angle of shearing resistanceφ′
- Atterberg limits
- Coefficient of curvatureCc
- Coefficient of earth pressure at restK0
- Coefficient of uniformityCu
- Dilatancy angleψ
- Effective stressσ′
- Grading curve
- Liquid limitLL
- Mohr circle
- Overconsolidation ratioOCR
- Plasticity chart
- Plasticity indexPI
- Pore water pressureu
- Undrained shear strengthsu
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