Engineering geology · hazards
Every building sits on geology. This is the part where the rock has to hold.
Engineering geology is the question “will it hold?” asked carefully. Read the boring log, work out the factor of safety, know what the ground does when it gets wet — and in Florida, know what’s dissolving underneath you.
A boring log lands on your desk with a proposed structure. Find the governing hazard — the one that decides whether this design survives.
The call
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The factor of safety is resisting force over driving force. Above 1, the slope stands; below 1, it moves. Watch what happens when you saturate it — this is why landslides come after rain.
Florida is a limestone platform with the water table running through it. Dissolve enough rock, remove enough support, and the roof comes down. Learn the three sinkhole types and what tips the odds.
Karst hazard
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A soil is three things at once — solid, water and air — and almost every number in a geotechnical report is a ratio between two of them. Get the phase relationships and the Atterberg limits, and you can classify a soil, predict whether it will swell, and estimate whether water moves through it.
Weigh the sample wet, dry it, weigh it again. That single pair of numbers — plus the density of the mineral grains — gives you the whole set. These are saturated soils, so every void is assumed full of water.
Phase diagram
Water content is referenced to the dry mass, not the total — which is why a saturated organic clay can honestly report a water content over 100%.
A fine-grained soil does not have one strength; it has a sequence of states that depend on how much water it holds. The Atterberg limits are the water contents at which it changes state, and they are the whole basis of fine-grained classification.
Four Casagrande trials. Water content falls as blow count rises; the relationship is linear against log N, so fit a line and read the liquid limit at 25 blows.
Plot PI against LL and the A-line separates clays from silts, while LL = 50 separates low from high plasticity. That single point is the USCS classification for any fine-grained soil.
| Plasticity index | Swelling potential | Likely dominant clay | Why |
|---|---|---|---|
| 0–15 | Low | Kaolinite | Layers hydrogen-bonded tightly together; water cannot get between them. Low surface area, low cation exchange capacity |
| 15–25 | Medium | Illite | Potassium ions lock the layers, but less rigidly than kaolinite. Intermediate on every measure |
| 25–40 | High | Illite–smectite mixtures | Some expandable layers present; behaviour scales with the smectite fraction |
| > 40 | Very high | Smectite (montmorillonite) | Water enters between the layers and forces them apart. Enormous surface area and very high cation exchange capacity — the soil that cracks in drought and heaves in wet |
The same property that makes smectite a foundation problem makes it good at holding contaminants: a high cation exchange capacity means dissolved metals adsorb onto the clay rather than travelling with the water. The next tab follows that through.
The question a site investigation actually has to answer is rarely “what is this soil”. It is will the contaminant move — and that depends on how fast water can travel through the pores, and how tightly the contaminant is held by the grains it passes.
You can estimate intrinsic permeability from three things you already measured: porosity, median grain size, and grain shape. Permeability scales with the square of grain size, which is why a small change in texture produces an enormous change in how water moves.
The shape factor
The Corey shape factor compares the short axis with the two longer ones:
with a long, b intermediate and c short. A sphere gives 1.0; well-rounded sand runs about 0.7–0.8; angular silt around 0.6–0.7; platy clay flakes can fall below 0.5. Flatter grains pack with more tortuous pore throats, so water has further to travel — which is what the factor is standing in for.
Also note k and K are different things. Intrinsic permeability k (m²) is a property of the rock alone. Hydraulic conductivity K (m/s) folds in the fluid: K = kρg/µ, so the same soil conducts warm water faster than cold, and water far faster than oil.
Two properties decide it, and they usually point the same way.
Excavation is expensive and it is not automatically the right answer. Work the cost, then weigh it against what the numbers above actually say about risk.
What the cost figure leaves out
Disposal is the easy number, and it is usually the smaller one. A real estimate also carries excavation and haulage, dewatering a saturated marsh soil, characterising the waste before a landfill will accept it, disposal at a hazardous rather than a clean-fill rate if lead exceeds threshold, replacement backfill, and site restoration.
And excavation itself creates exposure: digging a wetland mobilises material that was stable, and the truck route matters.
Monitored natural attenuation — leaving contamination in place and watching it — is a legitimate remedy where the soil demonstrably immobilises the contaminant. Making that case is exactly what the plasticity and permeability numbers are for. Round up truckloads: half a load still needs a whole truck.
The reference bench: soils by behaviour, foundations by problem, the tests that tell you what’s down there.
Grain size drives everything — drainage, strength, and how the soil behaves when it gets wet.
Two letters. The first says what the soil is, the second says how it is graded or how plastic it is. Learn the letters and you can read any geotechnical log.