Geotechnical Engineering for Marine and Waterfront Structures
Marine geotechnics governs foundation choice, the stability of dredged slopes, and the settlement that dictates how much reclamation fill you actually need.
Marine soils
- Estuarine and deltaic clays are normally to lightly overconsolidated, sensitive (often 3–6) and compressible; sampling disturbance can reduce measured strength by 20–30%, so good-quality samples matter more than test quantity.
- Tidal-channel sands may be loose to medium dense and susceptible to liquefaction under seismic or wave-induced cyclic loading.
Strength from in-situ tests
- CPTu: Su = (qt − σv0)/Nkt, with Nkt typically 14–20 for soft clays; calibrate against vane or triaxial results on the same site.
- SPT N-values require correction for overburden, rod energy and fines content before any correlation is applied.
- Vane shear gives peak and remoulded strength; sensitivity St = su(peak)/su(remoulded).
Foundations
- Driven piles in clay: shaft capacity by the alpha method (α ≈ 0.5–1.0, decreasing with increasing Su); in sand by the beta method (β ≈ 0.3–0.8). Open-ended pipe piles need a plugging check.
- Lateral capacity usually governs for berthing and mooring loads; p-y curves for soft clay differ markedly from sand and must match the soil model.
- Scour removes effective overburden and must be deducted before computing capacity; it can also unbalance a piled deck.
Slopes and reclamation
- Short-term stability of dredged slopes uses undrained strength; long-term uses effective stress. Dredging unloads the toe and can cause heave and base failure.
- Reclamation: Terzaghi one-dimensional settlement, accelerated by prefabricated vertical drains plus surcharge; check primary and secondary compression against the handover criterion.
Worked example
CPTu at mudline: qt = 900 kPa, σv0 = 60 kPa, Nkt = 15. Su = (900 − 60)/15 = 56 kPa. Under a 4 m fill (γ = 18 kN/m³, Δσ ≈ 72 kPa) and Nc = 5.14, the undrained bearing factor of safety is (56 × 5.14)/72 = 4.0, adequate short term - but consolidation settlement still has to be computed and scheduled.
Traps
- Using total stress parameters for long-term conditions in soft clay.
- Forgetting the pore-pressure correction: CPTu gives qt, not qc, and the difference can reach 10% of Su.