Which of the following statements is NOT correct?

1. Loose sand exhibits contractive behaviour upon shearing
2. Dense sand when under untrained condition, may lead to generation of negative pore pressure
3. Black cotton soil exhibits expansive behaviour
4. Liquefaction is the phenomenon where cohesion less soil near the downstream side of dams or sheet-piles loses its shear strength due to high upward hydraulic gradient

Liquefaction is due to cyclic load and not due to high hydraulic gradient.