Thixotropy and flow index

Thixotropy

Thixotropy is the property of some clay soils to lose strength when disturbed (remoulded) at constant water content, and regain it with time on standing at rest. The regain is caused by reorientation of water molecules and particles in the adsorbed layer. Example: pile driving in clay softens the soil, then the strength returns after some days (pile "freeze").

Flow index ($I_f$)

The slope of the flow curve, i.e. the line of water content against log of number of blows in the liquid limit test. It is the change in water content per one log cycle of blows:

$$I_f = \frac{w_1 - w_2}{\log_{10}(N_2/N_1)}$$

A high flow index means the shear strength falls quickly with increase in water content.