Thixotropy MCQ with answer and solution

Concept

Thixotropy is the property of certain clay minerals (like sodium bentonite) that undergo an isothermal gel-to-sol transition upon agitation and revert to a gel upon standing.

Step-by-step solution

1. Identify the primary function of bentonite slurry in borehole stabilization. 2. Recognize that the slurry must act as a fluid during pumping/circulation (sol state) and as a semi-solid gel when stationary to support the borehole sides. 3. Match this behavior (becoming fluid under agitation and setting into a gel at rest) to the definition of thixotropy. 4. Conclude that option (C) is the correct answer.

Explanation

Bentonite slurry is used to stabilize boreholes during deep foundation and piling works. When left undisturbed, the suspension forms a gel-like structure due to its thixotropic property, which prevents suspended soil particles from settling and supports the borehole walls. When agitated or pumped, it liquefies, making it easy to circulate.

Why the other options are wrong

A: Sensitivity relates to the loss of strength of clay upon remolding, not borehole support. B: Shrinkage is the reduction in volume upon drying, which is irrelevant for wet boreholes. D: Colloidal property helps in suspension, but the specific gelling action at rest is thixotropy.

Shortcut

Remember 'Thixotropic = Time-dependent gel-sol transformation', which is the exact mechanism required for drilling muds.

Common mistake

Confusing thixotropy with colloidal properties; while bentonite is a colloid, its specific stabilization action under rest versus motion is due to thixotropy.