The correct answer is: A. over-consolidated clay with a high over-consolidation ratio.
An over-consolidated clay is a clay that has been subjected to a greater effective stress in the past than it is currently experiencing. This can happen due to the weight of overlying soil or due to the removal of water from the clay. When an over-consolidated clay is unloaded, it will exhibit a large increase in stiffness and strength. This is because the clay particles are able to move closer together and interlock, which makes it more difficult to deform the clay.
The over-consolidation ratio (OCR) is a measure of the degree of overconsolidation of a clay. It is defined as the ratio of the current effective stress to the maximum past effective stress. A high OCR indicates that the clay has been subjected to a large amount of overconsolidation, while a low OCR indicates that the clay has been subjected to a small amount of overconsolidation.
Over-consolidated clays with a high OCR behave like dense sands. This is because the clay particles are able to move closer together and interlock, which makes it more difficult to deform the clay. As a result, over-consolidated clays with a high OCR are typically very stiff and strong.
Clays that are normally consolidated or under-consolidated do not behave like dense sands. This is because the clay particles are not able to move as close together and interlock, which makes it easier to deform the clay. As a result, normally consolidated or under-consolidated clays are typically less stiff and strong than over-consolidated clays with a high OCR.