The correct answer is: A. decrease in liquid limit and increase in plasticity index.
The liquid limit is the water content at which a soil will flow like a liquid. The plasticity index is the difference between the liquid limit and the plastic limit. The plastic limit is the water content at which a soil will just begin to crumble when rolled into a thread 1/8 inch in diameter.
When coarser particles like sand or silt are added to clay, the clay particles are less able to move around and interact with each other. This makes the clay less likely to flow like a liquid and increases the liquid limit. The coarser particles also fill in the spaces between the clay particles, which makes the clay less likely to crumble and decreases the plastic limit.
Therefore, the admixture of coarser particles like sand or silt to clay causes a decrease in liquid limit and an increase in plasticity index.
Here is a more detailed explanation of each option:
- Option A: decrease in liquid limit and increase in plasticity index. This is the correct answer. As explained above, the admixture of coarser particles like sand or silt to clay causes a decrease in liquid limit and an increase in plasticity index.
- Option B: decrease in liquid limit and no change in plasticity index. This is not the correct answer. The admixture of coarser particles like sand or silt to clay does cause a change in plasticity index.
- Option C: decrease in both liquid limit and plasticity index. This is not the correct answer. The admixture of coarser particles like sand or silt to clay does cause a change in liquid limit, but it does not cause a decrease in plasticity index.
- Option D: increase in both liquid limit and plasticity index. This is not the correct answer. The admixture of coarser particles like sand or silt to clay does cause a change in liquid limit, but it does not cause an increase in plasticity index.