Which type of loading is associated with primary consolidation settlement in clay soils?

Primary consolidation settlement in clay soils is primarily associated with long-term loading. During this process, clay particles reorganize and pore water is expelled from the soil pores over time, leading to a reduction in volume and resulting in settlement.

Long-term loading reflects sustained pressures on the soil that allows for the gradual dissipation of pore water pressure, facilitating the rearrangement of soil structure and effective stress changes. In clay soils, the consolidation process can take considerable time due to their low permeability, meaning that the effects of this prolonged loading may not be fully realized immediately.

Short-term loading typically creates conditions where immediate effects are observed without allowing sufficient time for consolidation to occur. This leads to rapid changes in pore water pressure without the accompanying volume change that characterizes primary consolidation. Immediate surface loading refers to the application of loads that can induce quick response in the soil, but does not allow for time-dependent settlement processes to manifest. Cyclic loading involves repeated loading and unloading, which can produce a different set of responses, commonly related to dynamic conditions rather than the slow drainage and consolidation associated with primary consolidation.

Thus, it is the long-term loading that initiates the gradual process of primary consolidation in clay soils, making it the correct choice.