Upon filling a large water tank on saturated clayey soil, which statements about pore water pressure and effective stress are true? Select two correct answers.

Focusing on the dynamics of pore water pressure and effective stress in saturated clayey soil when a large water tank is filled, the correct understanding hinges on the principles of soil mechanics.

Initially, when the water tank is filled, the pore water pressure in the surrounding soil increases rapidly due to the added load of water. This increase in pore water pressure can lead to a temporary situation where the soil's ability to support loads is compromised because a greater portion of the total stress is absorbed by the pore water rather than the soil skeleton. Over time, as excess pore water is dissipated, the effective stress, which is the stress carried by the soil structure itself, will start to increase.

The statement regarding effective stress decreasing at the beginning then slowly increasing is based on the understanding that with the initial rise in pore water pressure, effective stress decreases since effective stress is defined as total stress minus pore water pressure. Over time, as pore water drains out of the soil (if drainage is allowed), effective stress will gradually increase as the soil particles re-establish contact due to the reduction in pore water pressure.

Therefore, the phenomenon of pore water pressure initially increasing and then decreasing, along with the effective stress pattern of decreasing initially and then gradually increasing, reflects the transient behaviors