Bay Mud Settlement

I am not a geotechnical engineer, but I have a question concerning the settlement of bay bud with several feet of fill on top, and a high water table. I understand the logic concerning the long term settlement of the bay mud due to the weight of the fill, but my question is in regards to the effects of lowering of the water table in the fill. It has been described to me that by lowering the water table, the fill is now no longer bouyant and therefore heavier. This heavier weight then results in additional settlement of the fill. This does not make since to me, as the total weight on top of the bay mud is heavier with fill and water than with just fill. Am I missing something?

In addition, what happens when the water table is lowered into the bay mud?

Soil Mechanics / Geotechnical Analysis Geotechnical Investigations & In-Situ Testing Engineering Geology Groundwater Flow & Seepage

Asked 12/10/2021 01:00, updated: 13/04/2022 16:26

1 Answer

Votes: 2

Stephen Buttling

Yes Someone, I am afraid that, as you are not a geotechnical engineer, you are indeed missing something. Soil is a three phase system, made up of solid, fluid and gas. The solid is the soil particles, the fluid is normally water, and the gas can be air. Below the water table there is no air, so only solid and water. The water is continuously connected, so is subject to hydrostatic pressure, which we call the pore pressure. While gravity, and the soil above, is trying to push the particles closer together (total stress), the water is trying to push them apart. The difference between these two pressures, being the net pressure pushing the particles together, is what we call effective stress. When you lower the water table in the fill the total stress remains the same but the effective stress increases because there is less water pressure. This leads to consolidation. The same will happen if the water table is lowered into the Bay Mud, but that will be slower because of the lower hydraulic conductivity. Hope that helps.