True or False: for a particular soil, pressure just under the water table is negative
To determine whether pressure just under the water table is negative, you need to understand the concept of water table and soil mechanics.
The water table represents the underground boundary between the unsaturated soil above it and the saturated soil below it. It fluctuates based on rainfall, groundwater recharge, and other factors. When the water table is below the ground surface, the soil above it is unsaturated (contains air and water), and when the water table is at or near the ground surface, the soil above it is saturated (filled with water).
In general, the pressure in the soil increases with depth due to the weight of the overlying soil mass. So, above the water table, the pore water pressure (pressure of water within the soil pores) is generally positive. However, just below the water table, the pressure can be either positive or negative, depending on various factors.
If the soil has a high permeability, such as sandy or gravely soils, the rise or fall of the water table can lead to rapid drainage or infiltration of water. In this case, the pressure just below the water table tends to be negative, as the soil is partially dewatered due to the rapid drainage.
On the other hand, in low-permeability soils, such as clays, the water movement is slower, and the pressure just below the water table is generally positive. This is because the water is trapped within the soil pores, and the pore water pressure counterbalances the weight of the soil above.
Therefore, the statement "pressure just under the water table is negative" can be either true or false, depending on factors such as soil permeability and the rate of water movement. It is not universally true or false, and the specific conditions of the soil need to be considered.