What is negative pore water pressure in soil?
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The pressure exerted by the water in the pores on the soil is called a pour of water pressure. The negative and positive values are based upon the atmospheric pressure.
The soil below the groundwater table is fully saturated and hence the value off for pressure is greater than atmospheric pressure called a positive for water pressure.
The soil above the groundwater table is unsaturated and hence the pore pressure is less than the atmospheric pressure. Thus, the pore water pressure is negative.
Negative pore water pressure is due to the capillarity action of water in the soil. When the water in the soil is below the ground level, some parts of soil above water level gets saturated due to capillary rise. In this part of soil water is in tension whereas below the water table it is in compression.
Due to an increase in effective stress, shear strength of soil increases. Practically you can observe this near seashore. sand near shoreline have negative pore water pressure due to capillary action & due to an increase in shear strength you can easily walk over them.
In rock or soil, the pressure exerted on its surroundings by water which is held in pore spaces.
Types of pore water pressure:
Negative pore water pressure:
when the voids are partly filled with water in case of surface tension forces operate to achieve the suction effect and shear strength of soil will get an increase.
Generally, the pressure which is exerted by the fluid in a porous medium and composed of a solid framework, pores filled completely or partially with water or other fluid is called Pore Water Pressure.
Negative pore water pressure is obtained when the voids are partially filled with water. In this case, surface tension forces operate to gain suction effect and the shear strength of the soil is increased.
Hi, Pore water pressure is simply the pressure which is normally isotropic force per unit area exerted by the fluid in a porous medium on soil or rock which are composed of the solid framework and the pores are partially filled with water or any type of fluid. Negative pore pressure: it is water pressure from the outer surface or from below the concrete or pressure due to the ground is meant developed is known as negative water or negative power pressure. Causes of negative water pressure: the effective stress is greater than the total stress and the pore water pressure are in the state of partially saturated and in the soil. This occurs due to the surface tension of water throughout the vadose zone causing a suction pressure effect on particles present in the surrounding area.
Negative pore water pressure is a condition in soils when they are partially saturated.
What happens is, surface tension is developed on the surfaces of water in the pores. There are surfaces since the pores are partially filled. This surface tension pushes and pulls other particles in the soil and thus, effectively creates a pore water pressure in the downward direction.
This has an increasing effect on the effective stress.
σ’ = σ – (-μ)
So, the effective stress becomes even higher than the total stress. This means the soil will tend to provide even higher stress reactions than what it’ll do if completely dry.
There are two types of pore water pressures in soil, positive and negative. Soil existing below the ground water table is saturated, and pressure in the pore is greater than atmospheric pressure and hence we have positive pore water pressure. Above groundwater table, we have unsaturated soil and pressure in the pore is less than atmospheric pressure. In this region, we have negative pore water pressure.
Nafisa Nazneen Choudhury
Here the effective stress in soil is greater than total stress. This is because the voids in soil are partly filled with water so surface tension forces are generated that create suction and shear strength is increased.
The pressure is zero when the soil voids are filled with air, and is negative when the voids are partly filled with water (in which case surface-tension forces operate to achieve a suction effect and the shear strength of the soil is increased).