Question : What is meaning of positive and negative pore water pressure?

In the Subsurface condition , the soil is partially saturated at some zone and fully saturated in some zone. The phreatic surface i.e contact between full saturated and un saturated condition acts as a boundary between two zone i.e Positive pore water pressure and negative pore water pressure.In the Phreatic condition the pressure is equal to atmospheric pressure. Below the groundwater table, the soil is fully saturated, and the pore pressure is above atmospheric pressure and positive in value. Above the groundwater table where the soil is unsaturated, the pore pressure is below atmospheric pressure and hence is negative in value. In this zone, the pore water is continuous or semi continuous and the pore water pressure is below atmospheric pressure.

The magnitude of the negative pore pressure also called soil suction is controlled by surface tension at the air-water boundaries within the pores and is governed by grain size. In general, the finer the soil particles, the larger the saturation capillary head, and hence the higher the negative pore pressure. Rainfall infiltration from the ground surface add water to the ground and increase the saturation percentage of soil and hence may rapidly reduce the magnitude of negative pore pressure.

Thanking You.

Jyoti Khatiwada

Roll no. 10