positive and negative pore pressures.

1. Question: What is meaningof positive and negative pore water pressure?
In the Subsurface condition,the soil ispartiallysaturatedatsome zone andfullysaturatedinsome
zone.The phreaticsurface i.e contact betweenfull saturatedandunsaturatedconditionactsasa
boundary betweentwozone i.e Positive pore waterpressureandnegative pore waterpressure.Inthe
Phreaticconditionthe pressure isequaltoatmosphericpressure.Belowthe groundwatertable,the soil
is fullysaturated, and the pore pressure is above atmosphericpressure and positive invalue.Above
the groundwater table where the soil is unsaturated, the pore pressure is belowatmosphericpressure
and hence is negative in value. In thiszone,the pore wateriscontinuousorsemi continuousandthe
pore waterpressure isbelowatmosphericpressure.
The magnitude of the negative pore pressurealsocalledsoilsuctioniscontrolledbysurface tensionat
the air-waterboundarieswithin the poresandisgovernedbygrainsize. Ingeneral,the finerthe soil
particles,the largerthe saturationcapillaryhead,andhence the higherthe negative pore pressure.
Rainfall infiltrationfromthe groundsurface addwaterto the groundand increase the saturation
percentage of soil andhence mayrapidlyreduce the magnitude of negative pore pressure.
ThankingYou.
Jyoti Khatiwada
Roll no.10