Problem –1 . Calculate the gauge, and absolute water pressure at the menisci in capillary tubes having diameters of the same order of magnitude as the size of pores in clay, silt, and sand as demonstrated in the following example problem. Assume that pore diameter is about 1/20 of the grain size and contact angle θ = 0.

Solution The water pressure equation, Eq. (1), can be simplified as

,
after assuming the given contact angle. The required data is calculated using the values for surface tension of water at room temperature and magnitudes of the pore diameters of clay, silt, and sand as given in the following table.

Parameter	Sand	Silt	Clay
dgrain (mm)	2	0.075	0.0002
dPore (mm)	0.1	0.00375	0.00001
uw - u a (kPa)	-2.92	-77.87	-29200
uw (kPa)	+98.48	+23.53	-29099

Table 1. Minimum sustainable gauge and absolute water pressure in capillary tubes having representative diameters as the sizes of pores in clay, silt, and sand using,

It is clear from the Table 1 that the absolute water pressure can be negative when the pore size is sufficiently small, as in the case of clays. Therefore, pore water can sustain high tensile pressures in unsaturated fine-grained soils.