Effects of Capillarity
Description:
In silts and fi ne sands, the soil above the groundwater can be saturated by capillary action. You would have encountered capillary action in your physics course when you studied menisci. We can get an understanding of capillarity in soils by idealizing the continuous void spaces as capillary tubes. Consider a single idealized tube, as shown in Figure . The height at which water will rise in the tube can be found from statics. Summing forces vertically (upward forces are negative), we get
that is,
Solving for zc, we get
Capillary simulation
in soils
where T is the surface tension (force per unit length), u is the contact angle, zc is the height of capillary rise, and d is the diameter of the tube representing the diameter of the void space. The surface tension of water is 0.073 N/m and the contact angle of water with a clean glass surface is 0. Since T, u, and gw are constants,
For soils, d is assumed to be equivalent to 0.1 D10 where D10 is the effective size. The interpretation of Equation is that the smaller the soil pores, the higher the capillary zone. The capillary zone in fi ne sands will be larger than for medium or coarse sands.
The porewater pressure due to capillarity is negative (suction), as shown in Figure , and is a function of the size of the soil pores and the water content. At the groundwater level, the porewater pressure is zero and decreases (becomes negative) as you move up the capillary zone. The effective stress increases because the porewater pressure is negative. For example, for the capillary zone, zc, the porewater pressure at the top is 2zcgw and the effective stress is s9 5 s 2 (2zcgw) 5 s 1 zcgw. The approach we have taken to interpret capillary action in soils is simple, but it is suffi cient for most geotechnical applications. For a comprehensive treatment of capillary action, you can refer to Adamson (1982).