6.3.4 Matrix suction

Matrix suction is defined as the difference between the pore-air and the pore-water pressures (Fredlund and Rahardjo, 1993b).

It is associated with capillary action, the process where interface tension between water and air creates a curve interface boundary within a narrow opening, leading to a pressure difference between the two. The magnitude of the pressure difference between water and air is a function of the width of the gap between the solid surfaces.

The capillary rise that will occur within a soil is affected by its particle size and grading, since they affect the size of the pores within the soil mass. Fine grained soil with corresponding fine grained pore spaces are able to sustain large pressure differenc between pore water and air, allowing large capillary rises. in the other hand, coarse grained soil with larger voids tend to maintain lower pressure differences between air and water, resulting in a lower capillary rise.

The relationship between matric suction and degree of saturation may be presented graphically in the form of a soil water retention curve (also known as a soil moisture retention curve or Soil Water Characteristic Curve, SWCC). The form of these curves tends to be similar, regardless of soil type and are generally S shaped, although this shape is poorly defined in clayey soill. Figure 5 shows typical form of the SWCC with principal characteristics, while figure 6 shows some soil specific data showing effects of soil grading. This curve also depicts the relationship between soil water content and soil water pressure potential.









Figure 5: Typical absorption and desorption SWCC (Zhan and Ng. 2004)



Figure 6: SWCCs for some Dutch soils (after Koorevaar et. al.1983)