Soil suction or capillary pressure head can be measured either in the laboratory in an undisturbed sample of soil or directly in the field. Soil suction or total suction consists of the matric suction and the osmotic suction. Their magnitudes can range from 0 to 1 GPa (Rahardjo & Leong, 2006). Today no single instrument or technique exists that can measure the entire range with reasonable accuracy. Suction measurement instruments can only measure suction up to about 10 MPa. In the highway environment soil suction in the low range (0-100 kPa) or the mid range (100kPa-1 MPa) is of most concern. There are different measurement techniques depending on which component of suction one wants to measure, matric or total. Usually in geotechnical engineering it is the matric suction that is measured. Table 3.2 summarises techniques for measuring suction in terms of approximate measuring range and applicability in the laboratory or field (Lu & Likos, 2004; Rahardjo & Leong, 2006). As it is generally not necessary to take osmotic suction into account in routine geotechnical engineering practice, only the main in-situ methods for measuring matric suction are referred to. They include tensiometers, thermal conductivity sensors and contact filter paper techniques.
Direct measurement of suction in aggregates is more difficult than in finer grained soils as it is difficult to establish an effective contact between the measuring device and the pore space in the aggregates. In aggregates with a high proportion of fines, this may be achievable. Alternatively, indirect measures such as discussed in Chapter 2, Section 2.9, can be employed.
