Although far less common than in the laboratory, techniques for in-situ matric suction measurements using the contact filter paper (CFP) method and in-situ total suction measurements using the non-contact filter paper method have also been described (Greacen et al., 1989). The filter paper technique is, in theory, applicable over the entire range of total suction, but the method tends to be impractical for both extremely high and extremely low suction values. Reliable measurements tend to be limited to a range spanning about 0-10 MPa matric suction for the contact filter paper techniques and between 1 to 10 MPa of total suction for the non-contact technique.
The filter paper method is used as an indirect means of measuring soil suction. The advantages of the method include its simplicity, its low cost, and its ability to measure a wide range of suction. Although this technique is more often used in the laboratory, the filter paper method has also been used in the field to measure soil suction. The CFP technique relies on measuring the equilibrium water content of small filter papers in direct contact with unsaturated soil specimens. Figure 3.17 shows the filter paper setup and installation to put it in direct contact with the soil specimen. In the laboratory the filter paper is placed in contact with the soil specimen in an airtight container for seven days and thereafter the water content of the filter paper is determined and the matric suction of the soil specimen is inferred from a calibration curve. Filter paper not in contact with the specimen permits water exchange only in the vapour phase and therefore measures the total suction (Rahardjo & Leong, 2006).
The water content of the filter paper at equilibrium is measured gravimetrically and related to matric soil suction through a predetermined calibration curve for the particular type of paper used. Commonly used types of papers include Whatman No. 42 and Schleicher and Schuell No. 589. Calibration and test procedures for the measurement of matric suction using the contact filter paper technique are described in the ASTM Standard D5298-94 (1997). However, different researchers have suggested different calibration curves for the same filter paper (Leong et al., 2002; Rahardjo and Leong, 2006).
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Fig. 3.17 Contact filter paper setup procedure (in laboratory). Reproduced with permission of R. Bulut |
A considerable variety of test and assessment procedures are available for measuring the volumetric and gravimetric water contents of both laboratory and in-situ road construction and geotechnical materials. The simplest tests to perform are usually destructive, but sophisticated geo-physical techniques are becoming increasingly common and usable, not only as identification tools, but also as quantitative measurement techniques.
Suction, which has such a large effect on the mechanical properties of soils and aggregates, is probably the quantity most difficult to measure successfully and must usually be monitored indirectly by the response of, e. g., water content and vapour monitoring. As the relationships between these secondary responses and the primary cause, suction, may both be imprecisely described and hysteretic there is usually some uncertainty in value of suction determined.
Permeability, another major quantity that needs evaluating, is more readily measured using flow tests, but difficulties arise when measuring coarse-grained materials, such as road aggregates. Producing samples that are representative with respect to density and grading can be a challenge and the devices available for testing can allow water to preferentially flow along the edges, introducing further uncertainties into the assessment.
Nevertheless, a knowledge of permeability, suction and water content is indispensable for effective design and assessment of the movement of water in the highway and its adjacent environment.
