Significance and Use
5.1 The soil permittivity probe is used for the following purposes:
5.1.1 The test method described is useful as a rapid, nondestructive technique for bulk measurements of the water mass per unit volume of soil and soil-aggregate which may, in conjunction with an independent bulk density determination, be used in the determination of dry density.
5.1.2 The test method is used for quality control and acceptance testing of compacted soil and soil-aggregate mixtures as used in construction and also for research and development. The nondestructive nature allows repetitive measurements at a single test location and statistical analysis of the results.
5.1.3 Volumetric Water Content—The fundamental assumptions inherent in the test method are that the dielectric constants value measured by the system in a given test site composed of soil or soil-aggregate are directly correlated to the volumetric water content of the soil or soil-aggregate, and that the material is homogeneous. (See 6, “Interferences.”)
Note 2: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
Scope
1.1 This test method describes the procedures for measuring the water mass per unit volume of soil and soil-aggregate by use of an in situ permittivity probe. Measurements are taken at a depth beneath the surface of the soil determined by the design of the probe.
1.1.1 For limitations see Section 6 on Interferences.
1.2 The permittivity probe is inserted into a hole drilled or punched into the soil being measured. As its name indicates, the probe measures the dielectric permittivity of the soil into which it is placed. Two electrodes, connected to an oscillating circuit, are mounted a predetermined distance apart. These electrodes act as the plates of a capacitor, with the soil between the plates forming the capacitor dielectric.
1.2.1 The probe circuit creates an oscillating electric field in the soil. Changes in the dielectric permittivity of the soil are indicated by changes in the circuit’s operating frequency. Since water has a much higher dielectric constant (80) than the surrounding soil (typically around 4), the water content can be related by a mathematical function to the change in dielectric permittivity, and, consequently, the changes in the circuit’s operating frequency.
1.2.2 The construction, deployment, and operating principle of the device described in this test method differ from other methods that measure the dielectric constant, bulk electrical conductivity, complex impedance, or electromagnetic impedance (see Test Methods D6780/D6780M, D7698, and D7830/D7830M) of the soil and relate the results to water mass per unit volume and/or water content.
1.2.3 The water content of the soil measured by the permittivity probe is the volumetric water content, expressed as the ratio of the volume of water to the total volume occupied by the soil. This quantity is often converted, and displayed, by the probe in units of mass of water per volume of soil, or water mass per unit volume. This conversion is performed by multiplying the water content (in volume of water per volume of soil) by the density of water.
1.3 Water content most prevalent in engineering and construction activities is known as the gravimetric water content, ω, and is the ratio of the mass of the water in pore spaces to the total mass of solids, expressed as a percentage. To determine this quantity, the bulk density of the soil under measurement must also be determined.
1.4 Units—The values stated in SI units are to be regarded as the standard. Reporting the test results in units other than SI shall not be regarded as nonconformance with this standard.
1.5 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.
1.5.1 For purposes of comparing, a measured or calculated value(s) with specified limits, the measured or calculated value(s) shall be rounded to the nearest decimal or significant digits in the specified limits.
1.5.2 The procedures used to specify how data are collected/recorded and calculated in this standard are regarded as the industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
