Verification By Direct Methods.

Soil water content and dry density are indices most often used for controlling earthwork compaction quality. Presently two methodologies are used to measure these indices – direct or indirect methods. Direct methods of measuring density and obtaining material for water content determination include the sand cone method (ASTM D 1556), the drive tube method (ASTM D 2937) and to a lesser extent, the rubber balloon method (ASTM D 2167).

The reference method for water content determination is oven drying material for 24 hours at 105°C ± 5°C as prescribed in ASTM D 2216. Expedient water content methods are available (such as microwave, direct heat and calcium carbide); however, the user is reminded that these fast, convenient methods must be validated by the oven dried method.

Indirect methods for measuring dry density and water content include the MDI procedures described in the M+DI manual and various nuclear gauge devices. It is our recommendation that when using the MDI (or any other indirect method) that one or more direct methods cited above be used sufficiently to validate the results. This validation is best done at the beginning and at regular intervals for major earthwork projects or periodically for smaller projects especially when encountering new geological settings. The user is cautioned that using direct methods can result in wide variation of results or even erroneous results because the methods are highly dependent upon user experience and skill level and equipment calibration. Some will elect to use the nuclear gauge as the reference device which uses one method for determining the density and another method for water content. The water content determination by a nuclear gauge is a back scatter method with 50% or more of the indicated value resulting from the upper 2 to 3-in of material. In addition certain minerals in the soil can create false readings by a nuclear gauge. If the user relies on the nuclear gauge as the referenced device, we recommend that the gauge wet density be recorded and excavate a representative sample of the material beneath the gauge for subsequent water content determination by D2216. This will permit computation of the associated dry density for comparison.