A geophysical approach to construction quality assurance testing of compacted soil using electrical conductivity measurements

Construction quality assurance (CQA) testing is typically performed in compacted fine-grained soils to measure in situ values of dry density (pd) and water content (w). The nuclear gauge is the most common method. However, electrical conductivity (σ) measurement is an attractive alternative. Previous researchers have developed relationships between σ and volumetric water content (θ), but the effect of compaction effort has not been investigated. Other researchers have related σ to degree of saturation (S), and the relationship is independent of compaction effort. However, specimens compacted at the same S can possess different values for θ, so σ cannot correlate equally well to both parameters. Therefore, a laboratory study was performed to assess the effects of compaction effort, S, and θ on σ in specimens of fine-grained soil. By measuring σ over a range of w and compaction effort, it was observed that σ correlates well with θ independent of compaction effort, while the correlation between σ and S is dependent upon compaction effort. It was also observed that θ is a reasonable indicator of whether or not a soil specimen is wet or dry of w _opt, a common CQA acceptance criterion. Therefore, a new CQA method based on θ is proposed. Since θ can be predicted based on the measurement of σ, field geophysical measurement of θ may be used to quickly determine whether or not soil has been compacted in accordance with specifications.