Accurate and timely moisture measurement of earthwork during compaction of foundation layers is crucial to proper construction and long-term durability of the pavement structures. Since the traditional methods for measuring moisture are point specific, expensive and/or time consuming. None of the existing methods have been able to deliver real-time, continuous moisture measurements that would aid engineers in delivering more durable pavement structures.

In a recently completed study, funded by the National Road Research Alliance, researchers documented the current state of knowledge concerning field moisture measurement during pavement foundation construction, and field tested two prototypes

Several methods for soil moisture measurement have been developed throughout the years. A direct approach (i.e., the gravimetric method) entails collecting a soil sample from the site under investigation and weighing it before and after drying. This approach is regarded as the “gold standard” for determining the moisture content of substances. Indirect approaches do not measure the soil water content directly; they use a radiation source or a probe to measure a parameter significantly related to the moisture content. The advantage of these methods is that they are more rapid and often nondestructive.

This research had two goals:

1. Evaluate existing devices that can read moisture in a laboratory and field environment.
2. Develop and demonstrate a prototype device that can continually measure moisture during pavement foundation construction.

What did we do?

The research team documented the current state of knowledge concerning field moisture measurement during pavement foundation construction. The team has also provided critical information about the state of the art and the state of practice related to the most effective moisture measurement devices suitable for improving compaction near structures such as retaining walls and bridge abutments.

Six soil types from fine to coarse grained were subjected to traditional and large-scale laboratory testing for water content using the methods described in the literature review. A systematic evaluation of the laboratory prototype of the proposed equipment final prototypes was also done using laboratory and small-scale specimens prepared from diverse geomaterials.

Two prototypes were also field tested at the MnROAD test track pavement facility: one using a Wenner Array (WA) and the other a rolling equatorial array (REA) to measure continuous moisture measurement. The WA was used for the static complex resistivity measurements, and the REA was used for the rolling complex resistivity.

What did we learn?

Even though the study demonstrated the feasibility of the complex and traditional resistivity methods in estimating in situ moisture contents, further hardware and software development is necessary to implement the concept in the day-to-day operation of highway agencies.

The primary conclusion from the field study is that resistivity or complex-resistivity measurements with a rolling four-electrode array, with any given geometry, are probably not feasible for the control of moisture content for the following four reasons:

1. The CR polarization signal arising from the pore-scale relaxation is overwhelmed by lower-frequency polarization from moisture and compaction heterogeneity at the electrode spacing scale.
2. The apparent sensitivity of the CR measurement to electrode pressures on different soil types is difficult to predict or compensate for.
3. An incompatible tradeoff between a large electrode contact area and short array spacing is necessary to measure shallow, less dense specimens without deforming the specimen.
4. The CR measurement will become sensitive to larger heterogeneity scales with a larger electrode spacing.

More Information

• Road Research/NRRA project overview
• Final report