Monitoring and Modeling Subgrade Soil Moisture for Pavement Design and Rehabilitation in Idaho Report uri icon



  • Environmental changes have a direct impact on the structural capacity of the pavement, and consequently its performance. While the subgrade soil and the unbound materials are sensitive to moisture variation, the Asphalt Concrete (AC) layers are more sensitive to temperature variations. Quantifying the effect of these two environmental factors, moisture and temperature, is necessary for incorporation in the pavement design process. 

    The main goal of this research was to quantify the variation of subgrade moisture and asphalt surface temperature at various sites in Idaho and determine their effects on the structural capacity of the pavement layers, and hence determine their influence on the pavement performance. In addition, the impact of the existence of a rockcap base layer on the moisture regime in the subgrade and its effect on the overall pavement structural capacity was to be evaluated. 

    The research methodology included instrumentation of several pavement sites in northern region (Pack River, Worley, Moscow and Lewiston) and in southern region at Weiser. The Moscow and Weiser sites included adjacent sections of rockcap and aggregate bases to compare the effectiveness of these two types of base materials. Instrumentation sensors used were similar to those used in the FHWA Long Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP). Time domain reflectometry (TDR) probes were installed to measure volumetric moisture content, MRC thermistors were used to measure temperature at various depths, and ABF resistivity probes were installed to determine frost conditions. Piezometers were also installed to monitor ground water level (GWL) at the instrumented sites. Structural capacity was evaluated using Falling Weight Deflectometer (FWD). The moisture, temperature, resistivity and the GWL data were collected on a monthly basis for almost three years. However, the FWD data, which was collected by the ITD materials, was performed approximately once a year along with the ITD normal FWD testing schedule. This resulted in a great shortcoming in monitoring the seasonal variation of the pavement structural capacity at the instrumented sites. Therefore, the research relied on the LTPP-SMP database to acquire seasonal FWD data for many sites across the country. 

    Moisture and temperature data at the instrumented Idaho sites were analyzed to determine the seasonal variability of these two parameters. Historical climatic data were also obtained from weather stations, and augmented with the moisture and temperature data to develop seasonal timing at the various sites. The resistivity data, however, were found erratic and were not considered in any part of the analysis. 

    Data acquired from the LTPP-SMP database were analyzed to develop correlation models that quantify the variation of the resilient modulus of unbound materials and relate it to moisture variation. Similarly, correlation models to relate the modulus of asphalt concrete layers to the temperature variation were also developed. The developed models showed dependency of the modulus on many other factors such as material type, mix design, climatic region, and other design related parameters. The developed models were then checked using the collected data at the specific sites instrumented in Idaho. Then the models were incorporated in a mechanistic-empirical pavement design process to quantify the effect of the seasonal variation on pavement performance. 

    Results of the mechanistic analysis, which incorporated the developed models, indicated that the incorporation of the seasonal variation in pavement design process leads to the prediction of significantly shorter pavement service life. This finding is critical to pavement designers, since the lack of consideration of such seasonal variations could result in a premature failure. To determine the rockcap base layer effectiveness, moisture data at the Moscow and Weiser sites were analyzed. Results showed conflicting trends. In Moscow site, the subgrade experienced more moisture under the rockcap base while the opposite was observed in Weiser. It is believed that the extension of the rockcap layer to the open side ditches, as in Weiser site, allows the surface water to drain away relieving the subgrade from the excess moisture. On the other hand, where the rockcap is enclosed, as in Moscow site, the water in rockcap is entrapped and it drains downward causing the subgrade moisture to increase.
    However, the mechanistic analysis performed at these two sites, showed that the section with rockcap layer was consistently stronger than the section with aggregate base, even though the subgrade moisture content under rockcap layer was greater. The predicted rutting life, for the pavement section with rockcap layer, was about 5 times greater than the other section with aggregate base. Thus, the presence of rockcap base was always effective in increasing the pavement structural capacity and increasing the fatigue and rutting service lives.

    To facilitate the use of the research results, the developed models were applied to the specific conditions tested at the instrumented sites, and moduli seasonal adjustment factors (SAF) were calculated. Algorithm and Tables for these factors at the different regions were developed and provided in this report. 

publication date

  • July 2004



  • pavement design


report identifier

  • ITD Project SPR 0010 (025) 124

    UI-NIATT Project KLK459