Tjal2004, developed by VTI, monitors temperature at every 50 mm down to a depth of 2 m. Temperatures are collected twice an hour and distributed via the Internet. The temperature sensors are calibrated to give highest possible accuracy close to 0°C where freezing starts. Trucks, having mobile Internet, pick up the current freezing situation from the installed Tjal2004 along the intended roads to travel. Road owners give truckers allowance to use roads as long as the upper part of the pavement is frozen down to a certain depth. This means that in spring, load restrictions are imposed and removed automatically and very frequently. During periods in spring with clear weather the situation might change daily. In the daytime the solar radiation thaws the upper layers, which is followed by re-freezing during the clear and cold night. This means that the heavy loading of trucks is allowed in the early morning but prohibited in the afternoon. Figure 8.13 shows typical repeated freezing and thawing during the period March 4 — April 22 of the year 2008 monitored by one of the Tjal2004 installed in Sweden. The total frost depth is close to 1.5 m.
8.2 Conclusions, Implications, Recommendations
Field observations indicate clear and significant variations of moisture in subgrades. This is true both for moderate climates as well as for cold regions where it is related to temperature. In particular, thawing may induce strong increases in moisture levels. The mechanical behaviour as observed in-situ is strongly affected by moisture variations: the wetter the state the lower the stiffness (up to a factor 2 or more), the lower the stiffness then the higher the deflection.
Therefore one may conclude that an efficient drainage system is crucial in order to reduce the road structure’s ageing. An analytical assessment of this relationship between moisture and mechanical performance will be undertaken in the following two chapters.
