Spring-thaw load restrictions are often imposed to avoid severe pavement deterioration during periods of reduced bearing capacity. Equipment that enables monitoring of the pavement strength situation is very important for a road’s traffic carrying capacity, as restrictions could be lifted as soon as the pavement regains its capacity. There are projects with this aim being run in both Sweden and Finland. In Finland the Percostation is used to monitor dielectric value, electrical conductivity and temperature with depth. The Swedish approach is to monitor temperature profile only and to distribute this via the Internet enabling direct access from trucks to frost depth readings that are updated twice an hour. Both approaches are now described.
8.3.5.2 The Finnish Percostation
The Percostation monitors dielectric value, electrical conductivity and temperature at different depths through a maximum of eight channels. Both dielectric value and electrical conductivity are sensitive to the amount of unfrozen water, that is, it is clearly indicated when water freezes and when ice melts. Temperature is, of course, also related to freezing and thawing. Figure 8.12 shows dielectric values at depths 0.15 and 0.30 m together with air temperature during the thawing period of the year 2000 at the Koskenkyla Percostation. From the figure it is clear that the dielectric value approximately equals 5 at both depths when the soil is frozen and that it increases at thawing.
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Fig. 8.12 Dielectric values and air temperature monitored by the Koskenkyla Percostation during spring 2000 (Saarenketo et al., 2002). Reproduced by permission of T. Saarenketo |
The increase starts at depth 0.15 m and is somewhat delayed at the lower depth as thawing takes place from the surface and downwards. In late spring the dielectric value is highest at the greater depth indicating higher frost susceptibility and higher amount of ice lenses melting. Around April 10-12 the dielectric values show a peak followed by a continuously ongoing decrease when surplus water drains and bearing capacity recovers. The data monitored by the Percostation is of great value when imposing and removing spring-thaw load restrictions.
