Freezing Induced Water Flow

The significance of the coupling between heat and water transport will be illustrated using a freezing experiment performed by Mizoguchi (1990). He packed four iden­tical cylinders with Kanagawa sandy loam. Each cylinder was 20 cm long and had an internal diameter of 8 cm. The samples were prepared for the freezing test by bringing them to the same initial state involving a uniform temperature of 6.7 °C and a close to uniform volumetric water content of 0.33 throughout the cylinders. Water and soil in each cylinder was subjected to freezing from the top down, since their top covers were exposed to a circulating fluid with a temperature of -6 °C. One cylinder was used to obtain initial values, and the other three were removed from freezing after 12, 24 and 50 hours respectively. The cylinders were then cut into 1 cm thick slices for which the total water content (ice + liquid water) was

Fig. 11.6 The simulated volumetric water content, 9, in a model road (a) after a light rainfall event (top), (b) after a heavy rainfall event (middle), (c) after a moderate rainfall event using small fracture-zone permeability (bottom) The particles illustrate the flow paths of the infiltrated rainwater. The vertical scale is exaggerated for clarity

determined. The experimental procedure thus described was then reproduced in a computer model in order to test the model.

As described in Chapter 4, Section 4.6, water flows towards freezing fronts where it changes phase from liquid to solid. This process is clearly evident in Fig. 11.7 where the total water content in the upper half of the cylinder increases as the col­umn freezes (Hansson et al., 2004). Since freezing is a relatively quick process, extremely high hydraulic gradients emerge and can lead to sometimes very rapid upward flow of water. The freezing front is clearly visible in Fig. 11.7 as the depth interval where the total water content decreases rapidly. The calculated results are in fair agreement with the measured values. Specifically, the rapid decrease in the total water content at, or immediately below, the freezing front and the gradual recovery deeper in the columns is well predicted.

It is the dramatic redistribution of water caused by the freezing that causes frost heaving, which may damage roads even though the largest problems occur in con­nection with thaw weakening. It should, however, be pointed out that the computer

model used here neglects effects of frost heave. If the conditions for frost heave had been met during the simulation, the result had been different since the liquid pressure head would have changed as an effect of a relative ice pressure not equal to zero.