Effects of dynamic changes of desiccation cracks on preferential flow:experimental investigation and numerical modeling

Preferential flow induced by desiccation cracks (PF-DC)has been proven to be an important hydrological effect that could causevarious geotechnical engineering and ecological environment problems.Investigation on the PF-DC remains a great challenge due to the soilshrinking-swelling behavior. This work presents an experimental andnumerical study of the PF-DC considering the dynamic changes of desiccation cracks. A soilcolumn test was conducted under wetting-drying cycles to investigate thedynamic changes of desiccation cracks and their hydrological response. The ratios between thecrack area and soil matrix area (crack ratio), crack aperture and depth weremeasured. The soil water content, matrix suction and water drainage weremonitored. A new dynamic dual-permeability preferential flow model (DPMDy)was developed, which includes physically consistent functions in describingthe variation of both porosity and hydraulic conductivity in crack andmatrix domains. Its performance was compared to the single-domain model(SDM) and rigid dual-permeability model (DPM) with fixed crack ratio andhydraulic conductivity. The experimental results showed that the maximumcrack ratio and aperture decreased when the evaporation intensity wasexcessively raised. The self-closure phenomenon of cracks and increasedsurficial water content was observed during low-evaporation periods. Thesimulation results showed that the matrix evaporation modeled by the DPMDyis lower than that of the SDM and DPM, but its crack evaporation is thehighest. Compared to the DPM, the DPMDy simulated a faster pressure headbuilding-up process in the crack domain and higher water exchange rates fromthe crack to the matrix domain during rainfall. Using a fixed crack ratio inthe DPM, whether it is the maximum or the average value from the experimentdata, will overestimate the infiltration fluxes of PF-DC but underestimateits contribution to the matrix domain. In conclusion, the DPMDy betterdescribed the underlying physics involving crack evolution and hydrologicalresponse with respect to the SDM and DPM. Further improvement of the DPMDyshould focus on the hysteresis effect of the soil water retention curve andsoil deformation during wetting-drying cycles.