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Crosta and di Prisco 1069Fig. 13. Sand box model of the slope simulating the 3D seepage within a homogeneous, initially dry, dense sand stratum: (a) lateralview; (b) section showing the saturated and partially saturated domains.Numerical modellingTo qualitatively reproduce and interpret the evolution ofthe hydraulic system of the entire slope, more numericalanalyses were performed using the SEEP/W computer program(see Fig. 12). The characteristic suction – water contentcurves for the different materials, as well as thedependency of the hydraulic conductivity on the water content,were taken as shown in Fig. 10. We must observe thatthe characteristic curve, which links the saturation index andthe suction pressure, is assumed to be reversible. A parametricstudy was performed by changing the dimensions and positionof both superficial incisions and tension cracks and© 1999 NRC Canada
1070 Can. J. Geotech. Vol. 36, 1999Fig. 14. Sketch showing the shape of the saturation bulb under a point water source as observed during the sand box experiments. Thebroken lines show the limits of the seepage zone on the slope. gwt, groundwater table.the initial distribution of the piezometric head. The lowerboundary of the domain was assumed to coincide with thelower water table shown in Fig. 7. It can be concluded thatthe presence of the 20–30 cm deep incisions allow the waterto infiltrate into the lower soil layers (Fig. 12a). Withoutthese incisions the seasonal rice field flooding could not explainthe saturation conditions of the slope.Moreover, simulations show that if tension cracks withinthe consistent grey silt were absent, the water would flowonly within the upper more permeable layer and would notaffect the lower sandy layer. Then, only a superficial erosionwould be possible and not the deep-seated failure mechanismas observed.In the following the seepage within the layered slope willbe discussed by taking into consideration both superficial incisionsand tension cracks as illustrated in Fig. 12. The infiltrationfront develops with time. Its shape, in particular withinthe upper part of the slope, is influenced by the sequence oflayers with different permeabilities, as was also reported byRulon and Freeze (1985). The toe of the slope is reached bythe saturation front almost simultaneously with the establishmentof steady flow conditions. A parametric study has beenperformed to quantitatively describe the influence of floodingof the rice field on the saturation of soil layers. The computedtime period necessary for saturation of the lower sandylayer was comparable to the annual flooding time (about4 months). Nevertheless, this time interval cannot be quantitativelyrevealing because of (i) the imposed 2D geometricalconditions, and (ii) the difficulty in defining the permeabilityversus saturation ratio curve and its hysteretic behaviour.When the water flow reached the base of the slope(Fig. 12b), erosion at the toe of the slope took place. Thewater flow within the slope was made more rapid and thetunnel scouring in the upper soil strata more catastrophic.Evidently, only if the seepage is sufficiently developedwithin the slope does the phenomenon becomes self-feedingand instability occurs.Physical modellingTo qualitatively describe the shape of the saturated domain,when the water source is restricted, some simple laboratory-scalemodel tests were performed. The model slopeswere reproduced within a sand box measuring 80 × 30 ×45 cm and having two glass windows. The slope model wascreated by pluviating dry homogeneous Hostun RF sand toachieve a relative density Dr of 40%. The lower boundarywas kept saturated during the test, whereas the sand waskept dry initially. A perched water table was established onthe top by placing a small basin on the top of the modelslope with a hole at its base to induce a localized seepagesource.The shape of the saturated domain is evident in Fig. 13. Alateral view of the slope is illustrated in Fig. 13a, and the finalsection of the partially saturated bulb is shown inFig. 13b. This was obtained by brushing away the dry sandat the end of the test. Even though the deposit was homogeneous,the material was initially not partially saturated, andno tension cracks or tunnel scouring were observed, the experimentalobservations were useful in describing the 3Dshape of the seepage domain. The initial and final positions© 1999 NRC Canada
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