On slope instability induced by seepage erosion - Ingegneria ...

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Crosta and di Prisco 1059Fig. 3. Photographs of the Groppello area after the slope failure. (a) Aerial view. (b) Outward view of the V-shaped gully. (c) Scaroriginated as a consequence of the 1993 event at Groppello and view of the soil profile with a large conduit (pipe). The location wherea tractor wheel track cut through the impervious bottom of the rice field can be seen just above the pipe. The pipe is located close totheDPSH1siteinFig.4.(d) Two superficial incisions above pipes and about 2 m apart close to the DPSH 1 site.event was very rapid, as indicated by damage to cars andbuildings at the toe of the slope (Fig. 6b).Soil stratigraphyStratigraphy, thickness, and lateral extent of the soil strataare the main controlling factors with regard to piping andsoil liquefaction phenomena. Therefore, particular attentionwas paid to obtain a precise description of soil features andtheir changes along the profile. The stratigraphy exposedalong the main scarp caused by the slope failure was subsequentlychecked by one vertical sounding (Fig. 4) and twodynamic probing super heavy penetrometric tests (Meardi’s© 1999 NRC Canada
1060 Can. J. Geotech. Vol. 36, 1999Fig. 4. Sketch representing the geometry of the Groppello event, with location of in situ tests, the main erosion pipes, and the pipeline.DPSH, Meardi dynamical probing super heavy penetrometer.DPSH; weight 73.5 kg, penetration length 30 cm, diameter34 mm).Figures 7 and 8 show a representative stratigraphic profileof the site which summarizes the information obtained fromfield and borehole survey and laboratory tests. The observedsequence, starting from the surface, consists of alluvial depositswith a wide range of grain sizes (Fig. 9a).A30cmthick layer of organic topsoil forms a seal at the surface ofthe rice field. The underlying 70 cm sandy layer consists ofa finer grained upper half, with ochre and brownish oxidation,and a lower half which is colourless. The colourlesszones demonstrate the occurrence of water seepage throughthem. Below this stiff silty layer, cross-laminated cleansands are present with subangular to subrounded quartz andfeldspar grains, with subordinate micas (Figs. 9a, 9b).Within the channel, close to the terrace edge, the strata previouslyidentified in the depletion zone are substituted by alight brown silty sand soil, possibly a result of erosion andweathering of the previously described materials.Some stratigraphic observations were conducted withinthe accumulation zone along a small trench. Bedding wasmore evident in the upper part of the deposit, gently dippingin the direction of flow as indicated by alternating fine andslightly coarser sand layers.Geotechnical characterizationSampling and soundingA series of disturbed and undisturbed soil samples wascollected along the main failure scarp just after the event(Fig. 8). Hydraulic and geotechnical properties of the soillayers were determined and are summarized in Fig. 8 andTable 1. Three more samples have been collected from thechannel flanks and one sample from the accumulation area.The single borehole located 30 m from the scar (Fig. 4), atthe terrace edge and on the opposite side of the pipeline,shows a stratigraphic sequence similar to that in the failurezone. The groundwater table was at a depth of 20 m belowthe level of the upper terrace. In situ tests consisted of threeLefranc permeability tests within the borehole and two penetrometrictests (DPSH) (Fig. 8). The Lefranc variable-headtests (CIRIA 1986) were conducted only in the silty layers.© 1999 NRC Canada
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Page 9 and 10: © 1999 NRC CanadaTable 1. Summary

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