Crosta and di Prisco 1057Fig. 1. Sketches of possible mechanisms at the origin of sapping, tunnel scouring, and <strong>seepage</strong> <strong>erosion</strong> according to definitionscommonly found in the literature.© 1999 NRC Canada
1058 Can. J. Geotech. Vol. 36, 1999frictional strength, or <strong>by</strong> the shear failure of the continuum(Fig. 1). The latter is likely on steep <strong>slope</strong>s where <strong>seepage</strong><strong>erosion</strong> may trigger or evolve into mass movement. Seepage<strong>erosion</strong> can advance easily in soils (Jones 1990) with macroporesalong which water flow occurs and where a conspicuouswater supply (intermittent or continuous) and wellconcentratedwater infiltrations take place.Tunnel scour occurs when water enters a hole or crackwithin or at the surface of a soil deposit. The original conduit(Fig. 1) may be due to biogenic factors or tensile crackingalong the margins of gullies and landslide blocks,shrinkage in clay-rich soils, <strong>seepage</strong> <strong>erosion</strong>, or human activity.This paper illustrates a case history from the Po Plain ofnorthern Italy which is characterized <strong>by</strong> the following phenomena:<strong>seepage</strong> <strong>erosion</strong>, tunnel scouring, landslides, andliquefaction processes. The difficulty of interpreting the phenomenonis due to (i) its complex time sequence, (ii) itsthree-dimensional (3D) geometry, (iii) the unsaturated conditionsof collapsed materials, and (iv) liquefaction of the soilstrata. To clarify the phenomenon, the authors used standardgeotechnical design methods. Numerical programs and physicalmodels have been used to study flow localizationthrough unsaturated soils and to interpret the occurrence ofthe failure mechanism and its particular geometrical shape.Case historyGeological and geomorphological settingThis paper presents a case history of a failure which occurredat Groppello in the central Po Plain near the town ofPavia (Lombardy, Fig. 2) and the confluence of the Ticinoand Po rivers. In this area the boundaries of the Ticino Riverterraces are oriented in a east–west direction. The depositsforming the terraces are fluvial sediments from the recentPleistocene “diluvium,” mainly consisting of interlayeredsand, silt, and clay strata with occasional gravel. The presentTicino River bed is characterized <strong>by</strong> alluvial sandy gravelfrom the intermediate Holocene “alluvium.”The fluvial terraces exhibit flat top surfaces. The upperedge of the alluvial terrace is located at 89 m above sea level(asl), coinciding with the general elevation of the Po Plain.The terrace is bounded <strong>by</strong> a 20 m high, densely vegetated<strong>slope</strong> with an average inclination of 35° (Figs. 3a, 4). Thetoe of the <strong>slope</strong> is located on the edge of a lower alluvial terrace,at an elevation of 69 m as1. Both terraces are characterized<strong>by</strong> extremely low gradients, generally less than 1%.The upper terrace is used for rice and corn crops. A ricefield is located close to the <strong>slope</strong> failure in which a waterlevel of 20 cm depth was maintained at the time of the event<strong>by</strong> a system of interconnected ponds. Small concavitiesprobably representing the trace of old slides or <strong>erosion</strong> featurescan be observed.The original morphology has been slightly reworked <strong>by</strong>human activity. Agriculture is among the most important activitiesin the area. The commonly encountered rice fieldsare filled for 3 months per year but there are also corn crops.Description of the phenomenaThe Groppello <strong>slope</strong> failure took place between June 27and 28, 1993, along the upper terrace of the west bank of theFig. 2. Location map of the studied areas.Ticino River (Fig. 2). Interest in the phenomenon was due tothe presence of a pipeline running from Genoa to Milan.Economic damage to the rice crops was low and therefore ofsecondary importance.Rainfall was very limited during June 1993, even thoughprecipitation occurred just before the event. Moderate rainfallrecorded on June 23 and 24 (15 mm at Bereguardo,31 mm at Mortara, and 49.2 mm at Pavia) had a return periodof less than 2 years according to the statisticalGumbel’s method.The <strong>slope</strong> failure resulted in the formation of a bottleneckshapedgully (Figs. 3, 4). The maximum depth was about10 m, with a subcircular depletion area subdivided <strong>by</strong> narrowridges into three small secondary basins (Fig. 3a, 4).The intermediate zone was characterized <strong>by</strong> a narrow V-shaped channel (Fig. 3b) which started almost tangential tothe upper depletion area. The channel was almost 25 m long,with steep flanks (40°) and a longitudinal profile with an averageinclination of 6°, varying between 3° and 18°.At the base a large symmetrical cone almost 100 m inlength had formed (Fig. 4). The average cone inclinationranged between 1° and 3° (light grey shaded area in Fig. 4);the steeper apex (6°–8°; dark grey shaded area in Fig. 4) extendedup to 23 m from the channel outlet. The cone wascharacterized <strong>by</strong> a maximum width of 80 m at a distance of20 m from the channel outlet, along a preexisting ditch. Theaverage width was 60 m. Reeds growing on the lower terracewere only obliterated within 30 m of the channel outlet,while farther away no disturbance was observed.The existing pipeline, placed at an average depth of0.95 cm below the upper terrace tread, is subparallel to thechannel axis. It is about 7 m from the depletion area and almost45 m from the channel at the base of the <strong>slope</strong> (Fig. 4).The geometric and morphological features of theGroppello <strong>slope</strong> failure are similar to those of the rapidevent which occurred at Sairano in 1980 (Fig. 2). This eventoccurred along a vegetated terrace <strong>slope</strong> with a rice field atthe top and involved a 1.5 m deep pipeline. The terrace was10 m high with an average <strong>slope</strong> of 32° (Figs. 5, 6a). Thefailure surface had a maximum length of 50 m and amaximum depth of 7 m (Fig. 5). The narrow and elongatedchannel had an average <strong>slope</strong> ranging between 4° and 6°.Unlike the previous case, the pipeline was parallel to the<strong>slope</strong> and directly affected <strong>by</strong> the depletion zone. The Sairano© 1999 NRC Canada
Change language