Soil compressibility in transient unsaturated seepage analyses

Most levee underseepage and uplift analyses are based on steady-state seepage and can yield conservative results. Although computations are simpler and steady-state seepage parameters are easier to determine and readily available, transient unsaturated seepage analyses are more representative of levee seepage conditions because boundary conditions acting on the levee or floodwall and saturation change with time, which induce pore-water pressure and seepage changes with time in the embankment and foundation strata. In addition, these boundary conditions, e.g., flood surge or storm event, are rapid such that steady-state conditions may not have time to develop in the embankment and some foundation materials. Transient seepage analyses using a floodwall case study indicate that as soil compressibility of the underseepage layer decreases, rapid landside pore-water pressures increase and can approach steady-state values. The transient results also indicate that uplift factors of safety during the flood event are about 22% higher than those at steady state. The effect of soil compressibility can delay or accelerate the onset of uplift water pressure increase from the initial steady-state conditions.

Influence of infiltration on the periodic re-activation of slow movements in an overconsolidated clay slope

In Orvieto (central Italy), overconsolidated clay slopes are affected by intermittent slow movements at the top of the clay formation and within the landslide debris cover. Monthly data from inclinometers, Casagrande piezometers, and rainfall gauges show that velocity, pore pressure, and rainfall are closely related. A relationship is suggested to predict slope re-activation using rainfall history alone, once a pore pressure threshold has been reached and response of pore pressures to rainfall is understood. Pore pressures have been continuously monitored through vibrating wire cells. The threshold for shallow movements, critical for infrastructures and buildings, was identified by comparing displacement histories of a shallow movement, representative of many other ones recognized over the slope, and pore pressure, both measured at the centre of the sliding mass. The impact of infiltrated rainfall on groundwater flow was investigated through transient seepage analyses. Seepage analyses performed using hydraulic properties from laboratory and in situ tests do not fully depict the observed pore pressures because field data miss some structural characters and lithologic variations. The hydraulic properties of the shallower model layers were refined, based on field observations and interpretation of monitoring data, to have a good match between computed and measured pore pressures. Once the model is tested at different locations along a slope, it could be used to predict movement re-activation using only rainfall data.