Experimental and Numerical Investigations of a River Embankment Model under Transient Seepage Conditions

The evaluation of riverbank stability often represents an underrated problem in engineering practice, but is also a topical geotechnical research issue. In fact, it is certainly true that soil water content and pore water pressure distributions in the riverbank materials vary with time, due to the changeable effects of hydrometric and climatic boundary conditions, strongly influencing the bank stability conditions. Nonetheless, the assessment of hydraulic and mechanical behavior of embankments are currently performed under the simplified hypothesis of steady-state seepage, generally neglecting the unsaturated soil related issues. In this paper, a comprehensive procedure for properly defining the key aspects of the problem is presented and, in particular, the soil characterization in partially saturated conditions of a suitably compacted mixture of sand and finer material, typical of flood embankments of the main river Po tributaries (Italy), is reported. The laboratory results have then been considered for modelling the embankment performance under transient seepage and following a set of possible hydrometric peaks. The outcome of the present contribution may provide meaningful geotechnical insights, for practitioners and researchers, in the flood risk assessment of river embankments.

Isoparametric Element Analysis of Two-Dimensional Unsaturated Transient Seepage

A FEM for unsaturated transient seepage is established by using a quadrilateral isoparametric element, considering the fact that the main permeability does not coincide with the axis situation. It creates a function by using the element’s node hydraulic head and shape function instead of the real head in the Richard seepage control equation. With the help of the Galerkin weighted residual method, a FEM equation is given for analyzing 2-dimensional transient seepage problem. Further, based on the Jacobi matrix and Gauss numerical integral, it determines the elements of stiffness and capacitance matrices. This FEM equation considers not only the anisotropic of soil but also the uncoincidence between permeability and the axis. It is a common form of transient seepage. In the end, two examples illustrate the node accuracy of the quadrilateral element and the correctness of this FEM equation.

The importance of accounting vegetation on unsaturated slopes prone to rainfall-induced instabilities – a case study in Norway

<p>In unsaturated steep slopes, the upper unsaturated zone may have a crucial role in the slope stability. In this work we studied a natural slope located adjacent to a railway track in Eastern Norway. Due to its steep inclination, the factor of safety should be always kept under observation. In addition, the climate in Norway is expected to become wetter and warmer, with increased snow melting. Thus the rainfall/snowmelt infiltration play an important role and needs to be monitored properly to avoid any failure. The slope is instrumented since 2016 and both volumetric water content (VWC) and the pore water pressure regime are monitored.</p><p>The modulus SEEP/W of the commercial software GeoStudio (GEO-SLOPE International, Ltd.) was used to model the transient seepage conditions of the slope for a 7-month monitoring period (from June 2019 to December 2019). Several analyses were carried out by changing the initial conditions and the boundary climate conditions of the slope. Regarding the initial conditions, two series of simulations were carried out, one with an initial calibration of the VWC distribution, another one without calibration, hence, by only locating the ground water table at a specific depth and by indicating the maximum negative head (as required by the model). The calibration, instead, consisted in starting the simulation considering a VWC distribution as closer as possible to the in-situ value.</p><p>For each series, a total of three simulations were carried out with different boundary climate conditions, respectively considering only rainfall/snowmelt (R), considering both rainfall/snowmelt and evaporation (Cl), and considering rainfall/snowmelt and evapotranspiration due to vegetation (V). Indeed, the slope is all covered by relatively dense vegetation, with shrubs and birch trees. For the simulations including evaporation and vegetation (Cl and V), the land-climate interaction boundary condition was adopted. Climate functions, such as the pairs temperature-time, relative humidity-time, wind speed-time, were obtained from a close meteorological station. The evaporation was determined by using the Penman-Monteith equation, including vegetation features in the case of vegetated slope. Preliminary results show that the initial calibration is important for the correct back-analyses of the measured data, and that the model is more accurate when accounting for climate boundary conditions and vegetation, which influence also the slope stability conditions.</p><div>The work has been part of the working package "Landslide triggered by hydro-meteorological processes" within the Center for Research-based Innovation (CRI) programme KLIMA2050 (), financed by the Research Council of Norway.</div>

Moisture Migration and Control of New Embankment for Reconstruction and Expansion Project in Southern China

In hot and humid regions of southern China, the volumetric moisture content of the embankment after opening to traffic for a period of time reaches a stable state, and it is higher than the design value. When it was widened, the humidity gradient and exchange were formed due to the difference in moisture content between the existing and new embankment. To reveal the moisture migration of the existing and new embankment and control the rise of volumetric moisture content in new embankment, six frequency domain reflectometry sensors were installed in existing and new embankment to monitor the volumetric moisture content. A finite element model for the embankment was established and verified with the measured data. And seven numerical analyses of transient seepage in the new embankment of the cushion, cover, and partition using capillary barrier by sand were simulated. The results show that the volumetric moisture contents of the new embankment in southern China gradually increase and eventually reach an equilibrium state. The increase in water comes from the slope, the foundation, and the existing embankment. Early in the first 1∼2 years, the water mainly comes from the foundation and the existing embankment. After that, as time goes by, the water comes mostly from the slope infiltration and gradually migrates to the foundation and the existing embankment. Finally, the volumetric moisture content and the water storage gradually reach equilibrium. The volumetric moisture content of the new embankment using capillary barrier by sand at the cushion, the cover, and the partition is maintained as the construction volumetric moisture content. This combination is a very effective method to control the humidity stability of the new embankment in southern China.