Investigation into quantitative visualisation of suffusion

Abstract

Suffusion is the process whereby seepage water removes fine grains from a soil, which can result in failure of the soil body. This poses a risk for structures founded on soils that are subjected to large hydraulic gradients, such as encountered near hydraulic dams or river levees. Currently, most experimental work on this topic is geared towards quantifying, both the hydraulic gradient at which suffusion initiates, and the flux of eroded material. The reported values vary widely among experiments. The variation in the results can be explained by taking into account the effect of different experimental conditions. The flux of eroded material is the result of the interplay between particle erosion and filtration within the soil. Visualisation experiments allow for the direct observation of this. The effect of experimental conditions on the individual mechanisms of filtration and erosion, as well as the interaction between these, can be studied. Thereby, visualisation experiments complement methods targeted at quantifying the mass flux leaving the sample. In this work, the movement of fine grains and the resulting change in the structure of the sample are studied. Common laboratory equipment is used to design a visualisation experiment. The acquired images are analysed using three different quantitative image analysis techniques, with the objective of gaining further insight into the mechanism of suffusion. Particle image velocimetry (PIV) is an Eulerian method that is applied to determine velocity fields in fluid mechanics and granular flows. During suffusion, the velocity field is discontinuous; fine grains move whilst the coarse grains form a relatively fixed skeleton. This makes PIV less useful for the study of suffusion. To determine the displacement of individual particles, a Lagrangian method of particle tracking is considered. In the experimental setup used, fine grains are only tracked for a short length of time. This is due to both the large particle displacement between successive images, and the fact that other grains obscure the tracked particles from the camera. These difficulties can be remediated by improvement of the experimental procedure; the former by a higher acquisition rate, and the latter by use of a transparent granular medium where only the tracer particles are visible. With the apparatus used in this work, the temporal resolution is such that particle displacement cannot be studied unambiguously. Instead, a method of image subtraction (IS) is used that is geared towards quantifying the amount of material that moves. This yields data that can be interpreted to study both how much movement occurs, and where the movement occurs. Furthermore, IS is used to quantify the total change in the structure of the soil sample. Tests indicate that the load history plays an important role during suffusion. Erosion and filtration cause the soil structure to change, which has a direct effect on further particle transport in the sample. Therefore, the relation between three parameters: the number of moving particles, the location where they move, and the progression of the experiment, is key to understanding the process of suffusion. This is studied by plotting the movement in a 1D section of the sample over time. It can be concluded that visualisation experiments complement existing outflow experiments to study suffusion. The results of IS can be related to conceptual models that are currently used to describe erosion and filtration processes; these concepts are applicable also to the process of suffusion. Improvement of the experimental setup is required to establish whether the observations reported in this work have a general validity.Section Geo-EngineeringCivil Engineering and Geoscience