Keywords: clay, erosion, flow, hydraulic conductivity, microstructure, porewater, smectite.

Top liners of landfills of hazardous waste are often made of clay covered by erosion-protecting coarse-grained soil (silt to crushed rock). By proper granular and mineralogical composition percolation of the liner can be so designed that wetting/drying is confined to the liner and leaves the underlying waste nearly dry for very long periods of time. This can be achieved by using smectitic clay compacted to sufficiently high density. The microstructural constitution is heterogeneous, which causes significant variation in the rate and distribution of porewater flow that takes place under the prevailing hydraulic gradient. High flow rates can cause local erosion of fully water-saturated smectite clay and the perspectives from physical models and experiments show that a rate of E-3 m/s is sufficient to produce dislodgement of particles with a size of 0.5 µm, E-4 m/s for 1 µm particles, E-5 m/s for 10 µm particle aggregates, the order being determined by the number of interparticle bonds and particle weight. The porewater flow in a water saturated element of smectite clay takes place channel-wise, which is validated by experiments, but numerical modelling has indicated that the average hydraulic conductivity is significantly higher than indicated by experiments conducted under commonly used high hydraulic gradients. Detailed numerical modelling of flow on the microstructural level using the finite element method and assuming a hydraulic gradient of 1 m/m (meter water head per meter flow length) has given the distribution of flow rate in vector form. It shows that the flow rate in several micrometer wide voids is of the order of 2E-6 m/s for this gradient of 1 and increases to 2E-4 m/s when the gradient is raised to the commonly used gradient 10,000 m/m. This flow rate disrupts soft clay gels in the channels and causes migration of small particle aggregates, which can cause clogging of the most narrow parts. In practice, the hydraulic gradient rarely exceeds 1-5 m/m, which implies much lower flow rates and no risk for erosion and clogging. Experiments with permeation under a hydraulic gradient of 30m/m that was increased to 9,000 m/m after reaching steady state flow have verified that the evaluated hydraulic conductivity may rise by two orders of magnitude.

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