Series Of Infiltration Tests Research Articles

Effects of technological voids and hydration time on the hydro-mechanical behaviour of compacted bentonite/claystone mixture

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a promising sealing/backfilling material in deep geological repository for high-level radioactive waste in France. When the pre-compacted blocks are emplaced in the gallery, technological voids can remain. After the infiltration of groundwater from the host rock, they will swell freely, filling the technological voids, and then undergo further hydration under constant-volume conditions. From the perspective of the storage safety, it is essential to understand the effects of technological voids and hydration time on the hydro-mechanical behaviour of such pre-compacted blocks. In this work, a series of infiltration tests at various hydration times was carried out on compacted MX80 bentonite/COx claystone mixture with different technological voids. The evolutions of the swelling pressures in axial and radial directions as well as the hydraulic conductivity were monitored while wetting. After the predetermined hydration times, the dry density, water content and suction at different positions were determined, together with the microstructure investigation using mercury intrusion porosimetry. It was observed that the soils close to initial voids swelled and filled the initial voids upon contact with water, with significant increases in large-pore and medium-pore void ratios. From the variation of dry density profile with time, compression and swelling zones could be identified: in the compression zone, the soils corresponding to the initial soils were subjected to compression, with decrease in large-pore and medium-pore void ratios over time, whereas in the swelling zone, the soils with a higher dry density than the expected final one underwent further swelling, with large-pore void ratio increasing until saturation and then a slight decrease due to water redistribution in the soil. Owing to the soil density heterogeneity, the axial swelling pressure and hydraulic conductivity of samples with voids were slightly larger than those of samples without voids; contrarily, the radial swelling pressure was lower than that of the samples without voids.

Effective Hydraulic Properties Determined from Transient Unsaturated Flow in Anisotropic Soils

An important requirement for accurate modeling of field‐scale unsaturated transport using local‐scale parameters is a method to account for multiscale heterogeneity and the connectivity of the facies that control flow. A series of infiltration tests were conducted at the Hanford Site along a 60‐m‐long transect, instrumented at 1‐m intervals to measure water content (θ) and capillary pressure head (h) to quantify the spatial correlation structure of hydraulic properties and assess connectivity of the facies. Hydraulic parameters including the pore connectivity–tortuosity tensor (Li) were inversely estimated using the Subsurface Transport Over Multiple Phases (STOMP) numerical simulator coupled with the parameter estimation code, UCODE. Results show that six of eight parameters required for a modified van Genuchten–Mualem model could be inversely estimated using θ measured during transient infiltration from a surface line source and approximated prior information. Soils show evidence of saturation‐dependent anisotropy that was well described with the connectivity tensor. Variability of the vertical saturated hydraulic conductivity, Ksv, was larger than the horizontal, Ksh The autocorrelation ranges for Ksh, Ksv, the inverse of the air‐entry value, α, and the horizontal connectivity, Lh, were between 2.4 and 4.6 m, whereas the van Genuchten shape parameter, n, and saturated water content, θs, showed no autocorrelation. Accurate upscaling of hydraulic properties requires the correct assessment of the connectivity of facies.

Drying and wetting characteristics of a two-layer soil column

The drying and wetting characteristics of a two-layer soil column consisting of a 50 cm thick soil mixture over a 50 cm thick gravelly sand were investigated in this study. A series of infiltration tests were conducted in the laboratory through an infiltration column apparatus equipped with a tensiometer–transducer system, time-domain reflectometry, and water flow measurement system. Numerical analyses were performed using a finite element model to simulate water infiltration into the laboratory model under steady-state and transient conditions. The experimental data and numerical simulation results showed that static nonequilibrium profiles of pore-water pressure head were developed in the gravelly sand shortly after a drying process was started. The low unsaturated permeability of the gravelly sand resulted in a delay in downward water flow into the lower soil layer during a wetting process portion of the rainfall test.Key words: drying and wetting characteristics, soil column, infiltration, numerical analyses, soil-water characteristic curve, permeability function.

Hydro-mechanical response of a bentonite pellets/powder mixture upon infiltration

A series of infiltration tests on a 50/50% bentonite pellets/powder mixture compacted at different dry densities are reported and analysed. This mixture was proposed as sealing material in the Belgian concept for radioactive waste disposal. The tests have been performed with the French FoCa bentonite at two laboratories following analogous methodologies. The water intake, swelling pressure and axial strain have been measured during the tests. A common pattern of swelling pressure development has been found in all the tests, regardless the density of the mixture, the dimensions of the specimen, or the initial fabric, since a similar behaviour has also been found for specimens of compacted powder. At the beginning of saturation the swelling pressure increases sharply and then decreases, what gives place to an initial peak pressure. This decrease is interpreted as a collapse of the macrostructure on suction reduction. As saturation proceeds the swelling pressure increases again and eventually reaches a steady final value, what is a result of the redistribution of water towards the microstructure. The dry density and the initial water content of the mixture are two key factors affecting the particulars of this behaviour. After full saturation, the mixture becomes homogeneous with respect to water content and dry density, and the swelling pressure developed is analogous to that expected for a specimen of powder compacted at the same dry density.

Infiltration tests on a granite/bentonite mixture: Influence of water salinity

A series of infiltration tests on a 70%/30% granite/bentonite mixture compacted at a dry density of 1.70 g/cm3 are reported and analysed. This mixture is proposed as backfill material in the Swedish concept for radioactive waste disposal. The tests have been carried out at laboratory temperature for different periods of time, between 10 and 100 days, and with two kinds of infiltration water, to check the influence of the salinity on the hydration behaviour. The water intake has been measured during the tests, and the distributions of water content, dry density and soluble salts of the material have been determined at the end of the tests. It has become patent the difficulty in accurately measuring these parameters in such heterogeneous materials. The influence of the permeant salinity on the intrinsic permeability of the material has been highlighted, as well as the swelling capacity of this mixture. The ionic species distribution depends on the kind of infiltration water and the duration of the test. Dissolution, advection and diffusion are the mechanisms identified.

Infiltration Tests on Fractured Compacted Clay

Desiccation and freeze-thaw of compacted clay barriers may result in cracks that serve as preferential flow paths. A series of infiltration tests on compacted kaolin samples was conducted to explore the importance of preferential flow paths during infiltration, and their effect on the infiltration rate. Clod size at the time of compaction was found to have a strong influence on both the rate and depth of infiltration. We suggest that flow and infiltration through fractured clay may be described in terms of two stages: an initial dynamic stage in which the infiltration rate is initially high but decreases rapidly due to the clay swelling and closing fractures, and a steady-state stage usually characterized by ksat, during which the infiltration rate is relatively constant. Our study has shown that cracks do not fully heal upon hydration and readily reopen during subsequent dehydration. Infiltration rates during the dynamic stage of infiltration, while cracks are closing, are orders of magnitude higher than the steady-state rate used to estimate ksat, for barrier evaluation.

Studies of the infiltration and migration of brine in potash tailings

A series of infiltration tests were conducted on the potash tailings pile at the Lanigan Division Potash Mine in Saskatchewan. These tests included single ring infiltrometer, shallow pit, open caisson and trench type infiltration tests. The fluid retention curves for the tailings were established in the field using tensiometers and neutron probes to monitor in situ negative fluid pressures and fluid contents respectively. These field-measured curves were then compared with the laboratory-evaluated fluid retention curves.The responses of the field instrumentation during the trench infiltration test were also compared with the results of a computer simulation of infiltration, which utilized the laboratory-evaluated fluid retention curve and the calculated permeability-suction curve as input data. The good agreement obtained between the measured and simulated field responses validates the application of saturated–unsaturated flow theory to the analysis of the flow of brine within the unsaturated tailings. It also verifies the accuracy of the fluid retention curve determined in the laboratory as well as the calculated permeability-suction curve. Key words: potash tailings, infiltration testing, single ring infiltrometer, tensiometer, neutron probe, saturated–unsaturated flow, fluid retention curve, permeability-suction curve, finite-element flow model.