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Contaminant migration and immobilisation in stratified sands by Abir Al-Tabbaa, Cambridge University and J M Ayotamuno and R J Martin, Birmingham University

Most sites, whether consisting of natural soils or made ground, have heterogeneities. These are a major concern in the understanding of contaminant transport mechanisms and in the selection and success of remedial measures.

For this reason, the numerous complex factors related to site heterogeneities in terms of soil, contaminant and flow conditions have become a priority in research on contaminated land.

The complexity of the effects of heterogeneities means that simplified problems need to be addressed so that the effect of individual factors can be established. Some work has been reported on the transport mechanisms of contaminant species in various heterogeneous site configurations but very little has been reported on the effectiveness of various remedial measures.

The work reported here looks at the simplified problem of stratified sands. It investigates the migration of a single contaminant through the stratifications and then considers remediation of such heterogeneous soil using solidification/ stabilisation by soil mixing.

In situ solidification/ stabilisation of contaminated ground using soil mixing has recently emerged in the UK. Advan-tages of this technique are that it is cost effective and allows con-struction work to begin shortly after treatment, so research to validate this technology is timely.

Three sands with different particle size distributions and one conservative solute, sodium chloride, were used. The stratification configurations investigated were perpendicular, parallel and inclined to the main flow direction.

The experimental work was carried out in a laboratory-scale tank 0.45m long, 0.38m wide and 0.25m high. The solute flow conditions used were one-dimensional; either through all stratification or through the middle stratum only.

The first part of the project examined the transport of sodium chloride solution through the various sand stratifications and under the two different flow conditions in terms of breakthrough curves, concentration isochrones and dispersion parameters. Results were compared with those under homogenous configurations.

The porosity of the sands and the flow velocity in all the tests was kept constant so that the effect of the stratifications was examined purely in terms of the dispersion properties of the different sands. The work aims to relate the concentration distribution to the dispersivity and permeability of the sands, the stratification configurations and their inclination to the flow direction.

The second part of the project looked at remediation of horizontally stratified sands using in situ soil mixing with the use of laboratory- scale augers (roughly one-tenth scale of commercial augers) and cement- based grouts.

Two different auger designs were compared and the set-up of one of the augers is shown

in Figure 1. The effect of variables such as the sand layer thickness, degree of saturation, level of contamination, type of grout and the number of mixing cycles on the resulting degree of mixing of the soil layers and the physical integrity of the resulting column were investigated.

The constructed columns of layered sands (Figure 2) were allowed to cure for seven and 28 days before extrusion and testing for unconfined compressive strength, leachability, leachate pH and physical integrity. Each column was divided into two or three samples to examine any variability in the behaviour with depth. Manually mixed soil-grout samples were also prepared using the individual sands for comparison with the auger-mixed samples. The results will show how effective the augers and applied grouts are in the treatment of stratified sands. This will provide a correlation between initial laboratory treatability study and subsequent in situ application.

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