Deep Mixing Field Trials for Land Remediation: Some 3-Year Results from the SMiRT Project

"In-situ remediation of contaminated land with deep mixing can enable a multi-technique remedial strategy on the same site combining low permeability barriers, permeable reactive barriers, stabilisation/ solidification treatments as well as ground improvement, delivering significant advantages over alternative strategies. This was the core objective of Project SMiRT (Soil Mix Remediation Technology) which involved the execution of the largest research field trials in the UK to date to provide field validation for this technique. Around 800m3 of soil were treated on a contaminated site in Yorkshire in May 2011 with 6 different treatments, testing ~40 binder compositions and 10 different installation methodologies. Field monitoring, testing and coring activities at the site took place within the first 2 months after treatment and then again at 34 months after treatment. This paper presents an overview of the field activities to date and some 3-year results and assessment of a subset of the treated soil sections. Some good correlations are seen together with some clear differences between treatments. There is generally an observed consistency with depth, with the mixing effectiveness having a more profound effect than the binder composition or content, within the tested materials and ranges. These field trial activities are currently being used in the development of the full-scale commercial remedial strategy for the site.INTRODUCTION AND OVERVIEW OF PROJECT SMiRTWhile there are strong financial and environmental incentives to remediate and redevelop brownfield sites there are also barriers including fear of the unknown, regulatory control, delays, increased costs and the stigma associated with such sites. Hence the remediation process needs to be conducted in such a way to maximise the incentives and overcome the barriers, which is how deep mixing found its niche in the land remediation market. Soil mix technology is such a versatile technique that can perform a number of remediation treatments on the same site. These include low permeability in-ground barriers (cut-off walls), permeable reactive barriers (PRB), stabilisation/solidification (S/S) treatments, other additive related remediation, e.g. in-situ chemical oxidation and bioremediation, as well as soil amendments for greening and also ground improvement all on the same site (Al-Tabbaa et al, 2011). Placed in the context of alternative solutions, soil mix technology could potentially replace the five techniques shown in Figure 1, which would be required to deliver the same treatments mentioned above. Treatment trains strategy, where a combination of techniques in succession are used to both reduce the contamination levels and deal with residual contamination, is becoming common in land remediation and the use of one set of equipment to deliver this would bring about significant financial advantages. In addition, soil mix technology has many other advantages in terms of speed of implementation, elimination of exposure to contamination, elimination of off-site disposal and low cost. Soil mix technology for the remediation of contaminated land emerged in the UK in the mid 1980s and has since been employed in over 30 or so projects, which far exceeds its application to ground improvement for which only a handful of jobs have been carried out to date."