Mechanical and leaching behaviour of slag-cement and lime-activated slag stabilised/solidified contaminated soil.

Stabilisation/solidification (S/S) is an effective technique for reducing the leachability of contaminants in soils. Very few studies have investigated the use of ground granulated blast furnace slag (GGBS) for S/S treatment of contaminated soils, although it has been shown to be effective in ground improvement. This study sought to investigate the potential of GGBS activated by cement and lime for S/S treatment of a mixed contaminated soil. A sandy soil spiked with 3000mg/kg each of a cocktail of heavy metals (Cd, Ni, Zn, Cu and Pb) and 10,000mg/kg of diesel was treated with binder blends of one part hydrated lime to four parts GGBS (lime-slag), and one part cement to nine parts GGBS (slag-cement). Three binder dosages, 5, 10 and 20% (m/m) were used and contaminated soil-cement samples were compacted to their optimum water contents. The effectiveness of the treatment was assessed using unconfined compressive strength (UCS), permeability and acid neutralisation capacity (ANC) tests with determination of contaminant leachability at the different acid additions. UCS values of up to 800kPa were recorded at 28days. The lowest coefficient of permeability recorded was 5×10(-9)m/s. With up to 20% binder dosage, the leachability of the contaminants was reduced to meet relevant environmental quality standards and landfill waste acceptance criteria. The pH-dependent leachability of the metals decreased over time. The results show that GGBS activated by cement and lime would be effective in reducing the leachability of contaminants in contaminated soils.

Process envelopes for stabilisation/solidification of contaminated soil using lime-slag blend.

PURPOSE: Stabilisation/solidification (S/S) has emerged as an efficient and cost-effective technology for the treatment of contaminated soils. However, the performance of S/S-treated soils is governed by several intercorrelated variables, which complicates the optimisation of the treatment process design. Therefore, it is desirable to develop process envelopes, which define the range of operating variables that result in acceptable performance. METHODS: In this work, process envelopes were developed for S/S treatment of contaminated soil with a blend of hydrated lime (hlime) and ground granulated blast furnace slag (GGBS) as the binder (hlime/GGBS = 1:4). A sand contaminated with a mixture of heavy metals and petroleum hydrocarbons was treated with 5%, 10% and 20% binder dosages, at different water contents. The effectiveness of the treatment was assessed using unconfined compressive strength (UCS), permeability, acid neutralisation capacity and contaminant leachability with pH, at set periods. RESULTS: The UCS values obtained after 28 days of treatment were up to ∼800 kPa, which is quite low, and permeability was ∼10(-8) m/s, which is higher than might be required. However, these values might be acceptable in some scenarios. The binder significantly reduced the leachability of cadmium and nickel. With the 20% dosage, both metals met the waste acceptance criteria for inert waste landfill and relevant environmental quality standards. CONCLUSIONS: The results show that greater than 20% dosage would be required to achieve a balance of acceptable mechanical and leaching properties. Overall, the process envelopes for different performance criteria depend on the end-use of the treated material.

Relating monolithic and granular leaching from contaminated soil treated with different cementitious binders.

This work employed a clayey, silty, sandy gravel contaminated with a mixture of metals (Cd, Cu, Pb, Ni and Zn) and diesel. The contaminated soil was treated with 5 and 10% dosages of different cementitious binders. The binders include Portland cement, cement-fly ash, cement-slag and lime-slag mixtures. Monolithic leaching from the treated soils was evaluated over a 64-day period alongside granular leachability of 49- and 84-day old samples. Surface wash-off was the predominant leaching mechanism for monolithic samples. In this condition, with data from different binders and curing ages combined, granular leachability as a function of monolithic leaching generally followed degrees 4 and 6 polynomial functions. The only exception was for Cu, which followed the multistage dose-response model. The relationship between both leaching tests varied with the type of metal, curing age/residence time of monolithic samples in the leachant, and binder formulation. The results provide useful design information on the relationship between leachability of metals from monolithic forms of S/S treated soils and the ultimate leachability in the eventual breakdown of the stabilized/solidified soil.

Initial investigation into the use of GGBS-MgO in soil stabilisation

Portland cement (PC) is the most widely used binder for ground improvement. However, there are significant environmental impacts associated with its production in terms of high energy consumption and CO2 emissions. Hence, the use of industrial by-products materials or new low-carbon footprint alternative cements has been encouraged. Ground granulated blastfurnace slag (GGBS), a by-product of the steel industry, has been successfully used for such an application, usually activated with an alkali such as lime or PC. In this study the use of MgO as a novel activator for GGBS in ground improvement of soft soils is addressed and its performance was compared to the above two conventional activators as well as PC alone. The GGBS:activator ratio used in this study was 9:1. A range of tests was performed at three curing periods (7, 28 and 90 days), including unconfined compressive strength (UCS), permeability and microstructure analysis. The results show that the MgO performed as the most efficient activator yielding the highest strength and the lowest permeability indicating a very high stabilisation efficiency of the system. © 2012 American Society of Civil Engineers.

Properties of two model soils stabilized with different blends and contents of GGBS, MgO, lime, and PC

This paper addresses the use of ground granulated blast furnace slag (GGBS) and reactive magnesia (MgO) blends for soil stabilization, comparing them with GGBS-lime blends and Portland cement (PC) for enhanced technical performance. A range of tests were conducted to investigate the properties of stabilized soils, including unconfined compressive strength (UCS), permeability, and microstructural analyses by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influence of GGBS:MgO ratio, binder content, soil type, and curing period were addressed. The UCS results revealed that GGBS-MgO was more efficient than GGBS-lime as a binder for soil stabilization, with an optimum MgO content in the range of 5-20% of the blends content, varying with binder content and curing age. The 28-day UCS values of the optimum GGBS-MgO mixes were up to almost four times higher than that of corresponding PC mixes. The microstructural analyses showed the hydrotalcite was produced during the GGBS hydration activated by MgO, although the main hydration products of the GGBS-MgO stabilized soils were similar to those of PC. © 2014 American Society of Civil Engineers.

Comparisons of operating envelopes for contaminated soil stabilised/solidified with different cementitious binders

This work initiated the development of operating envelopes for stabilised/solidified contaminated soils. The operating envelopes define the range of operating variables for acceptable performance of the treated soils. The study employed a soil spiked with 3,000 mg/kg each of Cd, Cu, Pb, Ni and Zn, and 10,000 mg/kg of diesel. The binders used for treatment involved Portland cement (CEMI), pulverised fuel ash (PFA), ground granulated blast furnace slag (GGBS) and hydrated lime (hlime). The specific binder formulations were CEMI, CEMI/PFA = 1:4, CEMI/GGBS = 1:9 and hlime/GGBS = 1:4. The water contents employed ranged from 13 % to 21 % (dry weight), while binder dosages ranged from 5 % to 20 % (w/w). We monitored the stabilised/solidified soils for up to 84 days using different performance tests. The tests include unconfined compressive strength (UCS), hydraulic conductivity, acid neutralisation capacity (ANC) and pH-dependent leachability of contaminants. The water content range resulted in adequate workability of the mixes but had no significant effect on leachability of contaminants. We produced design charts, representing operating envelopes, from the results generated. The charts establish relationships between water content, binder dosage and UCS; and binder dosage, leachant pH and leachability of contaminants. The work also highlights the strengths and weaknesses of the different binder formulations. © 2013 Springer-Verlag Berlin Heidelberg.

The effect of inclined soil layers on surface vibration from underground railways using a semi-analytical approach

Ground vibration due to underground railways is a significant source of disturbance for people living or working near the subways. The numerical models used to predict vibration levels have inherent uncertainty which must be understood to give confidence in the predictions. A semi-analytical approach is developed herein to investigate the effect of soil layering on the surface vibration of a halfspace where both soil properties and layer inclination angles are varied. The study suggests that both material properties and inclination angle of the layers have significant effect ( ± 10dB) on the surface vibration response. © 2009 IOP Publishing Ltd.