Context
In recent years, several environmental issues linked to the use of soils in civil engineering have been raised. For example, in infrastructure projects about 30% of the total cost is due to the handling and hauling of both excavated soil and construction materials from quarries [1]. According to the official statistics from APAT (Agency for the Environmental Protection and Technical Services, Italy), until 2008, more than 80% of inert waste in Italy is landfilled, resulting in significant amounts of energy consumption and CO2 emissions. Beside this, the environmental problem due to the disposal of dredged sediments should be addressed. In fact, about 150 million m3 of contaminated dredged sediments are produced yearly in Europe [2]. In Italy, a total amount of 120 million cubic metre should be dredged in the main Italian harbors for their navigability. A good environmental policy should be oriented to maximize the reuse of soils. Studies indicate that saving potentials for re-use of excavated soils and rocks saves up to 14 kg CO2 per ton and 250 MJ/ton of non-renewable energy [3] as transport, landfilling, and collection from quarries are reduced. Moreover, the European Community since 1972 through Oslo and London Protocols and Directive 2008/98/CE defined dredged sediments as “waste” only if their reuse is proved not to be possible.
Although this environmental awareness pushes for improving reuse of soils, engineering applications require high-quality characteristics. In this respect, natural soils are often characterized by low mechanical properties, or may sometimes be contaminated (it could be the case of dredged sediments), making their reuse not possible without their improvement. In this context, geotechnical engineering can contribute to reach the soil reuse goal by finalizing suitable methods for improving their hydraulic and physico-mechanical properties. One of the possible answers to this need is the stabilization of soils with traditional binders, such as lime or cement.
Lime can improve the properties of fine-grained soils to obtain proper mechanical characteristics for earthen structures [4,5]. The addition of quicklime to a clayey soil-water system improves soil workability by reducing plasticity and swelling potential, increases hydraulic conductivity and improves strength, compressibility, and durability (e.g. [6 ,7,29]). Conflicting opinions have been found in literature about times, sequence, and modes of reactions [5,8]. Lime also revealed a good potential for the treatment of dredged sediments because, besides the geotechnical improvements [9,10], in case of contamination, the stabilization/solidification treatment by lime is capable to immobilize inorganic contaminants [11,12,13] but the performance in a marine environment (rich of sulfates) need to be further investigated [14].
Cement can be also used to stabilize and improve the properties of a wider range of soils, indeed the hydration of the constituents of cement causes the set and the hardening with time, due to pozzolanic reactions. Sandy soil can be always successfully treated with cement, whereas silty and clayey soil may result in systems with excessive shrinkage properties, sensitivity to weather conditions, requiring a too high percentage of binder to be stabilized [5]. The effects of cement addition on the characteristics of soils include modifications in the stress-strain behaviour [15,16], effectiveness in terms of resistance against chemical attack [17] and reduction in compressibility and plasticity [18]. Cement can also mix with bentonite to induce an increment of the strength of soils and reduction of their hydraulic conductivity. The mechanical performance [27], chemical compatibility [28], and hydraulic behavior [25,26] are the relevant aspects of these cement-bentonite mixtures that are often used to build slurry cutoff walls for containing pollutants within contaminated areas.
Several studies [e.g. 6,19,20,21,30,31] have highlighted that a lot of factors can affect mechanical, hydraulic, and microstructural characteristics of the soil-binder mixtures, such as binder type and amount, curing time and curing conditions, characteristics of the soil to be treated, compaction water content. Moreover, the combination of different stabilizing agents (e.g. lime with high alkali and slag cement or fly ashes) can improve geotechnical properties of soils [22] and dredged sediments too, making them suitable for large fill applications [e.g.23]. Fly ash and blast furnace slag are promising components of the mix design to limit the detrimental effect of sulfate-bearing soils [e.g. 24].
The research aims to further enhance the sustainable feature of soil stabilization technique, by focusing on both the binder types, mainly by-product from industry, and the material to be reused.
References
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