SAFE is short for Sustainable calcium Abstraction Fixation Engineering and is a project funded by the European Commission (EU SATURN Action 2 Lot 16b).
The SAFE project aims to improve the understanding of the implications of interactions between the lithosphere, atmosphere, and anthroposphere (in form of carbon sequestration) on soil young re-precipitated carbonates polymorph type and stability in urban settings. We explore how formation of modern-day collapsible soils relates to interactions between the three environmental spheres, with emphasis on loose calcareous loamy soils.
Three polymorphs of authigenic and anthropogenic carbonates are experimentally synthesised in artificial loam soil samples through carbon sequestration. Micromorphological, geochemical, mechanical and hydromechanical properties of these samples are then determined at natural and weathered states. The overarching hypothesis here is that the detection of the collapse risk can improve if calcareous soils – that may have a scope for mineral dissolution – are given a simple diagnostic geochemical test.
The construction rubble (e.g. cement and gypsum from crushed concrete, plaster and stucco) in soil is a source of Ca/Mg silicates or hydroxides. The Ca2+ cations from these silicates gradually interact with carbon dioxide in soil and form secondary carbonates. Soils mixed with rubble are highly basic (soil solution pH ranging between 6 to 8) and broadly abundant in urban settings. The highly basic environment is however jeopardises the stability of young re-precipitated Ca2+ minerals. Some workers use the term “anthropo-calcretisation in urban regions” and suggested that these human-induced calcretes contain calcitic coatings and infills and needle-fibre calcites, ranging from 1-4µm (micrite) to 15-40µm (sparite) and more. Natural pedogenic carbonates can also dissolve and re-precipitate into metastable carbonates. Pedogenic carbonates are abundant in shallow soils and occur particularly near root structures, inferring their close association with evapotranspiration during prolonged dry climates. Rapid dissolution of carbonates leads to soil structural collapse and rapid change in stress state. When calcareous soils are overlying shallow cavities, that sudden collapse can trigger larger scale instabilities. We recently published on a general tendency of silty and loamy soils for heaving following seasonal wetting-drying. This leads to a gradual increase in soil’s global void ratio, easier carbon circulation and precipitation pf secondary carbonates, which then leads to further expansion of the packing and a an increase in collapse potential.
Because of carbon sequestration through silicates and carbonates in urban ground, it is estimated that 290 Mt a-1 of carbon is captured. This is arguably equivalent to 12.5 kg C a-1 per every ton or 8.5 kg C a-1 per every metre cube within the top 1m urban soil. The uncertainties around stability of secondary carbonates however prevents any robust evaluation of the the rate at which carbon is stored in urban ground upon sequestration into stable carbonates. Little is known about links between soil pH, pCO2 (i.e. carbon dioxide partial pressure), and dissolved inorganic carbon DIC fluxes, and implications of anthropogenic driven carbon sequestration.
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Assadi-Langroudi, A., Ng’ambi, S. and Smalley, I., 2018. Loess as a collapsible soil: Some basic particle packing aspects. Quaternary International, 469, pp.20-29.
Assadi-Langroudi, A. and Jefferson, I., 2013. Collapsibility in calcareous clayey loess: a factor of stress-hydraulic history. Int J Geomate Geotech Constr Mater Environ, 5(1), pp.620-626.
The early detection of ground collapsibility in urban settings would improve if all calcareous urban soils suspected of having a scope for mineral dissolution are given a simple diagnostic test. Sudden hydro-collapse is generally believed likely in loose cemented granular soils and shallow drift deposits that overly subsurface cavities or abandoned disused mines. For the likelihood of hazard to be measured, these are first given a geophysical survey, followed by direct Single or Double Oedometer Tests (SOT/DOT) or in some cases, a field wetted Plate Load test. Geophysical mappings (including the more advanced microgravity and micromagnetic techniques) are generally restricted to detecting anomalies in vertical and surficial profile data and can help to detect one or a network of subsurface cavities. Yet, these techniques are predominantly based on wave signal transmission in low-resolution environments and hence cannot be considered as definitive methods. In some cases, reliance on geophysical measurements may lead to missing the detection of ground loss risk altogether or to failure in reliably distinguishing between cavities that are surrounded by well-cemented mineral particles and those surrounded by particles bound by readily soluble minerals. The SAFE project has looked into the mechanics of calcareous geomaterials. The team has experimentally simulated the formation of an array of natural and anthropogenic secondary carbonate. Findings offer a new insight into possible links between hydromechanical properties of carbonates in soil and the environment in which they crystallise. Through controlled carbon sequestration (near 25 °C and 1 atm pressure) of different carbonate polymorphs, two geochemical state parameters are found to have a major influence on stablity of young carbonates.