A simple laboratory method to simulate calcite-bonded loose-structured soil samples for collapsibility study

Abstract

Calcite is one of the commonest bond elements in natural collapsible soils. Where calcite occurs in significant amounts, the soils are considered calcareous. Collapsible soils are characterised by high porosity, high-void ratio, and low-dry density, with the soil particles held together in a honeycomb structure by a bonding agent like clay, suction or calcite. Collapse usually occurs when the bonding agent is lost through wetting, dissolution and/or stressing. Thus, understanding the behaviour of a bonding agent in the collapse phenomenon is critical to design safe and economic foundations built on collapsible soils. For a better interpretation, laboratory-simulated soils have the advantage to ensure controlled soil properties and somewhat homogeneity as against natural soils. Unfortunately, there is no standardized procedure for simulating calcite-bonded collapsible soils. A novel reaction setup developed in-house has been adopted to precipitate calcite bonding through lime gassing. Samples of silt-clay-lime mixes of differing proportions were prepared into varying dry densities to achieve collapsible soil features, and then, CO2 was passed through the mixes placed in the reaction setup. Gassing was done for 24 h. Samples were labelled A1, A2, A3, B1, B2, B3, C1, C2 and C3, where A, B and C represented silt/clay initial proportioning of 50/50, 35/65 and 20/80, respectively, while 1, 2 and 3 represented lime contents of approximately 9, 20 and 33%, respectively. After gassing, three different methods (acid–base colour indicator, calculated mass gain (CMG), and experimental determination of CaCO3 content) were used to confirm the presence or content of CaCO3 precipitate. Results revealed high levels of successes in terms of lime conversion to calcite and relative homogeneity of samples with collapsible features.