Closure to “Quantitative Estimation of Clay Mineralogy in Fine-Grained Soils” by Bhaskar Chittoori and Anand J. Puppala

Abstract

The authors’ contribution is a welcome addition to the literature. It brings out the importance of clay mineralogy in solving engineering problems connected with soil improvement techniques. The engineering behavior of fine-grained soils is controlled by the clay mineralogical composition of the soil. As discussed by the authors, soils of similar plasticity characteristics can contain different dominant clay minerals. Hence, it is correctly indicated by the authors that identical stabilization design or treatment cannot be given to soils with identical plasticity characteristics without the proper knowledge of the dominant clay minerals composing the soils. In fact, this is the major limitation of the plasticity chart used to classify finegrained soils. Hence, it is important to recognize the dominant clay mineral composition of soils in any project dealing with finegrained soils. The authors have proposed a methodology of quantifying the clay mineral composition of soils, which considers montmorillonite, illite, and kaolinite to be the major and stable clay mineral types, for which they deserve appreciation. The study of the paper under discussion reveals the following points: • The quantification of clay minerals in the soils with the available procedures such as X-ray diffraction, scanning electron microscopy, or infrared spectroscopy is not that simple to adopt. Sophisticated instrumentation and interpretation techniques are required. • The quantification of the dominant clay minerals in the soil by the method proposed by the authors requires information such as cation exchange capacity (CEC), specific surface area (SSA), and total potassium content (TP). The determination of these quantities, particularly SSA, is also not that simple. Hence, the proposed method is not user friendly. • There is no definite relationship between quantities such as CEC or SSA and the quantities of montmorillonite illite and kaolinite minerals predicted. In this context, the discussers feel that the exact percentage quantification of the clay minerals present in the soil is not required to judge the clay soil behavior. They opine that it is sufficient enough to classify the soils in to different groups based on their degree of expansivity, which is an indirect representation of the dominant clay mineralogy of soils. Sridharan and Prakash (2000) conducted a detailed analysis of the swelling capabilities of a number of soils of varying plasticity characteristics and varying clay mineralogical composition obtained from oedometer swell tests on soils, which give the most useful and reliable assessment of swell capacity of soils (Chen 1975; Winterkorn and Fang 1986) from air dry to saturated conditions under a surcharge of 7 kPa, in the compacted condition, and from other soil parameters such as liquid limit, plasticity index, and activity. Their analysis indicated that liquid limit, plasticity index, and activity cannot satisfactorily predict soil expansivity, because they do not consider the effect of soil clay mineralogy. On the basis of that analysis, they proposed a very simple and user-friendly methodology for the prediction of degree of soil expansivity and dominant clay mineral type present in the soil based on the free swell ratio (FSR), which compares exceedingly well with those from the oedometer swell tests. The FSR is defined as the ratio of the equilibrium sediment volume of a 10-g oven-dried soil sample passing through a 425mm sieve placed in a 100-mL graduated measuring jar containing distilled water (Vd) to that in carbon tetra chloride/kerosene (Vk), thoroughly mixed, and left for the sediment formation, after an equilibration period of a minimum of 24 h