A soil with 60% silt and 37% clay was evaluated in the laboratory and was found to plot above the A-line and was classified as CH (clay of high plasticity). Is such a classification valid? What are the possible reasons for such results? ASTM D2487 gives a table wherein inorganic silts and clays can be given the symbol CH with fat clay as group name. I would like to know under what circumstances such a classification is possible?
Yes, it can give such a classification. Firstly, the soil in question is predominantly fine-grained. According to the ASTM D 2487 (Standard Practice for Classification of Soils for Engineering Purpose), a fine-grained soil is classified as being of high plasticity if its liquid limit is greater than or equal to 50%, otherwise it is of low plasticity. Its plot on, above or below the A-Line on the plasticity chart defines whether it is regarded as clay or silt, assuming the soil is inorganic. If the mineralogy of the clay fraction of this soil is such that it is predominantly montmorillonite, for instance, then the soil may have high liquid limit (greater than or equal to 50%) and plasticity index such that it plots on or above the A-Line on the plasticity chart and become classified as CH.
Dear Dr. Akinwumi,
Thank you so much for your explanation. Based on your explanation, I have got more clarity on the issue. I needed an expert opinion to confirm my understanding. The USCS takes into account the liquid limit and plasticity for soil classification but not the particle size distribution. The former two are influenced by the mineralogy of the clay present. The soil in contention, if classified by textural classification, which is based on particle size distribution, would be classified as silty clay. I guess that's the reason for a soil having high silt content being classified as clay of high plasticity. And you are bang on regarding mineralogy, the XRD of the soil showed peaks corresponding to montmorillonite. Thank you so much once again. I would be real glad if you approve of my explanation or correct/improve upon it if needed.
Dear Dr. Akinwumi,
Thank you for your confirmation. Appreciate you for taking time to reply and clarify my doubts.
I would like to ask here why the A-line in the casagrande plasticity chart separates inorganic (above the line) from organic (below the line). It is clear to me why it separates clay (above) from silt (below), but what if you have organic clays? where would be the point (above because of clay or below because of organic)