Effect of clay mineral content on soil strength parameters

The impact of clay mineralogy on engineering properties relevant to road design, such as the strength and stiffness of road materials, has recently been the subject of intensive investigations, aiming at providing indirect measures of soil performance out of mineralogical data. This study deals with the effects of clay minerals on soil strength, expressed in terms of California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS), and stiffness, represented by the constrained modulus, which are commonly used as engineering properties for pavement materials. The study is performed on six clayey soils (S1-S6) of different geographic origins. Five of the six soils are highly plastic, potentially swelling, and one is kaolinitic soil. The mineralogical characteristics of the samples were analyzed through X-ray diffraction (XRD). The testing program included Atterberg limits, sieve analysis, CBR, UCS, and consolidation tests. The measured properties of the samples were compared in light of the mineralogical composition of the soils. The investigation showed that the kaolinite mineral is significantly more effective than the montmorillonite in reducing the plasticity of the clays. Higher strength indicators (CBR and UCS) were observed for samples with high kaolinite content (S4, S5, and S6). Moreover, the kaolinitic soil samples are characterized by a larger constrained modulus (Ec) than the other samples.