Clays are used widely in sanitary landfills, embankment dams, highway embankments, hydraulic barriers, and foundations. In most of these applications, clays are compacted at maximum dry density (MDD) and optimum water content (OWC). Density and water content have a profound effect on the strength and deformation behavior of compacted clays. However, this effect has not been quantified in detail, especially the water content at which transition from brittle to plastic behavior occurs for low, medium, and high plasticity clays. The objective of this research was to investigate the effect of varying water content and density on the strength and deformation behavior of low, medium, and high plasticity clays, and to quantify the transition water content between brittle and plastic behavior for each type of clay.
Initially, six samples each of low, medium, and high plasticity clays were compacted, three on the dry side and three on the wet side of OWC, to establish their compaction curves. The compacted samples were failed axially under unconfined compression and were visually inspected to determine the water content at which transition occurred between brittle and plastic deformation. Additionally, three samples of each type of clay were compacted at different water contents and failed using the direct shear test. The stress-strain curves from both tests were used to determine the transition water content between brittle and plastic behaviors.
The MDD values for low, medium, and high plasticity clays were found to be 102.5 lb/ft3 (1.64 Mg/m3), 95 lb/ft3 (1.52 Mg/m3), and 89.5 lb/ft3 (1.43 Mg/m3), with the corresponding OWC values of 18%, 25%, and 27%, respectively. The compressive strength values for the low, medium, and high plasticity clays at MDD and OWC were 54 psi (344.8 kPa), 59 psi (413.8 kPa), and 60 psi (420.7 kPa), respectively. The unconfined compressive strength first increased and then decreased with increasing water content, with the change in trend occurring within 5% of OWC for each type of clay. The high plasticity clay had the highest cohesion while the low plasticity clay had the highest friction angle. The transition between brittle and plastic behavior for the low, medium, and high plasticity clays occurred between 19-20%, 27-29%, and 30-32% water content, respectively.
This study was aimed at determining the transition water content as it relates to both brittle and plastic deformation. Earthquakes can cause failure of embankment dams in the form of cracking due to displacements or differential settlements from the vibrations. To prevent such failures from occurring, a homogenous embankment dam consisting of low plasticity clay (CL) or the clay core of a zoned embankment dam, must be compacted so that the clay material behaves more like a plastic material, i.e. deforms without a well developed failure plane. This study shows that, to ensure structural integrity of embankment dams in seismically active areas, the clay should not only be compacted wet of the OWC, but also on the wet side of the transition water content marking the boundary between brittle and plastic deformations.