The objective of this study was to investigate the effect of density, water content, drying temperature, layer thickness, and plasticity index on the nature of shrinkage cracks that result from drying of wet clay. Clay soils are widely used in construction of embankment dams, levees, highway embankments, sanitary landfills, hydraulic barriers, and foundations. For most of these projects, the clay is compacted at maximum dry density (MDD) and optimum water content (OWC). The climatic conditions can change repeatedly during the service life of structures made primarily of clay, resulting in corresponding changes in water content. This is particularly the case in Africa, Central America, and South Asia where prolonged periods of sunshine and intense heat follow monsoon season, characterized by intense, and occasionally prolonged, periods of precipitation. A decrease in water content upon drying causes a decrease in volume of clay, resulting in development of shrinkage cracks.
Five samples each of low plasticity clay (CL), medium plasticity clay (CM), and high plasticity clay (CH), as indicated by the plasticity index values for each clay, were compacted at water contents on both sides of the OWC to establish the compaction curves. The MDD values for CL, CM, and CH samples were found to be at 101.9 lb/ft3 (1.63 Mg/m3), 97.6 lb/ft3 (1.56 Mg/m3), and 94.2 lb/ft3 (1.51 Mg/m3), respectively, whereas the corresponding OWC values were 20.7%, 25.2%, and 40.2%, respectively.
The compacted samples were oven-dried at temperatures of 10oC (50oF), 20oC (68oF), 30oC (86oF), 40oC (104oF), and 50oC (122oF). Upon attaining a constant dry weight, the nature of any cracks developed in the samples (i.e. crack length, crack aperture, crack area, etc.) was examined and photographs of the samples were taken.
In addition to the compacted samples, uncompacted (loose) clay layers of varying thicknesses (5 mm, 7 mm, 10 mm, 20 mm, and 30 mm) of the three types of clay were saturated and oven dried at the aforementioned temperatures to investigate the effect of clay layer thickness on shrinkage crack parameters like crack length, crack aperture, and crack area. Compacted samples simulate the behaviour of clays as used in their engineering applications, whereas uncompacted samples simulate their behavior in undisturbed natural state.
The length, aperture, and area of the developed cracks for each clay type were correlated with dry density, initial water content, drying temperature, clay layer thickness, and plasticity index. Compacted samples of only CH showed clear signs of shrinkage crack development whereas compacted samples of CL and CM did not develop shrinkage cracks. Therefore, the CL and CM samples could not be analysed for crack length, aperture, and area.
Compacted CH samples exhibited an increase in crack length, crack aperture and crack area with an increase in dry density and water content. For uncompacted samples, the crack length and crack area reduced with increasing layer thickness, and crack aperture increased with increasing layer thickness.