Determination of optimum moisture content and degradation of shear strength over time for Hamilton ash materials

Abstract

Hamilton Ash is one of the widely used materials in earthwork projects throughout the Waikato region. Regardless of its frequent and widespread usage, limited research has been undertaken regarding its geotechnical properties. It has been observed that after the completion of fill construction, Hamilton Ash unit tends to saturate overtime thus leading to a gradual loss of shear strength and ultimately resulting in ground instability. Common instabilities associated with soil fills are settlement and subsidence which in excess can result in distortion and damage to structures, services and infrastructures that are founded on the material subjected to the movement. The compaction stage, wherein the problem originates, can be optimized if better understanding of the geotechnical properties of Hamilton Ash is attained.

My project focused on determining the key compaction parameters of Hamilton Ash which are the optimum moisture content and maximum dry density. Further investigation into the degrading influence of prolonged exposure to moisture on the shear strength is also undertaken. The use of laboratory compaction tests have been recognized as mandatory procedures in determining the crucial parameters for field-based applications. The Standard Proctor test was implemented in this research to help determine the key compaction parameters; Optimum Moisture Content and Maximum Dry Density of Hamilton Ash. Prior to the compaction tests, the important index properties of the soil that influence compaction were identified via several soil characterization tests, namely Atterberg Limits, grainsize and mineralogical analyses. Laboratory vane shear testing was also employed in this research to determine the strength of the compacted soil before and after wetting periods.