The mineralogical and engineering characteristics of Cretaceous and Tertiary shales in the lower Benue Trough were determined with a view to establishing how they affect civil engineering construction, with emphasis o...The mineralogical and engineering characteristics of Cretaceous and Tertiary shales in the lower Benue Trough were determined with a view to establishing how they affect civil engineering construction, with emphasis on road pavements in the area. Shale samples from the geologic formations of Imo, Enugu and Awgu shales were subjected to the following laboratory tests： clay mineral content, organic matter content, Cation Exchange Capacity and Plasticity according to methods specified by the British Standard Institute. The shales were classified based on Plasticity Index, liquid limit and Cation Exchange Capacity. The class of shales ranged from non-plastic to extremely plastic and low to high reactivity. The moisture content and plasticity values are related to the degree of weathering. The higher the weathering grade, the higher the moisture content and plasticity values. The organic matter content of the shales is generally low （0.2% to 1 !.2%） and influences the durability of the shales in an inverse manner. The clay mineral composition from x-ray diffraction consists of lllite-montmorillonite mixed layers, illite, and kaolinite. The iUite-montmorillonite mixed layer clays are most prominent in road sections with most severe pavement failures. In contrast, sections with kaolinite as the dominant clay mineral experienced less severe and limited pavement failure. The contrasting engineering behaviour of these clay minerals is due to their structures. The study showed that the presence of clay minerals derived from underlying shales is a major contributory factor to the behaviour and performance of roads built over shale subgrades, that any effective remediation work must take cognizance of the amount and type of clay minerals present.展开更多
文摘The mineralogical and engineering characteristics of Cretaceous and Tertiary shales in the lower Benue Trough were determined with a view to establishing how they affect civil engineering construction, with emphasis on road pavements in the area. Shale samples from the geologic formations of Imo, Enugu and Awgu shales were subjected to the following laboratory tests： clay mineral content, organic matter content, Cation Exchange Capacity and Plasticity according to methods specified by the British Standard Institute. The shales were classified based on Plasticity Index, liquid limit and Cation Exchange Capacity. The class of shales ranged from non-plastic to extremely plastic and low to high reactivity. The moisture content and plasticity values are related to the degree of weathering. The higher the weathering grade, the higher the moisture content and plasticity values. The organic matter content of the shales is generally low （0.2% to 1 !.2%） and influences the durability of the shales in an inverse manner. The clay mineral composition from x-ray diffraction consists of lllite-montmorillonite mixed layers, illite, and kaolinite. The iUite-montmorillonite mixed layer clays are most prominent in road sections with most severe pavement failures. In contrast, sections with kaolinite as the dominant clay mineral experienced less severe and limited pavement failure. The contrasting engineering behaviour of these clay minerals is due to their structures. The study showed that the presence of clay minerals derived from underlying shales is a major contributory factor to the behaviour and performance of roads built over shale subgrades, that any effective remediation work must take cognizance of the amount and type of clay minerals present.
