Geotechnical assessment of some charnockites from Nigeria as construction materials

Charnockites are a distinctive type of granite characterized by coarse-grained texture and the presence of pleochroic hypersthene and perthitic feldspars. They occur worldwide mainly in the Precambrian shield areas but are not as abundant as other rocks of granitic affinity. Laboratory test results for three charnockites from Nigeria indicate good quality materials that are fairly comparable in many aspects with the more tried and tested granites. However, the coarse-grained texture, the relatively high percentage loss values in the aggregate crushing value test as well as the Los Angeles abrasion and sulphate soundness values, which only just meet acceptance limits seem to indicate possible poor performance, particularly where they are to be subjected to significant impact, cavitation and scour. The compressive strengths of the samples fall below 200 MPa, which is generally considered a good average value for aggregate for most civil engineering works. However, the disparity between test results and performance records of some rocks, local availability of a rock and the high ratio of transportation to production costs of crushed stone ail militate against any disadvantage these somewhat poor test results may indicate charnockites have when compared with more established rocks.