keywords: Engineering, shale, cotton soil, compressibility, compaction, subgrade
Structural failure along the Numan-Gombe road around Guyuk area is manifested in pavement distress and this is due mainly to insufficient strength of the Numanha black cotton soil that covers that area. Twelve (12) samples randomly collected from the soil were subjected to standard laboratory test specified by British Standard (BS) 1377. X – ray diffraction analysis using Philips - PW 1011 model diffractometer was also done to ascertain the suitability of the soil for road pavement. The results showed that the soil is poorly graded sandy clay and classified as A – 7 – 5 type; mineralogical analyses portray illite as the dominant clay mineral. Liquid limit, plastic limit and plastic index and linear shrinkage showed average values of 68, 32, 35 and 15%, respectively. Maximum dry density and optimum moisture content averages at 1769 Kg/m2 and 16.7%, respectively. Average in-situ void ratio of 0.665 decreased to an average value of 0.385 while coefficient of compressibility decreased from average values ranging from 0.553 – 0.048 m2/KN as pressure increased. Results which suggest that the soil exhibits poor engineering qualities because of its high optimum moisture content, low dry density, low permeability, high compressibility and shrinkage. This is due to its high clay proportion possessing the tendency to swell and shrink repeatedly during alternate wet and dry seasons and hence do not meet the Nigerian standard for pavement materials.