Geological and Geotechnical Investigations of Artisanal Mining Subsidence Around Kuru and Guratopp Areas Within the Jos Plateau Tin Fields, North-Central Nigeria

Artisanal tin mining activities on the Jos Plateau tin field is currently characterized by incessant mine subsidence amongst the many other environmental degradations associated with the mine activities. Geological/geophysical and geotechnical tin field techniques have been applied in the case study of the mine subsidence. Geological studies revealed the tin mining fields are characterized by variable thickness of alluvium hosting the tin minerals with little or no rock outcrops. Geo-electrical resistivity soundings across the mining sites revealed variable thickness (3-27m) geo-electrical layers/lithological units of: (1) top lateritic soil, (2) sand/sandy clays, (3) weathered granite, and (4) slightly weathered to fresh granite. Laboratory test of sub-base soil samples for geotechnical parameters of: moisture content, atterberg limits, specific gravity, shear triaxial test and allowable bearing pressures were carried out. The results showed that moisture content is highest at 56% and lowest at 25%. Similarly, highest plastic limit (PL) is 43% and lowest is 19%; plastic index (PI) at highest value is 13% with lowest at 6%. These values indicate moderate expanse and shrinkage of the fine grained soil which contribute moderately to the mine shaft collapse. Allowable bearing pressure test results in KN/m2 range between 86 and 279 KN/m2 with an average of 135KN/m2. There is wide pressure variation with highest at 279KN/m2 and lowest at 22KN/m2 and these values are not consistent and can cause the collapse of the mine pits and tunnels. From particle size analysis; coefficient of uniformity (Cc) of Kuru Jenta mine shows that Cc=0, Cu=12, 21 and that of Guratopp shows Cc=0, Cu=6. The particle size distribution curves show narrow ranges of soil particle sizes and suggest a gap graded/poorly graded soil with poor compaction hence weak in stability and leading to mine shafts and tunnels collapse. The tin bearing alluvial materials has a wide range of particle sizes which are gap graded, poorly compacted, weak and hence leads to roof flow collapse in the cause of shafting and tunneling. The artisanal underground mining using shaft and tunneling techniques in the thick alluvial deposits must ensure intensive support and the dewatering to prevent collapse and navigate weak ground.