Insights from the Engineering Geological Mapping of Four Basement Rocks Derived Soils

Abstract

Due to the rapid expansion and associated construction of civil engineering structures on the Federal University of Technology, Akure (FUTA) campus, there arose an urgent need for an engineering geological mapping of the underlying soils (residual soils). Generalized geological mapping revealed four types of basement rocks namely migmatite-gneisses, granites, quartzites and charnockites. Results from the fifty (50) soil samples from twenty-five test pits collected all over the spread of the campus coverage of 6.4 km2 revealed that the campus is underlain by soils of granular and clayey composition, generally lateritic, having reddish to brownish colour. Engineering geological tests such as natural moisture content, particle size analysis, consistency limits, California bearing ratio and consolidation were carried out on the soils following standard procedures revealed that the values of natural moisture content do not generally follow a consistent pattern and varied from location to location. The grain size characteristics curve, displayed 84% and 16% subsoils are of well graded and poorly graded type respectively. The soils were grouped into CL (low plasticity), CI (medium plasticity) and CH (high plasticity) from consistency limits results. Compaction characteristics of the subsoils revealed 36% and 64% representative of fair to good and poor to very poor foundation materials respectively. Soils with settlement rates greater than 1mm/year were designated as high settlement subsoils. 72% and 28% of the subsoils fell into hard to stiff and soft categories from the shear strength characteristics respectively, and classified as c-ø soils. California Bearing Ratios values range from 10 – 70, indicating their suitability for pavement construction. Conclusively, areas underlain by migmatite-gneiss and charnockite-derived soils, and granite and quartzite-derived soils possessed low and high strength characteristics respectively which can be attributed to their textural characteristics. The subsoils of the entire campus spread are however capable of bearing very substantial loads.

Evaluation of plasticity and consolidation characteristics of migmatite–gneiss-derived laterite soils

“…Pegmatite and quartz vein occur as intrusions in nearly all the various rock types, and they cut across the different rocks concordantly or discordantly and sometimes occupy the joint spaces [24]. Texturally, the migmatite-gneisses are medium-to coarse-grained rock with light gray varying to pinkish rocks [25]. The differences in textural and mineralogical characteristics of parent rocks could be responsible for significant differences in the engineering properties of the derived soils [26].…”

Assessment of Gneiss-Derived Residual Soils as Materials Used in Road Pavement Structures

“…In harmony with FMWH [11] standard specification for plastic limit indicate that all the soil samples satisfy the specification. High plastic soils tend to be susceptible to high compressibility, swelling on moisture influx, low bearing capacity and leading to low permeability Olabode and Asiwaju-Bello [2]. Eighteen percent (18%) soil samples were consistent with FMWH [23] specification of less than 20% maximum for plasticity index while eighty two (82%) percent did not fall within the specification.…”

“…Therefore, the soil samples cannot be used as subgrade materials for any engineering design. Olabode and Asiwaju-Bello [2] stated that high amount of fines (greater than 35%) and low amount of coarse contents (less than 65%) suggest high amount of micas and feldspars that is mostly found in gneisses and charnockite. This largely influences the behaviour and the engineering properties of the soils in proportion to its abundance.…”

“…The formation of residual soils is through physical and chemical weathering of bedrock in situ with little or no movement of the individual soil particles [1]. The soils tend to be characterized by angular to sub-angular particles, mineralogy similar to parent rock, and the presence of large angular fragments within the overall soil mass [2]. They are highly structured soils which may be cemented depending on the degree of weathering.…”

“…The shear strength behaviour of residual soils are known to be affected significantly by the bonds between the particles which defines the cohesive nature of the soils. Previous research works have been limited to investigating the engineering properties of residual soils and the effects of soil structure in the engineering properties of the soils [3,4,2]. Many of these research works have been carried out on a broad scale.…”