ABSTRACT
An evaluation of geologic conditions and geotechnical parameters that influence gully formation in Ihiteowerri and Obibi communities in Orlu, South Eastern Nigeria was carried out. The study area is underlain by Ameki Formation (Age Eocene to Oligocene) of Anambra Basin. The lithology is mainly lateritized unconsolidated sands, clays and silts with yellowish brown colour. No hard rock exposure was observed in the location.The area is on a gentle drainage slope that dips from Obibi (120) NE to Ihiteowerri (320) SE, with sparse vegetation cover. The annual rainfall is between 2,00 to 2,500mm yr-1. Field geological study of the area revealed that gully erosion propagation and associated hazards has remained active for the past twenty (20yrs) without adequate control measure from the Government, except few control attempts from the communities. The gullies propagation and associated hazards have ravaged rural roads, threat to the community secondary buildings and private buildings in the area. Five (5) gully sites were studied from the two communities namely: Three of the gully sites are located at Ihiteowerri, namely Abor Gully site (ABS), Ogbaka gully site (OGB), and Umuchukwu gully site (UMU), while two other gullies sites are located at Obibi, namely: Mbonu Orji gully site (MOS) and Obibi gully site (OBUS).Three (3) composite samples were collected from each gullies (total 15 samples) at different depths of the between (1.0 to 1.5meters) top, (4.0 to 6.0 meters) middle and (8.0 to 15.0 meters), with one undispersed sample. The samples geotechnical properties were analyzed for grain size distribution, atterberg limits, compaction, shear strength and specific gravity. The mean result of geotechnical properties and measurements for the Five (5) gullies show that the soil is predominantly fine sand.
Abor gully (ABS) is (15.5mts) depth with near vertical slope angle of (75.50). Surface area eroded is about (960m2), with (10,872m3) volume of soil washed off. Mean Particle size distribution showsfine sand (73.3%), silt/clay fractions (23.2%), coarse sand (3.5%) zero gravels and uniformity coefficient (Cu) (11). Atterberg limit result indicates show liquid limit (LL) (25.5%), plastic limit (PL) (14.96%) and plasticity index (PI) (10.6%). Compaction result show dry density (1.79 mg/ cm3), optimum moisture content (OMC) (10.77%), porosity (n) (10.47%) and permeability (K) (1.37x 10-1 C m3/sec.). The soil has Cohesion (6.0 KN/M3) with friction angle (17.70), and Specific gravity (2.54). Erodibility characteristic indicate stable mechanical ratio (MR) (3.33), but highly erodible with dispersion ratio (DR) (83%) and erosion ratio (ER) (3.33).
