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This study was carried out to evaluate the influence of soil consistency limits and shear strength on the relative proliferation of gullies on three geological sediments, namely: the Upper Coal Measures (
_{UCM}), the Ajalli Sandstones (AS), and the Lower Coal Measures (LCM), which has been ranked as AS > UCM > LCM. Soil samples were collected from a depth range of 60 - 90 cm of sampling pits dug at a selected location on each of these texturally homogenous and unique formations. These were analysed for consistency limits and shear strength using standard methods and procedures. Results showed that the plasticity index (PI) of the UCM (PI
_{UCM}) was 24.49%; PI
_{AS}, 5.89%; and PI
_{LCM}, 44.85%. The shear strength (S) of the UCM (S
_{UCM}) was 314.74 KN/m
^{2}; SAS, 196.23 KN/m
^{2}; and SLCM, 321.72 KN/m
^{2}. The results of the study show that rankings of the respective contributions of these two geotechnical properties to the vulnerability of the sediments to gully (soil) erosion follow the same order of proliferation of gullies on the plateau landscape: AS > UCM > LCM. Hence, consistency limits and shear strength parameters partly influence and explain the relative proliferation of gullies on the three formations. Further studies are required to determine other soil factors of gully erosion in the area in order to evolve appropriate management strategies for these sediments.

Gullies proliferate the landscape of the Idah-Ankpa Plateau (IAP) of the Anambra Basin, Nigeria, which is underlain 100% by three geological sediments [

Soil erodibility is a measure of a soil’s vulnerability to erosion [

Soil erosion studies on the IAP are few and these have been concentrated on the AS. [

sites in the southeastern states of Nigeria are located either on the Nanka Sands or the Ajalli Sandstones. They observed that these two geological formations bear striking textural similarities from the gully erosion point of view. [

The geological causes of the proliferation of gullies at Ankpa, a growing semi- urban town located on the AS, was the subject of investigation by [

However, studies and data on the geotechnical properties of each of the geological sediments, especially the UCM and the LCM, are lacking. Availability of these data is required to explain the relative proliferation of gullies on these sediments. This study, therefore, was a part attempt to satisfy this need. The consistency limits and shear strength of the three geological units were determined to assess their roles as factors influencing the disproportionate differentials in the proliferation of gullies on the plateau landscape.

The Idah-Ankpa Plateau (IAP) of the Anambra Basin of Nigeria comprises the Western Ankpa Plateau and the Idah Flood Plains. It has been so named because the latter consists of an insignificant percentage of the whole area [

Notably, the textural homogeneity and uniqueness of each unit of the three

geological sediments have been established [

The samples were subjected to consistency limits tests to determine their liquid limits, plastic limits, and plasticity indices. The Cassagrande apparatus was used in the determination of the liquid limits while the plastic limits were obtained using the glass surface/palm rolling method. The shear strength was determined using the direct shear box apparatus.

When water is removed from a fine-grained soil, it passes through a number of states, namely: liquid, plastic, semi-solid, and solid. The moisture contents of the soil as it passes from one state to the other are known as the consistency limits. The range of moisture contents over which the soil is liquid is known as the liquid limit index, and the range over which it is plastic is the plastic limit index. The plasticity index is derived as:

PI = LL − PL (1)

where PI = Plasticity index

LL = Liquid limit index

PL = Plastic limit index

It is well known that a soil with a higher PI is more resistant to erosion than one with a lower value [

The shear strength of a soil is determined using a direct shear box apparatus and is computed as follows:

S = C + σ tan Φ (2)

where, S = shear strength

C = cohesion

Φ = soil’s angle of internal friction

σ = normal stress

σ = N Surfaceareaofring (3)

where, N = vertical load.

The shear strength of a soil is influenced by cohesion, the normal stress acting in the soil, and the angle of internal friction. The cohesion is determined by the amount of binding materials, such as clay and organic matter, present in the soil, and the normal stress is dependent on the imposed vertical load. The growth of a gully is controlled by the shear strength of the soil [

The results of laboratory tests carried out to determine the plasticity indices (PI) of the three geological sediments are shown in

Location and consistency limits | Ajalli Sandstones (AS) (60 - 90 cm depth) | Upper Coal Measures (UCM) (60 - 90cm depth) | Lower Coal Measures (LCM) (60 - 90 cm depth) |
---|---|---|---|

Location | Ankpa | Dekina | Owukpa |

Liquid limit (%) | 24.61 | 44.00 | 66.50 |

Plastic limit (%) | 18.72 | 19.51 | 21.65 |

Plasticity index (%) | 5.89 | 24.49 | 33.85 |

The 5.89% PI value of the AS betrays its sandy, erodible nature. Although sandy soils are generally and characteristically non-plastic, [

The PI of the LCM is the highest with a value (44.85%) greater than 40% and, therefore, can be rated as plastic. It retains the highest amount of water, transmits the least water through its sediments, sheds the highest amount of runoff, and is obviously the least erodible of the three formations. The ranking of the PI values of the three sediments can, therefore, be expressed as PI_{AS} < PI_{UCM} < PI_{LCM}. Because PI is inversely related to soil erodibility, the vulnerability to erosion of the three geological sediments can be expressed as AS > UCM > LCM. This agrees with the ranking of the relative proliferation of gullies on the IAP stated in the introduction (AS > UCM > LCM). The study clearly shows that the wide differences in the PI values of the sediments are factors that partly influence and explain the relative proliferation of gullies on the Idah-Ankpa Plateau (IAP).

The angle of internal friction (Φ) in the AS is the highest with a value of 37.20˚, and the LCM has the least (19.23˚), followed by the UCM (22.36˚) (

However, the low value of cohesion, C, in the AS (24.22 KN/m^{2}) confirms the paucity of binding materials in the sediments. The UCM has a higher value (74.52 KN/m^{2}), while the LCM has the highest (110.34 KN/m^{2}). Soil cohesion is affected by the amount of binding materials present in the soil, such as clay and organic matter (OM). The more the binding materials, the higher is the soil’s

Location and shear parameters | Ajalli Sandstones (AS) (60 - 90 cm depth) | Upper Coal Measures (UCM) (60 - 90cm depth) | Lower Coal Measures (LCM) (60 - 90 cm depth) |
---|---|---|---|

Location | Ankpa | Dekina | Owukpa |

Φ | 37.20˚ | 22.36˚ | 19.23 |

C (KN/m^{2}) | 24.22 | 74.52 | 110.34 |

Shear strength, S, value (KN/m^{2}) | 196.23 | 314.74 | 321.72 |

C-value. In a study of the textural characteristics of the sediments of the IAP, [^{2}). This is followed by the UCM (314.74 KN/m^{2}), and the LCM has the highest (321.72 KN/m^{2}).

Therefore, results of the triaxial shear tests show that the Φ-values of the three sediments follow the same order of vulnerability to erosion of the sediments: Φ_{AS} > Φ_{UCM} > Φ_{LCM}. On the other hand, the cohesion (C) and the shear strength (S) follow reverse orders of increase of the soils’ vulnerability to erosion: C_{AS} < C_{UCM} < C_{LCM} and S_{AS} < S_{UCM} < S_{LCM} respectively. Since the C and S values are inverse functions of the erodibility of the sediments, the rankings agree with the relative vulnerability to erosion of the sediments and, hence, the resulting order of proliferation of gullies on the IAP: AS > UCM > LCM.

The study again shows that the angle of internal friction, Φ, the cohesion, C, and the shear strength, S, of the sediments are soil geotechnical properties that partly influence and explain the relative proliferation of gullies on the geological sediments of the IAP.

This study leads to the following conclusions:

1) The plasticity indices of the geological sediments of the Idah-Ankpa Plateau can be ranked as Pl_{AS} < Pl_{UCM} < Pl_{LCM}.

2) Because Pl is inversely related to soil erodibility, the vulnerability to erosion of the sediments can be ranked as AS > UCM > LCM based on the PI criterion.

3) The angle of internal friction, Φ, is ranked as Φ_{AS} > Φ_{UCM} > Φ_{LCM}. Since Φ is directly related to soil erodibility, the vulnerability of the sediments to erosion based on the Φ factor can be ranked as AS > UCM > LCM.

4) Soil cohesion (C) and shear strength, S, are each inversely proportional to soil erodibility and are respectively ranked as C_{AS} < C_{UCM} < C_{LCM} and S_{As} < S_{UCM} < S_{LCM.}

5) Based on the C and S criteria, the vulnerability to erosion of the sediments can be ranked as AS > UCM > LCM.

6) Therefore, the consistency limits and shear strength of the sediments partly influence and explain the relative proliferation of gullies on the three geological formations underlying the IAP.

Further studies are needed to investigate other soil factors that influence the relative proliferation of gullies on these sediments. Management practices adopted for each of these sediments should reflect their relative fragility.

Oparaku, L.A., Iwar, R.T. and Edeh, J.E. (2018) Relative Proliferation of Gullies on Three Geological Sediments: The Influence of Soil Consistency Limits and Shear Strength. Journal of Environmental Protection, 9, 25-33. https://doi.org/10.4236/jep.2018.91003