1 Stability, Hydrogeological and Geospatial Characteristics of Gully

2 Slopes: A Case Study in Southeastern Nigeria.

5 Gully slope stability study is an essential part of gully erosional study as it helps determine a gully's
6 erodibility potential. The southeastern part of Nigeria has faced several environmental challenges, which
7 account for gully erosion and landslides. This research paper studied the gully slope geotechnical
8 parameters to help characterize the erodibility and slope failure potential. The field mapping shows that the
9 gully slopes were > 350. The geotechnical analysis reveals that the gully slope materials lack finer materials,
10 have moderate to high infiltration capacity, loose and collapsible soil, non-plastic to low plastic, low
11 cohesion capacity, and low friction angle. Based on the results, the gully slopes have poor geotechnical
12 characteristics. The principal component analysis (PCA) helped establish interrelationships among the
13 analyzed geotechnical parameters based on the multivariate statistical analysis. In contrast, the hierarchical
14 cluster analysis (HCA) helped categorize gully slopes based on their landslide potential. The obtained
15 Factor of safety for the gully slopes ranged from 0.50 to 1.10 for saturated conditions and 0.72 to 1.25 for
16 unsaturated conditions, revealing that the slopes are more stable in the dry season than the rainy season.
17 The hydrogeological survey obtained possible aquifers at a depth of 5.9 to 61.9m.

18

19 Keywords: Slope stability analysis. Hydrogeological characteristics. Principal component analysis (PCA).
20 Hierarchical cluster analysis (HCA). Geotechnical analysis. Factor of Safety (FS).

21

22 1 Introduction

23 Among all the geoenvironmental disasters experienced all over the globe, gully erosion and landslides are
24 highly persistent in some parts of Nigeria. These disasters account for the death of hundreds of people and
25 damage to properties worth over millions (Igwe 2012; Nebeokike et al. 2020; Egbueri and Igwe 2020;
26 Egbueri et al. 2021). Gullies are geologic hazards caused by running water which leads to persistent loss of
27 soils. On the other hand, landslides are slides of large masses of rock down a slope (Fig.1), which may
28 happen suddenly or slowly over a long period. These two geologic hazards always occur in association with
29 the other. The gullying process accounts for the destruction of engineering structures (building and
30 transportation), degradation of arable lands, abandonment/migration of communities, deterioration in water

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31 quality, and occasional loss of lives. Soil erosion through runoff is a severe environmental challenge faced
32 by over two-quarter of the world, and it's majorly triggered through natural and anthropogenic processes.
33 It remains one of the world's largest ecological problems threatening humans, plants, and animals
34 (Abegbunde et al., 2006). The soil erosion processes are attributed to some factors like; geology,
35 geomorphology, land use, land cover, hydrologic conditions, mining activities, and improper water disposal
36 (Gelagay and Minale, 2016; Okorafor et al., 2017; Egbueri and Igwe, 2020). Studies by Humberto and Lal
37 (2008) reported that erosion by water affects over 1billion hectares of soil worldwide, on an average of 56%
38 representing the total degraded land area. However, 28% of degraded land accounts for wind erosion
39 (Okorafor et al., 2017).

40 Gully slope failure plays a major or significant role in the expansion of gullies. Presently, it has been of
41 interest to several fields of study such as Engineering Geologists, Geotechnical Engineers,
42 Environmentalist, Civil Engineers, etc. Due to the present situation of gully development in many parts of
43 the globe, it narrows down to southeastern Nigeria. The stability of gully slopes has attracted interest
44 because it has been noted that many massive gullies enlarge by the failure of gully walls and slopes. In the
45 geotechnical study, the collapse of soil mass occurs along a plane or curved surface when a large mass of
46 soil slides in respect to the remaining mass. Slope failure occurs when the force causing failure becomes
47 greater than the soil mass's shearing resistance (shear strength). Two factors that may lead to slope failure
48 include; (1) increase in shear stress due to slope steepness either by excavation or natural erosion and (2)
49 loss of shear stress due to an increase in water content known as "pore pressure." The slope stability study
50 aids in revealing the potentiality of slopes to failure or collapse. The term factor of safety (FS) is generally
51 used in slope study as it defines the ratio of soil shear strength along a possible slip surface (Igwe and
52 Chukwu 2018; Egbueri et al. 2021). It's important in the gully slope study because it reveals the possibility
53 of a sliding soil mass and identifying the stability of gully slopes for both constructions and hazard
54 mitigation planning (Ilori et al., 2017; Igwe and Una 2019).

55 Gully erosion in southeastern Nigeria (Enugu and Anambra states) has been studied extensively by several
56 authors which may include; Egboka and Nwankwor, 1985; Okagbue, 1992; Igwe, 2012; Nwajide, 2013;
57 Chikwelu and Ogbuagu, 2014; Obiadi et al., 2014; Okoyeh et al., 2014; Igwe and Egbueri, 2018; Emeh and
58 Igwe, 2018. All have attributed the persistence of these gullies to the geological, hydrogeology, and poor
59 geotechnical properties of the soils. A recent study by Egbueri and Igwe 2020 studied the impact of
60 hydrogeomorphological characteristics on gullying processes in Anambra state, and this was done without
61 considering the gully slope stability. However, Igwe and Una (2019) and Nebeokike et al. (2020) assessed
62 the strength of some gully slopes using limit equilibrium stimulation was a very important study, although
63 they were limited to gullies within Ekwulobia, Nanka, and Udi areas. Egbueri et al., 2021 further assessed

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64 the slope stability of some gully slopes in some parts of Anambra state. Although these are important
65 studies, there is a need to study the gully slopes and the impact of hydrogeology on gully erosion
66 development through gully slope failure. Presently, no study has placed significance in simultaneously
67 assessing the slope stability and hydrogeological impacts of gullies with the Ajali and Nanka geological
68 formations in southeastern Nigeria.

69 Therefore, this present study is focused on assessing the slope stability, hydrogeological, and geospatial
70 characteristics of gully slopes in southeastern Nigeria, putting into consideration the following objectives;
71 (1) determine the gully slope distribution within the geologic formations; (2) develop a deterministic gully
72 slope failure model, which will be based on the hydrological and geotechnical properties; (3) integrate
73 multivariate statistical analysis to identify the key geotechnical association of the analyzed parameters; (3)
74 produce a factor of safety map for erosion risk ranking (4) determine the hydrogeological impact on the
75 erosional processes. Through the following objectives, it is hoped that this research would better understand
76 the stability of gully slopes and the role of hydrogeology on the gully slope processes and aid in mitigation
77 planning and erosion control measures.

78

79 2 Study area description

80 In this study, gully slopes within the western part of Enugu State and the southern part of Anambra State in
81 southeastern Nigeria were both mapped. These locations account for having one of the highest numbers of
82 gullies, and it's been underlain by the friable Ajali and Nanka formations, which several authors have
83 reported to be erosion-prone geological formations (Igwe and Egbueri, 2018; Egbueri et al. 2021). In the
84 study area, high rainfall and activities like excavating sands for construction from the gully slopes are
85 believed to play a part in the instability of the slopes and landslides. The study region lies within latitude
86 5º55' to 6º29' N and longitude 6º58' to 7º26' E with an elevation ranging from 169 to 433m (Fig. 2) above
87 the sea level. The study region is accessed through major roads known as federal roads and minor known
88 as state roads. However, most of the gullies are accessed through footpaths (Fig. 3).

89 The study area is within the tropical rainforest zone been influenced by two climatic conditions; the rainy
90 and dry seasons. The rainy season lasts between April to October, and it's been characterized by
91 thunderstorms, while the dry season lasts between November and March with high temperatures and a dusty
92 atmosphere. The mean monthly temperatures in the study area vary from 22°C to 28°C in the rainy season
93 and between 28°C and 32°C in the dry season (Igwe, 2017; Egbueri and Igwe 2020).

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94 The study area is characterized by v-shaped valleys and flat hilltops with greenish vegetation and uneven
95 or undulating topography. The vegetation is characterized by a tropical rainforest belt having short-tall trees
96 (palms, iroko, mahogany, and banana trees), shrubs, and grasses. Still, most areas have been subjected to
97 severe deforestation due to natural and anthropogenic activities (Nnebeokike et al., 2020). The drainage
98 system is characterized by streams and rivers, indicating a dendritic drainage pattern (Fig. 3).

99 The study area comprises two geologic formations, the Ajali and the Nanka, in the Anambra Basin. The
100 western part of Enugu mapped in this study is underlain by the Ajali Formation, whereas the Nanka
101 Formation covers the southern portion of Anambra State (Nwajide, 2013) (Fig. 3). The Ajali Formation is
102 dated early-Maastrichtian in age. The formation is characterized by a friably, unconsolidated, and poorly-
103 cemented mixture of sandstone and siltstone with a wide range of lithologic colors (Nwajide, 2013). The
104 Mamu Formation underlines the Ajali Formation; above the Ajali Sandstone lies the Nsukka Formation,
105 which is coal-bearing (Reyment, 1965; Murat, 1972; Obi, 2000) (Fig 3). The Eocene Nanka Formation is a
106 member of the Ameki Group comprising the Ameki Formation, Nanka Sand, and Nsugbe Sandstone. The
107 Nanka Formation is characterized by fine to coarse-grained tidally influenced fluvial and fluvial sandstones
108 at the basal part. The Nanka Formation is overlain by intercalating clay, shale, and limestone, with coarse-
109 grained cross-bedded sandstones and clays at the uppermost region (Nwajide, 2013; Emeh and Igwe, 2018).
110 The Ameki Group (Ameki Formation, Nanka Sand, and Nsugbe Sandstone) is underlain by the Imo
111 formation and overlain by the Ogwashi-Asaba Formation (Odunze and Obi, 2011; Nwajide, 2013). Both
112 geologic formations are notorious for their proneness to gullying, and the devastation is wrecking within
113 southeastern Nigeria.

114

115 3 Materials and methods

116 3.1 Fieldwork, soil sampling, and laboratory analysis

117 The field mapping was carried out in November 2019. During the mapping, various gully sites were visited,
118 and their slopes were carefully studied. Coordinates for different gully sites were recorded accordingly.
119 Features like lithology were studies, gully slope geometry like slope angle, average slope width, depth, and
120 lateral extent were determined using a portable Brunton handheld compass. A total of sixteen (16) soil
121 samples were carefully collected using a hand-auger. Its geotechnical parameters were analyzed, eight for
122 each geologic formation (the Ajali and Nanka formations). However, for the slope study, one hundred and
123 sixty-five (165) gully slopes were measured in this study (Table 1). Seventy-nine (79) and eighty-six (86)
124 gully slopes were recorded for the Ajali and Nanak formations. The soil samples from the gully slopes were
125 investigated for geotechnical parameters such as particle size analysis, consistency test, natural moisture

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126 content (NMC), compaction, permeability, porosity, and shear strength (C and Ø). The laboratory tests were
127 carried out based on the relevant American Society for Testing and Materials (ASTM) standard of soil
128 testing as described in a soil mechanics laboratory manual Kalinski, 2011.

129

130 3.2 Multivariate statistical and slope stability analyses

131 The IBM SPSS software (v. 25.0) was utilized in this study to understand the interrelationships among
132 analyzed slope geotechnical parameters. The multivariate analysis was performed differently for the two
133 geologic formations considered in this study to help establish a better understanding of the slope
134 geotechnical parameters. The multivariate statistical considered in this study were the principal component
135 analysis (PCA) and Hierarchical cluster analysis (HCA). For the PCA, component class values (> ±0.7)
136 were considered as strong components, and (< ±0.5) were considered as weak components. However,
137 different cluster and sub-cluster classes were obtained for the HCA based on the interrelationship among
138 different slope samples in the study. Furthermore, the Geostudio software (v. 2016) was also helpful in this
139 study for modeling a 2D gully slope failure mechanism. Gully slope properties such as slope height, slope
140 angle, unit weight, cohesion, and frictional angle were utilized in slope stability analysis. The Morgenstern-
141 Price limit equilibrium method was preferred because it allowed various user-specified interslice force
142 functions (Igwe and Una, 2019; Egbueri et al., 2021). Also, the Factor of Safety (FS) for all stimulated
143 gully slopes were extracted. The FS aided in determining the ratio of available shear strength of the slope
144 material to the shear resistance required to maintain equilibrium is highly important (Nebeokike et al. 2020).

145

146 3.3 Hydrogeological survey

147 The vertical electrical sounding (VES) method was employed in this study to determine the hydrological
148 characteristics of both formations and their impact on the gully occurrence in the study area. The VES
149 method was done based on estimating the electrical conductivity or resistivity of the hydrostratigraphic
150 sequences. The VES principle method is done by inserting an electric current of known intensity through
151 the ground with the help of two electrodes (power electrode – AB) and measuring the electric potential
152 difference with another two electrodes (measuring electrode – MN). The sounding depth is proportional to
153 the distance between (spacing) the power electrodes (Egbueri and Igwe 2020). It implies that as the power
154 electrode is spaced farther away, more depth information is obtained.

155 In this study, four VES stations were recorded. Two for each of the formation and was done along the major
156 gullies in Obioma, Iva valley (Ajali Formation), Nanka and Agulu (Nanka Formation), to delineate the

5
157 groundwater level around these gullies (Igwe, 2017; Okoyeh et al., 2014; Egbueri and Igwe; 2020). These
158 four stations were chosen due to their proximity to others. For the Ajali Formation, the VES at Obioma may
159 cover gully locations for the 9th Mile and Udi area, while the Iva valley may serve the Ngwo area. For the
160 Nanka Formation, the VES at Nanka may cover gully locations at Ekwulobia, Umuchu, and Uga; the one
161 at Agulu may also cover Igbo-Ukwu Oraukwu and Nimo areas.

162

163 4 Results and discussion

164 4.1 Field observation and gully slope geometry

165 The field observations and gully slope geometry are presented in Table 1. The field observation noted that
166 the erosional intensity of all visited gully sites is of moderate to very high intensity, showing high active
167 gullies (Table 1). The gullies within the Nanka Formation have larger and more threatened gully geometry
168 than the Ajali Formation. However, it was noticed that the Ajali area are more associated with landslide
169 than the Nanka area. Furthermore, observations like collapse slopes failed drainage channels were also
170 noted. On a general note, all visited and studied gullies have slopes ranging from 330-880, showing possible
171 landslide evidence. The topographic configuration of the studied areas has an undulating or uneven
172 topography (Fig. 2). Studies have shown that topography plays a major role in slope failure and landslides
173 in tropical soils within southeastern Nigeria (Okoyeh et al. 2014; Igwe and Egbueri 2018; Egbueri et al.
174 2021).

175 Further field observation shows that most of the gullies' toes contained quicksand, which indicates
176 liquefaction of soil materials under high rainfall infiltration. Infiltration capacity was observed to increase
177 with the sand content of the soil. As sand percentage increases, the amount of silt and cohesive clay
178 decreases in the slope material. Soil having a high percentage of sand will have less clay, lower cohesive
179 strength, and be less resistant to erosion by flowing water.

180

181 4.2 Geotechnical characteristic of the gully slope material

182 The gully slope materials were analyzed in other to determine their geotechnical characteristics. Based on
183 the obtained results shown in Table 2, it summarizes that the gully slope materials are vulnerable and
184 subjective to failure. It's indicative that the particle size analysis reveals a soil material composed of sandier
185 materials than finer materials like clays which could have served as natural binders resisting shearing force.

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186 From the table that the content of gravel, sand, and fines are in a range of 0−3%, 7.75−95.0%, and 5−22.5%
187 respectively for Ajali Formation and 4.42−17.7%, 66.5−89%, 4.7−22.2% respectively for Nanka Formation
188 (Table 2). It also reveals a sand-dominated material with low cohesive slope material observed in the field.
189 The particle size distribution curves in Figure 4 have shown a similar S-shaped for all gully slopes,
190 confirming a similar slope failure pattern/trend similarity. Egbueri et al. 2021, reported that the presence of
191 gravel in a soil material enhances shearing mobility in soils, especially when there is a force subjecting the
192 slope to failure.

193 Furthermore, the gully slope materials of the analyzed soils were classified into four soil types based on the
194 Unified Soil Classification System scheme (USCS). They include silty sand (SM) and poorly graded sands
195 (SP), well-graded sand (SW), and clayey sand (SC) (Table 2). Studies have shown such soil classes as soils
196 with a high amount of sand in the slope material and attribute a strong significance for gully occurrence
197 and development.

198 Permeability and porosity are factors that determine the influence of rainfall on soil material and its
199 infiltration capacity (Todd 1980; Igwe and Egbueri 2018; Nebeokike et al. 2020). The obtained results for
200 permeability and porosity are presented in Table 2. The k ranged from 1.13 𝚡 10- to 5.25 𝚡 10-4m/s, and n
201 ranges from 36 to 51% (Table 2). The soil permeability is moderate to highly permeable as reported by
202 Igwe et al. (2013), k in the ranged 10-7 –10-5m/sec are characterized as moderately to highly permeable.
203 Following the results of permeability and porosity in this study, it is suggested that the gully slopes materials
204 are generally loose, porous, and moderately permeable, indicating a high infiltration capacity. It also agrees
205 with the particle size distribution result revealing a high amount of sand in the gully slopes.

206 To further understand the nature of a gully slope material, there is a need to determine its compressibility
207 (Nebeokike et al. 2020, Egbueri et al. 2020; Egbueri and Igwe 2020). In doing this, a compaction test was
208 carried out to understand how loose or compacted the slope materials are at optimum moisture content. In
209 this study, the compaction test results are presented in Table 2. Based on the results, the maximum dry
210 density (MDD) ranged from 1.69 to 2.10g/cm3, and the optimum moisture content (OMC) ranging from
211 11.0 to 18.10%. The result reveals that the gully slope materials have a low to moderate maximum dry
212 density (MDD) and low optimum moisture content (OMC). Which are within the range of sandy slope
213 materials, and it's suggestive of a very loose slope material requiring little force to erode. The compaction
214 curve shown in Figure 5 has a similar n-shape for all samples, and it's indicative of a very loose soil material
215 with the tendency of collapse. Similar studies on slope stability within the southeastern region have reported
216 similar compaction characteristics revealing the nature of the materials as loose and collapsible (Maduka
217 et al. 2016; Igwe and Chukwu 2018 and Egbueri et al. 2021). It is believed that the loose nature of these

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218 slope materials, as revealed in the compaction test, can be attributed to the water holding capacity and
219 hydraulic conductivity of the materials (Igwe and Egbueri, 2018; Nebeokike et al., 2020).

220 The natural moisture content (NMC) result ranged from 2-9% (Table 2). It shows that the gully slope
221 materials have low water content revealing an unsaturated soil state. As reported by several researchers,
222 low moisture content reduces the effective stress and cohesiveness of soil particles, therefore making these
223 soil materials easy to erode (Isikwue et al., 2012; Nebeokike et al., 2020; Egbueri et al., 2021). The results
224 of the Consistency test are presented in Table 2. The liquid limit (LL) ranged from NP-28%, plastic limit
225 (PL) ranging from NP-15%, and plasticity index (PI) ranged from NP-10%. Based on the results, it was
226 observed that the slope materials generally have zero to low plasticity characteristics, which makes them
227 susceptible to erosive agents and landslides. These soil materials can be classified under problematic gully
228 slope, attributed to low LL (Msilimba and Holmes 2005; Fauziah et al. 2006; Baynes 2008; Egbueri et al.
229 2021). Egbueri et al. (2021) suggested that soil materials with LL values > 25% are problematic soils. In
230 this study, most of the gully slopes are within the category of problematic soils; however, some which could
231 seem safe by exceeding 25% may have other factors which will possibly subject them to sliding or collapse.

232 The shear strength parameter (cohesion and friction angle) in the gully slope study gives a better
233 understanding of the stability of the gully slope soils. An increase in the shear strength parameters increases
234 the stability of slopes (Muthreja et al., 2012; Coulibaly et al., 2017; Egbueri et al., 2021). The shear strength
235 of the analyzed slope materials in this study is presented in Table 2. The values of the cohesion ranged from
236 0-7kPa and 23-380 for the friction angle. It shows a similarity in the ranges of cohesion and friction angle.
237 Similar values were obtained by researchers that have worked on tropical slope soils within southeastern
238 Nigeria, which have reported a minimal to low shearing resistance. It could be attributed to the high sandy
239 content and low proportion of finer material, as revealed by the particle size analysis. (Egbueri et al. 2017;
240 Emeh and Igwe 2017; Igwe and Egbueri 2018; Igwe and Chukwu 2018; Nnebeokike et al 2020; Egbueri et
241 al. 2021). Summarily, based on the geotechnical results and characteristics of the gully slopes, it is believed
242 that rainfall infiltration has a high tendency in the gully slope failure due to variation in the rainy and dry
243 seasons. Whereby the slopes get saturated in the rainy season and dry up during the dry season, and as this
244 goes on, it reduces shearing resistance due to suction and strength. At a stage, a little to no force acted upon
245 the gully slopes (quarrying or vibrations from domestic appliances) will lead to collapse or results in a
246 landslide (Crosta and Frattini 2008; Igwe 2014; Igwe and Fukuoka 2014; Behara et al. 2016; Egbueri et al.
247 2021).

248

249 4.3 Multivariate statistical analysis of geotechnical parameters

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250 4.3.1 Principal component analysis

251 The principal component analysis (PCA) was done differently for the Ajali and Nanka formations to show
252 key components and relationships with the geotechnical parameters. The PCA results are presented in
253 Tables 3 and 4, respectively, for Ajali and Nanka formations. For the Ajali Formation, four PC classes were
254 obtained with their eigenvalues > 1 (Table 3). For the four PC classes, a total of 95.932% variability was
255 obtained. PC class 1, having the highest variability of 49.622% with its loading on the sand, fines, LL, PI,
256 NMC, MDD, OMC, cohesion (C), and friction angle (Ø) (Table 3). Parameters with PC loading within the
257 range of ±0.5−0.7 are moderately loaded, while PC > ±7 are highly loaded. The loading PC1 is of both
258 negative and positive loadings. Sand, fines, LL, NMC, OMC, C, and Ø are highly negatively and positively
259 loaded. Although PC class 1 is believed to be of special influence in this study, high loading plays a major
260 role in the gully slope failure. Egbueri et al. 2021 stated that parameters with positive loadings or negative
261 loading have a closer association. It implies that in this study, the C and Ø are influenced by the sand and
262 fines content, whereby an increase in sand content reduces the angle of internal friction, and a deficiency
263 in finer materials affects the cohesion values of the slope materials sliding action. Also, in this study, the
264 fines were observed to have a close association with the LL, NMC, and C, which was in agreement with
265 several authors suggesting that increasing fines content directly influences the LL, NMC, and C (Nath and
266 Dalal 2004; Isik and Keskin 2008; Sudha Rani and Phani Kumar 2011; Sen and Pal 2014; Egbueri et al.
267 2021).

268 A total variability of 23.978% was observed for PC class 2, with high loading on PI, MDD, OMC, and
269 porosity (n) with a moderate loading on permeability (k) (Table 3). Positive loading was observed on PI,
270 MDD, OMC, and k. The only negative loading in this class was observed on n. It suggests that the soil PI
271 has a direct influence on the MDD, OMC, and k. It could be attributed to the deficiency in finer materials,
272 as revealed by the particle size analysis. The total variance for PC class 3 and 4 was 13.503% and 8.828%,
273 respectively (Table 3). Gravel was observed to influence the soil PL for the PC 3. It suggests that gravel
274 content plays a role in the plasticity limit of slope material. For PC 4, moderate negative loadings were
275 observed with the PL and a moderate positive loading with the Ø. These parameters play a lesser role in the
276 gully slope failure than the parameters enlisted in the PC 1 and 2.

277 Three PC classes were obtained for the Nanka Formation with a total variance of 87.726% (Table 4). Total
278 variance of 55.376% was obtained for PC 1 with positive loadings on fines, LL, PL, PI, NMC, and C, while
279 negative loadings were observed on the sand content, MDD, and Ø. It suggests that fines content has a
280 major influence on most of the parameters in this class because it influences the LL, Pl, PI, NMC, and C.
281 However, the sand content influences the MDD and Ø. It agrees with the earlier suggestion that increased
282 fines content directly influences the LL, NMC, and C (Nath and Dalal 2004; Isik and Keskin 2008; Sudha

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283 Rani and Phani Kumar 2011; Sen and Pal 2014; Egbueri et al. 2021). It shows that a decrease in sand
284 content as observed in the particle size distribution influences the PI, NMC, and C. These factors are
285 important in landslide study since they have been captured to have a major influence on the gully slope
286 failure within the area (Nebeokike et al. 2021; Egbueri et al. 2021).

287 The PC 2 was observed to have a total variance of 23.019%, while PC 3 explains a total variance of 9.331%
288 with its loading on OMC (Table 4). For PC 3, significant loadings were observed on gravel, LL, MDD,
289 OMC, and k (Table 4). From the observation, gravel negatively influences LL and dry density, while the
290 OMC positively influences the k. The parameters enlisted in PC 2 have a lesser influence on the sliding
291 activity of the gully slopes. However, parameters enlisted in PC 1 have a major influence.

292

293 4.3.2 Hierarchical cluster analysis (HCA)

294 The hierarchical cluster analysis (HCA) was done differently for the Ajali and Nanka formations on the
295 sixteen samples using the gully slopes geotechnical parameters. It was in other to determine the association
296 of each sample regarding its landslide tendency, i.e., to classify the samples based on the analyzed gully
297 slope geotechnical parameters to help understand the degree to which the gully slope is more threatened.
298 Figure 6 shows a dendrogram using Ward's linkage method and Z score standardization to reduce bias. The
299 two dendrograms in Figure 6 represent the studied gully slope for the Ajali and Nanka formations. For the
300 first dendrogram (Fig. 6a), two cluster groups were obtained. The first cluster group signifies samples with
301 high sliding potential, including AJ8, AJ3, AJ7, and AJ1. However, the second cluster group signifies
302 samples with moderate and considerable sliding tendency, including AJ2, AJ5, AJ6, and AJ4. For the field
303 observations, the gully slopes enlisted in the first cluster group were observed to have a high slope gradient
304 and situated on high elevation (Table 1). It is also believed that these samples have poor geotechnical
305 parameters compared to the enlisted samples in the second cluster group.

306 The second dendrogram (Fig. 6b) represents samples within the Nanka Formation. In this dendrogram, two
307 cluster groups were obtained and two sub-cluster groups. The first cluster enlisted samples NK3 and NK4,
308 and they signify samples with very high sliding tendency. The two samples enlisted in the first cluster group
309 cover the Nanka gully complex and Agulu gully complex. As noted in the field, these two gully sites have
310 the highest gully geometry with an average lateral extent of over 750-1000m. The second cluster group was
311 further subdivided into two sub-cluster groups. It was based on Euclidean differential distances from each
312 other. The first sub-cluster group contains samples NK7 with a little difference in sliding tendency from
313 the NK1 and NK5, which are believed to have a moderate and considerable sliding tendency. The second
314 sub-cluster group contains samples NK2, NK6, and NK8. The Euclidean distances between samples NK2

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315 with NK6 and NK8 show little sliding tendency, which it's believed that NK6 and NK8 have the lowest
316 sliding tendency, and these samples may pose little to no hazard over a long period.

317

318 4.4 Gully Slope Stability Analysis

319 The stability analysis of the sixteen gully slopes within the study area was evaluated and computed along
320 with their Factor of Safety (FS). These gullies were the same as the ones analyzed for geotechnical
321 characteristics. The FS in this study was compared with the stability classification chart adopted by
322 Hadmoko (2004). Little modification was done on this classification chat to suit properly for tropical soils.
323 Based on this chart, a Factor of Safety (FS) < 0.9−1 indicates an unstable slope, while FS > 0.9 indicates a
324 purportedly stable slope (Nebeokike; 2020).

325 A summary of the stability analysis is presented in Table 5 for both saturated and unsaturated conditions,
326 respectively. Furthermore, Figures 7 and 8 show slope stability models for both conditions. In this study,
327 gullies AJ3, AJ5, and AJ8 represent the Ajali Formation, while NK3, NK5, and NK7 represent the Nanka
328 Formation were shown for saturated and unsaturated conditions (Figs. 7 & 8 respectively). The computed
329 FS values for saturated conditions ranged from 0.54 to 1.35 with a mean of 0.78 (Table 5). Based on the
330 results, 81.25% of the gully slopes are unstable whereas, 18.75% are critically stable. However, the obtained
331 FS values for unsaturated conditions range from 0.61 to 1.78 and a mean of 1.05 (Table 5). Based on the
332 results, 68.75% of the gully slopes are stable to critically stable whereas, 31.25% are unstable. It implies
333 that a greater percentage of the gully slopes are unstable during the rainy season than the dry season.
334 Therefore, these gully slopes are less hazardous during the dry season than in the long run of the rainy
335 season when the gully slopes are partial to fully saturated. These results further imply that the instability of
336 a slope is dependent on the Factor of safety, which decreases with an increase in precipitation, thereby
337 changing the volumetric water content of the materials as reported by several authors (Onda et al. 2004;
338 Rahardjo et al. 2005; Igwe, 2015; Igwe and Chukwu, 2018).

339 In comparing the simulated FS models for both saturated and unsaturated conditions (Figs. 7 and 8),
340 several disparities could be indicated, as revealed in the model. The sliding mass ranged from 151.25 to
341 215.55m3 and 1.62.3 to 225.86m3 (Ajali and Nanka Formations respectively) for saturated condition and
342 124.25 to 171.51m3 and 137.63 to 195.51m3 (Ajali and Nanka Formations respectively) for unsaturated
343 condition (Figs. 7 and 8). It reveals that a greater mass of sediments is eliminated during the rainy season
344 than in the dry season. In an outcome of possible gully erosion, a relatively rotational slip surface passing
345 slightly below the toe of the slope, having a depth of about 4m and a sliding mass of about 151.25m 3 as
346 shown in AJ3 (Figs. 7 and 8). Furthermore, Figures 7 and 8 indicate that a drop in rainfall infiltration during

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347 the dry season improved the FS of the gully slope. It illustrates that a decline in FS from 0.77 to 1.06 reduces
348 the sliding mass from 151.25 to 124.25m3 for AJ3 (Ajali Formation) and FS from 0.65 to 0.72 reduces
349 sliding mass from 162.3 to 137.63m3 for NK3 (Nanka Formation) (Figs. 7 and 8). It agrees with the reports
350 of some researchers that slope failures with these magnitudes of sliding masses could be described to
351 accompany an intermediate to high landslide (Maduka et al., 2016; Igwe and Chukwu, 2018).

352 The soil pore-water pressure presented in Figures 9 and 10 shows that in a saturated condition, the gullies
353 have a range of -160 to 200kPa and -800 to 600kPa for Ajali and Nanka formations, respectively (Fig. 8).
354 It reveals a high accumulation of pore-water pressure in the gully slope during the rainy season. During this
355 season, rainfall infiltration increases, and the gully slopes are partial to fully saturated with water and are
356 unstable, thereby posing a great hazard. These findings equally validate the results of FS, which suggests
357 that during the wet season, the FS is less than 0.9. However, during the dry season, when there is little to
358 no rainfall and a decrease in infiltration capacity, the pore-water pressure drops from -190 to 170kPa and -
359 1200 to 200kPa for Ajali and Nanka formations (Fig. 10). These results reveal that the gully slopes contain
360 little water during the dry season and a drop in the pore-water pressure. The presence of negative pore-
361 water pressure for both saturated and partially saturated soil agrees with the findings by Nnebokike et al.
362 2020, who also noted that the fundamental cause of negative pore-water pressure appears to be from osmotic
363 and adsorptive effects and the surface tension of water. Nebeokike et al., 2020 also noted that the principal
364 cause of negative pore-water pressure in saturated soils is probably the osmotic effect, hydrostatic stresses
365 rising from a dilating tendency on shear, and hydrostatic effects resulting from the stresses carried by
366 bent/distorted soil particles.

367 A safety map was generated for each gully slope in an attempt at several trial slip surfaces, as presented
368 in Figures 10 and 11. Igwe and Chukwu, 2018a stated that a safety map reflects a zone of potential failure
369 in which the Factors of Safety of the trial slip surfaces are similar. The FS during saturated condition were
370 observed at AJ3 (0.77–0.88), AJ5 (0.76–0.81) and AJ8 (1.10–1.15) for Ajali Formation and NK3 (0.65–
371 0.77), NK5 (0.50–0.62) and NK7 (1.35–1.40) for the Nanka Formation (Fig. 10). For the unsaturated
372 condition, FS was improved from 1.06–1.11 (AJ3), 0.99–1.04 (AJ5), and 1.25–1.30 (AJ8) for the Ajali
373 Formation and 0.72–0.77 (NK3), 0.61–0.70 (NK5) and 1.78–1.83 (NK7) for Nanka Formation (Fig. 11).
374 Safe zones are the areas within the red band (Figs.11 and 12), and all the Factors of safety within the location
375 indicated fairly good stability.

376 A spatial distribution map for the Factor of safety values was produced to illustrate areas of varying
377 stability conditions. Figures 13 and 14 present the spatial distribution map for both saturated and
378 unsaturated seasons within the study area. The result reveals that all gully slopes are relatively unstable
379 during saturated conditions due to high pore-water pressure, which decreases the soil matric suction. At this

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380 period, gully enlargement and expansion take place. Following the spatial distribution map (Figs. 13 and
381 14), towns like Umuabi, Obioma, Ekwulobia, Nanka, Uga, and Oraukwu obtain the least FS values. These
382 areas are relatively unstable to critically stable in both rainy and dry seasons.

383

384 4.5 Hydrogeological Characteristics

385 The results obtained from the geoelectric surveys are presented in Table 6, and the VES models are
386 shown in Figure 15. The aquiferous layers are highlighted in bold (Table 6). The aquiferous zones in this
387 study have resistivity ranging from 928.08 to 4397.00 Ωm with a thickness and depth which ranges from
388 2.7 to 66.33m and 3.2 to 84.4m, respectively. The VES curves comprise the k-type curves that usually
389 indicate saturated zones (Egbueri and Igwe, 2020). Figure 15 reveals that the major kicks on the curves
390 represent saturated zones. The minor kicks are inferred to be intercalations of clay and sand, with little or
391 no saturation. The study areas are characterized by shallow groundwaters, which the abundance of surface
392 water may influence. It is believed that the abundance of surface water networks influences the shallowness
393 of the water table, which suggests that the recharge and discharge mechanisms are jointly experienced in
394 the study area. Factually, this reveals that the hydrological process (groundwater hydrology working in
395 conjunction with the surface water hydrology) influences the gullying processes in the study area (Egboka
396 & Nwankwor, 1985; Egbueri and Igwe, 2020).

397 A model correlating the aquiferous layers for both formations is shown in Figure 16. It reveals that the
398 groundwater flows from the region of higher elevation (Obioma) down to the area with a lower elevation
399 (Agulu and Iva valley) westward, dependent on the topography. The Obioma, Nanka, and Agulu VES have
400 the shallowest aquiferous layer correlated to each other. These are termed to be perched aquifers relating to
401 the aquifer above the regional water table. It occurs above discontinuous aquitard, which allows
402 groundwater to mound above them. It correlates with the observations made during the field mapping as
403 water drips from some sandy overburden, especially in the Nanka – Agulu gully sites. It is believed that as
404 this water drips off, it also creates channels for rill erosion occurrence.

405

406 5 Conclusions and Recommendations

407 Southeastern Nigeria has faced several environmental challenges arising from the displacement of people
408 from their natural habitat, occasional loss of lives, and disruption of arable lands. All these happen as a
409 result of gully development and landslide activities. The study was conducted on the gully slopes further to

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410 understand the failure mechanism within the studied area. From the study, the following conclusion was
411 made;

412 • Based on the geotechnical parameter, the gully slopes were mostly dominated by sandier materials
413 and lacked finer materials, as revealed by the particle size analysis. However, the USCS classified
414 the slope soils as SM, SP, SW, and SC soils which have high significance for landslide action when
415 pore water pressure increases.
416 • The soil permeability and porosity ranged from 1.13 𝚡 10- to 5.25 𝚡 10-4m/s and 36 to 51%,
417 respectively, indicating moderate to high infiltration capacity.
418 • The soil compressibility test result suggests a loose and collapsible soil nature (with the dry density
419 ranging from 1.69 to 2.10g/cm3 and the optimum moisture content in the range of 11.0 to 18.10%).
420 • The slope materials are non-plastic to low plastic. However, the shear strength parameters revealed
421 that the slope materials are relatively low in cohesion (ranging from 1−7 kPa) and low friction
422 angle (ranging from 23−38º), suggesting a weak resistance to shearing forces.
423 • Furthermore, the PCA aided in identifying interrelationships among the slope parameters, while
424 the HCA helped categorize the gully slopes based on their landslide potential.
425 • The obtained Factor of safety (FS) for all gully slopes ranged from 0.54 to 1.10 and 0.88 to 1.25
426 for the Ajali Formation based on the slope stability analysis. Also, the Nanka Formation ranged
427 from 0.50 to 1.35 and 0.61 to 1.78 (for saturated and unsaturated conditions, respectively). It reveals
428 that the gully slopes are typically unstable in saturated conditions and critically stable in unsaturated
429 conditions. Therefore, the gully slopes are highly hazardous in the rainy season when the gully
430 slopes are partial to fully saturated than in the dry season.
431 • The spatial distribution maps illustrating some areas such as Umuabi, Obioma, Ekwulobia, Nanka,
432 Uga, and Oraukwu are severely critical to unstable slopes for rainy and dry seasons.
433 • The hydrogeological investigations revealed several shallow aquiferous layers. The shallowness of
434 the aquifers and availability of surface waters within the area as captured in the geological map
435 plays a major part in the saturation of the gully slopes.

436 With the recent developmental activities experienced in the southeastern region of Nigeria and the trend in
437 climatic changes, it's believed that several factors, if great care is not taken, will lead to rapid initiation and
438 expansion of more gullies within this area. It is thereby recommended that soil conservation programs
439 should be enforced and some slope control measures put in place, as has been recommended by several
440 authors (Maduka et al. 2017; Igwe and Chukwu 2018; Egbueri et al. 2021).

441

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