Result of the field study
From Table 2, the result of elevation of the gullies and slides range from 22.86m to 332.0m with mean elevation of 183.11m of the landslides and gullies. The angle of slope of the slides (slope steepness) ranges from 70 to 320 with mean slope angles of 20.250and these slope angles which are key parameters in estimating susceptibility to developing debris rock slides in the study area. The slopes angles range of 70 to 320indicate that the slope angles fall under the class of gentle to steep slope (Sikdar et.al 2004). The field result shows that the largest occurrences of landslide in the study area fall within an interval of slope angles ranging from 160 to 320. The slope angle of 32o which indicates a steep slope (Tables 2 & 3, Figs. 4 & 5) where the largest occurrence of landslides falls in the field coupled with high elevation of 330m with mean elevation of 183.11m respectively have contributed immensely to the high instability of the soils in the study area. This has resulted to landslide and gully erosion and this support the work of (Koko et al., 2005; Chen et al., 2012a, b; Igwe 2015). Slope angles of 32o at high elevation values measured in this work, correlates with the works of Chen et al. (2012a, b) and Koko et al. 2005. Chen et al. (2012a, b) identified slope failures that occurred after the 2008 Wenchuan earth quake, took place majorly on slope angles range of 30o -50o.. Koko et al. (2005) had similar observations that the hazards and risk correlated with rainfall induced landslide along railway occurs at high slope angles of (32o − 42o). This high slope values and elevation of the landslide and gullies in (Fig. 4, Table 2) the area is the cause of the mass sliding activities that are prominently trending in the northeast direction in the study area.
Table 2
Field measurements for gullies and slides in the study area
S/N
|
Longitudes (E)
|
Latitudes (N)
|
Elevations (m)
|
Lateral extents
(m)
|
Depth (m)
|
Width(m)
|
Slope Angle
(O)
|
Description of slope(Sikdar et.al 2004)
|
1
|
040 51’ 23’’
|
06030’4’’
|
332.00
|
80.0
|
68.7
|
23.2
|
32
|
Steep slope
|
2
|
04005’43’’
|
06027’41’’
|
310.80
|
40.2
|
68.2
|
20.4
|
31
|
Steep slope
|
3
|
040 43’16’’
|
06028’32’’
|
306.00
|
80.0
|
66.9
|
28.4
|
20
|
Moderately steep slope
|
4
|
040 48’ 10’’
|
06028’30’’
|
200.40
|
73.0
|
66.3
|
31.6
|
18
|
Moderately
steep slope
|
5
|
040 12’13’’
|
06024’00’’
|
120.00
|
64.0
|
64.7
|
30.4
|
16
|
Moderately
steep slope
|
6
|
040 13’13’’
|
06004’14’’
|
118.60
|
93.0
|
67.0
|
33.8
|
13
|
Moderate slope
|
7
|
050 32’07’’
|
06032’00’’
|
58. 40
|
69.4
|
67.1
|
33.7
|
10
|
Moderate slope
|
8
|
050 12’ 18’’
|
06037’01’’
|
26.30
|
80.2
|
20.3
|
72.6
|
09
|
Gentle slope
|
9
|
050 03’ 00’’
|
06037’00’’
|
24.70
|
50.2
|
4.1
|
46.7
|
07
|
Gentle slope
|
10
|
050 13’ 00’’
|
06027’01’’
|
22.86
|
43.0
|
2.6
|
30.4
|
07
|
Gentle slope
|
11
|
050 32’13’’
|
06024’00’’
|
120.00
|
64.0
|
60.7
|
40.4
|
16
|
Moderately
steep slope
|
12
|
04013’ 23’’
|
06034’14’’
|
118.60
|
93.0
|
65.0
|
34.8
|
13
|
Moderately
steep slope
|
13
|
050 44 16’’
|
06028’32’’
|
306.00
|
80.0
|
66.9
|
28.4
|
20
|
Moderately steep slope
|
14
|
050 42’ 10’’
|
06028’30’’
|
200.40
|
73.0
|
64.3
|
32.6
|
18
|
Moderately
steep slope
|
15
|
050 52’13’’
|
07034’00’’
|
120.00
|
64.0
|
64.7
|
32.4
|
16
|
Moderately
steep slope
|
16
|
050 51’ 19’’
|
06037’40’’
|
332.0
|
80.0
|
60.7
|
24.2
|
32
|
Steep slope
|
17
|
050 55 20’’
|
06 37’41’’
|
310.80
|
40.2
|
68.4
|
20.4
|
31
|
Steep slope
|
18
|
05053’ 16’’
|
06 38’32’’
|
308.00
|
80.0
|
66.9
|
30.4
|
32
|
Steep slope
|
19
|
050 54’10’’
|
06038’30’’
|
204.40
|
73.0
|
64.3
|
32.6
|
32
|
Steep slope
|
20
|
050 55’13’’
|
06040’00’’
|
122.00
|
64.0
|
63.7
|
30.4
|
32
|
Steep slope
|
|
|
|
MEAN
= 183.11
|
MEAN
= 87.30
|
MEAN
= 57.07
|
MEAN
= 32.24
|
MEAN
= 20.25
|
Remarks:Gentle
– Steep Slope
|
Table 3
Frequency of slope angles of the slides and gullies
Slopes Angles of the landslides (0)
|
Frequency of the landslides
|
70
|
2
|
90
|
1
|
100
|
1
|
130
|
2
|
160
|
3
|
180
|
2
|
200
|
2
|
310
|
2
|
320
|
5
|
The lateral extent ranging from 40.2 to 93.0m with mean lateral extent of 87.30m. The depth of slide ranging from 2.6m to 68.7m with mean of 57.07m and width value ranging from 20.4m to 72.6m with mean of 32.24m, indicate a high degree of erosion and sliding in the study area. The generated digital elevation model, slope map and land use map of the study area from 30m data -Spatial Resolution and Shuttle Radar Topographic Mission (SRTM) are shown (Figs. 3, 4 above and Fig. 6 below respectively). It shows that the elevation, the steepness of the slope, excess run off from heavy down pour and infiltration, deforestation, over grazing and the improper use of the land from the land cover map and poor drainage system contribute to the landslides and the exposure of the study area to other landslide related hazards in the study area. From the plot of the dimensions of the landslide against the slope angles of the landslide (Fig. 7), it shows that in this year the lateral extent and slopes angles of the landslides and gullies are sliding at almost the same rate in the north east direction in the field. During the early stage of gullying activities and sliding, the lateral extent of the slides and width of the slides were not proportionate. As the landslides and gullies become rapid this year, the lateral extent of the sliding become relatively proportional to the slope angles of the landslides (Fig. 7). The sliding in the area ranges from gentle to steep slopes at angle of 70 − 320 and the slides are trending in the north east direction in the study area (Figs. 8a & b). From Fig. 8b the blue colour shows initial stage of the sliding as very gentle to gentle slope. The red colour shows the advance stage of the sliding as moderately steep slope to steep slope in the study area. As the width of landslides increases, the lateral extent of the landslides increases relatively and these occur more on the mean slope angles of 22.5 o at mean elevations of the landslides and gullies of 183.11m. The lithology investigated in the field are upper most layer (dense lateritic soil), Ajali Formation and some clay materials at the slide walls and floor of the slides/gullies. The upper most layer is mostly of red earth with little clay material present. The Ajali Formation is characterized of medium to coarse grained, friable, unconsolidated to consolidated sands of 250cm thick in the landslides area. Also, within the landslide areas the units are interbedded by clay and fine materials.
Result of geotechnical analyses
The soil samples (table 4) from the landslides and gullies areas shows that soil samples SB1, SB2, SB6 and SB7 are mainly sandy clay with a plasticity indices of 30-36, coefficient permeability range of 3.5 ×104-cm/sec – 4.2 ×104-cm/sec, cohesion range of 27kƿa - 28kƿa and angle of internal resistance range of 27o - 30o respectively. Soil samples SB3, SB4, SB8 and SB9 are coarse-grained sand with no plasticity indices. The coefficient of permeability for samples SB3, SB4, SB8 and SB9) range of 2.8 × 104-cm/sec–3.2 ×104-cm/sec, cohesion 10kƿa - 18kƿa with angle of internal resistance of 24o- 26o respectively. Soil samples SB5 and SB10 are silty clay with plasticity index range of 40 - 41, coefficient of permeability of 4.6 × 104-cm/sec - 4.8 ×104-cm/sec, cohesion of 45kƿa - 46kƿa and angle of internal resistance of 10o-12o respectively. Soil samples (SB) 3, 4, 8 and 9 are non-plastic and they have liquid limits (LL) range of 24–26 with mean Liquid limit of 40 in the study area. The plastic limits (PL), liquid limit (LL) and plasticity index (PI) of SB5 and SB10 are 36 and 35, 76 and 76, 40 and 41 respectively. Plasticity index of 40 is high in the study area and this falls within the range of findings of Sowers and Sowers (1970). They reported that PI > 31 should be considered high and this indicates high content of expansive clay. SB5 and SB10 have liquid limit values of 76 and this conforms to the findings of Bell (2007), who classified clays with Liquid Limit (LL) range of 70– 90 as very high plasticity. The high plasticity values of 76 in SB5 and SB10 which are mainly of silt to clay layers because of their mineral composition will absorb a lot of water from rain fall and expand its volume within the slide and the gullies areas. The increase in the volume will create an upward force on the overlying coarse sand and medium sand layers. This alternating sequences of swelling and shrinking during wet and dry seasons will in turn give rise to the initiation of slopes failures and landslide activities in the study area. This has been able to explain the mechanism and root cause of the problem in the area. This may possibly explain why the clays in this area serve as glider/gliding plane to several landslides (Figs.9a&9b). In addition the very low permeability of silty clay units (SB5, SB10) that separates the sand units in the area have been identified to be the gliding plane for several landslides in the area of study (Okagbue 1992).
The cohesion values in the study area ranges from 10-40kƿa with a mean value of 25.6kƿa. The angles of internal resistance within the grains ranges from 100- 300 with mean of 23.70. The low cohesion values of 10kƿa with mean cohesion of 25.6kƿa suggest a low cohesion between the grains (a very loose compaction). The mean value of angles of internal resistance between the grains in the area is 23.70. This low value suggest very loose compaction (Surenda and Sajeev 2017). They classified angles of internal resistances between grains as follows: < 280 shows a very loose compaction, 280-300 displays loose compaction, 300-360 indicates a medium compaction, 360- 410 indicates dense compaction, > 41 shows very dense compaction. The low values of cohesion and angles of internal resistances resulted to cracks, fractures and slopes of various degree in study area. The non-plastic characteristics of the Ajali sands which display a very loose to loose compaction in the study area increases its susceptibility to erosion and landslides.
The permeability values range from 2.6 × 104-cm/sec - 4.8 ×104-cm/sec with mean permeability of 3.65 ×104-cm/sec in the Ajali Formation which is labelled (SB3, SB4, SB8, SB9) in the study area. The high permeability values of range 2.6 × 104-cm/sec – 3.2 ×104-cm/sec in the Ajali Formation depicts a high permeability due to non-plasticity of the Ajali Formation in the study area. The high permeability values shows that the Ajali Formation in the study area (Fig.10a &b) transmits enough volume of water to the underlying SB5 and SB10 (silt/clay layers) during period of raining season. Within this sand- clay boundary a high pore-water pressure will be developed. This excess water will be release at the boundary from the clay (Fig.9a) because of low permeability values of 4.6 × 104-cm/sec - 4.8 ×104-cm/sec from SB5 and SB10 (Igwe and Una 2019). The increase in the volume of water is as a result of rainfall in the study area which will in turn create an upward force on the overlying coarse Ajali Formation. Due to wet and dry seasons experience in the study area it will give rise to swelling during wet period and shrinking during dry season. This continuous alternation of swelling and shrinking will initiate cracks, fractures and landslides in the study area (Fig.11) and this corroborate the work of (Fukuoka 1980; Wieczorek 1996; Igwe et al. 2013; Ogbukagu 1976; Okagbue 1986; Egboka and Okpoko 1984; Okagbue 1992). This has been able to explain the mechanism behind the continuous sliding and gullies in the area. The nature of the Ajali Formation which is friable, highly porous, unconsolidated, prone to rapid disintegration, low shear strength (low cohesion and low angles of internal resistances between the grains), low plasticity of Ajali Formation coupled with high rainfall in the study area over the years resulted in the multiple slope failures and debris soil slide and spread in the study area (Figs.12a,b &13) using the classifications of (Varnes 1978; Crudes and Varnes 1996; Jakob et. 2006; Guzzetti et al. 2008) .