Slope Stability Analysis of Mine Waste Dumps at a Mine Site in Southeastern Nigeria

Slope Stability Analysis Of Mine Waste Dumps At A Mine Site In Southeastern Nigeria

Abstract: This paper investigates the stability of slopes of the mine tailing dumps at Enyigba, Southeastern Nigeria. The geotechnical parameters of the slopes were investigated, and applied in the computation of the factor of safety of the slopes using stability analysis. The numerical models were created with the Slope W Geostudio developed by Geo-slope International Limited. The slope materials showed particle size distribution: gravel (0 – 40%), sand (10.7 – 47.8%) and Fines (34.9 – 80.7%), with LL, PL and PI ranging from 24 – 49%, 24 – 35% and 9 – 26% respectively, while NMC ranges between3 – 13%, signifying low to medium plastic materials with low to medium moisture holding ability. These results agree with the soil low permeability (10^-4 – 10^-8 cm/sec) recorded. The MDD and OMC of the material ranged from 1.86 – 2.22g/cm³ and 13.45 – 17.05% while Cohesion and internal friction angle were recorded to be 13 – 34 kN/m² and 8 – 37° respectively; implying weak materials capable of shear failures. XRD confirmed the presence of swelling mineral (illite) in soil material, suggesting plastic clay, which is one of the factors influencing all forms of mass wasting. The slopes’ computed factor of safety ranged from 0.8 – 1.75, suggesting critical to poor slope stability, when predispose to landslide triggering agents .Hence, slope stabilization is required on the mine tailing dumps at Enyigba to prevent major landslide occurrence.  

Keywords: Mine tailing dumps, Enyigba, Slope stability, Factor of Safety, Landslide, Stabilization

Introduction:

Background

Mining is a process used for the production of economic minerals.  In Nigeria, open cast mining is the commonest method of mining for the extraction of ore minerals. Opencast mining includes rupturing of the upper layer of rocks with blasting, drilling to excavate the valuable mineral (Gupta and Paul 2016) beneath the earth. During this process massive amount of wastes or tailings are generated. These waste materials are heaped in a place for its future use or disposal, or dumped permanently (Das 2011). The storage could be internal (within the mine site) or external (outside the mine). Waste dumps usually possess steep angled slopes because the wastes have been deposited over from the top of the dump in an incessant and progressive manner (Bahadur and Sudeep 2013) and are continually moving downslope due to gravity.

The study area (Enyigba and Ameri Mines) have Pb-Zn mineral deposits which are locally mined by the old open-pit mining method, and mine tailings are indiscriminately dumped internally within the areas (Fig.1) which are in close proximity to the mines. The tailings are composed of ore and gangue minerals which when weathered release those composing heavy metals into the soil. (Ezeh and Anike 2009).The tailing dump-sites are scattered within the localities which cover an area of approximately 80km² and are strategically located at about 14km to Abakaliki (capital city of Ebonyi State, Nigeria) along the Abakaliki – Afikpo expressway (Igwe et al. 2012). The surface topography has been greatly distorted and there is severe environmental degradation.

As land available for mining activities has been a great problem to mining industries, the knowledge and study of dump design is highly needed to store dump wastes within a limited space (Kumar 2013). In any open cast mine, the management of the wastes generated during mining is vital, as it contributes greatly to the mine’s successful operation. Inappropriate management of the overburden (OB) dump can result in stability issues which in turn may affect the safety and the overall production of the mine (Poulsen et al. 2014). BlightandDa-Costa (2004), noted that abandoned waste heaps that do not receive proper treatment and maintenance can often experience partial losses of slope and surface erosion.

Slope movements could be catastrophic and it needs not to be very massive to be destructive. Disastrous landslides have occurred in several places in the world some of it consuming lives and properties, for instance: the 2015 Shenzhen catastrophic flow, killing at least 165 people with many collapsed buildings in China, as reported by CNN; in april 2013, a massive slope failure occurred at the open pit of the Bingham Canyon mine Utah .There are several types of slope failures namely: rotational failure, plane failure, wedge failure and toppling failure. Planar failure of rock occurs when a mass of rock in a slope slides down along a relatively planar failure surface.  This type of failure mostly occurs along structural discontinuities such as joints, faults, bedding planes. Wedge failure can occur in a rock mass with two or more intersecting discontinuities, which is approximately perpendicular to the strike of the slope, and dip towards the plane of the slope; Rotational failure occurs when a soil or rock mass slides along a curved surface, it is commonly caused by erosion. It may be translational if failure occurs in homogenous (isotropic) materials; toppling failure occurs when columns of rock formed by steep dipping discontinuities slips and assumes an outward and downward movement.

Failure of internal dumps is a complex problem, in addition to environmental considerations; it directly affects the mine safety (kainthola et al. 2011), deprivation of production, and additional cost for recovery, mine abandonment or premature closure and potentially loss of life in the study area. As a result, analysis of stability of operation or abandoned tailings is important. Hence this paper is aimed at applying different geotechnical parameters to evaluate the waste dump stability using numerical analysis, so as to suggest an economical, consistent and safe disposal of the mine wastes, and also design possible remedial measures for slope failures.

Understanding the factors that contribute to slope instability provides important information for safe mining operations. Due to continuous increase in dump heights, there is a possible threat to their stability and related problems (Gupta and Paul 2016). Shear strength of mine tailings has a major role to play in analyzing stability of tailings. Good understanding of the shearing behaviour of soil for stability design has involved a combination of laboratory-scale diagnostic testing and engineering judgment (Bradfield et al. 2013). Shear strength involves evaluating the cohesion and frictional angle of the slope material.

Slope stability is mainly determined by the slope geometry and the strength of the materials on it. Geometrical properties of the slope are the most important factor which affects its stability. Some of the key features of slope design parameters that affect the geometric properties are the height of the slope, slope angle, the width of the slope and the total area of failure surface. Slope stability is thus dependent on height- such that, as the slope height increases, stability decreases .More so, the stability of mine tailings depends upon several other factors which are: the compaction rate, the specific gravity of the slope material, moisture content and degree of saturation 

Geomorphology

The study area is located in Abakaliki local government area of Ebonyi state. It lies within longitudes 006^° 05´N and 006^°15´Nand008^° 05´E and 008^°10´E (Fig. 2).  It is accessed through major (federal highway: Abakaliki-Afikpo road – FUNAI road) and minor roads (footpaths and local road networks). It is about 14.2 km from Abakaliki, the capital city of Ebonyi State. It lies within one of the nine vegetation zones in Nigeria (Nweke 2015).It lies within the rainforest savannah region of south-eastern Nigeria (Blench 2004). Its vegetation is characterized by grasses and large trees of various ssizes. The topography of the area is generally undulating, with elevation not more than 400 m above-sea-level (Fig 3). The area is underlain predominantly by shale, which could be responsible for its low erodability.

The area is of humid tropical climate. The mean annual temperature stands at 28^°C (Nweke 2015). It experiences rainy season and dry season in a year (eight months of rainfall and four months of dryness). The area is drained by a major river known as Ebonyi River and its tributaries (iyiudene and iyiokwu). Both tributaries are perennial and usually overflow their banks at the peak of the rainy season. The drainage system of the area is of dendritic pattern (Fig. 2) controlled by the geology of the area. There are networks of major and minor tributaries that empty into the river; both the major river and their tributaries form network of narrow, meandering and feeder streams depicting a dendritic pattern of drainage (Igwe et al. 2012).

Geology of the area

The study area falls under the lower Benue Trough. Figure 4shows the geologic map of the study area. According to King (1950), Farrington (1952), Nwachukwu (1972), Murate (1970) and Cratchly and Jones (1965), Benue Trough originated as a failed arm at the time of the opening of the South Atlantic Ocean during the separation of the African plate and the South American plate. The area is part of the “Abakaliki anticlinorium” and it is underlain by the Abakaliki shales of the Asu River Group of the Albian-cretaceous sediments.

The Asu River Group consists of alternating sequence of shales, mudstone and siltstone with some occurrence of sandstone and limestone lenses in some places and attains an estimated thickness of 1500 meters (Agumanu 1989; Farrington 1952). Kogbe (1989) stated that the sediments consist of poorly-bedded sandy limestone. Much weathering and ferruginization have greatly transformed the black shales to a bleached pale grey shale with mottles of red, yellow, pink and blue (Orajaka 1965; Ukpong and Olade 1979).  The Lithostratigraphic succession of the area include: Albian- Cenomanian-Turonian-Conaician-Santonian-Campanian-Maastrichtian.  The rocks are extensively fractured, folded and faulted particularly following the series of tectonic episodes which have acted on them from Albian times (Benkhelil1986).

The fractures in the area serve as reservoirs for Lead-Zinc mineralization in the area. The highly indurated shales were interpreted by Obiora (2011) to have undergone low grade metamorphism, which is the reason for its usage as construction material. The areas studied generally consist of brown clay to reddish brown shales underlain by black shale which formed part of the components of the dumps.

Methodology

Fieldwork

Field investigations involving Reconnaissance survey and detailed geologic field mapping were carried out to carefully study the observable features on the mine tailings of Enyigba district, generate the slope Geometry, coordinates of the area and to collect samples for geotechnical analysis. The authors wish to state clearly that all Human and Animal Rights were preserved throughout the course of the field work. A total number of fifty (50) slopes were studied and seven (7) samples collected from abandoned and new mine tailings at Enyigba and Ameri mines as representative samples from different locations of the mine site. The samples are labeled AM 1 (OT), AM 2 (NT), EN 3 (OT), EN10 (OT), EN14 (NT), EN15 (OT) and EN16 (OT) respectively for the seven samples (Fig.3). The OT and NT attached to the labels signify old tailings and new tailings respectively.

Laboratory analyses

The samples gotten from the site were analyzed in the laboratory to reveal properties as natural moisture content, permeability, Atterberg limits, specific gravity, compaction, consolidation and shear strength of the materials. X-ray diffraction and X-ray fluorescence analysis were also conducted on the old and new tailings to reveal the component minerals of the clay material. The laboratory tests were carried out according to ASTM standard of soil testing.

Coordinates of the study area were used to develop digital elevation model of the area (Fig. 3).  The Slope/WGeostudio 2012 software developed by Geo-Slope International Limited was used to produce a modeled diagram of slope morphology and failure prediction (in terms of factor of safety) on the slopes. The analysis type used was the Morgenstern-Price limit equilibrium method. Parameters from the geotechnical tests conducted on the slope lithologies were imputed into the Geostudio software, and the parameters keyed in included the unit weight, cohesion, internal friction angle and the slope geometry (mainly the slope height and slope angles).

Results

The samples presented a wide range of variability in their geotechnical characteristics. The particle size distribution result revealed that the content of gravel, sand and fines in the Enyigba tailing dump units are 0 – 40.2%, 10.7 – 47.8% and 34.9 – 80.7% respectively (Table 1). The Atterberg limits indicated that liquid limit (LL) ranges from 24 to 49%, Plastic limit (PL) ranges between 24 and 35% and plasticity index (PI) ranges from 9 to 26% (Table 2). The permeability (k) of the samples ranges between 1.49 x 10^-4 cm/sec and9.04 x 10^-8cm/sec whereas natural moisture content (NMC) measured was observed to be between 3 and 13%. Compaction result showed maximum dry density (MDD) ranges between 1.86 and 2.22 g/cm³ while optimum moisture content (OMC) ranges from 13.45 to 17.05% (Table 3).Cohesion and friction angle of the shale ranges from 13 to 34 kN/m² and 8 to37°respectively. Samples specific gravity (Gs) falls within 2.58 and 2.70.

Discussion of result

Field description

The slope geometries and other measurements were taken on fifty slopes in the study area. Evidences of sliding surfaces and minor landslides were seen on the tailings (Fig.5). Field observations showed that the landslides were more evident on the old tailings compared to the new ones and consist of mainly translational slides. Mud cracks were seen on the natural ground which is an evidence of very fine clay materials in the area.

Most slopes were observed to be concave in curvature (Fig. 5), and from the field measurements, slope angles of the individual slopes ranged from 30-60^°. The steep concave natures of the slopes and the inherent weaknesses (joints and erosion) as observed at the two mine sites have important implications on the slope stability. Most landslide events have normally been reported to occur on concave slopes ranging between 36 and 58 slope angles(Mugagga 2011; Mugagga et al. 2011).A Spatial correlation between landslide occurrence and topographical concavity has been noted by Mugagga(2011).Igwe (2015a) also reported that positive correlation exist between frequency of landslide and slope angle, and however noted that landslides occur more on slopes with angles greater than 30 degree but less than 40 degree Thus, the slope geometry could be one of the causes of the sliding surfaces as shown in Fig. 5, since It presents moderate to high slope angles.

Laboratory test results

Grain size analysis

Results obtained from this test are presented in Table 1. The particle size distribution revealed that the slope materials comprises of gravel ranging from 0 – 40.2%, sand ranging between 10.7 – 47.8% and fines range from 34.9 – 80.7%. The results showed that there is high percentage of fines, low to medium percent of sand and gravel. This implies that fines dominate the slope materials. AM 2 has the lowest percentage of fines (34.8%) amongst the samples; signifying a less cohesive material. The high amount of fines in the slope material at the mine sites also has strong implications for landslide occurrence. Several workers like Knapen et al. (2006), Yang et al. (2007),Kitutu et al. (2009) and Wati et al. (2010) have reported the influence of high clay content on landslide occurrence.

The tailing dump samples have on the average clay content more than 55%, and this amount of clay content could have implications on the shrink-swell properties of the soil. Mugagga et al. (2011) noted that materials with clay content exceeding 20% exhibited expansion potential. Ten percent (10%) clay content has often been used as an indicator of the expansion potential  of materials (Baynes, 2008), thus 55.95% average clay content of the samples could imply medium to high expansive potential.

Moisture content and consistency limits

The result of Atterberg limits and moisture content tests are shown in Table 2.The materials’ natural moisture content ranges from 3-13%, signifying low to medium water content in the soil. The soil liquid limit (LL) ranges from 24 – 59% and the PL ranges from 24 – 35%, while PI ranges between 9 – 26%. These consistency limit values suggested that the soils possess low to high plasticity, which agrees with the soils’ low to moderate moisture content. Thus the soils can be classified as ML or CH according to the Unified Soil Classification System (USCS) (Fig. 6).It is observed that AM 2 and EN 14 which represents the new tailings have the lowest values of liquid limit, with LL = 24% and 36% respectively, whereas the old tailings have higher values with LL = 42 – 59%. This could be the outcome of higher water content in the old tailings as a result water infiltration over a longer period of time. Much emphasis have been made by many authors on the role of LL in characterizing the problematic soils (Msilimba and Holmes 2005; Fauziah et al. 2006; Baynes 2008). The high LL coupled with high clay content at the site could qualify the soils as problematic soils that are susceptible to landslides. The LL for all the sites is quite above the threshold of 25%, indicating medium expansion potential of the soils (Mugagga et al. 2011).

Sowers (1979) established that soils with a high PI tend to be clay, those with a lower PI are likely to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay. This is in line with the non-plastic nature of AM 2. The tailing dump materials have average value of PI to be 17%. The average values of plasticity index and liquid limit for most of the samples could be an indication of low to moderate swelling potential of the clay, such that the soils exhibit medium expansion potential and are hence, susceptible to landslides (Mugagga et al. 2011).A positive correlation has been observed to exist between high plasticity and fine-grained inorganic clay and silts (Isik and Keskin 2008). This swelling ability could signify the presence of swelling clay minerals. Due to the reduction of shear resistance, sliding have been found to be prevalent in plastic inorganic soils during rainfall events (Dai et al. 2002). Therefore, these plastic inorganic clays could be susceptible to sliding even under moderate rainfall events.

Permeability and bulk density

Infiltration capacity of soil depends on the permeability, degree of saturation, vegetation, and amount and duration of rainfall (Todd 1980). The permeability coefficient (k), determined by falling head permeability test, showed result ranging from 1.49 x 10^-4– 9.04 x 10^-8cm/sec (Table 3). This low k values imply that the soils are fine grained which agrees with the particle size distribution result, and suggest on the water holding ability of the soil. Low permeability can cause excess pore pressure build up during the rainy seasons due to high water levels. This in turn reduces the strength of materials and may result to dump failure.

Very fine-grained clayey materials have been observed to possess small pores that usually release water due to their high water retention capacity, thus rendering such clay susceptible to landslides (Yang et al. 2007; Jadda et al. 2009; Wati et al. 2010; Mugagga et al. 2011). The vulnerability of these clay soils are also aggravated by their low permeability (Wati et al. 2010).According to Glade (2002),Knapen et al. (2006) and Mugagga et al.(2011), deep rotational landslides on concave slopes can be attributed to the concentration of runoff and sub-surface water which reduces slope shear resistance.

The bulk density (γ_b) of the slope samples ranges between 1.79–2.21Mg/m³(Table 3), which falls within range of swelling clays. Seedman (1986) and Hong et al.(2012) noted that osmotic swelling occurs in clays with bulk density less than 2.45 Mg/m³. This is because very high activity of smectite minerals in clays enhances high water absorption ability, thus increasing the natural moisture content of the soil.

Compaction

Figure 7shows compaction curves for the samples. It explains the maximum compaction of the soil at optimum water content and dry density. The compaction result showed that the soil optimum moisture content (OMC) ranges between 13.45 and 17.05%, while the maximum dry density (MDD) ranges from 1.86 to 2.22 g/cm³ (Table 3). The result revealed intermediate OMC and low MDD. The curves represent that of low to medium plastic clays, which agrees with the consistency limits. Heavy clays of very high plasticity have very low dry density and very high optimum water content (Arora 2008).At very high water content, energy has little effect on the compacted density of a clay soil because the water is incompressible and takes the applied force without densifying the soil. Invariably high OMC and low MDD suggest a weak material.

Specific Gravity

The results of the tested samples are presented in Table 1. The values range from 2.58-2.70 (Table 3). Tuncer and Lohnes (1977) noted that the specific gravity of sandy soil, which is mostly made of quartz, may be estimated to be about 2.65, whereas for clayey and silty soils, it might vary from 2.60 to 2.90.Thus, the average specific gravity value of 2.63 confirmed the high clay content of the materials. However, rocks of specific gravity as low as 2.65 are usually weak and non-durable and could deteriorate at the engineering sites, especially on the influx of moisture (Reidenouer 1970). 

Shear strength

This is one of the key tests in establishing the stability of a slope material as it generates cohesion and angle of internal friction of the material. Gupta and Paul (2016) clearly stated that Slope stability is ultimately determined by two factors; the slope geometry and the strength of the materials on it. The range of the angle of internal resistance and cohesion are 8-37^°and 13– 34kN/m²respectively (Table 3). These results implied low-moderate values of cohesion and low values of internal friction angle for AM1, EN3, EN10, EN 15 and EN 16 (Table 3). Some of the samples’ failure envelopes are shown in Fig. 8.

It was observed that AM 2 is the least cohesive and has the highest angle of internal friction; this confirms the cohesion-less nature (low clay content and plasticity) of the material. The low shear strength of AM 2 could be as a result of presence of expansive minerals. Blyth and de Feritas(1984) postulated that the strength of any rock or soil where permeability prevents the draining of water in the voids would be reduced by any increase in hydrostatic pore pressure that develops within them. Water-infiltration weakens slopes and induce failure by reducing suction and strength (Crosta and Frattini 2008; Igwe and Fukuoka 2014; Igwe 2014). The strength reduction is experienced when the shear forces on the slope becomes greater than the mobilized resisting forces, thereby causing a considerable drop in the slope factor of Safety (Igwe 2015b). Steep slopes are mostly affected owing to the increasing shear stress against reducing shear strength (Yang et al. 2007; Wati et al. 2010; Mugagga et al. 2011). Behera et al. (2016) observed that a decrease in shear strength may trigger landslide in dump slope along weak plane.

Soil Minerals

The XRD result showing the minerals present in soil is shown in Fig. 9. The minerals present in soil are quartz (39 – 49%), illite (38 – 58%), magnetite (2 – 3%), anatase (2%) and millosevichite (8%). The presence of smectite mineral such as illite suggests the plasticity of the soil due to its swelling ability. There are evidences of shrink-swell ability of the soils in the forms of voids or desiccation cracks within the site (Fig. 10).The presence of clay minerals could give more plasticity to the soil, which is one of the most important causes of slope failure (Behera et al. 2016).Previous authors (Kitutu et al. 2009; Ohlmacher 2000; Yalcin 2007) have observed the role of dominant plastic clay minerals in landslide occurrence owing to their low shear strength and high swelling potential.

A comparison made between the old and new dumps revealed differences in the number and percentage of minerals. The old dumps have lesser number of minerals (3) but higher percentage of illite (58%) than the new dumps with 5 minerals and 38%illite(Fig. 9). This could be the effect of leaching and weathering respectively. The effects of the leaching and weathering could impact negatively on the strength of the slope materials, thus subjecting the slope to instability.

Rainfall pattern of the area

The stability of slopes could be compromised with the influence of external and internal factors. According to Inganga et al.( 2001); Dai et al. (2002); Nyssen et al. (2002);  Knapen et al. (2006); Claessens et al. (2007); NEMA (2007); Kitutu et al. (2009); Mugagga et al.(2011), intense rainfall, water level change, storm waves, rapid stream erosion, cultivation and excavation are some of the external factors that could cause failure of these slopes. Climatic conditions have particularly favored the prevalence of landslide in the tropics as noted by Baynes (2008),Mugagga et al.(2011) and Igwe (2015a). Some of the seeming stable slopes could be rendered susceptible to mass wasting due to problematic nature of the soils, excavation at the foot of the slope (removal of lateral support) and lack of vegetation, which exposes the slopes directly to adverse environmental conditions.

Landslide events in southeastern Nigeria were mostly reported in 2008 (Ugwueme landslides), 2010 (Iva Valley, Kwande and Imande-UkusuItulygh landslides) and 2013 (Ikwette, Obudu landslides) (Igwe et al. 2011; Igwe 2015a, b; Igwe et al. 2015; Brooks et al. 2014a, b). A lingering rainfall triggered 28 new shallow landslides in Enugu, and two major ones at Obudu in October 2013 (Igwe 2015a). Looking at the rainfall data of the area (Fig. 11), it is evident that such landslides occurred at high cumulative annual rainfall (about 1600mm), and normally after a drop in cumulative rainfall amount from the preceding year. Thus, anytime annual rainfall begins to rise above 1600mm in the area, there would be need to be mindful of landslide events that normally occur after such increase in annual rainfall, and to make adequate preparations for stabilization works in order to mitigate the occurrence of future landslides.

Stability Analysis of the dumps

The integration of experimental, numerical and stability studies have assisted extensively in the analyses of slopes subjected to rainfall infiltration (Griffiths and Marquez 2007; Igwe et al. 2012; Sarkar et al. 2012; Ma et al. 2013; Singh et al. 2013; Alemdag et al. 2014; Kalatehjari et al. 2014; Igwe 2015b).  Slope stability analysis has been carried out on the tailing dump slopes, using the limit equilibrium method (LEM), which presently is the most common stability approach in the field of slope stability analysis (Cheng et al. 2014).The analysis type adopted was the Morgenstern-Price limit equilibrium method. The basic requirement of this method is that equilibrium must be satisfied in terms of total stresses, and all the body weight and other forces acting upon the dump must be included in the analysis. At the end, a factor of safety is generated for individual slopes. Factor of safety is the ratio of the shear strength of the soil divided by the shear stress required for equilibrium. It is how much stronger a system is than it usually needs to be for an intended load.

Stability analyses of several slopes of the Enyigba mine dumps were evaluated, and their factor of safety (FS) computed. Figs. 12 and 13 are the stability analysis of slopes EN10 and EN14, and the computed FS are 1.28 and 1.14, suggesting fair and poor slope stability respectively. The FS of the other slopes analyzed ranges from 1.00 – 1.30, implying critical stability to fairly stable slopes(Table 4). These factors of safety were recorded during the dry season with little or no water in the dumps.  However, there are evidences of flooding (water-logs) at the sites during the rainy season (Fig 10). When the possible influence of groundwater and flooding within and around the old tailing slopes were considered in our analysis, there was a fall in FS from 1.28 to 1.22 (Fig. 12a, e) and further down to 1.14 (Fig. 12f), thus declining from fairly stable to poor stability. This fall in FS was also observed on the new mine tailing slope, where the FS dropped from 1.14 to 1.08 (Fig. 13a, e) and continued to 1.00 (Fig. 13f) as a result of groundwater and flooding influences respectively, thus degrading from poor stability to critical stability.

The drop in FS may remain continuous as water rises within and around the slopes until failure is initiated at FS < 1.0. At FS < 1(i.e activating forces greater than resisting forces), a mass wasting process such as landslide or mudflow is expected. It is important to note that water in and around the slopes could increase the volume of the sliding materials. The sliding mass increased from 173m³ to 185m³and 55m³ to 72m³as water increased within the slopes at the old and new tailing dumps as seen in Fig 12b, f and 13b, f respectively. These increases in sliding mass agree with the findings of Maduka et al. (2016).

The new mine dumps would likely undergo more of shallow landslides with lesser depth (7m) and sliding mass (55m³)when compared with the depth (12m) and sliding mass (173m³) of the old mine dumps (Figs 12b and 13b respectively). These values could be as a result of the differences in their level of compaction and cohesion. The friability of the new tailing dumps could be the reason for the shallow landslides observed in Fig. 5. However, in the event of possible slope failure, the old dumps are likely to have deep landslides (Fig. 12).

Several trial slip surfaces were analyzed on the modeled slopes (Fig. 12c and 13c) to produce a safety map for each. A safety map reflects a zone of potential failure in which the factor of safety of the trial slip surfaces is similar. Although the FS for the trial slip surfaces range from 1.28 to 1.60 for the old tailing dump and 1.14 to 1.71 for the new dump(Fig. 12c and 13c), the safety zone ranges between FS = 1.28 – 1.33 and 1.14 – 1.19 for the old and new tailing dumps respectively. The safe zones are the area covered by the red band (Fig. 12d and 13d). In the present analyses, all the factors of safety within the red bands indicated a fairly good to poor stability. It is important, however, to state that even slopes with FS >1 are conditionally stable (Mugagga et al. 2011). This could be the appropriate description of allegedly stable slopes because slope failure often occurs as a result of localized deformation in a thin zone of intense shearing. Consequently, this thin intense shearing force may not be duly represented by the overall stress–strain measurement (Gonghuiet al. 2010), owing to the influence of several physical and anthropogenic factors (Mugagga et al. 2011).

Moreover, in this era of erratic rainfall regimes, which is the principal trigger of slope instability (such as landslide and mudflows) coupled with the inherent weaknesses of the slope materials (low MDD and shear strength, low permeability and high clay content and OMC) and other visible triggers of landslides such as erosion and discontinuities (cracks, and joints), it would be impossible to rule out the occurrence of a massive landslide, capable of undermining the safety of lives and properties within the mine sites. Also, there have been reports on massive rainfall triggered landslide occurrences in south-eastern Nigeria in recent years (Igwe et al. 2011;Igwe et al. 2013;Igwe, 2015a, b; Brooks et al. 2014a, b). Therefore, stabilization of the slope is needful, especially on slopes in proximity to the mining pits (Fig. 14). Though, only shallow landslides are evident on the site (Fig. 5), results of the stability analysis suggests the possibility of a sliding mass ranging from 55m³ to 173m³. Landslides with such magnitude of sliding mass could be classified as intermediate to high landslide (Maduka et al. 2016). However, some shallow landslides at the site could be as a result of cyclic loading and vibrations on the slope from heavy vehicles and machines and blasting (Igwe 2015a).

Many authors have reported a significant drop in FS with increasing saturation (Onda et al. 2004; Rahardjo et al. 2005; Igwe 2015b). Hence, appropriate drainage control would drastically improve the stability of the mine tailing dump slopes. The stability of the slopes can be improved by dewatering (water removal) from the slope. But, this method of slope stability is expensive, time consuming and a tedious practice. Maduka et al. (2016) suggested the benching method (reduction on slope angles and slope heights) as a reliable, efficient and less expensive way of increasing the stability of slopes.

Conclusions

The physical properties of the Enyigba mines slope material have been characterized in this study, and their implications on the slope stability elucidated. Steep slopes, slope curvature, high clay contents as well as high water content and poor shear strength of the Enyigba tailing dumps have been pointed out as some of factors that would negate the stability of the slopes, coupled with the recent rise in annual rainfall lin southeastern Nigeria.

The slope materials at the Enyigba mine site have been characterized generally to be dark grey shale. The soil particle distribution, moisture content, permeability, compaction and shear strength indicated low to medium plastic soil, with low to medium water holding ability.These physical attributes could be as a result of soil mineralogy. The presence of illite minerals further in samples explains the reason for the soil weakness. Soils with the above attributes have been observed to exhibit low strength either as foundation or slope materials, and capable of causing engineering failures at sites. This agrees with the measured shear strength parameters (cohesion and internal friction angle), computed to be 13 – 34kN/m² and 8- 37^°respectively, signifying low to medium strength soil materials, capable of shear failure at engineering and mining sites.

The result of the stability analysis hints the possibility of landslide occurrences on the tailing dump slopes within the Enyigba mine sites. The studied mine tailing slopes were inferred to be critically stable to poor stability, with the computed factor of safety ranging from 1.00 – 1.30, suggesting possible failure of the slope. Therefore, there is a need for slope stabilization on the mine tailing dumps at Enyigba and Ameri.

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List of Figures

Fig. 1: Indiscriminate dumping in the mines

Fig. 2:  Location, Accessibility and Drainage map of the study area

Fig. 3:  3D topographic model of the area

Fig. 4:   Geologic map of the area

Fig. 5:  Conical hills of the dump at the mine site

Fig. 6:  Soil plasticity chart of the samples

Fig. 7:  Compaction curves of the samples

Fig. 8:  Failure envelop showing strength parameters

Fig. 9:  XRD showing the mineral components of the samples

Fig. 10:  Evidences of Smectite minerals, soil plasticity and flooding

Fig. 11:  Rainfall data of the study area

Fig. 12:  Slope stability model of an old mine tailing

Fig.13:  New mine tailing slope stability model

Fig. 14:  The dangerous closeness of the tailings to the mines

List of Tables

Table 1: Particle size distribution

Table 2: Soil index properties

Table 3: Geotechnical properties of the soil

Table 4: Stability analysis result

Table 1: Particle size distribution

Location	Am1	Am2	En3	En10	En14	En15	En16
Gravel(%)	24.37	30.73	40.15	0.00	27.98	0.00	2.46
Sand (%)	26.88	34.4	10.68	14.55	29.04	19.3	47.79
Fines (%)	48.75	34.87	49.17	85.45	42.98	80.70	49.75

Table 2: Soil index properties

Location	Sample No	Description	NMC	LL               PL              PI	ϒ (KN/m3)
Ameri	Am1	Old tailing	13	52               33             19	16.45
Ameri	AM2	New tailing	7	24               NP            NP	15.80
Enyigba	EN3	Old tailing	8	43               34             9	14.93
Enyigba	EN10	Old tailing	3	59               35             24	16.59
Enyigba	EN14	New tailing	4	36               24             12	15.41
Enygba	EN15	Old tailing	4	59               33             26	15.38
Enyigba	EN16	Old tailing	3	42               28             14	15.82

Table 3:Geotechnical properties of the soil

Sample No	C (kPa)	Φ (0)	MDD (g/cm2)	OMC (%)	Gs	ϒb(mg/m3)	k(cm/sec)
AM1	32	14	2.22	17.05	2.60	2.04	5.35×10-7
AM2	13	37	1.86	13.79	2.67	1.96	1.49×10-4
EN3	15	20	2.06	13.79	2.58	1.79	1.33×10-6
EN10	34	10	2.18	14.02	2.70	2.21	3.16×10-7
EN14	18	22	2.12	13.65	2.62	1.84	2.66×10-4
EN15	27	8	1.92	13.92	2.63	1.86	9.04×10-8
EN16	18	18	2.12	13.45	2.61	1.91	1.32×10-6

Table 4: Stability analysis result

Location	Slope description	Fs	Remark
Ameri	Old Dump	1.30	Fairly stable
Ameri	New Dump	1.00	Critically stable
Enyigba	Old Dump	1.30	Fairly stable
Enyigba	Old Dump	1.15	Poorly stable
Enyigba	Old Dump	1.24	Fairly stable