


































Energy and Earth Science 
Vol. 3, No. 2, 2020 

www.scholink.org/ojs/index.php/ees 

ISSN 2578-1359 (Print)   ISSN 2578-1367 (Online) 

61 
 

Original Paper 

Lion Mountain Landslide in Non-urbanized Terrain: Changing 

the Myth of Landslide Occurrence in Western Sierra Leone 

Yusuf A. Lahai
1
, Dauda M. Kamara

2
 & Chernoh M. Jalloh

1
 

1
 Department of Geology, Fourah Bay College, University of Sierra Leone, Freetown, Sierra Leone 

2
 School of Environmental Science and Engineering, Suzhou University of Science and Technology, 

Suzhou, China 

*
 Yusuf A. Lahai, Department of Geology, Fourah Bay College, University of Sierra Leone, Freetown, 

Sierra Leone  

 

Received: June 25, 2020       Accepted: July 8, 2020       Online Published: September 20, 2020 

doi:10.22158/ees.v3n2p61              URL: http://dx.doi.org/10.22158/ees.v3n2p61 

 

Abstract 

Freetown has documented one of the most devastating landslides in the world in 2017. Many debates in 

the media, few scientific papers and technical documents, have argued with eloquence ascertaining 

human factors, particularly deforestation and urbanization, as the dominant causative factor. This 

notion seems to be widely accepted for all other slides by the communities, government agencies and 

departments. Therefore, this work attempts to expand on existing public knowledge by demonstrating 

the less influential or insignificant human factors which can have impacts on certain landslide 

occurrences in the Freetown Layered-Complex.  

The representative landslide considered for this study occurred beyond the vicinity of urbanized zone. 

Therefore, to establish a clear understanding of the actual causative factors, fieldwork and laboratory 

investigations were undertaken. During the field survey, we assessed the rock type, discontinuities, 

geomorphology and hydrological influence of the landslide. The specific rock series underlying the 

landslide was confirmed through thin section analysis at the National Minerals Agency (NMA). DCP 

tests and laboratory analyses enhanced the derivation of geotechnical properties of the residual 

soil/regolith. 

This work systematically presented how natural conditions, such as: geology (rock types and tectonic 

signatures), geomorphology, hydrology and the geotechnical properties of the slope soil, have 

interplayed in the occurrence of the landslide event. In addition, the slip surface of the landslide 

occurred at a depth below the reach of plant activities (2.6 m). This information may help modify 

public messages by institutions and can be a source of useful information for the country’s Landslide 



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Disaster Management Department (LDMD). 

Keywords  

dynamic cone penetration, geotechnical properties, human factors natural conditions 

 

1. Background of the Study 

Freetown is located in the western area of Sierra Leone, which sits on the edge of the Atlantic Ocean in 

West Africa. Rapid population growth in this region became eminent during the civil war (especially 

from 1999 to 2001) because of its insulated nature against rebel activities. Economist intelligence unit 

(2002) and Kaldor and Vincent (2006) mentioned that one- third of the country‟s 2.6 million displaced 

persons and 500,000 farm families relocated to the safe haven (Western Area) (Gbanie et al., 2015). 

This has accounted for the rising population in Freetown (Weekes & Bah, 2017; Sesay et al., 2006; 

Gogra et al., 2010) amounting to enormous pressure on the small space between the mountains and the 

sea.  

The unavailability and affordability of suitable lands at coastal and inland settlements, have forced 

many people to settle on steep hills/mountains of the city. Over time, their migration to those terrains, 

has facilitated rapid encroachment into vital forestlands (once-protected forest highland) without any 

adherence to land policies and laws. As a consequence, the uncontrolled urban developments have 

caused several environmental issues, ranging from pronounced changes in natural channels from a 

significant increase in the storm water run-off and erosion (UNDP & EPA, 2017), to over-harvesting of 

timbers on the hillsides, leading to deforestation, and eventually causing soil erosion. In effect, these 

intense anthropogenic activities, mainly deforestation and urbanization, have contributed to landslide 

occurrences in the Freetown-Layered Complex, which are captured in few available documents/articles 

(e.g., Munro, 2009; UNDP & EPA, 2017; cui et al., 2019) as the dominant causal factors for the Regent 

rainfall-triggered landslide. This information on causal factors seems to be a widely accepted 

phenomenon for any landslide event within the Complex irrespective of the terrain/zone of occurrence, 

indicating an absolute dearth in knowledge regarding landslide causal factors. This necessitates 

scientific investigation of landslide occurrences in areas unaffected by deforestation and urbanization. 

Historically, landslide events have occurred in forested area (Redshaw et al., 2019), including an area 

formally designated as the Western Area Peninsula Forest (Sesay, 2005). Recent landslide inventory 

conducted through field surveys and the exploration of Google Earth satellite images (-accessed in 

2019 and 2020) in the area represented by Figure 1, has shown greater percentage of landslide scars in 

forested area than the urbanized zones (Lahai, 2020). The landslide used as a case study affected the 

Lion Mountain located 938m northeast of Fula Town community; hence, the name Lion Mountain 

landslide is adopted in this paper. It is a non-urbanized terrain: an area that is sparsely vegetated, but 

enough to preserve its beneficial action in terms of mechanical (root anchoring) and hydrological 

(suction generated by root water uptake) effects (Balzano et al., 2019). 

 



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Landslides have caused numerous destructions to forest in many parts of the world, and are seen to 

affect hugely the tropical areas due to the combination of intense rainfall and earthquakes. Studies done 

by Garwood et al. (1979); Martinez et al. (1995); Schuster and Highland (2007) have demonstrated 

elsewhere where the above triggering factors existed (i.e., rainfall and earthquakes). Similarly, the 

study area belongs to the tropical climate, which has a characteristic heavy rainfall, but the seismic 

hazard level in the entire country is very low and landslides triggered by earthquakes would be 

extremely unlikely within a 50- year return period (Arup et al., 2018). This makes the case unique, 

indicating strong relationship with geological instability and other geoenvironmental factors, which this 

study seeks to unravel.   

Furthermore, no attempt has been made to investigate landslide occurrence in non-urbanized areas and 

by extension the effects on its biodiversity. The few studies focused on landslide occurrences within the 

urban areas and specifically on Regent Landslide (e.g., UNDP & EPA, 2017; Arup et al., 2018; Cui et 

al., 2019; Redshaw et al., 2019; Lahai et al., 2019): which has accounted for the worst fatality in the 

country and the world during the year of occurrence, Madina Landslide (e.g., Sillah et al., 2011; Lahai 

et al., 2020), and Charlotte Landslide (Lahai et al., 2020). Information pertinent to these slides is not 

adequate and as such lacks the realistic basis to be extrapolated to other areas affected by landslides 

within the Freetown-Layered Complex, particularly in forested areas where little or no human activities 

are not experienced. This constraint in knowledge extension to other areas could be due to marked 

lithologic, topographic, hydrologic and tectonic variation across the terrain. Therefore, this work 

provides a comprehensive and better understanding of the actual causes of the Lion Mountain landslide 

through the following approaches: an intensive field assessment of the slide, laboratory analyses on soil 

samples, soil strength determination using Dynamic Cone Penetration (DCP) and data analyses 

obtained from the United States Geological Surveys (USGS) pertinent to the landslide location and its 

surroundings. 

This work presents detailed and accurate information on the geology, hydrology, geomorphic and 

geotechnical properties of the residual soil/regolith of the landslide. This is a significantly generated 

scientific fact on the conditions responsible for landslides in non-urbanized zones (forested areas). The 

information is hoped to eliminate knowledge gap and change the general notion on landslide causative 

factors, which will support the modification of public documents and government policies. 

 

2. Description of Landslide Area  

The area falls in the northwestern part of Freetown, which is located 0.93km northeast of Fula Town 

(8.430708N, 13.243174W) and 1.12km southwest of the Regent landslide. The geographic coordinates 

(latitude and longitude) of the landslide is recorded as 8.417325N and 13.230336W, which falls within 

the non-urbanized zone (forested area).  

 

 



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Figure 1. Location of Lion Moutain Landslide (a) Map Showing the Two Landslides and Area of 

Interest (Source: Google Earth-Accessed 2020), (b) Freetown Map with Inset of Interested Area 

Encompassing Zone 1, 2 and 3 of the Complex 

 

The area consists of mountain ranges trending from NNE to SSW direction, which are separated by 

fault planes  (they are seen as stream valleys, representing an area of discharge) as observed in Figure 

1.The landslide seems to have affected one of these mountainous slopes/faces (WSW) of the Freetown 

Peninsula. They have a vegetation cover ranging from thick to sparse forest, with patches of barelands 

(exposed rocks and soil), which represent landslide scars (Figure 2). 

 

 

Figure 2. Location of Landslide Scar in Relationship to Bareland, Sparse Forest and Thick Forest 

Reserve 

 

 

 

 



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Like any other locality in West African country, the landslide area experiences tropical and humid type 

of climate  that is strongly controlled by the tropical air mass blowing the entire sub region. 

Unfortunately, there is an absence of an accurate rainfall time-series data representing Freetown, which 

may prevent the determination of location-specific values (Redshaw et al., 2019), but analyses on data 

obtained from the country‟s Meteorological Agency present a general understanding of how rainfall 

conditions lead to both flooding and landslides. From the analyses, high amount of precipitation 

(100mm-1200mm) are received from July to September. This landslide event is reported to have taken 

place within this time bracket (especically August, 2018), and there has been a confirmed information 

on backward extension  of the landslide head wall during the rains in 2019. 

 

3. Methodology 

3.1 Field Work 

This survey was undertaken in March 2020,which coincided with the dry season, and included field 

assessment of the landslide with keen interest on the underlying geology, tectonic structures (fractures 

and joint sets), surface and subsurface hydrology and its geomorphology. The Dynamic Cone 

Penetration (DCP) was used to conduct in-situ soil (regolith) test. Only three tests at random intervals 

along the slope were undertaken to give an insight into the overall slope cover (residual soil) strength. 

This assumption is connected to the homogeneity of the underlying rocks (same chemical and physical 

composition) and also number of DCP blows  (NDCP) per penetration depth plots for the three testing 

points show high consistency (Coefficient of variance is less than 30%). Additionally, recording of 

vital information pertinent to the landslide (point coordinates of the landslide, landslide area, perimeter, 

length, width and slide volume) was achieved, and finally, description and classification of the slide 

done in accordance with Varnes (1978) and Cruden and Varnes (1996). Extraction and measurement of 

parameters (landslide‟s point coordinates, area and perimeter) from Google Earth image corresponded 

with field data obtained using the Global Positioning System (GPS). Information on the landslide‟s 

occurrence times and activities was derived from interviews with nearby local people who often visit 

the area for wood fetching prior the incident.  

Rock samples were logged using field-based approach (field description). The discontinuities (layering, 

fracture/crack and joints) on the landslide main body and adjacent surfaces were identified, their 

attitudes (strike and dip) measured using the Silva Compass and Clinometer and recorded appropriately. 

Other parameters noted include: diameter of boulders, thickness of bouldery debris, and slide volume. 

Both rock and  soil samples were later collected for thin section analysis and for geotechnical soil 

investigation at the National Minerals Agency‟s laboratory and the Sierra Leone Road Authority 

(SLRA) materials laboratory respectively.  

Hydrologically, we also assessed the bottom slope and nearby darinage system (current river and 

streams)  that may have affected the landslide or affected by the landslide materials in addition to the 

lineaments on post landslide surfaces (evidence of groundwater source); which provide clue to hillslope 



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hydrology and flow path during the rains. Key parameters noted were: the flow direction, proximity to 

landslide site and any erosive evidence on the stream bed. Extraction of these hydrological elements 

including drainage density of the landslide area and environs represented by the inset in Figure 1(b) 

were derived from the Digital Elevation Model (DEM). Finally, the landslide geomorphological 

elements recorded along its entire length are: elevation, degree of slope, slope aspect and they were 

compared with the DEM for validation.  

3.2 Laboratory Tests/Analyses 

Laboratory work was conducted on both rock (grab) and soil samples in different laboratories at NMA 

and SLRA respectively. The grab sample was cut into two parts using the slab cutter. One half of the 

grab sample was used for the preparation of slide (30-micron thickness) for thin section analysis, and 

the other for XRF analysis. For the purpose of this work, the result of thin section analysis formed the 

basis in ascertaining the specific gabbroic series underlying the landslide. 

Two soil samples (small and bulk) were obtained at 1.5m depth in each of the three trial pits within the 

landslide area for geotechnical soil investigation. The small samples were well preserved to prevent 

any loss of water component between the time of sampling and testing in the laboratory. These samples 

were used for the determination of moisture content using oven-drying method and the consistency 

limits of the soil. The bulk samples were utilized for the analysis of particle size distribution using the 

sieve method and the determination of specific gravity using 50ml or specific gravity bottle. Finally, a 

cone cutter (used in the field) and a weight balance were used to determine the bulk density at each of 

the three sampling sites. 

3.3 Data Analyses 

The DEM data, which we obtained from the United States Geological Surveys (USGS) were 

pre-processed and the 3-D analyst tool in the GIS environment utilized to generate classified maps of 

geomorphic factors (slope, aspect, curvature and elevation) and hydrological factors (drainage density, 

drainage system, flow direction). Also, the  location of the landslide (point coordinates) was 

integrated in each of the factor maps to establish it links with the terrain-specific factors, and Google 

Earth image utilized to enhance derivation of landslide dimensions that correlated with field data. 

Finally, geotechnical data were inputed, double checked for errors in data capture, arranged, and 

analyzed in the Microsoft Excel spreadsheet. This facilitated the presentation of data in tables and 

graphs.  

 

 

 

 

 

 

 



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4. Results 

4.1 Lion Mountain Landslide  

The Lion Mountain landslide with dimension 420m by 86.3m and a perimeter of 1,257m, is broadly 

translational involving rock fragments (angular and sub-rounded) and residual soil. It is a large 

landslide (area > 3000 m
2
) (Skrypczak et al., 2017), with a slide volume estimated as 94239.6 m

3 
using 

the method presented by Adegbe et al. (2014). The sliding  took place along the interface between  

bedrock (resistant gabbroic rock) and the overlying soil (weaker material) at few portions along the 

slope (upper and mid slope in Figure 2), and along discontinuities (joints and layered planes), with 

clear evidence at the landslide base. The failure surface is planar, which is persistent and slightly 

undulating at post landslide bedrock exposures. The estimated depth of rupture ranges from 2.5m to 

2.9m (below the zone affected by plant roots). 

The boulders found at the landslide  base were released either together with the debris or from the 

fractured rock mass. They bounced and slid along the slope to their point of deposition in the lower 

plain: a process refered to as boulder bounce. Also a proportion of debris and large boulders formed 

bulge with approximate elevation of ≈0.9m in the lower plain, whilst others (mainly debris) mixed with 

the surface runoff water to give rise to debris-laden flow (Flow-like behaviour of the landslide), which 

became enhanced by basal diverted streams, and carried into the major stream valley (a valley that 

contained a river named the Lion Mountain river). The total run-out is estimated as 1,273 m (1.28km), 

which spans from landslide headscarp to a point where  the Mountain Lion river  connects to the 

Babadorie valley (confluence point). Indigenes of the nearby community (Fula Town) confirmed the 

presence of water seepages on the slide‟s body for a couple of days following rainfalls, which could 

have emerged from the vertically dip fractures and joint sets. 

 

 

Figure 3. Landslide Tracjectory and Typology (a) Translational Landslide Body from Crown to 

Toe, (b) Debris-Laden Flow, (c) Lion Mountain Landslide with Implied Flooding Carrying 

Medium and Fine-Sized Particles (Debris-Laden Flooding) 

 



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4.2 Geological, Geomorphological and Hydrological Conditions of the Landslide  

4.2.1 Geological Conditions 

The landslide is underlain by a pegmatitic gabbro (very coarse-grained ultramafic rock) comprising up 

to 90% mafic minerals (pyroxene). The lithology is quite extensive, and belongs to zone 3 of the 

Freetown-layered Complex proposed by Chalokwu (2001) and Chalokwu et al. (2010). Identification 

and analysis of specific-rock series within the Freetown Complex unit affecting the landslide is 

particularly important than determining this lithologic unit with a very general approach. This approach 

unravels the distinct characteristic features corresponding to compositional variation within the unit. 

Post landslide observations revealed few fractures on the slightly undulating, but generally planar 

exposed fresh surfaces at the upper slide section and in the center; which are alternated by weathered 

and partially weathered materials along slope‟s length from top to base. They also show evidence of 

exfoliation joints, that could make the affected section appear to be flaking off in sheets, and linear 

marks (striation) indicating downslope rock sliding against each other (Figure 4). But much degree of 

rock fracturing are visible on weathered and partially-weathered rock surfaces at slide‟s base, starting 

just beneath the mid rock exposure, forming joint sets (NNW-SSE) to the toe (planar portion of the 

landslide). Some of the lineeaments are parallel to the slope while others are not and most of the dips 

are near vertical (~90 degree). The most obvious fracture has an offset of 15mm and it is orientated in 

the NNE-SSW direction with a vertical dip as well (Figure 5). The initial sliding surface may be 

associated to this weaker zone, followed by detachment of regolith from the lubricated surface of the 

underlying fresh rock (bedrock and overlying soil interface). However, this entire process of landslide 

seems difficult to track in post-disaster investigation.  

Also, the orientation of rock layering (ESE-WNW) and dip (43
0
 NW-almost the same as sliding plane) 

at the landslide site when compared with the strike and dip of outcrops on adjacent slopes and the 

mountain village (8.430708N, 13.243174W) showed correspondence. This aided in distinguishing 

in-situ rocks from boulders. The boulders with diameters ranging from 2m to 5.5m, were deposited at 

the bottom slope and along the stream valley underneath. They range from rounded old black surface 

colour weathered corestones to angular chunks released from the fractured rock mass.  

 



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Figure 4. Local Geology (a) Distinct Zones along Slope Face Showing Differential Weathering, 

Separated by a Tranverse Crack, (b) Typical Rock Type with Evidence of Exfoliation Joints, (c) 

Intensely Weathered Landslide Basal Section Where the Slipage/Failure may Have Initiated, (d) 

Parallel Proto-Joints in the Process of Developing as Full Mechanical Fractures but Maintaining 

Considerable True Cohesion 

 

The constituent rock is susceptible to chemical weathering, with clearly weathered zones at the ridge 

top (weathering profile is estimated as 2.6 m thick) and very thin along the flanks (1.5m-2.5m). Rocks 

at the upper landslide base are strongly affected by weathering (block weathring/rind weathering) 

(Figure 5a), and is seemed concentrating in the fractured planes/zones, which together with upward 

groundwater flow and increased pore-pressure on key joints may have initiated basal failure. 

 

 

Figure 5. (a) Joint Sets (Vertical Dip) in Highly Weathered Pegmatitic Gabbro, (b) The Largest 

Offset Fracture (15mm) within the Landslide Area 

 



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4.2.2 Geomorphological and Hydrological Conditions 

Geomorphologically, the landslide slope is cataclinal or a dip slope (i.e., topographic surface dips in the 

same direction and approximately by the same amount as the true dip of the underlying rock). Field 

data on landslide elevation and slope (recorded at the main scarp) are 335m and 35
0 
respectively, and 

the event affected west-south westerly (WSW) facing slope (slope aspect). These values correspond 

with those extracted from the DEM, with the slope belonging to class four (4) as seen in the figure 6 

below. Excluding the slope aspect, elevation and slope values decrease from landslide crown to toe. 

 

 

Figure 6. DEM for Geomorphic Factors (a) Slope Aspect, (b) Slope Curvature, (c) Slope Angle 

 

 

 

 



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The toe is characterized by two minor streams (quick stream) separated by 0.9 m accumulated debris, 

which eventually may have built significantly poor drainage properties, making it prone to the 

development of excess pore water pressure when saturated and sheared (undrained loading). These 

streams emerged due to barriers to natural flow path, causing the surface water body to diverge into 

two channels. The diverted streams could be traced up to 100m from the landslide toe to the confluence 

point (point of intersection with the Lion Mountain stream). They may have eroded the sediments 

(relatively loose materials) along their paths into the Lion Mountain stream, but could not significantly 

affect the centre material (sedimentary pile). 

The two minor stream channels at slope base and the Lion Mountain valley (main stream channel 

controlled by north westerly striking valley)-which connects the study area drainage system to 

Babadorie valley constitute the surface hydrology. Specifically, the landslide area is drained by 1
st
 

order streams and the entire area belongs to sixth Order basin, which joins rivers and tributaries based 

on the topography (Figure 7b). There is no evidence of effective erosional action (e.g., removal of 

material from the foot of the slope in the river bed by river bank erosion prior to the landslide event), 

which could have influenced any downslope movement. The drainage density of the slide area belongs 

to class three, which is moderate (19.88-2.81 km/km
2
) and may favour groundwater recharge (Figure 

7a). The main channel is separated from the landslide area by a relatively flat plain that is approxmately 

20m wide, which always become flooded during persistent rainfall. Following the landslide event, the 

basal accumulated sediments extending towards the river may have been  over-saturated with the 

flooded water leading to an increased pore pressure. This accounts for the loose nature of the materials 

when stepped into by nearby local people. 

 

 

 

 

 

 

 

 

 

 



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Figure 7. DEM for Hydrologic Factors (a) Drainage Density (b) Drainage System (c) Flow 

Direction 

 

However, evidence of water seepages/springs from open joints after rainfalls confirmed by local 

inhabitants of the nearby community, indicates interaction between precipitation and groundwater. The 

moderately and hghly weathered pegmatitic gabbro is attributed to the accumulation of the infiltrated 

rainfall water through the vertical joint sets (open fractures), thereby raising the groundwater level. 

Therefore, the landslide stability would have been impacted by the upward migration of groundwater 

through the joints to weaken the regolith above. This hydrological principle is supported with few 

studies that have established the influence of upward groundwater flow on slope stability (Weng et al., 

2018). The groundwater in the discontinuities would also have exerted an uplift force to the rock and 

undermined the stability of the weak stratum and rock-soil interface. 

 



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4.3 Geotechnical Assessment 

The properties of landslide‟s underlying geology are extremely important for the propensity for 

landslide occurrence. The decomposed/weathered zone acts as soil considering mechanical perspective, 

making the determination of its geotechnical properties necessary (Yalcin, 2011). This assessment was 

carried out using in-situ field test by DCP and laboratory tests for the purpose of evaluating the 

geotechnical properties of the slope cover soil (regolith). 

The DCP probed or revealed information on the thickness of weathered soil cover (depth > 1.5m) and 

its bearing strength. The strength factor, which according to UNDP & EPA (2017) was not considered 

by the affected populations at Regent prior construction in supposedly a non built-up areas. However, 

results of DCP tests presented in Figure 8 show variation in soil bearing strength with depth along the 

soil profile, which generally decrease towards the bed rock. The average number of  DCP blows 

(NDCP) plot also varies with depth and shows strong correlation with the weak to moderate gabbroic 

layering. This method evaluated the susceptibility of the bottom soil layer that slipped down the slope 

and extrapolated the potetial effects of overloading (e.g., precipitation/rainfall, infrastructures, sediment 

piling and dam) on landslide occurrence (i.e., implied anthropogenic impacts). 

 

 

Figure 8. Variation of Average Bearing Strength Capacity and NDCP in the Soil Profile 

Overlying the Bed Rock. UBA: Ultimate Bearing Capacity, ABC: Allowable Bearing Capacity 

and NDCP. Number of DCP Blows 

 

 

 

 

 



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The second part of the geotechnical assessment involves the determination of natural moisture content, 

specific gravity (G), grain size distributions consistency limits (PlasticLimit, Liquid Limit and 

Plasticity Index), and Liquidity Index (LI) of the landslide soil. Results of these parameters are 

presented in Table 1. The soil moisture content is within range, average specific gravity estimated as 

2.41, which is below the specific gravity of the fresh rock sample (2.8). The soil is well-graded, which 

have been grouped into three divisions corresponding to the major constituents. They include: gravel, 

sand and fines (clay and silt), with the fines exceeding the gravel component. The fine fraction can 

affect permeability and enhance pore pressure following periods of persistent rainfall. With the absence 

of X-Ray Diffraction (XRD) facility in the country to identify and determine clay fraction, analysis at 

the British Geological Survey (BGS) laboratories in Nottingham on Regent bulk debris samples, 

presented by Redshaw et al. (2019), has given an insight into the probable clay mineral type called 

halloysite. This clay mineral is linked to collapsible soil, which Moon (2016) has mentioned as one that 

can yield very suddenly when rapidly loaded. As noted by EL Jazouli et al. (2020), plasticity is 

regarded as one of the important parameters in the properties of water retention and corresponding 

swelling. Therefore, larger plasticity correlates to greater volume change that could impact landslide 

susceptibility. The Unified Soil Classification System (USCS) plasticity chart enabled further 

classification of the representative soil sample as seen in Figure 9. The laboratory-derived atterberg 

limits (PI=22.37%, and LL=53.46) aided in distinguishing the fines by plotting the values in the 

plasticity chart. The sample location falls below the “A” line (a line which Casagrande emperically 

plotted in his devised plasticity chart in 1948, separating inorganic clays and silt and organic soils), 

indicating that the soil sample contains silts and clays with large constituent of „rock flour (finely 

ground non-clay minerals). The Liquidity Index (LI) or a measure of the consistency of the soil is 

negative (-0.24), signaling drier nature of the soil than the plastic limit. This is due to the lower 

moisture content (23.70%), which correlates strongly to the period during which sampling was done 

(dry season), relative elevation, aspect and fine content (silt). 

 

 

 

 

 

 

 

 

 

 

 

 



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Table 1. Geotechnical Properties of Slope Soil Covering Lion Mountain Landslide  

Location and sample ID Lion Mountain Landslide (S9-FT) 

 
Properties Average/Mean Value 

 

Physical properties 

 

Natural Moisture content (%) 23.70 

Specific gravity 2.407 

Adsorption (%) 16.900 

Particle Size distribution (%) 

Gravel (63mm-2mm) 32.30 

Sand (2mm-0.063mm) 32.30 

Fines (Silt and Clay) <0.063 35.30 

Consistency limits (%) 

Plastic limit (PL) 31.09 

Liquid limit (LL) 53.46 

Plasticity index (PI) 22.37 

 
Liquidity index -0.24 

 

 

Figure 9. Plasticity Chart of the Representative Soil Collected from Lion Mountain Landslide 

 

 

 

 

 



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5. Discussion  

This study has presented the first detailed assessment of landslide occurrence in an area with little or no 

anthropogenic activities (e.g., urbanization and deforestation) in the country. On the basis of the 

analyses above, conclusion could be reached, designating natural conditions as the dominant causative 

factors for the occurrence of Lion Mountain landslide. 

This landslide shows characteristic signatures distinct from the others. This may be linked to the 

site-specific geo-environmental conditions, particularly the lithological variation and density of 

tectonic features (fractures, joints and cracks). The rock type is not only susceptible to chemical 

weathering (the degree of weathering reduces vertically with increasing depth), but shows evidence of 

discontinuities on the fresh rock and weathered surfaces (upper base and toe of the landslide). This 

makes the underlying material porous, permeable and direct receipient of the infiltrated rainfall water 

through percolation in the open joints and porosity. Hydrologically, the groundwater level may rise 

under this condition, which eventually migrates upward to lubricate/wet the overlying soil, thereby 

reducing its shear strength and increasing landslide susceptibility. At the basal section, there would 

have been an increment in the joints‟ pore-pressure, which further exacerbated slope instability and 

subsequent resulted to the failure. Geomorphologically, cataclinal slope are susceptible to translational 

landslides, and slope angle falls within the range of slope angle values that have accounted for many 

ground failures reported by Thomas (1983 &1998). 

Geotechnical investigation revealed the nature, characteristic and type of landslide soil cover. It is a 

well-graded soil with a percentage of fine fraction greater than gravels (see Table 1)-a characteristic 

contributing to landslide susceptibility. Laboratory-derived Atterberg limits have shown the soil to be 

plastic with high expansion potential (LL=53.46). The DCP tested the in-situ soil strength along the 

soil profile, with the two strength parameters (ultimate Bearing Capacity and Allowable Bearing 

Capacity) generally displaying reduction in their values towards the bedrock, indicating weaker layer at 

the bottom that is prone to to failure. This variation indicates different zones/layers marked by varied 

modal percentages. The weaker zone indicates greater tendency to slope failure than hard layers.  

The landslide occurs beyond the depth of plant activities, eliminating any influence of  plant root in 

the event. Also, very little or no human intervention exists at bot the slide area and surrounding (e.g., 

slope profile modification, usually by cut- and- fill in the area for houses, groundwater modification by 

a dam, a pipe leak or overflow path modification as in the case of Tacugama forest reserve, and bush 

burning). Also, average UBC (1.0744 Mpa/1074.4 KN/m
2
) determined from DCP are within presumed 

bearng values for the soil (>600 KN/m
2
). This should be rendering anthropogenic/human factors 

insignificant in the occurrence of landslide, thereby establishing a close association between 

geologically unstable ground and landslide events. To adress this, detailed ground investigation 

(Engineering Geological Mapping) is necessary. This is useful in understanding the relationship 

between the failed area and their main geological features. Also, Redshaw et al. (2019), convincingly 

presented scientific facts linking Regent landslide occurrence to deep-seated failures, further implying 



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evaluation of subsurface conditions. Finally, detailed geological studies to understand compositional 

and tectonic variations in the context of landslide occurrence should be considered to note their unique 

impacts on landslide characteristics. In addition to this, shear strength property of the soil should be 

determined, and confirmatory test for the clay type using XRD to further establish the material 

properties and behaviour. 

 

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