Assessing The Kekem Landslide

UNIVERSITY OF BUEA

FACULTY OF SCIENCE Department Of Geology and Environmental Science

Assessing the Kekem landslide

A research project submitted in partial fulfillment of the requirement for the award of a Bachelor of Science (BSc) Degree in Environmental Science

By Motaka Philip Asicho Etapo (UB024780)

Supervised by Dr. Ayonghe N. Samuel

June 2008

CERTIFICATION This is to certify that this work (long essay) “Assessing the Kekem Landslide” is the original work of MOTAKA PHILIP ASICHO ETAPO

Signature____________________ Motaka P. Asicho (Student)

Signature_______________________ Dr. Ayonghe N. Samuel (Supervisor)

Date_____________________________

Signature_______________________ Dr. Ayonghe N. Samuel (Head of Department)

Date_____________________________

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DEDICATION This work is dedicated to my father Mr. Asicho Henock Etapo for always being there for me; to my mother Mrs. Regina Bekeli Njomo for bringing me into this world. This work is also dedicated to my foster parents Late Mr. Ben Tarkem and Mrs. Alice Targem and all my foster brothers and sisters.

May God be with you all now and forever”

“

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ACKNOWLEGEMENT The success of this work is in the hands of a number of people whom I gratefully acknowledge. Special thanks go to my supervisor Dr. Ayonghe N. Samuel who took the time and effort to examine this work. He provided solutions to the problems I encountered and those I could encounter in the field. Great honor and appreciation goes to the VLIR Capacity Building Project in Geohazards Monitoring coordinated locally by Dr. Ayonghe N. Samuel for its financial aid to see that this project is accomplished. Special thanks also go to my partner in the field work Akerenwi. I am also grateful to Dr. James Paulson, John Antill, John Horn, Bev Horsman, Janet Cundall, Thevi Pather and the entire staff of Camosun College International, Victoria, Canada for their moral support in my academic carrier. I am also indebted to my lecturers; Dr. Yinda, Mme Manga, Dr. Oben, Dr. Nkwatoh, Dr. Suh, Dr. Mboudou, Mr. Mbotake and the others for their time and effort which they dedicated for my academic carrier. My thanks also go my friends; Otto, the Kometa family, Julius, Smith, Ayamba, Anslem, Ejani and Diane, whose contributions to my academic carrier cannot be left out. Special thanks go to my sister Tabi Augustina for always encouraging me to study.

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ABSTRACT On the 20th of October 2007, Kekem in the West Region of Cameroon experienced a landslide which left one person death and another person rendered handicapped. The landslide affected 16 families, destroyed farmland, a garden and destroyed the section of the Douala-Bafoussam main road beneath it. An assessment of the slide mechanism, soil type and the causes of the landslide are essential. To this effect, measurements were made on the landslide scar and the distance covered by the landslide, detailed field examination was done and questionnaires were used to get the views of the community. The results showed that the landslide was both rotational and translational; the soil was mainly lateritic due to intensive weathering and the heavy rainfall and steep slopes, abundant rock debris, poor vegetation cover, human activities along the slope all contributed to the eventual sliding of the area. Judging from field study, the hilly nature and the soil type of the region makes the region susceptible to future landslides and therefore mitigatory measures are proposed in order to avoid any loss of human life and property damage from any future landslide in the region. Hence, this work realized its purpose.

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Table of Contents Chapter I: Introduction……………………………………………………………………………………………………………….1 1.1. Background to the study ............................................................................................................. 1 1.2. Study area description ................................................................................................................ 1 1.2.1. Location of study area .......................................................................................................... 1 1.2.2. Topography and geology ...................................................................................................... 4 1.2.3. Climate and vegetation ........................................................................................................ 4 1.3. Problem statement ..................................................................................................................... 5 1.4. Importance of the study ............................................................................................................. 5 1.5. Objectives................................................................................................................................... 5 1.6. The rationale of the study ........................................................................................................... 5

Chapter II: Literature review…………………………………………………………………………………….……………….6 2.1. Definition of terms...................................................................................................................... 6 2.2. Velocity of landslides .................................................................................................................. 7 2.3. Causes of landslides .................................................................................................................... 8

Chapter III: Materials and method……………………………………………………………………………………….…10 3.1. Research methodology ............................................................................................................. 10 3.1.1. Slide mechanism ................................................................................................................ 10 3.1.2. Soil type ............................................................................................................................. 11 3.1.3 Slide factors ........................................................................................................................ 11

Chapter IV: Result, analysis and interpretation……………………………………………………………..………12 Chapter V: Discussion……………………………………………………………………………………………………………….23 Chapter VI: Conclusion and recommendation.……………………………………………………………………...27 6.1. Conclusion ................................................................................................................................ 27 6.2. Recommendation ..................................................................................................................... 28

Bibliography.................................................................................................................................. 29 Appendix……………………………………………………………………………………………………………………………………31 Questionnaire ................................................................................................................................. 31

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List of figures Figure 1.1: Map of the West Region locating the study area………………………………………………..2 Figure 1.2: Map of Kekem showing the area affected by the 2007 landslide………………………..3 Figure 4.1: Relative rapidity of chemical weathering of common igneous-rock forming minerals……………………………………………………………………………………………………………18 Figure 4.2: Soil profiles along landslide scarp……………………………………………………………………..19 Figure 4.3: Excavated soil profiles…………………………………………………………………………………..….19

List of tables Table 2.1: Velocity and types of mass wasting……………………………………………………………………7 Table 4.1: Scarp analysis……………………………………………………………………………………………………12 Table 4.2: Length analysis………………………………………………………………………………………………….13 Table 4.3: Width analysis……………………………………………………………………………………………..…...13 Table 4.4: Analysis of distance travelled…………………………………………………………………………….14 Table 4.5: Age group distribution of questionnaires…………………………………………………..….…..15 Table 4.6: Gender distribution of questionnaires………………………………………………………..….….15 Table 4.7: Respondents on vibration effect……………………………………………………………………..…16 Table 4.8: Respondents on duration of slide……………………………………………………………………...16 Table 4.9: Direction of fallen palms………………………………………………………………………………..….16

List of plates Plate 1: Area destroyed by landslide……………………………………………………………………………………4 Plate 2: Landslide scarp showing the soil profile……………………………………………………………..…20 Plate 3: Soil texture and color showing the coarse grains in fine material………………………….20

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CHAPTER I

Introduction 1.1. Background to the study A landside is a form of mass wasting or mass movement, which can be defined as “movement in which bedrocks, rock debris or soil move down slope in bulk or as a mass (Plummer et al., 1999)”. This movement down slope is basically under the influence of gravity. The movement varies in speed from very slow, referred to as “creep”, to very rapid, referred to as “rock fall”. Landslides are major geologic hazards. They affect humans in numerous ways, from killer landslides, to nuisance of an area slightly pulled apart by soil creep. Reports of the American Red Cross show that landslides cause up to two billion US dollars in damage and from 25 to 50 deaths annually in the United States (American Red Cross, 2008). Globally, they cause billions of US dollars in damage and thousands of deaths and injuries every year. Gravity is the factor driving landslide movement, with a combination of various factors that allow the force of gravity to overcome the resistance of earth materials to landslide which include; water, slope gradient, earthquakes, volcanoes, available debris, just to name a few. Integral to the natural processes of earth’s surface geology, landslides serve to redistribute soils and sediments in a process that can be in abrupt collapse or in slow gradual slides. Such is the nature of earth’s surface dynamics. Because the factors affecting landslides can be natural or human-made, they can occur in developed and undeveloped areas or any area where the terrain was altered for roads, houses, utilities, buildings and even for lawns in one’s backyard (United State Search and Rescue Task Force Landslide, 2000). Landslides are global hazards and they are also evident in Cameroon. Recent occurrences include: the Kekem landslide of 2007, the Bonduma landslide of 2006, the Mabeta landslide of 2001 (Ayonghe et al., 2004; Bonduma landslide, 2006)

1.2. Study area description 1.2.1. Location of study area Kekem is a small village located in the Upper Nkam Division of the West Region of Cameroon. The West Region is located between latitudes 4o85’N – 6o20’N and longitudes 9o63’E – 11o33’E Assessing the Kekem landslide

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Chapter I: Introduction

(fig 1). Kekem within the West Region is located between latitudes 5o06’N – 5o12’N and longitudes 9o98’E – 10o05’E. Kekem stands at a spot height of 1,016m above the sea level.

Figure 1.1: Map of the West Region locating the study area

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Chapter I: Introduction

Figure 1.2: Map of Kekem showing the area affected by the 2007 landslide

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Chapter I: Introduction

Plate 1: Area destroyed by the Kekem landslide

1.2.2. Topography and geology The topography of the region is mainly composed of hills and small mountains. This justifies the steep slopes of the region, which leaves the possibility of a landslide due to slope failure. Basically, the Upper Nkam region is composed of granitic rocks which are evident in the hills and small mountains. However, due to tropical variables like temperature and rainfall variation, there is intense weathering and metamorphism, which accounts for the dominant occurrences of weathered granite in Upper Nkam region.

1.2.3. Climate and vegetation Being located in the tropics, Kekem is expected to have typical tropical climate, characterized by high temperatures and rainfall. However, the hilly topography of the region provides for modification of the temperatures, which are slightly lower. Looking at the vegetation, the hilly nature of the region supports huge forest ecosystems, with very small portions of the land inhabited by the villagers. The forest further helps in modifying the climate due to its vegetation canopy.

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Chapter I: Introduction

1.3. Problem statement Kekem is a rural community in the Upper Nkam Division in the West Region of Cameroon. On the 20th of October 2007, the community experienced a landslide, after two days of heavy downpour within a week of continuous rainfall. The slide which began at about 3:30pm, caused the destruction of properties such as houses, farms, the main road and even the loss of one life, leaving another victim seriously injured. The slide also caused the exhumation of a corpse. Kekem last experienced a landslide in 1967, in a nearby area. Judging the susceptibility of the area with respect to steep slopes, the area is proned to landslides and this put the inhabitants of the area in constant fear. There is therefore a need to study the landslide so that proper mitigation measures can be put in place.

1.4. Importance of the study Bearing in mind the destruction associated with landslides, it becomes very necessary to study the cause, just as it is done with most landslides. This study will give useful information on the hazards associated with landslides within the area and this may go a long way to increase awareness on landslides within the community, and Cameroonian population as a whole.

1.5. Objectives The objectives of this fieldwork are therefore to:  Assess the mechanism of the slide.  Evaluate the soil type.  Assess the internal and external factors responsible for triggering the landslide.

1.6. The rationale of the study The rationale behind this study or research is therefore to investigate the potential causes of the landslide, and the role that each factor, whether internal or external factor, played in the eventual sliding of the land. It is aimed that through this work, potential mechanisms can be put in place to avoid or reduce population’s susceptibility to adverse effects, which might result from any future landslides. Above all, this work will go a long way to further knowledge out of classroom.

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CHAPTER II

Literature review Landslides are common natural hazards along the Cameroon Volcanic Line (CVL) which is a major morphological feature in West Africa (Ayonghe et al., 2001). The CVL is over 1,600km long, stretching from Pagalu to Oku via Sao Tome, Principe, Bioko, Limbe, all the way to the Madara Mountain (Ngole et al., 2007). The landslides occur mainly during the peak of rainy season from June to October. Most of the occurrences are however not recorded, except when people are killed and/or appreciable property is destroyed (Lambi, 1991; Ayonghe et al., 1999). Records of historical occurrences show that between 1988 and 2001, 64 people were killed by landslides along the CVL (Ayonghe et al., 2001)

2.1. Definition of terms There are many terms associated with landslides. They include: The term “flow” is used when the material moves as a viscous fluid. This usually results from the presence of excess amount of water. This often results when fine material get in contact with water. This is common during volcanic activity where the ash get in contact with water to form what is generally termed “lahar” (Foster, 1978; American Red Cross, 2008). The term “slide” is used if the material moves over a surface of the earth. This is typical when large amount of debris are force to move. The term “fall” is used if the material travels in air in a freefall, under the influence of gravity. This is typical with rocks, where broken pieces fly in air (Plummer et al., 1999). “Creep” is the slow, down slope movement of material and in some cases, bedrock. If the amount of water in the material increases, an earthflow will result. An earthflow differs from a creep in that; a distinct scarp is formed at the breakaway point of the parent rock (Foster, 1978). “Solifluction” is another term for landslide, which occurs when frozen ground melts from the top of a mountain or hill during warm spring days in temperate regions or during summer in permafrost (Leed et al., 1982). The term “translational slide” is used when the material carried away can be clearly distinguished from that left behind. There is a clear sliding surface – a surface which is impermeable and consistent throughout the area – which allows the material to slide down the Assessing the Kekem landslide

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Chapter II: Literature review

slope. This is very common in areas which have a high debris content, or loose material overlying bedrock. The presence of water in such material helps to lubricate the sliding surface, easing the movement of the loose material down the slope (Plummer et al., 1999). The term “rotational slide” is used when the material carried away can be clearly distinguished from that left behind, but in this case, there is no sliding surface. The material moves in a more or less circular manner. This is common in areas which have experienced intense weathering or deep weathering, such that there is no bedrock or the bedrock is far below to act as a sliding surface. This kind of landslide is usually caused by undercutting by rivers or penetration of underground water in weathered material, which tend to destabilize the upper material, resulting in downward movement. In the field, the plants lie in the upward direction (Plummer et al., 1999).

2.2. Velocity of landslides Landslides vary widely in terms of their velocities. It can be rapid or very slow at a rate of less than 1cm/year. The rapid landslides are usually very destructive, with little or no warning. For example; in Peru 1970, the town of Yungay was completely buried with about 18,000 deaths (Montgomery, 1992). The slow landslides are usually referred to as soil creep, which goes about unnoticeable, evident mostly by tilting of poles and fences (Foster, 1978). The rate of movement is greatly influenced by factors such as; the amount of water present, the steepness of the slope, and the factors which triggered the landslide (Duff, 1993). The velocity however, does not necessarily give the severity, as; slow slide over an extended area can be more destructive than a fast one in a limited area. Table 2.1: Velocity and types of landslides from Plummer et al., 1999 Slowest Type of movement Flow

<1cm/yr Creep (debris)

Slide

increasing velocity 1m/day – 1km/hr Earthflow

1 – 5km/hr Mudflow (water saturated with debris)

Fastest >5km/hr Debris avalanche Rock avalanche

Debris slide Rockslide

Fall

Rockfall Debris fall Landslides

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Chapter II: Literature review

2.3. Causes of landslides Landslides are caused by various factors, some of which can act singly or may result from cumulative effects of the factors. Some of the factors include:  Steepness of the slope: this factor is very essential since it determines the stability of the material. Critical angles which materials can attain – angle of repose – determine the stability of the material. This angle, which is usually 25o to 45o for consolidated material, depends on the grain size and also the shape of the particles (Montgomery, 1992). The Limbe landslide of 2001 had slopes with dips of 35o to 80o (Ayonghe et al., 2001).  Relief: this is the vertical distance between valley floor and mountain summit. Regions with high relief are more proned to slides than those of lower relief. This is because stability decreases with increasing distance from the base floor.  Presence of water: water plays a very important role in initiating a landslide. Landslides are usually associated with flood. The Kenya landslide of May 2002 which left 11 deaths resulted from a flood (BBC News Africa, 2002). In Cameroon, most of the landslides occur during the rainy seasons (Ayonghe et al., 2001). The effect of water is two fold; it increases the weight of the slope, and it lessens the internal cohesion of the material. The water tends to lubricate the sliding surface, which reduces the friction between the moving mass and the sliding surface, easing the slide (Foster, 1978).  Presence of loose rock and debris: when an area contains loose rocks or debris, there is the tendency for the material to move down the slope with the slightest instability. This loose material can easily be saturated with water, which tends to increase the weight of the material to overcome the frictional force preventing motion. Hence, they are easily carried away resulting to a slide. (Plummer et al., 1999).  Influence of earthquake: earthquakes have been known to cause landslides. Their tremendous vibrations tend to breakdown the forces holding the soil or rock particle together. The Alaskan slide of 1964 was the result of an earthquake (Foster, 1978). The devastating Peru landslide of 1970 was triggered by an earthquake which had an epicenter about 100km offshore (Montgomery, 1992).  Orientation of planes of weakness: in the course of human activities such as road construction, there is the tendency of slope cutting. If this cutting is done in manner in which the planes of weakness of the bedrock lie parallel, the above material becomes susceptible to sliding and any slight change in the stability results to a landslide (Plummer et al., 1999).  Undercutting by rivers: in some cases, a river flowing beside a cliff or any sloppy area may erode the toe of the slope, and with time, the extent of erosion becomes so great that the slope stability is altered, and a resultant slope failure occurs, leading to a landslide.

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Chapter II: Literature review  Frequent freezing and thawing: this is very common in the temperate regions whereby, as

the land freezes and thaws, there comes a point where not all the area thaws, and as a result, if layers within the bedrock or parent material thaws, since the surrounding mass is frozen and the resulting water cannot penetrate it, the water acts as a lubricant, carrying the frozen block down the slope, leading to a landslide. This is termed solifluction (Foster, 1978).

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CHAPTER III

Materials and method 3.1. Research methodology In the course of studying the Kekem landslide, the materials which were used included: a 3.6m tape, a cutlass, a 30cm ruler, a recording sheet, a pen, a camera and a pair of boots. Work in the field was done in accordance with the objectives. The steps and methods were carried out based on the said objectives.

3.1.1. Slide mechanism Here, the starting point involved identification of the slide scar and the scarp. Critical examination of the area was done to avoid any collapse while working. By observation, different areas of the scarp were chosen to be studied. After selection, the various areas were measured using a tape. The measurements were done from the top – surrounding highland – to the floor of the landslide. The results were recorded. The landslide surface was then observed for signs of striation. The length of the scarp – from the base of the scarp to the point which did not experience displacement – was measured using a rope of known length. The process was again repeated using a stick. The results were recorded. After that, the width was then measured, and this was done twice to obtain two results, at two slightly different positions. The results were recorded. Thereafter, the distance travelled by the material moved was then calculated. This is the distance from the base of the scarp to the toe of the regolith. This was done twice, and the results were then recoded. To obtain the time taken for the displacement, a small questionnaire was used, which was photocopied, and distributed to the following age groups: 15-25yrs, 26-50yrs, >50yrs. The slide area was then carefully examined to identify any slide surface, sub-surface seeps of water and any other findings. The regolith was then carefully examined. Further examination of the upland area was also done, for clues on the local geology of the area.

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Chapter III: Materials and method

3.1.2. Soil type Here, the area and slide material were analyzed for soil texture, by “feel”. The proportion of grain size distribution was also examined. The area was also studied to identify the parent material, and the possible mineral content was also assessed, bearing in mind the parent material. Soil profiles were analyzed at various points of the scarp, and the distance of the various horizons were calculated, using a tape and a meter ruler. Two soil profiles were dug at different areas, and the possible horizons were noted. Finally, analysis of the activities carried out on the soil was done. The soil was assessed for stability, in terms of vegetation and the level of its compaction. The soil was also examined for water content. This was done by simple scratching of the soil surface for evidence of wetness.

3.1.3 Slide factors Different factors could contribute to the slide. On the basis of these factors, investigations were done on the following; From the scarp, careful examination of the various layers was done in order to look for possibilities of planes of weakness, which might contribute to the slide. Also, from the questionnaires, information relating to the amount of rainfall before the actual slide was sorted. This might provide knowledge on the role of water in the sliding proper. Again, through the questionnaires, questions were asked relating to vibrations felt. This might actually determine if earthquake was a contributing factor to the slide. Furthermore, investigation was done concerning the steepness of the slope, by comparing the length of the scarp from the initial point of breakage – where the displacement began uphill – to the area which was not displaced downhill. Proper study of the area for loose rocks and debris was also done, in order to point out its contribution to the eventual landslide. The area was also examined for evidence of undercutting by human activities or excavation works. The area was examined for evidence of a cliff before the actual sliding. Finally, the vegetation was assessed in relation to soil stability, since plants play a very important role in holding the soil together.

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CHAPTER IV

Result, analysis and interpretation Based on observations and field work, and according to the method employed, the various results were obtained: Landslide scarp: Two regions of the slide were measured, using a tape, with the following results; Table 4.1: Scarp analysis Scarp Scarp 1 Scarp 2

Measurement (m) 5.85 5.30

Taking the average of the two; Scarp = (scarp 1 + scarp 2)/2 =

( .

.

)

= 5.575m

Length: Two lengths were measured, using a rope and a stick, at two different regions, and the following results were obtained; Region 1: Length = 22Rcm + 211cm R = length of rope = 360cm Length = (22 X 360 + 211) cm = 8131cm = 81.31m

Region 2: Length = (22Scm + 301cm) S = length of stick = 360cm Length = (22 X 360 + 301) cm = 8221cm = 82.21m Assessing the Kekem landslide

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Chapter IV: Results, analysis and interpretation Table 4.2: Length analysis Length Length 1 Length 2

Measurement (m) 81.31 82.21

Taking the average of the two; Length = (length 1 + length 2)/2 =

(

.

)

.

= 81.76m

Width: Two widths were also measured, using a rope and a stick, with the following results; Width 1 = 12Rcm + 296cm = (12 X 360 + 296) cm = 4640cm = 46.40m

Width 2 = 12Scm + 320cm = (12 X 360 + 320) cm = 4616cm = 46.16m Table 4.3: Width analysis Width Width 1 Width 2

Measurement (m) 46.40 46.16

Taking the average of the two; Width = (width 1 + width 2)/2 =

(

.

.

)

= 46.28m

Knowing the scarp, length and width of the area that was moved, the volume of material displaced can be calculated. This is as follows;

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Chapter IV: Results, analysis and interpretation

Volume = length X width X scarp Length = 81.76m Width = 46.28m Scarp = 5.575m Volume = (81.76 X 46.28 X 5.575) = 21,094.979m3

Hence, 21,094.979m3 of material was displaced from the area by the landslide. Distance travelled by the material: This distance was calculated at two different points, and the following results were obtained; From point 1: Distance = 53Scm + 350cm S = length of rope = 484cm Distance = (53 X 484 + 350) cm = 26002cm = 260.02m

From point 2: Distance = 71Rcm + 188cm R = length of rope = 360cm Distance = (71 X 360 + 188) cm = 25748cm = 257.48m Table 4.4: Analysis of distance travelled Distance travelled Distance 1 Distance 2

Measurement (m) 260.02 257.48

Taking the average of the two; Distance travelled = (distance 1 + distance 2)/2 =

(

.

.

)

= 258.75m Assessing the Kekem landslide

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Chapter IV: Results, analysis and interpretation

Time: Since most geologic hazards show little or no warning before occurrence, proper timing of the duration of such events becomes very complicated especially in areas like this, where there is no monitoring. Severe inquiries from eye-witnesses gave predominantly 20 – 30 minutes. Speed: Knowing the distance travelled by the material and the duration of the movement, the speed or velocity of the landslide can be calculated thus; Speed = distance travelled/time taken Distance travelled = 258.75m Time taken =

(

)

= 25mins = 1500secs Speed =

.

= 0.1725m/s = 621km/hr

Analysis of the questionnaires Table 4.5: Age group distribution of questionnaire Age group (yrs) 15 – 25 26 – 50 50 and above Sum

Number of males 18 9 6 33

Number of females 4 8 2 14

Total 22 17 8 47

Proportion (%) 46.81 36.17 17.02 100

Table 4.6: Gender distribution of questionnaires Gender Male Female Total

Respondents 33 14 47

Assessing the Kekem landslide

Proportion (%) 72.21 29.79 100

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Chapter IV: Results, analysis and interpretation Table 4.7: Respondents on vibration effect Vibration factor Vibration felt Vibration not felt Total

Respondents 9 38 47

Proportion (%) 19.15 80.85 100

Table 4.8: Respondents on duration of slide Duration of slide motion <20 minutes 20 – 30 minutes >30 minutes Total

Respondents 1 36 10 47

Proportion (%) 2.13 77.60 21.27 100

Critical examination of the surface of the landslide area revealed small channels of water, which appear to have subsurface sources. This is evident of underground seeps. Careful observation of the surface did not show any peculiar slide surface, though a critical look revealed some kind of gliding surface. However, the area left behind showed some significant difference from the material carried away. Hence, the slide can be said to have both a rotational and a translational nature. Looking at the area revealed an upward lying displacement of plants. Of the 15 palms displaced, 9 lay in the upward direction and 6 lay randomly. Hence, the slide can be said to have a rotational nature. Table 4.9: Direction of fallen palms Direction of fallen palms Upward lying palms Random lying palms Total

Number 9 6 15

Proportion (%) 60 40 100

Immediately below the surface, the region was wet, which suggest the abundance of water in the soil and the absorbing nature of slide material. Careful study of the upland region around the slide area revealed large fragments of metamorphic rocks (granite, gneiss) scattered all over the area. This suggests that the parent material of the region is predominantly granite which has undergone severe or intense weathering especially physical weathering which is evident in the dominant fragments of rocks.

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Chapter IV: Results, analysis and interpretation

Soil texture: Textural analysis of the soil done by “feel” revealed the dominance of fine grain material with spatial proportion of sand grains. Hence, the soil showed a mixture of fine and coarse grain material. However, there were also bolders and rock fragments scattered within the soil. Soil color: The appearance of the soil revealed a brown to light-brown coloration. This is typical of lateritic soils, with essentially clay minerals. Parent material: From the study of the local geology of the area, the rocks were mostly granite, and the dominance of fragments around indicates that the area has undergone intense weathering. The fine nature of the soil with some sand grains and the clayey appearance concludes the fact that the parent material is granite. Mineral content: Bearing in mind the occurrence of granite as the parent material, the mineral content of the soil is highly dependent on the mineral content of the parent material and their rate of weathering. The mineral content of granite according to Leet et al., 1982, in order of stability to weathering includes;  Quartz: This is highly resistant to chemical weathering, hence the most stable mineral of them all.  Feldspars: Plagioclase feldspars weather more rapidly than orthoclase feldspars, and anorthite (calcium plagioclase) tend to weather more rapidly than albite (sodium plagioclase).  Micas: Muscovite is more resistant to weathering than biotite mica. Biotite weather more slowly than other dark minerals.  Hornblende weathers slower than augite.  Augite weathers slower than olivine.  Olivine is the least resistant mineral in granite.

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Chapter IV: Results, analysis and interpretation

Quartz

Slow weathering

Muscovite

Orthoclase (K feldspar)

Biotite

Hornblende

Albite (Na feldspar)

Augite Rapid weathering

Olivine

Anorthite (Ca feldspar)

Figure 4.1: Relative rapidity if chemical weathering of common igneous rock-forming minerals modified after Leet et al., (1982)

The soil showed the presence of sand grains which are evident of the presence of quartz in the soil. There were also large fragments of rocks which were dominantly quartz. This is so because quartz is affected very slightly by chemical weathering. Hence, it is relatively stable. Feldspars crystallize from magma before quartz, hence when granite is exposed to weathering, the feldspars are the first to be broken down and the end product is clay minerals which is evident in the light-brown color of the landslide material. Hence, the soil is composed mainly of aluminosilicates, which are basically clay. Soil profile: A total of four profiles were sampled in order to assess the various horizons, at various regions of the landslide area. Two profiles were sampled on the landslide scarp at various areas, and the following results were obtained:

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Chapter IV: Results, analysis and interpretation

Profile 1

Profile 2

30cm

Humus layer

Humus layer

177cm

Slightly dark

Reddish brown

118cm

Light in color

Slightly dark with large amount of rock fragment

260cm

Reddish brown with rock fragments

Light in color with purely consolidated soil particles

Figure4.2: Soil profiles along landslide scarp

Two profiles were also sampled on the landside surface by digging at various points and the results presented below were obtained. Profile 3

28cm

82cm

Profile 4 12cm

Humus layer

54cm

Reddish mud

38cm

Fractured rocks

Humus layer

Light in color

Figure 4.3: Excavated soil profiles

Looking at the agricultural practices of the area, it was discovered that the area use to be a cocoa farm. Also, there was a garden of pepper below, all of which were destroyed by the landslide. Comparing the soil-vegetation stability for the area with a typical forest of cocoa revealed a possibility of instability since the vegetation was not sufficient to increase the extent to which the soil particles were held together. Looking at the fine property of the soil, the soil was quite compact and even appeared baked, probably due to the lateritic nature of the soil. Areas which were a little wet appeared as lumps. Assessing the Kekem landslide

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Chapter IV: Results, analysis and interpretation

Some areas showed a high water content just on scratching a few centimeters below, and some were actually damp or wet on the surface.

Rock fragment

End of scarp

Plate 2: Landslide scarp showing the soil profile

Lateritic soil

Rock bolder

Plate 3: Soil texture and color showing the coarse grains in fine material

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Chapter IV: Results, analysis and interpretation

Factors of the landslide The factors which were suspected to have contributed to the actual movement were classified into internal and external factors.

Internal factors Planes of weakness: Careful examination of the scarp and surrounding regions did not show any planes of weakness. The soil which showed almost a uniform composition was highly compacted, with no observable planes of weakness. Soil type: The soil type which was composed mainly of clay mineral though appeared compact, have a high affinity to water. Clay absorbs water and may even swell, which increases the volume and consequently the weight of the material. Clay is also known to have a low permeability; hence absorbed water is hardly given off. This is probably a contributing factor to the landslide. Rock fragments and debris: Study of the area showed that there was loose material especially rock fragments, which were very common in the area. This can be a contributing factor to the actual volume of material, which might have eventually contributed to the landslide. Slope steepness: The region which is a practically hilly area was observed to have a great deal of steepness, which might possibly result to a landslide if the material load increases. Hence, slope steepness can be considered as a contributing factor to the landslide. Soil water content: Clay normally absorbs water. Hence, with a potential supply of water, the water is normally retained and this can be a contributing factor. Vegetation: Due to the absence of potential forest trees which can hold the soil particles firmly, the soil was practically loose which therefore increase their susceptibility to movement under gravity. Hence, the lack of adequate vegetation is a contributing factor.

External factors Rainfall: Most slides are associated with periods of increased rainfall. With the month of October being a month with rains, this actually contributed to the landslide. It was discovered that before the actual sliding of the land, there was a week of continuous rain with the last two days having heavy rainfall. There is therefore no doubt that this contributed to the landslide. Road cutting: The area lies just adjacent to the Douala-Bafoussam main road. The road construction probably occurred along the slope of the area and there was cutting of the slope.

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Chapter IV: Results, analysis and interpretation

This has consequently affected the stability of the area with time. Hence, this is a possible contributing factor to the actual landslide which occurred. Earthquake and/or volcanism: Such processes are usually accompanied by vibrations. Vibrations were felt during the landslide. Critical investigation revealed that the said vibrations were experienced by only those who live immediately below the slope of the area. Such vibrations can be accounted for by the movement of the material of the slide as friction and other internal forces which tend to prevent the motion were broken down by the weight of the material under gravity. Hence, neither earthquake nor volcanism can be associated with the landslide. Human activities: The region actually served as farmland for the locals. In their effort to cultivate, there is constant clearing and tilling of the soil. This can be considered a contributing factor since in the process of farming, the soil is constantly made loose and this alters the stability of the slope, which makes it susceptible to a landslide. Cultural myth: There is a line of thought held by some of the locals. According to them, a corpse was buried in that same area and the population, who thought that it needed its location to be changed, attributed this to the landslide. During the landslide, the corpse was actually exhumed, which made the story more interesting. However, such a factor cannot be taken into consideration because geologic phenomena are scientific.

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CHAPTER V

Discussion Since landslides are geologic phenomena, there is therefore no doubt on the prevalence of certain factors. Conditions such as: the existence of a slope, inducing factor such as rainfall. Hence, there is no doubt that they would have had a role to play in the Kekem landslide. Based on the objectives of the study, we may proceed thus: Looking at the mechanism of the landslide, from the calculations, the volume of material displaced by the landslide is 21,094.979m3. This volume is quite great to cause reasonable damage. Looking at the velocity of the slide, we see that given the short time interval of 20 – 30 minutes, the material displaced covered a distance of 258.75m, giving it a velocity of 0.1725m/s. With such velocity, it is evident that the slide was fast. This gives an indication that the landside was actually an earthflow or a debris slide which from table 2.1 travelled with a speed of about 1m/day – 1km/hr. The Kekem landslide speed could actually be converted to a speed of 621km/hr, which sits conveniently in the range. Furthermore, given the fact that the slide occurred in a period of heavy rains which by implication, provides a huge quantity of water to the soil and the absorptive property of clay, which is the dominant composition of the soil, it can be concluded that the landslide was actually an earthflow. The wetness of the soil just immediately below the surface confirmed the abundance of water which further confirmed that it was actually an earthflow. Going back to the surface of the landslide area, the results reveal that there was no prominent slide surface. This is a clue to revealing the fact that the landslide had a rotational nature. However, the traces of a translational nature cannot be overlooked. Also, the fact that the trees in the field lay facing predominantly the upward direction further confirmed the fact that the landslide was rotational. This is a common factor or characteristic of rotational landslides. Finally, description from eye-witnesses was more or less some kind of intermittent motion, with a rolling motion. This seals the fact that the phenomenon actually had a rotational nature. Hence, the landslide can be said to be both a rotational and a translational landslide, probably beginning with a translational nature and proceeding in a rotational nature. Base on the soil type, the dominant fine grain texture of the soil and the light brown color to reddish-brown color is characteristic of lateritic soils. Hence, the soil can be termed a laterite.

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Chapter V: Discussion

The area is generally composed of weathered granite from which the clayey nature of the soil can be associated to. Granite is generally composed of quartz, feldspars and micas. The feldspars are the dominant composition of granite. Hence, the weathering of feldspars can be considered as the major contribution to the soil type of the Kekem area. Lateritic soils are generally rich in oxides of aluminum and iron. They usually form under conditions of good drainage. Bauxite for example, forms when intense and prolonged weathering removes the silica from clay minerals, leaving the clay as bauxite residue. The presence of water is very essential for the weathering of feldspars. Quartz is considered stable because it is only slightly affected by chemical weathering. Feldspars combine with water to form hydrous aluminum silicate which is the basis of clay minerals e.g. orthoclase. The role of carbon dioxide (CO2) cannot be left out. Water combines with CO2 to form carbonic acid thus: H2O(l) + CO2(g) →H2CO3(aq)

equation 5.1

H2CO3(aq) ↔ H+(aq) + (HCO3)-(aq)

equation 5.2

H2CO3(aq) ionizes thus:

When orthoclase feldspar comes in contact with the hydrogen ion (H+), clay is produced thus: 2K(AlSi3O8)(s) + 2H+(aq) + H2O(l) →Al2Si2O5(OH)4(s) + 2K+(aq) + 4SiO2(s)

equation 5.3

Potassium is liberated from the orthoclase to form the clay mineral. The potassium ion combines with the bicarbonate thus: K+(aq) + (HCO3)-(aq) →KHCO3 (S)

equation 5.4

The silica remains in the soil as sand grains (Tarbuck et al., 1996). Clay, according to Montgomery (1992), has a porosity of 45 – 55% and permeability less than 0.01m/day. This implies clay has a high affinity for water but at the same time, has a very low tendency to release water. Clay can absorb up to 20 times its weight of water and form a weak gel, which is susceptible to sliding. Clays generally tend to swell when they absorb water, increasing their volume and consequently, their weight. This actually has a role to play in the landslide since the dominant part of the slide material is composed of clay. However, when conditions are very dry, clay tends to shrink and even bake brick hard.

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Chapter V: Discussion

The nature of the soil profiles were all evidence of intense leaching in the area. They all showed good development as the various horizons were clearly distinct and the horizons were also very few. This is characteristic of lateritic soil. Considering the contributing factors and based on the Kekem landslide, the possible contributing factors include; rainfall, soil type, presence of rock fragments, the steepness of the slope, vegetation, human activities and road construction. These factors according to Terzaghi (1952) can be grouped into preparatory, activating and sustaining factors of the landslide. The preparatory factors prepare the slope for failure, the activating factors induce the failure and the sustaining factors keep the failed slopes in motion either continuously or intermittently. Looking at the Kekem landslide, the preparatory factor is basically the intense weathering which had been going on for a very long period of time. The weathering of the primary granite rocks into clay minerals rendered the area quite unstable taking into consideration the steep slopes of the area. This tends to reduce the intermolecular forces of attraction which are basically termed the shearing strength of the material. The construction of the road at the slope further helped to increase the steepness of the slope and also reduce the shearing strength of the material, making the area more susceptible to sliding. The absence of adequate vegetation to aid in holding the soil particles together can also be considered a preparatory factor as vegetation plays a contributory role in increasing the shearing strength of the material in the area. The activating factors basically include the heavy rainfall during the period before actual sliding. The soil type again and even the farming activities carried out on the area, contributed in activating the landslide. The area which is basically composed of clay soil due to rock decomposition had a high level of porosity. Hence, the area had the capacity of absorbing large volume of water. With excess water, the clay tends to swell due to its low permeability. The water sustained in the clay soil helped to increase the weight of the material. This increase in weight which is generally termed shearing stress increased the tendency of the weathered soil and rock fragments to move down the slope against the shearing strength. Hence, for the material to move, the shearing stress has to be greater than the shearing strength of the material (Montgomery, 1992). For slope failure; Shearing stress > Shearing strength

Assessing the Kekem landslide

equation 5.5

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Chapter V: Discussion

The farming practice also helped to reduce the shearing strength of the material due to constant tilling which leaves the soil loose. Hence, the landslide actually occurred at a point where the shearing strength could no longer hold the material in place. Sustaining the motion was actually the function of the excess water which accompanied the heavy rainfall. The sticky nature of the soil together with its high water content resulted in the intermittent motion of the material. The slope also had some contribution in sustaining the motion due to difference in elevation. Hence, different factors had different roles which they played in the eventual sliding of the material, which could be individual or contributory.

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CHAPTER VI

Conclusion and recommendation 6.1. Conclusion Landslides are geologic hazards which cannot be compromised. Their occurrences with little or no warning worsen their effect as they can take lives and cause intense damage within minutes. The Kekem landslide is just another manifestation of this geologic hazard and as usual, it was accompanied by property damage and even loss of life. The landslide, according to its mechanism was actually an earthflow, which was basically both a rotational and a translational landslide, with a dominant rotational nature. The soil type was basically clay which due to its absorptive and expansive properties played an important role in the eventual landslide. The contributing factors all played indispensable roles in the eventual sliding of the land. They include:      

Rainfall for a week with the last two days having heavy downpour. The highly weathered lateritic soil which altered the stability of the soil. The steep slopes reduced the shearing strength of soil and encouraged the sliding. The spatial vegetation cover failed to provide the binding support to the soil. Human activities such as the farming practice reduced the soil strength due to tilling. The abundant rock debris around the area added to the weight of the sliding material.

Looking at the Kekem area and the Upper Nkam Region in general, the region is quite hilly with steep slopes. The Kekem village is characterized by buildings on the slopes of the hills. Considering the weathered nature of the soil in the region and the poor nature of construction along the slopes gives a possibility of subsequent landslides in the future within the region. Hence, proper measures need to be put in place if there is any hope of avoiding dangers to human life and property loss in the future.

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Chapter VI: Conclusion and recommendation

6.2. Recommendation Seeing the possibility of future landslides in this region, it is essential that there should be; i. ii. iii. iv.

v.

Relocation of the villagers from the areas which might slide in the nearest future. Also, careful studies and differentiation of risk-proned zones and risk-free zones should be carried out so as to prevent any future risk to humans and property. Again, retaining walls with drain pipes should be constructed along areas with steep slopes especially beside human settlements and roads. Furthermore, education of the villagers on landslides and their hazards should be a necessity as this will have a great influence on their settlement patterns. This can be done through media like radio, television and direct contact. Finally, it is essential that the region should be carefully studied and areas which show a high degree of instability be cut down so that the future damage can be avoided when the land finally give away.

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Bibliography American Red Cross. (2008, march 14). Retrieved march 15, 2008, from http://www.redcross.org/services/disaster/0,1082,0_588_,00.html#topwww.redcross.org. Ayonghe, S. N., Mafany, G., Ntasing, E., & Samalang, P. (1999). Seismically activated swarm of landslides, tension cracks and a rockfall after heavy rainfall in Bafaka, Cameroon. International journal Natural Hazards , 13-17. Ayonghe, S. N., Ntasing, E. B., Samalang, P., & Suh, C. E. (2004). The june 27, 2001 landslide on volcanic cones in Limbe, Mount Cameroon, West Africa. Journal of African Earth Science vol 39 issue 3-5 , 435439. BBC News Africa. (2002, may 1). Retrieved march 15, 2008, from http://news.bbc.co.uk/1/hi/world/africa/1962450.stm.www.bbc.co.uk. Clay soil. (1999). Retrieved april 19, 2008, from http://www.burkesbackyard.com.au/factsheets.www.burkesbackyard.com. Clay: Properties and Classification. (1999). Retrieved april 19, 2008, from http://www.infoplease.com/ce6/sci/a0857390.htmlwww.infoplease.com. Duff, P. M. (1993). Holmes' Principle of Physical Geology 4th edition. london: Chapman & Hall. Foster, R. J. (1978). General Geology. Colombus; Toronto; London; Sydney: Charles C. Merrill Publishing Company. Huggett, R., & Cheesman, J. (2000). Topography and the Environment. Jovanovich, H. B. (1989). Earth Science. Lambi, C. M. (1991). Human Interference and Environmental Instability. In Cameroon Geography Review 1 (pp. 44-52). Landslide. (2004). Retrieved march 15, 2008, from http://en.wikipedia.org/wiki/landslidewww.wikipedia. Leet, L. D., Judson, S., & Kauffman. (1982). Physical Geology. Meinzer, O. E. (1942). Hydrology. New York: Dover Publications. Inc. Montgomery, C. W. (1992). Environmental Geology 3rd edition. Wm. C. Brown Publishers. Ngole, V. M., Georges-ivo, E. E., & Ayonghe, S. N. (June 2007). Physico-chemical, mineralogical and chemical considerations in understanding the 2001 Mabeta New Layout lanslide, Cameroon. Journal of Applied Science and Environmental Management vol 11 , 202 - 207. Assessing the Kekem landslide

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Plummer, C. C., Geary, D. M., & Carlson, D. H. (1999). Physical Geology 8th edition. WCB McGraw-Hill Companies. Soil Properties. (1999). Retrieved April 19, 2008, from http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/soil_systems/outline. Soil Texture and Water Quality. (2002, April 12). Retrieved April 19, 2008, from http://www.agr.gc.ca/pfra/water/facts/soiltexe.pdf. Tarbuck, E. J., & Lutgens, F. K. (1996). Earth An Introduction to Physical Geology 5th edition. New Jersey: Prentice Hall Inc. Terzaghi, K. (1952). Mechanisms of Landslides. Berkey Volume. Geol. Soc. Am . The Bonduma landslide. (2006). Retrieved Junw 09, 2008, from http://www.leffortcamerounais.com/2006/2008/geologist_expla.html. Thompson, G. R., & Turk, J. (1993). Earth Science and the Environment. United States Search and Rescue Task Force Landslide. (2000). Retrieved March 15, 2008, from http://www.ussartf.org/landslides.htm#top.

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Appendix Questionnaire Sorry for disturbing you, I am a student from the University of Buea. Can you kindly borrow me some of your time to answer these few questions? Tick (√) the right option 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Sex of the respondent: male female Age group: 15 – 25 26 – 50 >50yrs When did the landslide occur? 2005 2006 2007 Were you present when it occurred? Yes No At what time did it occur? 10 – 12noon 1 – 2:30pm 3 – 4:30pm >4pm Has this area been previously affected landslide? Yes No If yes, in what year……………. Was there any shaking before its occurrence? Yes No Was there rainfall before its occurrence? Yes No If yes, for how long? 1day 2days 3days others What was the intensity of the rain? Mild Heavy How long did the landslide last?.......... Has the area been used for human activities? Yes No If yes, what type of activities? Farming Rearing Construction Others Are some or all of these activities still taking place? Yes No Was there any damage cause by the landslide? Yes No If yes, what type of damage was observed? Destruction of farmland Houses Road Garden Was there any human casualties? Yes No If yes, how many people lost their lives? 1 2 Others How many were injured? 1 2 Others Did the affected receive any help after the landslide? Yes No If yes, what sort of help did they receive? Finance Relocation Material items Are you satisfied with the help so far? Yes No Do you remember any institution/organization that came to your aid? Yes No If yes, which ones? CERAC Red Cross State Others Will you accept relocation as a long term preventive measure for future landslides? Yes No Have any preventive measures been put in place? Yes No If yes, what are some of the measures? Prohibition of human activities Building retention walls Others Thanks for your cooperation.

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