The role of conditioning and triggering factors in the …madarevues.recherches.gov.mg/IMG/pdf/Madamines4_4_.pdfThe role of conditioning and triggering factors in the occurrence of

Madamines, ISSN 2220-0681, vol. 4, 2012 Page 42

The role of conditioning and triggering factors in the occurrence of lavaka: a case study in the vicinity of Antananarivo capital, central Madagascar

V. Ramasiarinoro1, L. Andrianaivo 2, R. RANDRIANJA 2

1 Département des Sciences de la Terre, Université d’Antananarivo, BP 906 Antananarivo 101 Madagascar, E-mail [email protected]

2 Ecole Supérieure Polytechnique, Université d’Antananarivo, BP 1500 Antananarivo 101, Madagascar E-mail [email protected], [email protected]

Abstract - "Lavaka" (Malagasy word) or large hillslope gully and landslides are triggered by geological structures, morphology, climate of the region, geotechnical properties of surface rock, engineering excavation for various purposes and waste water parameters. Weathering, a very important factor in lavaka occurrences, decreases the resistance of rock mass. Clayey soil, a weathering product of rock mass, contributes to lavaka occurrence because of its chemical and physical properties. One thinks that the intense rainfall during summer in the central region of Madagascar contributes to the weathering of the rock mass and increases also the water content in the clayey soils that leads to reduction in the stability of natural slopes. In this study, the lateritic soils forming as a result of completely weathering migmatitic rock in Antananarivo and surroundings were investigated in association with lavakas and determined mitigation measures for lavaka phenomena in Ambohimalaza and Ampangabe Villages. The type of clays was determined as illite using modified plasticity chart reviewed. Laboratory tests of the geotechnical properties for lateritic soils show that both stress increase and shear strength parameters such as cohesive strength and angle of internal friction decrease happen due to the increase of water content, thus a lavaka occurred and causes extensive damage to farmland or property in the vicinity.

Keywords: Lateritic soil, triggering factors, lavaka, mitigation measures, Madagascar.

Résumé – "Lavaka" (mot de Madagascar) ou grand ravin à flanc escarpé et glissements de terrain sont déclenchés par les structures géologiques, la morphologie, le climat de la région, les propriétés géotechniques de la roche en surface, les travaux d’excavation pour diverses utilisations et les paramètres des eaux responsables du dégât. L’altération, un facteur très important dans la formation des lavakas, abaisse la résistance de la masse rocheuse. Le sol argileux, un produit d’altération de la masse rocheuse, contribue à la formation de lavaka à cause de ses propriétés chimiques et physiques. On pense que les chutes de pluie intense pendant l’été dans la région centrale de Madagascar contribuent à l'érosion de la masse rocheuse et augmentent aussi la quantité d’eau dans les sols argileux qui entraine la réduction de la stabilité des pentes naturelles. Dans cette étude, les sols argileux formés, qui sont le résultat de l’altération complète de la roche migmatitique dans les environs d’Antananarivo et ses environs, ont été investigués avec les lavaka, et des mesures d’atténuation concernant les phénomènes de lavakisation dans les villages d’Ambohimalaza et d’Ampangabe ont été déterminées. Le type d'argiles a été identifié comme étant l’illite selon le diagramme de plasticité modifié qu’on a utilisé. Les essais de laboratoire des propriétés géotechniques des sols argileux montrent que lorsque les contraintes augmentent, les paramètres comme la cohésion et l’angle de frottement interne diminuent. Ceci est dû à l'augmentation de la quantité d'eau dans les sols latéritiques ; ainsi le phénomène de lavakisation peut se produire et peut causer un dégât étendu des terrains agricoles ou des propriétés dans les environs.

Mots clés: Sol latéritique, facteurs déclenchant, lavaka, mesures d’atténuation, Madagascar.


1. INTRODUCTION

Mass movements including landslides, "lavakas" or large hillslope gullies (Foucault and Raoult, 2001) and disperse erosion are frequent mainly in the central high plateau of Madagascar. Shallow translational landslides and lavaka phenomena are the most commonly observed failure mode involving lateritic soils slopes; they depend largely on the surface topography and are a recurrent problem. The soil types involved are quite varied, ranging from sand to clayey silt (Hutchinson, 1988). The triggering factor is represented by heavy rainstorms with high rainfall intensities and high duration. Several authors have studied these phenomena in order to determine the predisposing and triggering factors, to prevent future disasters (Van Asch et al., 1999; Antronico and Gullà, 2000; Campus et al., 2001). Despite the modest volume involved, shallow landslides and certain lavakas are very dangerous, due to the absence of incipient movement evidence and their high velocity.

This paper describes and analyses the results of the detailed study of the lavakas based on field and laboratory measurements and the monitoring of the slide area. This paper provides also a framework for evaluating risk assessment and for determining mitigation measures in response to lavaka phenomena disaster.

Ambohimalaza and Ampangabe Villages, in the vicinity of Antananarivo capital, represent the test sites and were studied for the design of remediation works concerning lavaka phenomena. They are located in the high plateau central of Madagascar and are characterized by sandy- clay mixture of lateritic soils on migmatite bedrock.

2. DESCRIPTION OF THE STUDY AREA

2.1. Geological and climatological setting of the study area

The territory of Ambohimalaza is located in the eastern part and those of Ampangabe in the southern part of Antananarivo capital (Figure 1). The whole area is characterized by a complex geological and structural setting. The geology is dominated by migmatitic complex of Precambrian age (Figure 1). The migmatite units are more or less fractured, with different sets of subvertical faults.

Figure 1: Location map and geological sketch of the study area

The study area is represented by lateritic soils, which cover the bedrock with a thickness ranging from 10 to about 15–20 m in topographic hollows. Sandy-clay, silty and/or clayey deposits formed by weathering and down slope transportation cover the migmatite bedrock units. The



Ambohimalaza basin is represented by alluvial deposits.

Elevation ranges from 1200 m to 1400 m above sea level. The structural–geological context influences the morphology. The ridges bounding the area are made up mainly of granitoids. Slope values are between 0° (alluvial plain) and 40°. There are a number of slopes that locally exceed 45° at the contact between intact granitoid and altered-eroded gneiss.

A medium water table is present. The depth to the water table varies between 5 and 13–14 m, the maxima (15–18 m) occur in September-October. The area has a mean annual rainfall of around 1300 mm. The general annual rainfall pattern is characterized by a summer maxima in January and a minima during the dry season mainly in October-November.

2.2. Weathered clay soils and lavaka phenomena in the study area

In most parts of the high plateau central of Madagascar the bedrock is represented by highly tectonized clayey and consisting of alternating layers of clay, clayey–silty, sandy-clay or clayey-sandy. Both represent structurally complex tectonized formations (Esu, 1977) having a predominant argillaceous component and they are very degradable material in tropical climate. The processes of chemical alteration and physical breakdown result in the weathering of the bedrock and the production of thick sequences of weathered materials, generally consisting in fine-grained soils (silt and clay), including diffused lapideous fragments of different size, with reduced physical and geotechnical properties. The degradation of the mechanical properties of the bedrock can be referred to processes concerning with water content changes, e.g. cycles of wetting–drying, softening (Picarelli, 2000), infiltration of different aqueous solutions (Calvello et al., 2005). Gravitational forces acting on these disaggregated materials cause them to move

down the slopes and accumulate in topographic depressions (colluvial deposits).

The most common slope instabilities in these lateritic soils are an initial shallow rotational or translational slide followed by flowage of the disturbed or undisturbed mass, which occur following periods of heavy rainfall. The infiltration of rain (saturation from above) and the formation and rise of a temporary perched water table, in contact with the less permeable bedrock, increase the pore pressure and cause a reduction in shear strength of the material (Ramasiarinoro, 2008). The failure surface may form within the weathered material (alloalterite), or corresponds to the point of contact between the lateritic soils and the bedrock.

3. METHODOLOGY AND PROCEDURE

3.1. The lavaka inventory

An inventory of lavaka from interpretation of aerial photographs and field surveys was used to document sites of instability. A lavaka and landslide inventory was completed in 2007 by satellite imagery and aerial photograph interpretation at 1:50,000 scale. The slopes are subjected to widespread instability and lavaka phenomena developed in recent years, due to high intensity rainfalls that mobilized some old dormant slides (lavaka). These events caused also the collapse of a few agricultural lands and property.

Different types of slope instability are present. The deepest kinds of movement, with a depth down to 10 m, concern the bedrock and are rotational slides, in the middle and lower part of the slope. At present most of them are dormant or stabilized. Multiple sliding surfaces have been recognized. Large lavakas involving lateritic soils mapped in Figures 2 and 3 indicate areas where many small instability phenomena are currently present. Shallow lavakas are the most commonly observed



failure mode involving alloalterite soils. Their identification and mapping is difficult due to their small size. Despite the modest volume involved, shallow lavaka appear very dangerous to roads, agricultural lands and property.

Figure 2: Field view of the lavakas in Ampangabe

Figure 3: Aerial photograph showing occurrence of lavaka in Ampangabe site

3.2. Field investigations

The field investigation consisted of soil profile description and soil sampling. This field investigation carried out in the vicinity of Antananarivo (Figure 3) revealed that much of the topsoil in the study area consisted of sand–clay mixture soil (Ramasiarinoro, 2008). These materials have been overlaid by migmatitic units. The migmatitic rocks consisting mainly of quartz and feldspars have been weathered to clay. Weathering in the area has presumably altered and broken down the upper part of

the bedrock through chemical decomposition and physical disintegration. In the decomposed rocks, inherent bonding in the parent rock has gradually deteriorated because cohesion due to bonding generally decreases as the material decomposes. Where there is limited cohesion this lateritic soil, if unprotected (i.e., subjected to saturation), is susceptible to erosion during heavy rainfall. The shear strength characteristics of such soils are dependent on factors including the material type, density, stress level and permeability.

For the geotechnical investigation laboratory, tests were conducted on samples taken up to a depth of 1–2 m from ground level in the lavaka scar (Figure 4). Knowing that clayey soil presents problems to geotechnical engineers due to its complex nature, their properties have been investigated for their effects on lavaka phenomena. This derives from its plasticity, its low permeability and thus, the time dependency of pore water pressure change and volume change, its structure, chemistry and mineralogy.

Figure 4: Disturbed and undisturbed soil sampling in the lavaka scar in Ampangabe

3.3. Geotechnical investigations

In this study, for the effects of clay on lavaka phenomena, representative disturbed (Figure 4) and undisturbed soil samples were collected for laboratory investigations.



Geotechnical investigation consisted of boreholes and laboratory tests including grain size analysis, consistency of clay, specific gravity, Atterberg limits, measurement of water content, unit weight, degree of saturation and direct shear test.

There are two separate procedures for obtaining the grain size distribution of soils: sieve analysis and sedimentation analysis (Craig, 1987). One of the most fundamental methods for determining particle size is to pass the material through a series of sieves. The particle size distribution is then reported as the weight percentage retained on each of a series of standard sieves of decreasing size and the percentage passed of the finest size. Sieves separate particles of more than 80 µm (gravel and sand) and sedimentation separates particles less than 80 µm (silt and clay).

Grain size analysis is widely used in classification of soils. The data obtained from grain size distribution curves are used in the design of filters for earth dams and to assess the suitability of the soil for various projects including road construction and air field, and also to determine lavaka hazards. Information obtained from grain size analysis can be used to predict soil water movement. Sieve analyses are carried out to designate grain size of soil in the lavaka area. The properties have been determined with grain size distribution curve drawing according to laboratory tests.

The general definition for specific gravity is the ratio between the unit weights of a substance and the unit weights of pure water at 4 °C. The average specific gravity of soil particles is about 26.8 KkN/m3, depending on the nature of the mineral constituents (Cernica, 1995). This average value supports, on a large scale, the overall mineralogical content of soils. Specific gravity is an important physical property used in volume, weight relationships for soil and rock. Specific gravity may be used to determine other important properties such as

void ratio, porosity, and degree of saturation.

For clay and silt the usual classification is derived from their engineering properties under varying conditions of moisture. The Atterberg limits are an empirically developed but widely used procedure for establishing and describing the consistency of cohesive soil. They can provide useful information regarding soil strength, behavior, stability, type, and state of consolidation, or as classification of an organic or inorganic clay. The small range between plastic and liquid limits, commonly presented by such materials, determines their ability to change rapidly from semi-solid to liquid state, resulting in the significant decrease of cohesion, angle of internal friction and bearing capacity after rain (Lambe and Whitman, 1979).

The poor water pressures contribute to most of the slope failures. The majority of lavakas and landslides occur during rainstorms (Ramasiarinoro, 2008). Effective stress is reduced by an increase in water pressure, and there is a consequent reduction in resistance to shear. So, direct shear tests were carried out in the laboratory to obtain the effective shear strength parameters of clayey soil. In these studies, direct shear tests performed in the 100 to 300 kPa stress range, allowed the measurement of shear strength parameters in terms of effective stresses. In this study, shear strength parameters (c and φ) were determined from shear box tests performed on 6 representative soil samples collected in the lavaka areas (Ampangabe and Ambohimalaza). Direct shear tests were applied on undisturbed 20×20×20 mm samples, recovered from block samples collected in the field. Percolation of surface water into the soil may soften the material especially during heavy rainfall periods and, therefore, may cause a decrease in the effective shear strength of the soil. Pore water pressures developed in clay soils may be released through a long drainage process



and, accordingly, may tend to stabilize, decreasing the effective shear strength of the soil. The long-term stability conditions, unconsolidated-undrained shear strength tests were carried out on the soil samples at a constant shearing rate of 0.05 mm/min, in a deformation controlled shear box that can record residual stress.

Figure 5: Field profile of the lavaka in Ampangabe site

4. RESULTS

The geotechnical profiles (Figure 5) show a slight variability of the geotechnical properties with depth.

The index properties of soil were determined by laboratory tests, and the results are given in Table 1. According to the results, The average specific gravity (γS) of soil is about 25.84 kN/m3, with a range from 25,17 to 26,10 depending on the nature of the mineral constituents. The water content (ω) ranged from a high of 33,3 % to a low of 10,7 %, with a mean value of 14.3 %. The average value of the fines (F) was 51% with a range from 45 to 57%. The permeability (k) varied from 1,7.10-5 m/s to 2,8.10-5 m/s, with a mean value of 2,2.10-5 m/s.

Plasticity is an important characteristic in the case of fine-grained soils; the term plasticity describes the ability of a soil to undergo unrecoverable deformation at constant volume without cracking or crumbling. Plasticity is due to the presence

of clay minerals or organic material. The physical state of a fine-grained soil at a particular water content is known as its consistency and this depends on the interaction between the clay mineral particles. Any decrease in water content results in a decrease in cation layer thickness and an increase in the net attractive forces between particles. For a soil to exist in the plastic state the magnitudes of the net interparticle forces must be such that the particles are free to slide relative to each other, with the cohesion between them being maintained. A decrease in water content also results in a reduction in the volume of the soil in the liquid, plastic or semi-solid state.

Table 1 gives also the consistency limits of soil which were determined in the clay occurring as a result of the weathering of Precambrian aged migmatitic rock. The clay soils tested were of intermediate and high plasticity, with liquid limits (ωL) ranging between 33% and 43%, with an average value of 38%. The plasticity index (IP) extended from 12% to 15%, averaging 13%, whereas the average plastic limit (ωP) value was 24%, ranging from 21% to 28%.

Table 1: Grain size and consistency limits test results

Sample % fines

γγγγS (kN/m3)

ω (%)

ωL (%)

ωP (%)

IP (%)

k (m/s)

MS-21 53 26,10 10,7 43,4 24,8 15,2 2,8 .10-5

MS-22 45 25,17 13,9 38,2 28,2 13,4 1,7 .10-5

MS-4 52 25,88 13,4 36,6 23,4 13,2 2,0 .10-5

MS-3 46 25,89 33,3 35,7 22,9 12,8 1,7 .10-5

MS-1 57 25,78 14,8 36,5 23,6 12,9 _

MS-2 48 25,80 25,2 33,4 21,4 12 _

The modified activity chart (Figure 6) underlines the absence of a certain amount of swelling minerals (smectite and vermiculite) and that classify the swelling–



shrinkage potential of these lateritic soils as low (Williams and Donaldson, 1980).

Figure 6: Modified activity chart showing the

swelling potential of the samples

More evident is the areal variation of the geotechnical properties. According to the plasticity chart (Casagrande, 1948; Costet et Sanglérat, 1969) in the figure 7, clay of low plasticity (CL) characterizes lateritic soils in the upper part of the slope (boreholes MS-21 for Ampangabe site, MS-4 and MS-1 for Ambohimalaza site). In the lower part of the slope (Figure 5) lateritic soils are represented by silty-sand (MS-22) with clayey-sand (MS-3 and MS-2) being classified as CL. This can be partially referred to a substantial difference in the bedrock of the Ambohimalaza migmatite where most of the shallow lavakas occur: from the top of this formation (borehole MS-4 and MS-1) to down (boreholes MS-3 and MS-2) the plasticity of the bedrock changes and decreases from medium to low (Ramasiarinoro, 2008).

Figure 7: Distributions of the samples on the

plasticity chart

In addition, the position that the Atterberg limits plot on a plasticity chart can be related to the clay mineralogy of a soil (Casagrande, 1948; Holtz and Kovacs, 1981; Ohlmacher, 2000). In this study, liquid limit and plasticity index for each sample were plotted on a plasticity chart, which contained the ranges for montmorillonite, illite, kaolinite, and chlorite (Figure 8) and showed that the samples were mainly composed of illite clay mineral.

Figure 8: Plasticity chart showing the relationship of the lateritic soil samples to clay mineralogy

Soil strength parameters (cohesion c, residual and effective friction angle φ) were measured in laboratory for these areas. They were derived from back analysis for the other regions.

Table 2: Shear strength parameters

Sample ω (%) φu (°) Cu (kPa)

21,79 9 12



MS-21 14,31 15 18

2,2 25 24

MS-22

32 9 9

21 12 12

15,98 15 16

MS-4 14,01 16 18

MS-2 26,53 5 16

In lateritic soils, which are involved in shallow lavakas, the residual friction angles (φu) is 5°, the lowest value, measured in laboratory, and correspond to the MS2 borehole (Table 2). Lateritic soils of depth less than 2 m show an effective friction angle (φu) ranging from values of 9°–25° (measured in laboratory on sample of the MS-21 borehole) and 9°–15° (MS-22 borehole), while the effective cohesion parameters (cu) are 12–24 kPa and 9–16 kPa respectively (Ramasiarinoro, 2008).

5. DISCUSSIONS

Lavakas are always associated with a disturbance of the equilibrium relationship which exists between stress and strength in material resting on the slopes (Ramasiarinoro, 2008). The relationship is determined by factors such as the height and steepness of the slope and the density, strength cohesion and friction of the materials making up the slope (Cruden and Varnes, 1996). Instability arises when the shear strength or maximum resistance of the material comprising the slope to shear stress is exceeded by a downslope stress. The shear strength depends on internal cohesion, which is independent of the weight of the overlying material. This cohesion is produced by the interlocking of particles that enables the material to rest at an angle and internal friction, the resistance of particles to slide across each other, which depends on the weight of overburden. Both are also highly dependent on material type and state, particularly with regard to the lubrication of internal spaces.

Generally, root systems contribute to soil strength by providing an additional cohesion component. Nevertheless the vegetation in the area is mainly represented by steppe, with root systems that vary widely in time and space, because of bush burning, lash-and burn agriculture, fuel-wood collection and destruction of native vegetation. Soil erosion results from deforestation and overgrazing. Due to the difficulty in estimating this parameter we take into account the most conservative scenario and we excluded root strength in the calculation (Ramasiarinoro, 2008).

It is believed that lavaka phenomena triggered by heavy rainfall are the most common throughout central Madagascar. In addition to heavy rainfall, appropriate geological materials are required for the lavakas to happen, in particular completely weathered materials. Precipitation is generally known to be one of the basic lavaka triggers and in the Antananarivo area observations have shown that lavaka occurrence is closely related to the precipitation regime (Ramasiarinoro, 2008). In that study, it was found that majority of the lavakas occurred during the summer wet season.

Slope, surface water and groundwater can have a major impact on slope erosion and stability. In lavaka prone areas, the presence of water and groundwater in the slope or surface water on the slope is usually the primary factor leading to a lavaka phenomenon. Severe lavakas typically occur, during or, shortly after periods of heavy rainfall. It is important to understand the magnitude of water flows that can be involved with slope properties. The water content of the soil that determines the geotechnical properties has been increased with discharging of precipitation, irrigation and sewerage water in the lavaka area. Thus, tension cracks occurred due to conditions such as extreme saturation, inclined slope and gravity furthermore. These cracks are expanded by continual



water ingress and the effect of gravity. Therefore, lavakas have occurred along with these cracks (Ramasiarinoro, 2008). In order to reduce lavakas occurrence measures should be taken to prevent water entering in the risk area. Drainage systems must be developed and retaining wall must be built at the bottom of the lavaka area.

6. CONCLUDING REMARKS

The results of studies related to clay and landslide show that the illite and montmorillonite clay minerals have lower shear strengths and higher swelling potentials and are more prone to lavaka and landslide problems than those composed of kaolinite and chlorite clay minerals (Ohlmacher, 2000). In the present study, the clay soils within the migmatitic formations play an important role in causing lavaka phenomena. The plasticity chart shows that the samples are composed of illite clay mineral which appears to be the dominant material in the soil environment.

The Atterberg limits of soil and the index properties were determined by laboratory tests and the results indicate that plastic and liquid limits increase as the water content increases. The soils have low plasticity and are inorganic in nature (CL). Shear strength parameters (c and φ) indicated that the clays have very low strength parameters when saturated with the water.

The weathering, land shortage and slope in the study area prevent people from abandoning the most lavaka prone areas. As population pressure increases, not only the stability of the slopes will be reduced, but people will also be forced to cultivate even more unstable slopes. As a consequence, the risk of damage by slope failure will increase.

The instability can be partly reduced by tempering the human impact. The excavation or terracing of slopes and the construction of structures concentrating water into vulnerable zones should be

avoided. Total reforestation with deep-rooted trees would reduce the lavaka phenomena risk in the region. This is realistic given the absence of forest due to bush burning, fuel-wood collection and destruction of native vegetation. Nevertheless, planting tree rows in risk zones could locally enhance stability. These measures can never completely impede the occurrence of lavakas and it seems that the search for solutions will only come to results by doing the conscious agricultural activity. Therefore, the water content of materials must be controlled in the area.

It is shown that weathering and soil erosion results from deforestation and overgrazing in the vicinity of Ambohimalaza and Ampangabe Villages have altered the nature and topography of the ground surface. Adverse human activities have contributed to the creation of lavakas, these include excavation works, irregular agricultural activity, embankments, and in particular drainage works. Water is one of the most predominant factors which cause lavakas which usually occur during periods of high precipitation. So, in natural slope areas, it is important to construct drainage systems for collecting water so that movement on the slopes can be reduced. Lavakas occur frequently in the high plateau central of the country. This is due to heavy rainfall, high inclination that contributes to rapid erosion and weathering of the rock mass causing reduction in the stability of natural slopes.

The field investigations and the geotechnical investigations carried out in the affected areas indicate that the overlying soil moves during the lavaka activity. Sliding along the tension crack occurred because of increased pore water pressure along boundary between the completely weathered migmatite and bedrock migmatite. The sliding of the overlying clay soil is associated with the saturation of clay pores and deep subsurface flows that generate pore water pressure in the materials that form the slopes. A few mitigation



measures have been suggested in this study. To reduce damage to property, these mitigation measures should be taken at the right time.

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Ramasiarinoro, V.J., (2008): Tectonique et morphopédologie de la région d’Antananarivo (Centre de Madagascar). Rhéologie des latérites et mécanismes de formation des lavaka. Thèse de doctorat nouveau régime, Université d’Antananarivo, 172p

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