16 Geotechnical Conditions and Stability Analysis of Landslide Prone Area

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Stability Analysis of Landslide Prone Area in Ethiopia
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  International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017 239 ISSN 2229-5518 IJSER © 2017 http://www.ijser.org  Geotechnical Conditions and Stability Analysis of Landslide Prone Area: A Case Study in Bonga Town, South-Western Ethiopia Damtew Tsige, Prof. Emer T. Quezon, Dr. Kifle Woldearegay  Abstract— There were several slides and associated ground subsidence which brought significant impact on cracking of walls and floor of several private and governmental buildings in Bonga Town. The principal and secondary roads were also affected by subsidence with vertical displacement up to 1m which hampered the traffic in the town. Water pipelines along the road were disturbed by the sliding which were later repaired. Cracking of the walls and floor of more than 120 private residences and more than 10 government buildings were recorded. The main highway that connects Bonga-Tepi- Masha via Alamo and Gatiba has been disrupted at four locations. This resulted in hampering in traffic for several days. This research aimed to evaluate the cause and failure mechanism as well as the stability condition of the landslides. The study involved the investigation of the Geotechnical parameters of soil and the terrain characteristics to be used for the stability analysis of the slope, including distribution and characteristics of soils, the groundwater table, and the depth and geometry of the failures. The Slope stability analysis is supplemented by using Geo-studio 2004 software. Soil samples were collected, and were tested for grain size analysis, distribution analysis (sieve & hydrometer) plastic limit, liquid limit, plasticity index, water content, unit weight of soil, specific gravity and shear strength parameters following the ASTM procedures. Based on the findings, the landslides were triggered by heavy rainfall. Therefore, the main factors controlling the stability of slope are soil type and characteristics, slope angle, water (surface and groundwater), and slope steepness. The design of retaining wall is recommended to mitigate the impact of landslides in the study area. Index Terms  —Geotechnical parameters; Groundwater table, Landslides; Mitigation measures, Retaining wall, Slope stability analysis. —————————    —————————— 1 I NTRODUCTION he termed landslide includes all varieties of mass movements of hill slopes and can be defined as the downward and outward movement of slope forming materials composed of rocks, soils, artificial fills or combination of all these materials along surfaces of separation of falling, sliding and flowing, either slowly or quickly from one place to another [1]. Although the landslides are primarily associated with mountainous terrains, these can also occur in areas where an activity such as surface excavations for highways, buildings and open pit mines takes place. Natural hazards can be divided into three main categories: atmospheric, endogenic and exogenic hazards. Atmospheric hazards are caused by processes of atmospheric nature, such as, tropical storms, hail storms, hurricanes and droughts. Endogenic hazards are results from internal earth processes, such as volcanoes and earthquakes. Exogenic hazards are caused by the operation of natural earth surface processes, including flooding, coastal erosion, mass movement and soil erosion. It is important to realize that natural hazards cannot always be categorized into one of these segments listed above. In many cases, the natural hazard could actually be a combination of two different types of the categorized hazards above. For example, a landslide is often triggered off by an atmospheric hazard, such as a tropical storm and an endogenic hazard such as an earthquake. However, this is a good method of separating the hazards into basic categories, making a differentiation between the geological hazards (endogenic and exogenic) and the atmospheric hazards [1]. Natural slope instability is a major concern in hilly terrain where failures might cause catastrophic destruction of the surrounding area. The failures might be triggered by internal or external factors that cause imbalance of natural forces. An internal triggering factor is the factor that causes failure due to internal changes, such as increasing pore water pressure and or imbalanced forces developed due to external load [2]. Landslide occurrence is on the increase worldwide the consequences of which can be loss of life, loss of livestock, damaging or destroying residential and industrial developments, villages or even entire towns, destroying agricultural and forest land and negatively influencing the quality of water in rivers and streams [3]. Geologic factors have also been found to cause mass movements on the slopes and these include shallow soils over hard, impermeable rocks or glacial till, soft, clay-rich rocks that produce thick plastic soil mantles, alignment of lineaments parallel to the ground slope and planar rock structures, unconsolidated or weakly consolidated deposits. (Sidle, R.et al, 1985). A close relationship exists between landslide activity and the amount of precipitation. The amount of rainfall has considerable influence on the moisture content and the pore water pressure in soils. Slope steepness is also a significant factor and the greater the height, steepness and concavity of slopes, the greater the volumes of the landslides [3]. The landslide in the study area was known to have the greatest impact on the Town due to damage to buildings and T  International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017 240 ISSN 2229-5518 IJSER © 2017 http://www.ijser.org  roads linking four different directions from the town. This prompted the researcher to investigate the causes of the landslide. 1.1. Objectives of the Study 1.1.1   General Objective The general objective of the research was to characterize the Geotechnical conditions of the site and analyze the stability of the landslide in Bonga town. 1.1.2   Specific Objectives    To evaluate the Geotechnical properties of soil/rocks in the landslide affected area.    To investigate the main causes, triggering factors for the occurrence of landslides in the study area.    To analyze slope stability of the affected areas and determine the factor of safety of the slopes using GeoStudio Software.    To recommend possible remedial measures in order to minimize risks from landslide in the area .   1.2   Research Questions The research questions that the researcher had sought to be answered as follows: 1.   What are the Geotechnical soil/rocks parameters that controls the initiations of the landslides and its failure mechanisms? 2.   What are the main causes, triggering factors for the occurrence of landslides in the study area? 3.   How the slope of the affected area's response to the effect of surcharge and its corresponding factor of safety using GeoSlope Software? 4.   What type of measures could be implemented in order to mitigate the landslide problems in Bonga town? 2   RESEARCH METHODOLOGY 2.1. Study Area Bonga town is located in the Southwestern part of Ethiopia, Southern Nation Nationalities and Peoples Regional State (SNNPS) . The town is an administrative center of Keffa zone. It is about 440 km from Addis Ababa passing through  Jimma Town. The Eastern and Southeastern part of the town was always affected by the landslide phenomena. The affected area of the landslide is bounded by 7°15'25 -7°16'00 Latitude and 36°14'55 - 36°15'25 Longitude, which can be seen from Figure 1. Figure 2.1: Map of the Study Area (Source: Google Map 2016) 2.2   Regional geological setting On the geological map of Ethiopia, the study area belongs to the Jimma volcanic (upper sequence) that consists of trachite, ignimbrite, rhyolite, and tuff with minor basalt. Whereas in the recent 1:50,000 scale engineering, geological map of the area (EGS, 1999) as shown in Figure 2, compiled in relation to the current landslide problem, basic lava flow and Pyroclastic materials formed due to tertiary volcanism dominated the area. The basic lava flows are fine-grained and porphyritic basalts, while fine and coarse Tuff and agglomerate represent the pyroclastic eruptions. The recent quaternary deposits are recognized as alluvial, colluvial and residual soils. Faults trending NE-SW direction displaces the volcanic rock units with a distinct scarp zone at the eastern and southeastern of the Bonga Town [4]. Figure 2.2:   Engineering geological map of BONGA and its surrounding. (Source: EGS, 1999) 2.2.   Regional Hydrogeology On the 1:2,000,000 Hydrogeological map of Ethiopia the Bonga town and its surroundings are characterized by  International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017 241 ISSN 2229-5518 IJSER © 2017 http://www.ijser.org  extensive aquifer, with fracture permeability of volcanic rocks (basalt, rhyolites, trachytes and ignimbrites) which are found in abundance. The water resource is widespread with moderate to large quantities of surface water and ground water. Most streams are perennial and cold springs are also common. The depth of the ground water varies from 0 to 100 meters, with a static water level around 1 to 4.5 meters. Rainfall is the main recharge of the groundwater, which reaches from 1000mm to 2000 mm annually, being one of the highest in the Ethiopia. While the recharge from runoff is relatively low and the discharge rate of groundwater is high. 2.3.   Climate Since the study area belongs to the Southwestern Ethiopian highlands, it is characterized by warm, humid and wet subtropical climatic conditions. According to the Bonga meteorological station, the mean annual rainfall of the area is 1628.8mm/year, i.e. for the years 1996-2013. Temperature during summer reaches up to 20°C. Even though, there is a rainfall distribution throughout the year, two major seasons of rainfall are common in the area. The first one is from July to September and the other is from April to May which shows biomedical characteristics of the rainfall. Based on the summary of hydrometric discharge data of Shite and Dincha at Bonga, there is a seasonal variation of stream flow, which reflects seasonal of the rainfall. The main rainy season in all the area is during winter, with a secondary maximum in the spring. Therefore, the peak stream flows occurred during winter months. It should be noted however that the duration of the rainfall and its distribution throughout the year can influence the conditions of surface runoff. 2.4.   Physiography The study area is characterized by rugged volcanic, mountainous terrain comprising of high to low relief hills. Bonga town is located at moderately to gently sloping and undulating topography. Elevation ranges from 1590 to 1880m above average sea level. 2.5.   Land use and settlement Bonga town is situated on the gentle and moderately steep to undulating slope of Sobra mountain. Recent settlements are still undertaking along the hill slopes. The main plantation around Bonga town is the coffee and maize, which cover the mid to lower hill slopes. The higher elevated areas are covered by dense tropical forest, mainly Junipers, Oak and Olive trees . 2.6.   Research Design The research was conducted by using both Experimental and Analytical methods as well as qualitative and quantitative study were employed in this study. Qualitative study forwards impression of the findings, whereas the quantitative study was used to describe the numerical aspects of the findings. 2.7.   Parameters for slope stability evaluation The parameters used in this study are grain size, specific gravity, permeability and shear strengthen (i.e. Cohesion, angle of internal friction and unit weight of soil), thickness of sliding mass, size of slide (i.e. Lengthen, depth and width). 2.8.   Field sampling and laboratory test preparation Soil samples collected to determine physical characteristics. Core sampling carried out using standard procedures of ASTM. For each sampling pit, undisturbed and disturbed samples collected Two sets of samples (one disturbed and one undisturbed) extracted at depths of 1.7m to 5.4m. The sampling pits (sites) were taken randomly in areas where landslides had occurred, specifically at the sides of the scar. Samples were collected at their moist condition using plastic bags. The plastic bags were tied to reduce loss of moisture. In-situ moisture contents were determined immediately. The samples were brought to the laboratory for testing using oven temperatures of 105°C for every test. Each sample was dried in oven until continuous weighing gives constant weight. Soil samples from 9 test pits were taken at various depths and analyzed to know the properties like cohesion, angle of friction, unit weight of soil, water content, liquid limit, plastic limit and plasticity index, specific gravity and grain size analysis (sieve and hydrometer). Then these properties were used in classification of soil and Slope stability analysis of landslide. The analysis was conducted through Numerical modeling software package GeoStudio-Slope/W 2004. Soil samples collected from the study area were brought to a Jimma University soil laboratory to substantiate the model provided by GeoSlope. 2.9.   Data collection procedures The research approach included: (a) review of previous studies and literatures on the subject matter; (b) Geotechnical investigation of soils, rocks and water pressure within slopes; (c) measurement of landslide features which includes (length, width, and depth) and failure mechanism; and (d) selection of appropriate slope stability analysis methods . 2.10.   Slope Stability analysis Modern limit equilibrium software such as Slope\W is making it possible to handle ever-increasing complexity in the analysis. Using the limit equilibrium, SLOPE/W could model homogeneous to heterogeneous soil types, complex stratigraphic and slip surface geometry, and variable pore-water pressure conditions using a large selection of soil models. Slope stability analyses can be performed using  International Journal of Scientific & Engineering Research, Volume 8, Issue 4, April-2017 242 ISSN 2229-5518 IJSER © 2017 http://www.ijser.org  deterministic or probabilistic input parameters. Stresses computed by a finite element stress analysis may be used in addition to the limit equilibrium computations, for the most complete slope stability analysis available. Therefore, GeoSlope Slope\W is one of the powerful tools of this integrated approach that opens the door to types of analyses of a much wider and more complex spectrum of problems, including the use of finite element computed pore-water pressures and stresses in a stability analysis. Not only does an integrated approach widen the analysis possibilities, it can help overcome some limitations of the purely limit equilibrium formulations [2]. The conventional Limit Equilibrium method is used to analyze the high embankment slope stability Program Geo-Studio (SLOPE/W) was formulated in terms of a moment and force equilibrium factor of safety equations. The Analysis provides a factor of safety, defined as a ratio of available shear resistance (capacity) to that required for equilibrium [5]. The limit equilibrium procedure for calculating the factor of safety involves comparing the available shear strength along the sliding surface with the force required to maintain the slope in equilibrium. 3 RESULTS AND DISCUSSION 3.1. Water content of the soil  A test was conducted to determine the water (moisture) content of soils. The water content is the ratio, expressed as a percentage of the mass of “pore” or “free” water in a given mass of soil to the mass of the dry soil solids. Water content tests were carried out on three samples. Table 3.1 shows the water content of the soils varying from 38.09%-38.92%. Table 3.1. The water content of the soils from landslide affected areas Slope profile Depth of sample taken (m) Water content in % SIBH1 1.9-2.4 38.77 S2BH2 1.8-2.6 38.09 S3BH3 1.7-2 38.92 3.2   Specific gravity of soil The specific gravity, Gs, is used in the determination of hydrometer analysis. In residual soils the specific gravity may  be unusually high or unusually low. The laboratory performed to determine the specific gravity of the soil by using a Pycnometer. Specific gravity is the ratio of the mass of a unit volume of soil at a stated temperature of the mass of the same volume of gas-free distilled water at a stated temperature. Specific gravity tests were also run on all samples and results of the test is summarized in Table 3.2. The soils were found to have specific gravity, which ranges from 2.68 to 2.7. Table 3.2. The specific gravity of the soils from landslide affected areas Slope profile Depth of sample taken (m) The average specific gravity SIBH1 1.9-2.4 2.7 S2BH2 1.8-2.6 2.68 S3BH3 1.7-2 2.69 3.3   Unit weight of soils  In-place density was determined for undisturbed soil obtained by pushing or drilling a thin-walled cylinder. The  bulk density is the ratio of mass of moist soil to the volume of the soil sample, and the dry density is the ratio of the mass of the dry soil of the volume the soil sample. The dry unit weight of natural, undisturbed samples ranges between 11.77 and 12 kN/m 3 and moist samples ranges between 16.68 and 16.97kN/m 3  , respectively. All of the tests were performed on samples according to ASTM standards. Table 3.3:   U nit weight of the soils from landslide affected areas Slope profile Depth of sample taken (m) The average specific gravity SIBH1 1.9-2.4 2.7 S2BH2 1.8-2.6 2.68 S3BH3 1.7-2 2.69 3.4   Grain size analysis result Soil sampling is the most important part in analyzing a landslide behavior. By knowing the properties of the soil of the landslide, a great deal of information can be obtained to determine the triggering mechanisms of the landslide. Soil tests ideal for the landslide analysis were particle size distribution and hydrometer analysis. There were three slope profiles that had been selected along the landslide area and a total of nine samples were collected with three samples from each slope profile. There were designated as Samples S1BH1, S1BH2, and S1BH3, of which all of these extracted from slope profile one at the depth of 2.4m, 2.1m and 1.9m, respectively. On the other hand, the Samples S2BH1, S2BH2, and S2BH3, were all taken from the slope profile to at respective depth of 2.6m, 2.5m and 1.8m. For Samples S3BH1, S3BH2, and S3BH3, were taken from the slope profile three at the depth of 1.9m, 2.0m and 1.7m, respectively . Table 3.4: Grain size analysis of the soils from landslide affected areas
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