River Bank Erosion of the Surma River Due to Slope Failure

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1 River Bank Erosion of the Surma River Due to Slope Failure Md. Shofiqul Islam, Farjana Hoque Department of Petroleum and Mining Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh Abstract The Sylhet city is located on the bank of the Surma River in Bangladesh is the most beautiful city with natural bounty. Bank erosion is a common problem for city dwellers and land owners due to unsupported bank. Dry and wet in-situ soil sample were analyzed in the laboratory to find out the major cause of bank erosion. Soil samples are silty clay with friction angle 40 and 24 at dry and wet condition, respectively. We the measured Poisson s ratio, cohesion, and Young s Modulus are 0.4, 2.75 KPa, and 20KPa, respectively, for dry samples. For wet condition, however, these values are 0.4, 2.75 KPa, and 20KPa, respectively. Finally, slope stability analyses using both liquid equilibrium and finite element method show that the river banks are eroding mainly due to slope failure in the wet condition. The factor of safety (FS) and shear Reduction Factor (SRF) values is more than 1 at dry condition while it is ~0.7 at wet condition. Keyword Surma River, Sylhet, Soil Sample, Slope Failure, Soil Erosion. I. INTRODUCTION The landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes, shallow debris flow. The gravity is the primary driving force for a landslide to occur, whereas other contributing factors that affecting the original slope stability. However, the major causes of landslides are groundwater pressure acting to destabilize the slope, loss of vegetation structure, soil nutrients, and soil structure, erosion of the toe of slope by rivers or ocean waves, weakening of a slope through saturation by snow melt, glaciers melting or heavy rains, earthquake adding loads to barely stable slope and volcanic eruption. Moreover, the human activities such as deforestation, cultivation, construction, vibrations from machinery, blasting, earthworks, agricultural activity, etc. cause landslides. Slope Stability analysis is the way to protect any structure from failure. If there are geological discontinuities, then we can protect it through slope analysis. Through analysis of the slope we may indicate the risky zone. The proper analysis data may show the preventive measures of such kind of soil. Study of slopes may help us which kind of support should be given to such quality soil. However, factors affecting slope stability are a strength of soil and rock, the type of soil and stratification, discontinuities and planes weakness, ground water tables and seepage through the slopes, external loading, and geometry of the slope. The geotechnical failure typically occurs due to stresses on the bank beyond the forces the bank can hold. However, the pore water pressure in the saturated bank reduces the frictional shear strength of the soil and increases sliding forces [1]. This type of failure is most frequent in fine grained soils because they cannot drain as rapidly as coarse grained soils [2]. This can be accentuated if the banks had already been destabilized due to erosion of cohesionless sands, which undermines the bank material and leads to bank fall down [1]. On the Surma River, significant river erosion has taken place due to slope failure. These causes widen the river channel, filling the river channel, losing the land, property, etc. As we know that Sylhet City is situated on the bank of the Surma River, it may hamper the safe living for the people who live near the bank. During the field visit, we have seen some vulnerable location in the river channel. High slope angle and water seepage also caused the slope failure in some places. However, some area has already been protected by concrete cubic block by the Water Development Board of Bangladesh, whereas, most of the area is unprotected from slope failure. In some cases landslides will take place due to heavy rainfall, earthquake or other anthropogenic activities. Moreover, there is no systematic analysis yet to be done on slope failure and its remedial measure of river bank of the Surma River. So there is an appeal to analysis of the slope of the area. In this project I aimed to 1) Analysis of the soil sample to determine Engineering Properties (unit weight of soil, Poison s Ratio, Young s modulus, Cohesion), 2) to analysis, slope at dry condition and wet condition and high groundwater level condition. II. Study Area The Study area is the Surma River bank, in and around the Sylhet city. We have collected samples from different location in the river bank. (Fig 1) 54

The different properties of soils such as grain size, unit weight of soil, internal angle of friction, cohesive strength, Poisson s ratio, and Young s modulus were measured ASTM standard, direct

2 Fig.1. The study area. Yellow asterisk showing the sample locations on the Surma River bank III. METHODOLOGY Total twelve (12) samples have been collected from three different locations from the Surma River bank (Fig.1). The different properties of soils such as grain size, unit weight of soil, internal angle of friction, cohesive strength, Poisson s ratio, and Young s modulus were measured ASTM standard, direct shear measurement, and unconfined compressive strength test describe below. 3.1 Measurement of unit weight of rock Weight (W) of the each sample was measured by electronic weight machine using ASTM standard. Unit weight (γ) of the samples was measured using the equation 1 γ = W (1) V Where, V is the volume of the cylindrical sampler. The unit weight values were used to determine factor of safety using SLIDE Direct Shear Measurement The test is carried out on remolded samples as well as in-situ samples in the laboratory of the department of Petroleum and Mining Engineering of Shahjalal University of Science and Technology using motorized direct shear apparatus (EDJ-2 Motorized Shear Appartus). The soil samples were compacted at optimum moisture content in a compaction mold and the assembled in the shear box. Then specimen for the direct shear test was obtained using the correct cutter provided. A normal load was applied to the specimen sample and the specimen was shared across the pre-determined horizontal plane between the two halves of the shear box. This procedure was repeated for four times, including one in-situ sample and three remolded samples. Measurements of shear load, shear displacement and normal displacement were recorded. From the results, internal angle of friction and cohesive strength were measured using Coulomb s shear strength equation (2). τ f = c + σ f tan φ (2) 55 Where τ f = shearing resistance of soil at failure, the c = apparent cohesion of soil, σ f = total normal stress on failure plane, φ = angle of shearing resistance of soil (angle of internal friction). The values of these parameters (cohesion and angle of internal friction) and unit weight were used to determine factor of safety using SLIDE Unconfined Compressive Strength test According to the ASTM standard, the unconfined compressive strength is the compressive stress at which an unconfined cylindrical specimen of soil will fail in a simple compression test. The test was performed at the Soil Mechanics Laboratory of Civil and Environmental Engineering Department of Shahjalal University of Science and Technology is using unconfined compressive strength tester (ELE International, Model ). Measurements of strains (%) and stresses were recorded. From the results, Poisson s ratio and Young s modulus were determined. These parameters along unit weight, cohesion and angle of internal friction of samples were used to determine shear stress reduction factor using PHASE Limit Equilibrium Method The conventional limit equilibrium methods for investigating the equilibrium of soil mass tending to slide down under the influence of gravity. Transitional or rotational movement is considered on assumed or known potential slip surface below soil or rock mass. In rock slope engineering methods may be highly significant to simple block failure along distinct discontinuities. All methods are based on comparison of forces (moments or stresses) resisting instability of the mass and those that causing instability (disturbing forces). Advantages Limit equilibrium methods are still currently mostly used for slope stability studies. These methods consist in cut- ting the slope into fine slices so that their base can be compared with a straight line, then to write the equilibrium equations (the equilibrium of the forces and/or moments). According to the assumptions made in the efforts between the slices and the equilibrium equations considered, many alternatives were proposed. In the most cases, differences between the values of the

3 safety factor obtained with the various methods are generally lower than 6% [3,4]. However, the disadvantages of limit equilibrium are that the factors of safety are assumed to be constant along the potential slope surface and load deformation characteristics are not explicitly accounted. The Ordinary methods of slices are applicable to nonhomogeneous slopes where slip surface can be approximated by a circle and based on the defining equation. The Bishop s modified method is also applicable to non-homogeneous slopes and cohesive soils where slip surface can be approximated by a circle. It is more accurate than the Ordinary Method of slices, especially for analyses with high pore water pressures. Moreover, the jumbo generalized procedure is applicable to non-circular slip surface. This method is also used for shallow Long planner failure surfaces that are not parallel to ground surface. Figure 2 shows the generalized force equilibrium of the multislice methods. Fig.2. The most common limit equilibrium techniques are methods of slices where soil mass is discretized into vertical slices Bishop s simplified method Bishop s simplified method [5] is a primary slope stability method where the interslice shear forces are neglected and regardless of whether the slip surface is circular or composite that is based on the equation (3) F = [c x+ W +pcosβ u xsecα tan φ ] m α (3) Wsinα M p R Where, x is the width of the slice, and m α is defined by the following equation (4), m α = cosα + sin αtan φ F Where, c and φ is the shear strength parameters for the center of the base of the slice, W is the weight of the slice, α is the inclination of the bottom of the slice, u is the pore water pressure at the center of the base of the slice Janbu s Simplified Method The Janbu s simplified (1956)[6] method is similar to the Bishop s simplified method except that the janbu s simplified method satisfies only overall horizontal force equilibrium but not overall moment equilibrium. The Janbu s simplified factor of safety is actually too low, even though the slices are in force equilibrium. Since force equilibrium is sensitive to the assumed interslice shear, as in the Junbu s simplified method, makes the resulting factor of safety too low for circular slip surfaces. F = c l+ P ul tan φ cos α P sin α ±A (4) Where, c is effective cohesion intercept, φ is effective angle of internal friction, l is the length of the failure surface at the base of the each slice, P is the total normal force on the base of the slice, A is the resultant external water forces, α is the angle between the tangent to the center of the base of each slice and the horizontal. 3.5 Finite Element Method- Shear reduction factor Finite Element (FE) approach to slope stability analysis over traditional limit equilibrium methods where no assumption needs to be made in advance about the shape or location of the failure surface. Failure occurs naturally through the zones within the soil mass in which the soil shear strength is unable to sustain the applied shear stresses. The FE method preserves global equilibrium until failure is reached. If realistic soil compressibility data are available, the FE solutions will give information about deformations at working stress levels. The FE method is able to monitor progressive failure up to and including overall shear failure Shear strength reduction method The shear strength reduction technique for slope stability analysis includes the systematic use of finite element analysis to determine a stress reduction factor or the factor of safety, which brings a slope to verge (end) of the failure. Moreover the shear strengths of all the materials in a FE model of a slope are reduced by the SRF. The conventional method, finite element analysis of this analysis of this model is performed until a critical SRF value which induces instability is attained. Within a specified tolerance a slope is considered unstable in SSR technique when it FE model does not converge to a solution. Shear strength reduction technique is widely used as it is readily available in many existing finite element softwares (e.g., Phase, Carsole, ABAQUS). It can be expressed in terms of principal stresses or normal stresses. Its linearity allows reduced parameters to be calculated readily while an original shear strength model is reduced by a factor F. Reduction of the shear strength envelope by a factor F, determination of new strength model parameters that conform to the lowered envelope, and use of the new parameters in conventional FE Elasto-plastic analysis. 3.6 Model Setup On the basis of soil properties determined from the laboratory analysis, we set up mathematical modeling shown in Fig. 3. The cross-section is assumed homogeneous and isotropic Elasto-plastic material. (5) Fig.3. Model set up for mathematical modeling 56

5 10 15 20 25 30 35 International Journal of Research and Innovations in Earth Science IV. RESULT AND DISCUSSION Collected in-situ soil samples are silty clay with an average unit weight of 11.

4 International Journal of Research and Innovations in Earth Science IV. RESULT AND DISCUSSION Collected in-situ soil samples are silty clay with an average unit weight of KN/m3 (Table 1). At dry condition, the measured friction angle, Poission s ratio, cohesion, and Young s Modulus are 40, 0.4, 2.75 KPa, and 20KPa, respectively (Table 1). However, at dry condition these values are 40, 0.4, 2.75 KPa, and 20KPa, respectively (Table 1). The significant decrease of friction angle, cohesive strength (cohesion) and Young s Modulus is due water pressure in the wet sample [7]. Wet soil sample Dry soil sample Table 1: The engineering properties of soil measured from laboratory analysis Sample No. Unit weight (KN/m 3 ) Slope angle ( ) Height (ft) Friction angle (φ, ) Cohesion (KPa) Poisson's ratio Young's Modulus (KPa) Avg Avg Safety Factor bishop simplified Surface Type: Circular Search Method: Grid Search Radius Increment: 10 Composite Surfaces: Disabled Reverse Curvature: Create Tension Crack Minimum Elevation: Not Defined Minimum Depth: Not Defined Every available surface 1.33 Factor of Safety: 1.33 Center: 4.481, Radius: Left Slip Surface Endpoint: , Right Slip Surface Endpoint: , Fig.4. Factor of safety using the Bishop s simplified method at dry condition Safety Factor bishop simplified Surface Type: Circular Search Method: Grid Search Radius Increment: 10 Composite Surfaces: Disabled Reverse Curvature: Create Tension Crack Minimum Elevation: Not Defined Minimum Depth: Not Defined Every available surface Factor of Safety: Center: 5.251, Radius: Left Slip Surface Endpoint: , Right Slip Surface Endpoint: , Fig.6. Factor of safety using the Bishop s simplified method at wet condition Fig.5. Shear reduction factor at dry condition Fig.7. Shear reduction factor at wet condition 57

Along with soil properties, the overall slope angle approximately 50 has used the SLIDE software, to measure the factor of safety for both wet and dry conditions yielding the values of 1.331 and 0.

5 Along with soil properties, the overall slope angle approximately 50 has used the SLIDE software, to measure the factor of safety for both wet and dry conditions yielding the values of and respectively in Bishop simplified method (Table 2, Fig. 4, 6). The other method (Junbu simplified method) also gives an analogous factor of safety and 0.69 for dry and wet condition, respectively (Table 2). Factor of safety is a term describing the structural capacity of a system beyond the expected loads or actual loads. Essentially, how much stronger the system is then it usually needs to be for an intended load. Safety Factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects. Moreover, SRF values were determined by finite element analysis using Phase2. The SRF values (Table 2) for both dry and wet conditions (Fig 5, 7) support liquid equilibrium analyses by Slide 2. The analytical results evident that river bank of the Surma River are stable in the dry season but unstable in the rainy season. The critical strength reduction factor is equivalent to the safety factor of a slope. The strength reduction factor is referred to as SRF and it is a finite element analysis is aid of commercial software. The measuring process is repeated for different values of Strength Reduction factor (SRF), until the model becomes unstable. This mainly determines the critical strength reduction factor or safety factor of slope. Since the water flow in the Surma River has less effect on bank erosion, it mostly fails due to slope instability. To avoid bank failure, CC block or other support is recommended for steep slope. Table 2 The factor of safety and SRF values from mathematical analysis Factor of Method safety/ Soil condition Liquid equilibrium Bishop simplified Junbu simplified Finite element (Shear reduction factor) SRF 1.33 Dry condition 0.77 Wet Condition 1.25 Dry condition Wet Condition 1.27 Dry condition 0.67 Wet Condition ACKNOLEDGEMENT The authors are grateful to the Department of Civil and Environmental Engineering, SUST for giving permission to use their soil mechanics laboratory to conduct the experiments. Authors are also thankful to Mr. AHM Humayun Kabir (Instrumental engineer, soil mechanics laboratory) for HIS help during experiments. REFERENCES [1] Modes and causes of bank failures, Chapter 3, Guidelines for Bank Stabilization Projects in the Riverine Environments of King County, King County, USA, 1993, pages 30 [2] M. H. Nasermoaddeli, E. Pasche, Modelling of Undercutting and Failure of Non-cohesive Riverbanks, Institute of River and Coastal Engineering, Technical University of Hamburg, Germany.pp8. [3] Duncan, J. M., 1996 State of the Art: Limit Equilibrium and Finite-Element Analysis of Slopes, Journal of Geotechnical Engineering, Vol. 122, No. 7, pp doi: /(asce) (1996)122:7(577) [4] Baba, K., Bahi, L., Ouadif, L., Akhssas, A., Slope Stability Evaluations by Limit Equilibrium and Finite Element Methods Applied to a Railway in the Moroccan Rif, Open Journal of Civil Engineering, 2( 1), Article ID: 17817, 6 pages DOI: /ojce [5] Bishop AW The use of the slip circle in the stability analysis of slopes; Géotechnique.; 5: 7-17,doi: /geot [6] Janbu N, Bjerrum L and Kjaernsli B. Soil Mechanics Applied to Some Engineering Problems (in Norwegian with English summary). Norwegian Geotech. Inst., Publication 1956; p16. [7] Collin BD and Sitar N 2009 Geotechnical properties of cemented sands in steep slopes; J. Geotech. Geoenviron. Engg. 135: pp AUTHOR S PROFILE Md. Shofiqul Islam is a professional academicial and researcher in the department of Petroleum and Mining Engineering, Shahjalal University of Scoence and Technology, Sylhet Bangladesh from December 01, The author earned his BSc (1999), MSc(2000) degree from University of Rajshahi with stood First class second position in the both examinations. Author also awarded MSc (2009) and PhD (2011) degree from University of the Ryukyus, Okinawa Japan. Author is working on Structural geology, Engineering Geology, Gepochemistry, Geophysics, Well Logging and Petroleum geology. V. CONCLUSION Slope stability analyses for dry and wet in-situ soil sample in the laboratory shows the major cause of bank erosion. Results show that the soil samples are silty clay with friction angle 40 and 24 at dry and wet condition, respectively. The slopes analyses using both liquid equilibrium and finite element method show that the river banks are eroding mainly due to slope failure in the wet condition. The factor of safety (FS) and shear Reduction Factor (SRF) values are similar and are more than 1 at dry condition while it is ~0.7 at wet condition. The steep slope of the Surma River is recommended proper training with support like CC blocks. 58

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