Untitled e ISSN 2582 5208 International Research Journal of Modernization in Engineering Technology and Science Volume 03/Issue 05/May 2021 Impact Factor 5 354 www irjmets com www irjmets com @Interna[.]
e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com STUDY COEFFICIENT STABILITY OF SLOPE WHEN CONSIDERING SATURATION OF LAND IN DONG NAI AREA Khoa- Pham Ngoc Dang*1, Hai- Do Thanh*2, Truyen- Phu Nhat*3 *1Dong *2,3Ho Nai Technology University Chi Minh University of Technology ABSTRACT In practice, there are many slopes that remain stable under normal conditions, but when using computational software, the coefficient of stability is often smaller, even unstable compared to the standard At the same condition of slope, when it rains for many days or the groundwater level rises, it will cause landslides This phenomenon is due to the water saturation of the soil, unsaturated soil often has the properties of stress deformation, pore water pressure variation, shear strength, permeability coefficient, according to the theory of saturated soil mechanics The shear strength of the unsaturated soil is very important to the stable state of the roof and the research to determine the shear strength of unsaturated soil is important and necessary Currently, to calculate the stability of the slope, we often apply the parameters of saturated soil to unsaturated soil to calculate and the results not reflect the reality In this paper, we use the finite element method to calculate the slope stability with parameters that best reflect its working conditions in the natural state, unsaturated soil The results shown that when applying Geostudio 2007 software suite for us to calculate slope stability coefficient for the two above soils is that when considering the parameters of unsaturated soil, especially negative cohesion The stability coefficient is always greater than the stability coefficient when calculated with saturated soil parameters At Dong Nai area, the coefficient of stability calculated using the soilwater curve (SWCC) is the largest and greater than the method with the Fredlund-Xing parameter 3.8%, which is greater than the hypothesis b = 14.40 is 11% and greater than 102% with the method assuming the soil is saturated Keywords : Slopes, saturated soil, unsaturated soil, GeoSlope, SWCC I INTRODUCTION Vietnam is located in a tropical monsoon region with abundant rainfall and humidity, especially our country's natural topography is hilly, accounting for a quarter of the country, mainly low hills Under such natural conditions, slope stability in nature becomes an important issue in the construction of infrastructure On the other hand, most of the slopes are located in areas with high terrain, low groundwater level, dry weather and rainfall that changes from season to season This type of terrain is typical for unsaturated soils, and the saturation of the soil changes regularly every month of the year In the past, soil structures were often designed on the basis of the theory of classical saturated soil mechanics Most researches in the field of Soil Mechanics focus mainly on saturated soil environment Unsaturated soils often have the properties of stress - deformation, pore-water pressure variation, shear strength, permeability coefficient, that not follow theories of saturated soil mechanics The complete use of available data on saturated soil parameters is difficult to meet the accuracy, as well as to reflect the actual working conditions of the slope in different lands in Vietnam Stemming from the above practical requirements and inheriting the research results of domestic and international authors, the author has focused on analyzing “Study of slope stability coefficient when considering storm Land in the Dong Nai region” II RESEARCH OVERVIEW 2.1 Slope roof The slope is a block of rock formed naturally or artificially, but with a tendency to decrease the slope for more stability The main destabilizing forces are gravity and permeability simultaneously caused by the slope shape and shear strength, also known as internal and external forces [1] The movement of the rock mass can be caused by destructive shear damage along an internal surface of a rock that partially or completely liquefies, with three different types of movement (failure, slide, slip) [1] www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1296] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com The object studied in this paper is a natural slope due to erosion or fragmentation in a hillside or valley Rain, people are the factors that promote the slope to destroy the existing sliding surface or create a new surface The commonly used method to assess stability is based on plastic limit equilibrium When the slope is unstable, there exist many different types that can occur locally or fully with the spherical or pivot surface Currently, most methods assume that the slip surface has a rotating cylindrical surface The shear strength is inversely proportional to the pore water pressure Currently, when analyzing the slope stability on saturated soil, people use effective anti-shear strength parameters (c 'and ') But in cases where the groundwater level is very deep compared to the natural ground, the strength of the shear will appear due to the negative pore water pressure above the groundwater Due to the difficult calculation, we often ignore, especially in the case of deep groundwater or when people are concerned about the possibility of a shallow destructive surface, negative pore water pressure cannot be ignored [1] 2.2 Analysis method of slope stability 2.2.1 Slope Balancing Model of Unsaturated Soil Currently, the degree of stability of the slope of the ground is usually evaluated based on the factor of safety Slope stability analysis has become a popular analytical tool for evaluating the stability coefficients of natural and man-made slopes To assess the stability of the slope, theoretically, there exist many methods of calculation such as limit equilibrium method, limit analysis method and numerical analysis method [1,2] In practice, the column division limit equilibrium method is widely used in practice to analyze and evaluate slope stability These methods are all based on the principle of electrostatics (ie static balance of force and / or torque), without considering displacement in soil mass The calculation method is based on the assumption that the sliding surface of the sliding block is cylindrical due to its consistency with reality and simplicity in calculation [2] The degree of stability of the slope is assessed quantitatively by a coefficient of stability Fs which is the ratio of the total moments of the holding forces Mg (anti-slip forces such as cohesion and internal friction of the soil) and the total moment of the forces Mt (external load, self weight of sliding block, water pressure, seepage pressure, ) [3] 𝐹𝑠 = 𝑀𝑔 𝑀𝑡 Here: ≥ [𝐹𝑠 ] (1) [Fs] - the coefficient of permissible anti-slip stability, depending on the grade of the building, is taken according to the code When slope stability takes into account unsaturated soil (negative pore water pressure), the "total cohesion" method can be used - applying the matric suction to the soil cohesion (Ching et al., 1984), find the equation for the coefficient of stability responding to both positive pore water pressure and negative pore water pressure (Fredlund, 1989, 1995; Rahardjo and Fredlund, 1993) or by nonlinear shear strength - adhesive www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1297] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com attraction (Rahardjo, Fredlund and Vanapali, 1992) In the “total cohesion” method, the cohesion of soil c is considered to increase as the cohesion of the soil increases The increase in that cohesion force from the adhesive attraction [ie (ua - uw) tg b] for different angles b [4] We already know the equation for shear strength of saturated soils We can rewrite the following: 𝜏𝑓 𝑓 = 𝑐 + (𝜎𝑓 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 ′ (2) Inside: Total cohesion 𝑐 = 𝑐′ + (𝑢𝑎 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 𝑏 (3) Thus, unsaturated soils are considered to have total cohesion, including effective cohesion and matric suction [4,5] 𝜏𝑓 𝑓 = 𝑐′ + (𝑢𝑎 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 𝑏 + (𝜎𝑓 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 ′ (4) 𝜏𝑓 𝑓 = 𝑐′ + (𝜎𝑓 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 ′ + (𝑢𝑎 − 𝑢𝑤 )𝑓 𝑡𝑎𝑛 𝜙 𝑏 Hay (5) This is also the equation for shear strength that Fredlund et al (1978) recommended use Torque Equilibrium Stability Formula For saturated soil [2] : 𝐹𝑚 𝑠 = ∑ ∑[𝑐′𝛽𝑅+(𝑁−𝑢𝑤 𝛽)𝑅 𝑡𝑎𝑛 𝜙′] (6) Wx−∑ 𝑁𝑓−∑ 𝑘𝑊.𝑒±[𝐷.𝑑]±𝐴.𝑎 For unsaturated soil [4,5] : 𝐹 𝑚 𝑠 = ∑∑[𝑐′𝛽𝑅+{𝑁−𝑢𝑤 𝛽 𝑡𝑎𝑛 𝜙𝑏 𝑡𝑎𝑛 𝜙𝑏 −𝑢𝑎 𝛽(1− )}𝑅 𝑡𝑎𝑛 𝜙′]] 𝑡𝑎𝑛 𝜙′ 𝑡𝑎𝑛 𝜙′ (7) ∑ Wx−∑ 𝑁𝑓+𝐴.𝑎 The method used to analyze the dam slope stability in the paper is the ingot limit balancing method using effective stress and pore water pressure to determine the slope stability coefficient 2.3 Software for calculating slope stability (Geostudio 2007) The commercial geotechnical software for analysis of ground stability transferred into our country today or independently is often built from the theoretical basis by the method of limit equilibrium distribution Geostudio 2007 software suite of GeoSlope International Company (Canada) is considered by many countries around the world to be the most powerful and most commonly used program suite today In the calculation process, the author used the SLOPE / W software in the GeoStudio 2007 GeoStudio software program of GeoSlope International Company (Canada) to solve the basic problem: slope stability SLOPE / W software is based on the Coulomb stable condition at the bottom of the soil rod to establish the expression of the stability coefficient according to the moment and (or) according to the applied force This software only assesses the stability of the rock roof strength There are many manure limit equilibrium methods used in SLOPE / W, the author chose to use the simple Bishop method to analyze natural slope stability III CALCULATION RESULTS To find out the change in slope stability coefficient of unsaturated soil, we start from the parameters of saturated soil and then add parameters of unsaturated soil The calculation of the slope is stabilized with GeoSlope software, we will use the analysis methods integrated in this software by changing the input parameters These parameters will first spread from the parameters of the saturated soil, then add new components of the unsaturated soil These new components determine whether we are simulating soils that are saturated or unsaturated, giving us the slope stability coefficients under different circumstances Using GeoSlope software, we consider the slope roof in (four) following methods: Method 1: Analysis with Saturated Soils ( b = 0, Ua-Uw = 0) Method 2: Analyzing with No Storm Soil Using b = 14.40 Method 3: Analysis with Unsaturated Soils Using a Soil - Water Curve (SWCC) www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1298] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com Method 4: Analysis with Unsaturated Soil Using Parameters (a, m, n) By Fredlund - Xing In these methods we will keep the geometric structure, the slip center mesh and the slip surface grid cell of the slope only change soil parameters 3.1 Characteristics and properties of the slope The slope roof used to calculate is the slope of a residential area in Ho Nai commune, Trang Bom district, Dong Nai province The calculated cross-section of the natural slope (Figure 1) consists of layers: Figure 2: Isometric line model for unsaturated soil and saturated soil in Geo - Slope Layer is the soil layer chosen to investigate the effect of saturation on the slope stability The above parameters will be changed and taken according to the methods of calculating stability coefficients using Geo Slope software The shear center and the slip surface grid of the slope are shown in Figure and the isotropic line in the slope is divided according to Figure The parameters are kept when calculating the stability coefficient using Geo Slope software Taking parameters from soil test results at different saturation levels and put into GEO-SLOPE module SLOPE / W software to calculate the slope coefficient with sliding surface is circular 3.2 Analysis with Saturated Soils ( b = 0, Ua-Uw = 0) 3.2.1 Parameters put into the model In this case, we use the parameter of soil layer with 80% saturation and consider it as the parameter of saturated soil And the parameters of class and are taken according to survey drilling documents The geometrical parameters of the slope are taken according to Figure 2, but only change the material properties of the layer In this case, we take b = 0, which means that there is no clay to the saturation parameter of the soil or tissue the unsaturated soil form now becomes a saturated soil model 3.2.2 Calculation results Figure shows that the minimum coefficient of stability in this case is 0.90 and equal to the stability factor in the case of saturated soil In both cases, the coefficient of stability is equal (0.9) and the slip surface is exactly the same Thus, in Geo - Slope, although negative adsorption is considered, when b = 0, that soil is still considered as saturated soil Figure 3: Results of slope stability in case of saturated soil www.irjmets.com b =0 @International Research Journal of Modernization in Engineering, Technology and Science [1299] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 3.3 Analysis For Unsaturated Soil Use Impact Factor- 5.354 b = www.irjmets.com 14.40 3.3.1 Parameters put into the model According to research results of many authors, value of b is nonlinear and is usually smaller than ' [2, 3, 5] We choose the value b when the suction pressure is greater than the intake air pressure, then the curve of b value is almost linear Similar to the analysis method with saturated soil ( b = 0), but the only difference is that the layer material has b = 14.40, this parameter is the average value from the direct shear test results with the same saturation and different pressure levels Accordingly, above MNN is unsaturated and below is saturated, the isotopes in this case are also divided as in Figure 3.3.2 Calculation results Figure shows that the smallest coefficient of stability in this case is 1,689 which is larger than the calculated result when b = (0.90) 82.1% Figure 4: Results of slope stability in case of saturated soil b = 14.40 3.4 Unsaturated Soil Analysis Using a Soil - Water Curve (SWCC) 3.4.1 Parameters put into the model Like the analysis method above, the soil above MNN is unsaturated and the lower is saturated, the isotropic lines in this case are also divided as Figure But in this case, for the non In saturation, we use parameters of soil layer with negative adhesive force based on the results of soil-water curve experiments to include in the model [6] As shown in Figure 5, When we enter the experimental parameters of the SWCC curves, Geo-Slope also draws SWCC curves and Geo - Slope uses this graph to calculate the unsaturated soil in the parts with changing negative cohesion Figure 5: SWCC curve calculated by Geo - Slope www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1300] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com 3.4.2 Calculation results Figure shows that the minimum coefficient of stability in this case is 1,820 and is (102%) times higher than the case regardless of the matric suction in unsaturated soil, which is 7.76% increase compared to the case b = 14.40 Figure 6: Results of slope stability using SWCC curve 3.5 Analysis of Unsaturated Soil Using Parameters (a, m, n) According to Fredlund - Xing 3.5.1 Parameters put into the model Like the method of analysis above, in this case, for unsaturated soils, we use the parameter of soil layer with negative matric suction based on the calculation results of the SWCC curve according to Fredlund - Xing ( w = 0.294, a = 12, m = 0.28, n = 2.5) [7] In Figure we see, when we enter parameters a, m, n calculated according to Fredlund - Xing from SWCC curve, Geo-Slope also draws SWCC curves almost the result Figure and Geo - Slope use this graph to calculate for unsaturated soil at areas with variable negative matric suction 3.5.2 Calculation results Figure shows that the minimum coefficient of stability in this case is 1.754 and an increase of 94.9% compared to the case of no adhesion in unsaturated soil, an increase of 7% compared to the case b = 14.40 and a decrease of 3.4% compared to the case using SWCC scores Figure 7: Curves According to Fredlund - Xing calculated by Geo - Slope www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1301] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com Figure 8: Results of slope stability with parameters Theo Fredlund - Xing IV CONCLUSION AND DISCUSSION Results of calculating slope stability according to methods are shown in Table and Figure Table 1: Results of slope stability according to methods Number Method Fs Assumption method does not use Assumption method Method using SWCC curve (Unsaturated soil) 1.820 Fredlund - Xing (Unsaturated Soil) Parameter Method 1.754 b b (saturated soil) = 14.40 (Unsaturated soil) 0.900 1.689 Figure 9: Comparison of slope stability coefficients according to methods From the above results, it shows that, under the same conditions of geometrical parameters, parameters of soil layers (saturation experiment), the same center grid, the same sliding arc, the same shear direction, we only add the parameters of zero soil When the slope is saturated, the slope has the smallest stability coefficient regardless of b, which means that the entire slope is considered saturated soil and the maximum stability coefficient when calculated by the soil-water curve (SWCC) when considering The slope is unsaturated soil When considering the unsaturated properties, the stability coefficient of the SWCC method is 3.8% larger and greater than the method with Fredlund-Xing parameters of 3.8%, than the hypothesis method b = 14.40 is 11% and more than 102% with the method assuming the soil is saturated Thus, in practice, there are many unsaturated slopes, but when calculating, we often take the parameters of saturated soil (Mechanical experiment) to calculate the stability coefficient is usually small At that time, we realize that the slope nail will not be stable, but in reality it still exists and is stable www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1302] e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science Volume:03/Issue:05/May-2021 Impact Factor- 5.354 www.irjmets.com From this result, we recommend to study the application of unsaturated soil to the stability tests for sloping roofs constructed in a short time, or constructed during the dry season in Vietnam (Like digging basements, digging culverts, ) At that time, the factor of safety due to calculations will be high, it will reflect the actual slope and reduce costs during construction V REFERENCES [1] Whitlow, R., “Soil mechanics” (translation), volume 1+2, Translated by: Uyen Nguyen, Cuong Trinh Van, Vietnam National University Ho Chi Minh City, (1996) [2] An Chau Ngoc “Soil mechanics”, Vietnam National University Ho Chi Minh City, 2012 [3] Thu Trinh Minh and Uyen Nguyen (2010) “Prevention of landslides in slopes and slopes” Vietnam National University, Hanoi [4] Fredlund, D.G., H Rahardjo, “Soil mechanics for unsaturated soils” (translation), volume 1+2, Translated by: Man Nguyen Cong, Uyen Nguyen, Vietnam National University Ho Chi Minh City, (1998) [5] Fredlund D.G., and Morgenstern, N.R, (1977), “Stress state variables for unsaturated soils”, Journal of the Geotechnical Engineering Division, Proceedings, American Society of Civil Engineering (GT5), 103:447 – 466 [6] ASTM D6836-02 (2002), "Standard Test Method for Determination of the Soil Water Characteristic Curve for Desorption Using Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, and/or Centrifuge”, Annual Book of ASTM Standards, Volume 04.09 [7] Fredlund, D.G and Xing, A., (1994) “Equations for the Soil-water Characteristic Curve”, Canadian Geotechnical Journal, Vol 31, No 4, pp 521-532 www.irjmets.com @International Research Journal of Modernization in Engineering, Technology and Science [1303] ... method of slope stability 2.2.1 Slope Balancing Model of Unsaturated Soil Currently, the degree of stability of the slope of the ground is usually evaluated based on the factor of safety Slope stability. .. according to the methods of calculating stability coefficients using Geo Slope software The shear center and the slip surface grid of the slope are shown in Figure and the isotropic line in the slope. .. properties of the slope The slope roof used to calculate is the slope of a residential area in Ho Nai commune, Trang Bom district, Dong Nai province The calculated cross-section of the natural slope