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Correlations between factor of safety with distributed load and crest length – Zariwam landslide as case study
Correlations between factor of safety with distributed load and crest length – Zariwam landslide...
Khan, Muhammad Israr
GEOLOGY, ECOLOGY, AND LANDSCAPES INWASCON https://doi.org/10.1080/24749508.2023.2167434 Correlations between factor of safety with distributed load and crest length – Zariwam landslide as case study Muhammad Israr Khan School of Resources and Civil Engineering, Northeastern University China, Shenyang city, Liaoning province, China ABSTRACT ARTICLE HISTORY Received 25 July 2022 This paper examines the impact on Factor of Safety (FS) value due to the variation in distributed Accepted 8 January 2023 load (L ), Crest length (C ), shear strength (τ), and shear stress (σ) both in seismic and non- D L seismic conditions. The main purpose of this paper is to develop correlations between these KEYWORDS parameters, which can be used in any slope stability analysis design project. Forty number of Slope stability; factor of analyses are performed by considering different soil material properties. Slope stability analysis safety; numerical modelling; is performed using Slide software and correlations are developed using Statistical Package for surcharge load; correlations Social Sciences (SPSS) software as well as with the help of Microsoft Excel. The analysis results indicate that the seismic slope stability analysis gives optimum value for slope FS and therefore it is highly recommended to perform and give preference to seismic slope stability analysis of any soil slope to compute and recommend FS value. The main novelty of this paper are the eight new correlations. These correlations can be used in slope stability projects like earthfill dams design, embankments, or any slope design project or case study to know about the slope factor of safety in detail. 1. Introduction There are many reasons which lead to the decrease in shear strength of soil and increase in shear stress, such It is important to understand slope stability analysis as, increased pore pressure due to raining, swelling, issues for two major reasons. First, for the purpose of cracking, development of slickensides, decomposition designing and constructing new soil slopes, it is of clayey rock fills, creep under sustained loads, leach- important to be able to identify changes in soil struc- ing, strain softening, weathering, cyclic loading, water tures and materials within the slope that may occur pressure in cracks at the top of the slope, increase in with the passage of time and the various loading and soil weight due to increased water content, excavation unloading conditions in which the slope will be set for at the bottom of the slope, drop in water level at the its lifetime. Second, in order to repair failed slopes, it is base of a slope, earthquake shaking, and loads at the important to understand the key elements of the situa- top of the slope. All these reasons are well explained in tion that lead to the failure, in order to avoid repetition one of the book (Duncan et al., 2014). Recently, many of failure. Experience is the best teacher and from researchers investigated seismic slope stability analysis experience the slope failure comes with important and provided very useful results (see, for example, lessons that what steps are needed to design, build, Bandara et al., 2018; Havenith et al., 2016; Ishii et al., and repair slopes so that it remains stable and safe. In 2012; HW Huang et al., 2018; Johari et al., 2015; Johari discussing the various causes of slope failure, it is & Khodaparast, 2015; Kalantari & Johari, 2022; Marc useful to begin by considering the basic requirement et al., 2016; Rodríguez-Ochoa et al., 2015; Xiao et al., for slope stability. The shear resistance of the soil must 2016; Serey et al., 2019; Tian et al., 2017; Wu, 2015). be greater than the shear stresses. Given this basic Even if the shear strength of the soil does not change, requirement, it follows that the most important cause the slopes may fail if the distributed loads acting on of instability is that, for some reason, the shear resis- slope changes, leading to increased shear stresses tance in the ground is less than the shear destabilizing within the soil. If the ground at the top of the slope forces required for equilibrium. This situation is is loaded, the shear resistance required for the slope to because of two reasons: withstand against the loads will increase. To avoid increasing the shear stresses on the slope, such loads (1) With a decrease in ground shear strength should be kept at a reasonable optimum distance of (2) With increasing shear stresses required for the slope. An acceptable distance can be determined equilibrium. through slope stability analysis. The crest length must CONTACT Muhammad Israr Khan email@example.com Room number 309, School of Resources and Civil Engineering, Northeastern University China,110819, China © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 M. I. KHAN be such that at least 1.5 value for Factor of Safety (FS) FS ¼ FS xR (3) 3Dm 2Dm 3D=2D is achieved, which is the minimum requirement for where FS is 3-dimensional slope factor of safety, 3Dm a safe slope as mentioned (Das et al., 2010). There is FS is 2-dimensional slope factor of safety, and 2Dm a strong correlation between the slope failure and the R is the ratio of 3d and 2d slope factor of safety. 3D/2D ground acceleration within the study area, which are This equation is valid only in high-risk areas where large mainly due to the different geographical features all uncertainty is found in the soil parameters. In case of over the world (Tiwari et al., 2017). Accurate and medium uncertainty in the soil properties and para- rapid forecasting of the stability of a landslide is cri- meters, the following correlations are developed in the tical for emergency response planning. However, cur- same work: rent methods of predicting rapid landslides cannot fully address the impact of soil size distribution FS FS 3D 2Dm FS ¼ FS þ (4) 3Dm 2Dm (Shan et al., 2020). As the material properties are FS 2Dm different at different places, the mapping slope stabi- A simplified version of Eq. (4), including R3D/2D, is lity predictions or analysis based on Geographical shown in Eq. (5): Information System (GIS) provided in many of the research papers (Ba et al., 2017; Ballabio & Sterlaccini, FS ¼ FS þ R 1 (5) 3Dm 2Dm 3D=2D 2012; Feizizadeh et al., 2013, 2014; L Liu et al., 2019) Similarly, for low uncertainty in soil parameters, the are not useful to apply for other regions. Every site following correlations are developed: must be properly investigated and tested to find out the material properties of that specific site. Boreholes FS FS 3D 2Dm FS ¼ FS þ (6) 3Dm 2Dm and non-destructive tests are required to compute the FS 3D mechanical properties of soil. Research (Deng et al., Assuming the 2D FS in R is the minimum 2D FS, 2016) results show that the external load calculation 3D/2D a simplified version of Eq. (6) is shown in Eq. (7): mode has little effect on slope stability. If the different external load patterns are equal, the slope stability R 1 3D=2D FS ¼ þ FS (7) under these external loads is the same, and if not, the 3Dm 2Dm 3D=2D external load leads to a better position of the slopes, as the position of the external load effect is closer to the Many other research works are done on the same topic lower slide slip point. FS value is dependent on the to compute the slope factor of safety in different con- surcharge load (S ) as well as the crest length (C ) of ditions and different soil properties. For example, L L the slope. some of the research papers titles on the same topic Keeping all these points in consideration, a pre- are as follows: defined soil slope is analyzed in this paper to examine and correlate FS with the variation of surcharge dis- (1) Discussion of “Probabilistic seismic slope sta- tributed load and its position on the slope surface. bility analysis and design” (W. Huang, 2019) (2) Influence of cross correlation between soil parameters on probability of failure of simple cohesive and c-ϕ slopes (Javankhoshdel & 1.1. Previous research on the topic Bathurst, 2015) Considering c-ϕ soil, Javankhoshdel and Bathurst (3) Evaluation of slope stability by finite element (2014) developed a correlation for slope factor of method using observed displacement of land- safety as slide (Ishii et al., 2012) (4) Revisiting strength concepts and correlations Fs ¼ (1) with soil index properties: insights from the γHN Dobkovičky landslide in Czech Republic where Su is undrained shear strength; γ is total unit (Roháč et al., 2020) weight; H is the height of slope; and Ns is a stability (5) Reliability analysis of slope stability under seis- number. mic condition during a given exposure time J Xiao et al. (2016) correlated the slope factor of (Huang et al., 2018) safety with the total elastic deformation energy (e ) (6) Development of empirical correlations for limit and ultimate deformation energy (e ) as equilibrium methods of slope stability analysis rffiffiffiffi (Moawwez et al., 2020) Fs ¼ (2) (7) Reliability approach to slope stability analysis with spatially correlated soil properties (Kim & Stark and Ruffing (2017) developed correlations Sitarb, 2013) between 2-dimensional and 3-dimensional slope sta- (8) Probabilistic slope stability analysis in sensitive bility analysis, such as clay area (Liu et al., 2015) GEOLOGY, ECOLOGY, AND LANDSCAPES 3 A very interesting conclusion is drawn by Shiferaw considered to correlate the slope factor of safety (2021) in one of the latest research works in 2021 as with different soil parameters. the research shows that the failure mechanism of the three soil types varies with the steepness and inclina- 1.3. Objective of the study tion of the slope. Toe slide is the most common kind of slope failure on clay and sandy clay soils. The failure The main objective of this study is to develop correla- mode of the slope slide is the most common for sandy tions between slope factor of safety with distributed soil. The failure mode in sandy clay and clay soils surcharge load and crest length of a slope in seismic tends to be base slide at lower heights (less than 2 and non-seismic conditions. m), while in sandy soil, the failure mode tends to be toe slide. The failure mode of sandy clay soils shifts 2. Materials and methods from slope slide to toe slide as the slope steepness increases. As a general rule, toe slip is the most com- Soil samples are collected from a slope at Mozishan mon failure mode in clayey soils. When the slope angle Park, which is local site at Shenyang city of China. is less than 18 degrees, the base slide takes place. Slope Figure 1 presents the site area having coordinates: failure is the dominant failure mode of sand. Failure of 41.666959, 123.477453. the base occurs at a steeper slope, namely at an angle of A local slope site is selected for analysis. Soil sam- 36.87 degrees. ples are collected from the site through boreholes at The factor of safety of slopes improves almost lin- various points. All required tests were performed to early with decreasing slope angle but increases at vary- compute the soil properties essential for the analysis. ing rates with decreasing slope height. The factor of The soil properties’ ranges are set for each and every safety can be increased by reducing either the slope analysis. The range of cohesion, friction, unit weight, angle or the height of the slope, depending on the and other input values is mentioned in the material specific slope in concern. To effectively increase properties section of every phase. Some of the impor- slope stability, it is important to understand the failure tant laboratory tests considered in this work are as mode and the effect of geometric change on the slope follows: factor of safety. A very useful related work can be checked in previous papers (Khan et al., 2019, 2022a, (a) Water content test 2022b; Khan & Wang, 2020a, 2020b, 2020c, 2020d, (b) Sieve analysis 2021a, 2021b, 2021c, 2021d, 2022a, 2022b). (c) Measurement of unit weight/specific gravity (d) Consolidation test (e) Atterberg’s limit test (f) Unconfined compression test 1.2. Existing problems and research idea to solve (g) Direct shear test the issue (h) Measurement of consistency limits The existing and normal trend to compute the slope (i) Falling head permeability test stability of any soil slope is to model a specific posi- tion of the soil slope, collect soil samples from that Figure 2 presents the boreholes and soil sampling at specific point, and then model it to check the stability site while Figure 3 presents the experiments per- of the slope. Main problem in such focused analysis is formed in the laboratory to determine the mechanical that the results are not applicable to any other point properties of soil which was brought from the site by on the same slope. For example, if a slope is 4 kilo- conducting four number of boreholes at different loca- meters in longitudinal direction, then the slope sta- tions of the site. bility analysis by collecting soils samples from only Three number of boreholes, twenty number of one position is not applicable to other point on the Atterburg’s limit test, eighteen triaxial tests, twenty same slope. To overcome this problem, correlations one direct shear test, and almost thirty sieve analysis are required between all the soil parameters such as tests were performed to compute the mechanical factor of slope safety, cohesion, friction, unit weight, properties of soil in detail. Similarly, all other tests slope angle, etc. to simply use those correlations and such as moisture content test, porosity, compaction, know about the slope safety at any point throughout and specific gravity tests were also conducted. All the length of let suppose 4 kilometers. This is the these tests were conducted repeatedly to make sure main idea behind this research work in which corre- the material properties are accurately computed. lations are developed between soil and slope para- Mechanical properties of soil after all the required meters in various multivariate conditions. Second, testings are mentioned in Table 1. Figure 4 shows the the results in case of seismic and non-seismic condi- slice details considered in the limit equilibrium tions are also not same; therefore, in this research 2-dimensional approach. It also explains the boundary both seismic and non-seismic conditions are conditions in both x and y directions. 4 M. I. KHAN Figure 1. Site location at Mozishan Park (Maps, accessed 14 January 2022). a. Borehole to collect soil sample b. Deep soil samples collected at site Figure 2. Boreholes and sampling at site. Details of other boreholes are available in the supple- ðCþ N tanϕÞ i¼1 F ¼ P P P (8) n n n mentary section. A A A 5 6 7 i¼1 i¼1 i¼1 where 2.1. Analysis method A ¼ ½Wð1 k ÞþU cosβþ Qcosσ� Rsinα (9) 5 u β Soil is not a homogenous material and hence its proper- � � ties vary from place to place and with time to time. � � Computing the soil properties is one of the most challen- A ¼ U sinβþ Qsinσ cosα