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International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 04, April 2019, pp. 528-535. Article ID: IJCIET_10_04_054 Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=04 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed IMPROVING THE ACCURACY OF KENNEY AND LAU METHOD IN ORDER TO ASSESS THE INTERNAL STABILITY OF GRANULAR SOILS Mohamed Ahmad ALsakran Ph.D. candidate, Institute of Geotechnical Engineering, Hohai University, 01 Xikang Road, Nanjing 210098, P. R. China Jun-Gao ZHU Ph.D., Professor, (a) Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, 01 Xikang Road, Nanjing 210098, P. R. China (b) Guest professor, Key Laboratory of failure mechanism and safety control techniques of earth-rockfill dam of the Ministry of Water Resources, Nanjing hydraulic research institute, 223# Guangzhou Road, Nanjing, P. R. China ABSTRACT Internal instability normally occurs in widely graded or gap graded soils. To be more specific, it may occur in soils that have a bimodal structure (i.e. the soil has two components, namely, coarse fraction and loose fine fraction). Internal instability occurs when the finer fraction particles can be washed out if their size is less than the size of the constrictions among the coarse fraction particles. A commonly used approach to evaluating the potential for internal instability in soils is that of Kenney and Lau. This method is used to assess the internal stability of cohesionless soils based on the shape of their GSD curves. The main objective of this paper is to propose a modification on Kenney and Lau method to increase the accuracy of this method using the critical particle sizes between the groups of the soils. Major type groups of the soils include; gravel (>4.75mm), sand (0.075-4.75mm) and fines (<0.075mm). The proposed modification was verified with a large number of experimental tests. Keywords: Kenney and Lau method, internal stability, suffusion, granular soils, soil type groups. Cite this Article: Mohamed Ahmad ALsakran and Jun-Gao ZHU, Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils, International Journal of Civil Engineering and Technology, 10(4), 2019, pp. 528-535. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=04 http://www.iaeme.com/IJCIET/index.asp 528 [email protected] Mohamed Ahmad ALsakran and Jun-Gao ZHU 1. INTRODUCTION Suffusion is an internal erosion process which involves washing out of loose fine particles through the voids of the primary soil. The fine particles are removed under the effect of seepage forces, leaving behind an intact soil skeleton formed by the coarser particles; hence the permeability and seepage maybe increased, Chang and Zhang (2011), leading to collapse of the soil skeleton, McCook (2004). Suffusion may result-in settlement or development of pipes or cracks. Experimental studies of Ke and Takahashi (2011), (2012) and numerical studies of Scholtes et al (2010) and Wood et al. (2010) had revealed that internal erosion problems can also cause a strength reduction. As a result, partial or complete failure of dams and embankments may occur soils (e.g., Fell etal.,2003; Zhang andChen,2006; Xu and Zhang, 2009; Zhang etal.,2009, 2011; Fujisawaetal.,2010; Pagano etal.,2010; PengandZhang,2012).. Suffusion also may occur in the granular filters that were constructed of internally unstable materials, renders those filters coarser, and accordingly decreases their ability to protect the core or foundations materials, Wan and Fell (2008). Many studies investigated the internal stability of cohesionless soils, for example, Kézdi (1979), Kenney and Lau (1985, 1986), Lafleur et al. (1989), Burenkova (1993), Skempton and Brogan (1994), Fannin and Moffat (2002), Moffat and Fannin (2006), Li and Fannin (2008), Ahlinhan et al. (2010), and Andrianatrehina et al. (2012). 2. THE KENNEY-LAU APPROACH The method of Kenney and Lau (1985, 1986) (KL method) is used to assess the internal stability of cohesionless soils based on the shape of their GSD curves. The finer percent (F) corresponding to an arbitrary particle diameter (D) is determined, as shown in Figure 1. (D) is the particle size that distinguishes the finer particles from the main soil skeleton; it’s called the delimiting particle size (DPS). The finer percent corresponding to the particle diameter (4D) is also determined, accordingly, the value of (H) can be easily determined as the difference of the finer percent between D and 4D. The internal stability is determined by calculating the H/F ratios in the range of F ≤ 0.2 for widely-graded soils, and in the range of F ≤0.3 for narrowlygraded soils. The soil is considered as internally unstable if the ratio (H/F) lies below the stability boundary (H/F=1.0) as shown in Figure 2. Figure 1 Determination of F and H of KL method http://www.iaeme.com/IJCIET/index.asp 529 [email protected] Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils Figure 2 The stability boundary of KL method Kenney and Lau (1985) initially defined the stability boundary as H/F = 1.3. Later, it was revised to be (H/F = 1.0) (Kenney and Lau, 1986) upon comments of Milligan (1986), Sherard and Dunnigan (1986), and Ripley (1986). The method assumes that the maximum possible finer content (i.e. erodible particles) for the widely graded soils (with Cu>3) is 20% and for the narrowly graded soils (with Cu<3) is 30%. For this reason the analysis is performed in the range of F <20% or F <30%. Li (2008) found out that the method of Kenney and Lau assesses the stability of “unstable gradations” correctly, while it provides a wrong assessment of some “stable gradations”. Accordingly, the method is conservative in evaluating the potential for internal stability. 3. MODIFIED KENNEY AND LAU METHOD USING SOIL CLASSES The range of particle sizes encountered in soil is very large from boulders with a controlling dimension of over 200mm down to clay particles less than 0.002mm. In the Unified Soil Classification System, soils are classified into named Basic Soil Type groups according to size, and the groups further divided into coarse, medium and fine. See Fig3. 1 Fines 0.8 Gravel Sand F% 0.6 0.4 Coar se Mediu m Fine 0.2 0 0.01 0.1 1 10 100 D mm Figure 3 Basic Soil Type groups http://www.iaeme.com/IJCIET/index.asp 530 [email protected] Mohamed Ahmad ALsakran and Jun-Gao ZHU For finding the expected (D) or the expected DPS particles that is located in-between of coarse and fine particles, see Fig 4. For soils that have three type groups (gravel, sand, fines), we need to determine two DPS particles (D1, D2). It is suggested to draw a line crosses the largest grain size and the critical particle size of gravel group and sand group (4.75mm), the intersection of this line with the diameter axis will determine the first DPS particle (D1). By drawing another line crosses the largest grain size and the diameter of grain size that represents the critical particle size of sand group and fines group (0.075mm), the second DPS particle (D2) will be obtained by intersection of this line with the diameter axis. 1 0.8 F% 0.6 0.4 0.2 0 0.01 D 2 0.075 0.1 mm D1 4.75mm 1 10 100 D mm Figure 4 The expected (D) for three type groups Fig5 shows, for soils that have two type groups (Gravel, Sand), two DPS particles (D1, D2) are needed. To determine (D1), it is suggested to draw a line crosses the largest grain size and the critical particle size of coarse aggregate and medium aggregate inside the sand group (2mm), the intersection of this line with the diameter axis will determine the first DPS particle (D1). By drawing another line crosses the largest grain size and the diameter of grain size that represents the critical particle size of gravel group and sand group (4.75mm), the second DPS particle (D2) will be obtained by intersection of this line with the diameter axis. As well for soils that have one type group (sand), it is needed to determine one DPS particle (D), see Fig6. It is suggested to draw a line crosses the largest grain size and the grain size that represents the critical particle size of sand aggregate and fine aggregate inside the sand group (0.425mm), the intersection of this line with the diameter axis will determine the DPS particle (D). 1 0.8 F% 0.6 0.4 0.2 0 0.01 0.1 D 2 D 1 1 D mm 2m m 10 100 Figure 5 The expected (D) for two type groups http://www.iaeme.com/IJCIET/index.asp 531 [email protected] Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils 1 0.8 F% 0.6 0.4 0.2 0 0.01 D 0.425m m 0.1 1 10 D mm Figure 6 The expected (D) for one type group 4. ANALYSIS AND RESULTS To get a clear idea about the accuracy of modified Kenney and Lau method that proposed here in this study, data published in the literature have been reanalyzed, namely: Skempton and Brogan (1994), Lafleur and Nguyen (2007), Wan and Fell (2004) and Wan and Fell (2008), Kenney and Lau (1985), Lafleur et al. (1989), Aberg (1993), Sadaghiani and Witt (2011), and Li (2006). In total 53 laboratory tests have been reanalyzed. In order to verify the modified Kenney and Lau method, the 53 data sets were analyzed and the delimiting particles size DPS (D) were determined and compared with (4D) to find H/F. Experimental tests data were analyzed according to the classical Kenney and Lau and modified Kenney and Lau methods. Table1 shows analysis soils data published in literature that have three type groups. There are 5 wrong predictions using the classical Kenney and Lau; where using modified Kenney and Lau, there are 2 wrong predictions. By analyzing the data in the literature that have two type groups and one type group, as we see in table2 and table3, respectively. For two type groups, there are 7 wrong predictions using the classical Kenney and Lau; where using modified Kenney and Lau, there are 3 wrong predictions. As same as for one type group, there are 2 wrong predictions using the classical Kenney and Lau; where using modified Kenney and Lau, there isn’t any wrong predictions. Table 1 Internal stability assessment of three type groups soils data No Gradat ion Soil type groups 1 X Gravel - Sand Fines H/F (DmaxH/F (Dmax4.75mm) 0.075mm) K-L (1985) 0.64 15.00 Classical K-L Modified K-L Laborat ory U U U Aberg (1993) 2 E 3 G 4 H Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines 1.48 1.20 U S S 1.00 3.67 S S S 0.37 29.00 U U U Sadaghiani and Witt (2012) http://www.iaeme.com/IJCIET/index.asp 532 [email protected] Mohamed Ahmad ALsakran and Jun-Gao ZHU No Gradat ion 5 Soil type groups Gravel - Sand Fines H/F (Dmax4.75mm) H/F (Dmax0.075mm) Classical K-L Modified K-L Laborat ory 0.33 5.74 U U U 1.40 S S S Li (2006) 6 HF10 Gravel - Sand Fines 1.00 Wan and Fell (2004,2008) 7 1,1A 8 2R 9 4R 10 9 11 10 12 A2 13 A3 14 B1 15 B2 16 C1 17 D1 Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines Gravel - Sand Fines 0.34 1.29 U U S 0.29 1.375 U U S 2.33 1.48 U S S 4.47 1.05 U S S 0.93 1.83 U U U 0.41 1.14 U U U 0.71 0.67 U U U 0.36 1.31 U U U 0.32 0.93 U U U 0.60 0.80 U U U 0.40 0.88 U U U Table 2 Internal stability assessment of two type groups soils data No Gradation Soil type groups 18 19 20 21 22 23 24 25 26 27 28 A Y YS AS DS 1 2 3 20 21 23 Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand 29 F Gravel - Sand 30 31 A B Gravel - Sand Gravel - Sand H/F (Dmax4.75mm) H/F (Dmax2mm) K-L (1985) 0.90 2.20 0.50 0.92 0.76 0.91 1.00 1.08 5.36 4.40 1.15 2.20 2.00 2.25 1.25 1.24 2.33 3.00 8.00 5.67 7.57 10.25 Aberg (1993) 3.47 8.00 Skempton and Brogan (1994) 0.67 13.00 1.17 6.00 http://www.iaeme.com/IJCIET/index.asp 533 Classical K-L Modified K-L Laboratory T U U T S S S S S S S U U U S S S S S S S S U U U S S S S S S S S S S S U T U S U U [email protected] Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils No Gradation 32 33 C D Soil type groups Gravel - Sand Gravel - Sand 34 35 HF01 HF03 Gravel - Sand Gravel - Sand 36 37 38 39 40 41 42 43 1 2 3 4 11 12 13 14 Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand Gravel - Sand 44 45 M6 M8 Gravel - Sand Gravel - Sand H/F (Dmax4.75mm) 2.25 3.50 H/F (Dmax2mm) 3.38 5.36 Li (2006) 3 2.75 0.47 0.33 Burenkova (1993) 0.78 0.18 2.00 0.33 0.33 0.25 0.63 0.31 2.04 3.00 1.92 1.55 1.14 1.14 0.75 0.71 Lafleur et al (1989) 0.47 0.52 1.33 0.84 Classical K-L S S Modified K-L S S U U S U U U U U U U U U U U U U U U S S S U U U U U S S S S U U U U U U Laboratory S S Table 3 Internal stability assessment of one type group soils data No Gradation Soil type groups 46 47 FR7 FR8 Sand Sand 48 49 50 51 52 53 G1-a G1-b G3-a G3-b G4-a G4-b Sand Sand Sand Sand Sand Sand H/F (Dmax-0.425mm) Li (2006) 0.00 0.07 Honjo et al (1989) 8 2.5 0.75 0 0 0.2 Classical K-L Modified KL Laboratory U U U U U U T T U U U U S S U U U U S S U U U U 5. SUMMARY AND CONCLUSIONS Analysis of a large number of experimental tests showed that the accuracy of classical Kenney and Lau model is somehow low (14 wrong predictions of 53 data sets). The modification that has been done on Kenney and Lau method, the internal stability criterion is applied only on one or two delimiting particle size depending upon the type groups of soils. By this modification, applying Kenney and Lau method is simpler and faster than the classical method; as well as, it was effective to enhance the accuracy of Kenney and Lau method to (5 wrong predictions of 53 data sets). Accordingly, the modified Kenney and Lau method is recommended to assess the internal stability of the granular soils. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from National Key R&D Program of China(2017YFC0404804),a research grant (No. 51479052) from the National Natural Science http://www.iaeme.com/IJCIET/index.asp 534 [email protected] Mohamed Ahmad ALsakran and Jun-Gao ZHU Foundation of China, and the Fundamental Research Funds for the Central Universities of China(No. 2017B20614). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] Aberg, B. (1992), Void ratio of noncohesive soils and similar materials, Journal of Geotechnical Engineering, Vol. 118, No.9, pp. 1315–1333. ASTM International (2006). Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) Burenkova, V. V.(1993), Assessment of suffosion in non-cohesive and graded soils. Filters in Geotechnical and Hydraulic Engineering, Balkema, Rotterdam, pp.357-360. Chang, D. S. and Zhang, L. M. 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