IMPROVING THE ACCURACY OF KENNEY AND LAU METHOD IN ORDER TO ASSESS THE INTERNAL STABILITY OF GRANULAR SOILS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 04, April 2019, pp Article ID: IJCIET_10_04_054 Available online at ISSN Print: and ISSN Online: 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 , 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 , 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 ( mm) 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 editor@iaeme.com

2 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 editor@iaeme.com

3 F % 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 Fig Sand Gravel Fine Mediu m Coar se D mm Figure 3 Basic Soil Type groups editor@iaeme.com

4 F % F % 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 D D1 4.75mm mm 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) D D 1 2m m D mm Figure 5 The expected (D) for two type groups editor@iaeme.com

5 F % Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils D m 1 10 m 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 1 X 2 E 3 G 4 H Soil type groups H/F (Dmax- H/F (Dmax- 4.75mm) 0.075mm) (1985) Classical Modified Laborat ory U U U Aberg (1993) U S S S S S U U U Sadaghiani and Witt (2012) editor@iaeme.com

6 Mohamed Ahmad ALsakran and Jun-Gao ZHU No 5 Gradat ion 6 HF10 7 1,1A 8 2R 9 4R A2 13 A3 14 B1 15 B2 16 C1 17 D1 Soil type groups H/F (Dmax- 4.75mm) H/F (Dmax mm) Classical Modified Laborat ory U U U Li (2006) S S S Wan and Fell (2004,2008) U U S U U S U S S U S S U U U U U U U U U U U U U U U U U U U U U Table 2 Internal stability assessment of two type groups soils data No Gradation Soil type H/F (Dmax- H/F (Dmax- Classical Modified groups 4.75mm) 2mm) Laboratory (1985) 18 A Gravel - Sand T U U 19 Y Gravel - Sand U U U 20 YS Gravel - Sand U U U 21 AS Gravel - Sand T S S 22 DS Gravel - Sand S S S 23 1 Gravel - Sand S S S 24 2 Gravel - Sand S S S 25 3 Gravel - Sand S S S Gravel - Sand S S S Gravel - Sand S S S Gravel - Sand S S S Aberg (1993) 29 F Gravel - Sand S S S Skempton and Brogan (1994) 30 A Gravel - Sand U U U 31 B Gravel - Sand T S U editor@iaeme.com

7 Improving the Accuracy of Kenney and Lau Method in Order to Assess the Internal Stability of Granular Soils No Gradation Soil type groups H/F (Dmax- 4.75mm) H/F (Dmax- 2mm) Classical Modified Laboratory 32 C Gravel - Sand S S S 33 D Gravel - Sand S S S Li (2006) 34 HF01 Gravel - Sand U S U 35 HF03 Gravel - Sand U U U Burenkova (1993) 36 1 Gravel - Sand U U U 37 2 Gravel - Sand U U U 38 3 Gravel - Sand U U U 39 4 Gravel - Sand U U U Gravel - Sand U S S Gravel - Sand U S S Gravel - Sand U S S Gravel - Sand U U S Lafleur et al (1989) 44 M6 Gravel - Sand U U U 45 M8 Gravel - Sand U U U Table 3 Internal stability assessment of one type group soils data No Gradation Soil type groups H/F (Dmax-0.425mm) Classical Modified K- L Laboratory Li (2006) 46 FR7 Sand 0.00 U U U 47 FR8 Sand 0.07 U U U Honjo et al (1989) 48 G1-a Sand 8 T S S 49 G1-b Sand 2.5 T S S 50 G3-a Sand 0.75 U U U 51 G3-b Sand 0 U U U 52 G4-a Sand 0 U U U 53 G4-b Sand 0.2 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(2017YFC ),a research grant (No ) from the National Natural Science editor@iaeme.com

8 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] Aberg, B. (1992), Void ratio of noncohesive soils and similar materials, Journal of [2] Geotechnical Engineering, Vol. 118, No.9, pp [3] ASTM International (2006). Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) [4] Burenkova, V. V.(1993), Assessment of suffosion in non-cohesive and graded soils. Filters in Geotechnical and Hydraulic Engineering, Balkema, Rotterdam, pp [5] Chang, D. S. and Zhang, L. M. (2011), A Stress-controlled Erosion Apparatus for Studying Internal Erosion in Soils, Geotechnical testing journal. Vol.34, No.6, pp [6] Fannin, R.J. and Li, M. (2006), A comparison of two criteria for internal stability on granular soil, 59th Canadian Geotechnical Conference, Vancouver, B.C., pp [7] Fannin, R.J. and Moffat, R. (2006) Observations on internal stability of cohensionless soils, Geotechnique, Vol. 56, No. 7, pp [8] Honjo, Y., Haque, M.A. and Tsai, K.A. (1996), Self-filtration behaviour of broadly and gap-graded cohesionless soils. Geofilters 96, BiTech Publications, Montreal, Canada, pp [9] Ke, L. and Takahashi, A. (2011), Strength reduction of gap-graded cohesionless soil due to internal erosion, Proceedings of the 5th Asia Pacific Conference On Unsaturated Soils, Pattaya, Thailand, pp [10] Ke, Lin and Takahashi, Akihiro (2012), Strength reduction of cohesionless soil due to internal erosion induced by one-dimensional upward seepage flow, Soils and Foundations, Vol.52, No.4, pp [11] Kenney, T., Chahal, R., Chiu, E., Ofoegbu, G., Omange, G., and Ume, C. (1985), Controlling constriction sizes of granular filters, Canadian Geotechnical Journal, Vol. 22, No.1, pp [12] Kenney, T.C. and Lau, D. (1985) Internal stability of granular filters. Canadian Geotechnical Journal, Vol. 22, No.2, pp [13] Kenney, T.C. and Lau, D. (1986) Internal stability of granular filters : Reply. Canadian Geotechnical Journal, Vol. 23, No.3, pp [14] Lafleur, J., Mlynarek, J. and Rollin, A.L. (1989), Filtration of broadly graded cohesionless soils, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 115, No.12, pp [15] Li, M & Fannin, RJ 2008, Comparison of two criteria for internal stability of granular soil, Canadian Geotechnical Journal, Vol. 45, No. 9, pp [16] Li, M. (2008), Seepage induced instability in widely graded soils, Ph.D. thesis, Department of Civil Engineering, The University of British Columbia, Vancouver. [17] McCook, DK (2004), A comprehensive discussion of piping and internal erosion failure mechanisms, Annual dam safety conference, ASDSO. Phoenix, Arizona. pp [18] Moffat, RA. and Fannin, RJ (2006), A Large Permeameter for Study of Internal Stability in Cohesionless Soils, Geotechnical Testing Journal, Vol. 29, No. 4, pp [19] Sadaghiani, Mohamad Reza Sahehi and Witt, Karl Josef (2012), Analysis of Internal Stability of Widely Graded Soils Based on Identification of Mobile Particles, Proceedings of the sixith International Conference on Scour and Erosion, Paris, France, pp [20] Skempton, A.W. and Brogan, J.M. (1994), Experiments on piping in sandy gravels, Geotechniqué, Vol. 44, No. 3, pp [21] Wan, C.F. and Fell, R. (2008), Assessing the Potential of Internal Instability and Suffusion in Embankment Dams and Their Foundations, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 134, No.3, pp editor@iaeme.com

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