Undrained Shear Strength of Sand-silt Mixture: Effect of Intergranular

KSCE Journal of Civil Engineering (2011) 15(8):1335-1342 DOI 10.1007/s12205-011-1051-x

Geotechnical Engineering

www.springer.com/12205

Undrained Shear Strength of Sand-silt Mixture: Effect of Intergranular Void Ratio and Other Parameters Mostefa Belkhatir*, Hanifi Missoum**, Ahmed Arab***, Noureddine Della****, and Tom Schanz***** Received November 12, 2009/Revised 1st: February 8, 2010, 2nd: July 14, 2010, 3rd: January 14, 2011/Accepted March 7, 2011

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Abstract The determination of the appropriate undrained residual shear strength of silty sands soils that are prone to liquefaction to be used in the assessment of the post-liquefaction stability of different types of earth structures is becoming a major challenge.The objective of this laboratory investigation is to study the effect of intergranular void ratio on undrained residual shear strength of loose, medium dense and dense (Dr = 12, 50, and 90% before consolidation) sand-silt mixtures under monotonic loading and liquefaction potential under cyclic loading. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with fine contents ranging from 0 to 50%. The confining pressure was kept constant to 100 kPa in all tests. The results of undrained monotonic tests indicate that the global void ratio does not represent the actual behaviour of the soil and the undrained residual shear strength decreases linearly with the increase of the intergranular void ratio and similar trend is happening with the fine content of the sand-silt mixtures up to 50% as well. Undrained cyclic test results show that the increase of the intergranular void ratio and the fine content accelerate the liquefaction phenomenon for the stress ratio under study and the liquefaction resistance decreases with the increase of the intergranular void ratio and the loading amplitude. Keywords: liquefaction, residual shear strength, intergranular void ratio, fines, mixtures ···································································································································································································································

1. Introduction During earthquakes, the ground shakes causing sandy soils to lose their strength and behave like a liquid. This phenomenon is called soil liquefaction and will cause settlement of buildings, landslides, the failures of earth dams, or other hazards. Liquefaction occurs due to an increase in the excess pore water pressure and a corresponding decrease in the effective overburden stress in a soil deposit. The understanding of liquefaction phenomena has significantly improved in recent years. Most liquefaction research was undertaken on clean sands with the assumption that the behaviour of silty sands is similar to that of clean sands. Recent researches on the nature of soil structure and its engineering stress-strain response indicate that the soil behaves as a collection of scale-level-dependent skeletons arranged in a particular manner (Thevanayagam, 1997; Thevanayagam and Nesarajah, 1998). However, several studies have mentioned

that the physical nature of silty sand is entirely different from that of clean sand (Zlatovic and Ishihara, 1995; Lade and Yamamuro, 1997; Thevanayagam et al., 1997; Thevanayagam, 1998; Yamamuro and Lade, 1998; Amini and Qi, 2000; Naeini, 2001; and Naeini and Baziar, 2004). They recognized that the undrained residual shear strength (Sus) response depends effectively on the void ratio as a state parameter. It is also anticipated that the global void ratio (e) cannot represent the amount of particle contacts in the sand-silt mixture samples. As the void ratio and proportion of the coarse grained soil or fine grained soil changes, the nature of their microstructures also changes. Due to a large grain size distribution range and availability of voids larger than some grains, at low fines contents, some of the finer grains may remain inactive and swim in the void spaces without affecting or contributing to the force chain. Therefore, it is quite important to use new index parameters such as the intergranular (Thevanayagam, 1998; Chu and Leong, 2002; and Monkul, 2005)

*Associate Professor, Laboratory of Materials Sciences & Environment, University of Chlef, BP 151 Route de Sendjes 02000 Chlef, Algeria (Corresponding Author, E-mail: [email protected]) **Associate Professor, Laboratory of Construction, Transports and Environment Protection, University of Mostaganem, BP 227 Route de Belacel 27000 Mostaganem, Algeria (E-mail: [email protected]) ***Associate Professor, Laboratory of Materials Sciences & Environment, University of Chlef, BP 151 Route de Sendjes 02000 Chlef, Algeria (E-mail: [email protected]) ****Associate Professor, Laboratory of Materials Sciences & Environment, University of Chlef, BP 151 Route de Sendjes 02000 Chlef, Algeria (E-mail: [email protected]) *****Professor, Laboratory of Foundation Engineering, Soil & Rock Mechanics, Ruhr University of Bochum, 44780 Bochum, Germany (E-mail: [email protected]) − 1335 −

Mostefa Belkhatir, Hanifi Missoum, Ahmed Arab, Noureddine Della, and Tom Schanz

and inter-fine (Naeini and Baziar, 2004) void ratios to assess the undrained residual shear strength response of sand-silt mixtures. Further investigations about the microstructure of soils including two submatrices (i.e. coarser grain and finer grain matrices) are therefore needed in order to gain insight regarding their influence on stress-strain and compression behaviour. It may be expected for soils or reconstituted mixtures with some amount of fine content that the soils or mixtures would exhibit the behaviour of the dominant grain matrix.

2. Experimental Program and Test Procedure The present laboratory investigation has been carried out in order to study the influence of intergranular void ratio on the undrained residual shear strength and liquefaction potential of sand-silt mixtures. For this purpose, a series of undrained triaxial tests under monotonic and cyclic loading conditions were performed on reconstituted saturated samples of Chlef sand with variations in fine content ranging from 0 to 50%. The samples were prepared into their loose, medium dense and dense states using undercompaction method of sample preparation and consolidated isotropically at an initial confining pressure of 100 kPa. Sand samples were collected from the liquefied layer of the deposit areas at a depth of 6.0 m (Fig. 1) close to the epicentre of Chlef earthquake (October 10th, 1980). Fig. 2 illustrates the typical subsidence location of the liquefied soil and sample collection.

The sand and silt used in this laboratory investigation were obtained by the dry sieve analysis and wet sieve analysis. The index properties of the soils used during this study are summarized in Table 1. The grain size distribution curves for the tested soils are shown in Fig. 3. Monotonic undrained compression tests were experimented on isotropically consolidated sand samples with 0, 10%, 20%, 30%, 40% and 50% non-plastic fines fraction. All specimens were prepared by first estimating the dry weights of sand and silt needed for a desired proportion into the loose, medium dense and dense state (Dr = 12, 50, and 90% before consolidation) using the undercompaction method of sample preparation which simulates a relatively homogeneous soil condition and was performed by compacting dry soil in layers to a selected percentage of the required dry unit weight of the specimen (Ladd, 1978). The concept of undercompaction is based on the fact that when successive layers of sand are placed without undercompaction, the compaction of each succeeding layer can further densify the sand below it. In order to avoid this difficulty, the lower layers were compacted to a lower density than desired for the final density. The specimens were 70 mm in diameter and 70 mm in height using smooth lubricated end plates. When the specimen was realized, a cap was placed on it and was sealed with O-rings. A partial vacuum of 20 kPa was applied to the sample to reduce the disturbances. Saturation was attained by purging the dry specimen with carbon dioxide for approximately 30 min. Deaired water was then introduced from the bottom drain line into the soil sample. Water was allowed to flow through it until an Table 1. Index Properties of Sand-silt Mixtures Material

Fc (%)

GS

D50 (mm)

Cu

emin

emax

Ip (%)

Clean Sand (S100M0)

0

2.680 0.68

3.36 0.535 0.854

-

Silty Sand (S90M10)

10

2.682 0.50

7.75 0.472 0.798

-

Silty Sand (S80M20)

20

2.684 0.43 15.26 0.431 0.748

-

Silty Sand (S70M30)

30

2.686 0.37 23.18 0.412 0.718

-

Silty Sand (S60M40)

40

2.688 0.29 27.33 0.478 0.732

-

Silty Sand (S50M50)

50

2.69

-

Silt (S0M100)

100

2.70

0.08 28.18 0.600 0.874 -

-

0.72 1.420

5.0

Fig. 1. Geotechnical Profile of the Soil Deposit at the Site

Fig. 2. Subsidence of the Chlef River Banks Due to Liquefaction − 1336 −

Fig. 3. Grain Size Distribution Curves of Tested Materials KSCE Journal of Civil Engineering

Undrained Shear Strength of Sand-silt Mixture: Effect of Intergranular Void Ratio and Other Parameters

amount equal to the void volume of the specimen was collected in a beaker through the upper drain line. A minimum Skempton coefficient-value greater than 0.96 was obtained at a minimum back pressure of 100 kPa. All test specimens were isotropically consolidated at a mean effective pressure of 100 kPa, and then subjected to undrained monotonic triaxial loading. In this investigation, all undrained triaxial tests were undertaken at a constant strain rate of 0.167% per minute. The loading rate was chosen so that pore pressure equalization throughout the specimen was fully ensured. All the tests were continued up to 24% axial strain. When granular soil contains fines, the global void ratio of the soil e can no longer be used to describe the behaviour of the soil. This is because, up to a certain fine content, Fc (the ratio of the weight of silt to the total weight of the sand-silt mixture), the fines occupy the void spaces only, and do not significantly affect the engineering behaviour of the sand-silt mixture. In this aim, the use of intergranular void ratio has been suggested (Kenny, 1977; Kuerbis et al., 1988; Mitchell, 1993). Mitchell (1993), and later on by Thevanayagam and Mohan (2000). They proposed to view the matrix of sand with fines as a combination of two submatrices: a coarser grain matrix and a finer grain matrix. Consequently, they suggested that for fine content Fc (i.e. expressed as a percentage of the total weight of the soil specimen) below a limit in the range of 20 to 30%, the contribution of fines in the force chain is minimal. In this idealisation, a simplifying assumption is proposed to consider the fine grain matrix as part of the voids between the coarser grains. Laboratory research of the fines effect on liquefaction has sometimes been based on the intergranular void ratio rather than the global void ratio. The concept of the intergranular void ratio calculates the void ratio while assuming that the volume occupied by the fines is part of the volume of voids. By neglecting the difference in the specific gravity of coarser and finer particles, an intergranular void ratio es was formulated as: e + ( Fc ⁄ 100 ) es = ---------------------------1 – ( Fc ⁄ 100 )

(1998). The concept of the intergranular void ratio suggests that the fines fill the voids formed between the sand grains and thus the behaviour of sand with a moderate amount of fines should be governed by the intergranular void ratio instead of the global void ratio. However, when the intergranular void ratio exceeds the maximum void ratio of the clean sand, there are sufficient fines to prevent grain-to-grain contact of the sand particles. In this case, the fines constitute the dominant structure and carry the shear forces while the coarse grains may act as reinforcing elements (Thevanayagam and Mohan, 2000). Fig. 4 shows the variation of the intergranular void ratio versus the fine content for the initial relative densities (Dr = 12%, 50%, and 90% before consolidation). As shown in this figure, the intergranular void ratio (es) increases hyperbolically with the increase of the fine content.

3. Monotonic Test Results 3.1 Undrained Compression Loading Tests Figure 5 shows the results of undrained compression tests carried out for different fine contents ranging from 0 to 50% at

(1)

where e is the global void ratio and FC is the fine content. Eq. (1) gives the intergranular void ratio defined by Thevanayagam

Fig. 4. Intergranular Void Ratio versus fine Content and Initial Relative Density Vol. 15, No. 8 / November 2011

Fig. 5. Effect of the Fine Content on the Undrained Shear Strength of the Sand-silt Mixtures (σ 3’=100 kPa, Dr = 50%): (a) Stress-strain Response, (b) Stress Path Curves

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Table 2. Summary of Monotonic Compression Triaxial Tests Results FC (%)

Dr (%)

γd (g/cm3)

e

es

Sus (kPa)

0

12 50 90

1.48 1.58 1.71

0.815 0.695 0.567

0.815 0.695 0.567

17.37 18.31 20.94

4 5 6

10

12 50 90

1.52 1.64 1.78

0.760 0.635 0.505

0.956 0.817 0.672

15.44 17.90 19.61

7 8 9

20

12 50 90

1.57 1.69 1.83

0.710 0.590 0.463

1.140 0.988 0.829

14.46 16.46 18.42

30

12 50 90

1.60 1.71 1.86

0.680 0.565 0.443

1.400 1.240 1.060

13.48 15.12 17.34

13 14 15

40

12 50 90

1.58 1.67 1.79

0.700 0.605 0.503

1.830 1.680 1.510

10.75 13.71 15.92

16 17 18

50

12 50 90

1.46 1.55 1.65

0.840 0.737 0.630

2.680 2.470 2.260

06.51 10.21 12.00

Test No Materials 1 2 3

10 11 12

Clean sand

Silty sand

e = Global void ratio es = Intergranular void ratio Sus = undrained residual strength

100 kPa confining pressure for the initial relative density (Dr = 50% before consolidation). In general, we notice that the increase in the amount of fines leads to a decrease in the deviatoric stress. This increase results from the role of the fines to increase the contractancy phase of the sand-silt mixture leading to a reduction of the confining effective pressure and consequently to a decrease in the peak resistance of the mixture as it is illustrated by Fig. 5(a). The stress path in the (p’, q) plane shows clearly the role of the fines increase in the reduction of the average effective pressure and the maximum deviatoric stress (Fig. 5b). In this case, the effect of fines on the undrained behaviour of the mixture is observed for the lower fine contents (0 and 10%), and becomes very pronounced beyond 20%. These results are in good agreement with the observations of Shen et al. (1977) and, Troncoso and Verdugo (1985). 3.2 Normalized Undrained Residual Strength When loose sand is subjected to undrained shearing beyond the point of peak strength, the undrained shear strength drops to a near constant value over large deformation. Conventionally, this shear strength is called the undrained steady-state shear strength or residual shear strength. However, if the strength increases after passing through a minimum value, the phenomenon is called limited or quasi-liquefaction. Even limited liquefaction may result in a large deformation and associated drop in soil resistance. The residual shear strength is defined by Ishihara (1993) as Sus =(qs/2)cosφ s = (M/2)cosφ s(ps’) M = (6 sinφ s)/(3 − sinφ s)

respectively. For the undrained tests conducted at the confining pressure and various initial relative densities, the deviator stress (qs) was estimated at the phase transition point along with the mobilized friction angle. Further more the residual shear strength was calculated using Eq. (2). Figure 6 illustrates the residual shear strength versus the fine content for three initial relative densities (Dr = 12, 50 and 90% before consolidation) at a confining pressure of 100 kPa. It is clear from this figure that the undrained residual shear strength decreases moderately with the fine content up to the value of 40%, and it continues to decrease but more sharply beyond that value. Figure 7 shows the residual shear strength versus the global void ratio and fine content. It is clear from this that the undrained shear strength decreases linearly as the global void ratio decreases and fine content increases for all densities (Dr = 12, 50, and 90% before consolidation) up to 30% fine content. It means that when decreasing the global void ratio and increasing the fine content, the undrained residual shear strength also decreases. In this case, it can be reported that the global void ratio does not represent the real behaviour of silty sand soil in the range of 030% fine content. Indeed, the global void ratio appears to be a parameter not as pertinent in sand-fines mixtures as in clean sands for characterizing the mechanical state of these materials. Beyond Fc = 30% the normalized undrained shear strength

Fig. 6. Residual Shear Strength versus Fine Content for Various Initial Relative Densities (σ 3’=100 kPa)

(2) (3)

where qs, ps’ and φ s indicate the deviator stress (σ 1’- σ 3’), the effective mean principal stress (σ 1’+ 2σ 3’)/3 and the mobilized angle of inter-particle friction at the Quasi-Steady State (QSS)

Fig. 7. Residual Shear Strength versus Global Void Ratio for Various Initial Relative Densities (σ 3’=100 kPa)

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KSCE Journal of Civil Engineering


continues to decrease almost linearly with increasing the global void ratio and the fine content for the initial relative densities (Dr = 12, 50 and 90%). Figure 8 shows the variation of residual shear strength (Sus) with the initial relative density (Dr measured before consolidation) at various fine contents. From this figure, it is clear that an increase in the relative density results in an increase in the residual shear strength at a given fine content. Thevanayagam et al. (1997) and Sitharam et al. (2004) report a similar behaviour of increasing residual shear strength with increasing relative density. The present laboratory study focuses on the effect of intergranular void ratio on the residual shear strength of sand-silt mixtures at various initial relative densities (Dr = 12, 50 and 90% before consolidation). It can be noticed from the results of this investigation that there is a significant decrease in the residual shear strength with an increase in the fine content or the intergranular void ratio, but there is a significant increase in the undrained residual shear strength with an increase in the initial relative density and a significant decrease of the residual strength with the increase of the fine content. A good agreement of our results is achieved with the experimental work reported by Ishihara (1993) on Tia Juana silty sand, Baziar and Dobry (1995) on silty sands retrieved from the Lower San Fernando Dam, and by Naeini and Baziar (2004) on Adebil sand mixed with different amounts of fines.

Fig. 8. Residual Shear Strength versus Relative Density for Various Fine Contents (σ 3’=100 kPa)

Figure 9 shows the normalized residual shear strength versus the intergranular void ratio. It seems that the variation of the residual strength due to the amount of fines is directly related to the intergranular void ratio in the range 0-50% fine content. In this case, the behaviour of sand-silt mixture samples is influenced by the contacts of coarser grains, which is solely quantified by the intergranular void ratio. By increasing the fine content in the range of 0-50%, the contact between sand grains decreases and therefore the intergranular void ratio increases and the residual shear strength decreases. Hence, in the range of 0-50% fine content, it can be proposed that the residual shear strength decreases linearly with the increase of the intergranular void ratio. In this case, the following expressions are suggested to evaluate the residual shear strength which is dependent on the intergranular void ratio (es) and on the initial effective confining pressure (σ c’) for the range of 0 to 50% fine content in normally consolidated undrained triaxial compression tests: Sus /σ c’ = 0.22 - 0.056(es) Sus /σ c’ = 0.22 - 0.046(es) Sus /σ c’ = 0.23 - 0.049(es)

for Dr = 12% for Dr = 50% for Dr = 90%

4. Cyclic Test Results 4.1 Compression-extension Tests Three series of stress-controlled cyclic triaxial tests (Table 3) were performed on isotropically consolidated soil specimens with different fine contents ranging from 0 to 40% and alternated symmetric deviator stress under undrained conditions to simulate essentially undrained field conditions during earthquakes. In the entire test program, a frequency of 0.5 Hz was maintained. In the first one, three alternated cyclic tests were experimented on clean sand samples (es = 0.695) with a relative density of 50% and an initial confining pressure of 100 kPa. The used cycles amplitudes (qm) were respectively 30, 50 and 70 kPa. The tests of the second one were realized on the mixture sand-silt samples with an intergranular void ratio of 0.817 and loading amplitudes of 30, 40 and 60 kPa. Samples with an intergranular void ratio of 1.680 and applied loading amplitudes of 20, 30 and 50 kPa were Table 3. Summary of Cyclic Triaxial Tests Results (Inter-granular void ratio effect)

Fig. 9. Normalized Residual Shear Strength versus Intergranular Void Ratio for Various Initial Relative Densities (σ 3’=100 kPa) Vol. 15, No. 8 / November 2011

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Relative Density Dr (%)

Fine Content Fc (%)

CSR

Number of Cycles Nc

0

0.15

158

0

0.25

12

N

Test

1

CUM3C

50

2

CUM2C

50

2

CUM1C

50

0

0.35

5

4

CUM4C

50

10

0.15

27

5

CUM5C

50

10

0.20

7

6

CUM6C

50

10

0.30

2

7

CUM7C

50

40

0.10

27

8

CUM8C

50

40

0.15

4

9

CUM9C

50

40

0.25

1


employed in the third test series. From the results obtained, it can be noticed that the intergranular void ratio affects considerably the liquefaction of the samples. Figure 10 illustrates the results of the test carried out on clean sand samples (es = 0.695) with a loading amplitude of 30 kPa. It is clear from this Figure that the pore water pressure increases during the cycles resulting in a reduction of the average effective pressure. The rate of an increase in the pore pressure remains low, because liquefaction was obtained only after 158 cycles; for the test with qm = 30 kPa and an intergranular void ratio of 0.817, we notice a significant increase in the pore water pressure during 27th cycle with a significant development of the axial strain (2.5%) leading to the liquefaction of the sample. For the applied loading amplitude of 30 kPa and an intergranular void ratio of 1.680, it can be noticed a significant increase in the pore water pressure during the 3rd cycle with a development of the axial strain leading to the liquefaction of the sample to the 4th cycle. This shows that the increase of the intergranular void ratio parameter in the range of (0.695 − 1.680) accelerates the potential

of liquefaction. 4.2 Influence of the Intergranular Void Ratio on the Potential of Liquefaction Figures 11(a) and 11(b) show the variation of the Cyclic Stress Ratio (CSR = qmax/2σ c’) and the cyclic liquefaction resistance (CLR) with the number of cycles (Nc) and the intergranular void ratio (es) respectively. According to Ishihara (1993), the Cyclic Liquefaction Resistance (CLR) is defined as the cyclic stress ratio causing the liquefaction at 15 cycles. We notice that liquefaction potential of the sand-silt mixtures decreases with the increase of the intergranular void ratio. These results are in good concordance with those found in monotonic tests showing that the increase in the intergranular void ratio amplifies the phase of contractancy. Consequently, the amplification of the contractancy phase induced a reduction in the liquefaction potential when the intergranular void ratio (es) increases. For the sand-silt mixtures, the variation of the intergranular void ratio amplifies the phase of contractancy resulting in a significant decline of the liquefaction

Fig. 10. Cyclic Test on Clean Sand (es = 0.695, e = 0.695, Dr = 50%, CSR = 0.15, σ 3’=100 kPa): (a) Stress-strain Response, (b) Pore Pressure-strain Response, (c) Stress Path Curve

Fig. 11. Effect of the Intergranular Void Ratio on the Liquefaction Potential of the Sand-silt Mixtures (σ 3’=100 kPa, Dr = 50%): (a) Stress Ratio-number of Cycles, (b) Cyclic Liquefaction-intergranular Void Ratio, (c) Number of Cycles-intergranular Void Ratio − 1340 −



potential. It should be noted that for the studied amplitude (qm= 30 kPa), the increase of the intergranular void ratio in the range of 0-50% fine content induces an acceleration of liquefaction phenomenon. Fig. 11(c) shows the cycles of loading till the liquefaction versus the intergranular void ratio. Again, we notice that the liquefaction resistance decreases with the increase of the intergranular void ratio. The samples sheared under higher loading level (CSR = 0.25) are more vulnerable to liquefaction than those sheared with smaller loading level (CSR = 0.15). 4.3 Effect of the Relative Density Undrained cyclic tests (Table 4) were performed on Chlef sand-silt mixture (Fc = 5%) for three initial relative densities (Dr = 12, 45 and 60% before consolidation). For each density, the loading amplitude has been changed in order to derive the liquefaction potential curve. The tests were undertaken out for the amplitudes qm = 30, 50 and 70 kPa. We note that the liquefaction was reached quickly for the higher amplitudes: after two and three cycles respectively for the loading amplitudes of qm = 70 and 50 kPa, whereas, the liquefaction under the loading amplitude qm = 30 kPa for the tests carried out with a relative density Dr = 12% (measured before consolidation) was reached at 24 Table 4. Summary of Cyclic Triaxial Tests Results (Relative density effect) Relative Density Dr (%)

Fine Content Fc (%)

CSR

Number of Cycles Nc

5

0.15

167

5

0.25

13

5

0.35

5

45

5

0.15

71

45

5

0.25

5

CUM15

45

5

0.35

3

7

CUM16

12

5

0.15

24

8

CUM17

12

5

0.25

3

9

CUM18

12

5

0.35

2

N

Test

1

CUM10

60

2

CUM11

60

2

CUM12

60

4

CUM13

5

CUM14

6

cycles; for the same loading amplitudes the liquefaction was obtained only after 3, 5 and 71 cycles for the tests with a relative density Dr = 45% (measured before consolidation). For the tests established with a relative density Dr = 60% (measured before consolidation) and for the same loading amplitudes, the liquefaction was reached at 5, 13 and 167 cycles respectively. Figure 12 summarizes the results of all these tests. Fig. 12(a) illustrates the influence of the relative density (measured before consolidation) on the liquefaction potential of sand-silt mixture (Fc = 5%). It shows clearly that the increase in the initial relative density leads to an increase in the liquefaction resistance of the sand-silt mixture. Fig. 12(c) shows the influence of the intergranular void ratio on the liquefaction resistance defined by the amplitude of the loading inducing liquefaction after 15 cycles for the Chlef sand-silt mixture. This figure shows clearly that the liquefaction resistance decreases with the increase in the intergranular void ratio and the loading amplitude. We note that the reduction in the liquefaction resistance of sand-silt mixture becomes very pronounced for the smaller cyclic stress ratios CSR = 0.15 and 0.25.

5. Conclusions The effect of the intergranular void ratio and other parameters on the undrained residual shear strength and liquefaction potential of sand-silt mixtures was investigated in this laboratory study. In the light of the experimental evidence, the following conclusions can be drawn: Undrained monotonic triaxial tests on loose, medium dense and dense silty sand indicate clearly that a correlation between the undrained residual shear strength (Sus) and the intergranular void ratio (es) can be obtained. For the range 0-50% of the fine content, it is shown that the undrained residual shear strength decreases linearly with the increase of intergranular void ratio. In fact, when the fine content is increased in the range of 0-50%, all the fine-grained soil mostly fills the voids among the sand-grain matrix and the contact between sand grains decreases and thus

Fig. 12. Effect of the Initial Relative Density on the Liquefaction Resistance of the Sand-silt Mixture (σ 3’=100 kPa, Fc = 5%): (a) Stress Ratio-number of Cycles, (b) Number of Cycles-relative Density, (c) Number of Cycles-intergranular Void Ratio Vol. 15, No. 8 / November 2011

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the intergranular void ratio increases. As the fine content increases, the undrained residual shear strength decreases up to 50% fine content. Undrained cyclic test results show that the liquefaction potential of the sand-silt mixtures decreases with an increase in the intergranular void ratio (fine content) and the samples sheared with higher level loading (CSR = 0.25) are more vulnerable to liquefaction than those sheared with smaller loading level (CSR = 0.15). It can also be concluded that the liquefaction resistance increases with the relative density but it decreases with the increase in the intergranular void ratio and the loading amplitude. We note that the reduction in the liquefaction resistance of Chlef sand-silt mixture becomes very pronounced for the smaller cyclic stress ratios CSR = 0.15 and 0.25. The cyclic test results are in good agreement with those found in monotonic tests. Indeed, for the different tested sand-silt mixtures, the intergranular void ratio amplifies the phase of contractancy resulting in a significant reduction of the liquefaction resistance. It should be noted that for the loading amplitude (qm= 30 kPa), the increase of the intergranular void ratio in the range of 0-40% fine content accelerates the liquefaction phenomenon of the studied sand-silt mixtures.

References Amini, F. and Qi, G. Z. (2000). “Liquefaction testing of stratified silty sands.” J. of Geotech. Geoenviron. Eng., Proc. ASCE, Vol. 126, No. 3, pp. 208-217. Baziar, M. H. and Dobry, R. (1995). “Residual strength and largedeformation potential of loose silty sands.” J. of Geotech. Eng., ASCE, Vol. 121, pp. 896-906. Chu, J. and Leong, W. K. (2002). “Effect of fines on instability behaviour of loose sand.” Geotechnique, Vol. 52, No. 10, pp. 751755. Dobry, R.., Ladd, R. S., Yokel, F. Y., Chung, R. M., and Powell, D. (1982). Prediction of pore pressure build up and liquefaction of sands during earthquake by cyclic strain method, National Bureau of Standards, N.B.S. Building Science Series 138. Ishihara, K. (1993). “Liquefaction of natural deposits during earthquakes.” Proc. 11th ICSMFE, San Francisco, Vol. 1, pp. 321-376. Ishihara, K. (1993). “Liquefaction and flow failure during earthquakes.” Geotechnique, Vol. 43, No. 3, pp. 351-415. Kenny T. C. (1977). “Residual strengths of mineral mixtures.” Proc., 9th Int. Conf. Soil Mech. And Found. Eng., Tokyo, Vol. 1, pp. 155-160. Kishida, H. (1969). “Characteristics of liquefied sands during MinoOwari, Tohnankai and Fukui earthquakes”. Soils and Foundations, Vol. 9, No. 1, pp. 75-92. Kuerbis, R., Nagussey, D., and Vaid, Y. P. (1988). “Effect of gradation and fines content on the undrained response of sand.” Proc. Hyd. Fill. Struc. Geotech. Spec. Publ. 21. ASCE, New York, pp. 330-345. Ladd, R. S. (1978). “Preparing test specimen using under compaction.” Geotech. Testing J., GTJODJ, Vol. 1, No. 1, pp. 16-23. Lade, P. V. and Yamamuro, J. A. (1997). “Effects of non-plastic fines on

static liquefaction of sands.” Canadian Geotech. J., Vol. 34, No. 6, pp. 918-928. Mitchell, J. K. (1993). Fundamental of soil behaviour, John Wiley Interscience, New York; 2nd ed. Monkul, M. M. (2005). Influence of inter-granular void ratio on one dimensional compression, M. Sc. Thesis, Dokuz Eylul University, Izmir, Turkey. Naeini, S. A. (2001). The influence of silt presence and sample preparation on liquefaction potential of silty sands, PhD Dissertation, Iran University of Science and Technology, Tehran. Naeini, S. A. and Baziar, M. H. (2004). “Effect of fines content on steady-state strength of mixed and layered samples of a sand.” Soil Dyna. and Earth. Eng., No. 0024, No. 3, pp. 181-187. Seed, H. B. and Idriss, I. M. (1981). Evaluation of liquefaction potential of sand deposits based on observations and performance in previous earthquakes, Pre-print No. 81-544, In Situ Testing to Evaluate Liquefaction Susceptibility, ASCE Annual Convention, St. Louis. Seed, H. B., Idriss, I. M., and Arango, I. (1983). “Evaluation of liquefaction potential using field performance data.” Journal of Geotechnical Engineering Division, ASCE, Vol. 109, No. 3, pp. 458-482. Shen, C. K., Vrymoed, J. L., and Uyeno, C. K. (1977) “The effects of fines on liquefaction of sands.” Proc. 9th Int. Conf. Soil Mech. and Found. Eng, Tokyo, Vol. 2, pp. 381-385. Sitharam, T. G., Govinda Raju, L., and Srinivasa Murthy, B. R. (2004). Cyclic and monotonic undrained shear response of silty sand from Bhuj region in India, ISET J. of Earthquake Technology. Paper No 450, Vol. 41, Nos. 2-4, June-December, pp. 249-260. Thevanayagam, S. (1997). “Dielectric dispersion of porous media as a fractal phenomenon.” J. of Applied Physics, Vol. 82, No. 5, pp. 2538-2547. Thevanayagam, S. (1998). “Effect of fines and confining stress on undrained shear strength of silty sands.” J. Geotech. Geoenviron. Eng. Div., ASCE, Vol. 124, No. 6, pp. 479-491. Thevanayagam, S. and Mohan, S. (2000). “Inter-granular state variables and stress-strain behaviour of silty sands.” Geotechnique, Vol. 50, No. 1, pp. 1-23. Thevanayagam, S. and Nesarajah, S. (1998). “Fractal model for flow through saturated soil.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 124, No. 1, pp. 53-66. Thevanayagam, S., Ravishankar, K., and Mohan, S. (1997). “Effects of fines on monotonic undrained shear strength of sandy soils.” ASTM Geotech Testing J., Vol. 20, No. 1, pp. 394-406. Troncosco, J. H. and Verdugo, R. (1985) “Silt content and dynamic behaviour of tailing sands.” Proc., 12th Int. Conf. on Soil Mech. and Found. Eng., San Francisco, pp. 1311-1314. Yamamuro, J. A. and Lade, P. V. (1998). “Steady-state concepts and static liquefaction of silty sands.” J. of Geotech. and Geoenvir. Eng., ASCE, Vol. 124, No. 9, pp. 868-877. Youd, T. L. and Idriss, I. M. (2001). “Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils.” Journal of Geotechnical and Geo-environmental Engineering, ASCE, Vol. 127, No. 10, pp. 297-313. Zlatovic, S. and Ishihara, K. (1995). “On the influence of non-plastic fines on residual strength.” Proc. of the first Int. Conf. on Earthquake Geotech. Eng. Tokyo, pp. 14-16.

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