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Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil
Ain Shams Engineering Journal (2014) xxx, xxx–xxx
Ain Shams University
Ain Shams Engineering Journal www.elsevier.com/locate/asej www.sciencedirect.com
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil Kamal Mohamed Hafez Ismail Ibrahim
*
Civil Engineering Dep., Suez Canal University, Egypt Received 29 May 2014; revised 1 October 2014; accepted 17 October 2014
KEYWORDS Gravel soil; Unsaturated shear strength; Soil water characteristic curve; Hysteresis; Fines and soil plasticity
Abstract Low plastic fines in gravel soils affect its unsaturated shear strength due to the contribution of matric suction that arises in micro and macro pores found within and between aggregates. The shear strength of five different types of prepared gravel soils is measured and is compared with a theoretical model (Fredlund et al., 1978) to predict the unsaturated shear strength. The results are consistent to a great extent except the case of dry clayey gravel soil. It is also found that on inundation of gravel soils containing plastic fines greater than 12% a considerable reduction in both the strength and the stiffness modulus is noticed. This 12% percentage is close to the accepted 15% percentage of fines given by ASTM D4318 (American society for testing material). The angle of internal friction that arises due to matric suction decreases with the increase of degree of saturation of soil. The hysteresis of some tested gravel soils is measured and found that it increases by increasing the percentage of fines. Ó 2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The unsaturated shear strength of soil is greater than the saturated strength due to increase in soil shear parameters as a result of rise in total suction. Total suction equals the sum of matric and osmotic suction. Osmotic suction is due to difference in pore water salt concentration within the soil while matric suction get rise due to capillarity action of micro and macro-pores in compacted soil. Total suction equals matric * Tel.: +20 1001525472. E-mail address: [email protected]. Peer review under responsibility of Ain Shams University.
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suction in case there is homogeneity in pore water salt concentration. The soil is often unsaturated and pore moisture stability takes place where no moisture flow or flux exists and when soil water content becomes constant with time. The shear strength equation of unsaturated soil proposed by Fredlund et al. [1] is as follows: 0
b
sf ¼ c þ ðrn ua Þ tan ; þ ðua uw Þ tan ;
ð1Þ
The shear parameters c, / and /b in the previous equation are determined from locating the shear envelope of unsaturated tested soil drawn in three axis (sf, rn ua and ua uw). c and / are the intercept and slope of shear envelope with respect to sf and rn ua axis while /b is the slope of shear envelope with respect to sf and ua uw axis. Modified direct shear box or modified triaxial cell is adapted to measure the shear strength of unsaturated soil at controlled suction.
http://dx.doi.org/10.1016/j.asej.2014.10.012 2090-4479 Ó 2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
2
K.M.H. Ismail Ibrahim
Nomenclature
/b uw ua (uauw) (rnua) hw hr hs OMC PI LL P200
effective cohesion of the soil effective angle of shearing resistance for saturated soil angle of internal friction with respect to the matric suction pore water pressure pore air pressure matric suction net normal stress volumetric water content residual water content saturated water content at zero suction optimum moisture water content plasticity index in % liquid limit % passing U.S. sieve # 200
Vanapalli et al. [2] emphasize that the soil water characteristic curve is closely related to the shear strength of unsaturated soil. Fredlund et al. [3] introduced the following empirical equation 0 0 hw hr ð2Þ sf ¼ c þ ðrn ua Þ tan ; þ ðua uw Þ tan ; hs hr The relationship between s and (ua uw) is assumed to be linear. Escario and Juca [4] determined that this relationship is actually non-linear. Later several other researchers observed a non-linear relationship between apparent cohesion (intercept of shear envelope with shear stress axis at zero normal stress) and matric suction (Fredlund et al. [5], Wheeler [6], Ridley [7], Ridley et al. [8]). Modified direct shear box and triaxial cells using axis translation techniques are examples of modified shear devices which can control soil suction. The friction angle decreases with increasing the size of direct shear box size and that is consistent with the decrease in friction angle with the increase in footing size found in model and prototype scale foundation tests (Amy and Alan [9]). According to ASTM D 3080-90, the direct shear box test has several particle-sizes to box-size requirements when preparing specimens for testing. It is recommended that the minimum specimen width should not be less than ten times the maximum particle-size diameter and the minimum initial specimen thickness should not be less than six times the maximum particle diameter. The minimum specimen width-to thickness ratio should be 2 to 1. Other works in the literature are much stricter on the particle-size to box-size requirement. Jewell and Wroth [10] suggest a ratio of shear box length to average particle size in the range of 50 to 300. Soil suction can be determined using various techniques. The filter paper method was developed in Europe in 1920 and was transferred to the United States in 1937 by Gardner [11]. The method requires a calibration for suction versus water content relationship of the filter paper. Mcqueen and Miller [12] introduced the calibration curve shown in Fig. 1 for filter paper water content versus suction. These curves convert the filter paper (Whatman 42 type) water content values to suction values.
gravimetric water content (ratio between the weight of water and weight of solids) e void ratio of soil Gs specific gravity of soil h matric suction = ua uw hr residual suction (the suction below which there is no free pore water (see residual condition in Fig. 2) SWCC soil water characteristic curve C(h) an adjustment factor which forces the SWCC to reach zero water content at high suction values 106 kPa (dry soil condition) af, bf and cf fitting parameters for SWCC GI group index of soil w soil water content wa air entry value w(h)
100000 10000
Suction (kPa)
c 0 /
1000 100 10 1 0
1
2
3
4
5
6
7
8
9
10
Filter paper water content (%)
Figure 1 Calibration curve for filter paper water content versus soil suction (Mcqueen and Miller [12]).
Figure 2 Illustration of the in situ zones of de-saturation defined by a SWCC (after Fredlund) [3].
Basically, the filter paper comes to equilibrium if sealed with the soil either through vapor (total suction measurement) equilibrium or through liquid contact (matric suction measurement) equilibrium. At equilibrium (water content of filter paper gets constant with time), the suction value of the filter paper and the soil will be equal. The filter paper water content is measured. By using the calibration curve of filter paper water content versus suction, the corresponding soil suction
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil value is found. This is the basic approach suggested by ASTM Standard test method for measurement of soil potential (suction) using filter Paper (ASTM D 5298). The soil water characteristic curve (SWCC) defines the relationship between the amount of water in the soil (gravimetric water content) or the volumetric water content and the soil suction. The air entry value (suction at water content just below full saturation where air begins to enter soil pores), inflection point and the residual suction (suction corresponding to absence of free water in soil) characterize the SWCC (Fig. 2). For clayey and sandy soils at the same water contents, the matric suction of the former is much bigger than the latter one, and the reason returns to clay particles that are smaller; increase in surface area; more number in micro-pore. In addition, the composition of mineral of the soil has great influence on water characteristic curves, which can be found on its affinity to water, Li Dan et al. [13]. Fredlund and Rahardjo [14] defined the volumetric water content (hw) as the ratio of volume of water, Vw, to the total volume, V, of the soil. The volumetric water content can also be expressed in terms of specific gravity, Gs, void ratio, e, and water content. wðhÞGs hw ¼ 1þe
ð3Þ
Hysteresis in the SWCCs indicates that the volumetric water content in the soil is not unique at a specific matric suction value but is related to the wetting and drying history of the soil. Total hysteresis is computed as the area between the drying and wetting SWCCs drawn on a logarithm scale (Hong Yang et al. [15]). The wetting SWCC can be obtained using a capillary rise open tube (Lambe and Whitman [16], Fredlund and Rahardjo [17]). Fredlund and Xing [18] found the SWCC which fits the experimental data is as follows hs
hw ¼ CðhÞ bf cf : ln exp ð1Þ þ ahf 3 ln 1 þ hhr 5 CðhÞ ¼ 41 6 ln 1 þ 10hr
ð4Þ
2
ð5Þ
Zapata [19] and Zapata et al. [20] developed an experimental correlation to allocate the SWCC for plastic soils (w PI > 0), these correlations are as follows
and in soil replacement, so a great need is required to study the unsaturated shear strength of gravel soils prepared with different proportions of low plastic fines. Plastic fines more than stated in specification requirements may cause reduction of strength and settlement problems on inundation. 2. Tested materials The prepared tested soil consists of different types of gravel soils; clean well graded gravel ‘‘GW’’, well graded with silt ‘‘GW-GM’’, well graded gravel with clay ‘‘GW-GC’’, silty gravel ‘‘GW’’ and clayey gravel ‘‘GC’’. Fig. 3 shows the grain size distribution for the tested soils. The gravel size is less than 10 mm. The fines are classified as low plastic fines according to plasticity chart. Table 1 shows the properties of different tested gravel soils. AASHTO M145-91, standard specification for classification of soils and soil-aggregate mixtures for high way construction defines the group index of soil (GI). GI ¼ ðP200 -35Þ½0:2 þ 0:005ðLL-40Þ þ 0:01ðP200 -15ÞðPI-10Þ
ð7Þ
For silty gravel (GM) or clay gravel (GC) GI ¼ 0:01ðP200 -15ÞðPI-10Þ
ð8Þ
Fig. 4 shows the modified proctor compaction curve for the tested soil. It is noticed that the optimum moisture content increases with the increase in percentage of fines. The optimum moisture content (O.M.C.) is 4%, 7.9%, 10%, 12% and 18% for GW, GW-GM, GW-GC, GM and GC soils while the maximum dry densities are 2.1, 2.15, 2.17, 2.18 and 2.13 t/m3 respectively. The soil suction intensity at wet of optimum goes to zero while it increases gradually in direction of dry of optimum zone. 3. Experimental program A series of five different types of soils ‘‘GC, GM, GW-GC, GW-GM and GW’’ are compacted and prepared at dry, and wet of optimum water content. It is required to determine the SWCC of prepared soils, studying the effect of hysteresis, measuring there shear parameters (c, / and /b) and unsaturated shear strength and verifying values with Fredlund et al. [1] model. Also it is required to test the effect of inundation on soil shear strength and its initial shear modulus. The filter
af ¼ 32:835 ln ðw PIÞ þ 32:438
100
bf ¼ 1:42 ðw PIÞ0:3185 Cf ¼ 0:2154 ln ðw PIÞ þ 0:7145 hr ¼ 500
90 80 70
ð6Þ
The ASTM D4318 specifies that for accepted gravel soil utilized in earth works and highways, the maximum permissible percentage of fines (P200) is 15% and liquid limit (L.L.) not exceeding 25 and plasticity index (PI.) not exceeding 5. This will be checked later on in the paper. Since gravel soil had been applied on large scale in many earth works such as sub-base soil in pavements, earth dams
% Passing
PI P200 w PI ¼ 100
3
GW
60
GW-GC
50
GW-GM
40
GC
30
GM
20 10 0 100
10
1
0.1
0.01
0.001
0.0001
Diameter (mm)
Figure 3
The grain-size distribution curve of the tested soils.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
4
K.M.H. Ismail Ibrahim Properties of tested gravel.
Table 1 Soil type
Texture
GW GW–GM GW–GC GM GC
%G
%S
%M
%C
55 45 50 55 59
40 44 40 38 20
5 11 0 17 2
– – 10 – 19
Cu
Cc
G.I
L.L.
P.L.
P.I.
w PI
40 100 70 5.8 1000
1.6 2.7 2.05 13.7 22.5
0 0 0 0 0.6
35 35 40 35 40
25 25 20 25 20
10 10 20 10 20
0.5 1.1 2.0 1.7 4.2
0.4
2.2
0.35
2.15 GW
2.1
GW-GM GW-GC
2.05
GM
2
GC
1.95 1.9 0
5
10
15
20
Volumetric water content ( θw )
γd (t/m3)
25
GW GW-GC
0.2
GW-GM GC
0.15
GM
0.1 0.05 0 0.01
wc %
Figure 4
0.3 0.25
1
100 10000 Soil suction (kPa)
1000000
Compaction curve of tested soils. Figure 6
SWCC of tested soils.
Table 2 Fredlund and Xing fitting parameters of SWCC for the tested soils. Soil type
af
bf
cf
hr
hs
GW GW-GC GW-GM GC GM
0.03 55.2 35.56 79.56 49.86
– 1.14 1.38 0.89 1.2
– 0.57 0.69 0.41 0.60
9 500 500 700 600
0.29 0.31 0.30 0.33 0.32
Figure 5 Shear box diagram with the system implemented to prevent evaporation. Volumetric water content (θ)
paper method was applied to measure the soil suction while the modified shear box was used to measure the shear strength and the unsaturated shear parameters. Fig. 5 shows a conventional shear box but it is modified to control suction by fixing the soil water content using a tight polyethylene bag to prevent water evaporation and to achieve vapor equilibrium during shearing (Gan [21]). The final water content of embedded filter paper in the soil is measured at the end of shearing where soil suction can be determined using the calibration curve (Fig. 2).
0.35 0.3 0.25 GW-drying
0.2
GW-wetting GC-drying
0.15
GC-wetting
0.1 0.05 0 0.01
1
100
10000
1000000
Soil suction (kPa)
Figure 7
Hysteresis curves for GW and GC soils.
4. Results and discussions Fig. 6 shows the SWCC of tested soils represented by volumetric water content with respect to soil suction. The saturated volumetric water content ‘‘hs’’ is 0.28, 0.29, 0.3, 0.33 and 0.34 for ‘‘GW’’, ‘‘GW-GM’’, ‘‘GW-GC’’, ‘‘GM’’ and ‘‘GC’’ soils respectively. Increasing w PI increases ‘‘hs’’, air entry value ‘‘wa’’, inflection point and increases the final dry soil
suction which is about 40 kPa in case of ‘‘GW’’ soil and reaches 1E6 kPa in case of ‘‘GC’’ soil. The soil–water characteristic curve fitting parameters of the tested soils are shown in Table 2. Fig. 7 shows soil hysteresis of two soils has extreme difference in percentage of fines ‘‘GW’’ and ‘‘GC’’ soil. The hysteresis is represented by SWCC in drying and wetting conditions.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil
τ (kPa)
5
φ b 45
40 35 30 25 20 15 10 5 0 0.01
2500
wc = 3%, σn = 200 kPa
2000
GW GW-GM
1500
GW-GC GM
1000
GC
500 0 0
0.05
0.1
0.2 ε %
0.15
Figure 8 Shear stress–strain distribution of tested soil at wc = 3% (dry of optimum).
It can also be represented by the area confined between drying and wetting SWCC. It is noticed that hysteresis in case of ‘‘GC’’ soil is greater than ‘‘GW’’ soil which means that increasing the percentage of fines increases the soil hysteresis. Fig. 8 show the shear stress–strain relationship for soils prepared at O.M.C then dried and sheared at wc equals to 3% while in Fig. 9 the prepared soils are sheared directly at O.M.C. The vertical applied stress is 200 kPa in both cases. The saturated cohesion (C) of all gravel soils is zero. In Fig. 8 the maximum shear strength is about 2700, 1470, 740, 170 and 166 kPa for GC, GW-GC, GM, GW-GM and GW soils respectively. The increase in strength of GC soil than GW soil returns to the contribution of higher suction induced in GC soil than GW soil. In Fig. 9 the GW soil has the highest strength about 162 kPa while ‘‘GC’’ has the lowest strength about 93 kPa because in both cases at O.M.C, soil suction is a trivial value and has no contribution in increasing the shear strength. To calculate the soil suction at O.M.C. determine e (void ratio) = 0.3, find wc at O.M.C from Fig. 4, calculate hw from Eq. (3), and get w from Fig. 6 it is found to be a trivial value). The presences of fines cause drop to the unsaturated dry strength on inundation. Fig. 10 shows the distribution of angle of shear resistance /b of tested soils with respect to soil suction. The angle of shear resistance /b of soil can be determined using the relation: Tan /b = Tan / * Sr (Vanapalli and Fredlund [22]), where Sr is the degree of saturation of soil. It is noticed that in case of ‘‘GW’’ soil the reduction in /b is steep due to limited percentage of fines. The angle of shear
GC GW GW-GC GW-GM GM
1
100
10000
1000000 ψ (kPa)
Figure 10 Angle of internal friction that arises due to matric suction (w).
resistance with respect to matric suction /b equals to the saturated angle of shear resistance 39° at full saturation (zero matric suction) and reaches about zero on complete dryness. In case of ‘‘GC’’ soil, /b equals 26° at zero suction and goes to zero at high matric value (zero water content). The reduction in /b on increasing soil suction is similar in shape to SWCC of soil. Fig. 11 shows the distribution of (Ds) which is the difference between unsaturated and saturated soil shear strength with respect soil suction. Clayey gravel ‘‘GC’’ recorded the highest difference (increase in unsaturated strength) at dry condition where Ds equals to 1210 kPa due to contribution of high suction of low plastic clay fines, while ‘‘GM’’ soil has less increase in shear strength about 1040 kPa due to contribution of lower suction of low plastic silt fines. In case of ‘‘GW’’ soil the contribution of soil suction in unsaturated shear strength is negligible due to limited percentage of fines which is less than 5%. Figs. 12 and 13 show a comparison between measured and predicted (Fredlund et al. [1] model given by Eq. (1)) unsaturated shear strength of tested samples prepared at dry of optimum wc = 3% and at O.M.C. respectively. The vertical applied stress is 200 kPa. Fig. 12 shows that the values are consistent to a great extent except the case of ‘‘GC’’ soil, the predicted unsaturated shear strength is (6180 kPa) considerably higher than the measured (2600 kPa) value. The deviation between both values may return to expected error in estimation of the high suction of ‘‘GC’’ soil that is induced at wc = 3%, also due to the presence of internal random cracks in clay fraction at that low water content which is not considered by the model.
τ (kPa) 180 160
σn = 200 kPa
Δτ (kPa) 1400
140 120
1200
100
1000
80
800
GC GW GW-GC
GW, O.M.C.=4% GW-GM, O.M.C.=8% GW-GC, O.M.C.=10% GM, O.M.C.=11.9% GC, O.M.C=18.8%
60 40 20 0 0
0.02
0.04
0.06
0.08
0.1
0.12
ε%
0.14
Figure 9 Shear stress–strain distribution of tested soils at optimum moisture content.
600
GW-GM
400
GM
200 0 0.01
1
100
10000
1000000
ψ (kPa)
Figure 11 The increase in saturated shear strength of tested soil with respect to matric suction.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
6
K.M.H. Ismail Ibrahim τ (kPa) 7000
means that partial dry ‘‘GC’’ soil at water content 3% will settle 13 times its settlement if it becomes saturated. In case of ‘‘GM’’ soil the shear modulus increases from 17,272 to 72,727 kPa at O.M.C and at wc = 3%; i.e. it increases about 4 times. So the increase in percentage of fines reduces the initial partial dry soil shear modulus on inundation and consequently increases the expected settlement. Also the type of fines has a considerable effect on reducing the initial shear modulus of soil if it becomes saturated; clay fines present in ‘‘GC’’ reduces the partial dry shear modulus than silt fines present in ‘‘GM’’.
Measured
6000
Fredlund et al [1]
5000 wc = 3% 4000 3000 2000 1000 0 GW
GW-GM
GW-GC
GM
GC
Figure 12 Measured and predicted unsaturated shear strength for tested samples prepared at wc 3%. τ (kPa)
180 wc = O.M.C.
160
Measured Fredlund et al [1]
140 120 100 80 60 40 20 0 GW
GW-GM
GW-GC
GM
GC
Figure 13 Measured and predicted unsaturated shear strength for tested samples prepared at O.M.C.
Comparing the results in Fig. 13, it can be noticed that they are relatively close because at O.M.C. the soil suction is low and its contribution in increasing the unsaturated shear strength is negligible, also the unsaturated shear strength of ‘‘GC’’ soil is relatively low because the saturated angle of internal friction is about 27° as shown in Fig. 10. Fig. 14 shows the value of initial shear modulus (ratio between initial shear stress and initial shear strain) of the tested soils at O.M.C. and at wc = 3%. It is noticed that the shear modulus of ‘‘GW’’ soil is not affected by the change in soil wc due to the limited percentage of fines 5%; it is 43636 and 45,454 kPa respectively. For ‘‘GC’’ soil the shear modulus is highly affected by the increase in soil suction; it increases from 13636 to 181818 kPa; i.e. it increases about 13 times, this
Soil shear mdulus (kPa)
200000
wc = optimum
180000
wc = 3%
160000 140000 120000
5. Conclusions Fredlund et al. [1] model can be applied to predict the unsaturated shear strength of tested gravel soils except the case of partial dry ‘‘GC’’ soil. The model overpredicts the unsaturated shear strength of ‘‘GC’’ soil as it does not consider the presence of random disconnected cracks and also due to error in either measuring or in real estimation of high suction values of SWCC at relatively low degrees of saturation. Dry and saturated shear strength of ‘‘GW’’ soil are approximately the same due to limited percentage of fines, but the dry shear strength of ‘‘GC’’ soil is considerably higher than that of ‘‘GW’’ due to matric suction contribution of fines in shear strength. On the other hand the saturated shear strength of ‘‘GC’’ soil is relatively less than ‘‘GW’’ soil due to loss in contribution of soil suction and also due to reduction in saturated angle of internal friction which is affected by the presence of clay fines. Soil hysteresis of gravel soil increases with the increase of percentage and plasticity of fines. The partial dry shear strength increases with the increase in percentage and in plasticity of fines. For percentage of fines less than 12% the saturated and unsaturated shear strength of gravel soils does not differ too much and this value is close to the maximum requirement (15% percentage of fines) given by ASTM D4318. The percentage of fines affects the saturated angle of internal friction /b that arises due to soil suction. The saturated friction angle is 39° for saturated ‘‘GW’’ and 34° for ‘‘GM’’ and 25° for ‘‘GC’’ soil. Also increasing the percentage of fines decreases the dry soil shear modulus on inundation about 4 times in case of ‘‘GM’’ soil and about 13 times in case of ‘‘GC’’ soil due to reduction in matric suction; i.e. increasing the expected soil settlement. At full saturation (zero soil suction) the angle of shear resistance with respect to suction /b equals to the saturated angle of internal friction ‘‘/0 ’’ and it decreases with the decrease of degree of soil saturation (increase of soil suction). In case of ‘‘GC’’ soil it is equal to 25° at full saturation and goes to zero at zero degree of saturation.
100000 80000 60000 40000
References
20000 0 GC
GM
GW-GC
GW-GM
GW
Figure 14 Initial shear modulus measured for tested granular soils at optimum and 3% water content.
[1] Fredlund DG, Morgenstern NR, Widger RA. The shear strength of unsaturated soils. Can Geotech J 1978;15(3):313–21. [2] Vanapalli SK, Fredlund DG, Pufahl DE, Clifton AW. Model for the prediction of shear strength with respect to soil suction. Can Geotech J 1996;33:379–92.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil [3] Fredlund DG, Xing A, Fredlund MD, Barbour SL. The relationship of the unsaturated soil shear strength to the soil– water characteristic curve. Can Geotech J 1995;33:440–8. [4] Escario V, Juca JFT. Strength and deformation of partly saturated soils. In: Balkema AA, editor, Proc 12th Int Conf on Soil Mechanics and Foundation Engineering. vol. 2, Bookfield, MA; 1989. p. 43–6. [5] Fredlund DG, Rahardjo H, Gan JKM. Non-linearity of strength envelope for unsaturated soils. In: Proc 6th Int Conf Expansive Soils. New Delhi; 1987. p. 49–54. [6] Wheeler SJ. An alternative framework for unsaturated soil behavior. Geotechnique 1991;41(2):257–61. [7] Ridley AM. Strength-suction-moisture content relationships for Kaolin under normal atmospheric conditions. In: Proc UNSAT, Paris; 1995, vol. 2, p. 645–51. [8] Ridley AM, Burland JB, Monroe AS. Unconfined compression tests with pore pressure measurements. In: Proc 11th African regional conference. SMFE. Cairo; 1995. [9] Cerato Amy B, Lutenegger Alan J. Specimen size and scale effects of direct shear box tests of sands. Geotech Test J 2006;29(6) [Paper ID GTJ100312]. [10] Jewell RA, Wroth CP. Direct shear tests on reinforced sand. Geotechnique 1987;37(1):53–68. [11] Gardner R. A method of measuring the capillary tension of soil moisture over a wide moisture range. Soil Sci 1937;43(4):277. [12] Mcqueen, Miller. The filter paper method for measuring soil suction. Fawcett & Collis-George; 1968. [13] Li Dan, Xiong Jun, Xue Qiang. The effect of different soil texture on SWCC of unsaturated soil. Flow in porous media, International forum on porous flow and applications, Wuhan city, China, April, 2009. [14] Fredlund DG, Rahardjo H. Soil mechanics for unsaturated soils. New York: John Wiley & Sons Inc.; 1993. [15] Yang Hong, Rahardjo Harianto, Leong Eng-Choon, Fredlund DG. Factors affecting drying characteristic curves of sandy soils. Can Geotech J 2004;41:908–20. [16] Lambe TW, Whitman RV. Soil mechanics, SI version. New York: John Wiley and Sons Inc.; 1979, p. 245–6.
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[17] Fredlund DG, Rahardjo H. The role of unsaturated soil behaviour in geotechnical engineering practice. In: Proceedings of the 11th Southeast Asian geotechnical conference, Singapore, Southeast Asian Geotechnical Society, Pathumthani, Thailand; 1993b. p. 37–49. [18] Fredlund DG, Xing A. Equation for the soil water characteristic curve. Can Geotech J 1994;31(3):521–32. [19] Zapata CE. Uncertainty in soil–water characteristic curve and impact on unsaturated shear strength predictions. Ph.D. dissertation, Arizona State University, Tempe, AZ, USA; 1999. [20] Zapata CE, Houston WN, Houston SL, Walsh KD. Soil–water characteristic curve variability. In: Proceedings of sessions of GeoDenver 2000, Denver ASCE Geo-Institute; 2000. p. 84–124. [21] Gan JKM. Direct shear strength testing of unsaturated soils. M.Sc. thesis, University of Saskatchewan, Saskatoon, Sask. [22] Vanapalli SK, Fredlund DG. Empirical procedures to predict the shear strength of unsaturated soils. Hong et al. editors, Eleventh Asian regional conference on soil mechanics and geotechnical engineering; 1999.
Kamal Mohamed Hafez Ismail Ibrahim is an Associate Professor in the Civil Department of Ismalia Faculty of Engineering, Suez Canal University, Egypt. He was born on January 1, 1963, in Cairo, Egypt. His research areas include seismic analysis of anchor-prestressed diaphragm walls, seismic displacement analysis of gravity retaining walls, properties of bentonite slurry grout, secant pile analysis, shear strength of consolidated clay, stabilisation of sand dunes with lime and silica fume, soil suction, heave and swelling pressure of expansive soil, settlement in tunnels, bearing capacity of granular soil underlain by soft clay, seismic displacement of gravity retaining walls and soil liquefaction.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Ain Shams University
Ain Shams Engineering Journal www.elsevier.com/locate/asej www.sciencedirect.com
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil Kamal Mohamed Hafez Ismail Ibrahim
*
Civil Engineering Dep., Suez Canal University, Egypt Received 29 May 2014; revised 1 October 2014; accepted 17 October 2014
KEYWORDS Gravel soil; Unsaturated shear strength; Soil water characteristic curve; Hysteresis; Fines and soil plasticity
Abstract Low plastic fines in gravel soils affect its unsaturated shear strength due to the contribution of matric suction that arises in micro and macro pores found within and between aggregates. The shear strength of five different types of prepared gravel soils is measured and is compared with a theoretical model (Fredlund et al., 1978) to predict the unsaturated shear strength. The results are consistent to a great extent except the case of dry clayey gravel soil. It is also found that on inundation of gravel soils containing plastic fines greater than 12% a considerable reduction in both the strength and the stiffness modulus is noticed. This 12% percentage is close to the accepted 15% percentage of fines given by ASTM D4318 (American society for testing material). The angle of internal friction that arises due to matric suction decreases with the increase of degree of saturation of soil. The hysteresis of some tested gravel soils is measured and found that it increases by increasing the percentage of fines. Ó 2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The unsaturated shear strength of soil is greater than the saturated strength due to increase in soil shear parameters as a result of rise in total suction. Total suction equals the sum of matric and osmotic suction. Osmotic suction is due to difference in pore water salt concentration within the soil while matric suction get rise due to capillarity action of micro and macro-pores in compacted soil. Total suction equals matric * Tel.: +20 1001525472. E-mail address: [email protected]. Peer review under responsibility of Ain Shams University.
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suction in case there is homogeneity in pore water salt concentration. The soil is often unsaturated and pore moisture stability takes place where no moisture flow or flux exists and when soil water content becomes constant with time. The shear strength equation of unsaturated soil proposed by Fredlund et al. [1] is as follows: 0
b
sf ¼ c þ ðrn ua Þ tan ; þ ðua uw Þ tan ;
ð1Þ
The shear parameters c, / and /b in the previous equation are determined from locating the shear envelope of unsaturated tested soil drawn in three axis (sf, rn ua and ua uw). c and / are the intercept and slope of shear envelope with respect to sf and rn ua axis while /b is the slope of shear envelope with respect to sf and ua uw axis. Modified direct shear box or modified triaxial cell is adapted to measure the shear strength of unsaturated soil at controlled suction.
http://dx.doi.org/10.1016/j.asej.2014.10.012 2090-4479 Ó 2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
2
K.M.H. Ismail Ibrahim
Nomenclature
/b uw ua (uauw) (rnua) hw hr hs OMC PI LL P200
effective cohesion of the soil effective angle of shearing resistance for saturated soil angle of internal friction with respect to the matric suction pore water pressure pore air pressure matric suction net normal stress volumetric water content residual water content saturated water content at zero suction optimum moisture water content plasticity index in % liquid limit % passing U.S. sieve # 200
Vanapalli et al. [2] emphasize that the soil water characteristic curve is closely related to the shear strength of unsaturated soil. Fredlund et al. [3] introduced the following empirical equation 0 0 hw hr ð2Þ sf ¼ c þ ðrn ua Þ tan ; þ ðua uw Þ tan ; hs hr The relationship between s and (ua uw) is assumed to be linear. Escario and Juca [4] determined that this relationship is actually non-linear. Later several other researchers observed a non-linear relationship between apparent cohesion (intercept of shear envelope with shear stress axis at zero normal stress) and matric suction (Fredlund et al. [5], Wheeler [6], Ridley [7], Ridley et al. [8]). Modified direct shear box and triaxial cells using axis translation techniques are examples of modified shear devices which can control soil suction. The friction angle decreases with increasing the size of direct shear box size and that is consistent with the decrease in friction angle with the increase in footing size found in model and prototype scale foundation tests (Amy and Alan [9]). According to ASTM D 3080-90, the direct shear box test has several particle-sizes to box-size requirements when preparing specimens for testing. It is recommended that the minimum specimen width should not be less than ten times the maximum particle-size diameter and the minimum initial specimen thickness should not be less than six times the maximum particle diameter. The minimum specimen width-to thickness ratio should be 2 to 1. Other works in the literature are much stricter on the particle-size to box-size requirement. Jewell and Wroth [10] suggest a ratio of shear box length to average particle size in the range of 50 to 300. Soil suction can be determined using various techniques. The filter paper method was developed in Europe in 1920 and was transferred to the United States in 1937 by Gardner [11]. The method requires a calibration for suction versus water content relationship of the filter paper. Mcqueen and Miller [12] introduced the calibration curve shown in Fig. 1 for filter paper water content versus suction. These curves convert the filter paper (Whatman 42 type) water content values to suction values.
gravimetric water content (ratio between the weight of water and weight of solids) e void ratio of soil Gs specific gravity of soil h matric suction = ua uw hr residual suction (the suction below which there is no free pore water (see residual condition in Fig. 2) SWCC soil water characteristic curve C(h) an adjustment factor which forces the SWCC to reach zero water content at high suction values 106 kPa (dry soil condition) af, bf and cf fitting parameters for SWCC GI group index of soil w soil water content wa air entry value w(h)
100000 10000
Suction (kPa)
c 0 /
1000 100 10 1 0
1
2
3
4
5
6
7
8
9
10
Filter paper water content (%)
Figure 1 Calibration curve for filter paper water content versus soil suction (Mcqueen and Miller [12]).
Figure 2 Illustration of the in situ zones of de-saturation defined by a SWCC (after Fredlund) [3].
Basically, the filter paper comes to equilibrium if sealed with the soil either through vapor (total suction measurement) equilibrium or through liquid contact (matric suction measurement) equilibrium. At equilibrium (water content of filter paper gets constant with time), the suction value of the filter paper and the soil will be equal. The filter paper water content is measured. By using the calibration curve of filter paper water content versus suction, the corresponding soil suction
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil value is found. This is the basic approach suggested by ASTM Standard test method for measurement of soil potential (suction) using filter Paper (ASTM D 5298). The soil water characteristic curve (SWCC) defines the relationship between the amount of water in the soil (gravimetric water content) or the volumetric water content and the soil suction. The air entry value (suction at water content just below full saturation where air begins to enter soil pores), inflection point and the residual suction (suction corresponding to absence of free water in soil) characterize the SWCC (Fig. 2). For clayey and sandy soils at the same water contents, the matric suction of the former is much bigger than the latter one, and the reason returns to clay particles that are smaller; increase in surface area; more number in micro-pore. In addition, the composition of mineral of the soil has great influence on water characteristic curves, which can be found on its affinity to water, Li Dan et al. [13]. Fredlund and Rahardjo [14] defined the volumetric water content (hw) as the ratio of volume of water, Vw, to the total volume, V, of the soil. The volumetric water content can also be expressed in terms of specific gravity, Gs, void ratio, e, and water content. wðhÞGs hw ¼ 1þe
ð3Þ
Hysteresis in the SWCCs indicates that the volumetric water content in the soil is not unique at a specific matric suction value but is related to the wetting and drying history of the soil. Total hysteresis is computed as the area between the drying and wetting SWCCs drawn on a logarithm scale (Hong Yang et al. [15]). The wetting SWCC can be obtained using a capillary rise open tube (Lambe and Whitman [16], Fredlund and Rahardjo [17]). Fredlund and Xing [18] found the SWCC which fits the experimental data is as follows hs
hw ¼ CðhÞ bf cf : ln exp ð1Þ þ ahf 3 ln 1 þ hhr 5 CðhÞ ¼ 41 6 ln 1 þ 10hr
ð4Þ
2
ð5Þ
Zapata [19] and Zapata et al. [20] developed an experimental correlation to allocate the SWCC for plastic soils (w PI > 0), these correlations are as follows
and in soil replacement, so a great need is required to study the unsaturated shear strength of gravel soils prepared with different proportions of low plastic fines. Plastic fines more than stated in specification requirements may cause reduction of strength and settlement problems on inundation. 2. Tested materials The prepared tested soil consists of different types of gravel soils; clean well graded gravel ‘‘GW’’, well graded with silt ‘‘GW-GM’’, well graded gravel with clay ‘‘GW-GC’’, silty gravel ‘‘GW’’ and clayey gravel ‘‘GC’’. Fig. 3 shows the grain size distribution for the tested soils. The gravel size is less than 10 mm. The fines are classified as low plastic fines according to plasticity chart. Table 1 shows the properties of different tested gravel soils. AASHTO M145-91, standard specification for classification of soils and soil-aggregate mixtures for high way construction defines the group index of soil (GI). GI ¼ ðP200 -35Þ½0:2 þ 0:005ðLL-40Þ þ 0:01ðP200 -15ÞðPI-10Þ
ð7Þ
For silty gravel (GM) or clay gravel (GC) GI ¼ 0:01ðP200 -15ÞðPI-10Þ
ð8Þ
Fig. 4 shows the modified proctor compaction curve for the tested soil. It is noticed that the optimum moisture content increases with the increase in percentage of fines. The optimum moisture content (O.M.C.) is 4%, 7.9%, 10%, 12% and 18% for GW, GW-GM, GW-GC, GM and GC soils while the maximum dry densities are 2.1, 2.15, 2.17, 2.18 and 2.13 t/m3 respectively. The soil suction intensity at wet of optimum goes to zero while it increases gradually in direction of dry of optimum zone. 3. Experimental program A series of five different types of soils ‘‘GC, GM, GW-GC, GW-GM and GW’’ are compacted and prepared at dry, and wet of optimum water content. It is required to determine the SWCC of prepared soils, studying the effect of hysteresis, measuring there shear parameters (c, / and /b) and unsaturated shear strength and verifying values with Fredlund et al. [1] model. Also it is required to test the effect of inundation on soil shear strength and its initial shear modulus. The filter
af ¼ 32:835 ln ðw PIÞ þ 32:438
100
bf ¼ 1:42 ðw PIÞ0:3185 Cf ¼ 0:2154 ln ðw PIÞ þ 0:7145 hr ¼ 500
90 80 70
ð6Þ
The ASTM D4318 specifies that for accepted gravel soil utilized in earth works and highways, the maximum permissible percentage of fines (P200) is 15% and liquid limit (L.L.) not exceeding 25 and plasticity index (PI.) not exceeding 5. This will be checked later on in the paper. Since gravel soil had been applied on large scale in many earth works such as sub-base soil in pavements, earth dams
% Passing
PI P200 w PI ¼ 100
3
GW
60
GW-GC
50
GW-GM
40
GC
30
GM
20 10 0 100
10
1
0.1
0.01
0.001
0.0001
Diameter (mm)
Figure 3
The grain-size distribution curve of the tested soils.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
4
K.M.H. Ismail Ibrahim Properties of tested gravel.
Table 1 Soil type
Texture
GW GW–GM GW–GC GM GC
%G
%S
%M
%C
55 45 50 55 59
40 44 40 38 20
5 11 0 17 2
– – 10 – 19
Cu
Cc
G.I
L.L.
P.L.
P.I.
w PI
40 100 70 5.8 1000
1.6 2.7 2.05 13.7 22.5
0 0 0 0 0.6
35 35 40 35 40
25 25 20 25 20
10 10 20 10 20
0.5 1.1 2.0 1.7 4.2
0.4
2.2
0.35
2.15 GW
2.1
GW-GM GW-GC
2.05
GM
2
GC
1.95 1.9 0
5
10
15
20
Volumetric water content ( θw )
γd (t/m3)
25
GW GW-GC
0.2
GW-GM GC
0.15
GM
0.1 0.05 0 0.01
wc %
Figure 4
0.3 0.25
1
100 10000 Soil suction (kPa)
1000000
Compaction curve of tested soils. Figure 6
SWCC of tested soils.
Table 2 Fredlund and Xing fitting parameters of SWCC for the tested soils. Soil type
af
bf
cf
hr
hs
GW GW-GC GW-GM GC GM
0.03 55.2 35.56 79.56 49.86
– 1.14 1.38 0.89 1.2
– 0.57 0.69 0.41 0.60
9 500 500 700 600
0.29 0.31 0.30 0.33 0.32
Figure 5 Shear box diagram with the system implemented to prevent evaporation. Volumetric water content (θ)
paper method was applied to measure the soil suction while the modified shear box was used to measure the shear strength and the unsaturated shear parameters. Fig. 5 shows a conventional shear box but it is modified to control suction by fixing the soil water content using a tight polyethylene bag to prevent water evaporation and to achieve vapor equilibrium during shearing (Gan [21]). The final water content of embedded filter paper in the soil is measured at the end of shearing where soil suction can be determined using the calibration curve (Fig. 2).
0.35 0.3 0.25 GW-drying
0.2
GW-wetting GC-drying
0.15
GC-wetting
0.1 0.05 0 0.01
1
100
10000
1000000
Soil suction (kPa)
Figure 7
Hysteresis curves for GW and GC soils.
4. Results and discussions Fig. 6 shows the SWCC of tested soils represented by volumetric water content with respect to soil suction. The saturated volumetric water content ‘‘hs’’ is 0.28, 0.29, 0.3, 0.33 and 0.34 for ‘‘GW’’, ‘‘GW-GM’’, ‘‘GW-GC’’, ‘‘GM’’ and ‘‘GC’’ soils respectively. Increasing w PI increases ‘‘hs’’, air entry value ‘‘wa’’, inflection point and increases the final dry soil
suction which is about 40 kPa in case of ‘‘GW’’ soil and reaches 1E6 kPa in case of ‘‘GC’’ soil. The soil–water characteristic curve fitting parameters of the tested soils are shown in Table 2. Fig. 7 shows soil hysteresis of two soils has extreme difference in percentage of fines ‘‘GW’’ and ‘‘GC’’ soil. The hysteresis is represented by SWCC in drying and wetting conditions.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil
τ (kPa)
5
φ b 45
40 35 30 25 20 15 10 5 0 0.01
2500
wc = 3%, σn = 200 kPa
2000
GW GW-GM
1500
GW-GC GM
1000
GC
500 0 0
0.05
0.1
0.2 ε %
0.15
Figure 8 Shear stress–strain distribution of tested soil at wc = 3% (dry of optimum).
It can also be represented by the area confined between drying and wetting SWCC. It is noticed that hysteresis in case of ‘‘GC’’ soil is greater than ‘‘GW’’ soil which means that increasing the percentage of fines increases the soil hysteresis. Fig. 8 show the shear stress–strain relationship for soils prepared at O.M.C then dried and sheared at wc equals to 3% while in Fig. 9 the prepared soils are sheared directly at O.M.C. The vertical applied stress is 200 kPa in both cases. The saturated cohesion (C) of all gravel soils is zero. In Fig. 8 the maximum shear strength is about 2700, 1470, 740, 170 and 166 kPa for GC, GW-GC, GM, GW-GM and GW soils respectively. The increase in strength of GC soil than GW soil returns to the contribution of higher suction induced in GC soil than GW soil. In Fig. 9 the GW soil has the highest strength about 162 kPa while ‘‘GC’’ has the lowest strength about 93 kPa because in both cases at O.M.C, soil suction is a trivial value and has no contribution in increasing the shear strength. To calculate the soil suction at O.M.C. determine e (void ratio) = 0.3, find wc at O.M.C from Fig. 4, calculate hw from Eq. (3), and get w from Fig. 6 it is found to be a trivial value). The presences of fines cause drop to the unsaturated dry strength on inundation. Fig. 10 shows the distribution of angle of shear resistance /b of tested soils with respect to soil suction. The angle of shear resistance /b of soil can be determined using the relation: Tan /b = Tan / * Sr (Vanapalli and Fredlund [22]), where Sr is the degree of saturation of soil. It is noticed that in case of ‘‘GW’’ soil the reduction in /b is steep due to limited percentage of fines. The angle of shear
GC GW GW-GC GW-GM GM
1
100
10000
1000000 ψ (kPa)
Figure 10 Angle of internal friction that arises due to matric suction (w).
resistance with respect to matric suction /b equals to the saturated angle of shear resistance 39° at full saturation (zero matric suction) and reaches about zero on complete dryness. In case of ‘‘GC’’ soil, /b equals 26° at zero suction and goes to zero at high matric value (zero water content). The reduction in /b on increasing soil suction is similar in shape to SWCC of soil. Fig. 11 shows the distribution of (Ds) which is the difference between unsaturated and saturated soil shear strength with respect soil suction. Clayey gravel ‘‘GC’’ recorded the highest difference (increase in unsaturated strength) at dry condition where Ds equals to 1210 kPa due to contribution of high suction of low plastic clay fines, while ‘‘GM’’ soil has less increase in shear strength about 1040 kPa due to contribution of lower suction of low plastic silt fines. In case of ‘‘GW’’ soil the contribution of soil suction in unsaturated shear strength is negligible due to limited percentage of fines which is less than 5%. Figs. 12 and 13 show a comparison between measured and predicted (Fredlund et al. [1] model given by Eq. (1)) unsaturated shear strength of tested samples prepared at dry of optimum wc = 3% and at O.M.C. respectively. The vertical applied stress is 200 kPa. Fig. 12 shows that the values are consistent to a great extent except the case of ‘‘GC’’ soil, the predicted unsaturated shear strength is (6180 kPa) considerably higher than the measured (2600 kPa) value. The deviation between both values may return to expected error in estimation of the high suction of ‘‘GC’’ soil that is induced at wc = 3%, also due to the presence of internal random cracks in clay fraction at that low water content which is not considered by the model.
τ (kPa) 180 160
σn = 200 kPa
Δτ (kPa) 1400
140 120
1200
100
1000
80
800
GC GW GW-GC
GW, O.M.C.=4% GW-GM, O.M.C.=8% GW-GC, O.M.C.=10% GM, O.M.C.=11.9% GC, O.M.C=18.8%
60 40 20 0 0
0.02
0.04
0.06
0.08
0.1
0.12
ε%
0.14
Figure 9 Shear stress–strain distribution of tested soils at optimum moisture content.
600
GW-GM
400
GM
200 0 0.01
1
100
10000
1000000
ψ (kPa)
Figure 11 The increase in saturated shear strength of tested soil with respect to matric suction.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
6
K.M.H. Ismail Ibrahim τ (kPa) 7000
means that partial dry ‘‘GC’’ soil at water content 3% will settle 13 times its settlement if it becomes saturated. In case of ‘‘GM’’ soil the shear modulus increases from 17,272 to 72,727 kPa at O.M.C and at wc = 3%; i.e. it increases about 4 times. So the increase in percentage of fines reduces the initial partial dry soil shear modulus on inundation and consequently increases the expected settlement. Also the type of fines has a considerable effect on reducing the initial shear modulus of soil if it becomes saturated; clay fines present in ‘‘GC’’ reduces the partial dry shear modulus than silt fines present in ‘‘GM’’.
Measured
6000
Fredlund et al [1]
5000 wc = 3% 4000 3000 2000 1000 0 GW
GW-GM
GW-GC
GM
GC
Figure 12 Measured and predicted unsaturated shear strength for tested samples prepared at wc 3%. τ (kPa)
180 wc = O.M.C.
160
Measured Fredlund et al [1]
140 120 100 80 60 40 20 0 GW
GW-GM
GW-GC
GM
GC
Figure 13 Measured and predicted unsaturated shear strength for tested samples prepared at O.M.C.
Comparing the results in Fig. 13, it can be noticed that they are relatively close because at O.M.C. the soil suction is low and its contribution in increasing the unsaturated shear strength is negligible, also the unsaturated shear strength of ‘‘GC’’ soil is relatively low because the saturated angle of internal friction is about 27° as shown in Fig. 10. Fig. 14 shows the value of initial shear modulus (ratio between initial shear stress and initial shear strain) of the tested soils at O.M.C. and at wc = 3%. It is noticed that the shear modulus of ‘‘GW’’ soil is not affected by the change in soil wc due to the limited percentage of fines 5%; it is 43636 and 45,454 kPa respectively. For ‘‘GC’’ soil the shear modulus is highly affected by the increase in soil suction; it increases from 13636 to 181818 kPa; i.e. it increases about 13 times, this
Soil shear mdulus (kPa)
200000
wc = optimum
180000
wc = 3%
160000 140000 120000
5. Conclusions Fredlund et al. [1] model can be applied to predict the unsaturated shear strength of tested gravel soils except the case of partial dry ‘‘GC’’ soil. The model overpredicts the unsaturated shear strength of ‘‘GC’’ soil as it does not consider the presence of random disconnected cracks and also due to error in either measuring or in real estimation of high suction values of SWCC at relatively low degrees of saturation. Dry and saturated shear strength of ‘‘GW’’ soil are approximately the same due to limited percentage of fines, but the dry shear strength of ‘‘GC’’ soil is considerably higher than that of ‘‘GW’’ due to matric suction contribution of fines in shear strength. On the other hand the saturated shear strength of ‘‘GC’’ soil is relatively less than ‘‘GW’’ soil due to loss in contribution of soil suction and also due to reduction in saturated angle of internal friction which is affected by the presence of clay fines. Soil hysteresis of gravel soil increases with the increase of percentage and plasticity of fines. The partial dry shear strength increases with the increase in percentage and in plasticity of fines. For percentage of fines less than 12% the saturated and unsaturated shear strength of gravel soils does not differ too much and this value is close to the maximum requirement (15% percentage of fines) given by ASTM D4318. The percentage of fines affects the saturated angle of internal friction /b that arises due to soil suction. The saturated friction angle is 39° for saturated ‘‘GW’’ and 34° for ‘‘GM’’ and 25° for ‘‘GC’’ soil. Also increasing the percentage of fines decreases the dry soil shear modulus on inundation about 4 times in case of ‘‘GM’’ soil and about 13 times in case of ‘‘GC’’ soil due to reduction in matric suction; i.e. increasing the expected soil settlement. At full saturation (zero soil suction) the angle of shear resistance with respect to suction /b equals to the saturated angle of internal friction ‘‘/0 ’’ and it decreases with the decrease of degree of soil saturation (increase of soil suction). In case of ‘‘GC’’ soil it is equal to 25° at full saturation and goes to zero at zero degree of saturation.
100000 80000 60000 40000
References
20000 0 GC
GM
GW-GC
GW-GM
GW
Figure 14 Initial shear modulus measured for tested granular soils at optimum and 3% water content.
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Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012
Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil [3] Fredlund DG, Xing A, Fredlund MD, Barbour SL. The relationship of the unsaturated soil shear strength to the soil– water characteristic curve. Can Geotech J 1995;33:440–8. [4] Escario V, Juca JFT. Strength and deformation of partly saturated soils. In: Balkema AA, editor, Proc 12th Int Conf on Soil Mechanics and Foundation Engineering. vol. 2, Bookfield, MA; 1989. p. 43–6. [5] Fredlund DG, Rahardjo H, Gan JKM. Non-linearity of strength envelope for unsaturated soils. In: Proc 6th Int Conf Expansive Soils. New Delhi; 1987. p. 49–54. [6] Wheeler SJ. An alternative framework for unsaturated soil behavior. Geotechnique 1991;41(2):257–61. [7] Ridley AM. Strength-suction-moisture content relationships for Kaolin under normal atmospheric conditions. In: Proc UNSAT, Paris; 1995, vol. 2, p. 645–51. [8] Ridley AM, Burland JB, Monroe AS. Unconfined compression tests with pore pressure measurements. In: Proc 11th African regional conference. SMFE. Cairo; 1995. [9] Cerato Amy B, Lutenegger Alan J. Specimen size and scale effects of direct shear box tests of sands. Geotech Test J 2006;29(6) [Paper ID GTJ100312]. [10] Jewell RA, Wroth CP. Direct shear tests on reinforced sand. Geotechnique 1987;37(1):53–68. [11] Gardner R. A method of measuring the capillary tension of soil moisture over a wide moisture range. Soil Sci 1937;43(4):277. [12] Mcqueen, Miller. The filter paper method for measuring soil suction. Fawcett & Collis-George; 1968. [13] Li Dan, Xiong Jun, Xue Qiang. The effect of different soil texture on SWCC of unsaturated soil. Flow in porous media, International forum on porous flow and applications, Wuhan city, China, April, 2009. [14] Fredlund DG, Rahardjo H. Soil mechanics for unsaturated soils. New York: John Wiley & Sons Inc.; 1993. [15] Yang Hong, Rahardjo Harianto, Leong Eng-Choon, Fredlund DG. Factors affecting drying characteristic curves of sandy soils. Can Geotech J 2004;41:908–20. [16] Lambe TW, Whitman RV. Soil mechanics, SI version. New York: John Wiley and Sons Inc.; 1979, p. 245–6.
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[17] Fredlund DG, Rahardjo H. The role of unsaturated soil behaviour in geotechnical engineering practice. In: Proceedings of the 11th Southeast Asian geotechnical conference, Singapore, Southeast Asian Geotechnical Society, Pathumthani, Thailand; 1993b. p. 37–49. [18] Fredlund DG, Xing A. Equation for the soil water characteristic curve. Can Geotech J 1994;31(3):521–32. [19] Zapata CE. Uncertainty in soil–water characteristic curve and impact on unsaturated shear strength predictions. Ph.D. dissertation, Arizona State University, Tempe, AZ, USA; 1999. [20] Zapata CE, Houston WN, Houston SL, Walsh KD. Soil–water characteristic curve variability. In: Proceedings of sessions of GeoDenver 2000, Denver ASCE Geo-Institute; 2000. p. 84–124. [21] Gan JKM. Direct shear strength testing of unsaturated soils. M.Sc. thesis, University of Saskatchewan, Saskatoon, Sask. [22] Vanapalli SK, Fredlund DG. Empirical procedures to predict the shear strength of unsaturated soils. Hong et al. editors, Eleventh Asian regional conference on soil mechanics and geotechnical engineering; 1999.
Kamal Mohamed Hafez Ismail Ibrahim is an Associate Professor in the Civil Department of Ismalia Faculty of Engineering, Suez Canal University, Egypt. He was born on January 1, 1963, in Cairo, Egypt. His research areas include seismic analysis of anchor-prestressed diaphragm walls, seismic displacement analysis of gravity retaining walls, properties of bentonite slurry grout, secant pile analysis, shear strength of consolidated clay, stabilisation of sand dunes with lime and silica fume, soil suction, heave and swelling pressure of expansive soil, settlement in tunnels, bearing capacity of granular soil underlain by soft clay, seismic displacement of gravity retaining walls and soil liquefaction.
Please cite this article in press as: Ismail Ibrahim KMH, Effect of percentage of low plastic fines on the unsaturated shear strength of compacted gravel soil, Ain Shams Eng J (2014), http://dx.doi.org/10.1016/j.asej.2014.10.012