True Triaxial Tests and Strength Characteristics Study on Silty Sand Liang MA and Ping HU

Transcription

1 217 2 nd International Conference on Test, Measurement and Computational Method (TMCM 217) ISBN: True Triaxial Tests and Strength Characteristics Study on Silty Sand Liang MA and Ping HU Department of Civil Engineering, University of Jinan, Jinan 2522, China Keywords: Silty sand, True triaxial tests, Intermediate principal stress, D, Strength. Abstract. In order to investigate the effect of intermediate principal stress on strength of sands, a series of true triaxial compression tests of Shanghai silty sand under different intermediate principal stress coefficient are performed on flexible true triaxial apparatus. Then the Lode angle dependent shape function of Mohr-Coulomb's strength criterion is modified based on true triaxial tests results and the true triaxial test results of Shanghai fine sands are simulated. The good predictions show that the strength criterion based upon true triaxial tests can describe the strength characteristics of sand in true D state preferably. Introduction For a long time, the conventional triaxial test has been used as the main test method to study the characteristics of soil. However, in the conventional triaxial tests, the sample can only be applied principal stress in two directions, so that the soil is always in a state of axisymmetric stress. It can only reflect the strength and deformation under axisymmetric stress while ignoring the influence of intermediate principal stress, so that it cannot describe the strength characteristics of sand in true D state preferably [1-2]. In order to overcome this defect, the true triaxial test is developed [-1]. Three principal stresses can be applied independently in the true triaxial tests, and the influence of different principal stresses on the strength of soil is analyzed. Therefore, the actual stress state of the soil can be modeled more accurately, which is helpful to study the D strength and constitutive relation of soil. Shanghai silty sand mainly consists of silt and silt-sand, mingled with a small amount of clay. There is no obvious critical state line in this sand, so it has a typical researching significance. In order to investigate the effect of intermediate principal stress on strength of silty sand, a series of true triaxial tests of Shanghai silty sand under different intermediate principal stress coefficients are performed. True Triaxial Tests of Shanghai Silty Sand Test Instrument The true triaxial instrument used in this test is the flexible triaxial instrument jointly developed by Tongji University and the Shanghai thinker Intelligent Instrument Technology Co. Ltd. The loading method is that the major principal stress is applied by the loading platform; the intermediate principal stress is applied on the flexible water bag through hydraulic cylinder driven by a stepper motor; the minor principal stress is provided by the air pressure from the pressure chamber. The biggest advantage of this instrument is that the rigid plate is replaced by the flexible water bag, which would eliminate the edge effect. In addition, the use of high-precision GDS automatic control device can improve the control accuracy of the intermediate principal stress. This instrument can carry out true triaxial tests under isotropic consolidation, non-isotropic consolidation and different principal stress ratios. Sand Preparation The soil samples used in this experiment were taken from Shanghai shipbuilding base in Changxing Island, which is widely distributed in the Yangtze River estuary area and used as foundation filling material. The sand gradation range is.25~.1 mm, the uniformity coefficient C u =.15, the 422

2 relative density of particles is 2.65, the maximum and minimum dry density are 1.57 g/cm and 1.25 g/cm respectively. In the tests, the soil samples were prepared into cubes with a dimension of 12 mm 7mm 7 mm. Test Program The true triaxial tests were performed both under the condition of isotropic consolidated-drained and isotropic consolidated-undrained. Under the drained condition, the consolidation pressure p is 5,1,15 kpa respectively and under the undrained condition, the consolidation pressure p is 5 kpa. At the same time, the different intermediate principal stress coefficients b was taken respectively. The expression of the intermediate principal stress coefficient b is: σ 2 σ b = σ1 σ (1) Where σ 1 is the major principal stress, σ 2 is the intermediate principal stress, σ is the minor principal stress. The intermediate principal stress coefficient b remained constant throughout the shearing process. The parameters of the tests are shown in Table 1. Table 1. Parameters of true D tests on silty sand. Consolidation state D r Initial void Initial confining intermediate principal stress / % ratio e pressure p / kpa coefficient b CD ,.25,.5,.75, 1. CD ,.25,.5,.75, 1. CD ,.25,.5,.75, 1. CU ,.25,.5,.75, 1. Where D e e e e r max max min Test Result Analysis = ( ) /( ) 1%. (1)Results of the true triaxial drained test The curves for stress versus major strain are as shown in Figure 1. Under true D stress condition, the deviatoric stress peak value increases with the initial confining pressure, while the relationship between the deviatoric stress peak value and the intermediate principal stress coefficient b is complex. It is shown that the peak value increases with b when b changes from. to.5. While when b =.75, the peak value decreased. When b = 1, the peak value has rebounded, but is still lower than the value of b =.5. (2)Results of the true triaxial undrained test The curves for deviatoric stress q versus major strain ε 1 at P = 5kPa are as shown in Figure 2. As can be seen from this figure, the deviatoric stress peak value does not always increase with the increase of initial confining pressure. The peak value would increases with b when b changes from. to.75. While when b = 1., the peak value would decreased. In Figure, the results show that the pore water pressure would initially increase and then decrease. In order to study the influence of intermediate principal stress on strength parameters of sand,a large number of true triaxial tests have been performed by scholars, and some test results have been obtained. In Figure 4, the ϕb - b curves of true triaxial tests in this paper are compared with the previous results [14-18],where ϕ b is the peak value internal friction angle corresponding with the intermediate principal stress coefficient b. It can be given as: σ1 σ sinϕb = σ + σ 1 (2) 42

3 6 4 b= q/ kpa b= (a) p =5 kpa (b) p =1 kpa (c) p =15 kpa Figure 1. Stress versus major strain. 4 b= b=. 2 u/kpa 1 b= Figure 2. Stress versus major strain Figure. Pore water pressure versus major strain. Under drained condition, the value of ϕ b can be taken as the averaged one of the three groups with different confining pressures. It is shown that the test laws of Shanghai silty sand under undrained condition are similar with that of Lade and Duncan s tests [6]. It is also shown that both of the peak value of internal friction angles would increase with the increasing of the intermediate principal stress from the minor principal stress, and then would decrease slightly when the intermediate principal stress is close to the major principal stress. However, under the condition of drainage, the test laws are slightly different. When the peak value of internal friction angle decreases to a certain extent, there is a rebound at b =.75, while the overall trend is similar to that of the undrained tests. Lade and Wang performed a series of true triaxial undrained tests [19] on Santa MonicaBeach sand with different relative densities and obtained the variation laws of the major strain ε 1 with the intermediate principal stress coefficient b at the time of failure, as shown in Figure 5. As can be seen from this figure, when Santa Monica Beach sand arrives at failure, the maximum principal strain can be obtained when the intermediate principal stress is equal to the minor principal stress. With the increase of b, ε 1 decreases gradually. When the intermediate principal stress is close to the major principal stress, ε 1 would first increase and then decrease. By comparison, it is found that the variation laws of the major principal strain in drainage tests is consistent with that of Santa Monica Beach sand when the sand arrives at failure. The major principal strain curve is gentler when the sand reaches the failure state in the undrained tests. With the increase of b, it gradually decreased to a certain extent, but the change was not significant, and there was only a slight increase when the intermediate principal stress near the major principal stress. 424

4 Figure 4. b -b curves of True Triaxial Tests on sands Figure b curves of True Triaxial Tests on sands Conclusions The strength characteristics of silty sand are closely related with intermediate principal stress. When the initial density and confining pressure are constant, the strength of silty sand would be different under different intermediate principal stress coefficients. However, the strength does not always increase with the increase of the principal stress coefficient. Generally speaking, with the increase of the intermediate principal stress, the strength of the sand would increase firstly, while when intermediate principal stress is close to the major principal stress, the strength of the sand would decrease slightly. Acknowledgments This research is financially supported by the National Natural Science Foundation of China under Grant No References [1] SUN Hong, YUAN Ju-yun, ZHAO Xi-hong. Study on soft soil by the true triaxial tests[j]. Journal of Hydraulic Engineering, 22, (12): [2] LIU Jin-long, LUAN Mao-tian, YUAN Fan-fan, et al. Evaluation of effect of intermediate principal stress on sand shear strength[j]. Rock and Soil Mechanics, 25, 26(12): [] BISHOP A W. The strength of soils as engineering materials[m]. 6th Rankine Lecture, Geotechnique, London, England, 1966, 16(2): [4] KO H Y, Scott R F. Deformation sand and failure[j]. Journal of the soil mechanics and foundations division ASCE, 1968, 94(4): [5] SUTHERLAND H B, MESDARY M S. The influence of the intermediate principal stress on the strength of sand[c]//proceedings of the 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico City, 1969: [6] LADE P V, DUNCAN J M. Cubical triaxial tests on cohesionless soil[j]. Journal of Soil Mechanics and Foundation Engineering Division, ASCE, 197, 99(1): [7] CHU J, LO S C R, LEE I K. Strain softening and shear band formation of sand in multi-axial testing[j]. Geotechnique, 1996, 46(1): [8] CALLISTO L, CALABRESI G. Mechanical behavior of a natural soft clay[j]. Geotechnique, l998, 48(4):

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