The responses of stress-strain behaviour of different soils under static loading may be different due to the structural orientation or arrangement of soil particles. Several literatures are available on the behaviour of soils under uniaxial loading at different testing conditions.
Miura and Toki (1982) have experimentally investigated the response of sand under different sample preparation techniques such as moist sample preparation, dry tapping and wet tapping as shown in Fig. 2.1. It was highlighted that the adoption of different sample preparation methodologies affects the behaviour and shear strength of soil. Similar observations have also been reported by several researchers, when the soil specimens were prepared by dry or wet pluviation, slurry deposition or water sedimentation and vibrations or moist-tamping method (Ladd, 1978; Chaney and Mulilis, 1978; Vaid and Negussey, 1988; Ishihara, 1993; Amini and
Literature Review 6 Qi, 2000; Vaid and Sivathayalan, 2000; Amini and Chakravrty, 2003; Juneja and Raghunandan, 2010; Della et al., 2011).
The test methodologies such as compression or extension shear of a saturated triaxial specimen at drained or undrained conditions affects the strength of soil. Seed and Lee (1966) have conducted the triaxial shearing at both drained and undrained conditions of the sand.
Typical the stress-strain responses reported, are presented in Fig. 2.2. It has been observed that the shearing under undrained condition reflects higher stress-strain response in comparison to the drained condition, because both soil particles and water are assumed to be incompressible and load taken by soil-water medium is higher at undrained shearing condition than the drained shearing.
Ishihara (1993) carried out the triaxial tests on saturated sand at different effective confining pressures and the responses are presented in Fig. 2.3. Fig. 2.3a represents that the strength (resistance to deform and failure) the soil sample increases with the increase of confining pressures. During undrained triaxial tests, the initially effective stress decreases due to the increase of pore water pressure (PWP), reflecting contractive behaviour of sand, and reaches the minimum deviator stress as shown in Fig. 2.3b. The state of minimum deviator stress is called as state of phase transformation, at which soil changes the phase from contractive to dilative (Ishihara, 1993). The contractive behaviour and excess PWP development were more prominent at high confining pressure. Tests on the saturated sand under undrained monotonic loading also revealed the possibility of static liquefaction (Castro, 1975; Kramer and Seed, 1988; Dash and Sitharam, 2011).
Della et al. (2011) carried out undrained triaxial compression tests at different degrees of saturation, represented by varying Skempton’s B-parameter (ranging between 32 and 90%), the results of which are presented in Fig. 2.4. It can be seen that the increase in the degree of saturation leads to a reduction in the resistance of the deviatoric stress (Fig. 2.4a) and an
Literature Review 7 increase in the pore-water pressure (Fig. 2.4b). The stress path plot in Fig. 2.4c also signifies the role of the degree of saturation in the reduction of the effective mean stress and the maximum deviatoric stress. Della et al. (2011) also indicated that the increase in degree of saturation (higher value of Skempton’s pore pressure coefficient ‘B’) reduces the soil dilatancy and amplifies the contractive phase. Based on undrained triaxial compression shear tests, Sarma et al. (2016) have also reported that with increase of degree of saturation, initial stiffness and resistance (peak deviator stress) of the soil decreases. Literatures also revealed that the increase in degree of saturation leads to a decreases in the liquefaction resistance of soil (Martin et al., 1978; Yoshimi et al., 1989; Ishihara et al., 2001, 2004; Yang and Elgamal, 2002; Yang et al., 2004; Bouferra et al., 2007; Arab et al., 2016).
Fig. 2.1 Typical plot of the effect of sample preparation method in triaxial testing at different testing conditions (Miura and Toki, 1982)
Drained tests results Undrained tests results
Literature Review 8
Fig. 2.2 Comparison of drained and undrained testing conditions on dense sand (redrawn after Seed and Lee, 1966)
Fig. 2.3 Typical plot of effect of confining pressure on (a) stress-strain response (b) stress path for sand (Ishihara, 1993)
0 5 10 15 20 25
0 5 10 15 20 25 30
Undrained static test Drained static test u, undrained static test
Axial strain (%)
Deviator stress (kg per sq. cm)
-8 -6 -4 -2 0 2
Dr = 70%
'3c = 5.0 kg per sq. cm
Change in PWP, u (kg per sq. cm)
Literature Review 9
Fig. 2.4 Typical plots representing the effect of saturation on undrained behavior of sand (Della et al., 2011)
Yamamuro et al. (2011) have investigated the effect of strain rates on sandy soil specimens. It was reported that with the increase in strain rates the elasto-plastic stiffness of sand increased significantly. It was also reported that with increasing strain rates the failure shear strength of sand increased moderately whereas, the axial strain at peak stress decreased significantly and the volumetric strains became more dilatant. Several other researchers have also reported the effects of strain rate on the soil behaviour for various practical purposes, such as design of pile foundation and stability of an earth fill dam during earthquakes, but no conclusive consensus were reported because of contradicting conclusions (Casagrande and Shannon, 1948; Whitman and Healy, 1962; Lee et al., 1969; Ito and Fujimoto, 1981; Yamamuro and Lade, 1993; Tatsuoka et al., 2008; Omidvar et al., 2012; Watanabe and Kusakabe, 2013;
Literature Review 10 Svoboda, 2013). Watanabe and Kusakabe (2013) reported the behaviour of undrained tests on saturated sand, in terms of the variations in deviator stress and excess pore water pressure with axial strain, at different loading rates as shown in Fig. 2.5. It was confirmed from Fig. 2.5a that the sand exhibits stiffer response and higher shear strength when it is subjected to faster strain rate. The peak strength, defined at the maximum deviator stress, also increases with the increase of strain rate. Fig. 2.5b presents the variations in excess pore water pressure with strain rate. It is seen that the rate of generation and redistribution of excess pore water pressure is reduced with the increase in strain rate was higher.
Fig. 2.5 Typical plots of (a) stress-strain response (b) excess PWP, with strain at different loading rates on saturated sand under undrained condition (Watanabe and Kusakabe, 2013)
Arab et al. (2016) carried out undrained tests on sandy specimens at different initial relative densities (Dr) and the results are presented in Fig. 2.6. Fig. 2.6 (a) presents the deviator stress versus axial strain, Fig. 2.6 (b) presents the pore pressure versus axial strain and, Fig. 2.6 (c) presents the mean effective stress (p’) versus deviator stress (σd). It was obserevd that the specimens reconstituted at Dr = 64 % and 78 % shows higher resistance to deform than the specimen prepared at Dr = 8%. It was also obserevd that loose sand (Dr = 8 %) shows a phase of contractancy (Fig. 2.6c), due to the generation of the high pore water pressure (Fig. 2.6b), followed by a phase of dilatancy characterizing the decrease of the pore water pressure (Fig.
Literature Review 11 2.6c). Similar observations were also been reported by Ishihara (1993) and De and Basudhar (2008).
Fig. 2.6 Undrained behaviour of sand: (a) Deviator stress versus axial strain (b) PWP versus axial strain (c) Mean effective (p') stress versus deviator stress (σd) (Arab et al., 2016)