Strain-softening behaviour of sand in strain path testing under plane-strain conditions

Abstract

Experimental data are presented in this paper to study the strain-softening behaviour of sand under planestrain conditions. K 0 consolidated strain path tests were conducted using a new plane-strain apparatus. The stressstrain behaviour of medium dense sand under plane-strain conditions was characterized. The test results show that the occurrence of pre-failure strain softening under planestrain conditions is affected by the void ratio, the strain increment ratio and the initial effective confining stress. This is consistent with previous findings established under axisymmetric conditions. However, a pre-failure strainsoftening behaviour in plane-strain tests conducted under high-confining stresses may consist of three stages, namely, material softening, banding softening, and ultimate state. This observation is different from that in triaxial tests where banding softening does not normally occur. List of symbols b(r 2 -r 3 )/(r 1 -r 3 ) de 1 axial strain increment de v volumetric strain increment de v /de 1 strain increment ratio e void ratio I 1 first principal stress invariant I 1 = r 1 + r 2 + r 3 I 3 third principal stress invariant I 3 = r 1 r 2 r 3 m constant in Lade's failure criterion in Eq. (5) p 0 mean effective stress p 0 ¼ 1 3 (r 0 1 þ r 0 2 þ r 0 3 Þ p a atmospheric pressure q deviatoric stress q ¼ 1 ffiffi ffi 2 p (r 1 ½ Àr 2 ) 2 þ (r 2 À r 3 ) 2 þ(r 3 À r 1 ) 2 1 2 Du excess pore water pressure b parameter in Eq.axial strain e s deviatoric strain, e s ¼ ffiffi 2 p 3 e 1 À e 2 ð Þ 2 þ e 2 À e 3 ð Þ 2 h þ e 3 À e 1 ð Þ 2 1 2 e v volumetric strain, e v ¼ e 1 þ e 2 þ e 3 / 0 effective friction angle of soil g effective stress ratio, g = q/p 0 g 1 symbol of Lade's failure criterion in Eqs. (4) and (5) g 1max constant in Lade's failure criterion in Eq. (4) M effective stress ratio g at critical state l Lode stress parameter, l ¼ 2r 0 2 À r 0 3 À r 0Lode strain parameter, m ¼ 2de 2 À de 3 À de 1 de 1 À de 3 r 0 1 major principal effective stress r 0 2 intermediate principal effective stress r 0 3

Stress-Dilatancy for Soils. Part III: Experimental Validation for the Biaxial Condition

“…quickly decreases from approximately 30° to the minimum value during shearing and remains almost constant for the pre-failure stage ( [5], [15], [27], [30]). …”

“…For the drained condition, the elasticity part of the strain increment can be treated as small, and equations (30) and (32) have the forms…”

“…Therefore, for comparison, it is assumed that  b = 15 as the mean value observed in drained biaxial compression tests. Figures 3 and 4 show that the best approximation of Rowe's and Bolton's equations (equations (30) and ( …”

“…The values of the maximum angle of friction max Φ for different initial porosities obtained experimentally and calculated from equations (26) and (27) Changi sand was extensively investigated by Wanatowski ( [28], [29], [30]) and Wanatowski and Chu [27] in plane strain conditions. Figure 7 shows the relationship between the stress ratio () and plastic dilatancy (D p ) for three tests [30]. The samples were anisotropically consolidated and sheared under drained conditions at a constant 3 and Tatsuoka [31] for Toyoura sand tested in plane strain conditions for different stress paths.…”

“…Most geotechnical structures such as slopes, embankments, retaining walls, and strip foundations can be simplified into plane strain conditions, which are modelled in a plane strain biaxial apparatus [2], [5], [6], [25], [26], [30] and others or in true triaxial tests [18].…”