Comparison of Cyclic Triaxial Behavior of Unbound Granular Material under Constant and Variable Confining Pressure

Abstract

Cyclic stresses due to passing wheels impose an accumulation of permanent strains in layers of unbound granular materials (UGMs) of flexible pavements. The hollow cylinder triaxial test would be the most appropriate test to simulate the in situ stress conditions but it is difficult to perform on UGMs due to their large maximum grain size. The simpler axi-symmetric cyclic triaxial test does not consider the shear stress components. It can be performed with a constant (CCP) or a variable confining pressure (VCP). CCP and VCP tests are commonly assumed to deliver similar residual and resilient strains as long as the average stress is the same. Thus, the simpler CCP test is mostly used in pavement engineering. However, this assumption is based on limited test data in the literature and may not be on the safe side. The present paper documents a comparative study of CCP and VCP tests on UGM. The study is mainly dedicated to the permanent deformations. The results show that only for some special stress paths both types of test deliver similar permanent axial or volumetric strains. For some other stress paths the CCP test may underestimate the permanent axial strain in comparison to the corresponding VCP test.

Effects of cyclic confining pressure on the deformation characteristics of natural soft clay

“…For example, based on many totally-drained tests on sands, Wichtmann et al [35] observed approximately double accumulation rates of permanent axial strain under certain stress paths with variable confining pressure, compared to the stress paths with constant confining pressure (CCP tests); Rondon et al [27] compared the drained behavior of unbound granular materials under several stress paths with and without cyclic confining pressure, and their results show that the conventional cyclic triaxial tests with constant confining pressure may underestimate the settlement induced by traffic loading in some cases. All these studies proved that the variable confining pressure plays an important role in the deformation behavior of soils, especially when drainage is allowed.…”

“…For the simulation of traffic loading, compressive cyclic deviatoric stress cycles have been proved to be insufficient by many researchers (Powrie et al [24]; Yang [25]; Grabe [26]; Rondon et al [27]; Cai et al [23]). As shown in Fig.…”

“…The existence of cyclic shear stress will result in the principal stress axes rotation, which can be investigated by hollow cylinder tests (Miura et al [29]; Tong et al [30]; Ishikawa et al [31]; Xiao et al [32]; Yang et al [33], Cai et al [34]). The coupling effects of cyclic vertical normal stress and cyclic horizontal normal stress can be simulated by the simultaneous application of cyclic deviatoric stress and cyclic confining pressure, using an advanced cyclic triaxial device which can apply variable confining pressure (Wichtmann et al [35]; Rondon et al [27]; Cai et al [23]). …”

“…Cyclic triaxial tests with variable confining pressure (VCP tests) have been employed many times for the research of coarse soils (Brown and Hyde [36]; Nataatmadja and Parkin [37]; Zaman et al [38]; Simonsen and Isacsson [39]; Wichtmann et al [35]; Chazallon et al [40]; Rondon et al [27]). For example, based on many totally-drained tests on sands, Wichtmann et al [35] observed approximately double accumulation rates of permanent axial strain under certain stress paths with variable confining pressure, compared to the stress paths with constant confining pressure (CCP tests); Rondon et al [27] compared the drained behavior of unbound granular materials under several stress paths with and without cyclic confining pressure, and their results show that the conventional cyclic triaxial tests with constant confining pressure may underestimate the settlement induced by traffic loading in some cases.…”

Influence of Nonlinear Resilient Models of Unbound Aggregates on Analysis and Performance of Road Pavements

“…[7,20]), non-destructive in-situ investigations and correlations with empirical parameters (e.g. [6,14]) were proposed to measure the resilient modulus of unbound granular materials.…”