The repeated load triaxial testing (RLT) (also known as the cyclic triaxial test) method is commonly used to establish the mechanical characteristics of granular materials. During the testing, a cylindrical specimen is compacted to a desired level and then tested by applying confining and vertical stresses. Two variants exist:
• a constant confining pressure (CCP) method; and
• a variable confining pressure (VCP) method.
In the CCP-method the sample is initially subjected to a hydrostatic confining pressure ac, which induces an initial strain ec (unmeasured in the test, but it is the same in
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Fig. 10.1 Stresses in an unbound granular material layer. (a) Typical pavement structure and stresses, (b) induced stresses in a pavement element due to moving wheel load (Erlingsson, 2007) |
all directions for isotropic material behaviour and, thus, can be estimated). The axial stress is then cycled at a constant magnitude q, which induces the cyclic resilient axial strain As. In the VCP-method both the axial and the radial stresses are cycled. The RLT can be used to obtain both the stiffness characteristics as well as the ability of the material to withstand accumulation of permanent deformation during pulsating loading (Gomes-Correia et al., 1999; Erlingsson, 2000). Figure 10.1 illustrates the general stress regime experienced in an unbound granular layer in a pavement structure as a result of a moving wheel load within the plane of the wheel track. Due to the wheel load, pulses of vertical and horizontal stress, accompanied by a double pulse of shear stress with a sign reversal, affect the element (Brown, 1996).
This stress regime associated with the vertical as well as the horizontal stress pulses can be established using the VCP-method in the RLT. Using the CCP-method the variation of the horizontal stresses is neglected, as the confining pressure is kept constant. For further details please refer to CEN standard EN 13286-7 (2004).
Resilient testing of granular material is usually divided into two phases: (i) a conditioning phase and (ii) a testing phase. During the conditioning phase 20,000 symmetric haversine load pulses are applied with the frequency of 5 Hz to stabilize the response from the specimen. Thereafter, different stress paths are applied to estimate the specimen’s response. Since the unbound granular materials show stress dependency behaviour, it is very important to apply a number of stress paths in order to observe such behaviour. During each stress path 100 symmetric haversine load cycles are applied with a rise time of 50 ms (total length of pulse 0.1 s) followed by a 0.9 s rest time. During the last ten load cycles data from the transducers as well as the axial load are collected to evaluate the specimen response, see Fig. 10.2.
For the permanent deformation estimation, constant amplitude symmetric haver — sine load pulses are applied in the axial direction usually with a frequency of 5 Hz, without any rest time between the pulses (Fig. 10.3). The stresses, cyclic numbers as well as the axial and sometimes the radial deformations are recorded during the test at appropriate intervals.
