Abstract
This paper presents the results of an experimental study of the particle scale mechanisms that underpin creep, on-going deformations under constant external load, in dry non-cemented sand under 1D oedometric compression loading at 2500 kPa. Traditional observations on the boundary of the sample are complemented with simultaneous measurements of the 3D kinematics of both the entire grain assembly and details of grain-scale mechanisms using synchrotron based X-ray tomography at two different spatial resolutions. Both the continuum response and the local grain scale response are captured using two spatial resolutions, i.e. {6.5},{upmu }hbox {m} and {0.65},{upmu }hbox {m} respectively. The results, for the first time, illustrate that small displacements measured at the boundary can be the result of rather pronounced fracturing at the individual grain scale.
Highlights
The rate-dependent behaviour, such as creep and relaxation, of naturally occurring granular materials is strongly linked to the micro-structure, micro-fractures, crushing and grain surface properties [17, 18, 21, 28]
We select 1D loading for simplicity of the experimental setup, for the relative ease of reaching stress levels greater than 1 MPa and because the cylindrical specimen shapes are convenient for X-ray tomography
I.e., 6.5 μ m and 0.65 μ m, high-speed X-ray tomography was applied to monitoring of a 2-h creep experiment in dry sand in oedometric conditions under 2.5 MPa of axial loading
Summary
The rate-dependent behaviour, such as creep and relaxation, of naturally occurring granular materials is strongly linked to the micro-structure, micro-fractures, crushing and grain surface properties [17, 18, 21, 28]. The general hypothesis in the discipline is that the inter-particle force distribution continues to change under constant external load, probably because of small variations of grain geometrical properties. Bowman and Soga [6] suggest that the distribution of force chains in the assembly play an important role. The main force chains, that transmit the majority of the load with only few (often larger) particles, is supported by secondary force chain structures that stabilise the system. Small changes at the grain level will accommodate a redistribution of the load
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