Creep and ageing of granular materials under isotropic pressure

Single- and multi-stage isotropic creep tests including bender element and acoustic emission measurements are carried out to investigate the relationship between ageing and creep in dense silica sand. In the considered pressure range, the experimental results show an inversely proportional relationship between ageing and creep: the small-strain shear modulus increases with decreasing isotropic pressure,, while the axial creep strain and the acoustic emissions show the opposite pressure dependence. In the multi-stage creep test, the small-strain shear modulus increases monotonically, while the rates of axial strain, number of acoustic emissions and small-strain shear modulus decrease with time according to a power law. In the single-stage creep tests, the ageing and creep indicators initially evolve as in the multi-stage creep test, but then the small-strain shear modulus reaches a peak value and decreases with time thereafter. At the same time, the rates of axial strain, number of acoustic emissions and small-strain shear modulus deviate from the power law. A conceptual model assuming a time-dependent behaviour of the normal and shear forces at the particle contacts in order to explain the experimental observations qualitatively is proposed. Accordingly, an increase in small-strain shear modulus results from homogenisation of the force chains, while a decrease in small-strain shear modulus results from a temporary formation followed by a time-delayed collapse of strong force chains.