I agree with Professor Andrew Schofield that the scree in his photo of the Grand Canyon may have remained stable for six million years but this does not mean that creep strain has never occurred at some of the contacts between the scree particles.
When the scree particles were deposited, the weight of the particles would have resulted in interparticle forces at the contacts.
Initially these forces may have resulted in contact stresses exceeding the yield stress of the scree material thus promoting viscous flow at some of the interparticle contacts.
One effect of the viscous flow would be shear strain at the interparticle contacts but this would be accompanied by merging of adjacent particles at their points of contact due to the compressive effect of the normal component of the interparticle forces.
With the passage of time, such merging would progressively reduce the interparticle stresses and consequently the creep strain rate. Ultimately, the interparticle stresses would be reduced to a value less than the yield stress of the scree material and creep strain would cease.
I very much welcome Arthur Penman's comment on the need for more research to develop reliable methods for predicting ground movements and I believe that to achieve that goal it is necessary to recognise the time dependent nature of the deformation behaviour of geotechnical materials.
Honorary Research Fellow